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Full text of "Evaluation of ground water resources, South Bay"

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California. Dept. of Water 
Resources. B\illetin. 




STATE OF CALIFORNIA 
The Resources Agency 

partment of Water Resources 

in cooperation with 
Alameda County Water District 



BULLETIN No. 118-1 



EVALUATION OF GROUND WATER RESOURCES: 
SOUTH SAN FRANCISCO BAY 

Volume II: ADDITIONAL FREMONT AREA STUDY 



NORMAN B. LIVERMORE, JR. 
Secretary for Resources 
The Resources Agency 



AUGUST 1973 



RONALD REAGAN 

Governor 

State of California 



DEC 06 1983 

f. QVT. DOCS.- LIBRARY 

WILLIAM R. GIANELLI 

Director 

Department of Water Resources 



Digitized by the Internet Arciiive 

in 2010 with funding from 

Kahle/Austin Foundation and Omidyar Network 



http://www.archive.org/details/evaluationofgrou11814cali 



STATE OF CALIFORNIA 
The Resources Agency 

Department of Wa ter Resources 

in cooperation with 
Alameda County Water District 

BULLETIN No. 118-1 



EVALUATION OF GROUND WATER RESOURCES: 
SOUTH SAN FRANCISCO BAY 

Volume II: ADDITIONAL FREMONT AREA STUDY 



Copies of this bulletin at $3.00 each may be ordered from: 

State of California 

DEPARTMENT OF WATER RESOURCES 

P.O. Box 388 

Sacramento, California 95802 
Make checks payable to STATE OF CALIFORNIA 
California residents odd soles tax 



AUGUST 1973 



NORMAN B. LIVERMORE, JR. 
Secretary for Resources 
The Resources Agency 



RONALD REAGAN 

Governor 

State of California 



WILLIAM R. GIANELLI 

Director 

Department of Water Resources 



The Bulletin No. 118 series, which is published by the Department of Water 
Resources for all interested agencies and the general public, includes: 



Bulletin No. 118-1 



Evaluation of Ground Water Resources; South Bay 
Appendix A: Geology, August 1967 
Volume I: Fremont Study Area, August 1968 
Volume II: Additional Fremont Area Study, 



Volume III: North Santa Clara County 
(now under study) 

Bulletin No. 118-2 Evaluation of Ground Water Resources; Livermore and 

Sunol Valleys (now under study) 

Appendix A; Geology, August 1966 

After completion of the evaluation studies, operations-economics studies of 
each ground water basin or study area will be scheduled and conducted 
cooperatively with local agencies. 



11 



FOREWORD 



The South Bay Ground Water Basin underlies south San Francisco Bay and the 
gently sloping lands adjacent to the Bay in Alameda, San Mateo, and Santa 
Clara counties. The ground water basin is divided into three main units: 
the Fremont study area, containing the Bay and southern Alameda County; the 
Santa Clara study area to the south; and the San Mateo study area to the 
west. 

In the Fremont study area, extractions exceeded recharge for many years, 
resulting in extensive salt water intrusion of the ground water aquifers. 
The Alameda County Water District has countered the salt water intrusion by 
augmenting the ground water supplies of the Fremont study area with imported 
water supplies from the South Bay Aqueduct of the State Water Project and 
the City of San Francisco's Sunol Aqueduct. Withdrawals from the basin were 
also reduced by using imported water from the Hetch Hetchy Aqueduct. 

This report is a supplement to Bulletin No. 118-1, "Evaluation of Ground 
Water Resources, South Bay, Volume I: Fremont Study Area", published in 
August 1968. The report presents the results of additional studies by the 
Department in cooperation with the Alameda County Water District, contains 
additional detailed geology of the area, and presents an accounting of recharge 
to and withdrawals from the ground water basin for the period October 1961 
through September 1970. 

During the period studied, actions of the local operating agency have resulted 
in a recovery of water levels in the ground water basin. However, the basin 
is still endangered by saline intrusion and preliminary design of a salt water 
barrier should be completed and construction started promptly. The conceptual 
plan for a salt water barrier is described in this report. Detailed planning 
for the barrier and testing of materials to be used for construction of the 
barrier are continuing as part of the cooperative study by the Department and 
the Alameda County Water District. 



William R. Gianelli, Director 
Department of Water Resources 
The Resources Agency 
State of California 
July 25, 1973 



iii 



TABLE OF CONTENTS 

Page 

BULLETIN NO. 118 SERIES ii 

FOREWORD iii 

ORGANIZATION vlii 

CALIFORNIA WATER COMMISSION .... ix 

ALAMEDA COUNTY WATER DISTRICT ix 

ACKNOWLEDGEMENT x 

ABSTRACT r • • ^ 

CHAPTER I. SUMMARY, FINDINGS AND RECOMMENDATIONS ... 1 

Study Objectives 1 

Study Results 1 

Findings 2 

Recommendations 3 

CHAPTER II. AQUIFER CHARACTERISTICS . 5 

Computer Assisted Subsurface Geologic Evaluation 5 

Sequences of Aquifers and Aquitards ..... 8 

CHAPTER III. AQUITARD CHARACTERISTICS 23 

Oxnard Plain Studies and Their Relationship to Fremont Area 23 

Current Investigation 25 

CHAPTER IV. SALINE WATER INTRUSION, STATUS AND CONTROL 29 

Extent of Saline Intrusion 29 

Volume of Saline Intrusion 32 

Effect of Saline Intrusion on Water Supply 33 

Control of Saline Intrusion 33 



iv 



TABLE OF CONTENTS (Continued) 

Page 

CHAPTER V. EVALUATION OF HISTORIC WATER SUPPLY AND DISPOSAL .... 37 

Study Area 37 

Ground Water Model 37 

Study Period 37 

General Conditions 40 

Precipitation 40 

Streamflow 40 

Land Use 40 

Imported Water 40 

Annual Deliveries 40 

City of San Francisco 45 

State of California 45 

Ground Water Inventory 46 

Direct Recharge of Precipitation and Delivered Water 46 

Depth of Recharge 49 

Annual Recharge 49 

Recharge from Streamflow 49 

Alameda and Dry Creeks Area 49 

Remainder of Study Area 51 

Subsurface Inflow 54 

Compaction of Clays 54 

Ground Water Pumpage ..... 54 

Saline Water Inflow 54 

Annual Inventory 55 

Change In Storage 55 



FIGURES 

Figure 

Number Page 

1 Location Map . Facing Page 1 

2 Geologic Sections 10 

3 Subsurface Depositional Patterns 15 

4 Salt Concentrations in Nevark Aquitard 26 

5 Intrusion of Salt Water into the Fremont Study Area ... 30 

6 Ground Water Contours and Isochlors 31 

7 Hydrographs at Selected Wells ... 35 

8 Conceptual Plan for Proposed Barrier 36 

9 Mathematical Model 38 

10 Cumulative Departure of Annual Precipitation from 

94-Year Mean 39 

11 Hydrologic System (Schematic) 47 

12 Relative Recharge Capability 52 

13 Clay Content of Upper Soil Strata 53 

14 Accumulated Change in Storage . .' 57 

TABLES 

Table 

Number " Page 

1 Specific Yield Values for Drillers Calls 7 

2 Salt Concentrations in Aquitard Pore Water 28 

3 Annual Amounts of Saline Intrusion ,...,.... 32 

4 Annual Amounts of Water Use 33 

5 Annual Precipitation and Index of Wetness 41 

6 Recorded Annual Runoff 42 



VI 



TABLES (Continued) 

Table 

Number Page 

7 Ungaged Tributary Hillside Runoff 44 

8 Land Use, Fremont Model Area 44 

9 Imported Water 45 

10 Agricultural Water Use Factors 48 

11 Depths of Applied Water 50 

12 Average Daily Evaporation Rates . 50 

13 Depths of Recharge and Runoff from Applied Water 

and Precipitation 51 

14 Ground Water Inventory 56 

15 Change in Storage 56 



PLATES 

Plate 

Number Page 

1 Land Use, 1972 Following Page 57 



vii 



State of California 

The Resources Agency 

DEPARTMENT OF V.'ATER RESOURCES 



RONALD R. REAGAN, Governor, State of California 

NORMAN B. LIVERMORE, JR., Secretary for Resources 

WILLIAM R. GIANELLI, Director, Department of Water Resources 

JOHN R. TEERINK, Deputy Director 



CENTRAL DISTRICT 
Robin R. Reynolds District Engineer 

This investigation was conducted 
under the supervision of 

Donald J. Finlayson Chief, Water Utilization Branch 

by 

Robert S. Ford Senior Engineering Geologist 

Edward E. Hills Associate Engineer 

In cooperation with 

ALAMEDA COUNTY WATER DISTRICT 

MATHEW P. WHITFIELD, General Manager and Chief Engineer 
STANLEY R. SAYLOR, Assistant Chief Engineer 

Under the supervision of 

Earl Lenahan : Senior Engineer 

Assisted by 

Joseph D. Newton Electrical Engineering Associate 

Houshang Poustinchi Junior Engineer 

James R. Reynolds Junior Engineer 

Allen Cuenca Engineering Technician IV 

James L. Ingle "■ Engineering Technician II 

Vernon J. Vargas Engineering Technician II 

Glenn D. Berry Engineering Technician I 

William B. Dewhirst Engineering Technician I 

Material on Aquitards was furnished by the 

Geotechnical Engineering Group 

Department of Civil Engineering 

University of California, Berkeley 

Under the supervision of 

Paul A. Watherspoon, Ph.D. .... Professor of Geological Engineering 

Assisted by 

Marcello Lippmaii Research Assistant 

Esteban Cremonte Research Assistant 



vili 



State of California 

Department of Water Resources 

CALIFORNIA WATER COMMISSION 



IRA J. CHRISMAN, Chairman, Visalia 
CLAIR A. HILL, Vice-Chalrman, Redding 

Mai Coombs Garberville 

Ray W. Ferguson Ontario 

William H. Jennings La Mesa 

Clare Wm. Jones Firebaugh 

William P. Moses San Pablo 

Samuel B. Nelson Northridge 

Ernest R. Nichols Ventura 

Orville L. Abbott 
Executive Officer and Chief Engineer 

Tom Y. Fujimoto, Staff Assistant 






ALAMEDA COUNTY WATER DISTRICT 

Harry D. Brumbaugh, President 
John R. Gomes, Director Clark W. Redeker, Director 

Frank Borghi, Director Carl H. Strandberg, Director 



IX 



FIGURE 



<r 



V- Ni-cv\v-. 




aywat'd, V 



/ 






LOCATION MAP 

LEGEND 

/■^/^-^ GROUND WATER BASIN 
BOUNDARY 

•''^•>w. SUBBASIN BOUNDARY 

Q*l"n ^ STUDY AREA 




CHAPTER I. SUMMARY, FINDINGS AND RECOMMENDATIONS 



The Fremont study area, shown on Figure 1, is located in southwestern Alameda 
County and occupies the northeastern portion of the South San Francisco Bay 
ground water basin. From the 1920' s to the present, saline water intrusion has 
been a problem in the area. The utility of the ground water reservoir has been 
preserved by the Alameda County Water District through the construction and 
operation of recharge facilities and the importation of water purchased from the 
State of California (State Water Project) and the City of San Francisco (Hetch 
Hetchy System) . 

Study Objectives 

Detailed geologic and hydrologic studies of the Fremont area were made in the 
1960 's and the results published in two Department reports: Bulletin No. 118-1, 
"Evaluation of Ground Water Resources, South Bay, Volume I: Fremont Study Area", 
August 1968; and Appendix A, "Geology", August 1967. In June 1968, the Department 
and the Alameda County Water District entered into an agreement to study the 
ground water resource on a cooperative basis. The objectives of the study were: 

1. Modification of the District's data collection program to provide greater 
areal coverage and increased reliability of data. 

2. Further definition of the subsurface geology and hydrology of the ground 
water basin based on additional data obtained from the modification of data 
collection networks, drilling of test holes and pump testing. 

3. Review of alternative methods of controlling saline water intrusion and the 
development of preliminary plans and costs for a proposed saline water 
barrier. 

4. Development of criteria for use and operation of artificial recharge 
facilities. 



Study Results 

The cooperative study during the 1968-72 period has accomplished these objectives 
with the exception of the fourth, relating to the operation of the recharge 
facilities. The continuing construction of the new Alameda Creek flood control 
channel through the recharge facilities has forced this portion to be postponed, 
although the ground water model being developed during the study will assist in 
determining operational plans for the recharge facilities. 

Modifications in the District's data collection program have been made during the 
study to take advantage of the more detailed information on the hydrology and 
subsurface geology of the ground water basin. The data collection program now 



-1- 



records changes in ground water levels and quality for each of the several 
aquifers and has been expanded to cover the entire study area. 

The result of the geologic study is a detailed mapping of the subsurface channels 
of Alameda Creek and adjacent streams, and is presented in Chapter II as an 
extension of information presented in Volume I and Appendix A of Bulletin 118-1. 
The detailed mapping was accomplished by a new approach, utilizing computer 
methods to evaluate subsurface geologic data. This work is significant in that 
it provides the basis for the location and design of an efficient salinity barrier, 
and indicates that the subsurface flow of water is highly directional, an 
important input for the successful modeling of the basin. The model of the basin 
will be used in planning the salinity barrier. Understanding the separate roles 
played by aquifers and by aquitards in the ground water system is a necessary 
preliminary to controlling saline water intrusion. Aquifer and aquitard 
characteristics are described in Chapters II and III. Each of these can be 
defined as: 

Aquifer - A porous, water-bearing geologic formation. Generally restricted 
to materials capable of yielding an appreciable supply of water. 

Aquitard - A geologic formation which, although porous and capable of 
absorbing water slowly, will not transmit it rapidly enough to furnish an 
appreciable supply for a well or spring. The permeability is so low that 
for all practical purposes, water movement is severely restricted. When 
separating extensive aquifers having a large head differential between 
them, it acts as a confining bed but the total water movement may be 
significant even though water movement per acre is insignificant. 

The results of the hydrologic studies are presented in Chapter IV as the status 
of saline water intrusion, and in Chapter V as an extension of the ground water 
inventory contained in Bulletin 118-1, Volume I, August 1968. 

Review of alternative ways of controlling sea water intrusion indicated that a 
series of shallow pumping wells placed in the center of the subsurface channels 
defined in the geologic study could intercept saline water flowing into the basin 
and at the same time establish a bayvard gradient in the intruded upper aquifer. 
This type of plan, called a pumping trough barrier, has been adopted as a basic 
plan. The preliminary location for the barrier reported on in Chapter IV uses 
the Coyote Hills as the central section and the easte^;n limits of the salt evapo- 
ration ponds as the north and south sections. As part of the continuing study, 
the District and Department have installed and tested one experimental well and 
are in the process of designing a second installation. Both agencies plan to 
continue developing a workable barrier design as rapidly as possible. 

Findings 

During the decade 1961-71, the amount of ground water in storage has been signi- 
ficantly increased by over 60,000 acre-feet and water levels have recovered 
approximately 55 feet in the forebay adjacent to the upper portion of Alameda 
Creek. During the same period average pumpage for beneficial uses has remained 
at approximately 40,000 acre-feet per year. Operation of gravel quarries during 



-2- 



the last three years of the study period involved pumping to lower water levels 
in the quarries. The water pumped by the quarries was wasted to San Francisco 
Bay. This practice was stopped in May 1971 by a Superior Court injunction 
obtained by the Alameda County Water District. The improvement in the ground 
water situation is primarily due to the importation and recharge by the Water 
District of large amounts of water through the State Water Project's South Bay 
Aqueduct. 

The Alameda County Water District has plans to reduce the total pumpage for con- 
ventional uses from the basin for the next five years. An 8.0 million gallons per 
day water treatment plant to treat South Bay Aqueduct water for the District's 
distribution system is scheduled for completion in 1974, and this plant will be 
operated to reduce the District's pumping. 

The District's full recharge capability has been used to meet pumping demands 
and to refill the ground water basin. By late 1972 the piezometric surface of 
the upper aquifer was at sea level. Recharge capability in excess of the 
requirements to maintain this level in the upper aquifer will be used to replace 
saline water that the District plans to pump from the basin. These plans are to 
pump saline water that is trapped in the Centerville, Fremont and deep aquifers 
into San Francisco Bay. If this saline water is not removed, it will spread to 
the usable parts of these aquifers and thus render them unusable. 

It is important to complete preliminary design of a sea water barrier and to begin 
construction of a barrier. There are three compelling reasons for prompt action: 
(1) any decrease in the supply to or the operation of the recharge facilities can 
cause large amounts of salt water to intrude the basin; (2) uncontrolled migration 
of saline water from the upper intruded aquifer to the lower producing aquifers 
will continue to lessen the utility of the entire basin (initial operation of the 
barrier would withdraw saline water from the upper aquifer); and (3) the 
necessarily long construction time required to complete the barrier. 

Recommendations 

It is recommended that the planning of the sea water intrusion barrier and develop- 
ment and testing of prototype barrier wells, which are part of the current 
Department-District study, be completed as soon as possible so that the District 
can make a decision on starting a long range barrier construction program as 
rapidly as possible. Barrier wells should be designed and installed one or two 
at a time, tested, and results used to improve design of the next series of wells. 



-3- 



CHAPTER II. AQUIFER CHARACTERISTICS 

The identification of horizontal and vertical boundaries of aquifers and aquitards 
is extremely difficult in most alluvial-filled valleys of California. In the 
past, this identification has been accomplished only on a gross scale and has 
been derived through the construction of geologic sections using drillers' logs 
of water wells as well as electric logs of oil and gas wells. Using this method, 
generalized formational boundaries and member boundaries can usually be deter- 
mined. The subsurface data presented in Volume I of Bulletin 118-1, August 1968, 
and in Appendix A, August 1967, of that volume were derived in this manner. 

This method of analysis does not provide the degree of detail that is required 
for operational studies of some ground water basins, particularly those in which 
older buried stream channels provide the media through which the major portion of 
ground water moves. Consequently, a new approach utilizing computer methods was 
developed to determine the continuity of the various aquifer systems present in 
the Fremont study area. In this approach, use was made of the now buried depo- 
sitional patterns which make up the Niles Cone. In the construction of a 
depositional feature such as the Niles Cone, the contributing stream (in this 
case Alameda Creek) has meandered back and forth across the up to 12-mile width 
of the cone, depositing stream-borne materials which range in size from coarse 
gravel and boulders down to clay. During periods of normal runoff, a stream 
course is established which contains the coarsest grained materials. These 
materials grade from large gravels and boulders at the apex of the cone to sand 
and silt at its distal end. Adjacent to the stream channel are clays and silts 
which grade outward to even finer grained materials. Periodically, during 
periods of storm runoff, the stream will abandon its course and seek a new route 
down the surface of the fan. It also may meander over short distances of less 
than a thousand feet, thus forming braided channel deposits. In time, as 
deposition continues, the abandoned stream channels become covered with younger 
materials. These materials usually are fine grained, thus isolating the old 
stream channel and converting it into a tabular aquifer. In a few cases, younger 
stream channels may form along or across older channels, thus creating areas of 
hydraulic continuity between different channel deposits. In a few cases, the 
older, buried channels may subsequently become warped or cut off due to regional 
tilting or faulting. 

Computer Assisted Subsurface Geologic Evaluation 

In the Fremont study area, a special computer program was developed to utilize 
information on the subsurface materials derived principally from logs of water 
wells. In analyzing these logs, it was found that the "calls" used by various 
drillers differed for the same material. It also was found that drillers' 
calls may be grouped, and thus a statistical analysis may be made based on these 
calls. This same approach was used by the U. S. Geological Survey, which grouped 
the drillers' calls by specific yield values in its study of the San Joaquin 
Valley. This grouping of calls, modified for the Fremont study area, is 



-5- 



presented on Table 1. The steps in the geologic analysis which utilized this 
grouping are briefly described below. 

1. The deepest well per quarter-quarter section (a one-quarter mile spacing) in 
the study area was identified and the values of the equivalent specific 
yield (ESY) tabulated for each material reported on the log. Equivalent 
specific yield is defined as being equal to the specific yield of a given 
niaterlal under unconfined conditions. The ESY of a material is a pure number 
and remains the same whether the material is presently under confined or 
unconfined conditions, as it relates to the relative grain size and not to 
the quantity of ground water which could be derived from it. 

2. The ESY values were averaged for 10-foot increments of elevation for each 
well used. 

3. The averaged ESY values were then converted to symbolic form for utilization 
in graphic presentation. Four symbols were used which represent the main 
types of depositional material: 

Range of Typical 

Symbol ESY Values Material 

1 to 7 Clay, Bay Mud, Silt 

8 to 12 Clay with Fine Sand 

+ 13 to 17 Sand with Clay Streaks 

18 to 25 Gravel, Coarse Sand 

4. Using a computer program, the sjnnbollc ESY values were printed out areally 
for each 10-foot increment of elevation at a horizontal scale of 1-inch 
equals 4,000 feet. Each of these "maps" were then printed on transparent 
media and prepared for viewing and analysis. 

5. Geologic interpretation of the several maps was then made by stacking them 
in ascending order of elevation. In this case, maps of the Fremont area 
were made for the intervals of -550 to -540 feet up to +190 to +200 feet. 
By viewing the maps from above, the traces of the^buried stream channels 
could be seen meandering down through the various levels. Also, areas of 
fine grained material could be identified as well as zones of hydraulic 
continuity between various levels. 

6. It was recognized that several layers of clay, or aquitards, exist in the 
Fremont area, and it is believed that much of this material was deposited 
during times of a higher sea level. Thus it was concluded that zones of 
aqueously deposited clay could be identified and traced, as these clays are 
predominantly colored blue, green, or gray due to the reduced state of the 
iron present in the clays. In contrast, terrestrially deposited clays tend 
to contain iron in an oxidized state and thus are colored yellow, brown, or 
red. 



-6- 



TABLE 1 

SPECIFIC YIELD VALUES 
FOR DRILLERS CALLS 



General Material Type 
and Specific Type 



Drillers Calls 



Crystalline Bedrock 
Specific Yield = 
00 Percent 

Clay and Shale 
Specific Yield = 
03 Percent 



Clayey Sand and Silt 
Specific Yield = 
05 Percent 



Cemented or Tight 
Sand or Gravel 

Specific Yield » 
10 Percent 



Gravel and Boulders 
Specific Yield = 
15 Percent 



Fine Sand 
Specific Yield 
15 Percent 



Granite 


Hard Rock 




Lava 


Rock 




Adobe 


Granite Clay 


Shale 


Boulders in Clay 


Hard Clay 


Shaley Clay 


Cemented Clay 


Hard Pan 


Shell Rock 


Clay 


Hard Sandy Shale 


Silty Clay Loam 


Clayey Loam 


Hard Shell 


Soapstone 


Decomposed Shale 


Muck 


Smearey Clay 




Mud 


Sticky Clay 


Chalk Rock 


Peat 


Sandy Clay 


Clay and Gravel 


Peat and Sand 


Sandy Silt 


Clayey Sand 


Pumice Stone 


Sediment 


Clayey Silt 


Rotten Conglomerate 


Shaley Gravel 


Conglomerate 


Rotten Granite 


Silt 


Decomposed Granite 


Sand and Clay 


Silty Clay 


Gravelly Clay 


Sand and Silt 


Silty Loam 


Lava Clay 


Sand Rock 


Silty Sand 


Loam 


Sandstone 


Soil 


Arcade Sand 


Cemented Sand 


Hard Sand 


Black 


Cemented Sand and 


Heavy Rocks 



Blue Sand 
Caliche 

Cemented Boulders 
Cemented Gravel 



Cobbles and Gravel 
Coarse Gravel 
Boulders 
Broken Rocks 



Gravel 
Dead Gravel 
Dead Sand 
Dirty Pack Sand 
Hard Gravel 



Gravel and Boulders 
Heaving Gravel 
Heavy Gravel 
Large Gravel 



Fine Sand 



Quicksand 



Lava Sand 
Soft Sandstone 
Tight Boulders 
Tight Coarse Gravel 
Tight Sand 

Rocks 

Sand & Gravel, Silty 
Tight Fine Gravel 
Tight Medium Gravel 
Muddy Sand 

Sand, Gravel, and 
Boulders 



Sand and Gravel 
Specific Yield = 
20 Percent 



Dry Gravel 
Loose Gravel 



Gravelly 
Gravelly Sand 
Medium Gravel 



Sand and Gravel 

Sand 

Water Gravel 



Coarse Sand and 
Fine Gravel 

Specific Yield = 
25 Percent 



Coarse Sand 



Fine Gravel 



Medium Sand 

Sand and Pea Gravel 



Based on Geological Survey Water Supply Paper 1169, "Ground Water Conditions and 
Storage Capacity in the San Joaquin Valley, California", 1959. 



-7- 



A separate computer program using the same well logs was developed to separate 
reduced and oxidized clays. The color of the materials was noted for each 
elevation increment and this information was put into a computer program which 
printed out the percent of reduced clay, ranging from for a 10-foot thickness 
of oxidized clay to 99 for a like thickness of reduced clay. Using these data 
in conjunction with the ESY data, it was found that certain zones of the fine 
grained materials were composed principally of reduced clay and thus probably 
were deposited subaqueously. Because of this, it may be assumed that the 
subaqueous clays are fairly continuous and serve as aquitards. 

Geologic sections which were prepared from well logs and the area printouts are 
presented as Figure 2. Figure 3 presents configurations of aquifer and aquitard 
materials at selected elevation intervals. Examination of the various maps and 
sections will show that the Fremont study area is roughly divisible into several 
aquifer zones and aquitards. From the ground surface downward, these zones, 
which are indicated on the geologic sections, are: Newark Aquitard, Newark 
Aquifer, Irvington Aquitard, Centerville Aquifer, Mission Aquitard, and Fremont 
Aquifer. 

For interpretative purpose, materials have been separated into aquifer and 
aquitard groups on the basis of having average specific yield values of under 
or over 7 percent. The transmissibility of the aquifer materials increases 
generally with increasing specific yield, with a low transmissibility rate for 
specific yields near 8 percent. 

Interpretation of the data uses average values for 10-foot elevation increments. 
As a result, the geologic sections may show aquifer or aquitard materials to be 
five feet thicker or thinner than the actual thickness. Surface exposures of 
aquifer material shown in the geologic sections should be interpreted as meaning 
that aquifer materials are present in the first ten feet of depth. This does not 
however, preclude the existence of extensive clay deposits of up to approximately 
seven feet thickness. In addition, the grain size of aquifer materials becomes 
finer with increased distance from the apex of the alluvial fan formed by Alameda 
Creek. This fan, called the Niles Cone, is the major physiographic feature of the 
Bay Plain portion of the Fremont study area. All of the aquifers and aquitards in 
this area are present as beds within this cone, as most of the materials were 
either derived from deposition by Alameda Creek or were influenced by it. 

Sequences of Aquifers and Aquitards 

The Newark aquitard is exposed at the ground surface throughout much of the 
Fremont area. This is the "clay cap" that is commonly spoken of by the various 
well drillers. The aquitard is composed of a mixture of fine material deposited 
subaqueously and on land, slopes gently bayward, and is expressed on Sheets 1 
and 2 of Figure 3 as the open area southwest, west, and northwest of the large 
area of aquifer material near Niles. Because some of the aquitard was transected 
by stream channels, several isolated bands of channel deposits are shown crossing 
it. 

Lying immediately below the Newark aquitard is the Newark aquifer, which shows 
its greatest expression on Sheet 3 of Figure 3, in the elevation interval -30 to 
-40 feet. Subsurface relationships of this aquifer are shown in the geologic 



-8- 



sections on Figure 2. A minimum of aquifer material is shown on Sheet 6 of 
Figure 3, representing the elevation interval of -120 to -130 feet. This is 
inferred to be the main zone of the Irvington Aquitard in the eastern portion 
of the Niles Cone, increasing in thickness to an interval of -120 to -160 feet 
in the portion of the Niles Cone southeasterly and northerly of the Coyote 
Hills. The eastern portion of the clay zone also contains stringers of channel 
material. The clay zone westerly of the Coyote Hills is primarily subaqueously 
deposited fine material. Below the Irvington Aquitard is the Centerville Aquifer 
which is depicted on the geologic sections shown in Figure 2. It attains its 
greatest expression in the interval from -180 to -190 feet, as shown on Sheet 8 
of Figure 3. 

Of major importance to the understanding of salt water intrusion and its control, 
are the locations of the subsurface channels connecting the Newark Aquifer with 
lands underlying the salt evaporation ponds and South San Francisco Bay. The 
locations of the subsurface channels connecting the various aquifers with the 
main recharge areas is important in planning recharge programs and in selecting 
well locations. The axes of the subsurface channels between elevations +30 
and -70 are shown on Figure 3, Sheets 1-4. 



-9- 



INDEX TO 
GEOLOGIC SECTIONS 



SECTION 


SHE 


F-0 


1 


F-l. F-2 


2 


F-3.F-4 


3 


F-6,F-6 


4 


F-7 


S 



LEGEND 




S^ AQUIFER (MATERIALS HAVING SPECIFIC YIELDS 
^~^ GREATER THAN 7 PERCENT) 



LIMIT OF DATA 



NORTHWEST 



+100 



-100 



= -200 



-30C 



-40a 



SOUTHEAST 
»I00 




100 



200 



300 



400 



20 25 30 

STATION IN 1.000 FEET 



FIGURE 2 - GEOLOGIC SECTIONS 



10 



WEST 



«I00 



-100 



-200 



-300 



-4001 




• 100 



100 



200 > 



-300 



400 



20 30 

SI/VTION IN 1 000 FEEI 



LEGEND 



AQUIFER (MATERIALS HAVING SPECIFIC 

YIELDS GREATER THAN 7 PERCENT) 

LIMIT OF DATA 



WEST 



EAST 



♦ 100 



2 -200r 




• 100 



100 



200 5 



-300 



400 



STATION IN 1 000 FEET 



FIGURE 2 -GEOLOGIC SECTIONS 



SHEET 2 of 5 



♦ 100 s 



-lOOl 




-3oa 



SIAtlON IN 1 000 FEEI 



SOUTHWEST 



NORTHEAST 




>I00 



100 



200 



3O0 



400 



STATION IN t 000 FEET 



FIGURE 2- GEOLOGIC SECTIONS 



SHEET 3 of 5 



12 





SOUTH 






^ 


t 


tlOO 




i 

u 






s 



NEWARK 


-^ 






;<s 


► ■" 


^ 


~ 





AQUIFER 



-lOOi 



S -zoo 



-joa 



-40W 




SECTION F.5 



»I00 



100 E; 



-200 si 



-JOO 



400 



'^^ SI&IION IN 1 000 FEET '° 



NORTH 



*I00 



-100 




• 100 



100 



200 



JOO 



400 



STATION IN 1 000 FEET 



FIGURE 2 - GEOLOGIC SECTIONS 



SHEET 4 of 5 



♦lOOrg- 



-100 



-200 



-3O0f 



-400f 




»I00 



100 E 



-200 



3O0 



-400 



SIAIION IN 1 000 FEEI 



EAST 



+100 



-100 



-200 



-300 



-400(- 




STATION IN 1 OOO FEET 



FIGURE 2 -GEOLOGIC SECTIONS 



SHEET 5 of 5 



14 



INDEX 






% 



DEPOSITION 
INTERVAL 

+ 30 TO +20 

TO - I 

-30 TO -40 

-60 TO -70 

-90 TO -100 

- 120 TO -130 
-150 TO - 160 

- 180 TO - 190 



SHEET 



h 



cx 



/ 



/ 



/■ 



/ 




/ 



/ 



/ \ 



\ 



\ 



\ 



\ 



/ 



/ 



\ 



\; 



f/ 



/ 



"•^. 



\ 



\ 



\ 



\ 



HAX 



r 




MOUNTAIN 
O VIEW 



LEGEND 
BEDROCK CONTACT AT GROUND SURFACE 
FAULT, DASHED WHERE INFERRED 
AREAL EXTENT OF AQUIFER 
AXIS OF SUBSURFACE DEPOSITION 

SCALE OF FEET 



Figure 3. SUBSURFACE DEPOSITIONAL PATTERNS 
SHEET I OF 8 ELEVATION INTERVAL +30 FEET TO +20 



FEET 



IS 



(J^ 







Ci 



cn 



/. 



>c 



.'^V 



/ 



/ 



/ 



PALO /\ 



/ 



/ 



NL 






MOUNTAIN 
O VIEW 



SHEET 2 OF 8 




L E G E N D 
BEDROCK CONTACT AT GROUND SURFACE 
AXIS OF SUBSURFACE DEPOSITION 
AREAL EXTENT OF AQUIFER 
AREAL EXTENT OF REDUCED CLAY 

SCALE OF FEET 



Figure 3. SUBSURFACE DEPOSITIONAL PATTERNS 



ELEVATION INTERVAL 



FEET TO 



16 




17 



^/.- 



*../*. 



■4C: 



(x> 




..^, 




\ 



\ 



MOUNTAIN 
O VIEW 



/ \ 



\ 



:y 



\ 



■.\ 



//. 



//. 



yxc ■ 



// 






J-. 



^c 



V 



^ 



\ 



X 



\ 



z^^^ 



1^ 



X 



*-• - r " 



\ 




/ 



\ 



/ 



/ 



z,- 



t \ 



/" 



\ 



/ 



L E G E N D 



\ 



\ 



\ 



BEDROCK CONTACT AT GROUND SURFACE 
'——•.' AXIS OF SUBSURFACE DEPOSITION 
•«^p«S!i|| AREAL EXTENT OF AQUIFER 
•"U / AREAL EXTENT OF REDUCED CLAY 



SCALE OF FEET 



Figure 3. SUBSURFACE DEPOSITIONAL PATTERNS 
SHEET 4 OF 8 ELEVATION INTERVAL -60 FEET TO -70 FEET 




^VM 



8 



<J^ 







f/ 



tr> 



o> 



Q> 




N 



\ 



/ 



/ 






^•^ 



/ 



PALO Av 
ALTO/^ 

/ 



\ 



7/, 



MOUNTAIN 
O VIEW 



/ 






/' 



L E G E N D 

BEDROCK CONTACT AT GROUND SURFACE 
AXIS OF SUBSURFACE DEPOSITION 
AREAL EXTENT OF AQUIFER 
^^/ ^ AREAL EXTENT OF REDUCED CLAY 



SCALE OF FEET 
«000 I2CXXI 




Figure 3. SUBSURFACE DEPOSITIONAL PATTERNS 

SHEET 5 OF 8 ELEVATION INTERVAL -90 FEET TO -100 FEET 



19 




MOUNTAIN 
O VIEW 



BEDROCK CONTACT AT GROUND SURFACE 
AXIS OF SUBSURFACE DEPOSITION 
AREAL EXTENT OF AQUIFER 
AREAL EXTENT OF REDUCED CLAY 

SCALE OF FEET 



Figure 3. SUBSURFACE DEPOSITIONAL PATTERNS 

SHEET 6 OF 8 ELEVATION INTERVAL -120 FEET TO -130 FEET 



20 




ALTO ^ 



MOUNTAIN 
O VIEW 



/ 

E 



BEDROCK CONTACT AT GROUND SURFACE 
AXIS OF SUBSURFACE DEPOSITION 
AREAL EXTENT OF AQUIFER 
AREAL EXTENT OF REDUCED CLAY 



SCALE OF FEET 



Figure 3. SUBSURFACE DEPOSITIONAL PATTERNS 
SHEET 7 OF 8 ELEVATION INTERVAL -150 FEET TO -160 FEET 



21 




MOUNTAIN 
O VIEW 



BEDROCK CONTACT AT GROUND SURFACE 
AXIS OF SUBSURFACE DEPOSITION 
AREAL EXTENT OF AQUIFER 
AREAL EXTENT OF REDUCED CLAY 



SCALE OF FEET 
4 000 12000 



Figure 3. SUBSURFACE DEPOSITIONAL PATTERNS 

SHEET 8 OF 8 ELEVATION INTERVAL -180 FEET TO -190 FEET 



2^ 



CHAPTER III. AQUITARD CHARACTERISTICS 

Nonsteady (fluctuating) flow of ground water to wells has traditionally been 
analyzed by considering each aquifer as an independent geologic and hydrologic 
unit. In the Fremont area at least three such aquifers exist, e.g., the Newark, 
Centerville, and Fremont Aquifers. Each of these aquifers is confined from 
above and below by layers that are of significantly less permeability. These 
layers, previously identified as aquicludes, have been found to possess definite 
permeability characteristics, to be compressible to some degree, and to release 
some water from storage. The descriptive term now applied to these confining 
beds is aquitards. Aquifers above or below the aquitards are termed leaky 
aquifers. 

Because leakage suggests that there is some degree of hydraulic continuity between 
aquifers that are separated by an aquitard, the behavior of each aquifer is 
closely related to the behavior of the entire system. Hence, the group of aqui- 
fers and aquitards in the Fremont area should be considered as a multiple aquifer 
system rather than a group of individual aquifers. 

Oxnard Plain Studies and 
Their Relationship to Fremont Area 

Recent studies in the Oxnard area of Southern California sponsored by the 
Department of Water Resources and reported on in Bulletin 63-4, "Aquitards in 
the Coastal Ground Water Basin of Oxnard Plain, Ventura County", September 1971, 
indicate that aquitards play a very important role in the overall ground water 
systems of coastal ground water basins. The layering of aquitards and aquifers 
at Oxnard are analogous to those in the Fremont area and the role of the 
aquitards in both areas have similarities. 

The aquitards in the Oxnard basin were found to have an average vertical permea- 
bility of about 10"^ cm/sec (0.02 gpd/ft^). Bulletin 81, "Intrusion of Salt 
Water into Ground Water Basins of Southern Alameda County", December 1960, 
reported a vertical permeability value range of 0.002 to 0.016 gpd/ft per foot 
of head for the Irvington Aquitard. Sensitivity analysis using the mathematical 
model of the Fremont study area made in 1967 indicated vertical permeability of 
the Irvington Aquitard separating the Newark and Centerville Aquifers (Figure 2) 
is in the 0.002 to 0.012 gpd/ft^ range. 

After giving consideration to distance from the apex of the depositional cones, 
the effect of the Coyote Hills and the depositional environment, it is estimated 
that the permeability of the Newark aquitard east of the Coyote Hills is at least 
10"^ cm/sec (0.2 gpd/ft^), while under the Bay it is assumed to be 10~° cm/sec 
(0.02 gpd/f t^) . The permeability of the deeper Irvington aquitard is believed to 
be 10"' cm/sec (0.002 gpd/ft^). There are two reasons for the differences in 
permeability: (1) the clays in the Newark aquitard are composed of mixtures of 



-23- 



reduced and oxidized clays, while those in the Irvington aquitard are primarily 
reduced clays; and (2) the Newark aquitard includes more small subsurface channels 
than the Irvington aquitard. With an assumed permeability of 10~° cm/sec 
(0.02 gpd/ft ) for the Newark aquitard and under a unit gradient of 1 ft/ft, about 
560,000 gpd, or 630 acre-feet per year, may move vertically across an aquitard 
having an area of one square mile. In the Fremont area, where there is a landward 
gradient in the Newark aquifer, it is possible for salt water from San Francisco 
Bay to enter the overlying aquifer zone, which crops out on the floor of the Bay. 
With a gradient of only 0.1 ft/ft, and a permeability of 10~° cm/sec (0.02 gpd/ft^) 
the amount of water that would pass through the aquitard underlying the Bay would 
be on the order of 60 acre-feet per year per square mile. Assuming that about 
100 square miles of aquitard are overlain by saline waters, about 6,000 acre-feet 
of Bay water could move into the aquitard each year provided there is a downward 
hydraulic gradient. 

With this amount of Bay water moving into the aquitard, the velocity of movement 
becomes of great importance, as this will set the time span for the water to pass 
through the aquitard and into the underlying aquifer. Assuming a vertical 
gradient of unity, and a permeability of 10"^ cm/sec, the Darcy velocity of water 
moving through the aquitard is one foot per year. Hence, in an aquitard which 
has a thickness of about 50 feet, and assuming a porosity of 50 percent, it would 
take about 25 years for water to pass through. However, if the vertical gradient 
is on the order of 0.1 ft/ft, the time factor is increased 10 times (25 to 250 
years). If the thickness is only 10 feet, then under the latter conditions, it 
would take 50 years for salt water to move through it. 

In addition to the movement of fluids through an aquitard due to purely hydraulic 
gradients, there is another force which may move ions through relatively imper- 
meable materials. This is the chemico-osmotic diffusion of chloride ion through 
an aquitard which has a high concentration of chloride on one side and a low 
concentration on the other. This may be the case under two conditions in the 
Fremont study area. First, it may occur in areas where saline water overlies 
zones of good quality water in the Newark aquifer but is separated from it by 
the Newark aquitard. Second, it may occur at inland areas of intruded Newark 
aquifer which are underlain by lower aquitards and aquifers containing fresh 
ground water. In cases such as these, there is a coupling between solute concen- 
tration gradient ground water flow, i.e. the mechanism by which a salt 
concentration gradient causes ground water flow and a hydraulic gradient causes 
salt flow. This phenomenon is termed chemico-osmoticT coupling. 

In the studies at Oxnard, it was found that an aquitard which had a permeability 
of 10"^ cm/sec (0.002 gpd/ft^) and separating a saline solution having 
36,000 ppm chloride from fresh ground water, underwent definite chemico-osmotic 
diffusion. Curves developed from the study showed that if the aquitard had a 
thickness of 30 feet and there was no difference in piezometric heads above and 
below it, then it would take about 800 years for the chloride ion to diffuse 
through the aquitard. However, impressing a head differential of 10 feet 
toward the zone of fresh water reduced this travel time to 250 years. 

The studies also showed that the rate of diffusion varies according to the square 
of the thickness of the aquitard. Hence, if the thickness of the aquitard was 
reduced from 30 to 10 feet, the 250-year travel time would be reduced to 30 years. 



-24- 



Furthermore, if the thickness was reduced to only one foot, the travel time 
would be very small, only 0.3 year. 

Finally, the time required for the concentration of chloride ion to increase 
to 1,500 ppm in an underlying aquifer was computed at Oxnard for various thick- 
nesses of aquitard, all at a hydraulic gradient of 1/3 ft/ft. With the 30-foot 
thick aquitard, it was found that it would take 1,050 years for the underlying 
aquifer to attain a concentration of 1,500 ppm chloride by chemico-osmotic 
diffusion. However, with a thickness of 10 feet, this time is reduced to 
70 years, and with a thickness of only one foot, the time is further reduced 
to only 4 years. 

Current Investigation 

During 1971-72, a study of aquitard properties in the Fremont area was started 
under the guidance of Professor Paul A. Witherspoon of the University of 
California at Berkeley. Five shallow test holes were drilled using augers of 
different types and sizes, depending upon depth and type of material to be 
drilled. The locations of the test holes are shown on Figure 4. 

During the drilling each change in lithology with depth was recorded, as well 
as a description of the material recovered. For each foot of hole drilled, a 
sample between three inches and one foot long was recovered from the auger. 
Care was taken to prevent contamination of the recovered cores from fresh water 
used in cleaning the auger or from surface soil and dust. The core sample 
immediately was placed into a labeled glass jar which was tightly capped. The 
samples obtained during a day's work were put in plastic bags and kept in the 
humidity room until the laboratory work could be done. The samples thus 
obtained are considered to be basically "undisturbed" and at field water content. 
During the laboratory procedures, care was taken to prevent evaporation. 

Each of the core samples was divided into two parts. One was used to determine 
the water content of the soil; the other was used for the actual determination 
of the pore fluid salt concentration. Laboratory work was done at 20°C, and the 
results were adjusted to standard resistivities at 25°C. 

The quantity of soluble salts (equivalent NaCl) in the pore fluid of the Newark 
aquitard materials, as estimated for several samples in each test hole, is pre- 
sented in Figure 4 as graphs of depth in feet versus total dissolved solids in 
parts per million. The maximum values of salt concentration for each test hole 
are shown in Table 2. 

There is a striking difference between the maximum salt concentrations of 
samples from test holes that are not in the area of salt ponds (but less than 
a mile away) and those that are directly in the area of salt ponds. The first 
two have a maximum salt concentration in the range of 2,500 ppm to 3,800 ppm 
(Test Holes A and B, Table 2), whereas the ones in the area of salt ponds (Test 
Holes C, D, and E) have salt concentrations that range from 17,500 to 60,000 ppm. 
The high values indicate that salt water has thoroughly invaded the aquitard 
layers. Fresh water is generally considered to contain less than 900 ppm 



-25- 



"^1 




LOCATION MAP 



r\ 




TEST 


HOLE 


NO. 


A 




CLAY 


/ 


3 








SILTY CLAY 




SANDY CLAY 


1 n 


SILTY SAND 




SANDY CLAY 


\ 


K 








CLAY 




SANDY CLAY 


^ C.KJ 

X 
H 
CL 
UJ 
Q 


CLAY 




\ 


V 






SANDY CLAY 






) 






CLAY 


40 - 


SANDY CLAY 




y 








CLAYISH SAND 





TEST 


HOLE 


NO.B 


FILL 




r 




^ 


SILT 




J 






CLAYISH SAND 


SILTY CLAY 


CLAY 


c^ 


\ 


b 























01234 01234 

EQUIVALENT NaCI CONCENTRATION (ppm x 10^) 

FIGURE 4: SALT CONCENTRATIONS IN NEWARK AOUITARD 

SHEET I OF 2 



26 



a> 

O) 



UJ 
Q 



20- 




u - 


FILL 




. 




TEST 


HOLE NO. 





- 












(feet) 

o 

1 1 


CLAY 




/ 
















/ 














X 

1- 

Q. 




















CLAYISH SAND 


















Q CU 




















30- 


SILTY CLAY 

AND 
CLAYISH SILT 


















CLAYISH SAND 
















10 20 30 40 50 

EQUIVALENT NaCI CONCENTRATION ( ppm x 10^ ) 



60 



FIGURE 4: SALT CONCENTRATIONS IN NEWARK AQUITARD 

SHEET 2 OF 2 



2 7 



chloride ion; ocean water approximately 19,000 ppm; South Bay waters range from 
11,000 to 18,000 ppm; and salt evaporation ponds up to 215,000 ppm. 

It appears that since some of the salt concentrations in the aquitard exceed 
the salt concentration in the South Bay waters, salt pond waters may constitute 
a source of degradation of the underlying aquifers. The mechanism for this 
salt water migration may be the result of a combination of two factors: 
chemico-osmotic diffusion, and a hydraulic gradient. 



TABLE 2 
SALT CONCENTRATIONS IN AQUITARD PORE WATER 



Test 


: Location 


: Maximum Salt** 
! Concentration 
: (ppm) 


: Formation 




Hole 
No.* 


: Type : 


Depth 
(feet) 


A 
B 
C 
D 

E 


Outside Salt Pond 
Outside Salt Pond 
In Salt Pond 
In Salt Pond 
Adjacent Salt Pond 


2,500 

3,800 

50,000 

17,500 

60,000 


Sandy Clay 

Silt 

Silt 

Clay 

Clay 


34 

3 

7 

10 

30 



* Locations shown on Figure 4. 
** Equivalent NaCl concentration. 



-28- 



CHAPTER IV. SALINE WATER INTRUSION, 
STATUS AND CONTROL 



Intrusion of saline water into the portion of the ground water area north of the 
Coyote Hills was evident by 1924. Degradation continued and ground water in the 
shallow, or upper, Newark aquifer became progressively more unsuitable for irri- 
gation use. The ranchers, in their search for suitable irrigation supplies, 
drilled wells deeper into the second, or Centerville aquifer, which is separated 
from the Newark aquifer by a nearly impermeable clay layer. Fresh water from 
deeper aquifers relieved the immediate problems, and the extent of the intrusion 
of saline water was not fully realized until 1950, when degraded water first 
began to appear in the Centerville aquifer. The salinity was first noticed in 
the Alvarado-Newark-Centerville area, and spread over a larger area. 

