Monitoring results from the first Boston Harbor Navigation Improvement Project confined aquatic disposal cell

Monitoring Results from the
First Boston Harbor Navigation
Improvement Project Confined
Aquatic Disposal Cell

Disposal Area
Monitoring System

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DISPOSAL AREA MONITORING SYSTEM

Contribution 124
January 1999

US Army Corps
of Engineerse

.

New England District ee?
Det
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Form roved
REPORT DOCUMENTATION PAGE

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1. AGENCY USE ONLY (LEAVE BLANK) 2. REPORT DATE 3. REPORT TYPE AND
January 1999 DATES Final Report

4. TITLE AND SUBTITLE Monitoring results from the first Boston Harbor Navigation Improvement Project _ 6. FUNDING NUMBERS
confined aquatic disposal cell
6. AUTHORS f
PEGGY MYRE MURRAY

7. PERFROMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING
Science Applications International Corporation ORGANIZATION REPORT
221 Third Street
Newport, RI 02840 SAIC No. 443

9. SRPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES) 10. SPONSORING/
US Army Corps of Engineers-New England District MONITORING AGENCY

696 Virginia Rd DAMOS Contribution
Concord, MA 01742-2751 Number 124

Available from : DAMOS PROGRAM MANAGER Regulatory Division, USACE-
696 Virginia Rd

11. SUPPLEMENTARY NOTES

oncord MA 0

12a. DISTRIBUTION/A VAIABILTY STATEMENT 12b. DISTRIBUTION CODE
Approved for public release; distribution unlimited

13. ABSTRACT

As part of the overall Boston Harbor Navigation and Improvement Project (BHNIP), shipping berths 11 and 12
at Conley Terminal in South Boston were deepened to -40 ft and -45 ft MLLW, respectively, in June-July 1997. In
phasel of the BHNIP, fine-grained maintenance sediment, classified as unsuitable for open-disposal, was dredged and
placed into an in-channel confined aquatic disposal (CAD) cell in Boston Harbor. The cell was extracted into the
existing federal channel, below the BHNIP channel depth of -40 ft MLLW. Following placement of maintenance
material into the cell, sufficient sand to cover the dredged material with a minimum of 3 ft capping layer was placed
using split-hull scows.

A monitoring survey was conducted by SAIC in October 1997 to assess the status of capped CAD cell. Survey
methods included one day of vibracoring, and one day of acoustic surveying including bathymetry, subbottom, and
side-scan sonar. Results of the survey indicated that most of the CAD cell was covered with a highly variable
thickness of sand, while the southern end had little to no cap material. This distribution was consistent with the
positioning of the split-hull scows used to dispose the sand. Sand disposal was permitted during the outgoing
(southerly) tidal cycle. Prior to the initiation of the project, preliminary modeling of sand transport due to Boston
Harbor tidal currents predicted that sand would be transported to the south, so no barge was placed directly over the
southern end of the cell. The results suggested that the sand remained in the convective state during placement, so that
all the cap material was placed directly below each positioned barge.

The monitoring results also suggested that postcap operations designed to level the sand served to enhance
mixing of the cap and underlying dredged material, and resulted in uneven sand coverage. A final videosled survey
was conducted by C.R. Environmental in December 1997. These data confirmed the presence of a thick layer of sand
covered with tunicates and other organisms in most of the cell, and the flat, fine-grained uncapped mud surface to the
south. Overall, the maintenance material was successfully placed in the cell, and capped with sand in all locations
where capping barges were located. The results of monitoring of Phase I provided guidance for operational and
monitoring modifications for Phase II of the BHNIP in 1998-99.

14. SUBECT TERMS 15.NUMBER OF PAGES
pAWA\

Boston Harbor, South Boston, capping, confined aquatic disposal (CAD), videosled survey re =

17. SECURITY CLASSIFICATION OF REPORT 18. SECURITY CLASSIFICATION }19. SECURITY CLASSIFICATION 20.LIMITATION OF
UNCLASSIFIED OF THIS PAGE OF ABSTRACT ABSTRACT

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4

Monitoring Results from the First Boston Harbor Navigation
Improvement Project Confined
Aquatic Disposal Cell

CONTRIBUTION #124

January 1999

Report No.
SAIC 443

Submitted to:
Regulatory Branch
New England District
U.S. Army Corps of Engineers
696 Virginia Road
Concord, MA 01742-2751

