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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
0 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 0 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
0
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
0 TO - I 0
-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 0 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
4000 0 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.
0
-
(feet)
o
1 1
CLAY
/
/
X
1-
Q.
CLAYISH SAND
Q CU
30-
SILTY CLAY
AND
CLAYISH SILT
CLAYISH SAND
0 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
\
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r
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V
\J
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A
C\l
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1
N
K
'
w
\j
t V
F
EAS
TOF
■lAYWA
WAIt
RDFA
R
JLT
<
>
(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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LEGEND
<i; /
/
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n CONTROL WELL
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O PIEZOMETER
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AXIS OF PROPOSED BARRIER
/ >^-*.
// '^>'
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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LEGEND
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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
0
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 0 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
0
1.7
11.7
1969-70
38.2
13.1
9.2
33.8
2.14
1.4
0
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
Co
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
Make
chec
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
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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.ij
^^
^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 ^ 0 ' ■'
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
0
1
MILES
1
1 1
1
?
i
0
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
0 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
0 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
0
0
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
0
0
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
0
0
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
0
0
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
0
0
7.00
240
73
66
606
245
255
78
50
15
7.00
260
79
67
451
183
250
76
0
0
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
0
0
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. 0
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. 0
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
0
0
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
0
0 .
70.0
86.3
46
26
29
7,333
2,235
2,917
889
0
0
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
0
0
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
0
0
300.0
370.1
95
33
32
5,167
1,575
11,042
3,366
0
0
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
0
0
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
0
0
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
0
0
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
0
0
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
0
0
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
0
-849
-169
-574
-445
-577
-267 -eee
-910
0
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
0 -32$
-727
99
-799
169
-459
-607
-1,000
0
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
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
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
0.14
0.22
0
0
0
0
0
0
0.03
0
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
0
0
0
0
0
0.37
0
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
0
0
1.75
0
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
0
2.04
0.42
0.39
0
1.41
7.12
E-5
2.8
0.37
0.07
0.87
0
0.93
0.20
0.18
0
0.65
3.28
E-6
4.7
0.62
0.12
1.45
0
1.56
0.32
0.30
0
1.08
5.45
E-7
3.4
0.39
0.15
0.89
0
1.08
0.43
0.22
0
0.68
3.84
E-8
9.6
1.16
0.43
2.52
0
3.05
1.23
0.63
0
1.92
in. 93
E-9
8.5
1.05
0.39
2.30
0
2. 78
1.12
0.57
0
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
0
35.79
2.21
3.32
0
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
0 . 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
0
0
2.09
0.59
6.74
5.88
1.66
6.15
1.05
0
0
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
0
0.50
0.14
1.62
1.41
0.39
1.48
0.25
0
0
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
0
0.63
0.18
2.02
1.77
0.50
1.85
0.32
0
0
0.77
0.22
2.48
31
1.15
0.32
1.20
0.20
0
0
0.50
0.14
1 .62
1.41
0.39
1.48
0.25
0
0
0.62
0.17
1.99
32
0.86
0.24
0.90
0.15
0
0
0.37
0.11
1.21
1.06
0.30
1.11
0.18
0
0
0.46
0.14
1.49
34
3.90
0.76
4.81
0.17
0.15
0
1.24
0.32
5.37
4.80
0.93
5.92
0.21
0.18
0
1.53
0.39
6.61
37
3.38
0.65
4.23
0.41
0.13
0
1.07
0.28
4.62
4.16
0.80
5.20
0.50
0.16
0
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
0.80
0
0
0.87
0.47
2.16
0.39
2.68
0
0.98
0
0
35
1 .03
0.54
2.53
0.47
3.15
0
1.16
0
0
1.27
0.66
3.11
0.58
3.87
0
1.43