Degradation of ground water by intrusion of saline water is probably caused by a 
combination of a number of conditions. The Newark aquifer is not in direct 
contact with San Francisco Bay except for localized areas where tidal currents 
or dredging may have scoured the bay mud and exposed the aquifer. Saline water 
may be entering the aquifer through openings in the bay mud and the clay cap, 
both of which overlie the aquifer, or the clay cap may have been breached by 
abandoned, unsealed wells. 

Intrusion is caused by saline water from the bay and salt ponds flowing through 
breaks in the clay cap and the clay cap itself and into the Newark aquifer, under 
the pressure differential existing between the bay surface and the aquifer. 
Although the downward flow of salt water per square foot of area is very small, 
the annual amounts over the total area of bay and salt ponds can be large. 

The hydraulic conditions allowing saline water intrusion and the paths of intru- 
sion are shown on Figure 5. Pumping from the Centerville and deeper aquifers 
created a hydraulic depression, or trough, in the water levels east of the Bay. 
Thus the hydraulic gradient in these aquifers is bayward from the forebay and 
landward from the bay. The forebay is connected to all of the aquifers and 
receives recharge from the surface. The hydraulic gradient in the Newark 
aquifer during periods of intrusion is landward from the bay to the forebay. 

Under these hydraulic conditions, saline water enters the portion of the Newark 
aquifer under the bay and the salt ponds. It then moves landward toward the 
forebay, and enters the lower aquifers by way of the forebay or by passing 
through the thin clay layers near the forebay. After the saline water has 
entered a lower aquifer, it then moves bayward down the hydraulic gradient toward 
the pumping depression. 

Extent of Saline Intrusion 

Figure 6 depicts lines of equal elevation of ground water and the status of salt 
water intrusion by isochlors (lines of equal chloride concentration in the ground 
water) in the Newark and Centerville-Fremont aquifers in the spring of 1970. The 



-29- 



Sea Level — 



Q -200 - 



Q -400 - 




Figure 5. INTRUSION OF SALT WATER INTO THE 
FREMONT STUDY AREA (SCHEMATIC) 



30 



NEWARK AQUIFER 
SPRING 1970 




LEGEND 

LINES OF EQUAL ELEVATION OF GROUND WATER IN FEET 
LINES OF EQUAL CHLORIDE CONCENTRATION IN PPM 



+ 40 
250 PPM 



CENTERVILLE 
FREMONT-AQUIFER 
SPRING 1970 







FIGURE 6 = GROUND WATER CONTOURS AND ISOCHLORS 



figures shoiold be considered as a graphic display of chloride concentration 
distribution rather than an exact comparison because the number of control points 
used and their locations are not constant. 

The area of the Newark aquifer with salt concentrations in excess of 250 ppm 
chloride decreased about 600 acres from approximately 21,100 acres in 1963 to 
about 20,500 acres in 1972. The area of the Centerville-Fremont aquifer with 
salt concentrations greater than 250 ppm chloride increased about 3,000 acres 
from approximately 8,800 acres in 1963 to approximately 11,800 acres in 1972. 

Volume of Saline Intrusion 

To determine the total volume of intrusion which has taken place, it is necessary 
to assign an average salinity to the intruding waters. The two sources of 
intrusion are: the Bay, with salinities varying between 10,600 and 18,900 ppm; 
and the salt evaporation ponds, with salinities varying from that of the Bay to 
215,000 ppm. A composite salinity averaging 21,000 ppm was chosen to represent 
intruding water, since this appears to be the average salinity of ground water 
in the upper aquifer around the perimeter of the Bay. 

The volume of salt water present in each of the aquifers in the spring of the 
years 1963 and 1972 are based on the isochlors, the salinity of Intruding water 
(21,000 ppm), and the storage capacities of the aquifers. The annual amounts of 
saline water intruding the ground water basin were estimated by prorating the 
total amount of saline water between 1963 and 1972 on the basis of water levels 
in the forebay area bayward from the Hajrward Fault. The annual amounts are 
listed in Table 3. 

Although the total amount of salt in the basin has increased between 1963 and 
1972, the annual rate of salt water entering the basin decreased from 1963 to 
1972 due to the Alameda County Water District's ground water recharge program. 
The reduction in annual salt water intrusion rates would have been greater except 
for pumpage and wastage of water from the basin by the gravel quarries for more 
economic gravel extractions, and the interruptions in the recharge operations 
caused by the construction of the Alameda Creek Flood Control Channel. The 
wastage of pumpage to the Bay has been stopped and the construction of the flood 
control channel has been completed. 

TABLE 3 

ANMJAL AMOUNTS OF SALINE* INTRUSION 
(In Acre-Feet) 



Year : Amount 



1961-62 


8,600 


1962-63 


6,600 


1963-64 


6,800 


1964-65 


5,400 


1965-66 


5,000 



Year : 


Amount 


1966-67 


3,100 


1967-68 


1,100 


1968-69 


1,100 


1969-70 


1,700 


1970-71 


1.700 



*Saline water at 21,000 ppm equivalent salinity. 



-32- 



Effect of Saline Intrusion on Water Supply 

During the study period the total amount of water supply available to the area 
has exceeded the total water use. The net result of this relationship and saline 
intrusion is shown by the well hydrographs in Figure 7 . Annual amount of water 
use is the sum of ground water pumped and direct delivery of imported water to 
customers, and is shown in Table 4. 

The hydrologic inventory in Chapter V shows that during the period 1961 to 1969, 
the total amount of water in storage increased by 76,000 acre-feet. Of this 
increase, 38,000 is attributable to saline intrusion and 38,000 to fresh water. 
During the two-year period 1969-71 there has been a decrease of water in storage 
of 11,000 acre-feet. This was the result of extractions exceeding fresh water 
recharge by 14,000 acre feet and a saline intrusion of 3,000 acre-feet. 

Although the water levels have recovered and water supply available has exceeded 
water use, a part of the water level recovery was due to saline intrusion and 
results in a continuing presence of salt water within the basin. The ground 
water basin is still endangered, not only from the large amount of salt water 
now present in the basin, but also from the probability of additional intrusion 
during future dry periods. 



TABLE 4 

ANNUAL AMOUNTS OF WATER USE 
(In Acre-Feet) 



Year 



Amount 



Year 



Amount 



1961-62 
1962-63 
1963-64 
1964-65 
1965-66 



43,800 
39,300 
45,400 
46,600 
49,200 



1966-67 
1967-68 
1968-69 
1969-70 
1970-71 



44,400 
48,500 
54,400 
53,700 
48,900 



Control of Saline Intrusion 

Various methods of protecting the ground water basin against further intrusion 
and for removal of the existing salts have been reviewed. A pumping barrier is 
recommended as the basic plan deserving further study and the plan which can be 
used to judge other alternatives. This type of plan is recommended because it 
will not cause saline water inland of the proposed barrier location to be forced 
farther inland into fresh water areas such as a recharge mound type of barrier 
would do, and the pumping barrier will assist in the removal of salt water from 
the upper aquifer. 

Previous work by the Department in both the Oxnard and Fremont areas assures 
that a pximping barrier is physically feasible. 



-33- 



The magnitude of the cost of installing a pumping barrier was arrived at by 
developing the conceptual plan shown on Figure 8. The barrier plan is anchored 
on the Coyote Hills and uses 14 pumping wells to form a protective arc. around 
the major production portions of the Newark aquifer. The capital cost of the 
system including wells, pumps, monitoring points and equipment, lands, discharge 
facilities and power service is estimated to be $1.2 million. The annual 
operations, maintenance and replacement costs are estimated to be $100,000. 



-34- 



r990 m 



HTD 1971 1972 




2 
3 



< 

O 






CONFINED GROUND WATER 

WEST OF HAYWARD FAULT 

LOWER AQUIFER 



























4S/IW-2IRt 

GROUND SUVKE ELCVftTIOH E7' 


















/Sn/ 


^. 


^., 


^, 




r\i 


\ 


/\ 


r\^ 
















A 


r^ 


V 










/ 






\/ 


''Vj 




K 


/ 


V. 


/\:j 


■ecoBDi 


A 


'N 


\ 


^/N 
r 


^/ 


V 






\J 


\J 


A 


C\l 






















V. 


1 




N 


K 


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F 
EAS 


TOF 


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WAIt 

RDFA 


R 

JLT 









< 
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(NO hecoud) 



FREE GROUND WATER 

WEST OF HAYWARD FAULT 

NEWARK AQUIFER 




CONFINED GROUND WATER 

WEST OF HAYWARD FAULT 

NEWARK AOUIFER 



1950 tSSI 952 I96J (954 i955 i956 l957 (958 (959 1960 '9€( (962 (965 (964 (965 (966 (967 (968 

CflLENOAfi YEAR 

Figure 7. HYDROGRAPHS AT SELECTED WELLS 



1969 1970 1971 197? 



35 



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AXIS OF PROPOSED BARRIER 


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Figure 8. 


CONCEPTUAL 


PLAN FOR PROPOSED 


BARRIER 





36 



CHAPTER V. EVALUATION OF 
HISTORIC WATER SUPPLY AND DISPOSAL 



The development of an inventory of supply to and disposal from the ground water 
basin provides a gross view of how the ground water basin is affected by climate 
and man's works. When the inventory is performed on many small pieces of the 
basin, as in modeling, the operational characteristics of the basin become clear. 
In both the gross inventory of the basin and in the modeling approach, supply 
and disposal are combined to obtain a theoretical change in storage. These 
changes are compared to the historic changes to verify the accuracy of the inven- 
tory and model. The model may then be used to test alternative plans for 
protection and operation of the ground water basin. 

Study Area 

The Fremont study area is the subsurface area influenced by Alameda Creek and 
adjacent smaller streams, and represents a manageable unit of the South Bay Ground 
Water Basin. For the purposes of this report the study area shown on Figure 1 
has been approximated by the ground water model shown in Figure 9. 

Ground Water Model 

The model configuration shown in Figure 9 is a modification of that described in 
Appendix E of the 1968 report. The area covered by the model has been enlarged 
to better approximate the study area. The arrangement of individual nodal areas 
(polygons) has been modified to conform to the more detailed geologic and hydro- 
logic interpretations. The southern end of the study area is an area of overlap 
of depositions of Alameda Creek and Santa Clara streams. This overlap condition 
has been simulated by using nodes 22 through 26 of the Fremont model in the model 
of the Santa Clara ground water area. 

For the purposes of this report, the amounts of recharge, pumpage and change in 
storage are shown for the total ground water basin. This information will be 
determined for each nodal area in the model, then verified and used for planning 
of the salinity barrier. 

Study Period 

In selection of a segment of time to use as a study period, it is desirable to 
specify certain criteria. The hydrologic condition during the study period should 
reasonably represent a long-time hydrologic condition. The time segment selected 
should begin at the end of a dry period and should end at the conclusion of a dry 
period in order to minimize the difference between the amount of water in transit 
in the zone of aeration between the beginning and end of the study period. The 
time segment should be within the period of available records, and if recent 



-37- 



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Figure 9. MATHEMATICAL MODEL 



38 



cultural conditions have been recorded, this information can aid in determination 
of the effect of urbanization on recharge to the ground water. 

The August 1967 report used a 16-year study period, water years 1949-50 through 
1964-65. This report uses a 9-year period, water years 1961-62 through 1969-70. 
The year 1961-62 was selected as the initial year because that year was the 
beginning of recharge of water from the State's South Bay Aqueduct and it was 
preceded by a year of below normal precipitation. The relative amounts of annual 
precipitation during the long term record, the base period, and the study period 
are shown on Figure 10. The long time average period of 94 years was not 
changed because the longer period of record now available did not change the 
average precipitation. 



94 -YEAR PERIOD 



400 



200 



UJ 

u 



bJ 
O 



< 

< 

UJ 

> 

-I 

2 

O-200 

1870 







/\ 








9 -YEAR 
PERIOD 


A 






VA^ 




HvJ 








a/ 



1890 



1910 1930 

OCTOBER I OF YEAR 



1950 



1970 



FIGURE 10 -CUMULATIVE DEPARTURE Of ANNUAL 
PRECIPITATION FROM 94 YEAR MEAN 



-39- 



General Conditions 

The general factors affecting the ground water basin are precipitation, 
streamf low, land use and imported water. 

Precipitation 

Precipitation for the entire period of record for gages in the vicinity of Niles 
is shown in Table 5. The 94-year average used in the 1968 report has been 
retained as long term average, since the additional record had no effect on the 
average. The 9-year period 1961-62 through 1969-70 has about the same average 
annual precipitation as the 94-year average. 

Streamf low 

Alameda Creek is the main stream traversing the forebay of the area. Flow 
measurements since 1891-92 are available for the creek where it enters the area 
near Niles and for three years, 1916-1919, for the lower end of the recharge area 
near Decoto. Main flows now leave the area by a new channel, Patterson Creek, 
but the old Alameda Creek continued to receive excess flows until 1967. Both of 
the outflow channels have been gaged since 1958-59. The Alameda Creek Flood 
Control Channel, which improved Patterson Creek, was completed beyond this point 
in 1967; thereafter all of the flows passed down that channel. Dry Creek, 
located near the upper end of the area and tributary to the Alameda Creek lower 
gage, is also measured. 

Flows of other streams tributary to the study area were estimated by correlation 
with gaged streams. Recorded amounts of runoff are shown in Table 6. Estimated 
amounts of annual runoff from ungaged tributary areas are shown in Table 7. 



Land Use 

The study area continues to be in transition from an agricultural to urban 
economy. The change in land use within the model area of 108,040 acres during 
the study period is shown in Table 8. Land use within the boundaries of the 
Alameda County Water District is shown on the plate following page 57. 



Imported Water 

Agencies in the study area purchase water from two suppliers of imported water; 
the City of San Francisco and the State of California. 



Annual Deliveries 

Amounts of water imported from the City of San Francisco's aqueducts and 
from the State of California's South Bay Aqueduct are listed in Table 9. 



-40- 



TABLE 5 

ANNUAL PRECIPITATION AND INDEX OF WETNESS 
1871-1970 





: : Index : 






: Index : 






: Index 


Water 


: a/: of b/: 


Water 




: of : 


Water 




: of 


Year 


: Inches : Wetness : 


Year 


: Inches 


. Wetness : 


Year 


: Inches 


: Wetness 



1871-72 


22.65 


125 


1905-06 


24.20 


133 


1940-41 


25.35 


140 


72-73 


14.31 


79 


06-07 


28.85 


159 


41-42 


21.23 


117 


73-74 


14.17 


78 


07-08 


15.12 


83 


42-43 


18.29 


101 


74-75 


11.74 


65 


08-09 


25.10 


138 


43-44 


15.38 


85 








09-10 


18.65 


103 


44-45 


16.82 


93 


1875-76 


25.88 


142 


1910-11 


27.59 


152 


1945-46 


14.39 


79 


76-77 


9.34 


51 


11-12 


15.80 


87 


46-47 


12.60 


69 


77-78 


24.67 


136 


12-13 


12.06 


66 


47-48 


14.72 


81 


78-79 


14.54 


80 


13-14 


22.95 


127 


48-49 


12.72 


70 


79-80 


17.70 


97 


14-15 


27.34 


150 


49-50 


14.00 


77 


1880-81 


20.14 


111 


1915-16 


21.38 


118 


1950-51 


20.21 


111 


81-82 


13.91 


77 


16-17 


13.50 


74 


51-52 


26.26 


145 


82-83 


14.07 


78 


17-18 


18.15 


100 


52-53 


15.50 


85 


83-84 


25.88 


142 


18-19 


17.49 


96 


53-54 


13.50 


74 


84-85 


10.36 


57 


19-20 


11.06 


61 


54-55 


14.90 


82 


1885-86 


23.35 


128 


1920-21 


20.62 


113 


1955-56 


23.85 


131 


86-87 


15.37 


85 


21-22 


19.85 


109 


56-57 


12.99 


71 


87-88 


14.67 


81 


22-23 


17.89 


98 


57-58 


28.30 


156 


88-89 


15.67 


86 


23-24 


8.63 


47 


58-59 


12.30 


68 


89-90 


36.36 


200 


24-25 


21.65 


119 


59-60 


13.83 


76 


1890-91 


14.04 


77 


1925-26 


16.35 


90 


1960-61 


14.03 


77 


91-92 


16.18 


89 


26-27 


18.79 


103 


61-62 


15.86 


87 


92-93 


23.72 


131 


27-28 


16.55 


91 


62-63 


22.58 


124 


93-94 


23.19 


128 


28-29 


14.48 


80 


63-64 


11.99 


66 


94-95 


26.63 


147 


29-30 


14.78 


81 


64-65 


18.14 


100 


1895-96 


20.33 


112 


1930-31 


12.22 


67 


1965-66 


14.02 


77 


96-97 


22.72 


125 


31-32 


18.87 


104 


66-67 


25.41 


140 


97-98 


13.58 


75 


32-33 


13.70 


75 


67-68 


15.06 


83 


98-99 


14.52 


80 


33-34 


10.66 


59 


68-69 


23.67 


130 


99-00 


19.30 


106 


34-35 


19.77 


109 


69-70 


15.30 


84 


1900-01 


25.22 


139 


1935-36 


16.69 


92 


1970-71 


19.96 


110 


01-02 


17.12 


94 


36-37 


19.78 


109 








02-03 


17.20 


95 


37-38 


21.80 


120 








03-04 


21.91 


121 


38-39 


13.33 


73 








04-05 


20.19 


111 


39-40 


22.20 


122 








Averages 






94 years 






9 years 












1871-1965 


18.17 


100 


1961-70 


18.00 


99 



a/ 1871-72 thru 1884-85 Weather Bureau's Niles Precipitation Station (SP Depot) 
1885-86 thru 1932-33 Niles 1 SW Precipitation Station 
1933-34 thru 1957-58 Niles 1 S Precipitation Station 
1958-59 thru 1969-70 Alameda County Corp. Yard Precipitation Station 

hj Index of Wetness is the percent of 94-year average. 



-41- 



TABLE 6 

RECORDED ANNUAL RUNOFF 
(In Acre-Feet) 



Alameda Creek Near Nlles 



Year 


Amount 


Year 


Amount 


Year 


Amount 






1920-21 


72,400 


1950-51 


115,200 


1891-92 


56,000 


21-22 


131,000 


51-52 


291,100 


92-93 


360,000 


22-23 


58,000 


52-53 


24,700 


93-94 


147,000 


23-24 


2,060 


53-54 


4,250 


94-95 


263,000 


24-25 


18,700 


54-55 


5,900 


1895-96 


118,000 


1925-26 


31,000 


1955-56 


214,100 


96-97 


204,000 


26-27 


48,300 


56-57 


7,880 


97-98 


7,020 


27-28 


30,100 


57-58 


245,700 


98-99 


64,100 


28-29 


5,240 


58-59 


14,660 


99-00 


51,700 


29-30 


19,200 


59-60 


11,940 


1900-01 


119,000 


1930-31 


1,220 


1960-61 


650 


01-02 


83,800 


31-32 


57,400 


61-62 


34,740 


02-03 


110,000 


32-33 


6,980 


62-63 


66,660 


03-04 


98,300 


33-34 


7,920 


63-64 


22,940 


04-05 


45,400 


34-35 


30,490 


64-65 


85,620 


1905-06 


203,000 


1935-36 


77,150 


1965-66 


26,320 


06-07 


324,000 


36-37 


100,100 


66-67 


140,000 


07-08 


46,500 


37-38 


286,000 


67-68 


41,510 


08-09 


239,000 


38-39 


15,220 


68-69 


110,100 


09-10 


84,200 


39-40 


92,580 


69-70 


58,120 


1910-11 


272,000 


1940-41 


200,000 


1970-71 


42,300 


11-12 


16,500 


41-42 


128,100 






12-13 


6,550 


42-43 


79,490 






13-14 


179,000 


43-44 


35,010 






14-15 


182,000 


44-45 


48,430 






1915-16 


233,000 


1945-46 


15,740 






16-17 


86,000 


46-47 


2,080 






17-18 


12,600 


47-48 


899 






18-19 


107,000 


48-49 


5,610 






19-20 


8,250 


49-50 


8,680 







-42- 



Table 6 (continued) 



Patterson Creek Near Union City 



Year 


Amount 


Year 


Amount 


Year 


Amount 


1958-59 


10,410 


1963-64 


4,240 


1967-68 


6,020 


59-60 


7,290 


64-65 


60,960 


68-69 


98,820 


60-61 


7,290 


65-66 


7,160 


69-70 


40,620 


61-62 


22,640 


66-67 


118,200 


70-71 


31,680 


62-63 


42,800 











Alameda Creek Near Decoto 



Year 


Amount 


Year 


Amount 


Year 


Amount 


1916-17 


74,000 


1917-18 


7,200 


1918-19 


91,400 



Alameda Creek at Union City 



Year 


Amount 


Year 


Amount 


Year 


Amount 


1958-59 


140 


1963-64 


99 


1967-68 


32 


59-60 


614 


64-65 


5,590 


68-69 


0.6 


60-61 





65-66 


560 


69-70 


160 


61-62 


1,300 


66-67 


266 


70-71 


723 


62-63 


3,860 











Dry Creek at Union City 



Year 


Amount 


Year 


Amount 


Year 


Amount 


1916-17 


957 


1961-62 


1,060 


1966-67 


2,930 


17-18 


61 


62-63 


1,970 


67-68 


612 


18-19 


1,330 


63-64 


224 


68-69 


3,580 


1959-60 


463 


64-65 


1,820 


69-70 


1,680 


60-61 


8 


65-66 


323 


70-71 


1,580 



-43- 



TABLE 7 

UNGAGED TRIBUTARY HILLSIDE RUNOFF 
(In Acre-Feet) 











Tributary to 


Node 










Year 


: la : 


2 : 


4 


: 3b : 


6 




8 


: 13 


: 14 


1961-62 


300 


80 


15 


140 


45 




375 


240 


480 


62-63 


2,610 


245 


40 


445 


140 


1 


,170 


755 


1,510 


63-64 


310 


30 


5 


50 


15 




135 


90 


175 


64-65 


565 


125 


20 


232 


70 




610 


395 


785 


1965-66 


600 


50 


10 


90 


25 




235 


155 


305 


66-67 


3,880 


365 


60 


662 


200 


1 


,745 


1,130 


2,245 


67-68 


650 


70 


10 


123 


35 




320 


210 


420 


68-69 


1,780 


285 


45 


520 


155 


1 


,370 


885 


1,765 


69-70 


80 


70 


10 


130 


40 




335 


215 


435 


1970-71 


80 


175 


30 


315 


95 




835 


540 


1,075 



a - Does not include gaged flow of Dry Creek at Union City (Table 6) . 
b - Does not include gaged flow of Alameda Creek near Niles (Table 6) 



TABLE 8 

LAND USE, FREMONT MODEL AREA 
(In Acres) 



Model Area - 108,040 Acres 







: Municipal 






: Dry Farm 




: Irrigated 


: and 


: Salt 


: Water 


: and 


Year 


: Agriculture 


: Industrial 


: Ponds 


: Surface* 


: Native 


1961-62 


12,850 


8,420 


24,200 


25,430 


37,140 


62-63 


11,990 


10,010 


24,200 


25,430 


36,410 


63-64 


11,520 


10,710 


24,200 


25,430 


36,180 


64-65 


11,100 


11,200 


24,200 


25,430 


36,110 


1965-66 


10,670 


11,700 


24,200 


25,430 


36,040 


66-67 


10,240 


12,200 


24,200 


25,430 


35,970 


67-68 


9,810 


12,690 


24,200 


25,430 


35,910 


68-69 


9,390 


13,190 


24,200 


25,430 


35,830 


69-70 


6,700 


14,610 


24,200 


25,430 


37,100 



^Includes San Francisco Bay 



-44- 



TABLE 9 

BIPORTED WATER 
(In 1,000 Acre-Feet) 





Source 




Total 






City 


of 


San Francisco ! 


State of 






Bunting 




Alameda 


: Hetch 


Total For 


Water 


Pit 




Creek 


: Hetchy 


California 


For Recharge 


All Uses 


Year 


(1) 




(2) 


: (3) 


(4) 


(5) = 


=(l)+(2)+(4) 


(6)=(3)+(5) 


1961-62 


- 2. 


33* - 


1.17 


5.47 




7.80 


8.97 


62-63 


1.12 




1.05 


0.82 


11.20 




13.37 


14.19 


63-64 


1.34 




0.46 


1.74 


18.23 




20.03 


21.77 


64-65 


5.31 




0.41 


1.80 


16.25 




21.97 


23.77 


1965-66 


2.57 




0.53 


3.10 


15.04 




18.14 


21.24 


66-67 


5.55 




1.60 


5.70 


8.21 




15.36 


21.06 


67-68 


4.04 




0.38 


3.46 


28.60 




33.02 


36.48 


68-69 


5.56 




1.17 


3.86 


13.41 




20.14 


24.00 


69-70 


3.64 




1.03 


3.59 


14.56 




19.23 


22.82 


1970-71 


3.18 




2.17 


5.57 


10.13 




15.48 


21.05 



*Suin of amounts for Bunting Pit and Alameda Creek, 



City of San Francisco 

Through its Hetch Hetchy Aqueduct, the City of San Francisco delivers 
treated water to the cities of Hayward and Milpitas and to the Alameda 
County Water District. All of this supply is served to customers of the 
local water systems, and is accounted for in the inventory as recharge of 
applied water. Alameda County Water District also receives small amounts 
of water from the City of San Francisco's Sunol Aqueduct. This water is 
delivered to the Bunting Pits (located on the south side of Alameda Creek 
west of Mission Boulevard) for recharge and to other users along Alameda 
Creek. 



State of California 

The South Bay Aqueduct of the California State Water Project has been a 
source of recharge water to the Fremont area since 1962, when the first 
section to be completed was put into operation. Water was released from 
the aqueduct at the Altamont Turnout and flowed through the Livermore 
Valley to Niles until 1965, when the remainder of the aqueduct was com- 
pleted. Since then water has been released to Alameda Creek at the 
Vallecitos Turnout, 

The ground water is recharged by water from the South Bay Aqueduct, released 
to flow in Alameda Creek, and then diverted into adjacent gravel pits near 
Niles. 



-45- 



Ground Water Inventory 

A schematic representation of the hydrologic system is shown on Figure 11. The 
reference, or free body, used in the ground water inventory is the ground water 
in storage. The inventory is made on an annual basis, and under the assumption 
that water which percolates below the root zone will reach the ground water mass 
during the same water year. The inventory can be represented by the simple 
equation: Supply - Withdrawal = Change in Storage. 

Items of supply, or recharge, to the ground water are derived mainly from preci- 
pitation, storm runoff, imported water, and pvimped ground water. Specifically, 
the items of supply are: 

1. Portion of precipitation percolating to ground water. 

2. Portion of storm runoff, or streamflow, including imported water released 
into Alameda Creek and adjacent gravel pits, percolating to ground water. 

3. Portion of applied (delivered) water percolating to ground water. (Applied 
water included pxomped ground water and imported water put directly into 
water distribution systems.) 

4. Subsurface inflow. 

5. Water released by compaction of clay beds. 

Withdrawals from the ground water consist of ground water pumpage and subsurface 
flow out of the basin. 

Change in storage is the anniial volume of ground water gained or lost from 
storage. 

Direct Recharge of Precipitation and Delivered Water 

The disposition of combined amounts of precipitation and applied water to evapo- 
transpiration, recharge, and runoff are computed for each type of land use. 
Starting at the beginning of a water year, and on a monthly-accounting basis, from 
October through April, the monthly amounts of precipitation and applied water are 
used to satisfy the soil moisture deficiency and potential evapo-transpiration 
consumptive use. The same process is followed during the summer growing season, 
but on a lump sum basis. During the growing season the amount of recharge must 
also be at least 20 percent of the applied water to allow for irrigation when 
roots had not developed their maximum ability to take moisture. Monthly potential 
evapo-transpiration rates, moisture holding content of soils, and effective 
rooting depths for crops are shown on Table 10. 

Since records on the amounts of water applied to individual crops are available 
only for 1972, data concerning annual amounts of applied water for the Northern 
Santa Clara County study area to the south were used for the Fremont area. As in 
the Santa Clara study, total irrigation during years before a pvimp tax was im- 
posed was assumed to be one irrigation greater than in years after the pump tax. 



-46- 



TMOSPHERE 



t 



I 



I 



i:: 






I 



t 



r 



31 



^ 



5 



T 

5 



I 



DELIVERED 
WATER 
SUPPLY 



SURFACE 
SOIL 




V V 



VEGETATION 



3. 2 



AND 



BOUNDARY OF GROUND 
WATER FREE BODY FOR 
INVENTORY^ 






1 



I 

• •••••• '•• 



> 



L, 



STREAMS 

AND 

PONDS 



^y/iSr£ WATER 



T 



is 

i 



I 



AQUIFERS 



<K 





^ 




^ 


: 

SUBSURFACE 
\ OUTFLOW 

9 
• 


BAY 


I INTRUSION 









SUBSURFACE FLOW 



RELATIVELY 
BEARING 



NON-WATER 
MATERIAL 



ADJACENT 

GROUND 

WATER 

AREA 



Figure II. HYDROLOGIC SYSTEM (SCHEMATIC) 



TABLE 10 
AGRICULTURAL WATER USE FACTORS 



Monthly Potential Evapo-Transpiration 
(In Inches) 





: Improved 


• 
• 


: Sugar 


: Deciduous 


: Nonirrigated 


Month 


: Pasture* 


: Alfalfa 


: Beets 


: Orchard 


: Barley 


October 


3.5 


3.5 


3.5 


2.7 


2.0 


November 


1.7 


1.7 


1.7 


1.1 


1.7 


December 


0.9 


0.9 


0.9 


0.9 


0.9 


January 


1.1 


1.1 


1.0 


1.1 


1.1 


February 


1.0 


1.0 


1.3 


1.4 


1.9 


March 


3.1 


2.9 


— 


2.1 


3.1 


April 


4.6 


4.1 


- 


3.2 


3.4 


May 


5.7 


5.1 


1.7 


4.6 


1.2 


June 


7.3 


6.5 


5.6 


6.2 


0.4 


July 


7.4 


6.8 


7.7 


6.8 


0.0 


August 


6.5 


6.2 


6.6 


5.8 


0.0 


September 


4.9 


4.8 


5.3 


4.3 


0.3 



*Evapo-transpiration of improved pasture considered equivalent to potential 
evapo-transpirat ion . 



Moisture Holding Content for Soils 
(In Inches per Foot of Soil) 



Soil Type : 


Available 
Water Content 


: 


Soil Type : 


Available 
Water Content 


Sand 
Clay 

Clay Loam 
Loam 


1.0 

1.0 to 1, 

1.4 

1.7 


,5 




Silty Clay 
Silty Clay Loam 
Silt Loam 
Silt 


1.7 
2.0 
2.3 
2.9 




Effective 
(Ir 


Rooting 
L Feet) 


Depth 




Irrigated Crop 


: Effective 
Root Depth 




• 


Irrigated Crop 


: Effective 
: Root Depth 


Pasture 
Alfalfa 
Sugar Beets 
General Field 
Walnuts 


2 
6 
5 
4 
8 






Misc. Truck 
Tomatoes 
Orchard, Mixed 
Vineyard 


3 

5 
6 

5 



-48- 



Annual amounts of applied irrigation water varied according to the amount of rain- 
fall occurring in February, March, and April, since rainfall in these months 
controls the moisture in the soil at the start of the growing season. Annual 
amounts of water applied to irrigated lands are listed in Table 11. Applied water 
on urban areas was assumed to be a depth of three feet on the pervious area. 

Annual amounts of rainfall becoming local runoff are computed as rainfall on 
impervious areas less evaporation. Average daily rates of rainfall evaporation 
are listed in Table 12. For irrigated and native lands, 10 percent is assumed 
to be impervious. For urban areas, 50 percent is assumed impervious. The depth 
of runoff is shown on Table 13. 



Depth of Recharge 

The maximum depth of recharge shown on Table 13 for each nodal area and year 
was computed for irrigated agricultural, native, and urban lands east of the 
salt evaporation ponds. For irrigated agriculture the value was computed 
for each nodal area based on the crop pattern of 1967. 

Annual Recharge 

The annual amounts of direct recharge (from rain and delivered water) are 
the products of the land use areas and the depth of recharge amount for the 
specific land use. The amount of recharge actually occurring will be less 
than this computed amount due to the high percentages of clay present in some 
portions of the area. To correct for the low permeability of the clay areas, 
the distance from the apex of the Alameda Creek cone were taken into account. 
The effect of distance from the apex of the cone is shown in Figure 12. The 
clay content for each node is shown on Figure 13. Annual amounts of recharge 
corrected by the recharge factors are listed in Table 14 on page 56. 

Recharge from Streamflow 

Streamflow available for recharge is the sum of flows originating in the hills to 
the west and local runoff from the surface of the study area. Local runoff origi- 
nating on the valley lands of the study area is that portion of precipitation not 
consumed or percolating to ground water. On its way to San Francisco Bay or a 
gaged channel, a portion of this local runoff may percolate. Due to the location 
of recharge facilities and gaging stations, the analysis of runoff has been 
divided into analysis of the gaged portion of the study area bounded by Alameda 
Creek, Dry Creek, and the hills to the northeast, and analysis of runoff in the 
remaining ungaged study area, less the Bay and the salt ponds. 

Alameda and Dry Creeks Area 

In the area bounded by Alameda Creek, Dry Creek, and the hills to the north- 
east, surface flows available for percolation include those passing the upper 
gage on Alameda Creek and the Dry Creek gage, tributary ungaged runoff from 
the hills to the north, and local runoff developed within this area. 



-49- 



TABLE 11 







DEPTHS OF APPLIED 


WATER 










(In Feet)* 








Water 


I 


: : 




5 


• 


Year 


: Deciduous 


: Pasture : 


Tomato 


: Cole 


: Average 


1961-62 


1.20 


2.40 


1.95 


2.40 


1.89 


62-63 


1.05 


1.95 


1.50 


1.95 


1.60 


63-64 


1.80 


2.56 


2.03 


2.56 


2.15 


64-65 


1.20 


2.03 


1.65 


2.03 


1.72 


1965-66 


1.80 


2.56 


2.03 


2.56 


2.15 


66-67 


1.05 


1.95 


1.50 


1.95 


1.60 


67-68 


1.50 


2.40 


1.95 


2.40 


1.12 


68-69 


1.20 


2.03 


1.65 


2.03 


1.72 


69-70 


1.35 


2.25 


1.80 


2.25 


1.94 



*Acre-feet per gross acre with 10 percent of gross area assumed as impervious. 



TABLE 12 

AVERAGE DAILY EVAPORATION RATES 
(In Inches) 



Month 


: During Storm 


: After Storm 


October 


0.040 


0.063 


November 


0.024 


0.038 


December 


0.014 


0.019 


January 


0.023 


0.024 


February 


0.037 


0.077 


March 


0.055 


0.121 


April 


0.074 


0.170 


May 


0.081 


0.191 


June 


0.063 


0.218 


July 


0.037 


0.183 


August 


0.073 


0.171 


September 





0.119 



-50- 



TABLE 13 

DEPTHS OF RECHARGE AND RUNOFF 
FROM APPLIED WATER AND PRECIPITATION 
(In Feet) 







Recharge From 




: Runoff 


From 


Water : 


Irrigated 


: Urban : 


Dry Farm 


: Urban 


: Nonurban 


Year : 


Land 


: Land : 


Land 


: Land 


: Land 


1961-62 


0.64 


0.56 


0.31 


0.40 


0.10 


62-63 


0.64 


0.44 


0.31 


0.56 


0.14 


63-64 


0.58 


0.38 


0.13 


0.25 


0.06 


64-65 


0.61 


0.42 


0.30 


0.44 


0.11 


1965-66 


0.65 


0.40 


0.24 


0.34 


0.08 


66-67 


0.81 


0.56 


0.67 


0.65 


0.16 


67-68 


0.77 


0.22 


0.09 


0.34 


0.08 


68-69 


0.89 


0.62 


0.74 


0.61 


0.15 


69-70 


0.59 


0.26 


0.28 


0.38 


0.09 



A portion of the flow in Alameda Creek is diverted into percolation pits by 
the Alameda County Water District. The only known surface diversions during 
the study period are those made by the District. During the last part of 
the study period, pumpage by gravel pit operators to control water levels in 
the pits was discharged to Alameda Creek. 

Recharge in the Alameda Creek-Dry Creek area is the total runoff available 
less outflow. The total runoff is the sum of flows in Dry Creek and Alameda 
Creek at the upstream boundary of the study area, plus local runoff and 
stream discharges produced within the area. The method of determining the 
amount of local runoff is described in the section on determining runoff in 
the remainder of the study area. Recharge from runoff in Alameda Creek, as 
shown on Table 14, contains releases from the South Bay Aqueduct of the 
State Water Project, and is computed on the basis of flows measured at Niles 
gage and Dry Creek gage. The amounts of recharge from runoff shown in 
Table 14 include recharge in the total area, including the pits. The amounts 
of South Bay Aqueduct water purchased by the Alameda County Water District 
are shown in Table 9. 



Remainder of Study Area 

The ungaged tributary hillside runoff and the runoff from precipitation are 
available for percolation on their way to the Bay. Local runoff is computed 
from land use in Table 5 and depth of runoff in Table 13. The ability of 
streamflow to become recharge to the ground water is regulated by the per- 
vious areas of the channels conveying the water, the length of time flow 



-51- 




MOUNTAIN 
O VIEW 



PERCENT REDUCTION IN RECHARGE 
DUE TO DISTANCE FROM APEX 
OF CONE OF DEPOSITION 



/^kf 



Figure 12. RELATIVE RECHARGE CAPABILITY 

52 




53 



takes place, and the surface and subsurface characteristics of the soil. 
In the more pervious portion of the area outside of the Alameda-Dry Creek 
area, percolation of runoff was determined for the sum of the following 
computations. 

40 percent of the flows of to 5,000 acre-feet 

30 percent of the flows of 5,001 to 10,000 acre-feet 

20 percent of the flows of 10,001 to 15,000 acre-feet 

Subsurface Inflow 

The combination of geologic interpretation of subsurface conditions in Node 8 
(Figure 9) and the depth of wells in Node 8 indicate that the majority of pumpage 
in the node is from the Santa Clara Formation underlying the alluvium. To account 
for this condition, 90 percent of pumpage in Node 8 was estimated to be subsurface 
inflow. 



Compaction of Clays 

Subsidence occurred in the South Bay Area during years prior to 1969. The center 
of subsidence is south of San Francisco Bay in Santa Clara County. Subsidence is 
associated with high amounts of pumpage in northern Santa Clara County and most 
of the water released by compaction of the aquitards is an inflow to aquifers in 
the Santa Clara County area. Shallow, thin aquifers belonging to the Fremont and 
Santa Clara areas overlap each other in the Alviso area and deeper aquifers of the 
two systems probably merge. This situation requires that a portion of the water 
produced by compaction of clays be assigned to the Fremont area. The annual amount 
of 500 acre-feet per year determined for the August 1968 report has been used for 
years, through 1966-67, and 200 acre-feet for 1967-68. Subsidence did not occur 
after 1968. 



Ground Water Pxunpage 

Ground water pumpage is made up of pumpage by Alameda County Water District, 
Citizens Utility Company, individual industries, individual domestics, and indi- 
vidual agricultural users. All except agriculture are based on information 
collected by Alameda County Water District. Estimates of agricultural pumpage 
are based on land use in Table 5 and unit applied water in Table 11. Annual 
amounts of pumpage are listed in Table 4. 



Saline Water Inflow 

Annual volumes of saline water entering the ground water system are computed in 
Chapter IV. 



-54- 



Annual Inventory 

An annual comparison of amounts of inflow to and outflow from the ground water 
system is shovm in Table 14. Inflow is the sum of recharge from rain, applied 
water and runoff, subsurface flow, and saline intrusion. Outflow is the sum of 
municipal, industrial, and agricultural pumpage. The net recharge is comparable 
to the change in the amount of water in storage. 

Change in Storage 

The change in storage is computed as the product of annual change in water levels 
in the unconfined ground water area and the specific yield of materials in the 
zone of change. For this computation clays were given a specific yield value of 
one percent. Annual amounts of change in storage and the comparison with amounts 
of net recharge are shown in Table 15. Net recharge is computed as the difference 
between withdrawals and additions of water to the ground water system, and 
includes pumpage, recharge from rain, runoff and applied water, subsurface inflow, 
water from subsidence and sea water intrusion. Change in storage and net recharge 
are computed independently and should be approximately equal. The overall trends 
of both computations, as shown by their summation plots on Figure 14, are similar 
and their differences within reasonable limits. 



-55- 



TABLE Ih 

GROUHD WATER INVENTORY 

(In 1,000 Acre-Feet) 





fumpa 


ge 


Rs 


charge From 




Sub- 
surface 
Flow 


Compac- 
tion 


Saline 
Intrusion 






Municipal,: 
Industrial: 


Agricul- 
tural 


Rain and 

Applied 

Water 


: Runo 


ff 




Year 


: Alameda 
:and Dry 
: Creek 


:R 


emalnder 
of 
Area 


Net 
Recharge 


1961-62 


114.5 


26.3 


13.5 


15.9 




2.2 


0.7 


0.5 


8.6 


0.6 


1962-63 


16.6 


19.8 


12.6 


29.14 




3.5 


1.14 


0.5 


6.6 


17.6 


1963-6i| 


18.5 


23.6 


9.5 


21,9 




1.14 


1.4 


0.5 


6.8 


- 0,6 


1964-65 


21.5 


19.6 


11.7 


29.1 




2.8 


0.8 


0.5 


5.4 


9.2 


1965-66 


22.0 


2i(.0 


11. iJ 


23.9 




2.0 


0.9 


0.5 


5.0 


- 2.3 


1966-67 


19.6 


17.1 


18.9 


38.9 




14.3 


0.5 


0.5 


3.1 


29.5 


1967-68 


26.7 


20.5 


7.2 


1414.7 




2.2 


0.5 


0.2 


1.1 


8.7 


1968-69 


314.3 


16.5 


20.5 


35.8 




4.0 


0.5 





1.7 


11.7 


1969-70 


38.2 


13.1 


9.2 


33.8 




2.14 


1.4 





1.7 


- 2.8 



TABLE 15 

CHANGE IN STORAGE 

(In 1,000 Acre-Feet) 





Change in 

Storage 

(1) 


Net 

Recharge 

(2) 


Accumulated 


Year 


Change in 

Storage 

(3) 


: Net : 
: Recharge : 
: (4) : 


Difference 
(5)=(3)-(4) 


1961-62 


- 4.3 


0.6 


- 4.3 


0.6 


- 4.9 


1962-63 


7.1 


17.6 


2.8 


18.2 


-15.5 


1963-64 


8.8 


- 0.6 


11.6 


- 17.6 


- 6.0 


1964-65 


6.6 


9.2 


18.2 


26.8 


- 8.6 


1965-66 


8.0 


- 2.3 


26.2 


24.5 


1.7 


1966-67 


20.9 


29.5 


47.1 


54.0 


- 6.9 


1967-68 


12.5 


8.7 


59.6 


62.7 


- 3.1 


1968-69 


16.7 


11.7 


76.3 


74.4 


1.9 


1969-70 


- 6.4 


- 2.8 


69.9 


71.6 


- 1.7 


1970-71 


- 5.1 




64.8 







56 



80 



iij 



cr 
o 

< 

o 
o 
o 



< 

tr 
o 



ill 
o 

z 
< 

X 

o 



t- 
< 



3 

o 
o 
< 



60 



40 



20 

















V 


^-■~., 












/ 


/^ / 
/ 
/ 
/ 












/ 


y 


/ 












/ 




y 


y 














,-' 



















0<^ 



1962 



1964 1966 1968 

OCTOBER I OF YEAR 



1970 



L E G E N D 



NET RECHARGE BY INVENTORY 



CHANGE IN STORAGE BY 
WATER LEVELS 



Figure 14. ACCUMULATED CHANGE IN STORAGE 



57 




state of California 
The Resources Agency 

Department of 
Water Resources 



UNIVERSITY OF CALIFORNIA • 
DAVIS 



DEC 06 1983 
GOV'T. DOCS. -LIBRARY | 



sa 
water 



Evaluation of Ground Water Resources 

South San Francisco Bay 

Vol. IV: South Santa Clara County Area 

Department of Water Resources 

in cooperation with Santa Clara Valley Water District 



Bulletin 118-1 
May 1981 




COVER: Off-stream ground c 
water recharge facilities operated by the 
Santa Clara Valley Water District. Some of 
these facilities have been in operation 
since before 1930. 



The attached errata sheets should be placed in the proper 
location in Bulletin 118-1, Volume IV, "Evaluation of Ground 
Water Resources, South San Francisco Bay, South Santa Clara 
County Area". 



Artificial recharge i 
to a ground water bas 
naturally. Artificia 
accomplished principa 
through impoundments 
releases into permeab 
surface reservoirs du 
season, thus affordin 
the channels of Uvas 
into the ground water 
Conservation District 
collectively referred 
recharging south coun 



s the practice of deliberately adding water 
in through means beyond that which would occur 
1 recharge in south Santa Clara County is 
lly by the Gavilan Water Conservation District 
in Uvas and Chesbro Reservoirs with timely 
le stream channels. Water is stored in the 
ring the wet season and released during the dry 
g an opportunity for the water to infiltrate 
and Llagas Creeks (see Figure 18A) and flow 
reservoir. In recent years, Gavilan Water 
has also used offstream percolation ponds, 
to as the Church Avenue Recharge Facility, for 
ty aquifers. 



Santa Clara Valley Water District artificially recharges water in 
south county by releasing water from Anderson Reservoir for infiltra- 
tion at the Main Avenue percolation ponds and or along the Madrone 
Channel (see Figure 17). 




Figure ISA MONTHLY RELEASE FROM UVAS AND CHESBRO RESERVOIRS 
(Daia from Gavilan Water Conservation District) 




ON THE COVER: Off-stream ground 
water recharge facilities operated bytfie 
Santa Clara Valley Water District, Some of 
these facilities have been in operation 
since before 1930. 



The attached errata sheets should be olaced In the prooer 
location in Bulletin 118-1, Volume IV, "Evaluation of Ground 
Water Resources, South San Francisco Bay, South Santa Clara 
County Area". 




ON THE COVER: Off-stream ground t 
water recharge facilities operated by the 
Santa Clara Valley Water District. Some of 
these facilities have been in operation 
since before 1930, 



ERRATA 
Pg. 99. Correction replaces the two paragraphs on lower half of page, 

Artificial Recharge 



Artificial recharge 1 
to a ground water bas 
naturally. Artificia 
accomplished principa 
through impoundments 
releases into permeab 
surface reservoirs du 
season, thus affordin 
the channels of Uvas 
into the ground water 
Conservation District 
collectively referred 
recharging south coun 



s the practice of deliberately adding water 
in through means beyond that which would occur 
1 recharge in south Santa Clara County is 
lly by the Gavilan Water Conservation District 
in Uvas and Chesbro Reservoirs with timely 
le stream channels. Water is stored in the 
ring the wet season and released during the dry 
g an opportunity for the water to infiltrate 
and Llagas Creeks (see Figure 18A) and flow 
reservoir. In recent years, Gavilan Water 
has also used offstream percolation ponds, 
to as the Church Avenue Recharge Facility, for 
ty aquifers . 



Santa Clara Valley Water District artificially recharges water in 
south county by releasing water from Anderson Reservoir for infiltra- 
tion at the Main Avenue percolation ponds and or along the Madrone 
Channel (see Figure 17). 