Prepared by:
Peggy Myre Murray

Submitted by:
Science Applications International Corporation
Admiral's Gate
221 Third Street
Newport, RI 02840
(401) 847-4210

US Army Corps
of Engineerss

New England District

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TABLE OF CONTENTS

Page

IETS OR LA BIEE S22 Wk Piece te ccees coc Eee Aetna eA OL ae Ree aes Se ke MRR RS iV
NETS TMORGEIGUIRES ccrncencnecucecemecenbeiicse econ se cueus une nu neoee auciant Se eRe RRSER Eien eac era cree Vv
EE © WINE. SWIMINIAIRN Greene aincsecs cle cnc toes aa cnoanee sas soe ne nen Seb resect tecie as Vi
EO sUIN RO IDC CIION 6 hice etcetera or tai RENO ceed eter. loulncticte ae ics 1
1.1 The Boston Harbor Navigation and Improvement Project ...................... 1

iQ backeround tothe Conley erminalsbrojectan ase teree eee eee eee eee 4

2 Excavation ofthe;CAD.Gelle ii. uc eRe EOE ona cer 4
ip2eDeDredgediMaterial)Disposali@perationsies-) eee ee eee eee eee eee eee 4

1263). Capping Operations. 7y.c secs agai seseeensasdads aces Meee nee Etec ee 7

130 “CAD Celli Monitoring Surveys aes coer so esec te os tee es sceueeee see reiareasee 7

PRO @SURVENOME HOD Sic @ seen A: Aen etnitee en ae een ISR IN dances WM eh Me eras banat 9
Dn ANaVIGatlOM ee. ha.8 ae seasoieaned soadonees aolsetinsrana ts oor eacmes mae mesma se ase ince 9

DEIN NA DTACOLE; SUIVEW Race se ane ec reeralh Uaya ere ele he hae te tA vera Oe eee cree y)

DLA Corne Operations: esria an. Sales sat cei ee eteep tear cen ee ne seeeaerers 9

2) a Nibracore) Collectionsandy Erocessinger ee seeee eee eee eee ee eee eee eee 10

Desp VGNCOUSHC: SUEVEY* OpPeravlOnseisy sans tia ooy a aacomiseeeicne etre cee eee ane neers WW

poe eobathymetuc Data Collectionand’Analiysisens--es eee eee eee eee acces 12

2.3.2 Subbottom Data Collection and Analysis........................200eeee es 13

Dasy oe SidescanyData CollectionrandyAnalySiseeeeeseeeeee eee eee eee eee eee 14

DRA S NAGEOSIEUUSUEVEN a acide lice Hes sae eae raee cea cients et acne nen nets eer 14

SM OMIRIE SUOIERS oss eecte canro ne ete carte aectui teeta law ome ternctene scree eval te ergy Weta ao fas aera 15
BI ASVGl Sayer ol Soporte XAT [aenneouncehoannens dcoocsucdcemosesaspoodanadaaddhscomaosounete 15

322.) BathymetricdResullts..2 72s uit ens teed clgeeer orient ist tauinl a arate eee eras 15

Beso NaIbTACOLE RESUMES Aree aun eo tatare cere mane ae aed cc cenitete epee eictae isin eet rae 19

35455 SUDDOLMLOMPRES ULES ie eee ict ati eit eaneinar alia isle act De alata ettacteleseeae Aaa 25)

BHP VV TMCOMRESUMES He, catiatale aca lereisevouie eva wicvetata cia iieteval Leiber ge Sg eve pth tue tte en oe eral ak 29

AN OERPDIS © UWSSTON eee. yeas teeter ice weal a ce stir peatarae Career reteset Seto reat ere othe a ra 32
4.1 Topography and Texture of the Sediment Surface of the CAD Cell ......... 32

AD Thicknessiofithe: Sand (Capiicsnrs vccekse eect Noe acts eer ee ieee ec ae ee oar 32

4.3 Implications of Cap/Dredged Material Mixing ......................2eeeeee eee 313

4.4 Consolidation of the Maintenance Material.....................eeeeeeeeeee ee eee 34

4.5 Implications of Erosion from the Unsupported Cell Walls..................... 34

TABLE OF CONTENTS (continued)