0
0
37
0.13
0.07
0.32
0.06
0.40
0
0.15
0
0
0.16
0.09
0.39
0.07
0.49
0
0.18
0
0
38
0.98
0.52
2.42
0.45
3.01
0
1.11
0
0
1.21
0.64
2.98
0.55
3.70
0
1.37
0
0
40
0.53
0.12
1.06
0.10
0.72
0
0.27
0
0.40
0.65
0.15
1.30
0.12
0.89
0
0.33
0
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
0.20
0.07
0.03
0
0.11
0.06
0.02
0.16
0
0.25
0.09
0.04
0
0.14
44
0.10
0.04
0.26
0
0.40
0.14
0.06
0.01
0.21
0.12
0.05
0.32
0
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
0.20
0.07
0.03
0
0.11
0.06
0.02
0.16
C
0.25
0.09
0.04
0
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
0.91
0.23
4.29
3.86
0.66
4.93
0.42
0.18
0
1.12
0.28
5.28
36
1.74
0.30
2.27
0.19
0.08
0
0.51
0.13
2.38
2.14
0.37
2.79
0.23
0.10
0
0.63
0.16
2.93
37
2.71
0.46
3.53
0.29
0.13
0
0.78
0.20
3.68
3.33
0.57
4.34
0.36
0.16
0
0.96
0.25
4.53
40
0.34
0.05
0.46
0.03
0.02
0
0.09
0.02
0.47
0.42
0.06
0.57
0.04
0.02
0
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 0 29.19
0.98 2.73 1.35 1.02 1.08 1.43 0 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
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
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
0
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
0
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
0
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
0
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
0
-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
0
-58
-1,136
426
26
73
-263
907
-907
720
0
146
-665
55
27
-76
-933
1,196
-673
755
-297
-351
-671
585
28
-634
-713
2,006
-2,245
1,476
0
-642
-1,140
-581
29
-131
-275
941
-807
672
0
-93
-583
272
30
-193
-429
1,595
-2,259
1,290
0
-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
0
458
0
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
0
60
504
67
-67
. -193
0
395
0
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
0
894
-289
44
257
-381
1,060
260
-676
-649
-123
197
-364
45
-51
213
116
61
-61
116
0
-260
-49
46
181
188
196
0
0
-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
0
634
57
131
-86
317
-494
874
-390
-263
-113
512
58
126
-41
190
-417
766
-175
7
-335
324
59
142
0
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
0
56
-112
225
-84
-169
0
225
64
5
20
69
-127
235
-96
-17
-47
33
65
20
-20
80
40
159
-30
-418
318
60
66
0
0
-42
-211
211
-84
0
-42
126
67
-31
-61
184
-18
273
-107
-6
-40
135
68
41
68
-149
86
0
-4
3
-9
15
69
15
15
-168
168
46
-46
15
0
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
0
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
0
-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
0
-72
-1.401
828
26
90
-324
1,119
-1,119
888
0
180
-820
68
27
-94
-1,181
1,478
-830
931
-366
-433
-828
722
28
-782
-879
2,474
-2,769
1,821
0
-792
-1.406
-717
29
-162
-339
1,161
998
329
0
-118
-719
336
30
-238
-829
1,967
-2,786
1,891
0
-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
0
868
0
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
0
74
622
83
-83
-238
0
487
0
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
0
1.103
-386
44
317
-470
1,308
321
-834
-801
-182
243
-449
48
-S3
263
143
78
-78
-143
0
-321
-60
46
223
232
242
0
0
-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
0
782
87
162
-106
391
-609
1,078
-481
-324
-139
632
88
188
-81
234
-814
948
-216
9
-413
400
89
178
0
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
0
69
-138
278
-104
-208
0
278
64
6
28
88
187
290
-118
-21
-88
109
68
28
-28
99
49
196
-99
-816
392
74
66
0
0
-82
-260
260
-104
0
-82
188
67
-38
-78
227
-22
337
-132
-7
-49
167
68
81
84
-184
106
0
-8
10
-11
19
69
19
19
-207
207
87
-87
19
0
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 0 , 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
0
0
» 1
1567
/ :
H 1
l<ibS
0
" 1
l'»6)
0
D
I I
19T0
c
0
» 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 0
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
0 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)
0 03937
25 4
inches (in)
0 3937
2 54
feet (ft)
3 2808
0 3048
miles (mi)
0 62139
1 6093
square inches (in')
000155
645 16
square feet (ft')
10764
0 092903
acres (ac)
24710
0 40469
square miles (mi')
0 3861
2 590
gallons (gal)
026417
3 7854
million gallons (10"^ gal)
0 26417
3 7854
cubic feet (ft')
35315
0 0283 1 7
cubic yards (yd^)
1 308
0 76455
acre-feet (ac-ft)
08107
1 2335
cubic feet per second
35315
0 028317
(ftVs)
gallons per minute
0 26417
3 7854
(gal/mm)
gallons per day (gal/day)
0 26417
3 7854
million gallons
0 26417
3 7854
per day (mgd)
acre-feet per day (ac-
08107
1 2335
ft/day)
pounds (lb)
2 2046
0 45359
tons (short. 2,0001b)
1 1023
0 90718
feet per second (ft/s)
3 2808
0 3048
horsepower (hp)
1 3405
0 746
pounds per square inch
0 14505
6 8948
(psi)
feet head of water
0 33456
2 989
gallons per minute per
0 08052
12419
foot drawdown
parts per million (ppm) 1 0
micromhos per centimetre 10
1 0
1 0
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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ARE SUBJECT TO RECALL AFTER ONE WEEK.
RENEWED BOOKS ARE SUBJECT TO
IMMEDIATE RECALL
filiEJUN 2 0)387
VCD KBPARY
I'D JAN 2 3 2001
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