- 3000 o 



Figure ISA MONTHLY RELEASE FROM UVAS AND CHESBRO RESERVOIRS 
{D^ia from Gavilan Water Conservation District) 



^ 



Department of Water Resources 

in cooperation with 

Santa Clara Valley Water District 

Bulletin 118-1 



Evaluation of Ground Water Resources 

South San Francisco Bay 

Vol . IV: South Santa Clara County Area 



May 1981 



Huey D. Johnson Edmund G. Brown Jr. Ronald B. Robie 

Secretary for Resources Governor Director 

The Resources State of Department of 

Agency California Water Resources 



FOREWORD 



This bulletin provides an evaluation of the ground water resources 
of South Santa Clara Valley, located in the southern portion of 
Santa Clara County. It also touches on the resources of a portion 
of the adjacent Hollister ground water basin in northern San 
Benito County. The bulletin is the result of a cooperative 
investigation undertaken by the Department of Water Resources 
(DWR) and the Santa Clara Valley Water District (SCVWD). 

The SCVWD service area is of special interest because it has one 
of the best-managed water resource programs in California. Ground 
water traditionally has been a major source of water supply in the 
area. As a result, SCVWD has developed a successful conjunctive 
water use program involving local surface water, artificial 
recharge, water conservation and waste water reclamation. The 
water district also receives imported water supplies from the 
State Water Project and the San Francisco Water Department (Hetch 
Hetchy) system; in the future it will receive water from the San 
Felipe Project, which is being constructed by the U. S. Water and 
Power Resources Service. Even with this well coordinated water 
program, population growth and increased industrial water use will 
reduce ground water in storage by the late 1980s, unless some 
corrective measures are taken. 

Because SCVWD pursues such a highly developed conjunctive water 
use philosophy, other water management agencies in California 
would do well to study the SCVWD blueprint as a model for their 
own management programs. Results of this study will be used by 
SCVWD to evaluate alternative management plans for the efficient 
use of surface, ground, and waste water and to evaluate the 
effects of various artificial recharge and ground water extraction 
strategies. In Santa Clara County and elsewhere, enlightened 
ground water basin management is essential to continued growth and 
welfare. 



Ronald B. Robie, Director 
Department of Water Resources 
The Resources Agency 
State of California 



111 



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ptes 


o( 


this bullet 


n or $4.00 each may be ordered 


from: 








State of California 

DEPARTMENT OF WATER RESOURCES 

P. 0. Box 388 

Sacramento, California 95802 




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ks 


poyable to 


DEPARTMENT OF WATER RESOURCES j 








California 


residents odd 6% sales tax. 





IV 



TABLE OF CONTENTS 

Page 

FOREWORD iii 

ORGANIZATION X 

CALIFORNIA WATER COMMISSION xi 

ENGLISH-METRIC EQUIVALENTS Inside back cover 

CHAPTER I . SUMMARY 1 

Area of Investigation 1 

Previous Investigations 2 

Current Investigation 3 

Major Findings 4 

Reconunendations 5 

CHAPTER II, GEOLOGIC FEATURES 7 

Physiographic Setting 7 

Geologic History 8 

Geologic Formations and Their Water-Bearing Properties . . 10 

Franciscan Formation 12 

Ultrabasic Rocks 24 

Great Valley Sequence 2 4 

Tertiary Marine Sediments 24 

Purisima Formation 25 

Santa Clara Formation 26 

Volcanic Rocks 26 

Valley Fill Materials 27 

Alluvial Fans 27 

Older Alluvium 28 

Younger Alluvium 28 

Basin Deposits 28 

Stream Deposits 29 

Landslides 29 

Base of Water-Bearing Materials 29 

Faults 34 

Paleodrainage System 36 

Lake Deposits 37 



TABLE OF CONTENTS 

Page 



CHAPTER III. GEOHYDROLOGY . 

The Ground Water Basin 

Water-Level Measurements, Contours, and Profiles 
Ground Water Occurrence 

Coyote Subbasin 

Llagas Subbasin 

Bolsa Subbasin 

Ground Water Movement 

Water-Level Fluctuations 

Coyote Subbasin 

Llagas Subbasin 

Bolsa Subbasin 

Ground Water Recharge 

Ground Water Quality 

CHAPTER IV. THE MATHEMATICAL MODEL 

Description of the Model 

Hydrologic Input ... 

Precipitation ".,... 

Tributary Runoff 

Artificial Recharge 

Stream Infiltration 

Coyote Unit 

Uvas Unit 

Llagas Unit .... 

Pacheco Unit 

Pajaro Unit 

Land Use , 

Pumpage 

Deep Percolation 

Change in Storage 

Historic Data 

Procedure 



VI 



57 


57 


60 


61 


61 


61 


76 


76 


76 


80 


82 


84 


84 


86 


89 


90 


95 


96 


96 


99 


100 


100 


102 


102 


104 


104 


104 


105 


10 8 


110 


110 


111 



TABLE OF CONTENTS 

Page 

Average Specific Yield 113 

Results 113 

Adjustment of the Model 113 

CHAPTER V. GROUND WATER BASIN 125 

Water Level Measurements 126 

Well Qualification 128 

Proposed Network 128 

Implementation of Network 129 

APPENDIXES 

A Bibliography of Geologic and Ground Water 

References 133 

B Glossary of Selected Geologic and 

Hydrologic Terms 139 

TABLES 

1 Description of Geologic Units, South Santa 

Clara Valley - Hollister Basin Area 11 

2 Post-Drought Water Level Recovery, 

South Santa Clara Valley 83 

3 Nodal Parameters, South Santa Clara Valley 

Ground Water Model 9 2 

4 Branch Parameters, South Santa Clara Valley 

Ground Water Model 9 3 

5 Net Annual Flows, South Santa Clara Valley 

Ground Water Model 97 

6 Tributary Runoff, South Santa Clara Valley 101 

7 Stream Infiltration, South Santa Clara Valley .... 103 

8 Land Use, South Santa Clara Valley 108 

9 Ground Water Pumpage, South Santa Clara Valley . . . 109 

10 Unit Values of Applied Irrigation Water, South 

Santa Clara Valley 110 

11 Deep Percolation in Pervious Soils, South 

Santa Clara Valley Ill 



vii 



TABLE OF CONTENTS 

Page 
TABLES (Continued) 

12 Deep Percolation in Impervious Soils, 

South Santa Clara Valley 112 

13 Changes in Ground Water Storage, South 

Santa Clara Valley 114 

14 Corrected Hydrologic Balance, South 

Santa Clara Valley 116 

15 Nodal Analysis of Ground Water Model, 

South Santa Clara Valley 117 

16 Existing Ground Water Monitoring Network, 

South Santa Clara Valley 127 

17 Proposed Ground Water Monitoring Network, 

South Santa Clara Valley 132 

FIGURES 

1 Area of Investigation Facing page 1 

2 Looking Back in Geologic Time 9 

3 Areal Geology, South Santa Clara Valley 13 

4 Geologic Section, South Santa Clara Valley .... 18 

5 Elevation Contours on Base of Alluvial 

Materials, South Santa Clara Valley 30 

6 Subsurface Deposition 39 

7 Ground Water Basin, Siobbasin, and Valley Floor 
Boundaries 5 8 

8 Elevation Contours of Water Levels in Wells, 

Fall 1914 and Fall 1974, South Santa Clara Valley 62 

9 Elevation Contours of Water Levels in Wells, 

Fall 1977 and Fall 1979, South Santa Clara Valley 66 

10 Water-Level Monitoring Wells and Preciitation 
Stations, South Santa Clara Valley 70 

11 Water Level Profiles, South Santa Clara Valley . . 74 

12 Hydrographs of Three Wells, Coyote Subbasin . . 77 

13 Hydrographs of Six Wells, Llagas Si±»basin .... 78 

14 Annual Precipitation at Two Stations, South 

Santa Clara Valley 80 



Vlll 



TABLE OF CONTENTS 

Page 
FIGURES (Continued) 

15 Monthly Stream Flow, by Calendar Year, 

Coyote Creek near Madrone 81 

16 Monthly Stream Flow, by Calendar Year, 

Llagas Subbasin Streams 85 

17 Ground Water Recharge Facilities, South 

Santa Clara Valley 86 

18 Monthly Releases, by Calendar Year, to 

Main Avenue Percolation Ponds 87 

19 Nodal Network , South Santa Clara Ground 

Water Model 90 

20 Isohyetal Contours, South Santa Clara 

Valley 98 

21 Accumulated Deviation from Mean Precipitation 

at Two Stations, South Santa Clara Valley 99 

22 Stream Percolation Units and Tributary 

Drainage Areas, South Santa Clara Valley 100 

23 Land Use, 1967 and 1974, South Santa 

Clara Valley 106 

24 Nodal Historic Periods of Record, South 

Santa Clara Ground Water Model 118 

25 Computer-Generated Hydrographs, South 

Santa Clara Ground Water Model 120 

26 Comparison of Historic and Model Generated 

Ground Water Elevation Contours 122 

27 Proposed Ground Water Monitoring Network, 

South Santa Clara Valley 130 



IX 



state of California 
EDMUND G. BROWN JR., GOVERNOR 

The Resources Agency 
HUEY D. JOHNSON, Secretary for Resources 

Department of Water Resources 
RONALD B. ROBIE, Director 

ROBERT W. JAMES MARY ANNE MARK GERALD H. MERAL CHARLES R. SHOEMAKER 
Deputy Director Deputy Director Deputy Director Deputy Director 

CENTRAL DISTRICT 

Wayne MacRostie Chief 

This investigation was conducted 
under the supervision of 

Donald J. Finlayson Chief, Investigations Branch 1/ 

Robert L. McDonell Chief, Investigations Branch 2/ 

by 

Robert S. Ford Supervising Engineering Geologist 

Assisted by 

Richard J. Lerseth ... Senior Engineer, W.R. 

Richard A. McGuire Water Resources Technician II 

Vera L. Doherty Editorial Technician 

Betty L. Swatsenbarg Geologic Aid 

Drafting services 
provided by 

Harry Inouye Senior Delineator 

Gayle Dowd Senior Delineator 

In cooperation with 

SANTA CLARA VALLEY WATER DISTRICT 

Under the supervision of 

David K. Gill Advanced Planning Manager 

by 

Eugene S. Watson Senior Civil Engineer 

Thomas I. Iwamura Engineering Geologist 

1/ Prior to March 1, 1979 
2/ After March 1, 1979 

X 



state of California 

Department of Water Resources 

CALIFORNIA WATER COMMISSION 



SCOTT E. \fRANKLIN, Chairperson, Newhall 
THOMAS K. BEARD, Vice Chairperson, Stockton 



Roy E. Dodson San Diego 

Alexandra C. Fairless Areata 

Daniel S. Frost Redding 

Merrill R. Goodall Claremont 

Donald L. Hayashi San Francisco 

Charlene H. Orszag Sherman Oaks 

James E. Shekoyan Fresno 



Orville L. Abbott 
Executive Officer and Chief Engineer 



Tom Y. Fujimoto 
Assistant Executive Officer 



The California Water Commission serves as a policy advisory 
body to the Director of Water Resources on all California water 
resources matters. The nine-member citizen Commission provides 
a water resources forum for the people of the State, acts as a 
liaison between the legislative and executive branches of State 
Government, and coordinates Federal, State, and local water 
resources efforts. 



XI 




FIGURE 1.--Area of Investigation, 



Xll 



CHAPTER I. SUMMARY 



Santa Clara County is a major water-consuming area which uses 
water supplied from surface storage reservoirs, ground water 
reservoirs, and imports. To obtain adequate information for the 
preparation of a series of water resource development plans in 
this area, the California Department of Water Resources (DWR) 
entered into an agreement with the Santa Clara Valley Water 
District (SCVWD) to study the water resources of Santa Clara 
County. 

This bulletin. Volume IV of the Bulletin 118-1 series, "Evaluation 
of Ground Water Resources: South San Francisco Bay", presents the 
geohydrologic conditions that affect the occurrence and movement 
of ground water in South Santa Clara Valley and a contiguous por- 
tion of Hollister Basin in San Benito County. The cooperative 
agreement called for equal sharing of costs of the study within 
Santa Clara County, with the State providing the entire staff and 
funding for the San Benito County portion. 

Plans have been drafted for additional studies of a wide range of 
management programs in the Santa Clara County portion of South 
Santa Clara Valley following the geohydrologic studies reported on 
in this bulletin. Parallel studies by DWR and SCVWD on possible 
use of waste water reclamation to extend the utility of present 
water supplies have been coordinated over the past several years 
and are continuing. In addition, a water quality management study 
has been conducted by the two agencies to provide information on 
cause-effect relationships and to form a basis for alternative 
water quality management plans. 

Area of Investigation 

The study area for this bulletin comprises South Santa Clara 
Valley, in Santa Clara County, and a portion of the contiguous 
Hollister Basin, in San Benito County. The area of investigation 
extends from Coyote Narrows, on the north, southward into San 
Benito County, as shown on Figure 1. The area is bounded on the 
west by the Santa Cruz Mountains and the Gabilan Range and on the 
east by the Diablo Range and the Calaveras fault. To the north, 
at Coyote Narrows, foothills of the Santa Cruz Mountains and 
Diablo Range nearly merge and form a constriction to ground water 
movement and, in turn, separate the study area from the remainder 
of the Santa Clara Basin to the north. The southern limit of the 
study area is formed by a narrow zone of water-bearing materials 
lying between the Calaveras fault, to the east, and the Lomerias 
Muertas, a group of Gabilan Range foothills to the west. 



Identification of Coyote Narrows as the northern limit of South 
Santa Clara Valley differs from the northern limit of the valley 
as defined in State Water Resources Bulletin 1, "Santa Clara 
Valley Investigation", June 1955. The presently defined northern 
limit was determined to be more appropriate from a basin manage- 
ment point of view than is the Bulletin 7 boundary, which crosses 
water-bearing materials and is coincident with the topographic 
divide at Cochran Road, just north of Morgan Hill. This concept 
is valid even though both surface and ground water to the north of 
the divide move northward toward San Francisco Bay, and to the 
south of the divide move southward toward Pajaro River and 
Monterey Bay. 

South Santa Clara Valley is composed of three subbasins: (1) 
Coyote subbasin, which extends from Coyote Narrows south to the 
topographic divide at Cochran Road; (2) Llagas subbasin, which 
extends from the Cochran Road topographic divide south to Pajaro 
River; and (3) Bolsa subbasin, which comprises the remainder of 
the study area. 

Previous Investigations 

Ground water has been a major source of water for domestic, agri- 
cultural, and municipal uses in South Santa Clara Valley for at 
least 90 years. Interest in this resource has resulted in the 
publication of a number of papers and reports dealing with this 
subject. The earliest known published reference to ground water 
in the South Santa Clara Valley was in a paper on the mineral 
resources of Santa Clara County by W. L. Watts, prepared for the 
Tenth Annual Report of the State Mineralogist in 1890. Watts dis- 
cussed the ground water conditions throughout the county and made 
reference to an artesian zone southeast of Gilroy, in which "at a 
depth of 320 feet, a good flow of water was obtained, which flowed 
5 inches above the edge of the 7-inch pipe". 

In 1914, W. 0. Clark began an extensive study of the ground water 
resources of the entire Santa Clara County area. Clark's work 
resulted in two publications by the U. S. Geological Survey. The 
earlier one, Water-Supply Paper 400, published in 1916, discusses 
geology and ground water conditions in the area from Coyote 
Narrows south to San Martin. The paper presents the location of 
251 wells, logs of 72 wells, water levels from 207 wells, and a 
number of streamflow measurements. 

Clark's second publication, Water-Supply Paper 519, published in 
1924, discusses the geology and ground water conditions throughout 
the county as well as south to Hollister. The report shows loca- 
tions of 466 wells in the present study area and also the limit of 
the zone of flowing wells as it existed in 1914. 



An agreement between the State and Santa Clara Valley Water 
Conservation District was signed in 1930 to study that District's 
water resources. That study, published in 1933 as Division of 
Water Resources Bulletin 42, evaluated the ground and surface 
water hydrology of the county as far south as the Cochran Road 
topographic divide, which was the southerly boundary of the Water 
Conservation District at that time. The study identified 10 wells 
and also provided some water level and streamflow data. No 
attempt was made to identify or define any geologic conditions. 

In 1948, a joint contract among the State, the County of Santa 
Clara, and the City of San Jose called for a new, detailed study 
of the ground water and surface water resources. The results of 
that study were published as State Water Resources Board 
Bulletin 7. The bulletin contained a general discussion of the 
geology and ground water resources of South Santa Clara Valley as 
well as that of the valley area to the north of Coyote Narrows. A 
map in that bulletin identified the Cochran Road topographic 
divide as being the boundary between South Santa Clara Valley and 
the ground water basin to the north. 

Beginning in 1962, DWR undertook a comprehensive study of the 
geology and ground water resources of the entire South Bay por- 
tions of Alameda and Santa Clara Counties. This study has 
resulted in the publication of three bulletins dealing with the 
evaluation of ground water resources: two are concerned with the 
resources of southern Alameda County, and one with resources of 
the northern portion of Santa Clara County. This bulletin, on the 
resources of South Santa Clara Valley, completes the evaluation of 
ground water resources of the South Bay area. 



Current Investigation 

The geohydrologic study contained in this bulletin was performed 
to provide a framework for the development of a mathematical model 
of the ground water basin. The model, in turn, will be used as a 
conceptual tool for the generation of a workable ground water man- 
agement plan. 

Early in the study it became apparent that there was a critical 
need for geologic work to define the aquifer system which pre- 
viously had been described as a heterogeneous mixture of permeable 
and impermeable strata. To this end, a statistical analysis was 
used to examine the large quantity of well logs and other sub- 
surface data which were available. The results of the geologic 
phase of the study include the analysis which helped to develop a 
3-dimensional concept of the subsurface features. 



Major Findings 

Nearly all of South Santa Clara Valley has underlying geologic 
formations which yield some water to wells. The water-bearing 
formations are faulted, and those faults traversing the valley 
floor appear to impede ground water movement to some degree. 
There is no indication of any total barriers to ground water move- 
ment except the Calaveras fault, which cuts across the San Benito 
County portion of the valley from San Felipe Lake to Hollister. 

The area between Gilroy and Hollister contains lake deposits in 
the near-surface strata, causing confined ground water conditions 
to occur. Because of this condition, flowing wells were once 
prevalent in some parts of this area. There is some indication 
from preliminary modeling results that the water-bearing zones 
below the confining lake beds do not react as a totally confined 
aquifer system, but rather more like a leaky system. There 
appears to be some ground water infiltration taking place, imply- 
ing that most of the lake beds are formed of discontinuous clay 
layers. 

The ground water model for South Santa Clara Valley is verified as 
well as available data will permit. However, the level of verifi- 
cation still is not adequate for reliable analysis of detailed 
management and operation p'ans. Nevertheless, the model can be 
used as a tool for a general analysis of operation plans if its 
present limitations are recognized. 

At the present time, there are two major differences between 
ground water levels generated by the model and historic levels: 

1. Historic water levels in the upper part of the Llagas subbasin 
imply that an impulse recharge was introduced in 1969; hydro- 
logic data calculations do not support this. Consequently, 
water levels generated by the model for this area do not agree 
with historic water levels for the period 1969 to 1971. 

2. Historic water-level data for the central portion of the 
Llagas subbasin indicate a steep gradient; levels generated by 
the model have a more gradual gradient. This difference 
implies that some restriction to ground water migration might 
be taking place. The ground water monitoring network proposed 
in Chapter V, augmented by an improved data network, would 
provide the necessary data for further adjustment of the 
model . 



Recommendations 

Based on the material presented in this bulletin, it is 
recommended that the Santa Clara Valley Water District: 

1. Complete verification of the ground water model developed 
in this study by: 

a. Redesigning the data collection system on the basis of 
geologic and hydrologic information. 

b. Testing the accuracy of the ground water model with the 
data collected during the first three to five years of 
operation of the redesigned data collection system. 

2. Use the presently unverified model to test the general 
response of the ground water system for a variety of 
alternative conjunctive operation plans. 

3. Continue to cooperate with other local water agencies in 
conjunctive operations of the surface and ground water 
resources available to the area. 

4. Take measures to assure that overdraft does not recur by 
securing new sources of water as needed and obtaining 
necessary legal authority to prohibit damaging overdraft. 



2—82239 



CHAPTER II. GEOLOGIC FEATURES 



Decisions affecting the mode, occurrence, quality, and use of 
ground water in South Santa Clara Valley must be based on a 
knowledge of the geologic and hydrologic aspects of the study area 
and its surrounding region. The geology as it pertains to ground 
water can be perceived by examining the physiographic setting, the 
geologic history, and the nature and water-bearing characteristics 
of the various geologic formations. 

Physiographic Setting 

South Santa Clara Valley is a northwest-trending feature roughly 
38 kilometres (km), or 24 miles (mi), in length; it ranges in 
width from 3 to 10 km (2 to 6 mi). Most of the valley is drained 
by the Pajaro River, which flows westerly along the southern 
boundary of the valley and empties into Monterey Bay, about 30 km 
(20 mi) to the west. Major tributaries include Llagas and Uvas 
Creeks, both of which enter South Santa Clara Valley from the west 
and flow southerly to the Pajaro River. The extreme northern 
portion of the valley is drained by Coyote Creek, which enters the 
valley 5 km (3 mi) northeast of Morgan Hill and flows northwest- 
erly to exit at Coyote Narrows. Coyote Creek then continues 40 km 
(25 mi) northwesterly across North Santa Clara Valley and empties 
into San Francisco Bay. The floor area of South Santa Clara 
Valley covers about 180 square kilometres (km ), or 70 square 
miles (mi ). Of this area, 155 km (60 mi ) are within the 
Pajaro River drainage area with the remaining area drained by 
Coyote Creek. 



The Hollister Basin is south of the Pajaro River, adjacent to 
South Santa Clara Valley, and wholly within San Benito County. 
This basin also is drained by the Pajaro River, with the major 
tributaries being Tequisquita Slough, Pacheco Creek, and the San 
Benito River. The study area portion of the Hollister Basin 
covers about 60 km (23 mi ) . Hollister Basin is transected 
by the Calaveras fault, a regional feature that branches from the 
San Andreas fault 20 km (12 mi) southeast of the basin and, after 
crossing the basin floor, extends northerly through the foothills 
east of South Santa Clara Valley. Because of ground water barrier 
conditions along the fault, that portion of Hollister Basin east 
of the fault was excluded from the study area. 

To the east of South Santa Clara Valley and Hollister Basin rise 
the foothills and mountains of the Diablo Range; promontories 
include Mt. Sizer, elevation 983 metres (3,225 ft), Mariposa 



Peak, elevation 1 055 m (3,461 ft), and Laveaga Peak, elevation 
1 159 m (3,802 ft). To the west, north of the Pajaro River, are 
the Santa Cruz Mountains, with Loma Prieta, elevation 1 160 m 
(3,806 ft) and Mt. Madonna, elevation 578 m (1,896 ft) being the 
principal promontories. South of the Pajaro River is the Gabilan 
Range, culminating at Fremont Peak, elevation 967 m (3,172 ft). 
Also to the west of Hollister Basin are the Lomerias Muertas, 
which attain a maximum elevation of 360 m (1,181 ft). 

The floor of South Santa Clara Valley attains a maximum elevation 
along the drainage divide near Morgan Hill. Here the elevation is 
1 45 m (476 ft) near the east side of the valley at a point imme- 
diately southwest of Anderson Dam. The lowest point is at an ele- 
vation of 35 m (115 ft) where the Pajaro River exits the valley. 
Coyote Creek leaves the valley at Coyote Narrows at an elevation 
of 75 m (246 ft) . 

The study area portion of Hollister Basin is a low-lying, 
extremely level area called The Bolsa. This intensely farmed area 
slopes gently upward from the Pajaro River, at elevation 35 m 
(115 ft), to elevation 76 m (250 ft) at the southern boundary of 
the study area. 

Geologic History 

The geologic history of South Santa Clara Valley can be traced to 
the latter part of the Jurassic Period, some 140 million years 
ago, as shown on Figure 2. Prior to that time, the history is 
largely known only by inference, as the record has been obscured 
by events of Jurassic and later periods. During the Jurassic and 
Cretaceous Periods, much of this part of California was dominated 
by a marine environment. Sediments accumulated on the ocean floor 
associated with outpourings of oceanic volcanic rocks and injec- 
tion of serpentine along zones of weakness. These rocks, now 
folded, altered, and deformed, comprise the Franciscan Formation 
now exposed in the Santa Cruz Mountains. To the east, sequences 
of nonvolcanic sands and clays also were deposited on the ocean 
floor with the resulting rocks now constituting the Great Valley 
Sequence which crops out to the east of South Santa Clara Valley. 

For the next 40 million years, the region was lifted above sea 
level, deformed by faulting and warping, and subjected to erosion. 
Beginning in the Miocene Epoch, about 25 million years ago, a por- 
tion of the area again became submerged and deposition of marine 
clays and sands resumed. These latter materials now form the 
Tertiary marine sediments found to the west of Gilroy. 

Near the close of the Miocene Epoch, much of the area again was 
uplifted and faulted over a period of about 2 million years, until 
the early part of the Pliocene Epoch, when marine deposition 
resumed. At that time, vast amounts of sand were deposited in the 
area from Lomerias Muertas west to Monterey Bay, creating what is 
now the Purisima Formation. Once again the land was uplifted 
above sea level and further deformed by erosion and faulting. 



^t,^T\V^At40ATOMICGEotOG/C tu 




HOTt 

IfMorifis not dratvn fo tco'c 

Chart Iran US.G.S 



Beginning of geologic record 
in South Santa Claro Valley 



,PP<'' 



26 



3^ ,^- ^^ ^tagyAy 



^,y!2^^ o^/<?ocs^^ 



/t/ZOC/TA'^ 






HOLOCENE 

PAST 10500 YEARS 




Serpentine 
Franciscan Formotion 
Great Valley Sequence 



Tertiory Marine 
Sediments 



Purisima Fm. 
Santo Clara Fm. 
Volcanic Rocks 



Alluvial Sequence 
Landslides 



FIGURE 2. --Looking Back in Geologic Time. 



Continental sedimentation began in the southern part of the area 
about 4 million years ago, followed some time thereafter by vol- 
canic activity which broke out in the vicinity of the Calaveras 
fault and produced the series of basalt flows now found east of 
Gilroy. The volcanic activity was associated with deposition of 
great thicknesses of continental clay, sand and gravel, forming 
the Santa Clara Formation. Deposition of the Santa Clara 
Formation continued throughout much of the Pleistocene Epoch, dur- 
ing which there was continued regional faulting and folding. 

During the latter part of the Pleistocene, movement along the San 
Andreas fault, to the west, apparently formed a natural dam and 
created a lake that filled both South Santa Clara Valley and 
Hollister Basin to a maximum elevation of 90 m (295 ft). This 
lake, named Lake San Benito, apparently was not the first lake to 
occupy the area, as old dissected terraces suggest that there has 
been at least one earlier lake with a water surface elevation of 
about 130 m (427 ft). Lake San Benito was in existence for a 
fairly long time, indicated by a maximum thickness of lake-bottom 
clays on the order of 75 m (246 ft). At times. Lake San Benito 
probably spilled to the north and drained to the sea by way of the 
depression that is now San Francisco Bay. At other times, the 
lake drained to the west through what is now Elkhorn Slough. This 
westerly drainaqe is believed to have contributed to the excava- 
tion of the well known Monterey Sea Canyon. Ultimately, fault 
movement removed the natural blockage, and Lake San Benito was 
drained. However, once again, fault movement associated with 
landsliding apparently blocked the outlet and a later lake, called 
Lake San Juan, was created with a water surface elevation of 60 m 
(197 ft). An additional 50 m (165 ft) of lake-bottom sediments 
accumulated in Lake San Juan before it too was drained, leaving 
the area much as it is today. 

Geologic Formations and Their Water-Bearing Properties 

A number of geologic formations in South Santa Clara Valley and 
Hollister Basin yield water to wells to some degree. The Pliocene 
to Holocene materials are the principal water-producing units; 
water derived from these materials usually is of excellent qual- 
ity, although local quality problems occur. In contrast, the pre- 
Pliocene rocks yield little water and the water may contain enough 
undesirable mineral constituents to make it unusable for most 
beneficial purposes. 

Each of the various geologic formations occurring in the South 
Santa Clara Valley area is briefly discussed below. The discus- 
sion includes a description of the general lithology, the water- 
yielding characteristics, and the general character of the water 
quality. Table 1 presents a brief description of the general 
character and water-bearing properties of the various geologic 
formations. The areal extent of each of the various geologic 
units is shown in Figures 3A, 3B, and 3C; geologic sections are 
shown in Figures 4A through 4D. 



10 



Table 1. Description of Geologic Units, 
South Santa Clara Valley-Hol lister Basin Area 



Geologic 
Age 


Geolor.lc Unit 


Map 

Symbol 

(FiCiure i) 


General Character, 
Location, and Thlcknesii 


Water-bearing 
Properties 


Holocene 


Landslides 


*ls 


Unstable masses of clay and 
rocks occurring on slopes east 
of Valley; may be as much as 
15 m (50 rt. ) thick. 


Not a reliable source ol ground 
water; locations of a number of 
springs and seeps . 




Stream Deposits 




Unconsolidated p;ravel and sand 
in and near stream channel 
areas and on terraces; may 
be subject to flooding. May 
be as much as 15 m (50 ft.) 
thick. 


May be good source of ground 
water Jn nonflooding areas; ground 
water Is unconflned. Most ground 
water in this unit is u'-.derflow. 




Basin Deposits 


-ib 


Unconsolidated clay, silt and 
ort^anlc materials occurring in 
flat, undrained portions of Valley; 
saline soils are present In some 
areas. May be subject to ponding. 
May be as much as 30 m (100 ft.) 
thick. 


Very low permeability; not a reliable 
source of ground water. Of no impor- 
tance to .ground water recharge. 




Younger Alluvium 


-Jy 


Unconsolidated floodplain deposits 
of clay, silt, and sand; contains 
some zones of sandy gravel. May 

be as much as 30 m (100 ft.) thick. 


Provides water to shallow wells. 
Important to ground water recharge. 
Ground water is generally unconflned. 




Alluvial fans 


Qf 


Unconsollaated to semlconsolldated 
sand, gravel, and clay occurring 
at edge of valley and at mouths of 
tributaries. May be as much as 37 m 
(125 ft.) thick. Deposits of clayey 
gravel underlying older alluvium 
probably belong to this unit. 


Generally yields large amounts water 
to properly-constructed wells. Most 
ground water is under some degree of 
confinement. 


Pllo- 
Plelstocene 


Older Alluvium 


Qo 


Unconsolidated older floodplain 
deposits of clay, silt, and sand 
with predominant clay subsoil. May 
be as thick as 37 m (1^5 ft.) 
near the axis of the valley- 


Provides some water to wells; most 
wells located on this unit produce 
water froiji underlying Tiaterials. 
Ground water varies from unconflned 
to confined. 




Santa Clara 
Formation 


TQs 


Folded and faulted beds of 
consolidated silt, clay, and sand; 
occasional zones of gravel. Exposed 
to east of valley; occurs at depth 
under valley floor. Up to 550 m 
(1,800 ft.) of stratlgraphlc 
thickness . 


A major water-bearing formation. Many 
deep wells in valley areas tap upper 
part of this formation, yielding 
large quantities of good quality 
water. 




Volcanic rocks 


T>iv 


Basalt and basic intruslves 
occurring In hills to east of 
valley. Occur interbedded with 
Santa Clara Formation; present in 
subsurface beneath floor of valley. 
Thickness not known. 


Of little importance t^ ground water. 


Pliocene 


Purislma 
Formation 


Tp 


Folded and faulted beds of 
massive micaceous siltstone, 
sandstone, conglomerate, and 
gypsiferous shale_ cropping out 
west of Hollister Basin; occurs 
at depth beneath some valley 
floor areas. Stratlgraphlc 
thickness Is as much as >* 600 m 
(15,000 ft.); most of formation 
Ic of marine origin. 


Uppermost 600 m (2,00C it.) contains 
good quality water under confined 
conditions; remainder of formation 
contains saline water. 


Miocene 


Tertiary Marine 
Sediments 


Tm 


Fosslllferous conglomerate and 
sandstone; siliceous shale and 
mudstone. All are of marine 
origin. Exposed in hills west 
of Gilroy. Of undetermined 
thickness. 


Generally contains saline water. A 
few low-yielding wells tap potable 
water contained in fractures and 
flushed zones. 


Cretaceous 


Great Valley 
Sequence 


K 


Folded, thinly-bedded shale, 
sandstone, and conglomerate; 
all of marine orlr.ln. Estimated 
thickness 12 000 m (^40,000 ft.) 


Contains saline and mineralized water. 


Jura- 
Cretaceous 


Franciscan 

Formation 


,TKf 


Folded, faulted, and sheared 
lithic sandstone and shale; altered 
basalt, diabase, and cuff; 
chert, greenstone, limestone, and 
melange. All of marine origin. 
Estimated thlcknes.--. 15 000 m 
(50,000 ft.) 


or no significant Importance to ground 
water. 




Ultrabaslc rocks 


ub 


Green to black serpentine. 
Of undetermined thickness. 


Of no importance to ground water. 



11 



Franciscan Formation 

Rocks of the Franciscan Formation are exposed in the Santa Cruz 
Mountains to the west of South Santa Clara Valley, at several 
isolated hills protruding through the valley floor, at a few 
locations in the foothills immediately east of the valley, and in 
the central portion of the Diablo Range. The formation also 
underlies South Santa Clara Valley and Hollister Basin at depths 
ranging from 50 m (160 ft) near Coyote to as much as 1 000 m 
(3,000 ft) in The Bolsa. 

The Franciscan Formation has been estimated by Bailey and others 
(1964) to be about 15 000 m (50,000 ft) in stratigraphic thick- 
ness. It is composed of a great variety of folded, faulted, and 
sheared marine sediments and related oceanic volcanic rocks. The 
most widespread rock type is a well indurated, poorly sorted sand- 
stone containing abundant grains of quartz and feldspar as well as 
many lithic fragments; this rock type frequently has been called a 
graywacke. The predominant color of the sandstone is gray; 
weathered exposures commonly are tan to brown. Exposures of the 
sandstone are usually mantled by a residual soil cover about one 
metre (3 ft) thick. 

Shale accounts for about 10 percent of the volume of the 
Franciscan Formation. It is commonly interbedded with the sand- 
stone and is usually gray to black. Volcanic rocks, such as pil- 
low basalt, diabase, tuff, and tuff breccia are common in most 
areas; many of these rocks have been altered to greenstone. Minor 
rock types include chert, limestone, silica-carbonate rock, and 
melange, the latter being a chaotic mixture of sandstone, green- 
stone, chert, and other rocks in a sheared, shaly matrix. 

Bedding in the Franciscan Formation is highly variable, with indi- 
vidual beds ranging from 2.5 centimetres (1 in.) to 6 metres 
(20 ft) in thickness. Fossils generally are rare, although 
locally abundant in beds of chert and limestone. The Franciscan 
Formation is of marine origin and was probably formed in the deep 
ocean in water depths ranging from 180 to 900 m (600 to 3,000 
ft). 

The Franciscan Formation is considered to be of no significant 
importance to ground water. In the entire South Santa Clara 
Valley area there are only 25 wells which are known to yield water 
from this formation and for which data are available. The wells 
range in depth from 30 to 100 metres (100 to 330 ft), and the 
depth to water at the time of drilling ranged from 4 to 55 metres 
(13 to 180 ft). Discharges from all of these wells are minimal, 
ranging from 10 to 190 litres per minute (L/m), or 3 to 50 gallons 
per minute (gpm) . Any ground water yielded from the Franciscan 
Formation is derived from secondary fractures rather than from 
primary openings. Water quality data are lacking from these 
wells. Because the wells are all used for domestic purposes, it 
can be assumed that the water is of acceptable quality. 



12 




INDEX MAP 



Xajxu5 



SYMBOLS 

GEOLOGIC CONTACT, BEDROCK UNITS 
GEOLOGIC CONTACT.ALLUVIAL UNITS 

LANDSLIDE 

FAULT, SHOWING DIRECTION OF MOVEMENT; 
DASHED WHERE PROJECTED. 
DOTTED WHERE CONCEALED. 



o^^oooooooooooo APPROXIMATE LIMIT OF LAKE SAN BENITO 

(Water-Surface Elevation 90 m3 

,>\MH./„„vuu.ui(niu./, APPROXIMATE LIMIT OF LAKE SAN JUAN 

CWater-Surface Elevation 60 m) 

A 1 

T- J LOCATION OF GEOLOGIC SECTION 

Note: SeeTable 1 For Description Of Geologic Units 

SOURCES OF DATA 

BEDROCK GEOLOGY: LANDSLIDES: ALLUVIAL GEOLOGY: 

T. W. Dibblee (1973), T. H. Nilsen (1972) D. Isgrig (1969). 

0. Kilburn (1972). W. C. Lindsay (1974) 
T. H. Rogers and J. W. Williams (1974) 



FIGURE 3A.--Areal Geology, South Santa Clara Valley. 



13 




14 



FIGURE 3B.--Areal Geology, 




South Santa Clara Valley. 



15 




FIGURE 3C.--Areal Geology, 



16 




South Santa Clara Valley. 



17 




1 ''~-^'-Sand ond Gfovel 
p^^ --^qepoiits J 





NOTE: See Figure 3B for 
locations of section. 



DISTANCE IN KILOMETRES 
2 3 



DISTANCE IN MILES 




DISTANCE IN MILES 



FIGURE 4A. --Geologic Sections A-A' and B-B' 
South Santa Clara Valley. 



18 



nniTO sssn - 133J ni noiiva3T3 




01 



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3 
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C/1 



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o 
<u 



en 
o 

(U 
CD 



CO 



on 

C3 



S3ai3W NI NOIiVA3"13 



19 



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pA|g Dssjai oiuDS - 







HMJO soBon ■ 



J - J U0)0»s - 







■i- 






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S3Hi3W NI NOI1VA313 



20 



nniva sosn-i33j ni NOiimjiB 




■o 




c 




(O 




- 


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o 


CU 


1 


^^ 


Q 


r— 




(T3 


to 


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c 




o 


(0 




t. 


+j 


IB 


o 


^— 


ai o 


OO 






to 


o 


+-> 


• r- 


c 


cn 


rtj 


O oo 


^— 




o ^ 


0) 


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ts 


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S3«i3W Nl NOI1VA313 



3—82239 



21 



COYOTE 







DISTANCE IN KILOMETRES 
4 5 



2 3 

DISTANCE IN MILES 




" - ~ Sond Ond 
Deposits 



— L'it 






Santo Cloro Formetion 



Bose o( well^ 
Log Oolo 



NOTE; See Figures 3Band 3C 
for location of section. 



DISTANCE IN KILOMETRES 
25 26 27 



DISTANCE IN MILES 



22 



FIGURE 4D. --Geologic Section F-f''-f2, 



ii> 



I il 



MORGAN ° 
I "'^^ I 



o • 



Post Lohr Sar< Benito Fluviol Deposits 




■', ;; 




Sand ond Gravel Deposits^ 



- LoKe Son Benito 
Delto-c Depoiiti (7) 



F'OnCiscon Formation 



DiSTANCt IN KILOMETRES 
14 15 16 



--I00 O 



DISTANCE IN MILES 



Approiifnoti Level of Lahe Son Benito 



/Alluwiufn 






Approiimote Level of Lake San Juon 



^ — ' — ''•' _ 



,-^ — .-•^- 



^ 
^ 



\.Dell 



<::::^^-"— 



C'J' 



._ -zr — ■ 



i/ 



Base of Well^ 
Log Dola 



DISTANCE IN KILOMETRES 
37 38 



J 



£-'- 



DISTANCE IN MILES 



South Santa Clara Valley. 



23 



Ultrabasic Rocks 

Ultrabasic rocks, which belong to the Franciscan Formation, crop 
out near Coyote Narrows, in the hills east of Coyote Creek, and at 
other locations in Franciscan terrain. These rocks also occur at 
depth beneath alluviated areas in association with Franciscan 
rocks. The ultrabasic rocks are of undetermined thickness and are 
composed principally of green to black serpentine which exhibits a 
reddish-brown soil cover. The serpentine is usually extensively 
fractured and sheared. No known wells tap these rocks, and it is 
not expected that they would yield measurable quantities of water 
to wells. Water quality data are not available, but ground water 
contained in ultrabasic rocks in other regions is generally of 
poor quality. 

Great Valley Sequence 

Rocks of the Great Valley Sequence, of Cretaceous age, crop out to 
the east of South Santa Clara Valley in a belt that is about 5 km 
(3 mi) wide between the Calaveras fault on the west and the 
Madrone Springs fault on the east. These rocks also are exposed 
to the east of Hollister Basin as well as at certain localities in 
the Santa Cruz Mountains. The rocks of the Great Valley Sequence 
differ markedly from those of the Franciscan Formation. Sand- 
stones tend to be clean and of uniform grain size; calcite cement 
is present locally. Shales are abundant and locally form more 
than one-half of the sequence. Conglomerate is generally present 
and may occur in very thick lenses. The total estimateS thickness 
of the Great Valley Sequence is about 12 000 m (40,000 ft). The 
Great Valley Sequence is thinly bedded; rhythmic alterations of 
sandstone and shale are common; beds generally have great con- 
tinuity. Unlike the Franciscan Formation, fossils are common to 
locally abundant. Also unlike the Franciscan Formation, the rocks 
of the Great Valley Sequence are only moderately to slightly 
deformed. All of the sequence is of marine origin, having been 
deposited in a nonvolcanic environment at depths significantly 
less than the deep ocean environment of the Franciscan Formation. 

Well data are totally lacking for the Great Valley Sequence. 
Wells drilled into these rocks would probably yield minimal 
amounts of water and the incidence of dry holes would be very 
high. Ground water contained in fractures in these rocks probably 
is of adequate quality, although areas of mineralized or saline 
water are known to occur throughout the sequence. 

Tertiary Marine Sediments 

Exposures of marine sediments of Miocene age are found to the west 
of Gilroy, in the area from Day Road south to Uvas Creek. The 
sediments consist of a sequence of fossilif erous conglomerate and 
sandstone belonging to the Temblor Formation, overlain by hard 



24 



brown siliceous shale and mudstone belonging to the Monterey 
Formation. . These sediments are of undetermined thickness. 

Like the older marine sediments, these materials are not con- 
sidered to be of any great significance as sources of ground 
water. Only four wells are known to yield water from the Tertiary 
Marine Sediments, and they produce only about 15 to 60 L/m (4 to 
16 gpm). The four wells range in depth from 50 to 90 metres 
(165 to 295 ft), and the depth to water at the time of drilling 
was about 15 metres (50 ft). Although no water quality data are 
available from these wells, the water produced is probably only 
marginally potable. The water derived from these sediments is 
probably contained in such secondary openings as fractures and 
shears; some potable water also may be derived from flushed zones. 
Because of the marine origin of these sediments, most water 
contained in primary openings may be expected to be saline. 

Purisima Formation 

The Purisima Formation, of Pliocene age, is of nonmarine and 
marine origin; it contains many zones of fresh water. The forma- 
tion is exposed to the southwest of South Santa Clara Valley and 
Hollister Basin. It also underlies the valley floor south of 
Gilroy at an undetermined depth. 

The lowermost member of the formation is exposed in the Sargent 
Hills and Lomerias Muertas from Tick Creek southward. The member 
is composed of sandstone with interbedded micaceous siltstone and 
is estimated to have a stratigraphic thickness of about 1 500 to 
2 100 m (4,920 to 6,890 ft). The member is folded and faulted, 
and of medium permeability. Kilburn (1972) reports that ground 
water in this member is saline. 

The middle member of the formation is exposed principally in the 
central portion of Lomerias Muertas. It has a stratigraphic 
thickness of not over 1 500 m (4,920 ft) and is composed of mas- 
sive sandstone, pebbly conglomerate, and gypsiferous shale. 
Kilburn states that this member probably contains poor quality 
saline water at depth, but shallow depths probably contain potable 
water. Ground water in this member may locally contain large 
concentrations of sulfate, and it may be under some degree of 
confinement. 

The uppermost member of the Purisima Formation also is exposed in 
the Lomerias Muertas. The member consists mostly of pebbly sand- 
stone and is estimated to be about 600 to 1 000 metres (2,000 to 
3,300 ft) in stratigraphic thickness. Most of the materials in 
this member are of continental origin. A few wells are known to 
tap this member, but well data are totally lacking. 

Kilburn reports that the member contains good quality ground water 
under confined conditions and yields large amounts of water to 
wells. 



25 



Santa Clara Formation 

Sediments belonging to the Santa Clara Formation, of Plio- 
Pleistocene age, are exposed in the hills bordering much of the 
east side of South Santa Clara Valley; related materials are 
exposed farther to the east. The formation also underlies much of 
South Santa Clara Valley, but the depth to the uppermost layers of 
the formation could not be determined because it is not possible 
to make a distinction between it and the overlying alluvial 
deposits from data presented on water well drillers' logs. 

The Santa Clara Formation consists of fairly well consolidated 
silt, clay, and sand; some zones of gravel are present. Most of 
the materials were deposited under fluvial conditions. The forma- 
tion is in fault contact with or lies unconformably on a fairly 
rugged surface of older rocks, notably those of the Franciscan 
Formation and Great Valley Sequence. After deposition, the forma- 
tion was folded into northwest-trending anticlines and synclines 
whose limbs dip from 5 to 40 degrees. The Santa Clara Formation 
has an estimated maximum stratigraphic thickness of 550 metres 
(1 ,800 ft). 

Data are available from only three wells completed in the Santa 
Clara formation. The wells, in the general area of Anderson 
Reservoir, range in depth from 12 to 114 metres (40 to 375 ft). 
At the time of drilling, the reported depth to water was about 
5 metres (16 ft); on test, the wells yielded from 45 to 375 litres 
per minute (12 to 100 gpm). Water quality data are not available 
for any of these wells; it may be assumed that the water produced 
is of acceptable quality as the wells are used for domestic pur- 
poses. The lower portions of many deep wells in the study area 
undoubtedly tap sediments of the Santa Clara Formation. A number 
of these wells produce excellent quality ground water used for 
irrigation and municipal purposes. 

Volcanic Rocks 

Volcanic rocks of late-Pliocene age crop out at a number of loca- 
tions to the east of South Santa Clara Valley. Similar rocks also 
occur in the subsurface in certain parts of the valley as indi- 
cated by well logs. The volcanic rocks consist principally of 
basalt, although local areas of basic intrusive rocks also occur. 
Dibblee (1973) indicates that the age of some of the volcanic 
rocks is 3.5 million years, placing them in the latter part of the 
Pliocene Epoch. Post-Pliocene deformation has folded these 
sequences into a series of gentle anticlines and synclines, all 
having a northwest trend. The volcanic rocks are in fault contact 
with older rocks of the Great Valley Sequence and Franciscan 
Formation, as well as with younger materials. In places, the 
volcanic rocks are interbedded with sediments of the Santa Clara 
Formation. The thickness of the volcanic rocks has not been 
determined. 



26 



No wells are known to have been completed in the volcanic rocks. 
Carefully located wells may yield adequate water for domestic 
purposes; however, the incidence of dry holes will be significant. 
Ground water contained in the volcanic rocks is probably of 
acceptable quality for most beneficial purposes. 

Valley Fill Materials 

Valley fill materials, of Holocene age, occur in the gently slop- 
ing to level valley floor portion of South Santa Clara Valley, 
Hollister Basin, and tributary valleys. The materials range in 
thickness from less than a metre (3 ft) to probably as much as 
30 to 50 metres (100 to 165 ft) near the axes of the valleys. 
Some of the valley fill materials are underlain by the Santa Clara 
and the Purisima Formations; identification of the contact between 
the valley fill materials and these other formations is not 
possible due to the marked similarity reported on well drillers' 
logs. Other parts of the valley fill materials are underlain by 
volcanic or pre-Pliocene rocks. 