Page

5.0. SUMMARY AND RECOMMENDATIONS ree. secnecceee sees scis-ertaae ict te eee eeere 36
Cyd eenme Wut roto oer HendrporiHecens daar Grondtondoanan ison osodsausosaacieccaqccuchsooness00. 26° 36

5.11. Sidescan is .2 3 cschs cual ddedenecee da dctsneds idee ste Meee ee ee oe hee eee 36

Sale Bath me tyne eee ee eet eeee eee eee acer eeecr ener crc eces eo eee ee eee eee 36

Syne STi Ole) ot Vase cewennaccob nan seodosonabancesoudndodadscnso ch dobudascadaseoosscacoces 36

5.1.4 Subbottomre gen. Qs eden ese ees ee, Se) eee Bi

5 .1-5.. Video.........Inaiets Jamies 2. We bees ae ee ee ee ES eae oF

5.2 _,..Recommendations.,-.<:o5..4. 149: 9h-0- oie ee Bee ee eaeee sok seis oeeeeeeee 37/

6.0, REFERENCES wisisscisicsis cod ntswsondlsiie ed douse sete «OES Ue Goer Mas. cnet ee eee Cee 41

APPENDIX

ili

Table 1-1.

Table 2-1.

LIST OF TABLES

Volumes of Material to be Dredged from Boston Harbor

Core; DatarSumimanyyeseene ewes ssece rine acccteee a eeeee emer!

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Figure 1-1.

Figure 1-2.
Figure 1-3.

Figure 1-4.

Figure 3-1.

Figure 3-2.

Figure 3-3.

Figure 3-4.

Figure 3-5.

Figure 3-6.

Figure 3-7
Figure 3-8

Figure 3-9

Figure 3-10

LIST OF FIGURES

Page
Boston Harbor Navigation Improvement Project location of main
Channels; toybeydned Sed ok seth Penn et sees. hs masz tea es igh bays Wires Ae e cee 2
Proposed location of in-channel confined aquatic disposal cells .................... 3
Location map of Phase 1 confined aquatic disposal cell........................00000 5)
Bathymetry of cell area following excavation, prior to disposal
@NiecksiMfarinesINAIDS Siete lacus Scene n uae alll ie listo oka DUAR err Uae iG reas 6
SiCle-Cezin Gomeir Mis OH SUMRIES Oi Cal .noadadéascasade ss ccocsedabeccobaodsdooneasdes 16
Bathymetry of cell area after completion of capping in a) July
(Weeks Marine); and b) October 1997 (SAIC; NAD83) ......................02... 18
3D view of postcap cell bathymetry in a) July (Weeks Marine);
and b) October 1997 (SAIC); view towards the southern end of
the Celle rast seren acne aetna cite oetich Rice eta nen te a ARAN DUNES ent TN SOUANERR MEER 20
Postcap bathymetry of cell collected in October 1997 (NAD83)
showing locations of cores and three cross Sections..............2...0.0eceee eee ees 21
Bathymetric transects along north-south lanes 6 and 7
(see Rigure SA yiman vated pie Mami ciiae Wall gS rs eee aah here rerun Mane 2D
Bhotographs of selectedicores»;scale mycmiepe qe hee eee 24
Subbottom'profile ofeane)Si(seeskisune3—4 in @ ye eee ee 26
Subbottomiprohleior Wanew/a(See iounes—4) penance eee eee eee eee 28
Video image of sand cap showing presence of tunicates and other
SDI S es see Ge ce es san MN ai era we Meta A en tr aR OU RE a 30
Videoumarerom Boston’Blue Clay icelliwallpeennss eee eee ee eee Sil

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EXECUTIVE SUMMARY

As part of the overall Boston Harbor Navigation and Improvement Project
(BHNIP), shipping berths 11 and 12 at Conley Terminal in South Boston were deepened to
MLLW -40 and -45, respectively, in June-July 1997. In phase 1 of the BHNIP, fine-
grained maintenance sediment, classified as unsuitable for open ocean-disposal, was
dredged and placed into an in-channel confined aquatic disposal (CAD) cell in Boston
Harbor. The cell was excavated into the existing federal channel, below the BHNIP
channel depth of -40 ft MLLW. Following placement of maintenance material into the
cell, sufficient sand to cover the dredged material with a minimum of a 3 ft capping layer
was placed using split-hull scows.

A monitoring survey was conducted by SAIC in October 1997 to assess the status of
capped CAD cell. Survey methods included one day of vibracoring, and one day of
acoustic surveying including bathymetry, subbottom, and side-scan sonar. Results of the
survey indicated that most of the CAD cell was covered with a highly variable thickness of
sand, while the southern end had little to no cap material. This distribution was consistent
with the positioning of the split-hull scows used to dispose the sand. Sand disposal was
permitted during the outgoing (southerly) tidal cycle. Prior to the initiation of the project,
preliminary modeling of sand transport due to Boston Harbor tidal currents predicted that
sand would be transported to the south, so no barge was placed directly over the southern
end of the cell. The results suggested that the sand remained in the convective state during
placement, so that all the cap material was placed directly below each positioned barge.

The monitoring results also suggested that postcap operations designed to level the
sand served to enhance mixing of the cap and underlying dredged material, and resulted in
uneven sand coverage. A final videosled survey was conducted by C. R. Environmental in
December 1997. These data confirmed the presence of a thick layer of sand covered with
tunicates and other organisms in most of the cell, and the flat, fine-grained uncapped mud
surface to the south. Overall, the maintenance material was successfully placed in the cell,
and capped with sand in all locations where capping barges were located. The results of
monitoring of Phase I provided guidance for operational and monitoring modifications for
Phase 2 of the BHNIP in 1998-99.

VI

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1.0 INTRODUCTION
1.1. The Boston Harbor Navigation and Improvement Project

The Boston Harbor Navigation and Improvement Project (BHNIP) involves
deepening of the main ship channel (in the Inner Confluence and the mouth of the
Reserved Channel), and three tributary channels (Mystic River, Chelsea Creek, and
Reserved Channel) in Boston Harbor (Figure 1-1). In addition to the channels, several
terminals and berth areas will also be dredged, for a total of 2.1 million yd* of material
(Table 1-1). All of the channels will be deepened to -40 ft MLLW, except for Chelsea
Channel, which will be dredged to -38 ft MLLW. BHNIP is a joint project between the
US Army Corps of Engineers, New England District (NAE) and the local sponsor, the
Massachusetts Port Authority (Massport). The first phase of the project, Conley Terminal,
was conducted in the summer of 1997.

Following extensive environmental review, the disposal options for both suitable
and unsuitable material were described in the Final Environmental Impact
Statement/Report (FEIS/R; NAE and Massport 1995). The plan included disposing the
unsuitable material in approximately 50 in-channel confined aquatic disposal (CAD) cells,
approximately 1.3 million yds’ of clean material will be dredged to create the cells
averaging 20 feet deep, dredged below the federal navigation channels in the Mystic River,
Chelsea River, and the Inner Confluence (Figure 1-2; NAE and Massport 1995; Demos
1997). Because of concerns over the environmental impact of such a large-scale project,
the state of Massachusetts negotiated for intensive environmental monitoring under the
auspices of the CWA 401 Water Quality Certificate (WQC; Babb-Brott 1997). The WQC,
granted by the Massachusetts Department of Environmental Protection, included the
stipulation that the unsuitable material must be dredged using an environmental (closed)
clamshell bucket, and capped by at least 3 ft of clean, granular material. One of the goals
of the WQC was to monitor the short and long-term integrity of the capped CAD cells, and
included review of all monitoring data by a state-sponsored Independent Observer (IO) for
the Coastal Zone Management Agency (CZM; ENSR, Acton, MA was selected as IO).
Survey results presented in this paper are only part of the extensive monitoring data
collected for the Conley Terminal project (ENSR 1997a,b; Section 1.2).

Concerns were raised by the BHNIP Technical Advisory Committee (TAC) prior to
capping that the density difference between the fine-grained maintenance sediment and the
coarse-grained sand cap would result in displacement of the cap, and that cap coverage
would be difficult to verify using bathymetric methods alone (ENSR 1997b). Verification
monitoring of the cell, in addition to the required monitoring by the WQC, was initiated in
the summer-fall of 1997. The survey data presented in this report were collected in order
to address these TAC concerns (Section 1.3).

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

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MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

Table 1-1
Volumes of material to be dredged from Boston Harbor

ieee meee) a sera ha ge pee
(cy) (cy)
Federal Channels 1,230,000 16,900 612,000 1,858,900

In-channel Cells* 1,300,000 1,300,000
Berths 95,000 181,500 276,500

2,625,000 16,900 793,500 3,435,400

*Assumes 54 cells as in Figure 1-2.