The valley fill materials are divided into two general groups: 
alluvium, which has a slope of less than 2 percent (i.e., a rise 
of 2 metres in 100 metres or 2 ft in 100 ft) and alluvial fan 
deposits, which have slopes greater than 2 percent. The alluvium, 
has been further subdivided into older alluvium, younger alluvium, 
basin deposits, and stream channel deposits based on a combination 
of their physiographic expression and their soil characteristics. 

The valley fill materials are the principal water produces in 
South Santa Clara Valley and Hollister Basin. Well yields vary 
widely depending on well construction and location. Yields from 
properly constructed wells are adequate to meet the needs of any 
beneficial use to which ground water is put. Quality and depth to 
water vary from point to point. 

Alluvial Fans . Alluvial fan deposits occur around the margin of 
South Santa Clara Valley, Hollister Basin, and near the mouths of 
tributary valleys. The fans are composed of a heterogeneous, 
unconsolidated to semiconsolidated mixture of clay, silt, and 
sand; gravel lenses and stringers are common. The alluvial fans 
range in thickness from less than a metre (3 ft) to as much as 
37 m (125 ft). Alluvial fan deposits rest on a variety of older 
materials, ranging from sediments of the Santa Clara Formation to 
rocks of the Franciscan Formation. In the valley, the fans are 
overlain by younger alluvial materials. Many of the zones of clay 
and gravel underlying the valley near Cochran Road belong to 
alluvial fan deposits which have become buried by younger alluvial 
materials. 

Because of their heterogeneity, the alluvial fans contain ground 
water that is usually under some degree of confinement. Water 
quality generally is not a problem, and the alluvial fan deposits 



27 



whether exposed or under a veneer of alluvium, usually yield large 
amounts of ground water to properly constructed wells. 

Older Alluvium . Deposits of older alluvium occupy the central 
portion of South Santa Clara Valley, from near Coyote south to 
Gilroy. Older alluvium consists of unconsolidated clay, silt, and 
sand which was formed as floodplain deposits. The older alluvium 
is characterized by a dense clayey subsoil which inhibits the 
downward movement of water; hence it possesses a very low recharge 
potential. Older alluvium is as much as 37 m (125 ft) thick near 
the axis of South Santa Clara Valley; it is underlain by alluvial 
fan deposits and a variety of older sediments, most notably those 
of the Santa Clara Formation and lacustral deposits from Lake San 
Juan. The older alluvium is overlain in a few places by younger 
alluvium and basin deposits. 

Ground water in the older alluvium ranges from unconfined to 
locally confined. It provides adequate yields of water to wells 
up to 30 m (100 ft) deep; deeper wells located on the older 
alluvium draw from underlying materials. Most water produced by 
the older alluvium is of acceptable quality. 

Younger Alluvium . Younger alluvium occurs in flat, well drained 
areas near Coyote and also from Gilroy south to the Hollister 
Basin. The younger alluvium is composed of unconsolidated 
deposits of silt, sand, and clay; zones of buried sandy gravel 
locally occur. In a manner similar to the older alluvium, younger 
alluvium has been formed as a floodplain deposit. In contrast to 
the older alluvium, however, the younger alluvium does not possess 
a well defined clay subsoil and thus water can percolate downward. 
The younger alluvium attains a maximum thickness of about 30 m 
(100 ft) and is generally underlain by alluvial fan deposits and 
older alluvium. 

Ground water in the younger alluvium is generally unconfined. It 
provides adequate water for domestic purposes to wells generally 
less than 30 m (100 ft) in depth; deeper wells located on this 
unit tap underlying materials. Ground water in the younger 
alluvium is generally of acceptable quality. 

Basin Deposits . Basin deposits occur in low-lying, undrained 
areas near Coyote and Gilroy in South Santa Clara Valley and in 
the Bolsa area of the Hollister Basin. The deposits consist of 
unconsolidated silty clay and sandy clay interbedded with zones of 
plastic clay and organic clay. All of these materials are very 
fine-grained and thus they have very low infiltration rates. As a 
result, ponding is prevalent during the wet season and saline 
soils are present in a number of areas. The basin deposits are as 
much as 30 m (100 ft) thick; they are underlain by alluvial mate- 
rials as well as bottom sediments deposited by Lake San Juan. 

The basin deposits are not a reliable source of good quality 
ground water. Because of their fineness of grain, they will yield 
only minimal amounts of water to wells; the water yielded may be 



28 



of poor quality. Wells situated on the basin deposits draw from 
underlying more permeable materials. Also because of the fineness 
of grain, the basin deposits act as a confining zone to underlying 
ground water and also inhibit any ground water recharge. The very 
low infiltration rate precludes any significant recharge from the 
Pajaro River in its course across the basin deposits. 

Stream Deposits . Deposits of unconsolidated sand, gravel, and 
cobbles, containing little or no silt and clay, occur in and adja- 
cent to the various stream channels. Related deposits, slightly 
elevated above the channel areas, also occur as stream terraces. 
Those deposits in the active channel areas are subject to movement 
during periods of high streamflows; during low flows, they are all 
nearly fully exposed. The stream deposits have a high infiltra- 
tion rate, and are of great value as areas for natural and delib- 
erate recharge. Because of their mobile nature during certain 
periods of the year, the stream deposits are not reliable sites 
for wells. Such wells, if constructed to preclude sanding, may be 
capable of providing fairly high yields from relatively shallow 
depths. Much of the ground water produced would be underflow from 
the adjacent stream. 

The stream deposits are as much as 15 m (50 ft) thick; they are 
underlain by a variety of alluvial materials as well as older 
sediments and rocks. All ground water is unconfined and is of 
good to excellent quality. 

Landslides 

Landslides of Holocene age occur in a number of areas to the east 
of South Santa Clara Valley. The slides are located on exposures 
of the Santa Clara Formation as well as on volcanic rocks; faults 
are associated with many of the slides. Most of the landslides 
are up to 15 m (50 ft) thick and consist of a heterogeneous mix- 
ture of clay and silt; slides evolved from volcanic rocks contain 
a substantial quantity of broken rock. Because of their relative 
instability, water wells have not been drilled into slide mate- 
rials. Many of the slides are saturated as shown by the springs 
and seeps found at their lower extremities. 

Base of Water-Bearing Materials 

Rogers and Williams (1974) presented a map showing the thickness 
of the alluvial materials in South Santa Clara Valley based on the 
analysis of well logs (31 logs in current study area) that bot- 
tomed in "bedrock". A modified map, shown on Figures 5A and 5B , 
presents elevation contours on the base of the alluvial materials 
derived from the analysis of logs from 89 wells intercepting bed- 
rock (shown on well logs as "rock", "hill formation", etc.). 
Figure 5 differs from the map presented by Rogers and Williams in 
that a buried hill and several buried promontories have been 
identified. 



29 



' ^Oo^ 



''Oo, 



LEGEND 

Ground elevation contours outside of valley floor 
area; contour interval 200 ft. (60 m) 

Subsurface elevation contours on base of alluvial 
materials; contour interval 100 ft. (30 m) 

Boundary of valley floor area 

Well extending below base of alluvial materials 
(intercepts "bedrock") 

Fault, dotted where concealed 




30 



FIGURE 5A. --Elevation Contours on Base of 




Alluvial Materials, South Santa Clara Valley. 



31 



M ILES 

I 

I 



1 1 

I 2 

KILOMETRES 




32 



FIGURE 5B. --Elevation Contours on Base of Alluvial Materials, 

South Santa Clara Valley. 



A buried hill, shown on Figure 5A, exists under Highway 101, 
roughly between Richmond and Kalana Avenues, a distance of about 
2 kilometres (1.2 mi). The hill is bounded on the south by the 
Shannon fault. Sediments overlying the hill are on the order of 
60 m (200 ft) thick, while they attain a thickness of from 90 to 
120 m (295 to 395 ft) in adjacent areas. The ancestral drainage 
apparently originated near what is now Coyote Narrows, flowed 
southerly around the hill and joined the ancestral south-flowing 
Coyote Creek near the intersection of Cochran Road and Madrone 
Channel. Subsequent deposition, principally by Coyote Creek, 
buried the hill. The drainage ultimately was reversed after 
Coyote Narrows had been formed, possibly by headward erosion and 
stream capture by an ancestral tributary to Guadalupe River in 
North Santa Clara Valley. 

Four buried bedrock promontories also are shown on Figure 5A. One 
is near the intersection of Cochran Road and Monterey Highway; it 
extends easterly from the hill front about 2 km (1.2 mi). About 
75 m (245 ft) of alluvial materials overlie it; alluvial materials 
are over 100 m (330 ft) thick on each side. The south side of the 
promontory appears to be quite steep. Channel deposits of the 
ancestral Coyote Creek occur adjacent to a portion of this buried 
escarpment. 

A second buried promontory occurs between Main and East Dunne 
Avenues, east of the Highway 101 Freeway. Here, sediments are on 
the order of 80 to 90 m (260 to 295 ft) deep overlying bedrock. 
Rock has not been intercepted by any wells on either side of the 
promontory, but it lies under a sedimentary cover of at least 
120 m (395 ft). This promontory apparently forced the ancestral 
Coyote Creek to flow along the west side of the valley in its 
southward course toward the Pajaro River. 

Two buried promontories, in part controlled by the Chesbro fault, 
occur near Llagas Creek. The crest of the lesser occurs south of 
the intersection of Monterey Street and Watsonville Road; here 
sediments are less than 40 m (130 ft) deep over the buried hill. 
The larger promontory is an extension of the east-trending hill 
that forms the south bank of Llagas Creek near San Martin. This 
latter promontory extends as far east as Highway 101 (South Valley 
Freeway), a distance of 1.5 km (1 mi); the depth of sediments 
overlying it is about 60 m (200 ft). Llagas Creek apparently 
flowed between these two promontories, as there are layers of 
buried stream channel materials along the present course of the 
creek to a depth of 75 m (245 ft). 

The alluvial materials making up South Santa Clara Valley rest on 
a now-buried bedrock trough. The axial line of this trough begins 
near Coyote at an elevation of about 30 m (100 ft); here, valley- 
fill materials are at most about 50 m (165 ft) thick. The axial 
line passes below sea level near Laguna Avenue, where the 
sediments are about 80 m (260 ft) thick. It then meanders south- 
easterly at an ever-decreasing elevation, but its depth and loca- 
tion cannot presently be determined because of a lack of deep 



33 



well data. Near Gilroy, the axial line is below elevation -150 m 
(-490 ft), based on a 213 m (700 ft) deep well in that city that 
did not penetrate bedrock. In the Bolsa area of San Benito 
County, a well bottomed in sedimentary material at an elevation of 
-290 m (-950 ft). Although no bedrock was encountered, it is 
probable that the lower portion of the well penetrated the 
Purisima Formation. Kilburn (1972) indicates that in the Bolsa 
area, the top of the lowermost member of the Purisima Formation 
(saline water-bearing) is at an elevation of about -500 m 
(-1,640 ft) and the elevation of the top of bedrock is about 
-900 m (-2,950 ft). These data place the top of the lowermost 
member of the Purisima Formation at a depth of about 550 to 600 m 
(1,800 to 1,970 ft). According to Kilburn, the base of fresh 
water is at some indeterminate depth above the top of the lower- 
most member of the Purisima Formation. 



Faults 

South Santa Clara Valley is an elongate feature situated roughly 
parallel and adjacent to a number of major fault zones. To the 
east, about 7 km (4 mi), is the Madrone Springs fault, an easterly 
branch of the Calaveras fault. The trace of the Calaveras fault 
ranges from 1 to 5 km (0.5 to 3 mi) east of South Santa Clara 
Valley; it crosses the floor of the Hollister Basin. Some 15 km 
(9 mi) west of South Santa Clara Valley is the San Andreas fault, 
which traverses the Santa Cruz Mountains. Associated with these 
faults are the nearly parallel Ben Trovato, Berrocal , Silver 
Creek, and Sargent faults. Compound movement along all of these 
faults has created a series of en echelon subsidiary faults, all 
exhibiting left-lateral displacement, which crosses diagonally 
beneath the floor of South Santa Clara Valley. 

Of all of these major faults, only the Calaveras fault has any 
significant effect on ground water movement in the study area. 
All of the others traverse upland areas outside of the limits of 
the South Santa Clara Valley-Hollister Basin. South from San 
Felipe Lake, the Calaveras fault extends across Hollister Basin. 
Here, the fault is indicated by a number of sag ponds strung out 
along Tequisquita Slough (see Figure 3C). According to Kilburn, 
the fault forms a barrier to any westward movement of ground 
water. Because of this, the Calaveras fault was picked as the 
eastern boundary of the study area. 

The Shannon fault has been shown by Bailey and Everhart (1964) as 
entering South Santa Clara Valley near the west end of Bailey 
Avenue. Well log data suggest that the fault crosses the valley 
and apparently joins the Coyote Creek fault near the east end of 
Burnett Avenue. The Shannon fault appears to have left-lateral 
displacement and may have caused the 2-km (1.2 mi) offset in the 
axis of the valley. Surficial materials to a depth of about 20 m 
(65 ft) do not appear to have been affected by movement along the 
fault. However, some buried stream channel materials below that 
depth appear to have been offset an undetermined distance. The 



34 



fault does not appear to be a significant barrier to ground water 
movement because of the great degree of interconnection between 
the various buried stream channel deposits. 

Dibblee (1973) mapped an unnamed fault passing under Chesbro 
Reservoir and a part of Paradise Valley, and possibly extending as 
far as Edmundson Avenue. Rogers and Williams (1973) identified 
this as the Chesbro fault. The Chesbro fault apparently continues 
eastward across South Santa Clara Valley at least as far as the 
intersection of Foothill and San Martin Avenues. It may continue 
eastward from that location, passing through exposures of the 
Santa Clara Formation and joining the Coyote Creek fault near the 
point where the latter makes an abrupt turn to the east. Like the 
Shannon fault, the Chesbro fault appears to be of left-lateral 
displacement and may be responsible for the change in direction of 
the axis of the valley as well as for the bedrock ridge along the 
right bank of Llagas Creek upstream from San Martin. Near the 
east and west sides of the valley, some of the water-bearing 
materials appear to have been offset, causing a restriction in 
ground water movement across the trace of the fault. 

A southeast-trending fault, mapped by Dibblee, passes to the north 
of Uvas Reservoir, crosses Hayes Valley, and ends near Santa 
Teresa Boulevard, about 5 km ( 3 mi ) north of Gilroy. Rogers and 
Williams identified this fault as a branch of the Ben Trovato 
fault zone. Analysis of well logs suggests that this fault 
extends across South Santa Clara Valley and intersects an unnamed 
fault near the east end of Leavesley Road. The latter fault has a 
trace roughly parallel to the Calaveras fault and is approximately 
750 m (2,460 ft) west thereof. The fault that crosses the valley 
apparently has left-lateral displacement as suggested by the sub- 
surface bedrock contours shown on Figure 5B. Well log data indi- 
cate that materials less than 50 m (165 ft) in depth have not been 
greatly affected by movement along the fault. Buried stream- 
channel materials east of Highway 101 are unaffected by the fault 
zone, as ground water appears to move down gradient unimpeded. In 
contrast, to the west of Highway 101, water-bearing materials may 
have been offset, as there is a restriction to ground water move- 
ment across the trace of the fault. 

Allen (1964) identified the Carnadero fault as having a south- 
easterly orientation and running along the base of the mountains to 
the southwest of Gilroy. Well logs suggest that this fault 
branches near Santa Teresa Boulevard. One branch apparently heads 
in the direction shown by Allen and joins the Calaveras fault south 
of Shore Road, in San Benito County. The trace of this fault in 
the Bolsa area is suggested from the lack of continuity between 
wells below depths of about 50 to 100 m (165 to 330 ft) and from 
noting that water level fluctuations on the east side and west side 
of the Bolsa area are markedly different. 

A northerly branch of the Carnadero fault leaves the main trace 
near Santa Teresa Boulevard and apprently joins the Calaveras 
fault in the vicinity of San Felipe Lake. Like the main trace of 



35 



the fault, this subsidiary feature does not cut any water-bearing 
materials closer than about 50 to 100 m (165 to 330 ft) below 
ground surface. There appears to be a restriction of ground water 
movement in the area east of Frazier Lake Road. 



Paleodrainage System 

To fully understand the geohydrologic system in South Santa Clara 
Valley, it was necessary to define and delineate the intercon- 
nected network of buried stream channels. This was accomplished 
through the use of the computer-assisted program of analysis of 
water well drillers' logs, called the GEOLOG program. One element 
of this program uses lithologic data shown on water well drillers' 
logs and converts these data to a series of maps of discrete 
subsurface intervals showing zones of sandy-gravel materials (the 
buried stream channels) and zones of fine-grained materials (the 
interstream clayey areas). These subsurface maps, showing the 
now-buried meandering stream channel materials, are presented as 
Figures 6A through 6J. Wells that penetrated the entire thickness 
of the alluvial fill and bottomed in bedrock also provided data on 
the configuration of the underlying bedrock surface. A detailed 
discussion of the GEOLOG program and its application to the 
paleodrainage, ground water storage capacity, and transmissivity 
of a ground water basin is presented by Ford and Finlayson (1974) 
and also by Ford and others (1975). 

The northern part of South Santa Clara Valley (that portion north 
of the drainage divide near Morgan Hill) contains a dual paleo- 
drainage system. The lower system is below an elevation of about 
zero, and is tributary to an ancestral southward-flowing Coyote 
Creek. Above the zero elevation, northward-trending Coyote Creek 
deposits are found in ever-increasing amounts. The creek appears 
always to have entered South Santa Clara Valley near its present 
entry location. It was probably shifted to the north by a combi- 
nation of construction of its own alluvial fan, deflection by an 
eastward-projecting promontory, and stream capture. 

From the drainage divide south to the Pajaro River, there appear 
to be a number of buried Coyote Creek stream channels which 
meandered over the floor of the valley. Many tributary streams, 
the most prominent of which were the ancestral Llagas and Uvas 
Creeks, entered Coyote Creek so that by the time it reached the 
Pajaro River (which was at about its present location). Coyote 
Creek was a stream of some consequence. It appears that the 
ancestral Coyote Creek entered the Pajaro River near the mouth of 
the present Carnadero Creek. 

Coyote Creek did not empty into the Pajaro River continuously; at 
times in the past it flowed directly into one of the lakes that 
occupied the lower portion of the valley. When this was the case, 
the deposits of stream channel materials terminate near the ances- 
tral shoreline; beyond that point are deposits of lake-bottom 
sands and clays. Other streams tributary to Coyote Creek (Uvas 



36 



Creek, Llagas Creek, etc.) also directly entered the lakes; some 
of these streams also probably constructed deltas at the point of 
entry into still water. 

Under the Bolsa area of San Benito County it is difficult to iden- 
tify many buried stream channel deposits due to the lack of ade- 
quate well data. It appears that a number of streams flowed 
westerly from the area east of the Calaveras fault, but these 
stream channel deposits could not be traced west of the fault. A 
broad subsurface channel converges on the Pajaro River from the 
south in the elevation interval from 15 to 30 m (50 to 100 ft). 
The origin of the channel may be from Santa Ana Creek or from the 
San Benito River. 



Lake Deposits 

South Santa Clara Valley has been the site of at least two large 
lakes. According to Herd and Helley (1977), the earlier lake. 
Lake San Benito, had a maximum water elevation of about 90 m 
(295 ft); the lake persisted at this level for some time during 
the Holocene, at least 5,000 years ago. In the valley today, at 
elevations below 90 m (295 ft), there are ever-increasing thick- 
nesses of lacustral clays and silts. Some of the more surficial 
clays underlying the portion of the valley below elevation 60 m 
(195 ft) can be attributed to deposition on the bed of a more 
recent, lower-stage lake, called Lake San Juan by Jenkins (1973). 

The lake-bottom clays appear to be fairly continuous and form a 
series of confining beds. The Lake San Benito clays extend as far 
north as San Martin Avenue, where they are at a depth of about 
50 m (165 ft). Underlying the lake-bottom materials are pre- 
lacustral sand and gravel deposits that may be correlative with 
the upper portion of the Santa Clara Formation. The lake-bottom 
clays slope southward and become progressively thicker until in 
the Bolsa area they are on the order of 80 m (265 ft) thick. 
Underlying these materials in this latter area are coarse-grained 
materials that may either belong to the uppermost portion of the 
Santa Clara formation or may be unnamed post-Santa Clara and pre- 
Lake San Benito sediments. 

The present upper limit of the Lake San Benito clays probably does 
not represent the uppermost level of lake-bottom deposition. Post- 
lake erosion and deposition has formed a zone of interconnected 
aquifer material some 20 to 40 metres (65 to 130 ft) thick over- 
lying the lake-bed deposits and extending from San Martin Avenue 
nearly to Bloomfield Road. Near Bloomfield Road, the coarse- 
grained materials grade laterally into progressively finer-grained 
materials. Occasional zones of granular material occur to the 
south and were probably formed as the result of lake-bottom deposi- 
tion of sandy material. 



37 



4—82239 



Overlying ths zone of post-Lake San Benito sediments, in valley 
areas below elevation 60 m (195 ft), is a zone of lacustral clay 
attributable to Lake San Juan. These clays attain a maximum 
thickness of about 60 m (195 ft) under the Pajaro River where they 
appear to rest directly on the older Lake San Benito clays. Like 
the older clays, these clays also have discrete zones of sandy 
material. 

Under the Bolsa area, where the Lake San Benito and Lake San Juan 
clays attain maximum thickness, the clays are present to a depth 
of about 140 m (460 ft), or to elevation -100 m (-330 ft). Partly 
because of the great thickness of clayey material that underlies 
an area extending from Bloomfield Road south to Shore Road (San 
Benito County) and from near Corporal east to the Calaveras fault, 
an area of about 64 km (25 mi ) , there appears to be very 
little hydraulic continuity between the Morgan Hill-Gilroy area 
and the Hollister area, except possibly at depths not presently 
tapped by most wells. 



38 



LEGEND 






Subsurface deposits of coarse-grained materials representing now-buried 
stream channels. Other areas represent fine-grained interstream 
materials. 



Subsurface extent of water-yielding materials outside of ground water 
basin for elevation interval shown (principally Santa Clara and 
Purisima Formations). 






Subsurface extent of volcanic rocks for elevation interval shown. 




Subsurface extent of nonwater-yielding rocks for elevation interval 
shown (principally Franciscan Formation, Great Valley Sequence, 
and Miocene marine rocks). 



Trace of fault crossing ground water basin. 



Trace of fault in rocks outside of ground water basin. 



•105 



Ground surface elevation contour (in metres). 



Present valley floor boundary. 



FIGURE 6A. --Subsurface Deposition, Legend for 
Figures 6B Through 6J. 



39 




FIGURE 6B. --Subsurface Deposition, +105m to +120m. 
Coyote and Llagas Subbasins. 



40 




I 2 

KILOMETRES 



FIGURE 6C. --Subsurface Deposition, +75m to +90m. 
Coyote Subbasin. 



41 




FIGURE 6D. --Subsurface Deposition, +75m to 



42 




90m, Coyote and Llagas Subbasins. 



43 




FIGURE 6E. --Subsurface Deposition, +45m to 



44 




+60m. Coyote and Llagas Subbasins. 



45 



46 



FIGURE 6F. --Subsurface Deposition, 




+45m to +60ni, Llagas Subbasin. 



47 




FIGURE 6G. --Subsurface Deposition, +15m to 



48 




I 2 

KILOMETRES 



+30m, Coyote and Llagas Subbasins, 



49 




y 



V 


' ~ -^,- 


---•- 


-^ 


- < 


ooo 


-xy\ 




f "^ 





. -v4 ^^ 



i 



'Ky'* 



^* i. i j 



^^ 



^M > > - Hi 

"7 




FIGURE 5H. --Subsurface Deposition, +15m to 



50 




+30m, Llagas and Bolsa Subbasins, 



51 




FIGURE 61. --Subsurface Deposition, Om to 



52 




-15m, Coyote and Llagas Subbasins. 



53 



f ^-?te>_.A 



r: 







FIGURE 6J. --Subsurface Deposition, Om to 



54 







/ 




'Pop-^S ■■') 



I 2 

KILOMETRES 



•15m, Llagas and Bolsa Subbasins. 



55 



CHAPTER III. GEOHYDROLOGY 



The term "geohydrology" refers to the study of flow character- 
istics of subsurface waters; the term is synonymous with ground 
water hydrology. Geohydrology includes such topics as the occur- 
rence, movement, and recharge of ground water, each of which is 
discussed below. Also included in this chapter are discussions of 
related topics such as the identification of the ground water 
basin and subbasin boundaries, water-level fluctuations, and the 
quality of ground water. 

The Ground Water Basin 

A ground water basin is defined as an area underlain by permeable 
materials capable of furnishing a significant supply of ground 
water to wells. A basin is three-dimensional and includes both 
its surface extent and all of its subsurface fresh-water-yielding 
materials. Ground water basins usually can be divided into a 
valley floor area and upland ground water terrain. The valley 
floor area normally constitutes the major part of a ground water 
basin, and it usually is an area of low-to-negligible relief. 
A valley floor area frequently can be divided into a number of 
subbasins. Upland ground water terrain is any contiguous upland 
area underlain by permeable, water-yielding materials possessing a 
high degree of hydroloqic continuity with the valley floor 
area. 

Ground water basins and subbasins can be separated from each other 
by any of the following features and conditions: 

1. Impermeable Bedrock . Impermeable bedrock includes rocks 
of very low water-yielding capability that are usually of marine 
origin; it also includes crystalline and metamorphic rocks. Rocks 
of this category that form a part of the boundary of Santa Clara 
Ground Water Basin include those of the Franciscan Formation, 
Great Valley Sequence, Tertiary Marine Sediments, and also 
serpentine and related ultrabasic rocks. 

2. Constriction in Permeable Materials . A narrow gap in 
impermeable bedrock, even though filled with permeable stream 
channel materials, can form a ground water subbasin boundary. 
Coyote Narrows is in this category, and it forms the separation 
between the South Santa Clara Valley and the North Santa Clara 
Valley. 



57 




^l.-Z^^,^Z2^^^Q^^S>]^^ 




i-UAGAS- 



■••^?^ 



Ground water basin boundary 
Ground water subbasin boundary 
Valley floor boundary 
Contiguous ground water upland 
Surface water and ground water outflow 
Surface water and ground water inflow 




FIGURE 7.— Ground Water Basin, 

3. Fault . A fault which crosses permeable materials may 
form a barrier to ground water movement as indicated by marked 
differences in water levels on either side. The Calaveras fault, 
where it crosses Hollister Basin, forms a ground water subbasin 
boundary separating the Bolsa Subbasin area from the remainder of 
Hollister Basin. 

4. Zone of Low Permeability . A zone of clay which has 
significant areal and vertical extent may create a partial barrier 
to ground water movement and thus may form a subbasin boundary. 
The lacustrine clays near the Pajaro River form such a basin 
boundary as they impede ground water movement between the Llagas 
and Bolsa Subbasins. 

5. Ground Water Divide . A ground water divide can form the 
boundary between two adjacent ground water subbasins; for example, 
the divide near Cochran Road, which forms the boundary between the 
Coyote and Llagas Subbasins. 

The ground water basin, subbasin, and valley floor boundaries for 
the Santa Clara-Hollister Basin area are shown on Figure 7. The 
boundary of the present study area generally coincides with that 
of the valley floor area except in the Hollister Basin, where the 
subbasin boundary formed by the Calaveras fault is used. 



58 




Subbasin, and Valley Floor Boundaries. 

The ground water basin as herein defined and as shown on Figure 7 
differs markedly from that shown in two previous bulletins. This 
difference stems from a need to identify a geohydrologically 
discrete area for use in the mathematical model. The differences 
are enumerated below: 

1. Bulletin 7 ; As discussed in Chapter I, State Water 
Resources Board Bulletin 7 (June 1955), identified two separate 
ground water basins in Santa Clara County: North Santa Clara 
Valley and South Santa Clara Valley. The boundary between these 
two basins was at the Cochran Road ground water divide, thus 
placing the present Coyote Subbasin in North Santa Clara Valley. 
The present study places a ground water subbasin boundary at 
Coyote Narrows, thus putting Coyote Subbasin in South Santa 
Clara Valley. 



2. Bulletin 11 



DWR Bulletin 118, "California's Ground 



Water" (September 1975), generally followed the nomenclature of 
Bulletin 7 and identified Ground Water Basin No. 2-9.02 as the 
South Bay Area of Santa Clara Valley. The areal extent of this 
basin extended south from San Francisco Bay to and including the 
present Coyote Subbasin. All of Basin No. 2-9.02 is within the 
San Francisco Bay Hydrologic Study Area (HSA). To the south, in 
the Central Coastal HSA, is Ground Water Basin No. 3-3, the 



59 



Gilroy-Hollister Valley. According to Bulletin 118, the dividing 
line between Basins Nos . 2-9.02 and 3-3 is at the Cochran Road 
ground water divide. 

Water-Level Measurements, Contours, and Profiles 

The determination of the occurrence, movement, and fluctuations of 
ground water is made through analysis of water-level data obtained 
from a number of key wells located throughout a ground water 
basin. Historic records of water level data are of great value in 
the determination of near-pristine hydrologic conditions. Hence, 
the records published by Clark in 1917 and 1924 provide insight as 
to ground water conditions in the South Santa Clara Valley area 
some 65 years ago. More recently, seasonal water level data have 
been collected, tabulated, and published by the Santa Clara Valley 
Water District (SCVWD) and its predecessor for Coyote Valley since 
1936, from Coyote Valley south to San Martin since 1948, and south 
of San Martin since 1969. Long-term water-level data are avail- 
able for only a few wells in the San Benito County portion of the 
study area. 

Most of these water-level data are actually of a composite nature. 
That is, they do not represent actual potentiometric conditions 
for any specific aquifer or water-bearing stratum. Instead, due 
to construction characteristics of monitoring wells, each water- 
level measurement represents only an average for all water- 
bearing strata intercepted by a particular well. More refined 
data can be obtained only from wells of known depth and specific 
perforations or from piezometers constructed to obtain data from a 
specific aquifer or water-bearing stratum. A water-level 
monitoring network utilizing such wells and piezometers is 
discussed in Chapter V of this bulletin. 

One interpretation of water-level data is a map depicting eleva- 
tion contours on the upper surface of the ground water body. 
Figures 8A and 8B show such contours for fall 1914, adapted from 
Clark (1917) and Clark (1924), which represent the ground water 
body in its original unstressed condition. The same figure shows 
elevation contours, derived from SCVWD data, for fall 1974 and 
illustrates the condition of the ground water body after some 60 
years of use. Figures 9A and 9B, also derived from SCVWD data, 
show elevation contours for fall 1977, when water levels were at 
their lowest due to the drought, and contours for fall 1979, which 
are indicative of postdrought recovery. Water level monitoring 
wells used to derive the elevation contours are shown on Figures 
10A and 1GB. 

One derivative of a water-level contour map is a water-level pro- 
file, such as shown on Figure 11. This profile shows the slope 
of the potentiometric surface, and hence the direction of ground 
water movement. Shown on Figure 11 are the profiles and direction 
of movement for fall 1914, fall 1974, fall 1977, and fall 1979. 



60 



Ground Water Occurrence 

Ground water in the South Santa Clara Valley-Hollister Basin area 
occurs in the alluvial materials and in the Santa Clara and 
Purisima Formations (see Table 1 and Figure 3). Older rocks, such 
as those belonging to the Franciscan Formation, are tapped only in 
and near the foothills and yield only minor quantities of water to 
wells. 

The major occurrence of ground water is in the valley floor area; 
that is, in the area underlain by alluvial materials. These mate- 
rials are underlain by the Santa Clara Formation and in the Bolsa 
Subbasin by the Purisima Formation. Ground water in much of the 
valley floor area is mostly unconfined. It occurs under essen- 
tially water-table conditions. Local areas of confinement occur, 
however, as indicated by water levels in certain wells that stand 
somewhat higher than those in nearby areas. In the subsurface, 
much of the ground water is partially confined. Movement of 
ground water is sufficiently restricted to cause slight differ- 
ences in head between differing depth zones during periods of 
heavy pumping. During periods of little draft, however, the 
various water levels all recover to nearly the same level. This 
condition results from the lenticular and discontinuous nature of 
sediments where zones of permeable sand and gravel are layered 
between less permeable beds of silt and clay. 

Coyote Subbasin 

Ground water in Coyote Subbasin occurs in the valley fill mate- 
rials principally under unconfined conditions. Water levels in 
the wells tapping the unconfined ground water body have generally 
been about 5 to 10 m (16 to 33 ft) below ground. Near Bailey 
Avenue, 1 km (0.6 mi) west of Highway 101, Clark (1924) reported 
two intermittently flowing wells in the 1914-1915 period. 
Although depth and stratigraphic data are lacking for these wells, 
their ability to flow during winter probably was caused by a sea- 
sonal rise in the water level coupled with local confinement. 

Llagas Subbasin 

Ground water in much of the Llagas Subbasin occurs under generally 
unconfined conditions. Local zones of confinement are present 
from San Martin south as noted by certain deeper wells that at one 
time flowed during the winter. South of Gilroy, in the area of 
extensive lake-bottom sediments, ground water is generally con- 
fined, and many wells originally flowed (i.e., were artesian). 
The potentiometric surface of the confined ground water body is 
now below the ground surface. To the south of Gilroy, there is 
also a perched to semiperched ground water body which occurs in 
the more permeable alluvial materials overlying extensive deposits 
of lake-bottom clays. 



61 



€ID 



LEGEND 

Elevation contours, in metres, Fall 1914 
Elevation contours, in metres, Fall 1974 
Basin boundary 
Nonwater bearing rock 



Metres Feet 






55 


180 


60 


197 


65 


213 


70 


230 


75 


246 


80 


262 


85 


279 


90 


295 


95 


311 


100 


328 




FIGURE 8A. Elevation Contours of Water Levels in Wells, 



62 






f 






^/^X 


X 



I 2 

KILOMETRES 



1914 data from Clark (1917) and 
Clark (1924); 1974 data from 
Santa Clara Valley Water District 



Fall 1914 and Fall 1974, South Santa Clara Valley. 



63 



LEGEND 

Elevation contours, in metres, Fall 1914 
" " ^ Elevation contours, in metres. Fall 1974 

Basin boundary 

\ ]]) Nonwater bearing rock 



Q'^: 



W. 



J 



\ 



A^\ \ 



HE 



yK\ 



xi <, 



IT 



/ C? 






/-' 



\> 



■"\ 



A H\ 



r 



\ 



.60 



^ER^USON 



(*d55 



^>. 



^" 



A 



\ i 



■^ 



%5^ 



-}^' 






■V'-TA 



/ / 



.V<r5 



/ 



P^afe^ 


==la^ 


f 


/ r^s^^'^ 


vi 1 


""^7^ 


--— ™, 


^2£S2i~-£[^^ <a \j / -/^^ 


o~ — i 


V/^ 


V- 




—-.^.Aeresa ^ 


\ 


1^ -^^ J / T^^^feit 


^~\V\ 


I 




"/^^^^""■^^^i-. rSX 






.1,1 1^^ ^ ,^^' ^ n^^ ^\ 








'^V'^ ^\ ^ 








-<^-C ^^ \ / 


Metres 


Feet 




y^n-^ X^ 


45 


148 




i ^ ' ■' 


50 


164 




L ■ V 


55 


180 




■■'■zy'.^ ,, -^ -'' -' '-' _/ 


60 


197 






65 


213 




' - /\'"V~^ 


70 


Z30 




/^ ^^ /^ ^^ / j^ ^ 


75 


246 




i/^"-^^^ ^ 


80 


262 




2/ / Uva8 //^ iK . 



/ 



(,• 



1^*^ 



, \, 



•1 



C^' 



efc** 



i2J<«. 



< / 

X 



r' 



V 



i.,. 



A J-'-x-. 



FIGURE 8B. --Elevation Contours of Water Levels in Wells, 



64 




-r 

I 2 

KILOMETRES 



1914 data from Clark (1917) 
and Clark (1924); 1974 data 
from Santa Clara Valley Water 
District 



Fan 1914 and Fall 1974, South Santa Clara Valley. 



65 



Metres 



Feet 



LEGEND 

— Elevation contours, in metres, Fall 1977 



" " ^ Elevation contours, in metres. Fall 1979 
Basin boundary 



dJ ]) Nonwater bearing rock 



60 


197 


65 


213 


70 


230 


75 


246 


80 


262 


85 


279 


90 


295 


95 


311 




FIGURE 9A. --Elevation Contours of Water Levels in Wells, 



66 




n r 

I 2 

KILOMETRES 



Data from Santa Clara Valley Water District 



Fall 1977 and Fall 1979, South Santa Clara Valley. 



67 



• 



LEGEND 

Elevation contours, in metres, Fall 1977 
Elevation contours, in metres, Fall 1979 
Basin boundary 
Nonwater bearing rocl< 




Metres 


Feet 


15 


49 


20 


66 


25 


82 


30 


98 


35 


115 


40 


131 


45 


148 


50 


164 


55 


180 


60 


197 


65 


213 


70 


230 



FIGURE 9B. --Elevation Contours of Water Levels in Wells, 



68 





1 


MILES 

1 
1 1 


1 


? 


i 















1 2 
KILOMETRES 




3 





t= 



/ 



Santa Clara County data from 
Santa Clara Valley Water District 



Fall 1977 and Fall 1979, South Santa Clara Valley. 



69 




FIGURE 10A.--Water-Level Monitoring Wells and 



70 




Precipitation Stations, South Santa Clara Valley. 



71 



Legend 



Z^ 



# >•.-;-■' 



■:\ .-^': 



^ Water - level monitoring wells, hydrographs 
" shown on Figures 12 and 13 \, 

O Other water - level monitoring wells 







FIGURE 10B.--Water-Level Monitoring Wells and 



72 







Precipitation Stations, South Santa Clara Valley. 



73 



1 10 



100 - 



COYOTE SUBBASIN 



E 

3 
*^ 
CB 

O 

CO 

o 

(0 

3 

(0 

« 

« 



> 
e 




90 


SAN MARTIN c LLAGAS 


S GILROY 




(0 


<D 




--. ""^^^--^ ^ 








*» 


80 


"""""""•-» ^*"^'*~««^ 




O 








> 
CO 




'^ "«. ^^""^s^ 


o 


70 


>LJ!^-.. ""--^^^"^---..,,.^^^ 


-1 












a ___^ ^^ ^^^^^^^^^^_ 






60 


^ — ~~____^^^^^--^^iGKir^ 


--- 


rrrrrrrr 




^""^^"""'^^^^'m^ • ""■ — --.^ 




50 


" ^^"'^^^^^^" "'■■•• • 


'•.... 


40 


^^ ° — ~"°~— -^ 


••••.. 


30 


^^ 


•^^^^ 


20 







22 23 24 25 26 27 28 29 30 31 

Scale in 
I I I I I I 



14 



15 



16 



17 



18 19 

Scale in 



FIGURE 11.— Water-Level Profiles, 



74 



ffl DC 



MADRONE 



MORGAN 

""-L LLAGAS SUBBASIN 



-|350 




Miles 



South Santa Clara Valley. 



75 



Bolsa Subbasin 

Most ground water in the Bolsa Subbasin occurs in permeable 
materials underlying the deposits of lake-bottom clays; it is 
typically under various degrees of confinement. Clark (1924) 
identified 36 continuously flowing wells, 8 intermittently 
(seasonal) flowing wells, and 28 nonf lowing wells in the Bolsa 
Subbasin during the 1915-1916 period. All of the flowing wells 
were located south of Shore Road. Clark identified other flowing 
wells to the east of the Calaveras fault, all apparently within 
the lakebed area of the "60-metre lake" described by Jenkins 
(1973) and Herd and Helley (1977). 

Ground Water Movement 

The determination of the direction of ground water movement is 
made through the analysis of water-level data obtained period- 
ically from a number of key wells located throughout a ground 
water basin. Many wells receive monthly measurements, and maps 
are prepared showing ground water elevation contours for the 
spring and fall of each year. Spring measurements purportedly 
show the configuration of the ground water surface during a time 
of minimum pumping; i.e., during a time when water levels stand at 
their highest in wells. Conversely, fall measurements ideally 
show the ground water surface while it is under greatest stress 
and water levels are at their lowest. These minimum elevations 
are usually attained during September and October at wells near 
recharge areas, after which levels begin to rise. Levels in wells 
more removed from recharge areas usually continue to decline an 
additional four to eight weeks, and minimum water level elevations 
are usually attained in late October or in November. 

Ground water moves from areas of recharge to areas of discharge, 
or in the case of confined ground water, from areas of high 
potentiometric pressure to areas of lower pressure. Under natural 
conditions, ground water in the South Santa Clara Valley-Hollister 
Basin moved in the same direction as the surface water drainage. 
Hence, ground water to the north of the Cochran Road topographic 
divide moved northward toward Coyote Narrows, while that to the 
south moved toward the Pajaro River. Some upward movement of 
ground water occurred through windows in the various confining 
beds in response to hydraulic pressure differentials between the 
underlying ground water and the overlying unconfined ground water. 
Ground water still generally follows this same pattern of movement 
except where modified by local pumping depressions. The general 
direction of ground water movement is indicated on Figures 8A, 8B, 
9A, 9B, and 11. 

Water-Level Fluctuations 

Typical water-level fluctuations in South Santa Clara Valley are 
shown on the hydrographs on Figures 12 and 13, which are 



76 



GROUND SURFACE ELEV. 72.5m (238 ft.) 




-220 



200 



1950 



1955 



I960 



1965 



1970 



1975 



GROUND SURFACE ELEV. 86.3m (283 fU 




1950 



WELL NO. 08S/02E-3SG01 

Depth: 45.7in (150 ft.) 

I I I I I I I I I 



1955 



I960 



1965 



1970 



1975 



GROUND SURFACE ELEV. 95.1m (312 ft.) 



< 
> 

LU 




rsio 



290 



270 



-250 



-230 



1950 1955 I960 1965 

DATA FROM SANTA CLARA VALLEY WATER DISTRICT 



FIGURE 12.--Hydrographs of Three Wells, Coyote Subbasin. 



77 



GROUND SURFACE ELEV. 117.7m (386 ft.) 




1950 



-270 



1955 



w 
O 
w 



GROUND SURFACE ELEV. 100.3m (329.1 ftj 




111 

ID 



310 z 



-290 



-270 



250 



-230 



-210 



190 



< 

> 

LLI 



1950 1955 I960 

DATA FROM SANTA CLARA VALLEY WATER DISTRICT 



FIGURE 13.— Hydrographs of 



78 



GROUND SURFACE 
ELEV. 79.6m (261 ft.) 



GROUND SURFACE 
ELEV. 63.4m (208 ft.) 



-240 



220 



< 

Q 

W 

o 

CO 

3 



I- 



< 
> 

UJ 




- 140 



1970 



1975 



GROUND SURFACE 
ELEV. 51.8m (170 ft.) 




WELL NO. 11S/04E-10D04 

Depth: 112.8m (370 ft.) 

__i I I J I I I 1 I 



170 



-150 



130 



-110 



90 



-70 




1970 



1975 



GROUND SURFACE 
ELEV. 45.7m (160 ft.) 



1970 



1975 




< 

Q 

CO 

O 
w 

3 



2 

g 
I- 
< 
> 



1970 1975 

D/ir/1 FROM SANTA CLARA VALLEY WATER DISTRICT 



Six Wells, Llagas Subbasin. 



79 



representative of hydrologic conditions in unconfined and confined 
aquifers. The hydrographs show long-term trends as well as 
seasonal responses to recharge and discharge. Locations of the 
wells represented by the hydrographs are shown on Figures 1 OA and 
10B. 



Coyote Subbasin 



During the 50 
in the Coyote 
was probably 
subbasin reco 
Avenue, 197 4 
1914. During 
Avenue declin 
ranged from 1 
Kalana Avenue 
they were in 
about 10 m (3 



-year pen 

Subbasin 
not steady 
vered at 1 
levels wer 
the droug 
ed as much 
to 20 m 
, water le 
fall 1964, 
3 ft). 



od from 1914 to 1 
from 3 to 5m (10 
From 196 4 to 1 
east to 1914 leve 
e 5 metres (16 ft 
ht years, 1975-77 

as 10 m (33 ft) ; 
(33 to 66 ft) bel 
vels in fall 1977 

and the depth to 



964, 


water 1 


to 


16 ft); 


974, 


levels 


Is. 


In fact 


) hi 


gher tha 


, levels sou 


in 


the fall 


ow g 


round. 


were slight 


water in fa 



evels declined 
the decline 
in much of the 
, near Kalana 
n they were in 
th of Bailey 

of 1977 they 
North of 
ly higher than 
11 1977 was 



GILROV No. 3417 




'MORGAN HILL 2E No. 5844 



1950 1955 I960 1965 



1970 1975 



FIGURE 14. --Annual Precipitation at 
Two Stations, South Santa Clara Valley. 



The ability of a ground water 
system to respond to the effects 
of precipitation and recharge is 
indicated through the comparison 
of well hydrographs with precip- 
itation and streamflow data. 
Examination of these data for the 
Coyote Subbasin indicates that 
wells in the subbasin respond 
with very little lag in time. 
For example, the hydrographs of 
wells Nos. 08S/02E-22D01, 08S/ 
02E-35G01, and 09S/02E-12B01 , 
shown on Figure 12, indicate a 
dramatic water level decline in 
fall 1961. This decline is 
matched by a period of minimum 
precipitation recorded at the 
Morgan Hill 2E station, shown on 
Figure 14, and a zero flow in 
Coyote Creek during the fall of 
that year, as shown in Figure 15. 
Similar low water levels and 
their corresponding minimum pre- 
cipitation and zero streamflows 
can be seen by comparing the data 
for fall 1964 and 1966. In con- 
trast, somewhat higher-than- 
normal water levels were recorded 
in the three wells during spring 
1969. These latter water levels 
correlate to a peak on the pre- 
cipitation graph as well as to a 
high streamflow. 



80 



The 1975-77 drought had very little long-term effect on the water 
resource of Coyote Subbasin. By fall 1977, water levels in wells 
had declined to all-time lows, but after a 6-month period of 
above-normal rainfall and associated streamflow, water levels had 
recovered to predrought conditions. In some areas, water levels 
were higher than fall 1974 levels by as much as 3m (10 ft). 

Table 2 provides data on the postdrought water-level recovery in 
Coyote Subbasin. Data for a shallow well. No. 08S/02E-27G0 1 , 
indicate that by spring 1978, water levels had recovered to within 
0.2 m (0.7 ft) of the predrought, spring 1975 level. Data for 
seven wells in the 25- to 45-m (82 to 148 ft) depth range indicate 
that most had recovered to the spring 1975 water-level elevation. 
Data for seven wells from 50 to 105 m (164 to 344 ft) deep show 
that most had equaled or exceeded the spring 197 5 level. 

The lack of long-term declines in water levels in the Coyote 
Subbasin suggests that the subbasin is not presently stressed 




1969 i960 




pA7\f\^rfe3n; 



1969 r970 1971 

Oato from u S Geological Survey 



1973 1974 1975 I97S 1977 I978 

Station MI70000 



FIGURE 15. --Monthly Stream Flow, by Calendar Year, 
Coyote Creek near Madrone 



81 



beyond its capacity. Most of the subbasin appears to be ade- 
quately recharged by Coyote Creek; controlled releases by SCVWD 
from Anderson Reservoir maintain the steady flow of surface water 
infiltrating to the ground water body. 