Chelsea River

re East Boston
-

Figure 1-2. Proposed location of in-channel confined
aquatic disposal cells.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

1.2. Background to the Conley Terminal Project

For the first phase of the BHNIP, an in-channel CAD cell was constructed for
containment of unsuitable dredged material from shipping berths at Conley Container
Terminal in South Boston by Weeks Marine (Camden, NJ). The dredged, fine-grained
sediments were disposed into the CAD cell and then capped with sufficient sand to cover
the deposit with a 3 ft thick layer of clean, granular material. Dredging and disposal
operations are summarized in the following sections.

‘

1.2.1 Excavation of the CAD Cell

The CAD cell, located in the main ship channel south of the Inner Confluence near
the East Boston shoreline (Figure 1-3), was designated Cell #2 in the FEIR/S for the
BHNIP project. The cell was excavated below the maximum channel depth anticipated for
Boston Harbor (40 ft MLLW) to an average total depth of 57.5 ft. First, the unsuitable
maintenance material from the cell area was removed and stored in a barge. Cell
excavation continued into Boston Blue Clay (BBC), a homogeneous, high strength greenish
gray clay with low water content and low permeability (CDM 1991). Bathymetric surveys
were conducted at all phases of cell construction and fill by the dredging contractor (Weeks
Marine; ENSR 1997a). Bathymetric data were provided from a survey conducted by
Weeks Marine following the dredging of the cell on 29 June 1997, and processed for
graphical purposes by SAIC (Figure 1-4). Results showed an irregular topography, with
depths of the cell floor that varied from minimum depths along the edges and in the north
central part (54-56 ft), to maximum depths in the SW corner (62-64 ft). The approximate
dimensions of the CAD cell were 500 ft long (north-south) by 200 ft wide (east-west).

1.2.2 Dredged Material Disposal Operations

Following the completion of the cell, the unsuitable maintenance material from both
the surface of the cell and from Conley terminal was placed in the cell from 29 June to
5 July 1997. Six gravity cores were collected throughout the cell by the NAE on 9 July
1997. The recovered cores ranged from 3.3 - 4.5 ft, and results showed that the majority
of the cores consisted of dark gray to black silt with a consistent sand component (13-
32%), and were relatively watery (moisture content 80-160%). The bottom 3-10 in of
each core consisted of gray clay (approximately 95% silt/clay) with low moisture content
(approximately 40%), consistent with the basement BBC.

Four bathymetric surveys were conducted after disposal of the unsuitable material (6, 7,
8, and 14 July). Results from the first postdisposal bathymetry survey (6 July) showed a
relatively uniform bottom within the cell with an average depth of 48.5 ft, resulting in an
average dredged material thickness of 9 ft (NAE 1997). These results indicated that the
material was relatively fluid and settled evenly over the cell floor. By the third survey (8 July),

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

Tobin Bridge

500 ft

Figure 1-3. Location map of Phase 1 confined aquatic disposal cell.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

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MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

the material had apparently consolidated approximately 2 ft. One week later (14 July), the
range of depths in the cell was still relatively narrow (approximately 47.5 - 50 ft), but the
deposit showed an average depth of approximately 49.5 ft, indicating an overall consolidation
of approximately 1 ft over a period of one week.

1.2.3 Capping Operations

Capping operations were conducted from 14 July to 25 July 1997. Sand was slowly
placed in the cell using a split-hull scow that was cracked open to slow the rate of material
deposition. Disposal operations were conducted only during the outgoing tidal cycle. The
scow was positioned using differential GPS (DGPS) in eight locations over the cell for a
total volume of 14,800 yd’ (Appendix; ENSR 1997a; NAE 1997). The sand was obtained
from Ossippee Aggregate. Grain size ranges of the sand were reported as dominated by
coarse sand (59%), with smaller fractions of gravel (15%), and fine-medium sand (23%;
grain size classifications from Wentworth 1922).

A bathymetric survey was conducted after seven loads of material were placed with
the scow oriented in north-south positions around the cell (Appendix). The bathymetric data
showed a large mound in the center of the cell. Modeling conducted prior to capping using
tidal current data predicted that the sand would spread towards the south (down current)
during sand disposal. The preliminary bathymetric data showing a flat, well defined
sediment surface was interpreted as indicating that the sand coverage in the southern end was
well distributed (resulting from settling from the southerly tidal current), but the northern
end of the cell was insufficiently covered. To rectify this, the contractor first unsuccessfully
used a sweep bar to even the coverage, and then used a clamshell bucket to redistribute the
material. Three 80-ft wide cuts were dredged from the central portion of the cell, and the
material was placed in the northern end of the cell. Following this operation, a final barge
of sand was placed in the cell; this scow was positioned along an east-west orientation in the
northern section of the cell for the final cap disposal event. Weeks Marine conducted a final
postcap bathymetric survey at the end of the project on 25 July 1997.

1.3. CAD Cell Monitoring Survey

Following completion of the cell, NAE and Massport were planning on accelerating
part of the monitoring required by the Water Quality Certificate. Coincidentally, samples
collected as part of a separate research project suggested that part of the cell contained
insufficient cap material (Shull and Fitzgerald 1997). Using this information and
recommendations of the technical advisory committee (TAC), NAE planned and
implemented an acoustic and coring survey as part of the Water Quality Certificate
monitoring to assess the success of capping of the first CAD cell, and to provide a resource
for operational and monitoring modifications for application to the remainder of the
BHNIP. In addition to the acoustic and coring survey, a follow-up video survey was

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

conducted by C. R. Environmental that confirmed some of the monitoring observations and
provided visual evidence of the surface of the CAD cell.

The acoustic and coring survey was planned and implemented to assess the success
of capping for two objectives of equal importance:

e to verify coverage of the first CAD cell and locate areas potentially requiring additional
cap material;

e to provide a resource for operational and monitoring modifications for application to
the remainder of the BHNIP.

The results of the monitoring survey presented here indicated that the majority of the
CAD cell was capped with a highly variable thickness of sand, but that the southern end had
little or no cap material. Postcap operations designed to level the sand cap appeared to have
resulted in highly uneven sand coverage, and potentially served to enhance mixing of the cap
and underlying dredged material. The acoustic data suggested that unsupported cell walls had
become less steep, and provided evidence that consolidated blocks of material had fallen from
the walls and settled on top of the dredged/cap material. Finally, the sediment placed in the
cell (maintenance material and cap) had continued to consolidate, resulting in a topography
that grossly mimicked the topography of the cell floor.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

2.0 SURVEY METHODS
2.1 Navigation

Vessel positioning and data integration were achieved with SAIC’s Portable
Integrated Navigation Survey System (PINSS) using a Magnavox 4200 GPS receiver. One
to 5-m DGPS accuracy was achieved to the GPS signals by applying corrections that were
acquired from the U.S. Coast Guard differential beacon located at Portsmouth, NH, using
a frequency of 288 kHz. During field operations, PINSS provided the navigator and vessel
operator with range and bearing to selected targets (1.e., beginning and end of survey
lines), signal quality, time of day, and selected data from environmental sensors including
the fathometer, subbottom towfish, and side-scan sonar. Core station and survey lane
positioning are discussed under the different survey operation descriptions below.

2.2 Vibracore Survey
2.2.1 Coring Operations

Target locations for the cores were selected in order to meet the goals of the
monitoring survey. The primary goal was to collect cores in potential areas of concern as
suggested by results of several surface sediment samples collected in the area of the cell
(Shull and FitzGerald 1997), including the southern end and the edges of the cell. In
addition, bathymetric data collected during the various phases of the project were used to
identify areas of potentially thinner cap. Therefore, the cores were collected beginning
from the southern end of the cell, and moving to the central and northern end of the cell as
the day progressed.

Long cores were collected with the goal of penetrating to the basement of the cell,
which consisted of Boston Blue Clay (BBC). With penetration into the BBC, a complete
stratigraphy (cap/dredged material/BBC) could be identified. The complete stratigraphy
was essential to determine if cap material had displaced dredged material during
operations. The final goal of core collection was to collect material to be used for
estimates of speed of sound in the layers of sediment detected by the subbottom data.

The sediment vibracoring survey was conducted on 9 October 1997. Cores were
collected using a two-vessel operation. PINSS navigation software and the GPS antenna
was configured on a workboat supplied by Boston Line and Service Co., Boston, MA.
Cores were obtained using a crane off of a 40 ft barge that was lashed to the workboat, and
anchored in a 2-point configuration. Actual core locations were calculated as a distance
and offset between the GPS antenna and the coring wireline.