Llagas Subbasin 

Because of the overall limited natural recharge capability of much 
of the Llagas Subbasin, it could become momentarily stressed due 
to a high dependency on ground water. 

In 1914, ground water to the north of Gilroy occurred below a 
depth of 5 to 10 m (16 to 33 ft) below ground surface. By 1964, 
demand on the ground water body had sent water levels in wells to 
a depth of from 15 to 30 m (50 to 100 ft). In the next ten 
years, levels recovered somewhat, and by 1964, ground water was 
only about 10 to 20 m (33 to 66 ft) below ground surface. 

The 1975-77 drought made a greater impact on the Llagas Subbasin 
than on the Coyote Subbasin. In the fall of 1977, water levels 
had been drawn down to an all-time low of 30 to 40 m (100 to 
130 ft) below ground. According to data from water level monitor- 
ing wells, recovery from the drought was only about 75 percent 
complete by the spring of 1978. 

Table 2 shows postdrought water-level recovery data for the Llagas 
Subbasin. Data from three shallow wells, all tapping essentially 
unconfined ground water, indicate that although water levels had 
recovered 11.7 m (38 ft) from the 1977 drought to spring 1978, 
levels still remained 11.0 m (36 ft) below those of spring 1975. 
The average of 28 wells in the 50- to 100-m (164 to 328 ft) depth 
range indicated that water levels had recovered 13.4 m (44 ft) by 
spring 1978, but still remained 8.7 m (29 ft) below those of 
spring 1975. Data from eight wells tapping confined ground water 
south of Gilroy indicate that although water levels came up an 
average of about 19 m (62 ft) by spring 1978, they still remained 
about 5 m (16 ft) below the spring 1975 level. 

In a manner similar to that in Coyote Subbasin, monitoring wells 
in Llagas Subbasin show responses to major departures from the 
precipitation norm. For example, well No. 09S/03E-26P0 1 is less 
than 2 km (1-1/4 mi) from Morgan Hill 2E Precipitation Station. 
The hydrograph from the well, shown on Figure 13, indicates unusu- 
ally high water levels in spring 1959 and 1969. The latter value 
coincides with a period of high precipitation; the former value 
also coincides, but with a one-year time lag. Similarly, the 
minimum values shown for fall 1955, 1964, 1966, 1968, and 1972 
have correlatable minimum points on the precipitation chart 
(Figure 14); in those cases the time lag varies from one to two 
years. 

Precipitation data from Gilroy Precipitation Station have a very 
rough correlation to water-level data recorded at well 



82 



Table 2. Post-Drought Water Level Recovery, 
South Santa Clara Valley 
(in metres) 



Uell Number 


Depth 


Ground 
Eleva- 
tion 


Spring 1975 


Lowest 1977 


Spring 1978 


Water-level Difference 


Recovery Rate 
Drought 1977- 
Spring 1978 
(Metres/Month) 


Hater Level 


Water Level 


Mater Level 


Spring 1975 

to 
Spring 1978 


Drought 1977 

to 

Spring 1978 


Date 


Eleva- 
tion 


Date 


Eleva- 
tion 


Date 


Eleva- 
tion 



COYOTE SUBBASIN 
Shallow Wells (Less than 10 metres deep) 



08S/D2E-27G01 


7.9 


77.7 


04/21 


75.9 


08/31 


71.0 


04/28 


75.7 


- 0.2 


♦ 4.7 


0.59 










Intermediate Well 


(25 to 45 metre 


deep) 








08S/02E-22D01 


26.2 


72.5 


03/19 


68.7 


12/01 


63.5 


04/04 


67.3 


- 1.4 


♦ 3.8 


0.91 


08S/02E-35G01 


45.7 


86.3 


03/20 


78.5 


09/26 


71.9 


03/29 


78.4 


- 0.1 


• 6.5 


1.06 


08S/02E-35M01 


27.4 


80.8 


03/20 


79.0 


09/15 


72.3 


03/29 


78.7 


- 0.3 


+ 6.4 


0.98 


09S/02E-01C01 


45.7 


91.1 


04/22 


85.3 


09/15 


77.6 


03/23 


86.3 


♦ 1.0 


+ 8.7 


1.38 


09S/02E-02J02 


34.7 


87.8 


04/30 


83.4 


08/31 


75.6 


04/28 


84.6 


♦ 1.2 


+ 9.0 


1.13 


O9S/O2E-02P02 


33.2 


85.3 


04/22 


82.5 


09/15 


75.1 


03/29 


91.9 


- 0.6 


♦ 6.8 


1.05 


09S/02E-11C01 


36.6 


87.2 


04/22 


85.8 


09/26 


77.0 


03/29 


85.6 


- 0.2 


♦ 8.6 


1.39 


AVERAGE. 


Intermediate 


Wells 














- 0.06 


• 7.1 


1.13 










Deep Wells (50 


to 105 metres deep] 








09S/02E-02C01 


83.8 


81.7 


04/21 


80.1 


09/26 


73.0 


03/29 


79.5 


- 0.6 


» 6.5 


1.05 


09S/02E-02G01 


68.6 


82.9 


04/30 


80.6 


08/31 


72.4 


04/28 


81.6 


* 1.0 


♦ 9.2 


1.15 


09S/02E-12B01 


54.9 


95.1 


04/22 


88.8 


09/15 


80.7 


03/23 


89.0 


+ 0.2 


♦ 8.3 


1.32 


O9S/02E-12EO1 


65.5 


90.8 


04/30 


86.0 


oa^3i 


77.3 


04/28 


87.1 


♦ 1.1 


• 9.8 


1.23 


09S/03E-07L02 


60.4 


100.6 


04/22 


93.5 


09/15 


83.5 


03/22 


94.0 


+ 0.5 


♦ 10.5 


1.67 


09S/03E-16C01 


91.7 


117,7 


04/22 


99.9 


09/15 


81.6 


04/28 


97.4 


- 2.5 


♦ 15.8 


2.11 


09S/03E-18B01 


102.7 


100.9 


03/30 


95.1 


12/01 


84.8 


03/22 


95.4 


* 0.3 


♦ 10.6 


2.84 


AVERAGE. 


Deep Wells 








LLAGAS 


SUBBASIN 






0.0 


♦ 10.1 


1.62 










Shallow 


Wells (less than 50 metres 


deep) 








10S/03E-01N02 


40.2 


86.9 


04/28 


78.9 


11/30 


55.4 


03/27 


65.7 


-13.2 


♦ 10.3 


2.64 


10S/04E-07E99 


48.8 


87.5 


04/24 


73.7 


09/14 


49.0 


06/16 


63.0 


-10.7 


♦ 14.0 


1.55 


10S/O1E-30P05 


36.6 


63.4 


04/30 


57.2 


09/01 


29.1 


04/27 


48.0 


- 9.2 


♦ 10.9 


1.37 


AVERAGE. 


Shalloa Wells 
















-11.0 


♦11.7 


1.85 










ntermediate 


to Deep Wells (50 to 150 metres deep) 








09S/03E-15F01 


76.2 


121.0 


04/29 


111.9 


11/30 


87.5 


03/28 


105.3 


- 6.2 


♦ 17.8 


4.53 


09S/03E-15L01 


61.0 


118.9 


04/29 


114.1 


11/30 


97.6 


06/16 


112.7 


- 1.4 


♦ 15.1 


2.29 


09S/03E- 16001 


121.9 


117.3 


04/29 


96.7 


11/30 


80.7 


03/28 


86.4 


-10.3 


+ 5.7 


1.45 


O9S/03E-2OHO1 


73.2 


107.6 


04/22 


97.9 


11/30 


83.0 


03/28 


90.2 


- 7.7 


♦ 7.2 


1.83 


09S/03E-21KO1 


68.6 


110.3 


04/22 


97.4 


09/08 


82.9 


03/28 


87.4 


-10.0 


♦ 5.5 


0.82 


09S/03E-22B03 


103.6 


113.4 


04/30 


94.9 


11/30 


77.8 


04/27 


85.1 


- 9.8 


♦ 7.3 


1.48 


09S/03E-23E01 


128.0 


110.9 


04/29 


90.4 


11/30 


71.7 


03/28 


81.6 


- 8.8 


♦ 9.9 


2.52 


095/03E-25P01 


75.9 


107.9 


04/29 


78.6 


09/15 


60.2 


06/16 


77.3 


- 1.3 


♦ 17.1 


1.87 


a9S/03E-26P01 


76.2 


100.3 


03/21 


88.2 


09/15 


68.0 


03/27 


78.8 


- 9.4 


♦ 10.8 


1.62 


09S/03E-33H01 


115.8 


96.0 


04/28 


86.3 


09/14 


66.3 


03/27 


76.1 


-10.2 


♦ 9.8 


1.51 


09S/03E-34D01 


114.3 


99.7 


05/01 


90.8 


09/01 


67.1 


04/27 


82.9 


- 7.9 


♦ 15.8 


1.98 


09S/03E-34N01 


57.3 


93.9 


04/28 


88.7 


11/30 


69.9 


03/27 


80.2 


- 8.4 


♦ 10.3 


2.64 


09S/03E-34001 


59.4 


95.7 


04/28 


90.3 


11/30 


71.3 


03/27 


82.8 


- 7.5 


♦ 11.5 


2.95 


095/03E-36F01 


144.8 


98.1 


04/29 


78.1 


11/29 


57.0 


03/27 


64.5 


-13.6 


♦ 7.5 


1.91 


09S/03E-36H01 


61.0 


94.5 


04/29 


82.2 


09/15 


61.2 


03/27 


69.9 


-12.3 


♦ 8.7 


1.35 


10S/03E-03C01 


67.1 


107.6 


04/28 


100.7 


09/26 


80.8 


03/27 


93.9 


- 6.8 


♦ 13.1 


2.16 


10S/03E-13D03 


75.9 


79.6 


04/30 


71.7 


09/30 


46.0 


04/27 


64.4 


- 7.3 


♦ 18.4 


2.64 


10S/03E-23J02 


78.6 


71.6 


04/24 


68.9 


09/12 


44.2 


05/30 


58.8 


-10.1 


♦ 14.6 


1.87 


10S/03E-36A05 


64.6 


63.7 


03/24 


56.9 


11/28 


34.0 


03/22 


44.1 


-12.8 


♦ 10.1 


2.66 


10S/04E-17K02 


76.2 


90.2 


04/24 


63.0 


09/01 


38.5 


05/30 


51.5 


-11.5 


♦ 13.0 


1.43 


10S/O4E-20MO1 


64.3 


67.1 


04/28 


60.8 


09/14 


34.0 


05/31 


50.5 


-10.3 


♦ 16.5 


1.91 


10S/04E-31G04 


100.0 


60.7 


04/01 


52.2 


08/12 


43.0 


05/25 


43.0 


- 9.2 


♦ 17.4 


1.83 


llS/04E-02Dnl 


86.9 


69.8 


04/01 


51.8 


09/01 


14.1 


04/27 


43.6 


- 8.2 


♦ 29.5 


3.70 


11S/ME-03J01 


126.5 


59.7 


03/24 


51.3 


09/26 


26.5 


03/23 


40.4 


-10.9 


♦ 13.9 


2.34 


11S/04E-06D01 


143.3 


63.7 


04/01 


49.7 


09/02 


25.3 


05/25 


43.3 


- 6.4 


♦18.0 


2.04 


11S/04E-06H01 


105.5 


59.1 


04/01 


50.6 


09/02 


23.7 


05/25 


42.0 


- 8.6 


♦ 18.3 


2.07 


11S/04E-06P02 


92.0 


62.2 


04/01 


51.2 


09/02 


24.1 


05/26 


43.0 


- 8.2 


♦ 18.9 


2.14 


11S/04E-11C01 


131.1 


53.3 


03/24 


49.0 


09/26 


25.5 


03/23 


39.9 


- 9.1 


♦ 14.4 


2.43 


AVERAGE, 


Intermediate 


to Deep Wells 












- 8.7 


♦ 13.4 


2.11 






Wells 


in Area 


of Lakebed 


lay--Conf 


ned Ground Water 


(20 tolls 


metres deep) 






11S/04E-08K01 


__- 


54.3 


03/21 


45.1 


09/13 


23.9 


03/22 


36.3 


- 8.8 


♦ 12.4 


1.96 


11S/04E-10D04 


112.8 


51.8 


04/01 


49.5 


09/01 


19.6 


04/27 


47.8 


- 1.7 


♦28.2 


3.54 


riS/04E-15J01 


— 


43.9 


03/24 


44.6 


09/15 


24.3 


03/22 


38.8 


- 5.8 


♦ 14.5 


2.31 


11S/04E-17M01 


24.4 


54.9 


03/31 


50.0 


11/28 


30.8 


03/22 


41.2 


- 8.8 


♦ 10.4 


2.74 


11S/04E-21P01 


— 


47.2 


04/01 


45.1 


09/01 


14.2 


04/27 


42.2 


- 2.9 


♦28.0 


3.51 


11S/04E-21Q01 





47.2 


03/31 


43.9 


08/01 


16.8 


03/30 


38.1 


- 4.8 


♦21.3 


2.64 


11S/04E-22N03 


67.1 


45.7 


03/31 


43.4 


09/13 


19.0 


03/22 


37.4 


- 6.0 


♦ 18.4 


2.91 


11S/04E-27E02 


— 


44.2 


03/31 


42.7 


08/03 


16.8 


03/22 


36.9 


- 5.8 


♦20.1 


2.61 


11S/04E-32R02 


— 


42.7 


03/31 


40 8 


09/26 


20.4 


03/22 


36.6 


- 4.2 


♦ 16.2 


2.75 


AVERAGE. 


Wells in Area 


of Lakebed Clay 












- 5.4 


♦ 18.8 


2.77 



83 



No. 10S/04E-30P05, shown on Figure 13. This 37-m (120-ft) deep 
well appears to have about a one-year response lag to maximum and 
minimum precipitation. Nearby well No. 1 1 S/04E-06D0 1 has only 
minimal correlation; this is probably due to the fact that the 
well is 143 m (470 ft) deep and taps confined ground water. The 
hydrograph of well No. 1 1S/04E-10D04 indicates that confined 
ground water shows little response to changes in precipitation; 
the well is in the area of lakebed clays. Seasonal fluctuations 
in precipitation prior to the 1975-77 drought caused only a slight 
water level fluctuation in the well. The low precipitation period 
of 1971-72 apparently caused a lowering of levels during fall 
1972, but levels returned to near normal the next spring. The 
1976-77 drought, however, again caused a lowering of fall levels 
in the well. 

It was not possible to determine the degree of water level 
response attributable to fluctuations of streamflows in Llagas or 
Uvas Creeks. The flows, which are shown on Figure 16, occur 
mostly during the nonirrigation winter months. During a very few 
years, 1974 on Llagas Creek for example, minor streamflows 
occurred during the irrigation season. Some of this flow may have 
infiltrated and sustained water levels in nearby wells. Because 
much of the ground water in this area exists under confined con- 
ditions, water level responses in wells are greatly affected by 
pumping of other wells tapping the same system. 

Bolsa Subbasin 

Only a minimum of water-level data are available for the Bolsa 
Subbasin. Clark made no attempt during the 1914-1916 study to 
determine the elevation of the pressure surface at any of the 
flowing wells. Water-level data are available from five wells for 
fall 1974 and 1977. These data indicate that the ground water 
depression identified by Kilburn (1972) continues to exist.- The 
depth to water at Shore Road in fall 1974 was 37.7 m (124 ft); in 
fall 1977, it was only 33.0 m (108 ft), which might be attribut- 
able to a slight increase in potentiometric pressures in some of 
the confined aquifers rather than an actual rise in water levels 
in an unconfined aquifer. Conversely, at a well located 2 km 
(1.2 mi) south of Shore Road, the depth to water in fall 1974 was 
33.3 m (108 ft) and 44.5 m (146 ft) in fall 1977. This drop of 
the potentiometric surface may have been due to a decrease in 
pressure in the underlying confined aquifers. 

Ground Water Recharge 

Recharge to the ground water body is derived from the following 
five sources: natural recharge along streams, seepage along 
canals and other waterways, deep percolation of precipitation and 
excess irrigation water, artificial recharge, and subsurface 
inflow from the Santa Clara Formation (often called "hidden 
recharge"). The amount of water recharged to the ground water 



84 





LLAGAS CREEK BELOW CHESBRO DAM (Stofion 153500) 

(Data from Santa Clara [/alley Wafer District) 



■ 


















- 


■ 


















- 




















k ; 


■ 


















- 


- 








1 










- 


■\j 


J 


I. 


-^ ^ 


u 




l\ 






I ■ 



1972 1975 1974 i975 

UVAS CREEK NEAR GILROY (Station III54200) 

fOota from U 5 Geological Survey} 



FIGURE 16. --Monthly Stream Flow, by Calendar Year, 
Llagas Subbasin Streams. 

body from these different sources varies widely from year to year, 
as the controllinq factor in most areas is precipitation. In 
years of abundant precipitation and its resultant runoff, recharge 
is larqe; conversely, in dry years such as the 1977 drouqht, there 
is little precipitation, little runoff, and consequently little 
recharge other than that derived from reservoir releases. 

Ground water in Coyote Subbasin is recharged principally by Coyote 
Creek. Flow in the creek, which is maintained by releases from 
Anderson Reservoir, infiltrates the streambed to recharge to the 
ground water body. A lesser amount of recharge also is afforded 
from several streams draining the mountainous area to the west. 

In Llagas Subbasin, natural recharge is afforded by Llagas and 
Uvas Creeks, which enter the subbasin from the west. Coyote Creek 
provides little direct natural recharge to this subbasin; however, 
some Coyote Creek water, after infiltrating to the ground water 
body, may percolate laterally and move into the subbasin by way of 
subsurface inflow. The Pajaro River, although flowing about 
10 km (16 mi) along the southern boundary of the subbasin. 



7—82239 



85 












fMAIN AVE PERCOLATION PONDS 



KILOMETRES 

J-' tf" 12 3 4 




SANTA CLARA VALLEY WATER DISTRICT 





vr^» 









UoQos , '^ 

'^*' >V-»*^ \^ GAVILAN WATER < 
CONSERVATION.^ 



DISTRICT 






\, 



FIGURE 17. --Ground Water Recharge Facilities, 
South Santa Clara Valley. 

affords very little natural recharge because of underlying beds of 
nearly impermeable lake-bottom clays. 

A number of ground water recharge facilities augment the natural 
recharge to Llagas Subbasin. Santa Clara Valley Water District 
operates such facilities at the Main Avenue Percolation Ponds and 
the Madrone Channel; a number of percolation ponds along Llagas 
and Uvas Creeks are operated by Gavilan Water Conservation 
District. The locations of these artificial recharge facilities 
are shown on Figure 17. Monthly releases to the Main Avenue ponds 
from 1959 to 1978 are shown on Figure 18. 

Very little direct recharge is afforded to the Bolsa Subbasin from 
precipitation or streamflow due to the nearly impervious nature of 
the clayey materials. Most recharge occurs by way of underflow 
from such areas as the contiguous ground water terrain in the 
Lomerias Muertas to the west, or buried permeable materials which 
enter the subbasin from the south. 



Ground Water Quality 

A recent study of the South Santa Clara Valley area by Morgester 
and McCune (1980) indicated the following ground water quality 
characteristics: 



86 








A 




Dofo from Santa Clara Valley Water District 



FIGURE 18. --Monthly Releases, by Calendar Year, to 
Main Avenue Percolation Ponds. 

Ground water generally is hard, with samples from only a few 
wells showing hardness values of less than 200 milligrams per 
litre (mg/L) . 

Samples from only a very few wells had concentrations of boron 
in excess of 0.5 mg/L. 

Samples from only 8 percent of the wells indicated an adjusted 
sodium adsorption ratio greater than 6. 

Samples from 38 of the 198 wells sampled contained nitrate in 
excess of 45 mg/L. 

Electrical conductivity (EC) values for 68 of the wells 

sampled were greater than 750 microsiemens per centimetre 

(uS), and six of these wells had EC values greater than 
1 500 us. 

The majority of wells with potential boron, sodium, and 
salinity problems were in the southerly portion of the area, 
while most of the wells with high nitrate levels were in the 
central portion of the area. 



87 



CHAPTER IV. THE MATHEMATICAL MODEL 



One of the objectives of the study of South Santa Clara Valley was 
the development of a digital computer model to be used as a tool 
in a water management program for this portion of Santa Clara 
County. The computer program used to perform the ground water 
simulation was originally developed in 1970; it has been used 
in a number of other ground water basins in this part of 
California, the most recent being North Santa Clara Valley. 

The computer simulation of an aquifer system is based on a math- 
ematical approximation of the basic ground water flow equation. 
The solution to this equation is obtained by applying a finite 
difference approach. To apply the finite difference approach, 
a nodal network, shown on Figure 19, is superimposed upon the 
ground water basin. The center of each element, or cell, of this 
network is called a node and is identified by a discrete number. 
The ground water model assumes that all physical and hydrologic 
characteristics of a particular cell are located at the node 
point. Ground water flow between adjacent cells is treated in the 
same manner as a spill from a reservoir. That is, once the head 
rises above some minimum level, ground water begins to spill into 
an adjacent cell. The quantity and velocity of flow are con- 
trolled by the transmissivity of each cell boundary. 

The basic ground water flow equation in finite difference form is 
written for each node in the model. A large system of simulta- 
neous equations is created having the hydraulic head for each node 
and flows through cell boundaries as unknowns. This system of 
equations is solved by an iterative procedure that is repeated 
until it has converged to a solution. 

Input to the model is in the form of data which describe the phys- 
ical conditions of the various nodes in the ground water basin. 
Nodal parameters, shown on Table 3, indicate for each node its 
surface area, surface and bedrock elevations, average specific 
yield, and elevation of the potentiometric surface for the initial 
model run. Connections between nodes are made by numbered 
branches, each of which has its own characteristics, such as 
width, length, the elevation below which transmissivity is assumed 
to be zero (the check elevation), and estimated transmissivity. 
These branch parameters are shown on Table 4. 

The nodal parameters and configuration for the Coyote Subbasin 
portion of the model are identical with those previously estab- 
lished by the Santa Clara Valley Water District for their model 
for Coyote Subbasin; only the node and branch numbers have been 
changed. 



89 




FIGURE 19.— Nodal Network, South 



Description of the Model 

The ground water model of South Santa Clara Valley is comprised of 
69 cells (see Figure 19). Of these, cells 1 through 35, and cells 
37, 38, and 48 are in Llagas Subbasin; cells 36, and 39 through 47 
are in Bolsa Subbasin; and cells 49 through 69 are in Coyote 
Subbasin. 

The entire boundary of the model network, with one exception, has 
been assumed to be a no-flow boundary. This exception is the 
northernmost side of cell 69, which is at Coyote Narrows and 
across which some ground water outflow from the model occurs. 
There may be other boundary segments across which some ground 
water may move; however, the quantity of flow is minor and, as far 
as the model is concerned, does not occur. Such points might be 
inflow at Llagas Creek (cell 14), inflow at Uvas Creek (cell 30), 
and outflow at the Pajaro River (cell 41). There also may be some 
inflow to the model from upland areas of the Santa Clara 
Formation, particularly to the east of the valley (cells 4, 7, 9, 
15, 17, 20, 23, 27, 31, and 34), and from the Purisima Formation 
(cells 41, 43, 45, 46, and 47). Any inflow that would take place 
from those areas probably would affect only deeper wells and would 
have little if any effect on model operation. 



90 




1 — I — I — I — I — I — I — I — I 

123456 78 
KILOMETRES 



Santa Clara Ground Water Model 



Orientation of the nodal network was in part controlled by faults 
which transect the floor of South Santa Clara Valley and which may 
have some effect on ground water movement. One such fault is the 
Chesbro fault, which lies adjacent to the common boundary between 
cells 8 and 11, 10 and 13, and also 9 and 12. The unnamed fault 
which transects the valley near Rucker has been used as the common 
boundary between cells 17 and 20, 18 and 21, and also 19 and 
22. 

The ground water divide near Cochran Road has been defined in the 
model as the boundary between cells 1 and 49, 2 and 50, and 3 and 
51. Ground water moves to the north and to the south from this 
divide; ground water normally will not move across this divide 
except in response to nearby pumping. 

Because the southern portion of the South Santa Clara Valley area 
has been the site of a number of extensive lakes in the geologic 
past, widespread deposits of lake-bottom clays exist. To simulate 
the ground water confinement present in the area of lake-bottom 
sediments, cells 34 throuah 47 were defined as being entirely 
confined at the ground surface. The model treats the remaining 
cells as containing unconfined ground water. 



91 



Table 3. Nodal Parameters, South Santa Clara Valley 

Ground Water Model 



Node 
Number 


Surface Area 


Surface Elevation 


Bedrock Elevation 


Average 

Specific 

Yield 

(Percent) 


Initial Water 
Level Elevation 


Acres 


Hectares 


Feet 


Metres 


Feet 


Metres 


Feet 


Metres 



1 


363 


14? 


400 


122 








5.00 


329 


100 


2 


428 


17 c 


380 


lie 


80 


24 


9.00 


288 


88 


3 


514 


203 


360 


110 


150 


46 


5.50 


323 


98 


4 


T.oei 


4Z9 


380 


116 


-130 


. 40 


7.00 


274 


84 


5 


541 


210 


360 


110 


180 


55 


6.30 


278 


85 


6 


710 


28? 


360 


110 


200 


ei 


4.50 


288 


88 


7 


1,001 


405 


350 


107 


-150 


. 46 


9.00 


262 


80 


8 


805 


326 


340 


104 


-150 


- 46 


15.00 


271 


83 


9 


1,065 


431 


330 


101 


-100 


. 30 


3.60 


235 


72 


10 


685 


27? 


330 


101 


-100 


. 30 


3.00 


259 


79 


n 


671 


272 


320 


98 


90 


2? 


7.80 


257 


78 


12 


852 


34C 


290 


38 


- 20 


. 6 


10.20 


227 


69 


13 


670 


271 


310 


94 


80 


24 


5.50 


267 


81 


14 


862 


346 


330 


101 


60 


le 


10.00 


303 


92 


15 


1,415 


573 


290 


33 


-120 


- 3? 


6.50 


217 


66 


16 


1,143 


463 


290 


33 


10 


3 


6.10 


234 


71 


17 


1,093 


442 


270 


32 


- 90 


- 2? 


5.00 


195 


S9 


18 


1,208 


439 


260 


79 


-100 


- 10 


10.00 


208 


63 


19 


1,053 


426 


270 


82 


- 90 


- 2? 


8.80 


226 


69 


20 


1,355 


548 


240 


73 


- 80 


- 24 


17.50 


190 


58 


21 


1,2(10 


436 


240 


73 


- 90 


- 27 


4.10 


202 


62 


22 


945 


382 


260 


79 








7.30 


217 


66 


23 


1,790 


724 


220 


07 


-100 


- 30 


7.90 


170 


52 


24 


1,460 


531 


210 


64 


-140 


- 43 


6.00 


177 


54 


25 


846 


342 


240 


73 


-100 


- 30 


6.40 


196 


60 


26 


457 


185 


240 


73 


20 


6 


7.00 


180 


55 


27 


1.621 


656 


190 


58 


-350 


-107 


5.80 


174 


5^ 


28 


1,401 


567 


210 


64 


-480 


-146 


8.00 


170 


52 


29 


482 


195 


240 


73 








7.70 


193 


59 


30 


1,373 


556 


230 


70 


20 


6 


7.80 


189 


53 


31 


1,676 


678 


180 


55 


-260 


- 79 


5.00 


159 


48 


32 


1,443 


584 


180 


55 


-170 


- 52 


6.70 


151 


46 


33 


1,459 


590 


210 


64 


- 70 


- 21 


6.40 


163 


SO 


34 


1,927 


780 


180 


55 


-470 


-143 


6.80 


159 


48 


35 


2,555 


1,034 


180 


55 


-200 


- 61 


6.20 


151 


46 


36 


2,290 


927 


160 


49 


-170 


- 52 


4.00 


110 


34 


37 


1,620 


656 


160 


49 


- 80 


- 24 


8.50 


138 


42 


38 


1,907 


772 


190 


53 


-180 


- 55 


6.20 


143 


44 


39 


1,870 


757 


160 


49 


-550 


-168 


6.50 


101 


31 


40 


1,400 


567 


140 


43 


-490 


-149 


6.00 


128 


39 


41 


1,700 


688 


175 


53 


-550 


-168 


5.00 


142 


43 


42 


1,620 


656 


170 


52 


-340 


-104 


5.60 


85 


26 


43 


1,840 


745 


160 


49 


-300 


- 91 


15.00 


113 


34 


44 


1,380 


558 


180 


55 


-160 


- 49 


8.00 


84 


26 


45 


675 


273 


180 


55 


-550 


-168 


5.00 


79 


24 


46 


1,340 


542 


250 


76 


-700 


-213 


10.00 


76 


23 


47 


800 


324 


300 


91 


-350 


-107 


6.50 


141 


43 


48 


1,259 


510 


190 


53 


-190 


- 58 


11.00 


173 


S3 


49 


225 


91 


395 


120. 


200 


61 


9.90 


294 


90 


50 


365 


148 


380 


116 


-150 


- 46 


11.80 


291 


89 


51 


325 


132 


350 


10? 


150 


46 


9.50 


312 


95 


52 


378 


153 


350 


107 


250 


76 


9.50 


305 


93 


53 . 


495 


201 


350 


107 


-100 


- 30 


11.80 


294 


90 


54 


546 


221 


330 


101 








11.80 


303 


92 


55 


379 


153 


320 


98 


50 


IS 


11.80 


288 


88 


56 


483 


195 


320 


98 


-100 


- 30 


9.60 


284 


37 


57 


457 


185 


305 


93 


50 


IS 


9.60 


288 


88 


58 


392 


159 


300 


91 


100 


. 30 


9.30 


276 


34 


59 


408 


165 


295 


90 


-100 


- 30 


9.70 


272 


83 


60 


424 


172 


285 


87 


175 


S3 


9.00 


270 


82 


61 


337 


136 


280 


85 


- 50 


- 15 


9.90 


253 


7? 


62 


414 


168 


270 


82 


- 50 


- IS 


12.00 


256 


78 


63 


321 


130 


270 


82 


150 


46 


8.90 


266 


81 


64 


343 


139 


270 


32 


- 25 


- 8 


7.00 


248 


76 


65 


309 


125 


250 


76 








7.00 


240 


73 


66 


606 


245 


255 


78 


50 


15 


7.00 


260 


79 


67 


451 


183 


250 


76 








7.00 


240 


73 


68 


174 


70 


250 


76 


50 


15 


7.00 


241 


73 


69 


166 


67 


240 


73 


100 


SO 


9.90 


230 


70 



92 



Table 4. Branch Parameters, South Santa Clara Valley 

Ground Water Model 





Branch 


Connecting 


w 


dth 


1 


Length 


Fault Check 


Elevation 


Transmlssivity* 
1 1 


Number 




Nodes 


Feet 


1 Metres 


Feet 


1 Metres 


Feet 1 


Metres 


A. F. /year 


dam /yr. 


I 


1 


2 


3,458 


1,004 


4,333 


T ^^'2 


- 50 




15 


150.0 


13S.0 


2 


2 


3 


2.083 


636 


5,875 


1,791 


50 




15 


150.0 


185.0 


3 


1 


4 


2,792 


861 


7,750 


2,361 










220.0 


271.4 


4 


2 


4 


2,750 


S3S 


6,583 


2.006 


- 50 


- 


15 


130.0 


160.4 


5 


2 


5 


5,925 


1,806 


6,583 


2,006 


-100 


- 


30 


130.0 


160.4 


6 


3 


5 


1,708 


SZl 


5,625 


1,715 


50 




15 


130.0 


160.4 


7 


3 


6 


3,458 


1,054 


4,375 


1,334 


200 




61 


70.0 


86.3 


8 


6 


5 


4,583 


1,397 


4,333 


1,321 


50 




15 


130.0 


160.4 


9 


5 


4 


5,292 


1,613 


5,208 


1,587 


- 50 


- 


15 


180.0 


222.0 


10 


4 


7 


6,750 


2,0i? 


5,917 


1.804 


- 50 


- 


IS 


200.0 


246.7 


11 


4 


8 


1,583 


482 


8,333 


2,540 


- 50 


_ 


15 


130.0 


160.4 


12 


5 


8 


3,417 


l,04i: 


6,333 


1,930 


-100 


- 


30 


130.0 


160.4 


13 


6 


8 


3,833 


1,168 


6,250 


1,905 


50 




15 


70.0 


86.3 


14 


8 


11 


5,833 


1,778 


4,583 


1.397 


- 50 


- 


15 


120.0 


148.0 


15 


8 


10 


4,792 


1,461 


5,875 


1,791 


-100 


- 


30 


150.0 


185. 


16 


8 


7 


2,083 


635 


7,792 


2,375 


-100 


_ 


30 


150.0 


135.0 


17 


7 


10 


6,000 


1,829 


4,875 


1,486 


-100 


- 


30 


150.0 


185.0 


18 


7 


9 


4,417 


1,346 


7,833 


2,387 


-100 


- 


30 


150.0 


185.0 


19 


10 


9 


4,917 


1,499 


6,083 


1,854 


-100 


- 


30 


150.0 


185. 


20 


10 


13 


6,000 


1,829 


4,875 


1,486 


- 50 


- 


IS 


120.0 


148.0 


21 


11 


13 


2,833 


863 


5,917 


1.804 










120.0 


148.0 


22 


11 


14 


5,167 


1,S7S 


4,875 


1,486 


- 50 


- 


IS 


100.0 


123.4 


23 


14 


13 


5,000 


1,S24 


5,083 


1,549 


50 




15 


100.0 


123.4 


24 


13 


16 


1,000 


305 


8,417 


2,566 


- 50 


- 


IS 


100.0 


123.4 


25 


13 


12 


5,250 


1,600 


5,958 


1,816 


- 50 


- 


15 


150.0 


les.o 


26 


9 


12 


8,542 


2,604 


4,792 


1,461 


-100 


_ 


30 


120.0 


148.0 


27 


9 


15 


1,000 


305 


11,000 


3,353 


-100 


- 


30 


100.0 


123.4 


28 


12 


15 


4,583 


1,397 


8,083 


2.464 


-100 


- 


30 


120.0 


148.0 


29 


12 


16 


6,792 


2,070 


4,583 


1.397 


-100 


- 


SO 


100.0 


123.4 


30 


16 


19 


8,700 


2,652 


5,250 


1,600 


-100 


- 


30 


100.0 


123.4 


31 


16 


15 


3,750 


1,143 


6,625 


2,013 


-100 


_ 


30 


100.0 


123.4 


32 


15 


19 


2,083 


635 


6,875 


2,096 


-100 


- 


30 


150.0 


185.0 


33 


15 


18 


8,125 


2,477 


5,792 


1,765 


-100 


- 


30 


210.0 ■ 


259.0 


34 


19 


18 


5,583 


1,702 


5,792 


1.765 


-100 


- 


30 


190.0 


234.4 


35 


19 


22 


7,833 


2,387 


7,417 


2.413 


- 50 


- 


15 


100.0 


123.4 


36 


22 


25 


4,917 


1,499 


7,250 


2.210 


- 50 


_ 


IS 


70.0 


86.3 


37 


22 


21 


5,417 


1,651 


5,750 


1,753 


- 50 


- 


IS 


120.0 


148.0 


38 


18 


21 


7,667 


2,337 


7,417 


2,261 


-100 


- 


30 


150.0 


185.0 


39 


18 


17 


7,500 


2,286 


9,583 


2,921 


-100 


- 


30 


250.0 


308. 4 


40 


15 


17 


1,250 


381 


12,583 


3,835 


-100 


- 


30 


100.0 


123.4 


41 


17 


20 


10,417 


3,175 


5,125 


1,562 


-100 


- 


30 


200.0 


246.7 


42 


21 


20 


3,542 


1,080 


10,125 


3,086 


-100 


- 


30 


150.0 


185.0 


43 


21 


24 


5,708 


1,740 


8,750 


2,667 


-100 


- 


30 


200.0 


246.7 


44 


21 


25 


6,458 


1,968 


7,417 


2,261 


- 50 


- 


IS 


120.0 


148.0 


45 


25 


26 


9,083 


2,768 


2,667 


813 







. 


70.0 


86.3 


46 


26 


29 


7,333 


2,235 


2,917 


889 










150.0 


185.0 


47 


29 


30 


2,625 


800 


7,167 


2,185 


- 50 


- 


15 


205.0 


252.9 


48 


28 


30 


6,583 


2,006 


7,583 


2,311 


- 50 


- 


15 


250.0 


308.4 


49 


29 


28 


4,166 


1,270 


8,333 


2,540 


- 50 


- 


15 


250.0 


308.4 


50 


26 


28 


1,083 


330 


9,583 


2.921 


- 50 


- 


IS 


250.0 


308.4 


51 


25 


28 


625 


191 


10,542 


3,213 


-100 


_ 


SO 


250.0 


308.4 


52 


25 


24 


3,125 


953 


10,000 


3.048 


•100 


- 


30 


200.0 


246.7 


53 


20 


24 


7,583 


2,311 


6,958 


2.111 


-100 


- 


30 


250.0 


308.4 


54 


20 


23 


7,833 


2,387 


8,542 


2.f}4 


-100 


- 


30 


100.0 


123.4 


55 


24 


23 


3,250 


991 


10,208 


3.1'1 


-100 


- 


SO 


200.0 


246.7 


56 


23 


48 


1,125 


343 


11,000 


3, 333 


-100 


_ 


SO 


220.0 


271.4 


57 


24 


48 


6,250 


1,905 


6,958 


2,121 


-100 


- 


30 


350.0 


431.7 


58 


24 


28 


4,500 


1,372 


8,000 


2,438 


-100 


- 


30 


330.0 


407.1 


59 


28 


48 


8,833 


2,692 


4,875 


1,486 


-100 


- 


30 


320.0 


394.7 


60 


28 


33 


3,750 


1,143 


10,833 


3,302 










250.0 


308.4 


61 


30 


33 


4,583 


1,397 


9,083 


2,768 


50 




IS 


200.0 


246.7 


62 


27 


32 


4,166 


1,270 


8,167 


2,489 


-100 


- 


SO 


300.0 


370.1 


63 


23 


27 


11,875 


3,620 


5,750 


1.753 


-100 


- 


SO 


150.0 


IBS.O 


64 


27 


31 


10,208 


3,111 


6,833 


2,083 


-100 


- 


30 


175.0 


215.9 


65 


32 


31 


5,083 


1,549 


7,750 


2,362 


-100 


- 


30 


300.0 


370.1 


66 


31 


35 


3,750 


1,143 


9,792 


2,985 


-100 


_ 


SO 


300.0 


370.1 


67 


32 


35 


8,000 


2,438 


7,208 


2,197 


-100 


- 


SO 


420.0 


518.1 


68 


33 


35 


6,146 


1,873 


11,667 


3,556 


-150 


- 


46 


300.0 


370.1 


69 


35 


38 


5,333 


1,625 


11,458 


3,492 


-150 


- 


46 


480.0 


592.1 


70 


35 


37 


6.750 


2,057 


9,917 


3,023 


-150 


- 


46 


350.0 


431.7 



93 



Table 4. Branch Parameters, South Santa Clara Valley 
Ground Water Model (Continued) 



Branch 
Number 


Connect 
Node! 


ng 


Width 


Lenqth 


Fault Check 


Elevation 


Transmissi vi ty* 


A. F. /year 


dam ./yr. 


Feet 1 


Metres 


Feet 


1 Metres 


Feet 


Metres 


71 


35 


34 


2,083 


635 


11,629 


3,545 


-150 


- 46 


300.0 


370. i 


72 


31 


34 


10,917 


3,328 


7,500 


2, 2S6 


-150 


- 46 


200.0 


246.7 


73 


34 


37 


7,000 


2,134 


3,833 


2,692 


-200 


- 61 


260.0 


320.7 


74 


36 


34 


7,833 


2,387 


9,583 


2,921 


-150 


- 46 


230.0 


283.7 


75 


36 


37 


3,542 


1,080 


9,167 


2, 794 


-150 


- 46 


270.0 


333.0 


76 


36 


40 


6,167 


1,880 


7,917 


2,413 


-150 


- 46 


300.0 


370.1 


77 


37 


40 


8,167 


2,489 


6,083 


1,854 


-200 


- 61 


350.0 


431.7 


78 


37 


38 


6,333 


1,930 


9,792 


2,985 


-200 


- 61 


420.0 


518.1 


79 


38 


41 


9,417 


2,870 


6.250 


1,905 


-250 


- 76 


600.0 


740. : 


80 


40 


41 


7,000 


2,134 


9,375 


2,858 


-250 


- 76 


540.0 


666.1 


81 


43 


41 


4,500 


1,372 


12,333 


3,-'59 


-300 


- 91 


600.0 


740.1 


82 


43 


40 


8,750 


2,667 


7,500 


2,286 


-250 


- 76 


450.0 


55S.1 


83 


39 


40 


667 


203 


10,833 


3,302 


-200 


- 61 


350.0 


431.7 


84 


39 


36 


9,792 


2,98S 


8,458 


2,578 


-150 


- 46 


300.0 


370.1 


85 


39 


43 


7,583 


2,311 


7.583 


2,311 


-200 


- 61 


450.0 


555.1 


86 


42 


39 


8,250 


2, SIS 


7,583 


2,311 


-150 


- 46 


400.0 


493.4 


87 


42 


43 


1,000 


SOS 


1 1 ,000 


S,3S3 


-250 


- 76 


400.0 


493.4 


88 


44 


43 


9,083 


2,768 


8,667 


2,642 


-250 


- 76 


450.0 


SS5.1 


89 


45 


44 


12,083 


3,683 


4,167 


1,270 


-200 


- 61 


300.0 


370.1 


90 


44 


42 


8.333 


2,540 


8,333 


2,540 


-250 


- 76 


400.0 


493.4 


91 


46 


42 


5,083 


1,549 


12,083 


3,683 


-250 


- 76 


600.0 


740.1 


92 


46 


44 


4,708 


1,435 


11,792 


3,594 


-200 


- 61 


500.0 


616.8 


93 


47 


46 


3,167 


965 


8,333 


2,540 


-200 


- 61 


720.0 


888.1 


94 


48 


33 


500 


152 


12,292 


3,747 








300.0 


370.1 


95 


33 


32 


5,167 


1,575 


11,042 


3,366 








300.0 


370.1 


96 


48 


32 


7,667 


2,337 


7,417 


2,261 


-100 


- SO 


400.0 


493.4 


97 


48 


27 


4,833 


1,473 


9,000 


2,743 


-100 


- SO 


280.0 


345.4 


98 


1 


49 


3,250 


991 


1,667 


508 


100 


30 


10.0 


12.3 


99 


2 


50 


4,833 


1,473 


1,667 


508 


-125 


- 38 


3.0 


3.7 


100 


3 


51 


3,083 


940 


1,917 


584 


175 


S3 


3.0 


3.7 


101 


49 


50 


3,583 


1,092 


3.333 


1,016 


25 


8 


70.0 


86.3 


102 


50 


51 


2,750 


838 


5,083 


1,549 








120.0 


146.0 


103 


49 


52 


1,750 


533 


6.625 


2,019 


225 


69 


15.3 


18.9 


104 


50 


52 


2,167 


661 


6.250 


1,905 


50 


IS 


89.3 


110.2 


105 


50 


53 


1,833 


559 


6.583 


2,006 


-125 


- 38 


113.9 


140. S 


106 


51 


53 


2,917 


889 


5.875 


1,701 


25 


- 8 


123.3 


152,1 


107 


51 


54 


1,750 


533 


6,167 


1,880 


75 


23 


25.5 


31.5 


108 


53 


52 


5,417 


1,651 


2,333 


711 


75 


23 


127.5 


167. S 


109 


54 


53 


4,917 


1,499 


3,792 


1,156 


- 50 


- 15 


158.1 


19S.0 


110 


52 


55 


1,916 


584 


5.750 


1,763 


150 


- 46 


25.5 


31. S 


111 


53 


55 


1,667 


508 


5.750 


1,753 


- 25 


- a 


127.5 


157.3 


112 


54 


56 


3,500 


1,067 


5,917 


1.804 


- 50 


- IS 


146.2 


180.3 


113 


54 


57 


1,500 


457 


7,083 


2,159 


25 


8 


12.8 


15.8 


114 


56 


55 


5.500 


1,676 


2,833 


863 


- 25 


- 8 


148.8 


183.5 


115 


56 


57 


5,333 


1,625 


3,125 


953 


- 25 


- 8 


63.8 


78.7 


116 


55 


58 


2.000 


610 


5,875 


1,791 


75 


23 


43.4 


SS.6 


117 


i6 


58 


1.833 


559 


5,417 


1,651 








198.1 


244.4 


118 


56 


59 


750 


229 


5,333 


1,625 


-100 


- 30 


89.3 


110.2 


119 


57 


59 


2.583 


787 


4,583 


1.397 


- 25 


- 8 


56.1 


69.2 


120 


57 


60 


2.917 


889 


4,583 


1,397 


110 


34 


3.0 


3.7 


121 


59 


58 


5,583 


1,702 


2,833 


863 








230.0 


283.7 


122 


60 


59 


3,917 


1,194 


3,750 


1,143 


135 


41 


48.5 


59.8 


123 


58 


61 


1,833 


559 


6,208 


1,892 


25 


8 


51.0 


62.9 


124 


59 


62 


3,417 


1,042 


5,208 


1,587 


- 75 


- 23 


129.2 


159.4 


125 


60 


63 


4,166 


1,270 


4,167 


1,270 


165 


SO 


2.0 


2.5 


126 


63 


62 


3.833 


1,168 


3,083 


940 


50 


IS 


85.0 


104.8 


127 


62 


61 


5.500 


1,676 


2,917 


889 


- 50 


- IS 


221.0 


272.6 


128 


61 


64 


4.166 


1,270 


4,000 


1.219 


- 40 


- 12 


86.7 


106.9 


129 


66 


62 


2.917 


889 


4,417 


1,346 








112.2 


138.4 


130 


66 


65 


1,458 


444 


3,958 


1,206 


25 


8 


112.2 


138.4 


131 


65 


64 


4,500 


1,372 


2,833 


863 


- 15 


- S 


255.0 


314. S 


132 


64 


67 


2,500 


762 


4,667 


1,423 


- 15 


- 5 


102.0 


125.8 


133 


65 


67 


4,500 


1,372 


3,667 


1,118 








131.8 


162.6 


134 


68 


67 


3,666 


1,117 


3,667 


1,118 


25 


8 


238.0 


293.6 


135 


67 


69 


2.583 


787 


4,458 


1,359 


50 


IS 


170.0 


209.7 


136 


63 


66 


2,250 


686 


4,583 


1,397 


50 


15 


8.5 


10. S 


137 


62 


65 


917 


280 


6,042 


1,842 


- 50 


- 15 


153.0 


188.7 


138 


53 


56 


1,083 


330 


6,792 


2,070 


-150 


- 46 


140.3 


173.1 


139 


69 


70 


2,400 


732 


14,620 


4,456 


75 


23 


200.0 


246.7 



94 



Hydrologic Input 

Construction of a ground water model requires hydrologic input, in 
the form of an inventory, for each cell of the model for each year 
of the selected study period. The inventory was determined by 
combining all inflow to the basin and all outflow from the basin, 
by year, thus obtaining the net annual flow. This flow was then 
apportioned to each node as shown on Table 5. 

The reaction of a ground water basin under changing conditions 
depends not only upon the geologic framework of the basin, but 
also upon the basin's hydrologic balance for a particular time 
period. This balance takes into account precipitation, evapora- 
tion, evapotranspiration, recharge, discharge, and consumptive 
use. The analysis of a ground water basin is based on the amount 
of water in storage, which is reflected by ground water levels 
throughout the basin. When the change in the amount of ground 
water in storage from one point in time to another during a given 
study period matches the computed hydrologic balance of the basin 
for that same time period, the resulting inflows and outflows can 
be used as input to the mathematical model. 

The current study has been based on an inventory of the following 
flows to and from the Santa Clara ground water basin: 

Inflows 

Deep Percolation 

Stream Percolation 

Pond Percolation (artificial recharge) 

Subsurface Inflow 

Outflows 

Agricultural Pumpage 
Urban Pumpage 
Subsurface Outflow 



In order to derive a hydrologic inventory, the following two 
criteria must be observed: 1) the inventory must result in a 
hydrologic balance for the entire basin, and 2) the inventory must 
determine the net flow for each individual node as representative 
of the hydrologic balance for that node. 

The hydrologic balance resulting from the inventory reflects the 
theoretical change in the amount of ground water in storage. The 
accuracy of the inventory can be estimated by comparing the change 
in storage derived by this method to that calculated from changes 
in historic water levels. 

Certain items in the ground water inventory were measured 
directly, a few were calculated, and some were measured for only a 
part of the study period and calculated for the remainder. Of 



95 



those items that were calculated, most were prepared on a water 
year basis (October 1 through September 30). The principal excep- 
tion was ground water pumpage, which was prepared on a calendar 
year basis. However, because a calendar year and a water year 
both contain the same summer period, during which the greatest 
variation in pumpage occurs, the use of the calendar year for 
determining pumpage has only a minor effect on the calculations. 

Net annual flows determined for the study period (1965-73) are 
shown on Table 5. These, coupled with the initial ground water 
elevations for each node, shown on Table 3, were used as hydro- 
logic input to the model. 



Precipitation 

An isohyetal map showing the variation of mean annual precipita- 
tion in the study area is shown on Figure 20. The precipitation 
data and the isohyets were adapted from data provided by the Santa 
Clara Valley Water Distrct. Base stations used to develop the 
isohyetal map were selected based on the length and reliability of 
station records, representative geographic and topographic condi- 
tions in the area, and orographic storm pattern. A common 52-year 
span, 1919-1970, was used in the preparation of the isohyetal 
map. 

The yearly amounts of rainfall at Gilroy and Morgan Hill stations, 
from 1948 through 1975, are shown on Figure 14. The accumulated 
percent deviation from the mean for Morgan Hill and Hollister sta- 
tions is shown on Figure 21. 

Tributary Runoff 

Only a small portion of the drainage area tributary to South Santa 
Clara Valley is gaged. Runoff from the remaining area was esti- 
mated by developing runoff-precipitation relationships for the 
gaged areas and applying these relationships to the ungaged areas. 
The locations of tributary drainage areas are shown on Figure 22. 
Table 6 lists the tributary drainage areas and their estimated 
amounts of annual runoff for the years 1965 through 1973. 

For developing correlation curves used in estimating tributary 
runoff, known seasonal runoff was plotted against seasonal pre- 
cipitation. The straight line relationship between seasonal 
runoff and seasonal precipitation was used to determine the amount 
of precipitation that would be required to initiate runoff along 
ungaged streams. 

Seasonal runoff from an ungaged area can be computed from the 
following formula when nearby runoff and precipitation data are 
available: 



96 



Table 5. Net Annual Flows, South Santa Clara Valley 
Ground Water Model 





Node 
Num- 
ber 


Water Year 




Water Year 




1964- 
1965 


1965- 
1966 


1966- 
1967 


1967- 
1968 


1968- 
1969 


1969- 
1970 


1970- 
1971 


1971- 
1972 


1972- 
1973 


1964- 
1965 


1965- 
1966 


1966- 
1967 


1967- 
1968 


1968- 
1969 


1969- 
1970 


1970- 
1971 


1971- 
1972 


1972- 
1973 












(Acre-Feet) 
















(Cuiic 


DekametreBl 








1 


3.166 


2.682 


3,654 


2.657 


3,333 


2,968 


2,922 


2,613 


3,197 3,301, 


3,308 


4.507 


3.277 


4,111 


3.667 


3.604 


6.223 


3,943 


2 


-540 


-738 





-849 


-169 


-574 


-445 


-577 


-267 -eee 


-910 





7.047 


-208 


-708 


-549 


-772 


-329 


3 


1.220 


690 


1,580 


692 


1,425 


925 


1,014 


595 


1.592 1,505 


851 


1,949 


854 


7.758 


7.747 


7.257 


734 


1,964 


4 


1.330 


619 


1,683 


915 


2,295 


1,362 


1,279 


1,022 


2.362 I,64J 


764 


2,076 


7.729 


2.837 


7.680 


7.678 


7.261 


2,914 


5 


-452 


-669 


-252 


-626 


163 


-309 


-449 


-597 


-49 -558 


-825 


-311 


-772 


207 


-387 


-554 


-736 


-60 


6 


414 


30 


734 


-i - 


640 


74 


151 


-178 


663 51! 


37 


905 


-66 


789 


97 


-786 


-220 


818 


7 


-1.167 


-1.486 


-276 


-1.553 


38 


-939 


-959 


-1,512 


-150 -1,439 


-1,833 


-340 


-7.976 ■ 


47 


-1,158 


-7.783 


-7.865 


-185 


8 


-409 


-746 


115 


-694 


427 


-360 


-408 


-635 


285 -505 


-920 


142 


-856 


627 


-444 


-603 


-783 


352 


9 


-353 


-1,079 


176 


-1.213 


484 


-716 


-706 


-,1389 


75 -436 


-7,337 


277 


-7.496 


597 


-883 


-877 


-7.773 


93 


10 


-781 


-1,171 


-585 


-1.102 


51 


-643 


-772 


-963 


-85 -963 


-1,444 


-722 


-7,359 


63 


-793 


-952 


-1,199 


-105 


11 


545 


99 


982 


32 


999 


345 


375 


-30 


953 e?2 


722 


7.277 


39 


7,232 


426 


463 


-37 


1,176 


12 


-301 


-1,140 


116 


-1.106 


47 


-772 


-616 


-934 


846 -371 


-7,406 


743 


-1,364 


58 


-952 


-760 


-7.162 


7.044 


13 


280 


-751 


737 


-774 


278 


-466 


-278 


-776 


1.137 345 


-928 


909 


-955 


343 


-575 


-343 


-967 


7.402 


14 


8.775 


4,659 


9,747 


4.086 


4,636 


3,697 


5,881 


3,872 


11.447 10,824 


5,747 


72,023 


5.040 


5,719 


4.660 


7,254 


4.776 


14,120 


15 


-988 


-1,899 


-218 


-1,878 


411 


-987 


-982 


-1 ,646 


306 1,219 


-2,342 


-289 


-2.377 


507 


-7.217 


-7.277 


-2.030 


377 


16 


659 


-749 


1.279 


-827 


692 


-391 


-29 


-827 


1,936 813 


-924 


7.578 


-7.020 


864 


-482 


-36 


-1,020 


2,388 


17 


-246 


-999 


189 


-980 


400 


-539 


-383 


-874 


450 -303 


-1,232 


233 


-7.209 


493 


-665 


-472 


-1,078 


556 


18 


150 


-1,436 


764 


-1,556 


48 


-1,061 


-512 


-1,308 


2,015 785 


-1,771 


942 


-7.979 


69 


-7.309 


-632 


-1,613 


2.486 


19 


206 


-1.117 


985 


-1,297 


138 


-886 


-481 


-1 ,444 


1 ,450 254 


-1,378 


7,275 


-1,600 


770 


-7.093 


-693 


-1,781 


1.789 


20 


263 


-1,640 


726 


-1,959 


-450 


-1.580 


-872 


-2,004 


1,704 324 


-2,023 


896 


-2,416 


-556 


-7.949 


-7.076 


-2.472 


2.102 


21 


-653 


-1,605 


-282 


-1.521 


147 


-1.025 


-915 


-1,419 


592 -eoi 


-1,980 


-348 


-1,876 


787 


-7.264 


-7.729 


-1.750 


730 


22 


-682 


-1.238 


216 


-1,354 


311 


-768 


-792 


-1,507 


64 -841 


-7.527 


266 


-1,670 


384 


-947 


-977 


-7.869 


79 


23 


290 


-1.591 


1,041 


-2,009 


289 


-1,345 


-762 


-2,156 


1 ,462 3iS 


-7.962 


7.284 


-2,478 


358 


-7.659 


-940 


-2,669 


7.803 


24 


-2.164 


-2.704 


-1 ,303 


-2,880 


-761 


-2,035 


-2,152 


-2,852 


-1.217 .2,869 


-3.335 


-1,607 


-J. 562 


-939 


-2.670 


-2.664 


-3,678 


-7.607 


25 


-385 


-772 


83 


-816 


465 


-335 


-387 


-826 


145 .47S 


-952 


102 


-1,007 


574 


-413 


-477 


-1,019 


779 


26 


102 


-233 


433 


-282 


500 


-25 


-35 


-369 


340 ,28 


-287 


534 


-348 


677 


-31 


-43 


-455 


479 


27 


-828 


-2541 


-181 


-2.919 


-1,169 


-2,429 


-1,789 


-2,991 


360 -1-021 


-3,734 


~223 


-3', 601 


-7.442 


-2,296 


-2,207 


-3,689 


444 


28 


-222 


-542 


417 


-606 


822 


181 


-203 


-556 


370__ -!?4 


-089 


514 


-749 


1,014 


-223 


-250 


-686 


466 


29 


94 


-346 


499 


-375 


563 


-37 


30 


-293 


59o' lie 


-427 


616 


-461 


694 


-46 


37 


-361 


728 


30 


6.280 


2.458 


8,623 


3,059 


5.147 


3,454 


4.280 


404 


8.896 ?,?4e 


3,032 


10,636 


3,773 


6.349 


4.267 


5.279 


498 


10,973 


31 


2.598 


1.058 


3,341 


-1,479 


719 


-558 


172 


121 


3.277 3,205 


7.305 


4,121 


-7.324 


S«7 


-688 


212 


149 


4,042 


32 


-875 


-1 .994 


-251 


-2,331 


-806 


-1,770 


-1.435 


-2.126 


106 -1,079 


2,460 


-310 


-2.875 


-944 


-2.783 


-1.770 


-2.622 


737 


33 


1.746 


-657 


4,299 


-571 


3.700 


-192 


906 


-1,944 


2.774 2.754 


-810 


5,303 


-704 


4.664 


-237 


1,118 


-2, 399 


3,422 


34 


2.284 


-1.887 


3,536 


-2,610 


-776 


-2,294 


-1.327 


-3,511 


3,315 2,877 


-2,328 


4.362 


-3,279 


-957 


-2,830 


-7,637 


-4.331 


4,099 


35 


304 


-1 .428 


3,389 


-3,038 


3.528 


-1 ,950 


-673 


-2,594 


527 375 


-1, 787 


4.780 


-3. 747 


4.352 


-2, 406 


-830 


-3. 200 


650 


36 


459 


-1.692 


1,390 


-1,647 


189 


-1,300 


-1.044 


-2,065 


1 .565 568 


-2,087 


7,776 


-2.032 


233 


-1.604 


-7.288 


-2,547 


1,930 


37 


1.465 


-1.906 


3,019 


-2,330 


-353 


-2,050 


-994 


-2,709 


2,929 7,807 


-2,347 


3,724 


-2,874 


-435 


-2. 629 


-7,226 


-3. 342 


3,613 


38 


-450 


-2.151 


2,325 


-2,355 


2.764 


-2,016 


-754 


-3,021 


-379 -555 


-2.653 


2,868 


-2,905 


6.409 


-2,497 


-930 


-3. 726 


-467 


39 


-2.091 


-2.521 


-1 ,666 


-2,376 


-1.191 


-1,961 


-2.183 


-2,710 


-1,974 -2.579 


-3,770 


-2.055 


-2,937 


-7.489 


-2.419 


-2,693 


-3,343 


-2,435 


40 


-20 


-1.152 


918 


-1,171 


916 


-871 


-466 


-1 ,485 


36 -25 


-7,427 


7.732 


-1,444 


7.730 


-7.074 


-575 


-1,832 


44 


41 


947 


-1.611 


4,482 


-1,838 


4.101 


-1,287 


-339 


-2,718 


1,003 J,)68 


-7.987 


5.529 


-2,267 


5.059 


-7.588 


-478 


-3,353 


7.237 


42 


-1.610 


-2.019 


-1,204 


-1 .978 


-588 


-1,517 


-1.730 


-2,337 


-1.431 -7,986 


-2,490 


-7.485 


-2,440 


-726 


-1.871 


-2,734 


-2, 983 


-7.766 


43 


-610 


-1.252 


-267 


-1,175 


4«6 


-769 


-1.125 


-1,909 


-836 -7S2 


-7.544 


-329 


-1,449 


599 


-949 


-7.388 


-2,355 


-1,031 


44 


-937 


-1 .396 


-527 


-1,395 


109 


-908 


-1.174 


-1,677 


-847 -7,158 


-7,722 


-650 


-1,721 


134 


-1,120 


-7.448 


-2,069 


-7.046 


45 


227 


-387 


1,031 


-450 


1.650 


343 


23 


-473 


876 280 


-477 


7.272 


-566 


2,035 


423 


28 


-583 


7.087 


46 


-2,059 


-2,598 


-1,510 


-2,857 


-1.278 


-2,043 


-2,192 


-2,838 


1.723 -2,540 


-3, 205 


-7.863 


-3,524 


-7.576 


-2,520 


-2,704 


-3,501 


-2.726 


47 


5,274 


5.391 


6,049 


5,229 


6.295 


6,289 


5,265 


4,794 


5.456 6,505 


6,650 


7.467 


6.460 


7.765 


6,524 


6,494 


5,913 


6,730 


48 


-1.171 


-1,490 


-512 


-1,558 


-214 


-1,082 


-1.154 


-1,612 


-542 -1,444 


-1,939 


-632 


-1,922 


-284 


-7.336 


-1,423 


-1,998 


-669 


49 


2.380 


1,709 


2,444 


1,847 


2,482 


2.414 


2,182 


2,039 


2,347 2,936 


2,108 


3.075 


2,279 


3,062 


2.978 


2,691 


2.676 


2,995 


50 


-364 


-532 


-261 


-615 


-114 


-447 


-489 


-606 


-223 -449 


-656 


-322 


-769 


-747 


-567 


-603 


-748 


-275 


51 


82 


-120 


299 


-150 


368 


4 


27 


-163 


321 101 


-748 


369 


-785 


454 


6 


33 


-201 


396 


52 


1.245 


280 


1,311 


442 


1,374 


1.19S 


893 


641 


1.148 1.536 


345 


1,617 


545 


7.695 


7.474 


1,102 


791 


1,416 


53 


-541 


-721 


-357 


-750 


-71 


-463 


-410 


-501 


-67 -667 


-889 


-440 


-925 


-88 


-571 


-SOS 


-818 


-83 


54 


-401 


-737 


-3 


-775 


174 


-429 


-422 


-718 


115 -495 


-909 


-4 


-956 


276 


-529 


-527 


-996 


142 


55 


1.220 


286 


1,311 


459 


1,414 


1.232 


899 


637 


1.115 7,505 


353 


1,617 


5SS 


7.744 


1.620 


7.709 


-786 


1.376 


56 


-434 


-694 


-210 


-830 


27 


-292 


-387 


-542 


-143 -535 


-856 


-259 


-7.024 


33 


-360 


-477 


-669 


-176 


57 


-310 


-694 


110 


-722 


162 


-386 


-357 


-693 


187 -382 


-856 


136 


-897 


200 


-476 


-440 


-855 


237 


58 


1.089 


182 


1,209 


333 


1,275 


1.064 


790 


556 


1.102 7,343 


224 


1,491 


477 


1,573 


1.312 


974 


686 


1.369 


59 


-585 


-749 


-400 


-821 


-168 


-562 


-600 


-800 


-344 -722 


-924 


-493 


-7.073 


-207 


-693 


-740 


-987 


-424 


60 


-267 


-589 


71 


-648 


136 


-372 


-411 


-811 


-32$ 


-727 


99 


-799 


169 


-459 


-607 


-1,000 





61 


895 


73 


1,003 


195 


1,089 


901 


614 


364 


863 7,304 


90 


1,237 


247 


1,343 


1,111 


757 


499 


1,065 


62 


-548 


-697 


-366 


-765 


-168 


-528 


-563 


-743 


-338 -676 


-860 


-457 


-944 


-207 


-651 


-694 


-976 


-417 


63 


127 


-448 


8 


-468 


95 


-255 


-26? 


-474 


51 757 


-553 


70 


-577 


777 


-316 


-326 


-686 


63 


64 


1.285 


-23 


857 


82 


924 


780 


480 


213 


657 1,585 


-28 


7.057 


101 


7.740 


962 


692 


263 


810 


65 


-420 


-511 


-258 


-541 


-151 


-390 


-418 


-537 


-244 -578 


-630 


-378 


-667 


-786 


-491 


-576 


-662 


-301 


66 


-51 


-446 


230 


-483 


330 


-255 


-223 


-536 


219 -63 


-550 


284 


-696 


407 


-315 


-275 


-661 


270 


67 


177 


-412 


252 


-374 


485 


209 


47 


-177 


288 278 


-SOS 


377 


-467 


598 


268 


58 


-213 


356 


68 


66 


-204 


231 


-214 


99 


-138 


-32 


-170 


296 81 


-252 


285 


-264 


722 


-770 


-39 


-270 


365 


69 


833 


276 


860 


439 


878 


874 


674 


550 


747 1.028 


340 


7.067 


542 


7.083 


7.078 


831 


678 


927 


TOTAL 


24,562 


-40.418 


67,865 


-48,077 


52,091 


-18.358 


-7,189 


-53,442 63,698 30.300 


-49,852 


83.772 


-59.302 


64.266 


22,643 


-8,969 


-65,920 


78.673 



97 




«<e, ' -"^-J^x "v^ ($0) 

NOTE: Contours are Average Annual Rainfall in Milimetres, C Inches 5 



FIGURE 20.— Isohyetal Contours, South Santa Clara Valley. 



where 



R,, = 



R^ = 



Pu = 



P„ = 



Au 
Ag 



Rg (P^^/Pg)(Au/Ag), 

Seasonal runoff from ungaged area 

Seasonal runoff from representative gaged area 

Seasonal precipitation on the ungaged area 

Seasonal precipitation on the representative 
gaged area, and 

Area of ungaged area 

Area of gaged area. 



Similarly, the annual basin precipitation can be estimated by the 
following formula, if mean seasonal precipitation data are 
available: 



where 



a 
P'. 



P'i 



(Pi-p'3)/(P'^-p-i) 

Annual basin precipitation 

Mean seasonal basin precipitation, estimated 
from isohyetal map 

Seasonal precipitation at nearby index station, 
and 

Mean seasonal precipitation at nearby index 
station. 



98 



z 

< 200 



O 



z 
o 

1- 
< 

> o 

Ui 

o 





















n 








A 


























I\ 


'I 










L 




























\ 




k 




V 


593 mm 
(23.4lnche> 


) 














1 


Y 






\ 


^ 










i 
















1 


! 








\ 


1 




/ 


\ 




































\a 


1 

■a 




— 






























V 


o 


• 
I 






J 


\ 
























f^^ 




^ 


\A 




/ 


\ 


r 
















- 






W 


t7 
if 


Vl 


\1 


^ 


































M 
































- - 








































: jop Year Average- 553mm (21.8 Inches) 























































































































TTTT 


T" 








































































■ 
























352 mm 
(13.9 Inches! 




































1 






















































j] 


^ 
















1 
a 
















1 








■ '' 


' 


















'S.': 






{ 


V 












r 




'" 


h 


a>< 


k 












V 
















\ 


h 




in 


ii 


\l 






a 




■ 






" 




















1 




J 


\ 


Ki 














1 






■'• 












V 




































100 Yeor Average- 329 mm (12.9 Inches) 




" 













































































































-200 



WATER YEAR 
MORGAN HILL 

^ Estimated Prior to 1944 



WATER YEAR 
HOLLISTER 



FIGURE 21 .--Accumulate(j Deviation from Mean Precipitation at 
Stations, South Santa Clara Valley. 



Two 



Artificial Recharge 

Artificial recharge is the practice of deliberately ponding water 
for direct infiltration to the ground water body. Artificial 
recharge in South Santa Clara Valley is composed principally of 
waters released from Anderson Reservoir by Santa Clara Valley 
Water District for infiltration either at the Main Avenue 
Percolation Ponds or along the Madrone Channel (see Figure 17). 
This operation is the only well defined artificial recharge 
practiced in South Santa Clara Valley. 

Gavilan Water Conservation District (GWCD) practices ground water 
recharge using waters released from Uvas and Chesbro Reservoirs. 
The GWCD stores the water during the wet season of the year and 
releases it during the dry season, thus affording a greater 
opportunity for the waters to infiltrate the channels of Uvas and 
Llagas Creeks. 



99 




1-Hr 



6 , 



I I MILES 

r I 'i i' 



5 6 7 8 9 



KILOMETRES 



LEGEND 



W2, E4- Tributary drainage areas. 
^lumber refers to listing on Table 6 

• Stream gage. 



FIGURE 22.— Stream Percolation Units and Tributary Drainage Areas, 

South Santa Clara Valley. 

Stream Infiltration 

Stream infiltration in the South Santa Clara Valley was determined 
by the use of the gaging stations within the area. The basin was 
subdivided into stream infiltration units that reflected the 
positions of the gaging stations. These units are: Coyote Unit, 
Uvas Unit, Llagas Unit, Pajaro Unit, and Pacheco Unit. 

The stream infiltration values derived for these units were 
divided into subunits coincident with the nodal areas of the 
ground water model (Table 7). The difference of flow between the 
inflow gaging station and the outflow gaging station of a stream 
infiltration unit is assumed to be the total stream infiltration 
for that unit. Stream infiltration values are applied to each 
node of the ground water model, based on the ratio of streambed 
distance within each node to total distance within the unit. 

Coyote Unit 

The Coyote Unit includes all of the Coyote Subbasin. There are 
two main channels in this unit, Coyote Creek and Coyote Canal, 



100 



Table 6. Tributary Runoff, South Santa Clara Valley 



Drain- 
age 
Area* 



Area 
(mi 2) 



Water Year 



1964- 


1965- 


1966- 


1967- 


1968- 


1969- 


1970- 


1971- 


1972 


1965 


1966 


1967 


1968 


1969 


1970 


1971 


1972 


1973 



Total 



E-1 

E-2 

E-3 

E-4 

E-5 

E-6 

E-7 

E-8 

E-9 

E-10 

E-11 

W-1 

W-2 

W-3 

W-4 

W-5 

W-6 

W-7 

W-8 

W-9 

W-10 

W-11 



;i ,000 Acre-Feet) 



5 
1 
2 
2 
1 

1.8 
1.3 
3.7 
3.3 
9.0 
142.0 

5.8 

19.6 

30.4 

12.0 

13.4 

9.0 

13.6 

16.9 

2.0 

9.8 

6.4 




0.35 
0.55 
0.65 
0.30 
0.50 
0.32 
0.94 
0.85 
1.02 
3.60 

1.54 
8.88 
27.81 
12.52 
9.54 
8.53 
9.14 
8.55 
0.84 
4.48 
1 .04 




0.14 
0.22 
0.13 
0.06 
0.10 
0.12 
0.35 
0.32 
0.44 
1.80 

1.35 
5.05 
8.55 
2.31 
1.95 
1.52 
?.13 
1.99 
0.14 
0.74 
0.44 



1.42 
0.38 
0.60 
1.54 
0.71 
1.18 
0.72 
2.05 
1.87 
2.54 
18.20 

4.14 

20.98 

54.99 

18.45 

15.53 

12.40 

16. l'^ 

15.14 

1.21 

6.49 

2.51 




0.14 
0.22 












0.03 



1.89 
8.35 
10.18 
2.44 
2.21 
1.73 
2.37 
2.22 
0.16 
0.87 
■0.04 



6.02 
0.62 
0.98 
1.66 
0.76 
1.27 
0.88 
2.48 
2.26 
3.90 
29.10 

9.52 

24.05 

50.43 

13.94 

13.63 

11.16 

13.94 

13.06 

1.09 

5.85 

3.96 



0.54 
0.21 
0.33 
0.34 
0.16 
0.26 
0.35 
1.00 
0.91 
1.38 
1.80 

0.85 
9.20 
30.04 
10.70 
8.95 
6.94 
8.68 
8.13 
0.77 
4.13 
1.40 



0.58 
0.21 
0.33 
0.32 
0.15 
0.24 
0.18 
0.51 
0.46 
0.57 
2.70 

0.92 
5.65 
16.24 
3.62 
3.45 
2.31 
4.00 
3.73 
0.22 
1.19 
0.58 



0.08 
0.04 
0.04 












0.37 



0.13 
4.24 
5.73 
1.01 
1.06 
0.65 
1.21 
1.19 
0.06 
0.34 
0.06 



1.82 
0.59 
0.98 
1.15 
0.53 
0.88 
0.55 
1.56 
1.42 
2.11 
29.10 



10.46 
2.68 
4.25 
5.79 
2.67 
4.43 
3.12 
8.89 
8.09 
12.06 
86.30 



2.37 23.21 
21.47 107.87 
43.72 247.74 



12.01 

11.54 

8.52 

11.79 

11.84 

U.83 

4.42 

2.14 



77.03 
67.86 
53.76 
69.42 
65.85 
5.32 
28.51 
12.17 



Drain- 
age 
Area* 


Area 


Water Year 


Total 


1964- 
1965 


1965 
1966 


1966- 
1967 


1967- 
1968 


1968- 
1969 


1969- 
1970 


1970- 
1971 


1971- 
1972 


1972- 
1973 



(1,000 Cubia Dekametres) 



E-1 


17.6 








1.75 





7.40 


0.66 


0.71 


0.10 


2.24 


22.87 


E-2 


3.4 


0.43 


0.17 


0.47 


0.17 


0.76 


0.26 


0.26 


0.05 


0.73 


3.30 


E-3 


5.2 


0.68 


0.27 


0.74 


0.27 


1.21 


0.41 


0.41 


0.05 


1.21 


5.23 


E-4 


6.2 


0.80 


0.16 


1.89 





2.04 


0.42 


0.39 





1.41 


7.12 


E-5 


2.8 


0.37 


0.07 


0.87 





0.93 


0.20 


0.18 





0.65 


3.28 


E-6 


4.7 


0.62 


0.12 


1.45 





1.56 


0.32 


0.30 





1.08 


5.45 


E-7 


3.4 


0.39 


0.15 


0.89 





1.08 


0.43 


0.22 





0.68 


3.84 


E-8 


9.6 


1.16 


0.43 


2.52 





3.05 


1.23 


0.63 





1.92 


in. 93 


E-9 


8.5 


1.05 


0.39 


2.30 





2. 78 


1.12 


0.57 





1.75 


9.95 


E-10 


23.3 


1.25 


0.54 


3.12 


0.04 


4.80 


1.70 


0.70 


0.09 


2.60 


14.83 


E-11 


367.8 


4.43 


2.21 


22. 39 





35.79 


2.21 


3.32 





35.79 


106.15 


W-1 


15.0 


1.89 


1.66 


5.09 


2.32 


11.71 


1.05 


1.13 


0.16 


3.53 


28.55 


W-2 


50.8 


10. 92 


6.21 


25.81 


10.27 


29.58 


11.32 


6.95 


5.22 


26.41 


132.68 


W-3 


78.7 


34.21 


10.52 


67.64 


12.52 


62.03 


36.95 


19.93 


7.11 


53.78 


304.72 


W-4 


31.1 


15.40 


2.84 


22.69 


3.00 


17.15 


13.16 


4.45 


1.24 


T4.77 


94.71 


W-5 


34.7 


11.73 


2.40 


19.10 


2.72 


16.76 


11.01 


4.24 


1.30 


14.19 


83.47 


?■/-? 


23.3 


10.49 


1.87 


15.25 


2.13 


13. 73 


8.54 


2.81 


0.80 


10.48 


66.12 


W-7 


35.2 


11.24 


2.62 


19.88 


2.92 


17.15 


10.63 


2.84 


1.49 


14.50 


85.39 


W-8 


43.8 


10.53 


2.45 


18.62 


2.73 


16.06 


10.00 


4.59 


1.46 


14.56 


81.01 


W-9 


5.2 


1.03 


0.17 


1.49 


0.20 


1.34 


0.95 


0.27 


0.07 


1.02 


6.54 


W-10 


25.4 


5.51 


0.91 


7.98 


1.07 


7.20 


5.08 


1.46 


0.42 


5.44 


35.07 


W-11 


16.6 


1.28 


0.54 


3.09 


0.05 


4.87 


1. 72 


0.71 


0.07 


2.63 


14.97 



*See Figure 24 for location of 
tributary drainage areas. 



Data for areas E-1 through E-9 
from Santa Clara Valley Water 
District. 



8—82239 



101 



both of which flow north from Anderson Reservoir. Inflow into the 
Coyote Unit is by way of runoff from eastern and western slopes 
(tributary areas E- 1 and W- 1 ) , as well as flow from Anderson 
Reservoir through Coyote Creek and Coyote Canal. 

With the exception of 1969, the flow in Coyote Creek was deter- 
mined from the gaging station just below Anderson Dam. Because of 
spill from Anderson Reservoir in 1969, the gaging station at 
Coyote Creek near Madrone was used for that year. The flow in 
Coyote Canal initially was determined from data from the gaging 
station at Metcalf Road. These data were modified to include 
leakage from Coyote Canal, estimated by SCVWD, and estimates of 
inflow from the east and west slopes. 

Outflow from the Coyote Unit is along Coyote Creek, at Metcalf 
Road, and along the Coyote Canal, at the Diversion Dam. Estimated 
stream percolation for the Coyote Unit ranges from about 5 000 to 
12 000 cubic dekametres (4,100 to 9,800 acre-feet) annually. 

Uvas Unit 

The Uvas Unit includes the drainage area of Uvas Creek from the 
western slopes to Gilroy. The gaged inflow to the unit from the 
western slopes was based on the three following gaging stations: 
Bodfish Creek near Gilroy (U. S. Geological Survey), Little Arthur 
Creek at Redwood Retreat Road (SCVWD) , and Uvas Creek above Uvas 
Reservoir (USGS). 

Evaporation from Uvas Reservoir was estimated from evaporation 
data and coefficients provided by the Santa Clara Valley Water 
District. Uvas Creek water exported to the Llagas Creek watershed 
was deleted from the total. Outflow from the Uvas Unit was by way 
of Uvas Creek at the USGS gaging station at Thomas Road. Except 
for 1972, when percolation was zero, stream percolation has ranged 
from about 11 200 to 32 200 dam^ (9,100 to 26,100 acre-feet 
annually. 

Llagas Unit 

The Llagas Unit encompasses the drainage area of Llagas Creek from 
Cochran Road south to the Pajaro River. Inflow to the Llagas Unit 
includes Llagas Creek, runoff from the eastern slopes southerly 
from Anderson Reservoir, runoff from the western slopes from 
Cochran Road to Gilroy, and water imported from Uvas Creek. 

Inflow from Llagas Creek was based on flows at the gaging station 
just below Chesbro Reservoir. These flows were taken from USGS 
records for the study period except for 1972 and 1973, which were 
derived from SCVWD records. Estimates of inflows from eastern 
slopes were from data provided by SCVWD. The stream percolation 
ranges from 3 800 to 46 700 dam"^ (3,100 to 37,900 acre-feet) 
annually. 



102 



Table 7. Stream Infiltration, South Santa Clara Valley 



Node 
Num- 
ber 








Water Yea 


r 
















Mater Yea 


r 








1964- 
1965 


1965- 
1966 


1966- 
1967 


1967- 
1968 


1968- 
1969 


1969- 
1970 


1970- 
1971 


1971- 
1972 


1972- 
1973 


1964- 
1965 


1965- 
1966 


1966- 

1967 


1967- 
1968 


1968- 
1969 


1969- 
1970 


1970- 
1971 


1971- 
1972 


1972- 
1973 










(Acre 


-Feet) 


















(Cuii^- 


Dekamet 


res) 


























COYOTE UNIT 


















1 


0.27 


0.14 


0.25 


0.19 


0.27 


0.32 


0.23 


0.23 


0.20 0.33 


0.17 


0.31 


0.23 


0.33 


0.39 


0.2S 


0.28 


0.24 


49 


0.92 


0.46 


0.83 


0.62 


0.90 


1.08 


0.79 


0.78 


0.68 1.13 


0.S7 


1.02 


0.76 


1.11 


1.33 


0.97 


0.96 


0.84 


52 


1 .21 


0.60 


1.08 


0.81 


1.17 


1.41 


1.03 


1.02 


0.88 1.49 


0.74 


1.33 


1.00 


1.44 


1.73 


1.27 


1.2S 


l.OS 


55 


1.21 


0.60 


1 .08 


0.81 


1.17 


1.41 


1 .03 


1.02 


0.88 1.4S 


0.74 


1.33 


1.00 


1.44 


1.73 


1.27 


J. 25 


l.OS 


56 


0.18 


0.09 


0.17 


0.12 


0.18 


0.22 


0.16 


0.16 


0.13 0.22 


0.11 


0.21 


O.IS 


0.22 


0.27 


0.20 


0.20 


0.16 


58 


1 .11 


0.55 


0.99 


0.75 


1.08 


1.30 


0.94 


0.93 


0.81 1.3? 


0.63 


1.22 


0.92 


1.33 


i.eo 


1.16 


1.14 


1.00 


61 


1.02 


0.51 


0.91 


0.68 


0.99 


1 .19 


0.86 


0.86 


. 74 1.2i 


0.63 


1.12 


0.B4 


1.22 


1.46 


l.OS 


1.06 


0.91 


64 


1.02 


0.51 


0.91 


0.68 


0.99 


1 .19 


0.86 


0.86 


0.74 1.2i 


0.63 


1.12 


0.84 


1.22 


1.46 


1.06 


1.06 


0.91 


67 


0.64 


0.32 


0.58 


0.43 


0.63 


0.75 


0.54 


0.54 


0.47 0.7S 


0.39 


0.71 


0.53 


0.77 


0.92 


0.66 


0.66 


0.i8 


69 


0.74 


0.36 


0.67 


0.50 


0.72 


0.87 


0.63 


0.62 


0.54 




0.91 


0.44 


0.82 


0.62 


0.89 


1.07 


0.77 


0.76 


0.66 



8.32 4.14 7.47 5.59 8.10 9.74 7.07 7.02 6.07 



10.23 S.IO 9.1$ 6.89 9.97 11.96 8.70 8.62 7.46 



LLAGAS UWIT 



4 


0.50 


0.40 


0.40 


0.80 


0.70 


0.70 


0.60 


0.80 


0.90 


0.62 


0.49 


0.49 


0.97 


0.86 


0.86 


0.74 


0.97 


1.11 


5 


0.10 


0.10 


0.10 


0.20 


0.20 


0.20 


0.10 


0.20 


0.20 


0.12 


0.12 


0.12 


0.25 


0.25 


0.25 


0.12 


0.25 


0.25 


8 


0.10 


0.10 


0.10 


0.20 


0.10 


0.10 


0.10 


0.20 


0.20 


0.12 


0.12 


0.12 


0.25 


0.12 


0.12 


0.12 


0.25 


0.25 


10 


0.20 


0.10 


0.10 


0.30 


0.30 


0.30 


0.20 


0.30 


0.40 


0.26 


0.12 


0.12 


0.37 


0.37 


0.37 


0.25 


0.37 


0.49 


12 


0.87 


0.39 


0.90 


0.52 


0.30 


0.30 


0.49 


0.48 


1.34 


1.07 


0.48 


1.11 


0.64 


0.37 


0.37 


0.60 


0.59 


1.65 


13 


0.86 


0.22 


0.80 


0.24 


0.10 


0.10 


0.33 


0.19 


1.18 


1.06 


0.27 


0.9S 


0.30 


0.12 


0.12 


0.41 


0.23 


1.46 


14 


4.78 


1.36 


5.00 


0.85 








2.09 


0.59 


6.74 


5.88 


1.66 


6.15 


1.05 








2.57 


0.73 


8.29 


15 


0.10 


0.10 


0.10 


0.20 


0.20 


0.20 


0.10 


0.20 


0.20 


0.12 


0.12 


0.12 


0.25 


0.25 


0.25 


0.12 


0.25 


0.25 


16 


1.15 


0.30 


1.10 


0.29 


0.10 


0.10 


0.46 


0.23 


1.58 


1.42 


0.37 


1.35 


0.36 


0.12 


0.12 


0.57 


0.28 


1.95 


18 


1.73 


0.63 


1.80 


0.67 


0.40 


0.40 


1.07 


0.59 


2.76 


2.13 


0.78 


2.20 


0.83 


0.49 


0.49 


1.32 


0.73 


3.40 


19 


1 .15 


0.32 


1 .20 


0.20 








0.50 


0.14 


1.62 


1.41 


0.39 


1.48 


0.25 








0.62 


0.17 


1.99 


20 


2.01 


0.64 


2.10 


0.54 


0.20 


0.20 


0.93 


0.43 


2.90 


2.47 


0.79 


2.58 


0.66 


0.25 


0.25 


1.14 


0.53 


3.57 


21 


0.87 


0.36 


0.70 


0.60 


0.50 


0.40 


0.55 


0.57 


1.31 


1.07 


0.44 


0.86 


0.74 


0.62 


0.49 


0.68 


0.70 


1.61 


23 


1.25 


0.42 


1.30 


0.30 


0.10 


0.10 


0.60 


0.24 


1 .72 


1.54 


0.52 


1.60 


0.37 


0.12 


0.12 


0.74 


0.30 


2.11 


27 


1.44 


0.41 


1 .50 


0.26 








0.63 


0.18 


2.02 


1.77 


0.50 


1.85 


0.32 








0.77 


0.22 


2.48 


31 


1.15 


0.32 


1.20 


0.20 








0.50 


0.14 


1 .62 


1.41 


0.39 


1.48 


0.25 








0.62 


0.17 


1.99 


32 


0.86 


0.24 


0.90 


0.15 








0.37 


0.11 


1.21 


1.06 


0.30 


1.11 


0.18 








0.46 


0.14 


1.49 


34 


3.90 


0.76 


4.81 


0.17 


0.15 





1.24 


0.32 


5.37 


4.80 


0.93 


5.92 


0.21 


0.18 





1.53 


0.39 


6.61 


37 


3.38 


0.65 


4.23 


0.41 


0.13 





1.07 


0.28 


4.62 


4.16 


0.80 


5.20 


0.50 


0.16 





1.32 


0.34 


5.66 


TAL 


26.40 


7.81 


28.34 


7.10 


3.48 


3.10 


11.93 


6.19 


37.89 


32.48 


9.59 


34.84 


8.75 


4.28 


3.81 


14.70 


7.61 


46.61 





















PAJARO 


UNIT 


















33 


0.71 


0.38 


1.75 


0.32 


2.18 





0.80 








0.87 


0.47 


2.16 


0.39 


2.68 





0.98 








35 


1 .03 


0.54 


2.53 


0.47 


3.15 





1.16 








1.27 


0.66 


3.11 


0.58 


3.87 





1.43 








37 


0.13 


0.07 


0.32 


0.06 


0.40 





0.15 








0.16 


0.09 


0.39 


0.07 


0.49 





0.18 








38 


0.98 


0.52 


2.42 


0.45 


3.01 





1.11 








1.21 


0.64 


2.98 


0.55 


3.70 





1.37 








40 


0.53 


0.12 


1.06 


0.10 


0.72 





0.27 





0.40 


0.65 


0.15 


1.30 


0.12 


0.89 





0.33 





0.49 


41 


7.75 


1.83 


15.79 


1.51 


11.38 


0.28 


4.10 


0.01 


5.90 


9.53 


2.25 


19.42 


1.86 


14.00 


0.34 


5.04 


0.01 


7.26 


43 


0.05 


0.02 


0.13 





0.20 


0.07 


0.03 





0.11 


0.06 


0.02 


0.16 





0.25 


0.09 


0.04 





0.14 


44 


0.10 


0.04 


0.26 





0.40 


0.14 


0.06 


0.01 


0.21 


0.12 


0.05 


0.32 





0.49 


0.17 


0.07 


0.01 


0.26 


45 


0.42 


0.18 


1.03 


0.01 


1.58 


o.se 


0.23 


0.03 


0.86 


0.52 


0.22 


1.27 


0.01 


1.94 


0.69 


0.28 


0.04 


1.06 


46 


0.05 


0.02 


0.13 





0.20 


0.07 


0.03 





0.11 


0.06 


0.02 


0.16 


C 


0.25 


0.09 


0.04 





0.14 


47 


0.21 


0.09 


0.51 


0.01 


0.79 


0.28 


0.12 


0.01 


0.43 


0.26 


0.11 


0.63 


0.01 


0.97 


0.34 


0.15 


0.01 


0.53 



TOTAL 11.96 3.81 25.93 2.93 24.01 



.40 8.06 0.06 8.02 



74.72 4.68 31.90 3.59 29.53 1.72 9.91 





















PACHECO 


UNIT 


















34 


3.14 


0.54 


4.01 


0.34 


0.15 





0.91 


0.23 


4.29 


3.86 


0.66 


4.93 


0.42 


0.18 





1.12 


0.28 


5.28 


36 


1.74 


0.30 


2.27 


0.19 


0.08 





0.51 


0.13 


2.38 


2.14 


0.37 


2.79 


0.23 


0.10 





0.63 


0.16 


2.93 


37 


2.71 


0.46 


3.53 


0.29 


0.13 





0.78 


0.20 


3.68 


3.33 


0.57 


4.34 


0.36 


0.16 





0.96 


0.25 


4.53 


40 


0.34 


0.05 


0.46 


0.03 


0.02 





0.09 


0.02 


0.47 


0.42 


0.06 


0.57 


0.04 


0.02 





0.11 


0.02 


0.58 



TOTAL 7.93 1.35 10.27 0.85 0.38 



2.29 0.58 10.82 



1.66 12.63 



2.82 0.71 13.32 



30 
33 



17.97 8.04 22.18 10.96 8.31 8.75 11.55 
1.80 0.80 2.22 1.10 0.83 0.88 1.16 



TOTAL 19.77 8.84 24.40 12.06 9.14 9.63 12.71 



23.73 
2.38 

26.11 



22.10 
2.21 



9.99 27.28 13.48 10.22 10.76 14.21 29.19 

0.98 2.73 1.35 1.02 1.08 1.43 2.93 



24.31 10.87 30.01 14.83 11.24 11.84 15.64 



103 



A pipeline diversion from Anderson Reservoir provides inflow to 
the unit. Twenty percent of the water is assumed routed to 
percolation ponds at Main Avenue and Hill Road; the remainder is 
percolated along the Madrone Channel. The total amount of 
artificial recharge ranges from 1 850 to 4 800 dam (1,500 to 
3,900 acre-feet) per year for the study period. 

Pacheco Unit 

The Pacheco Unit encompasses that portion of the Bolsa Subbasin 
that is bounded by Bolsa Road on the west, the Pajaro River on the 
north, and the Calaveras fault on the east. 

Inflow to the Pacheco Unit includes outflow from San Felipe Lake 
into the Pajaro River and runoff from a small portion of the 
eastern slopes. 

Inflow to San Felipe Lake is from Pacheco Creek using Pacheco 
Creek gaging station near Dunneville (USGS) and overflow from 
Tequisquita Slough. Outflow from San Felipe Lake was arbitrarily 
set at this inflow minus 11 000 dam (9,000 acre-feet) per year 
for the purpose of this modeling effort. 

Stream percolation for the Pacheco Unit ranges from zero to about 
13 300 dam^ (zero to 10,800 acre-feet) annually. 

Pajaro Unit 

The Pajaro Unit encompasses the valley floor between the western 
slopes and the Southern Pacific Railroad, from Gilroy to the Pajaro 
River, and also between the western slopes and Bolsa Road, from 
the Pajaro River south to Hollister. 

Inflow to the Pajaro Unit includes Llagas Creek from the north, 
the Pajaro River from the east. Tick Creek and Tar Creek from the 
west, and the southwestern slope runoff. Outflow is at the Pajaro 
River near Chittenden (USGS). Stream percolation ranges from about 
74 to 32 000 dam-^ (60 to 26,000 acre-feet) annually. 

Land Use 

Land-use data are used to estimate deep percolation and pumpage 
from the agricultural and urban lands. Although the study period 
extends from 1964 through 1973, land-use surveys are available 
only for 1967 and 1974; these surveys are shown on Figure 23. 
Certain assumptions and adjustments in land-use data were made to 
bring those data in line with the study period. Land use for the 
period 1964-67 is assumed to be similar to that of the 1967 survey. 
Land use for the period 1967-70 is based on the 1967 survey modified 
by dat^ from the "Atlas of Urban and Regional Change" published in 
1970 by the U. S. Geological Survey. Interpretation of the 1970 
data assumed changes in distribution of land use in each node to 



104 



be linear. A similar interpolation was made for 1970-75, based on 
the 1970 Atlas combined with the 1975 survey. 

The land-use data were divided into areas contiguous to the 69 
nodes of the ground water model. Certain irrigated lands external 
to the nodal network are supplied by water pumped from wells 
internal to the model. These lands were added to the nodal areas 
to determine pumpage. 

Three major groupings of land use were defined: agricultural, 
urban, and native vegetation. The total annual area for the three 
major land-use groups for the study period is shown in Table 8. 

Pumpage 

Agricultural and urban pumpage were determined separately for 
South Santa Clara Valley and are summarized in Table 9. 

Agricultural pumpage (applied water) was estimated by determining 
the area for each crop from the land-use data and estimating the 
unit amount of applied water used for each crop; this value is 
defined as unit applied water. The value of the unit applied 
water is based on rooting depth, available soil moisture, poten- 
tial evaporation, and precipitation. The maximum soil moisture is 
assumed to occur at the beginning of each year. This value is 
used to obtain the amount of applied water needed during the crit- 
ical months to supplement the amount of precipitation required for 
maximum growth. The value of applied water then was increased by 
20 percent to offset losses in application. 

The unit values of applied water are shown on Table 10. These 
values become a part of the hydrologic inventory by multiplying 
them by the area of each respective crop grown in South Santa 
Clara Valley. It should be emphasized that the data are based on 
the full soil moisture profile during the critical months, and 
therefore constitute the maximum pumpage expected. 

Urban pumpage was determined by multiplying the area of urban 
lands by the unit urban pumpage. Water delivered by Gilroy was 
to urban areas in cells 28, 32, 33, and 48 and that delivered by 
Morgan Hill was to urban areas in cells 6, 8, and 11. Urban areas 
in these seven cells were totaled and then divided into the total 
deliveries to obtain an amount of annual use per node. 

Some thought was given to domestic water use throughout the 
remainder of South Santa Clara Valley (areas of 0.7-0.8 hectare or 
1-2 acres) not designated as urban areas. About 28 percent of the 
population of the model area is not served municipal water by 
either Gilroy or Morgan Hill. The land-use distribution shows 
discrete urban areas in all but 14 cells. Domestic water use in 
those outlying areas is not considered significant. 



105 




FIGURE 23. --Land Use, 1967 and 



106 







1974 LAND USE x / 




1974, South Santa Clara Valley. 