An Aqua Surveys Inc. (ASI) electric motor vibracorer was used to acquire sediment
core samples. The corer was deployed off of the barge using the crane, and lowered to the

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

10

seafloor for vibracore collection. Two types of liners were used, flexible plastic liners that
were processed during the survey, and hard lexane liners (internal diameter of 3.5 in) that
were transported to a shore-based core processing facility (Section 2.2.2). Replicate cores
(one with a soft liner, one with a hard liner) were collected at each station. A variety of
core catchers and nose cones were used throughout the coring day to maximize sediment
recovery. In addition to personnel from SAIC, ASI, and Boston Line and Services, two
Massachusetts Institute of Technology (MIT) SeaGrant Program students were on-board, as
well as the independent observer (IO, ENSR) for the Massachusetts CZM.

2.2.2 Vibracore Collection and Processing

Sediment cores were acquired at a total of seven stations (Table 2-1). Two
replicates were collected at each station from CAD-1 through CAD-5. Replicate A from
each was collected in a flexible liner and processed on-board, and replicate B was collected
in a hard liner and relocated to a shore-based core processing facility. The final two core
locations, CAD-6 and CAD-7, were located in areas of thicker sand as suggested by the
several feet of sand coating the recovered barrel. Recovery of 6A was low and a loss of
material was indicated, so two soft liner cores were collected at this station (CAD-6A, 6B).
Two cores became stuck in the sediment, so that no core was recovered at CAD-6C and
CAD-7A.

Immediately following retrieval of the vibracore at each station, the flexible core
liners were placed in a core cradle that was pre-marked with a scale interval (cm), being
careful to keep the core oriented (top to bottom). The soft liner was split open with a
utility knife, and the core catcher was placed at the bottom of the core. Cores were
described and photographs were collected every 20 cm of the core. Samples were
collected by MIT student observers.

The replicate cores collected with a hard liner were removed from the core barrel,
carefully capped to prevent loss of sediment and/or water, marked with core numbers, and
“top” and “bottom” labels on the core, and stored horizontally for transport back to
laboratory coring facilities. Several cores were stored vertically for several hours because
the top of the core consisted of very fluid mud, and it was difficult to determine the
boundary of the sediment/water interface. After settling, these cores were re-cut at what
was determined to be the interface.

Cores were transported to the coring facilities at the Graduate School of
Oceanography, University of Rhode Island in Narragansett, RI. They were stored
horizontally in a core refrigerator for 4-7 days prior to processing. Cores liners were split
longitudinally using the GSO’s core splitter which uses two razors to score the outside of
the liner. The caps were cut using a utility knife, and then a piano wire was used to split
the core into two longitudinal halves. One half was described, and eight samples were

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

I]

Table 2-1

Core Data Summary
Core Total Liner Latitude Longitude Top

Name Length (cm) Type (cm)
42.37904 -71.04436

42.37902 -71.04442

42.37893 -71.04449

42.37895 -71.04448

42.37863 -71.04383
42.37869 -71.04395
42.37863 -71.04408

42.37869 -71.04408
42.37927 -71.04391

42.37925 -71.04400

42.37956 -71.04406

42.37958 -71.04385

hard 42.37960 -71.04379 Core not recovered.
long core 42.37958 -71.04407 Core not recovered.

DM = Black silty clay or clayey silt, industrial smell, assumed to be dredged material.

Sand = Brown medium to coarse sand, assumed to be sand cap.
Mixed = Black medium to coarse sand with silt component, assumed to be mix of above.
BBC = Boston Blue Clay.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

12

collected for potential future analysis of grain size. The other half was photographed and
archived in a refrigerated coring facility for potential future evaluation.

2.3 Acoustic Survey Operations

Survey operations were conducted aboard the survey vessel Cyprinodon (CR
Environmental) on 10 October 1997. Those involved in the survey included SAIC, the IO,
one MIT student, and two pilots from CR Environmental. The PINSS navigation system
was used for all navigation. Survey lanes were designed using the estimated locations for
the four corners of the cell (ENSR 1997a). Survey lanes for bathymetry (Section 2.3.1)
and subbottom (Section 2.3.2) were planned using the PINSS survey planning module to
cover the area of the cell and one survey lane outside in all four directions. PINSS
computed towfish position for the subbottom and side-scan sonar fish using a cable layback
calculation and provided this position to the data collection system. Side-scan sonar
operations were conducted following the bathymetry and subbottom surveys on the same
day (Section 2.3.3).