107 



Table 8. Land Use, South Santa Clara Valley 



Water 
Year 


Agriculture 


Native 
Vegetation 


Total 
Agriculture 

& Native 
Vegetation 


Urban 


Valley 
Total 



(Acres) 



1965 


39,480 


24,290 


63,770 


2,370 


66,140 


1966 


39,190 


24,280 


63,470 


2,280 


65,750 


1967 


39,100 


24,320 


63,420 


2,580 


66,000 


1968 


38,630 


23,930 


62,560 


2,880 


65,440 


1969 


39,010 


23,640 


62,650 


3,190 


65,840 


1970 


38,910 


23,180 


62,090 


3,500 


65,590 


1971 


39,920 


20,530 


60,450 


3,830 


64,280 


1972 


41,600 


20,620 


62,220 


4,160 


66,380 


1973 


42,760 


19,240 


62,000 

CHea tares) 


4,550 


66,550 


1965 


15,977 


9,830 


25,807 


959 - 


26, 766 


1966 


1 5, 860 


9,826 


25,686 


923 


26,608 


1967 


15,823 


9,842 


25, 665 


1,044 


26,710 


1968 


1 5, 630 


9,680 


25,310 


1,170 


26,480 


1969 


1 5, 790 


9,570 


25, 360 


1,290 


26,650 


1970 


15, 750 


9,380 


25,130 


1, 420 


26,550 ■ 


1971 


16,160 


8,310 


24,470 


1,560 


26,030 


1972 


1 6, 840 


8,340 


25,180 


1,680 


26,860 


1973 


17,300 


7,790 


25,090 


1,840 


26,930 



Most water used by manufacturers is sel f -produced , and is 
estimated to be about 6 200 dam^ (5,000 acre-feet) annually. 
Because no definitive amounts are available for the study period, 
this value has not been included in the inventory. 

Deep Percolation 

Deep percolation was determined by inventory using data on land 
use, precipitation, evaporation, transpiration, and irrigation 
(applied water). The theory used to develop the inventory is 
summarized below. 

A surface water balance is first determined for a specific posi- 
tion and land use with respect to time. This balance is composed 
of rain and applied water minus the evaporation and 
transpiration. 



108 



Table 9. Ground Water Pumpage, South Santa Clara Valley 



Water 
Year 


Agriculture 


Urban 


Total 



1965 
1966 
1967 
1968 
1969 
1970 
1971 
1972 
1973 

Total 



1965 

1966 
1967 
1968 
1969 
1970 
1971 
1972 
1973 

Total 





(Acre- 


-feet) 


95,940 




18,020 


115,270 




22,660 


86,280 




20,470 


130,680 




19,940 


96,650 




19,310 


95,710 




17,670 


100,350 




18,000 


127,360 




18,380 


98,190 




16,510 



946,430 170,960 

(Cubic Dekametres) 



118,342 

142,185 
106,426 
161,194 
119,218 
118,058 
123,782 
157,099 
121,117 

1,167,421 



22,228 

27,951 
25,249 
24,595 
23,819 
21, 796 
22,203 
22,672 
20,365 

210,878 



113,960 
137,930 
106,750 
150,620 
115,960 
113,380 
118,350 
145,740 
114,700 

1,117,390 



140,570 

170,138 
131,675 
185,789 
143,037 
139,854 
145,985 
179,771 
141,482 

1,378,299 



Each type of crop grows within a certain depth of soil, called the 
root zone. The net water from the surface water balance is 
infiltrated from the surface to the root zone and becomes a part 
of the soil moisture in storage. The crop has a capability of 
using stored moisture within its root zone for transpiration. If 
the plant cannot use all the water percolating into the root zone, 
some of the water will be stored in the root zone for later use. 
The portion of water in excess of that held by the soil percolates 
downward below the root zone and eventually recharges the ground 
water body. Once the water has moved below the root zone it 
becomes deep percolation, and is no longer available to crops. 

Table 11 gives the deep percolation through pervious soils for 
each crop for each year of the study period. Table 12 gives 
similar data for impervious soils. 



109 



Table 10. Unit Values of Applied Irrigation Water, 
South Santa Clara Valley 



Crop 








Wate 


r Year 










1965 


1966 


1967 


1968 


1 1969 


1970 


1971 


I 1972 


1973 








(Acre-feet 


per acre) 








Deciduous 


2.31 


2.91 


2.33 


3.68 


2.46 


2.43 


2.46 


3.23 


2.20 


Grain 





0.32 


0.42 


0.10 


0.21 


0.21 


0.21 


0.32 


0.21 


Pasture 


3.68 


3.98 


3.13 


3.75 


3.73 


3.70 


3.84 


4.09 


3.75 


Misc. Row 


2.43 


2.74 


1.88 


2.85 


2.47 


2.45 


2.58 


2.96 


2.49 


Sugar Beets 


2.50 


2.79 


2.38 


2.99 


2.49 


2.58 


2.48 


3.42 


2.53 


Tomatoes 


2.29 


2.93 


1.74 


2.99 


2.34 


2.33 


2.45 


3.18 


2.34 


Vineyard 


0.91 


1.79 


1.13 


2.43 


1.74 


1.48 


1.87 


2.35 


1.35 






(Cub 


ic dekametres 


per heotojpe) 








Deciduous 


7.04 


8.86 


7.10 


11.21 


7.50 


7.40 


7.50 


9.84 


6.70 


Grain 





0.98 


1.27 


0.30 


0.64 


0.64 


0.64 


0.98 


0.64 


Pasture 


11.21 


12.13 


9.54 


11.43 


11.37 


11.27 


11.70 


12.46 


11.43 


Misc. Row 


7.40 


8.55 


5.73 


8.68 


7.53 


7.47 


7.86 


9.02 


7.59 


Sugar Beets 


7.82 


8.50 


7.25 


9.11 


7.59 


7.86 


7.56 


10.42 


7.86 


Tomatoes 


6.98 


8.93 


5.30 


9.11 


7.13 


7.10 


7.47 


9.69 


7.13 


Vineyard 


2.77 


5.45 


3.44 


7.40 


5.30 


4.51 


5.70 


7.16 


4.11 



Change in Storage 

Change in storage at any given node is the product of: 1) the 
change in depth to ground water between the beginning and the end 
of the study period; 2) the area of the cell; and 3) the specific 
yield at the average depth of water level. The change in storage 
may be represented by the equation: 



AS = Ad*s*A, 
where AS = Change in storage. 

Ad = Change in depth to ground water, 
s = Average specific yield, and 
A = Surface area of the cell. 



Historic Data 

The change in depth to ground water of each node was developed 
from historic ground water levels of the wells within each 
cell. 



110 



Table 11. Deep Percolation in Pervious Soils, 
South Santa Clara Valley 



Water 
Year 


Deciduous 


Beets 


Grain 


Truck and 
Field 


Tomatoes 


Pasture 


Vineyard 


Irrigated 
Total 


Native 
Vegetation 


Urban 











(Ac 


re-Feet) 












1964-65 


6,261.80 


375.68 


1.43 


2,515.35 


1,822.13 


3,158.93 


203.30 


14,338.62 


5,546.64 


465.45 


1965-66 


7,731.45 


424.78 


1.70 


2,206.32 


2.415.70 


3,259.89 


293.48 


16,333.32 


2,236.93 


243.92 


1966-67 


14,108.03 


787.17 





3,767.05 


2,130.83 


4,581.73 


547.57 


25,922.38 


12,487.05 


1,380.15 


1967-68 


15,038.81 


468.39 


4.54 


2,598.22 


2,612.95 


2,238.62 


916.20 


23,877.73 


52.06 


172.98 


1968-69 


25,679.89 


1,151.52 


751.68 


8,807.78 


6.020.03 


6,601.91 


1,426.32 


50,439.13 


23,499.93 


2,321.41 


1969-70 


7,576.87 


549.43 


279.62 


3,525.70 


2,355.49 


3,499.66 


527.79 


18,314.56 


5,330.42 


624.46 


1970-71 


5,809.99 


483.23 


486.51 


4,070.75 


2,741.56 


3,788.72 


413.14 


17,793.90 


5,058.81 


752.14 


1971-72 


6,693.11 


635.50 


124.93 


3,714.23 


3,922.60 


2,703.88 


826.11 


18,620.36 


442.75 


291.34 


1972-73 


10,001.43 


1,434.29 


1,927.42 


9,312.82 


5,864.55 


5,840.94 


528.37 


34,909.82 


11,827.24 


2,034.45 



TOTAL 98,901.38 6.309.99 3,577.83 40.518.22 29,885.84 35,674.28 5,682.28 220.549.82 66,481.81 8,286.31 

(Cubic Dekametres) 



1964-eS 


7,723.93 


463.40 


1.76 


3,102.69 


2,247.60 


3,896.54 


250.77 


17,686.69 


6,841.78 


574.13 


1965-66 


9,536.74 


523.97 


2.10 


2,721.50 


2,979.77 


4,021.07 


362.00 


20,147,15 


2,759.25 


300.88 


1966-67 


17,402.26 


970.97 





4,646.66 


2,628.38 


5,651.56 


675.43 


31,975.26 


15,402.78 


1,702.42 


1967-68 


18,550.37 


577.76 


5.60 


3,204.91 


3,223.07 


2,761.34 


1,130.13 


29,453.18 


64.22 


213.37 


1968-69 


31,676.14 


1,420.40 


927.20 


10,864.40 


7,425.71 


8,143.46 


1,759.36 


62,216.67 


28,987.16 


2,863.46 


1969-70 


9,346.07 


677.72 


334.91 


4,348.95 


2,905.50 


4,316.83 


651.03 


22,591.01 


6,575.07 


770.27 


1970-71 


7,166.62 


596.06 


600.11 


5,021.27 


3,381.71 


4,673.39 


509.61 


21,948.77 


6,240.04 


927.76 


1971-72 


8,255.95 


783.89 


154.10 


4,581.50 


4,838.53 


3,335.23 


1,019.01 


22,968.21 


546.13 


359.3? 


1972-73 


12,336.76 


1,769.20 


2,377.47 


11,487.36 


7,233.92 


7,204.80 


651.75 


43,061.26 


14,588.90 


2,509.49 


TOTAL 


121,994.84 


7,783.37 


4,413.25 


49,979.24 


36,864.19 


44,004.22 


7,009.09 


272,048.20 


82,005.33 


10,221.15 



The following two sources were used to develop these data: 



1. 



2. 



Ground Water Level Data, 1924-77, published by 
Valley Water District (SCVWD) in August 1977. 



the Santa Clara 



A detailed report consisting of historic ground water level 
data of wells located throughout the major valley areas of 
Santa Clara County. Hydroqraphs and ground water contour maps 
obtained from SCVWD were also used in developing the required 
ground water elevations for the Coyote Basin and the portion 
of South Santa Clara Valley included in the study area. 

Ground water level measurement data from the San Joaquin 
District, Department of Water Resources. _ 

Used in developing ground water elevations for that portion of 
San Benito County included in the study area. The well mea- 
surements were made either by San Benito County or by the San 
Joaquin District of DWR. 



Procedure 

Measurement data showing ground water elevations ideally should be 
complete for the entire study period. These data should include 



111 



1964-65 
1965-66 
1966-67 
1967-68 
1968-69 

1969-70 
1970-71 
1971-72 
1972-73 

TOTAL 



Table 12. Deep Percolation in Impervious Soils, 
South Santa Clara Valley 



Water 
Year 


Deciduous 


Beets 


Grain 


Truck and 
Field 


Tomatoes 


Pasture 


Vine- 
yard 


Irrigated 
Total 


Native 
Vegetation 


Urban 



1,158.40 
733.98 

1,786.57 
570.88 

1,643.40 



924.39 
736.31 
383.73 
947.38 

8,885.04 



56.88 
35.78 
90.69 
30.57 
91.17 



0.31 

0.39 



9.87 

60.47 



57.89 55.63 

55.2b 68.81 

33.98 54.37 

99.61 193.71 



307.71 
196.19 
490.91 
180.84 
633.82 

413.88 
389.39 
241 .87 
709.42 



(Acre-Feet) 

305.88 
195.83 
497.04 
167.40 
573.88 

358.55 
346.10 
216.70 
642.93 



211.66 
132.95 
333.31 
106.75 
330.73 

191.74 

172.28 

98.98 

268.04 



70.30 
43.98 

108.55 
36.40 

107.16 

58.40 
47.20 
24.69 
61.49 



2,111.14 
1,339.10 
3,307.07 
1,102.71 
3,440.63 

2,060.48 
1 ,815.35 
1,054.32 
2,922.58 



843.45 
538.08 

1,337.63 
431.42 

1,359.62 

794.35 
631.49 
343.63 
877.23 



165.76 
103.87 
282.90 
105.33 
361.39 

237.34 
221.59 
133.57 
402.27 



551.83 443.56 3,564.03 3,304.31 1,846.44 558.17 19,153.38 7,156.90 2,014.02 



(Cubic Dekametres) 



1964-65 


1,428.89 


70.16 


0.38 


379.56 


377.30 


261.08 


86.72 


2,604.09 


1,040.40 


204.46 


2965-66 


905.36 


44.14 


0.48 


242.00 


241.56 


163.99 


54.25 


1,651.78 


663.72 


128.12 


1966-67 


2,203.73 


111.86 





605.54 


613.10 


411.14 


133.90 


4,079.27 


1,643.97 


348.96 


1967-68 


704.18 


37.71 


12.17 


223.06 


206.49 


131.68 


44.90 


1,360.19 


532.16 


129.92 


1968-69 


2,027.13 


112.46 


74.59 


781.82 


707.88 


407.96 


132.18 


4,244.02 


1,677.09 


445.77 


1969-70 


1,140.23 


71.41 


68.62 


510.52 


442.27 


236.51 


72.04 


2,541.60 


- 979.83 


292.76 


1970-71 


908.24 


68.16 


84.88 


480.31 


426.91 


212.51 


58.22 


2,239.23 


778.94 


273.33 


1971-72 


473.33 


41.91 


67.07 


298.35 


267.30 


122.09 


30.45 


1,300.50 


423.37 


164.76 


1972-73 


1,168.60 


122.87 


238.94 


875.07 


793.04 


330.63 


75.85 


3,605.00 


1,082.06 


496.20 


TOTAL 


10,959.69 


680.68 


547.13 


4,396.23 


4,075.85 


2,277.59 


688.51 


23,625.68 


8,828.04 


2,484.30 



spring measurements, which are vitally important because maximum 
ground water elevations during February through June are used in 
calculating the change in depth to ground water during the period 
of minimal pumping. Furthermore, during the spring months, the 
ground water basin recovers from any excessive pumping that might 
have occurred during the preceding fall. In addition, cones of 
depression are minimized, and the measured water levels thus tend 
to reflect an essentially unstressed condition. 

Ideally, a well used for measuring ground water elevations should 
be located at or near the node point, or center of the cell. 
However, in many cases, this was not possible. Therefore, data 
from the well closest to the node point were assigned to represent 
the respective cell. If there were two or more wells in a cell, 
all located away from the node point, the historic maximum spring 
ground water elevations from these wells were used to interpolate 
the ground water elevation at the node point. 

In some cases historic spring ground water elevations or levels 
were not available. Water-surface elevations for these cells were 
synthesized by using either trends indicated by hydrographs of 
nearby wells, by using ground water contour maps, or both. 

The yearly change in ground water level is the difference in the 
spring ground water elevations for two consecutive years under 



112 



consideration. The chanqe is positive or negative depending upon 
whether there is an increase or decrease of ground water in 
storage. 

Average Specific Yield 

Average specific yield values for all cells except those in Coyote 
Valley were obtained from the GEOLOG computer program (Ford and 
Finlayson, 1974), which computes these values from data in the 
water well log file. Average specific yield values for Coyote 
subbasin (Cells 49 through 69) were obtained from data developed 
by the Santa Clara Valley Water District for its Coyote ground 
water model. 



Results 

Yearly changes in ground water storage and the total change in 
storage for the study period are shown in Table 13. 

Adjustment of the Model 

A computer model is an idealized simulator of a prototype system; 
however, normally it will not accurately simulate the prototype 
the first time that it is run. Therefore, the model is adjusted 
until it satisfactorily simulates the historic hydrologic record. 
At that point the model is considered verified, and it can then be 
used as a planning tool by superimposing new conditions or new 
hydrologic events to describe future conditions and responses 
within the ground water basin. 

In the process of adjusting a ground water model, storage coeffi- 
cients, specific yields, and transmissivities are adjusted within 
acceptable ranges until the model accurately simulates the proto- 
type basin within the required margin of error. Changing the 
physical dimensions of each cell or branch, and/or the hydrologic 
input data, are necessary only when these adjustments fail to 
bring the model into verification. 

Ground water elevations for each node are used as indicators of 
adjustment. Historic ground water elevations for the study period 
are matched against the calculated elevations for each node. The 
model is then adjusted until the calculated ground water eleva- 
tions match the historic elevations throughout the study period. 

One major problem with full verification of the model is the 
determination of the correct net annual flows. The flows that 
were used were compiled from the computerized hydrologic data 
developed for the basin. These calculated flows were out of bal- 
ance with the measured change in storage by about 222 000 dam 
(180,000 acre-feet) for the total study period or an average of 
24 600 dam (20,000 acre-feet) per year. Two methods were used 



113 







Table 


13. Changes in Ground 
South Santa Clara 


Water 
Valley 


Storage, 
















Water Year 










Node 
Number 




















1964-65 


1965-66 


1966-67 


1967-68 


1968-69 


1969-70 


1970-71 


1971-72 


1972-73 












(^cre-Feet) 










1 


-52 


33 


173 


-242 


239 


-113 


111 


-173 


185 


2 


-138 


76 


254 


13 


357. 


-478 


-166 


-212 


583 


3 


-175 


■ 35 


200 


-185 


554 


36 


-12 


-777 


916 


4 


-472 


-108 


504 


350 


1,961 


-519 


1,060 


-302 


1,037 


5 


-386 


39 


1 ,040 


-17 


995 


-403 


-463 


-611 


690 


6 


-65 


40 


379 


-104 


494 


-33 


-245 


-407 


491 


7 


-535 


-7 


1,390 


-163 


3,456 


-2,171 


-321 


-1,216 


416 


8 


-260 


73 


466 


-67 


2,239 


• -656 


-920 


-322 


178 


9 


-286 


337 


654 


-283 


1,051 


-107 


-305 


-1,182 


519 


10 


-240 


-523 


162 


-205 


1,074 


-346 


-396 


-729 


541 


11 


-70 


64 


356 


-203 


576 


-40 


-317 


-337 


260 


12 


48 


81 


583 


841 


1,132 


-74 


-342 


-1,016 


406 


13 


-210 


5 


917 


-306 


1,240 


-333 


-296 


-918 


869 


14 


-112 


74 


708 


-339 


835 


-189 


-279 


-616 


578 


15 


-225 


-273 


1,920 


-1,579 


1,570 


58 


-332 


-1.334 


113 


16 


-289 


-249 


2,454 


-1,621 


1,653 


-76 


-710 


-1.190 


810 


17 


-66 


-361 


1,363 


-989 


921 


-152 


-106 


-864 


192 


18 


-340 


-765 


2,178 


-2,095 


2,011 


-1.676 


1,020 


-754 


510 


19 


74 


-965 


2,079 


-1,782 


1,931 


-772 


-39 


-1,025 


854 


20 





-776 


2,182 


-2,091 


2,132 


-1 ,096 


-232 


-1,390 


1,267 


21 


126 


-1,345 


3,667 


-3,667 


3,505 


-4,003 


2,452 


-727 


1,147 


22 


-77 


-1,161 


2,232 


-1,693 


2,291 


-2,474 


1,578 


-1,185 


1,486 


23 


-36 


-315 


2,422 


-1 ,431 


1,365 


-1,979 


1,141 


-867 


277 


24 


108 


-324 


2,552 


-2,552 


2,119 


-585 


-78 


-1,336 


411 


25 


-135 


-470 


1,774 


-965 


965 





-58 


-1,136 


426 


26 


73 


-263 


907 


-907 


720 





146 


-665 


55 


27 


-76 


-933 


1,196 


-673 


755 


-297 


-351 


-671 


585 


28 


-634 


-713 


2,006 


-2,245 


1,476 





-642 


-1,140 


-581 


29 


-131 


-275 


941 


-807 


672 





-93 


-583 


272 


30 


-193 


-429 


1,595 


-2,259 


1,290 





-411 


-894 


680 


31 


190 


-690 


871 


-759 


1.205 


-90 


-369 


-456 


400 


32 


-266 


-142 


1,103 


-964 


373 


-360 


-617 


-607 


729 


33 


-542 


-111 


1,451 


-1,005 


1,437 


-287 


-353 


-329 


829 


34 


.481 


-1 ,663 


1,045 


-1,350 


1,350 


2,703 


-1,414 


-332 


1,247 


35 


-195 


1,311 


-546 


-769 


1,311 


2,267 


-2,631 


-949 


769 


36 


-83 


-83 


3,476 


195 


-110 


-254 





458 





37 


99 


-635 


846 


-669 


1,235 


-147 


-236 


-316 


222 


38 


-145 


-434 


507 


-684 


1,665 


941 


-1,376 


-434 


507 


39 


-468 


1,403 


701 


623 


340 


-1,466 


468 


-234 


-234 


40 





60 


504 


67 


-67 


. -193 





395 





41 


622 


-974 


-581 


-1 ,020 


5,573 


510 


-408 


-408 


403 


42 


667 


309 


34 


139 


-139 


-395 


221 


-1,001 


-231 


43 


232 


-762 


762 


212 


-696 


-1,178 





894 


-289 


44 


257 


-381 


1,060 


260 


-676 


-649 


-123 


197 


-364 


45 


-51 


213 


116 


61 


-61 


116 





-260 


-49 


46 


181 


188 


196 








-196 


-543 


-478 


2,315 


47 


-188 


-137 


-514 


-143 


1,044 


-1,323 


-1,342 


631 


133 


48 


632 


970 


-164 


326 


-330 


-656 


-647 


-1,090 


453 


49 


-99 


99 


177 


99 


414 


-374 


-59 


-59 


335 


50 


-210 


325 


333 


223 


1,051 


-976 


-123 


-124 


377 


51 


89 


30 


238 


-209 


596 


-387 


-268 


233 


596 


52 


-70 


37 


316 


-236 


362 


-303 


-326 


-316 


606 


53 


105 


580 


316 


-316 


1,107 


-527 


-527 


-264 


1,001 


54 


165 


-198 


790 


-732 


1,351 


-716 


-390 


-481 


1,330 


55 


116 


155 


155 


-310 


776 . 


-272 


-349 


-78 


582 


56 


195 


-98 


142 


-498 


1,005 


-268 


-386 





634 


57 


131 


-86 


317 


-494 


874 


-390 


-263 


-113 


512 


58 


126 


-41 


190 


-417 


766 


-175 


7 


-335 


324 


59 


142 





283 


-566 


944 


-189 


-142 


-330 


425 


60 


78 


39 


272 


-43 


532 


-155 


-427 


39 


427 


61 


7 


66 


101 


-295 


590 


-232 


-135 


-28 


243 


62 


31 


53 


66 


-207 


392 


-132 


-79 


-88 


216 


63 


-28 





56 


-112 


225 


-84 


-169 





225 


64 


5 


20 


69 


-127 


235 


-96 


-17 


-47 


33 


65 


20 


-20 


80 


40 


159 


-30 


-418 


318 


60 


66 








-42 


-211 


211 


-84 





-42 


126 


67 


-31 


-61 


184 


-18 


273 


-107 


-6 


-40 


135 


68 


41 


68 


-149 


86 





-4 


3 


-9 


15 


69 


15 


15 


-168 


168 


46 


-46 


15 





15 


I 


-2,573 


-9,918 


53,863 


-37,466 


63,152 


-25,019 


-15,205 


-31,162 


31 ,790 



114 



Table 13. Changes in Ground Water Storage, 
South Santa Clara Valley (Continued) 



Node 
Number 










Water Year 






- 




1964-65 


1965-66 


1966-67 


1968-68 


1968-69 


1969-70 


1970-71 


1971-72 


1972-73 



(Cubic Dekametres) 



1 


-64 


41 


220 


-299 


298 


-146 


137 


-220 


228 


2 


-170 


94 


313 


22 


1,087 


-890 


-208 


-282 


719 


3 


-216 


43 


247 


-228 


683 


44 


-18 


-988 


1.130 


4 


-882 


-133 


622 


432 


2,419 


-640 


1,308 


-989 


1,279 


S 


-476 


48 


1,283 


-21 


1,227 


-497 


-877 


-784 


881 


6 


-80 


49 


467 


-128 


609 


-102 


-302 


-802 


608 


7 


-660 


-9 


1,718 


-207 


4,263 


-2,673 


-396 


-1.800 


813 


8 


-321 


90 


878 


-83 


2,762 


-809 


-1,138 


-397 


220 


9 


-383 


416 


807 


-349 


1,296 


-132 


-376 


-1,488 


640 


10 


-296 


-661 


200 


-283 


1,328 


-427 


-488 


-899 


667 


11 


-86 


79 


439 


-280 


710 


-49 


-391 


-416 


321 


12 


89 


100 


719 


1,037 


1,488 


-91 


-422 


-1,283 


801 


13 


-289 





1,131 


-377 


1,830 


-411 


-368 


-1.132 


1,072 


14 


-138 


91 


873 


-480 


1,092 


-233 


-344 


-760 


713 


IS 


-278 


-337 


2,368 


-1,948 


1,937 


72 


-410 


-1.648 


139 


16 


-386 


-307 


3,027 


-2,000 


2,039 


-94 


-876 


-1,463 


999 


17 


-81 


-448 


1,631 


-1,220 


1,136 


-187 


-131 


-1,066 


237 


18 


-419 


-944 


2,687 


-2,884 


2,481 


-2,067 


1,288 


-930 


629 


19 


91 


-1,190 


2,884 


-2,198 


2,382 


-982 


-48 


-1,264 


1,083 


20 





-987 


2,691 


-2,879 


2,691 


-1,382 


-236 


-1,718 


1,863 


21 


188 


-1,689 


4,823 


-4. 823 


4,323 


-4,938 


3,028 


-397 


1,418 


22 


-98 


-1,432 


2,783 


-2,088 


2,826 


-3,082 


1,946 


-1,462 


1,833 


23 


-106 


-339 


2,938 


-1,768 


1,634 


-2.441 


1,407 


-1,069 


342 


24 


133 


-400 


3,148 


-3,148 


2,614 


-722 


-96 


-1.648 


807 


28 


-187 


-880 


2,188 


-1,190 


1,190 





-72 


-1.401 


828 


26 


90 


-324 


1,119 


-1,119 


888 





180 


-820 


68 


27 


-94 


-1,181 


1,478 


-830 


931 


-366 


-433 


-828 


722 


28 


-782 


-879 


2,474 


-2,769 


1,821 





-792 


-1.406 


-717 


29 


-162 


-339 


1,161 


998 


329 





-118 


-719 


336 


30 


-238 


-829 


1,967 


-2,786 


1,891 





-807 


-1,103 


839 


31 


234 


-881 


1,074 


-936 


1,436 


-111 


-488 


-862 


493 


32 


-328 


-178 


1,367 


-1,189 


460 


-444 


-761 


-749 


899 


33 


-669 


-137 


1,790 


-1,240 


1,773 


-384 


-442 


-1,023 


1,023 


34 


893 


-2,081 


1,239 


-1,668 


1,668 


3,334 


-1,744 


-1,026 


1,838 


38 


-241 


1,817 


-673 


-949 


1,617 


2,796 


-3,248 


-1,171 


949 


36 


-102 


-102 


4,288 


241 


-136 


-313 





868 





37 


122 


-783 


1,044 


-828 


1,823 


-181 


-291 


-390 


274 


38 


-179 


-838 


628 


-844 


2,084 


1,161 


-1.697 


-838 


628 


39 


-877 


1,731 


368 


778 


419 


-1,808 


877 


-289 


-289 


40 





74 


622 


83 


-83 


-238 





487 





41 


767 


-1,201 


-717 


-1,288 


6,874 


629 


-80S 


-803 


803 


42 


823 


381 


42 


171 


-171 


-487 


273 


-1,238 


-288 


43 


286 


-940 


940 


262 


-889 


-1.483 





1.103 


-386 


44 


317 


-470 


1,308 


321 


-834 


-801 


-182 


243 


-449 


48 


-S3 


263 


143 


78 


-78 


-143 





-321 


-60 


46 


223 


232 


242 








-242 


-670 


-890 


2.886 


47 


-232 


-169 


-634 


-176 


1.288 


-1,632 


-1.688 


778 


164 


43 


780 


1,196 


-202 


1,019 


-407 


-809 


-798 


-1,348 


889 


49 


-122 


122 


218 


122 


811 


-461 


-73 


-73 


413 


80 


-289 


401 


417 


278 


1,309 


-1,204 


-188 


-183 


1.082 


81 


110 


37 


294 


-288 


738 


-477 


-331 


294 


738 


82 


-36 


46 


390 


-291 


1,063 


-374 


-402 


-390 


748 


S3 


130 


718 


390 


-390 


1,368 


-680 


-680 


-326 


1.238 


84 


204 


-244 


974 


-903 


1,666 


-883 


-481 


-893 


1.641 


6S 


143 


191 


191 


-382 


987 


-336 


-430 


-96 


718 


86 


241 


-121 


178 


-614 


1.240 


-331 


-476 





782 


87 


162 


-106 


391 


-609 


1,078 


-481 


-324 


-139 


632 


88 


188 


-81 


234 


-814 


948 


-216 


9 


-413 


400 


89 


178 





349 


-698 


1.164 


-233 


-178 


-407 


824 


60 


96 


43 


336 


-83 


718 


-191 


-827 


43 


827 


61 


9 


81 


128 


-364 


723 


-286 


-167 


-38 


300 


62 


38 


68 


31 


-288 


484 


-163 


-97 


-109 


266 


63 


-38 





69 


-138 


278 


-104 


-208 





278 


64 


6 


28 


88 


187 


290 


-118 


-21 


-88 


109 


68 


28 


-28 


99 


49 


196 


-99 


-816 


392 


74 


66 








-82 


-260 


260 


-104 





-82 


188 


67 


-38 


-78 


227 


-22 


337 


-132 


-7 


-49 


167 


68 


81 


84 


-184 


106 





-8 


10 


-11 


19 


69 


19 


19 


-207 


207 


87 


-87 


19 





19 


Z 


-3,174 


-12,234 


66,439 


-46.214 


84,068 


-30,861 


-18.706 


-39.672 


39,213 



115 



1964-65 
1965-66 
1966-67 
1967-68 
1968-69 

1969-70 
1970-71 
1971-72 
1972-73 

Total 



Table 14. Corrected Hydrologic Balance, 
South Santa Clara Valley 



Water 
Year 


Deep 
Perco- 
lation 


Stream 
Perco- 
lation 


Artificial 
Perco- 
lation 


Waste 

Water 

Disposal 


Subsurface 
Inflow/ 
Outflow 


Appl ied 
Water 


Urban 
Use 


Sum 
(Balance) 


Change 

in 

Storage 



23,471 
20,795 
44,717 
25,742 
81,422 

27,362 
26,273 
20,886 
52,974 

323,642 



47,650 
16,180 
60,010 
15,530 
28,220 

14,250 
26,850 
1 , 1 00 
55,290 

274,080 



2,100 
1,600 
1,500 
3,500 
3,200 

3,100 
2,100 
3,500 
3,800 

24,400 



(Acre-Feet) 



6,400 
6,690 
6,690 
3,200 
3,400 

3,600 
4,090 
6,000 
5,890 

45,960 



34,997 
24,663 
39,056 
24,450 
28,617 

24,029 
28,167 
22,667 
37,506 

264,152 



75,715 
92,216 
67,879 
104,542 
77,324 

76,565 

80,278 

101,889 

78,550 

754,958 



14,337 
18,132 
16,231 
15,954 
15,445 

14,137 
14,396 
14,705 
13,211 

136,548 



24,565 
-40,420 

67,863 
-48,074 

52,091 

-18,360 

- 7,194 

-53,441 

63,698 

40,728 



- 2,573 

- 9,918 
53,862 

-37,466 
68,152 

-25,019 

-15,165 

-32,162 

31,790 

31,501 



{Cubic Dekametres) 



1964-65 


28,951 


58, 776 


2,590 


7,894 


43,168 


93,394 


17,685 


30^301 


- 3,174 


196S-66 


25,651 


19,958 


1,974 


8,252 


30,422 


113,748 


22,366 


-49,858 


-12,234 


1966-67 


55,158 


74,022 


1,850 


8,252 


48,175 


83, 729 


20,021 


83,709 


66,439 


1967-68 


31,753 


19,156 


4,317 


3,947 


30,159 


128.953 


19,679 


-59,299 


-46,214 


1968-69 


100,434 


34,809 


3,947 


4,194 


35,299 


95,379 


19,051 


- 64,254 


84,065 


1969-70 


33,751 


17,577 


3,824 


4,441 


29,640 


94,443 


17,438 


-22,647 


-30,861 


1970-71 


32,408 


33,120 


2,590 


5,045 


34, 744 


99,023 


17,757 


- 8,874 


-18,706 


1971-72 


25, 763 


12,458 


4,317 


7,401 


27,960 


125,680 


18,139 


-65,919 


-39,672 


1972-7S 


65,343 


68, 200 


4,687 


7,265 


46,263 


96,891 


16,236 


78,571 


39,213 


Total 


399,212 


338,076 


30,096 


56,691 


325,831 


931,240 


168,432 


50,238 


38,856 



to adjust this hydrologic imbalance. First, both the agricultural 
and urban pumpage were reduced by 20 percent. Second, a 
subsurface inflow value of 12 300 dam^ (10,000 acre-feet) per 
year was added on a pro rata basis to each cell based on the ratio 
of length of external cell boundary divided by the entire length 
of the model boundary. The adjusted values for the hydrologic 
balance are shown on Table 14. 



Present Status of the Model 

The South Santa Clara Valley ground water model could not be veri- 
fied due to a lack of historic data. The availability and quality 
of historic data are shown on Table 15 and on Figure 24. In spite 
of these shortcomings, the model, as it is now developed, can be 
used as a tool for general analysis of basin operational plans. 

Figure 25 shows a number of ground water hydrographs for a 
selected set of nodes in the final adjustment run of the model. 
Figure 26 shows contour plots of the historic and calculated 
ground water levels for the fifth and ninth years of the study. 



116 



Table 15. Nodal Analysis of Ground Water Model 
South Santa Clara Valley 



Node 
Um- 
ber 


Historic 
Water- 
level 

Record 


laximum Deviation of Computed Water Level 


Remarks 


Below Historic Record 


Above Historic Record 


Feetl Metres 1 Year 


Feetl Metres 1 Year 



1 

2 
3 


1965-69 


25 


8 


1966 


-- 


- 


- 


1965-73 


31 


9 


1970 


__ 


II 


._ 


4 


1965-73 


20 


6 


1969 


21 


P 


1972 


5 


1965-73 


26 


8 


1969 


3 


2 


1972 


6 


1965-73 


24 


7 


1969 


6 


2 


1972 


7 


1965-73 


40 


12 


1969 


6 


2 


1972 


8 


1965-70 


28 


9 


1%9 


-- 





-- 


9 


1965-73 


-- 





-. 


16 


i 


1972 


10 


1965-73 


26 


8 


1969 


13 


4 


1966 


11 


1965-73 


-- 







32 


10 


1972 


12 


1965-73 


29 


$ 


1969 


11 


S 


1965 


13 


1965-73 


28 


9 


1969 


-_ 


__ 


-- 


14 


1965-73 


30 


9 


1969 


-- 


__ 


.. 


15 


1969-73 


34 


10 


1969 


-- 


.- 


-- 


16 


1969-73 


41 


12 


1969 


__ 





__ 


17 
18 
19 
20 


1969-73 


32 


10 


1969 


-- 


— 


-- 


1969-73 


44 


IS 


1969 


-- 


-- 


- 


21 
22 
23 


1969-73 


21 


e 


1969 


- 


-- 


- 


24 
25 


1969-73 


28 


9 


1969 


-- 


— 


— 


26 


1972-73 


20 


6 


1972 


.. 




.. 


27 


l%9-73 


26 


S 


1970 


-- 





— 


28 
29 


1970-73 


8 





1970 


-. 





-- 


30 
31 


1969-73 


19 


6 


1971 




;; 




32 
33 
34 


1969-73 


10 


3 


1973 


- 


- 


" 


35 
36 


1965-71 


7 


— 


1963 


13 


4 


1965 


37 


1969-73 


16 


6 


1972 


-- 


— 


-- 


38 


— 


-- 


— 


-- 


-- 


— 


-- 


39 


— 


— 


-- 


-- 


-- 


— 


-- 


40 


--- 


— 


— 


— 


-- 


— 


-- 


41 


— 


-- 





— 


-- 


._ 


.. 


42 


1%5-73 


— 


_ 


-- 


47 


14 


1971 


43 





— 





-- 


.. 





.. 


44 


1968-73 


-- 





-- 


53 


16 


1970 


45 


--- 


-- 


— 


-- 


-- 


— 


-- 


46 


— 


-- 





-- 


— 





-- 


47 


l%7-73 


-- 


— 


-- 


57 


1? 


1971 


48 


— 


-- 


— 


-- 


— 


— 


-- 


49 


— 


-- 





— 


-- 





-- 


50 


1965-73 


24 


7 


1969 


10 


3 


1971 


51 


— . 


-- 





-- 


— 





-- 


52 


l%5-73 


34 


10 


1%9 


— 


— 


— 


53 





-. 





-- 


— 


— 


— 


54 


1965-73 


35 


11 


1969 


-- 


— 


— 


55 


— 


— 


— 


-- 


-- 


— 


-- 


56 


l%5-73 


11 


i 


1969 


9 


3 


1971 


57 


1965-73 


23 


7 


1969 


-- 





-. 


58 


l%5-73 


16 


6 


1969 


— 


— 


-- 


59 
60 


... 


-- 


— 


-- 


-- 


— 


-- 


61 


1965-73 


". 




.! 


12 


4 


1971 


62 


1965-73 


— 


— 


— 


9 


3 


1971 


63 


— 


— 


__ 


-- 


— 





-- 


64 


1965-73 


— 





— 


13 


4 


1971 


65 


... 


-- 


— 


— 


-- 


-- 


-- 


66 





.- 





.- 


-- 





— 


67 


1968-73 


.. 


_ 


-- 


11 


I 


1971 


63 


1968-73 


-- 


— 


-- 


18 


i 


1971 


69 


— 


.- 


— 


— 


-- 


— 


.. 



78 feet) below. 



Fair match, 2-8 metres (8-25 feet) below. 

No historic data. 

Fair match, 5-9 metres (15-31 feet) below. 

Matched, 1965-68. 

Matched, 1965-68. 

Poor match, 1965-68. 
Matched, l%5-68 and 1972-73. 
Matched, 1965-68. 
Matched, 1968-71. 
Matched, 1972-73. 

Matched, 1969-70, otherwise below. 

Matched, 1967 only. 

Matched, 1-2 metres (3-5 feet) below, 1965-63 and 1972-73. 

Matched, 1965-67 and 1971-73. 

Matched, 7-10 metres (23-34 feet) below. 

Matched, 9-12 metres (31-41 feet) below. 
Unmatched, 0-10 metres (0-32 feet) below. 
No historic data. 
Unmatched, too low. 
No historic data. 

No historic data. 
No historic data. 
Unmatched. 
Unmatched, 9 metres 
No historic data. 

Matched, 6 metres (20 feet) below. 
Matched, 8 metres (26 feet) below. 
Matched, 2 metres (8 feet) below. 
No historic data. 
No historic data. 

Fair match, about 6 aietres (19 feet) below. 
Matched, 1969-72. 
No historic data. 
No historic data. 
No historic data. 

Matched, 1966-71. 
Fair match, 2-5 metres 
No historic data. 
No historic data. 
No historic data. 

No historic data. 

Unmatched, 2-14 metres (6-47 feet) above. 

No historic data. 

Unmatched, 3-16 metres (10-53 feet) above. 

No historic data. 

No historic data. 

Unmatched, 4-17 metres (12-57 feet) above. 

No historic data. 

No historic data. 

Matched, 1965-68. 

No historic data. 

Unmatched, 3-10 metres (11-34 feet) below. 

No historic data. 

Unmatched, 4-11 metres (12-35 feet) below. 

No historic data. 

Matched, 1965-68. 

Unmatched, 1-7 metres (2-23 feet) below. 

Poor match, 2-5 metres (5-15 feet) below. 

No historic data. 

No historic data. 

Poor match, 2-4 metres (5-12 feet) above. 
Matched, 1965-70. 
No historic data. 
Matched. 1965-66. 
No historic data. 

No historic data. 

Unmatched, computed level 3 metres (10 feet) above <)round. 
Unmatched, computed level 3 metres (10 feet) above ground. 
No historic data. 



;8-16 feet) below. 



9—82239 



117 




FIGURE 24.— Nodal Historic Periods of Record, 



Two major differences between the model and historic ground water 
level records still preclude the model from being verified and are 
discussed below. 



Difference 1 ; 
(Cells 1 , 4, an 
to the subbasin 
this. As a res 
with the histor 
after. Histori 
two years to re 
appears to move 
basin. The det 
be refined, and 
unaccounted spi 
calculations. 



Historic' water levels in the upper Llaqas Subbasin 
d 9) imply that an impulse-like recharge was made 

in 1969; the hydrologic calculations do not verify 
ult, the model-generated water levels do not agree 
ic water levels for 1969 and for two years there- 
c water levels in this subbasin appear to require 
ach equilibrium after the large impulse. The water 

slowly southward toward the lower part of the sub- 
ailed hydrology in the upper Llagas Subbasin should 

data for Anderson Reservoir should be reviewed for 
lis to be used in streambed percolation 



118 



LEGEND 
I I Complete Water-Level Record 1965-1973 

I I Water-Level Record Only For Certain Years 

I I No Historic Water-Level Data 




South Santa Clara Ground Water Model. 




In addition, in the southern portion of the model there is a 
depression formed in the historic water levels; the model was not 
able to simulate this. This condition also implies some deqree of 
fault restriction. Imposing a fault boundary in this area should 
alleviate this problem. 



119 



Lttl 


C 2^0 


ibo 


260 


300 


120 


}••(. 


• 


* 








N '1 






. 








„ j 


1966 














» 1 


1567 




/ : 








H 1 


l<ibS 













" 1 


l'»6) 





D 








I I 


19T0 




c 










» 1 


1971 




. * .* 








"' i 


197; 




•^ 








" ; 


UT3 


I 1 


• 


1 


, 


1 — 


11 



260 




260 




259 




2i9 




258 




257 




256 




29S 




253 




?52 





Zii 2 •• C 



Node 9 
NODE TYPE 1, 




Node 62 

Full period of record; Fair to good agreement between historic 
and computed levels. Nodes 9, 56, 58, 62, 63, and 64. 




Node 3 



I 268 


286 


I 2e« 


288 


I 287 


267 


I ?B0 


267 


I 2TB 


286 


I 2T6 


268 


I 276 


290 


I 276 


291 


I 277 


293 


I 279 


290 


I 2 79 


26B 


I 276 


265 


1 276 


28 2 


I !7S 


287 


I 275 


292 


I 276 


296 


1 278 


301 


I 260 


299 


I 282 


297 


1 zez 


295 


I 293 


293 


I 283 


292 


I 2e* 


290 


1 2B« 


289 


I 2B« 


267 


I 285 


266 


I 284 


28b 


I Z83 


285 


I 2BZ 


28* 


I 282 


267 


I 282 


290 


I 283 


293 


I 285 


296 



!••€ 26C 



Node 57 
NODE TYPE 2. 



30 120 1*0 


16 


18 


Node 36 






00 no ^-.o 


lb- 


280 



133 

133 
133 



Node 68 

Six to nine years of historic record; Up to seven years of fair to 
good agreement between historic and computed levels. May 
have up to 4 years of nonagreement. Nodes 3, 13, 36, 50, 57, 
67, and 68. 



FIGURE 25. --Computer-Generated Hydrographs, 



120 




Node 4 
NODE TYPE 3. 



Node 37 

Four to nine years of historic record; Up to five years of poor to 
good agreement between computed levels. May have up to 6 years 
of nonagreement. All nodes except those of types 1, 2, 4, or 5. 




1«TJ I 

II •• 



ZflT 



Node 11 
NODE TYPE 4. 



Node 54 

Five to nine years of historic record; Up to four years of 
poor to fair agreement between historic and computed levels. May 
have 2 to 9 years of nonagreement. Nodes 5, 1 1, 23, 42, 52. and 54. 



LEGEND 

+ HISTORIC WATER LEVEL 
ACTUAL 

/ HISTORIC WATER LEVEL 
r ESTIMATED 

* COMPUTED WATERLEVEL 

COMPUTED AND HISTORIC 
WATER LEVELS MATCHED 



H GROUND SURFACE 



NOTE: 

Scale on hydrographs. 
Horizontal elevation 
in feei. Vertical in years 



NODE TYPE 5. 



Node 44 

Two to seven years of historic record: Little or no agreement 
between historic and computed levels. Nodes 26, 44, and 47. 



South Santa Clara Ground Water Model 



121 




200 



•Gilroy 




50 



00 



HISTORJC 




300 



250 



200 



(Gilroy 




COMPUTED 



WATER YEAR 1968-1969 



FIGURE 26. --Comparison of Historic and Model 



122 



Contour Interval 
10 ft. (3m) 




300 



250 



200 



c 




120 



HISTORIC 




350 



300 



250 



200 



150 



>6ilroy 




WATER YEAR 1972-1973 



Generated Ground Water Elevation Contours. 



123 



CHAPTER V. GROUND WATER BASIN SURVEILLANCE SYSTEM 



During the 1950s, surveillance of ground water in Santa Clara 
County was limited to that area north of San Martin and consisted 
of measuring depths to static and pumping ground water levels and 
analyzing the water being pumped. Because the intent of the pro- 
gram was only to monitor the water coming from the well, little 
attention was given to understanding the individual aquifer or 
group of aquifers that produced the water. Since that time, there 
has been an ever-increasing interest and concern about the ground 
water resources of Santa Clara County. More knowledge is needed 
about the physical conditions of the ground water resource — how 
water infiltrates the ground water body, how and by what paths it 
moves from point to point within the ground water body, how it can 
become polluted or degraded, and what the effects of its extrac- 
tion are. 

Data required to monitor the ground water resource of South Santa 
Clara Valley include the following nine items: 

1 . Pumpage . Metered ground water pumpage by water year (October 
through September) is necessary to enable the accurate deter- 
mination of an annual water balance; metered pumpage also is 
necessary to formulate operational plans, because the ground 
water resource is intensely used and responds rapidly to 
changes in pumping rates. 

2. Unconfined Water Levels . Periodic ground water elevation data 
for selected locations in the unconfined ground water zone are 
necessary to accurately determine changes in storage. Most 
elevation determinations can be seasonal, but a few continuous 
recorders are necessary to identify periods of maximum stress 
of the ground water system. 

3. Confined Water Levels . Elevation data of the confined 
potentiometric surface should be developed on a seasonal 
basis. These data are needed to provide information on pres- 
sure differences between the various aquifers and also to 
determine conditions of water supply. 

4. Surface Inflow . A sufficient number of gaging stations along 
the perimeter of the ground water basin are required to form 
reliable estimates of tributary inflow. 

5. Local Runoff . Tracts representing differing natural and 
developed areas should be instrumented with precipitation and 
flow instruments to determine contribution of valley areas to 
local runoff. 



125 



6. Artificial Recharge . Accurate inflow and outflow measurements 
for all percolation facilities, including both ponds and 
streams, are necessary to provide reliable data on the 
quantity of water deliberately recharged to the basin, 

7. Surface Outflow . A sufficient number of gages on streams 
draining the valley are required to provide reliable estimates 
of quantities of surface water leaving the basin. 

8. Transmissivity . A program of field testing selected water 
wells will provide accurate data on aquifer transmissivities . 

9. Water Quality . Quality monitoring of surface and ground water 
is necessary to detect possible degradation before it proceeds 
beyond control. Quality data for surface water measuring 
stations, taken for a wide range of flows, will provide infor- 
mation on fluctuations of mineral constituents entering and 
leaving the basin. Similar data from monitoring wells will 
provide data on the mineral characteristics of the various 
parts of the aquifer system. The frequency of sampling and 
the analysis for specific mineral constituents will vary 
widely. 

Water Level Measurements 

A data gathering system that will provide information on the 
elevation of the upper surface of the unconfined ground water Ixjdy 
must be based on adequate knowledge of: 1) the subsurface 
geology, 2) the subsurface hydrology, and 3) construction details 
of each monitoring well. The first two requirements have been met 
by the study reported on in this bulletin. An appraisal of the 
existing ground water level network was made in order to evaluate 
the third requirement. Wells measured for ground water levels 
between 1936 and 1978 were reviewed. Both a driller's log and 
construction details for each measurement well are necessary so 
that a relationship between water levels and aquifers can be 
developed. Of the 118 wells for which water-level data are 
available, only 17 have construction details available. Of the 
101 remaining wells, 21 are of unknown depth. 

A further requirement in the determination of the configuration of 
the unconfined ground water surface is that the monitoring wells 
should tap only those aquifers which do not have any significant 
degree of confinement. In general, wells in South Santa Clara 
Valley reaching depths greater than about 85 metres (280 ft) draw 
water from aquifers under some degree of confinement. 

Table 16 lists all wells that were measured from the study period 
through 1978. Because of the general lack of adequate construc- 
tion data for many of the wells measured, it is not possible to 
incorporate the majority of them into a meaningful water-level 
measurement network. Hence, a modified water-level measurement 
network should be implemented. 



126 



Table 16. Existing Ground Water Monitoring Network, 
South Santa Clara Valley 



Uell 
Location 
Number 


Period 

of 
Record 


Depth 

in 

Metres 


Perforated 

Interval 
in Metres 


Remarks 


b8S/02E-22D01 


1936-78 


26.2 


... 


No construction data 


08S/02E-22F01 


1968-78 


74.7 


... 


No construction data 


08S/02E-26M02 


1947-77 


45.7 


... 


No construction data 


08S/02E-27G01 


1968-78 


7.9 




No construction data 


08S/02E-28H02 


1968-78 


— 


... 


Depth unknown 


08S/02E-28H03 


1974-77 


9.1 


... 


No construction data 


08S/02E-31Q01 


1969-78 


17.1 


... 


No construction data 


08S/O2E-34AO1 


1936-70 


21.3 




Destroyed 


08S/02E-34E01 


1968-75 


... 


... 


Depth unknown 


08S/02E-35G01 


1937-78 


45.7 


... 


No construction data 


08S/02E-35M01 


1959-78 


27.4 


... 


No construction data 


09S/02E-01C01 


1938-78 


45.7 


... 


No construction data 


09S/02E-01E99 


1936-39 


33.5 


... 


Destroyed 


09S/02E-01G98 


1936-38 


... 


... 


Destroyed 


09S/02E-01JOI 


1937-77 


41.1 


... 


No construction data 


09S/02E-02C01 


1937-78 


83.8 


... 


No construction data 


09S/02E-02G01 


1939-78 


68.6 


... 


No construction data 


O9S/O2E-02J02 


1948-78 


34.7 


... 


No construction data 


09S/02E-02P02 


1972-78 


C 


C 


Confidential log 


09S/02E-02P99 


1937-58 


30.5 


... 


Destroyed 


09S/02E-11C01 


1958-78 


36.6 


... 


No construction data 


O9S/02E-12B01 


1937-78 


54.9 


... 


No construction data 


CI9S/02E-12E01 


1937-78 


65.5 


... 


No construction data 


O9S/02E-12F99 


1937-46 


36.6 


... 


No construction data 


09S/03E-07H03 


1968-77 


91.4 





No construction data 


09S/03E-07L02 


1953-78 


60.4 




No construction data 


O9S/03E-07L99 


1937-53 


60.4 




Destroyed 


09S/03E-08J02 


1937-76 


91.4 


... 


No construction data 


09$/03E-1SF01 


1968-78 


76.2 




No construction data 


09S/03E-15L01 


1955-78 


61.0 




No construction data 


09S/03E-16A01 


1958-74 


42.4 


-.- 


Destroyed 


■09S/03E-16C0I 


1936-78 


91.7 


... 


No construction data 


09S/03E-16J01 


1948-78 


121.9 


... 


No construction data 


09Sr/03E-17C01 


1968-78 


... 


... 


Depth unknown 


09S/03E-17K99 


1936-57 


54.9 


... 


Destroyed 


09S/03E-18B01 


1958-78 


C 


c 


Confidential log 


095/03E-20E99 


1948-54 


85.7 


... 


Destroyed 


09S/03E-20F01 


1954-76 


38.4 


... 


No construction data 


09S/03E-20F99 


1972-76 


... 


... 


Depth unknown 


09S/03E-20H01 


1948-78 


73.2 


... 


No construction data 


09S/03E-21K0I 


1948-78 


68.6 


... 


No construction data 


09S/03E-22803 


1948-78 


103.6 


... 


No construction data 


09S/03E-22P99 


1972-78 


... 


... 


Depth unknown 


09S/03E-23E01 


1948-78 


128.0 


... 


No construction data 


09S/03E-25P01 


1948-78 


75.9 


... 


No construction data 


09S/03E-26P01 


1948-78 


76.2 




No construction data 


09S/03E-27COI 


1948-70 


91.4 




Destroyed 


O9S/03E-33G01 


1948-56 


50.6 


... 


Destroyed 


09S/03E-33H01 


1957-78 


115.8 


... 


No construction data 


09S/03E-33H03 


1972-77 


... 


... 


Depth unknown 


09S/03E-31A99 


1972-78 


... 


... 


Depth unknown 


09S/03E-34D01 


1958-78 


114.3 


... 


No construction data 


09S/03E-34D99 


1948-58 


42.7 


... 


Destroyed 


09S/03E-34K01 


1975-78 


... 


... 


Depth unknown 


09S/03E-34Q01 


1948-78 


59.4 


47-56 




09S/03E-35N01 


1958-78 


57.3 


... 


No construction data 


09S/03E-35N80 


1948-54 


48.8 


... 


No construction data 


09S/03E-35P99 


1972-78 


-.- 


... 


Depth unknown 


09S/03E-36F01 


1962-78 


144.8 


... 


No construction data 



Well 
Location 
Number 


Period 

of 
Record 


Depth 

in 

Metres 


Perforated 

Interval 
in Metres 


Remarks 


09S/03E-36M01 


1948-78 


61.0 




No construction data 


10S/03E-01E02 


1976-77 


C 


C 


Confidential log 


10S/03E-01N02 


1969-78 


40.2 




No construction data 


10S/03E-03C01 


1948-78 


67.1 


--- 


No construction data 


105/03E-04G01 


1948-69 


51.8 


.-- 


Destroyed 


10S/03E-05L02 


1976-78 


7.3 


... 


No construction data 


10S/03E-11G01 


1975-77 


85.3 




No construction data 


10S/03E-13DO3 


1969-78 


C 


C 


Confidential log 


10S/03E-14D01 


1972-78 


61.0 


... 


No construction data 


10S/03E-14R02 


1968-71 


27.4 


... 


Destroyed 


10S/03E-23J02 


1968-78 


78.6 


... 


No construction data 


10S/03E-24M01 


1972-77 


C 


c 


Confidential log 


10S/03E-26J01 


1975-77 


C 


c 


Confidential log 


10S/03E-36A05 


1972-77 


64.6 


... 


No construction data 


10S/04E-06P01 


1969-78 


C 


c 


Confidential log 


10S/04E-07E99 


1973-78 


48.8 


... 


No construction data 


10S/04E-07F01 


1969-74 


79.2 


... 


No construction data 


10S/04E-07F02 


1974-78 


... 


... 


Depth unknown 


10S/04E-17F01 


1975-77 


55.2 


... 


No construction data 


10S/04E-17K02 


1969-78 


76.2 


.-- 


No construction data 


10S/04E-17N02 


1969-78 


... 


... 


Depth unknown 


10S/04E-18G02 


1975-77 


56.1 




No construction data 


10S/04E-18J01 


1975-77 


C 


c 


Confidential log 


10S/04E-18N99 


1971-78 


74.4 


... 


No construction data 


1OS/04E-2OMO1 


1969-78 


64.3 


... 


No construction data 


10S/04E-21H01 


1969-78 


... 


... 


Depth unknown 


10S/04E-30P05 


1969-78 


36.6 


... 


No construction data 


10S/04E-31G04 


1969-78 


C 


... 


Confidential log 


10S/04E-3IR99 


1971-78 




... 


Depth unknown 


10S/04E-33M99 


1971-76 


... 


... 


Depth unknown 


10S/04E-34E02 


1969-78 




.- 


Depth unknown 


10S/04E-34L05 


1975-77 


49.7 


— ■ 


No construction data 


11S/04E-02D01 


1969-78 


86.9 


16-85 




11S/04E-03J01 


1972-78 


126.5 


... 


No construction data 


115/04E-04C03 


1969-78 




... 


Depth unknown 


11S/04E-05D01 


1969-70 






Depth unknown 


11S/04E-05L99 


1971-78 


... 


... 


Depth unknown 


11S/04E-06B01 


1969-78 


213.7 


20-210 




11S/04E-06D01 


1969-78 


143.3 


33-140 




11S/04E-06H01 


1969-78 


105.5 


30-105 




11S/04E-06N01 


1968-76 


82.9 


... 


Destroyed 


11S/04E-06P02 


1969-78 


C 


... 


Confidential log 


11S/04E-08I<01 


1969-78 


91.4 




No construction data 


11S/04E-08K02 


1975-77 


... 




Depth unknown 


11S/04E-09K02 


1975-77 


41.5 




No construction data 


11S/04E-09P01 


1975-77 




... 


Depth unknown 


11S/04E-10D04 


1969-78 


112.8 




No construction data 


11S/04E-10K01 


1971-77 


109.7 




No construction data 


11S/04E-11C01 


1969-78 


131.1 


_._ 


No construction data 


11S/04E-15001 


1969-78 


136.6 


121-136 




11S/04E-16J01 


1971-75 


53.3 




No construction data 


11S/04E-17C03 


1969-71 


... 




Destroyed 


11S/04E-17M01 


1968-78 


24.4 


... 


No construction data 


11S/04E-21B02 


1977 


C 




Confidential log 


115/04E-21P01 


1969-78 


... 


... 


Depth unknown 


11S/04E-21001 


1972-78 


... 




Depth unknown 


11S/04E-22N03 


1972-78 


67.1 




No construction data 


11S/04E-27E02 


1971-78 


... 




Depth unknown 



C - Confidential well log; data are on file with the Department of Water Resources but not available for public release (Water Code Sec, 13752). 



127 



Well Qualification 

The first step in selecting wells for a modified measurement net- 
work is determining what aquifer, or group of aquifers, the mea- 
surements of the well should represent; this step is called well 
qualification. A qualified well is defined as being one that 
meets all of the following criteria: 

1. The well is accurately located. An essential factor because 
several wells may be grouped in a cluster and measurements may 
not always be for the same well. 

2. A well log is available and on file with the agency performing 
monitoring operations. An electric log of the well, although 
not entirely necessary, is desirable. 

3. Well construction data are available to the agency performing 
the monitoring operations. 

4. A fairly long period of record of measurements is available. 
Although not as essential as the first three criteria, a 
historic water-level record is preferable to a new one. 

Using the above data, personnel with an understanding of 
subsurface geology and ground water hydrology can determine that 
water-level measurements from a particular well reflect the 
potentiometric surface of a specific aquifer, or group of 
aquifers. Where this is done, fluctuations of the water levels in 
the particular well become meaningful data. 

Qualified monitoring wells should be located with the aid of 
information on the buried stream channels, shown on Figures 5A 
through 5 J, augmented by knowledge of lake-bottom clay deposits. 
Also, the ideal monitoring network will contain not only repre- 
sentative wells tapping principal aquifers, but additional wells 
reflecting effects of faults. 

Proposed Network 

The development of the proposed network of monitoring wells 
involved a detailed examination of the subsurface geology dis- 
cussed in Chapter II. Monitoring well locations thus selected 
should reflect water levels for given zones. 

Geohydrologic data reveal an essentially unconfined ground water 
zone in the northwest portion of the valley between Coyote and 
Madrone. Minor confinement occurs locally at depth due to the 
presence of discontinuous clay lenses. The thickness of valley 
fill materials increases southeasterly toward Hollister. 

Between Morgan Hill and Gilroy, impermeable lacustrine clays 
divide the ground water into a shallow unconfined zone and a 
deeper confined zone. Confinement increases to the southeast. 



128 



where lacustrine clays separate the deeper confined aquifer into 
at least two zones in the Bolsa area. 

Two types of monitoring wells are recommended. Shallow wells in 
the northwest and middle sections of the area will monitor depths 
of 85 m (280 ft) or less. Multiple wells will monitor both the 
shallow unconfined zone and deeper confined zones in the middle 
section of the area. Multiple wells in the southeastern section 
of the area will monitor both intermediate and deep confined 
aquifers, and should be about 200 m (650 ft) deep. 

The recommended minimum network contains 20 shallow wells and 16 
multiple-completion wells. Figure 27 shows the areal distribution 
of the proposed network and Table 17 presents location and 
monitoring interval data for the proposed monitoring wells. 

The water-level monitoring network coincides with the proposed 
water quality monitoring network to minimize the number of wells 
to be monitored. 



Implementation of Network 

Several steps should be taken in establishing the modified moni- 
toring network: 

1 . Search records and make a field canvass to locate all well 
data in the vicinity of proposed monitoring well locations. 

2. Determine if an existing well can be used or modified for use 
as a monitoring well. 

3. If Step 2 is negative, or if cost is excessive, drill a test 
hole and construct therein a single-completion or multiple- 
tube piezometer. 

4. Locate monitoring wells beyond the area of interference of 
large municipal and industrial wells. Consideration also 
should be given to restricting the placement of new high- 
capacity wells that may adversely affect existing monitoring 
wells. 

5. Keep the continuity of measurements in existing wells unbroken 
until there is some overlap of record with those of new or 
replacement facilities. 

Many of the water-level measurements now available are taken by 
the agency that operates the well. Such measurements should be 
continued by such agencies for their own operating reasons. 



129 




FIGURE 27. --Proposed Ground Water Monitoring 



130 



LEGEND 



[~| New Shallow Well, Water - Level Only 

I New Shallow Well, Water-Level and Quality 

/\ Existing Shallow Well, Water-Level Only 

j; ^^ Existing Shallow Well, Water-Level and Ouality 




Q New Deep or Composite Well, 
Water-Level Only 

9 New Deep or Composite Well. 
Water-Level and Quality 

-O- Existing Deep Well, 
Water-Level Only 



Y 
/ o 



^•^^... 



\ 



\ 



u 



-'/■ 



o 



I 2 3 

■ ' I l ' I 'l I 
I 2 3 4 5 6 



J MILES 

KILOMETRES 



Network, South Santa Clara Valley. 



131 



/ 



Table 17. Proposed Ground Water Monitoring Network, 
South Santa Clara Valley 



Location 


Type* 




Monitoring Interval 
(Approximate Depths) 


Existing Wells 


Water 
Level 


Water 
Qual- 
ity 


Feet 


Metres 


SCVUD 


1 Type* 


1 DWR 


1 Type* 


08S/02E-27R 


S 100 


■200 


30- 


-61 


— 


. 


08S/02E-27R 


S 


X 


X 


08S/02E-35G 


S 100 


-200 


30- 


-61 


08S/02E-35G01 


S 


08S/02E-35H02 


S 


X 




08S/02E-34L 


S 50 


-120 


IS- 


-37 


... 


- 


... 


- 


X 




09S/02E-01R 


S 50 


-180 


15 


-55 





. 


09S/02E-01R02 


S 


X 


X 


09S/02E-02Q 


S 65 


-115 


20 


-55 





- 


09S/02E-02Q04 


S 


X 




09S/03E-08M 


M 100 


■1C5 


30- 


-SO 





_ 


. 


_ 


X 






215 


■265 


66- 


■81 


— 


- 


— 


. 






09S/03E-10N 


M 100 


■150 


30- 


■46 


— 


- 





- 


X 


X 




230 


■280 


70- 


-85 





. 


— 


- 






09S/03E-16G 


S 115 


■230 


35- 


-70 





. 







X 




09S/03E-20F 


S 100- 


200 


30- 


-61 


09S/03E-20F01 


s 


— 




X 


X 


09S/03E-21B 


S 100 


200 


30- 


-61 


— 


- 


09S/03E-21B01 


s 


X 




09S/03E-22J 


M 100 


-150 


30- 


■46 


— 


. 


— 


. 


X 


X 




200- 


250 


61- 


-76 


— 


. 


— 


- 






09S/03E-26P 


S lOO 


-250 


30- 


-76 


09S/03E-26P01 


s 


— 


. 


X 




09S/03E-28R 


S 100 


-250 


30- 


-76 


— 


- 


— 


. 


X 


X 


09S/03E-34Q 


S 100 


-190 


30- 


-58 


09S/03E-34Q01 


s 


... 


- 


X 




10S/03E-02J 


M 100 


-150 


30- 


-46 





. 


10S/03E-02R03 


s 


X 


X 




200- 


250 


61- 


■76 





. 


10S/03E-01E02 


D 






10S/03E-04K 


S 80 


-150 


24- 


-46 





. 


10S/03E-03D 


s 


X 


X 


10S/03E-12N 


S 115 


-250 


3S- 


-76 


10S/03E-13D03 


s 


10S/03E-12N01 


s 


X 




10S/03E-24G 


S 130- 


210 


40- 


■64 


— 


. 


10S/03E-24G 


s 


X 




10S/03E-25L 


S 100 


-210 


30- 


-64 


— 


- 


10S/03E-25L02 


s 


X 


X 


10S/04E-18C 


M 90 


-180 


27- 


-55 





. 


10S/04E-18C 


s 


X 






220 


-300 


67- 


■91 


— 


. 


— 


- 






10S/04E-27N 


S 80 


-260 


24- 


-79 


— 


- 


10S/04E-27F 


s 


X 


X 


10S/04E-30J 


S 80 


■260 


24- 


-79 


10S/04E-30P05 


s 


--- 


- 


X 




11S/03E-01R 


S 80- 


-200 


24- 


-61 


... 


- 


11S/03E-02H01 


s 


X 


X 


11S/04E-05D 


M 100- 


-200 


30- 


■61 







11S/04E-05D01 


s 


X 






300- 


400 


91- 


-122 


— 


. 


11S/04E-03G 


D 






11S/04E-08M 


S 100 


-250 


30- 


-76 


— 


- 


11S/04E-08N 


S 


X 


X 


11S/04E-12N 


M 180- 


230 


55- 


-70 


— 


. 


— 


- 


X 


X 




295 


-395 


90- 


120 


— 


. 


— 


- 






11S/04E-16A 


M 100- 


230 


30- 


■70 


— 


- 


11S/03E-15A 


D 


X 


X 




320 


-400 


98- 


-122 


— 


- 


— 


. 






11S/04E-26L 


H 330 


-410 


101- 


■125 


— 


. 


— 


. 


X 






574- 


640 


1?S- 


-195 


— 


. 


— 








nS/04E-27F 


M 180- 


-245 


55- 


■75 


— 


. 


11S/04E-28A 


S 


X 


X 




330 


-410 


101- 


-125 


— 


. 


11S/04E-27D 


D 






11S/04E-33L 


S 131 


-295 


40- 


-90 


— 


. 


— 


- 


X 




11S/04E-36P 


M 250 


■350 


76- 


■107 


— 


- 




- 


X 


X 




450- 


-650 


137 


■199 


... 


- 


--- 


- 






11S/05E-32N 


M 200 


-350 


61- 


■107 


... 


_ 


..i* 


. 


X 






400- 


-600 


122- 


■183 


— 


- 


— 


- 






12S/04E-12R 


M 130 


-290 


40- 


-88 


..* 


. • 





. 


X 






400 


-600 


122- 


-183 


— 


- 


'" 


- 






12S/05E-08D 


M 200 


■300 


61- 


-91 





_ 


12S/05E-08D01 


S 


X 


X 




400 


-600 


122- 


-183 


— 


. 


— 


- 






12S/05E-16D 


M 200 


■ 300 


61- 


■91 


— 


. 


— 


- 


X 






400 


-600 


122- 


-183 


— 


. 


— 


- 






12S/05E-21J 


M 200 


■300 


61- 


■91 


— 


- 


— 


- 


X 


X 




400 


-600 


122- 


-183 














♦Type: S - 


Shallow well 




D - 


Jeep wel 1 




















M - 


Multiple-complet 


on well 

















132 



Appendix A 

BIBLIOGRAPHY OF 
GEOLOGIC AND GROUND WATER REFERENCES 



133 

10—82239 



APPENDIX A 
BIBLIOGRAPHY OF GEOLOGIC AND GROUND WATER REFERENCES 



Akers, J. P., 1977, Sources of Emergency Water Supplies in Santa 
Clara County, California: U. S. Geol. Survey Water- 
Resources Investigation 77-51, 21 p. 

Allen, J. E., 1946, Geology of the San Juan Bautista Quadrangle, 
California: Cal. Div. Mines Bulletin 133, 112 p., 2 maps 
1 :62,500. 

Averett, R. C. , Wood, P. R. , and Muir, K. S. , 1971, Water 

Chemistry of the Santa Clara Valley, California: U. S. 
Geol. Survey, Open-file Report, 24 p. 

Bailey, E. H., and Everhart, D, L. , 1964, Geology and Quicksilver 
Deposits of the New Almaden District, Santa Clara County, 
California: U. S. Geol. Survey Professional Paper 360, 
206 p., 18 plates, maps 1:24,000. 

Brown, R. D. , Jr., and Lee, W.H.K., 1971, Active Faults and 

Preliminary Earthquake Epicenters (1969-1970) in the Southern 
Part of the San Francisco Bay Region: U. S. Geol. Survey 
Misc. Field Studies, Map MF-307, 1:250,000. 

Clark, W. 0., 1917, Ground Water for Irrigation in the Morgan 

Hill Area, California: U. S. Geol. Survey Water-Supply Paper 
519, 209 p., 18 maps 1:125,000. 

Clark, W. O. , 1924, Ground Water in the Santa Clara Valley, 
California: U. S. Geol. Survey Water-Supply Paper 519, 
209 p. , 18 maps 1 : 125,000. 

Davis, F. F., and Jennings, C. W. , 1954, Mines and Mineral 

Resources of Santa Clara County, California: Calif. Jour. 
Mines and Geology, vol. 50, pp. 320-430, 1 map 1:125,000. 

Dibblee, T. W. , Jr., 1973, Preliminary Geologic Map of the Gilroy 
Quadrangle, Santa Clara County, California: U. S. Geol. 
Survey Open-file map, 1:24,000. 

Dibblee, T. W. , Jr., 1973, Preliminary Geologic Map of the Mt. 
Sizer Quadrangle, Santa Clara County, California: U. S. 
Geol. Survey Open-file map, 1:24,000. 

Dibblee, T. W. , Jr., 1973, Preliminary Geologic Map of the Morgan 
Hill Quadrangle, Santa Clara County, California: U. S. Geol. 
Survey Open-file Map, 1:24,000. 

Dibblee, T. W. , Jr., 1973, Preliminary Geologic Map of the 
Gilroy-Hot Springs Quadrangle, Santa Clara County, 
California: U. S. Geol. Survey Open-file Map, 1:24,000. 



135 



Dibblee, T. W. , Jr., 1973, Preliminary Geologic Map of the Mt. 
Madonna Quadrangle, Santa Clara and Santa Cruz Counties, 
California: U. S. Geol. Survey Open-file Map, 1:24,000. 

Division of Water Resources, 1933, Santa Clara Investigation: 
Bulletin 42, 271 p., 3 maps 1:125,000. 

Faye, R. E., 1976, Mathematical Model of the West Bolsa Ground 
Water Basin, San Benito County, California: U. S. Geol. 
Survey Water-Resources Investigation 76-71, 54 p. 

Ford, R. S., 1975, Evaluation of Ground Water Resources, Sonoma 

County: Department of Water Resources, Bulletin 118-4, Vol. 

1 - Geologic and Hydrologic Data, 177 p., 13 maps 1:350,000, 

2 maps 1 : 125,000. 

Ford, R. S., and Finlayson, D. J., 1974, Use of a Computer in the 
Delineation of Subsurface Features in a Ground Water Basin: 
Department of Water Resources, Technical Memorandum 52, 
24 p. 

Ford, R. S., Mitchell, W. B. , Jr., Chee , L. , and Barrett, J., 
1975, Evaluation of Ground Water Resources, South Bay: 
Department of Water Resources, Bulletin 118-1, Vol. Ill - 
Northern Santa Clara County Area, 133 p., 20 maps 1:140,000, 
2 maps 1:46,000, 1 map 1:316,800. 

Hansen, W. R., and Ford, R. S., 1967, Evaluation of Ground Water 
Resources, South Bay: Department of Water Resources, 
Bulletin 118-1, Appendix A: Geology, 153 p., 1 map 1:35,800, 
1 map 1:102,000, 1 map 1:125,000, 8 maps 1:140,000. 

Helley, E. J., 1967, Data for Observation Wells in San Benito 
County, California: U. S. Geol. Survey Open-file Report, 
36 p. 

Helley, E. J., and Brabb, E. E. , 1971, Geologic Map of Late 
Cenozoic Deposits, Santa Clara County, California: San 
Francisco Bay Region Environment and Resources Planning 
Study, Basic Data Contribution 27, 3 maps 1:62,500. 

Hem, J. D. , 1959, Study and Interpretation of the Chemical 
Characteristics of Natural Water: U. S. Geol. Survey 
Water-supply Paper 1473, 269 pp. 

Herd, D. G., and Helley, E. J., 1977, Holocene Lake San Benito, 

San Benito and Santa Clara Counties, California — Dammed by a 
Great Landslide on the San Andreas Fault Zone: Geol. Society 
of America Abstracts, p. 1012. 

Isgrig, D. , 1969, Soil Survey of San Benito County, California: 
U. S. Department of Agriculture, Soil Conservation Service, 
111 p. , 111 maps 1 :20,000. 



136 



Jenkins, 0. F. , 1973, Pleistocene Lake San Benito: California 
Geology, July 1963, pp. 151-163. 

Kapple, G. W. , 1979, Digital Model of the Hollister Valley Ground 
Water Basin, San Benito County, California: U. S. Geol. 
Survey Water Resource Investigations 79-32, 17 p., 11 maps 
1 :48,740, 1 map 1 :79,200. 

Kilburn, C. , 1972, Ground Water Hydrology of the Hollister and San 
Juan Valleys, San Benito, California: U. S. Geol. Survey 
Open-file report, 44 p. , 9 maps, 1:48,000. 

Lindsey, W. C, 1974, Soil Survey of Eastern Santa Clara Area, 
California: U. S. Dept. of Agriculture, Soil Conservation 
Service, 90 p., 48 maps, 1:24,000. 

Morgester, T. E. , and McCune, W. J., 1980, South Santa Clara 

Valley Ground Water Quality Investigation: Department of 
Water Resources Memorandum Report, 35 p., 6 maps 1:150,000. 

Nilsen, T. H., 1972, Preliminary Photointerpretation Map of 

Landslide and other Surficial Deposits of Parts of the Los 
Gatos, Morgan Hill, Gilroy Hot Springs, Pacheco Pass, Quien 
Sabe, and Hollister 15-Minute Quadrangles, Santa Clara 
County, California: U. S. Geol. Survey, Misc. Field Studies 
Map MF-416, 1 :52,500. 

Rogers, T. H. , and Williams, J. W. , 1974, Potential Seismic 
Hazards in Santa Clara County, California: California 
Division of Mines and Geology, Special Report 107, 39 p 
6 maps 1:62, 500 . 



• f 



Santa Clara Valley Water District, 1977, Ground Water Level Data, 
1924-1977. 423 pp. 

State Water Resources Board, 1955, Santa Clara Valley Investiga- 
tion: Bulletin 7, 154 p., 9 maps 1:140,000, 1 map 
1 :156,680. 

Taliaferro, N. L. , 1949, Geology of the Hollister Quadrangle, 
California Division of Mines Bulletin 143, map 1:62,500. 

Watts, W. L., 1890, Santa Clara County: California State Mining 
Bureau, Tenth Annual Report of the State Mineralogist, pp. 
609-618. 

Webster, D. A., 1972, Map Showing Ranges in Probable Maximum Well 
Yield from Water-Bearing Rocks in the San Francisco Bay 
Region, California: U. S. Geol. Survey Misc. Field Studies, 
Map MF-341 , 1 :250,000. 



137 



Webster, D. A., 1972, Map Showing Areas in the San Francisco Bay 
Region where Nitrate, Boron and Dissolved Solids in Ground 
Water May Influence Local or Regional Development: U. S. 
Geol. Survey Misc. Field Studies, Map MF-432, 8 p., 3 sheets, 
1:125,000. 

Williams, J. W. , Armstrong, C. F. , Hart, E. W. , and Rogers, T. H. , 
1973, Environmental Geologic Analysis of the South County 
Study Area, Santa Clara County, California: California 
Division of Mines and Geology, Preliminary Report 18, 41 p., 
2 maps 1 :24,000. 

Wright, R. H., and Nilsen, T. H., 1974, Isopleth Map of Land- 
slide Deposits, Southern San Francisco Bay Region, 
California: U. S. Geol. Survey Misc. Field Studies, Map 
MF-550, 1:125,000. 



138 



Appendix B 

GLOSSARY OF 
SELECTED GEOLOGIC AND HYDR0L06IC TERMS 



139 



Appendix B 
GLOSSARY OF SELECTED GEOLOGIC AND HYDROLOGIC TERMS V 



ftnticline . A fold, the core of which contains stratigraphical ly 
older rocks; it is convex upward. Cf, Syncline. 

ftquifer . A body of geologic materials that is sufficiently satu- 
rated and permeable to conduct ground water and to yield eco- 
nomically significant quantities of ground water to wells. 

Artesian . An adjective referring to ground water confined under 
some degree of hydrostatic pressure. 

Artificial recharge . The act of deliberately placing water 
underground. 



asalt . A dark-colored, fine-grained igneous rock, commonly of 
extrusive origin (i.e., ejected onto the surface of the earth). 



Basic intrusive rock . A group of dark-colored, crystalline igne- 
ous rocks having a relatively low sil ica content and emp laced at 
some depth below the surface of the earth. 

Bedrock . A general term for sol id rock that under 1 ies soil or 
other unconsolidated, surf icial material . 



Cell . A discrete onit, or part of a ground water model; of poly- 
gonal shape and containing a node at Its center. 

Chert . A hard, extremely dense sedimentary rock consisting domi- 
nantly of silica; it is tough and may be variously colored. The 
term "flint" is essentially synonymous. 

Clay . An earthy, extremely fine-grained sediment composed primar- 
ily of hydrous aluminum silicate minerals (i.e., montmoril- 
lonite, etc. ) ; grain size is less than 0.005 mm. 

Confined ground water . Ground water under pressure signifi- 
cantly greater than that of the atmosphere and whose upper 
surface is the bottom of a bed of distinctly lower permea- 
bility than the bed in which the water occurs . Cf . Unconf ined 
ground water. 

Conglomerate . A coarse-grained sedimentary rock composed of 
rounded fragments larger than 2 mm in diameter set in a fine- 
grained matrix of sand, silt, or natural cement. 

Consumptive (Jse . The difference between the total quantity of 
water withdrawn from a basin and the quantity of water returned 
to the source. It includes water transpired from plants, evapo- 
rated from the soil, and diverted from one watershed to another. 

Contact . The surface between two different types or ages of 
rocks. 

Continental origin . Said of geologic materials deposited on a 
continental mass as opposed to those deposited in an oceanic 
environmnent . Cf. Marine origin. 

Crystalline rock . A rock consisting wholly of crystals or frag- 
ments of crystals: e.g., an igneous rock. 



Fault contact . A contact between two different types or ages of 
rocks that is formed by a fault. 

Feldspar . A group of abundant rock-forming minerals belonging to 
the aluminum si 1 icate group. Feldspars are the most widespread 
mineral group and constitute 60 percent of the earth's crust. 
Orthoclase and plagioclase are two common feldspar minerals. 



which the 



Fine-qrained . (a } Said of a crystalline rock 

individual minerals have an average diameter of less than 1 mm. ; 
(b) Said of a soil in which silt or clay predominate. 

Flushed zone . A zone of geologic materials deposited under a 
marine environment and now containing fresh water. 

Fluvial ■ Produced by the action of a stream or river. 

Formation . The basic rock unit in the local classification of 
rocks, consisting of a body of rock generally characterized by 
some degree of homogeneity or distinctive features. Formations 
are combined into groups and subdivided into members. 



Geohydrology . A term referring to the hydrologic character- 
istics of subsurface waters. Often used interchangeably with 
hydrology. Synonym: Ground water geology. 

Gravel . An unconsolidated, natural accumulation of rounded rock 
fragments, consisting predominantly of particles larger than 
2 mm, such as boulders, pebbles, or cobbles; the unconsolidated 
equivalent to conglomerate. 

Greenstone . A dark green, compact altered basic to ultrabasic 
rock owing its color to sutSi minerals as chlorite and 
hornblende. 

Ground water . (a ) That part of the subsurface water that is in 
the zone of saturation ; (b) Loosely, all subsurface water as 
distinct from surface water. 

Ground water basin . A valley-like area underlain by permeable 
materials which are capable of furnishing a significant suppl y 
of potable ground water to wells. 



Ground water basi: 



gement . The planned use of a ground water 

basin as to yield, storage space, transmission capability, and 
ground water in storage. It includes: (1) Protection of natu- 
ral recharge and use of artificial recharge; ( 2 ) Planned varia- 
tions in amount and location of pumping over time; (3) Use of 
ground water storage conjunctively with surface water; and (4) 
Protection and planned maintenance of ground water quality. 

Ground water body . All ground water, whether unconfined or con- 
fined, contained within a ground water basin. 



Ground water divide . A i 
tiometric surface from 
directions. 



idge in the water table or other poten- 
which ground water moves away in both 



Ground water pumpage . The quantity of ground water pumped. 

Ground water subbasin . A discrete unit of a ground water basin. 

Gyps ifer ous shale . Shale containing significant quantities of 
gypsum, a hydrous calcium sulfate. 



Deep percolation . Precipitation or applied water moving downward 
below the root zone toward storage in the ground water body. 

Diabase . An intrusive igneous rock whose main components are the 
minerals labrador ite (a feldspar ) and pyroxene. 

Dip. The angle that a bed or a fault plane makes with the 
horizontal. 



Evapotranspiration . The combined loss of water through transpira- 
tion of plants and evaporation from the soil. 

Extrusive . Said of igneous rock that has been e]ected onto the 
surface of the earth. Extrusive rocks include lava flows and 
volcanic ash. Cf. Intrusive. 



H 



Head . (a) The pressure of a fluid on a given area, at a given 
point caused by the height of the fluid above the point; (b) The 
water-level elevation in a well, or elevation to which water of 
a flowing well will rise in a pipe extended high enough to stop 
the flow. 

Hydrologic Balance . An accounting of the inflow to, outflow from, 
and storage in a hydrologic unit; the relationship between evap- 
oration, precipitation, runoff, and the change in storage, 
expressed by the hydrologic equation; the hydrologic budget . 



Hydrologic equation 
budget; P = F + B 
transpiration, R 
storage (whether 



The equation that balances the hydrologic 

S, with P as precipitation, E as evapo- 
runoff, and S as change in ground water 
negative or positive). 



1/ Principal reference: American Geological Institute, Glossary 
of Geology, )977. 



141 



Igneous . Said of a rock that solidified from molten material. 

Impermeable . A condition of a geologic material that renders it 
incapable of transmitting significant quantities of water. 
Synonym: Impervious. Cf. Permeable. 

Indurated . Said of a compact rock or soil hardened by the action 
of pressure, cementation, or heat. 

Infiltration . The movement of surface water downward into a geo- 
logic material through its natural openings. Cf. Percolation. 

Intrusive . An igneous rock solidified from molten material below 
the earth's surface. Cf. Extrusive. 

Isohyet . A line connecting points of equal precipitation. 

Isohyetal map . A map showing isohyet contours. 



Lacustrine . Pertaining to, produced by, or formed in a lake 
environment. 

Lens . A geologic deposit bounded by converging surfaces (at least 
one of which is curved), thick in the middle and thinning out 
toward the edges , resembl inq a convex lens . 

Lenticular . Resembling a lens in shape, especially a double- 
convex lens. 

Limb . The side of a geologic fold. 

Lithic . Said of a medium-grained sedimentary rock containing 
abundant fragments of previously formed rocks. 



Perched . Unconfined ground water separated from the main ground 
water body by an unsaturated zone. 



M 



Marine origin . Said of geologic materials deposited in an oceanic 
environment as opposed to those deposited in an onshore condi- 
tion. Cf . Continental origin. 

Melange . A heterogeneous chaotic mixture of rock materials; 

specifically a body of deformed rocks consisting of fine-grained 
material thoroughly mixed with angular blocks of dissimilar 

materials. 

Member . A discrete portion of a formation distinguishable from 
adjacent parts of the formation by color, hardness, composition, 
or other features. A member may be subdivided into a number of 
beds. 



atural openings of 



Percolation . The flow of ground water through 
a geologic material. Cf. Infiltration. 

Permeable . A condition of a geologic material that renders it 

capable of transmitting a significant quantity of water without 

impairment of its structure. Synonym: Pervious; Cf . 

Impermeable. 

Physiography . A description of the surface features of the earth; 
synonymous with physical geography and comparable to 

qeomorphology. 

Piezometer . A facility emp laced to measure and record changes in 
ground water levels. 

Pillow basalt . An oceanic basalt characterized by discon- 
tinuous, cTose-f ittinq, pi 1 low-shaped masses ranging in size 
from a few centimetres to a metre or more in diameter. Pillow 
structures are considered to be the result of under-water 
volcanic action. 

Poorly sorted . Said of a sediment that is not sorted or that con- 
sists of particles of many sizes mixed together in an unsystem- 
atic manner so that no one size predominates. In engineering 
usage, equivalent to well -graded. Antonym: Well -sorted. 

Potentiometric surface . An imaginary surface representing the 
static head of ground water and defined by the level to which 
water will rise in a well. The water table is a potentiometric 
surface. Synonym: Piezometric surface. 

Precipitation . The discharge of water (as rain, snow, or hail) 
from the atmosphere upon the earth's surface. It is measured as 
a liquid regardless of the form in which it originally occurred; 
in a sense, it may be called rainfall. 

Primary opening . The original openings (pores, fractures, etc. ) 
created at the time that a particular geologic material was 
formed. Cf. Secondary opening. 



Quartz . Crystal 1 ine silica, an important rock- forming mineral . 
It is, next to feldspar, the commonest mineral . Forms the major 
portion of most sands and has widespread distribution in igne- 



ous, metamorphic, and sedimentary rocks. 



Metamorphic rock . Any rock derived from preexisting rocks by 
mineralogical, chemical, and structural changes, essentially in 
the solid state, in response to marked changes in temperature, 
stress, and chemical environment while at depth. 

Micaceous . Consisting of, containing, or pertaining to mica, a 
group of platy aluminum silicate minerals. 

Mudstone . An indurated mud having the texture and composition, 
but lacking the lamination of shale; a blocky or massive, fine- 
grained sedimentary rock in which the proportions of clay and 
silt are approximately the same. 



Node. The point within the cell of a mathematical nodel at which 
all conditions are assumed to occur ; the geometric center of a 



Oceanic volcanic rocks . Extrusive igneous rocks formed in a 
marine environment, commonly of basaltic composition. 

Orographic . (a) Pertaining to mountains; (b) Said of the precipi- 
tation that results when moisture -laden air encounters a moun- 
tain range. 



Recharge . The processes involved in the absorption and addition 
of water to the ground water body. 

Residual soil . A soil that has developed in place in the absence 
of any significant transport. 



Sag pond . A sm 
formed where 



ill body of water occupying an enclosed depression 
'ault movement has impounded drainage. 



Sand . (a) A rock fragment or particle in the range of 0.074 to 
4.76 mm diameter, and being somewhat rounded by abrasion in the 
course of transport; (b) A loose aggregate of mineral or rock 
particles of sand size predominantly composed of quartz; also a 
mass of such material, such as a t)each. 

Secondary opening . An opening (pore, fracture, etc. ) created in a 

geologic material some time after the material had been formed 
and caused by faulting, weathering, chemical solution, etc. Cf . 
Primary opening. 

Seep . An area, generally small, where water percolates slowly to 
the land surface; the flow is generally less than that of a 
spring. 



142 



S cont. 



Setniconf ined . A condition of an aquifer, or group of aquifers, in 
which ground water movement is sufficiently restricted to cause 
slight differences in head between differing depth zones during 
periods of heavy pumping and no head differences during periods 
of little draft. 

Sequence . A major informal rock group that is greater than a 
formation. 

Serpentine . A rock consisting almost wholly of serpentine-group 
minerals; e.g. , antigorite, chysotile, etc. , and derived from 
the alteration of previously existing Eerromagnesian minerals 
such as olivine and pyroxene. Synonym: Serpentinite. 

Shale . A fine-grained, indurated sedimentary rock formed by 
consolidation of clay, silt, and mud, and characterized by 
finely stratified structure that is parallel to the bedding. 

Shear . A surface along which differential movement has taken 
place. 

Silica-carbonate rock . A rock type developed through the 

alteration of serpentine; it is very hard and is composed of 
such minerals as quartz, dolomite, opal, and chalcedony. 

Silt . A particle in the size range between sand and clay, 
specifically between 0.005 and 0.075 mm. 

Siltstone . An indurated silt having the texture and compo- 
sition, but lacking the fine lamination of shale; a massive 
mudstone in which silt predominates over clay. 

Soil moisture. Water, or moisture contained in the soi 1 or root 



Transmissivity . The rate at which ground water is transmitted 
through a unit width of an aquifer or group of aquifers. This 
term replaces the former term "transmissibil ity'. 

Tuff. A compacted deposit of volcanic ash and dust that may con- 
tain up to 50 percent of other materials such as sand or clay. 
The term is not to be confused with tufa, a chemical sedimentary 
rock formed along certain lake shores. 



Unconfined ground water . Ground water that has a free water 
table: I.e., water not confined under pressure beneath rela- 
tively impermeable materials. Cf. Confined ground water. 

Unconformable . Said of strata that exhibit a substantial break or 
gap in the geologic record; i.e., a geologic unit that is 
directly overlain by another that is not the next in strati- 
graphic succession. A condition which results from a change 
that caused deposition to cease for a considerable span of time; 
it normally implies uplift and erosion with a loss of some of 
the previously formed geologic record. 

Unconsolidated deposits . A sediment that is loosely arranged or 
unstratif led, or whose particles are not cemented together. 

Upland ground water terrain . An upland area underlain by water- 
yielding materials and located adjacent to a ground water basin 
and possessing a high degree of hydrologic continuity with the 
valley floor. 



Spring . A place where water flows freely and naturally from a 
rock or the soil onto the land surface or into a body of water. 

Stratigraphic thickness . The thickness of a geologic unit mea- 
sured at right angles to the direction of extension of the unit; 
the thickness measured perpendicular to both the strike and dip 
of a unit. 

Stream capture . The natural diversion of the headwaters of one 
stream into the channel of another stream having greater ero- 
sional activity and flowing at a lower level ; diversion affected 
by a stream eroding headward at a rapid rate so as to tap and 
lead off the waters of another stream. 

Strike . The direction that the bedding or a fault plane takes as 
it intersects the horizontal. 

Stringer . A thin sedimentary bed. 

Subsoil . The soil below the surface soil or topsoil. 

Subsurface inflow . Ground water movement through the subsurface 
into a ground water basin. 



Subsurface outflow. 



ound water movement through the subsurface 



out of a ground water basin. 

Syncline . A fold, the core of which contains stratigraphically 
younger rocks; it Is concave upward. Cf . Anticline. 



Valley floor . The central portion of a ground water basin; an 
area of low-to-negligible relief suitable for agricultural or 

urban development . 

W 

Hater-bearing . The capability of a geologic material to yield 
supplies of ground water of potable quality adequate for most 
beneficial purposes. 

water quality . (a) The fitness of water for use; (b) Loosely, the 
chemical and biological characteristics of water. 

Water table . That surface in a ground water body at which the 
water pressure is atmospheric. It is defined by the levels at 
which water stands in wells that penetrate the water body ^ust 
far enough to hold standing water. 

Water year . 

Well constru 



October 1 to September 30. 

The physical characteristics of a water well; 



e.g. , method of dr 1 1 ling well , depth and diameter of casing, 
.depth and type of perforations, size and extent of filter enve- 
lope, length of well seal , etc. 



Zone of saturation . A subsurface zone in which all openings in 
the geologic materials are filled with water. Under most condi- 
tions, the upper surface of this zone is the water table. 



82239—950 4-81 700 



143 



CONVERSION FACTORS 



Quantity 



To Convert from Metric Unit 



To Customary Unit 



Multiply Metric 
Unit By 



To Convert to Metric 

Unit Multiply 
Customary Unit By 



Length 



Area 



Volume 



Flow 



Mass 

Velocity 

Power 

Pressure 

Specific Capacity 



millimetres (mm) 

centimetres (cm) for snow depth 

metres (m) 

kilometres (km) 

square millimetres (mm') 

square metres (m') 

hectares (ha) 

square kilometres (km') 

litres (L) 
megalitres 
cubic metres (m^) 
cubic metres (m^) 
cubic dekametres (dam') 

cubic metres per second (mVs) 

litres per minute (L/min) 

litres per day (L/day) 
megalitres per day (ML/day) 

cubic dek:,metres per day 
(damVday) 

kilograms (kg) 
megagrams (Mg) 

metres per second (m/s) 

kilowatts (kW) 

kilopascals (kPa) 

kilopascals (kPa) 

litres per minute per metre 
drawdown 



Concentration milligrams per litre (mg/L) 

Electrical Con- microsiemens per centimetre 

ductivity (uS/cm) 



inches (in) 


03937 


25 4 


inches (in) 


3937 


2 54 


feet (ft) 


3 2808 


3048 


miles (mi) 


62139 


1 6093 


square inches (in') 


000155 


645 16 


square feet (ft') 


10764 


092903 


acres (ac) 


24710 


40469 


square miles (mi') 


3861 


2 590 


gallons (gal) 


026417 


3 7854 


million gallons (10"^ gal) 


26417 


3 7854 


cubic feet (ft') 


35315 


0283 1 7 


cubic yards (yd^) 


1 308 


76455 


acre-feet (ac-ft) 


08107 


1 2335 


cubic feet per second 


35315 


028317 


(ftVs) 






gallons per minute 


26417 


3 7854 


(gal/mm) 






gallons per day (gal/day) 


26417 


3 7854 


million gallons 


26417 


3 7854 


per day (mgd) 






acre-feet per day (ac- 


08107 


1 2335 


ft/day) 






pounds (lb) 


2 2046 


45359 


tons (short. 2,0001b) 


1 1023 


90718 


feet per second (ft/s) 


3 2808 


3048 


horsepower (hp) 


1 3405 


746 


pounds per square inch 


14505 


6 8948 


(psi) 






feet head of water 


33456 


2 989 


gallons per minute per 


08052 


12419 



foot drawdown 

parts per million (ppm) 1 

micromhos per centimetre 10 



1 
1 



Temperature 



degrees Celsius (°C) 



degrees Fahrenheit (°F) 



(18 X °C) + 32 (°F-32)/l-8 



state of California— Resoui 
Department of Water F 

P.O. Box 388 

Sacramento 

95802 



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