2.3.1 Bathymetric Data Collection and Analysis

Depth soundings were collected with an Odom DF3200 Echotrac® survey
echosounder using a 208 kHz transducer with a 3° beam angle. The Odom simultaneously
displayed water depth data on a chart recorder and transferred the digital sounding data to
the PINSS. The echosounder collected 6-8 soundings per second and transmitted an
average value to the PINSS at a rate of one sounding per second. Depth soundings were
collected along pre-configured survey lanes with 15 meter spacing in both E-W (14 lanes)
and N-S (8 lanes) orientations.

A Seabird Electronics, Inc. Model SBE 19-01 conductivity-temperature-depth
(CTD) profiler was used to acquire a vertical profile of sound velocity in the water column
during the day. These data were used to correct the bathymetry data for speed of sound
during post-processing.

Using SAIC’s Hydrographic Data Analysis System (HDAS), bathymetric soundings
were edited for outliers and corrected for sound velocity, transducer draft, and tidal
variation. Tidal data from the Boston Harbor tide station (station #8443970 located near
the Northern Avenue bridge) were obtained from the NOAA Ocean and Lakes Levels
Division (OLLD) web-server (http://www.olld.nos.noaa.gov). Following the application
of all correctors, the depth soundings were spatially averaged to produce a grid of cells.
The gridded bathymetric data were used to produce the various topographic maps included
in this report.

All graphics have been plotted in NAD83 latitude/longitude coordinates. Depth
values are relative to Mean Low Water (MLW) in order to compare with data provided by

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

13

Weeks Marine. Water depths are indicated with specific contour intervals noted on the
figures. Both contour plots and three-dimensional relief plots were produced using Golden
Software’s Surfer® program; a vertical exaggeration has been added to the 3D plots shaded
relief in order to highlight small topographic gradients.

2.3.2 Subbottom Data Collection and Analysis

High resolution subbottom profile data were acquired with an Edgetech X-Star™
Model 216S Full Spectrum Digital Subbottom Profiler. Subbottom profile data were
collected simultaneously with bathymetry, and therefore along the same survey lanes
(Section 2.3.1). Subbottom seismic profiling is a standard technique for determining the
presence of sediment layers below the sediment/water interface. The X-star system emits a
swept-frequency pulse; the frequency of the transmitted pulse changes linearly with time,
and is therefore called a chirp system. The depth of penetration and the degree of
resolution is dependent on the frequency and pulse width of the seismic signal, and the
characteristics of the penetrated material.

The narrow beam (13°) transducers of the X-Star system, mounted in a towfish,
were lowered using the winch aboard the survey vessel Cyprinodon, and trailed the vessel
by approximately 15 m. The X-Star system generated a frequency-modulated pulse that
was swept over an acoustic range of 2 to 10 kHz during the subbottom survey. The pulse
rate was set to 6 pulses per second for optimum performance of the output devices. At 6
pulses per second, traveling at an average vessel speed of 4-5 knots, a subbottom
measurement was acquired every 34-43 cm along the vessel track. The return signals were
transmitted via a data cable through an analog to digital (A/D) signal converter to an on-
board Sun Sparc II Workstation for data display and archive. Data were stored on Exabyte
tapes, and continuous profile data were printed on an Alden thermal printer.

Penetration of sound in sediment is both a function of system frequency and the
impedance contrast between the water column and sediment. Acoustic impedance, the
product of velocity and density of sound in a layer, is also affected by differences in
surface roughness, porosity, and grain size, among other factors (Hamilton 1970; LeBlanc
et al. 1992). In general, sound penetrates further into fine-grained sediment because the
impedance of high-water content silt and clay is closer to that of the water column. The
ability to detect subbottom layers is similarly dependent on the acoustic impedance contrast
between sediment layers. Subbottom has been used to accurately map the lateral and
vertical coverage of a sand cap over dredged material because of the contrast between the
sand cap and underlying fine-grained dredged material (e.g., Murray et al. 1994a).

Subbottom layers were not digitized due to the difficulty in identifying continuous
reflectors below the surface of the dredged material. The thermal paper printouts were
scanned and several representative sections are included in this report. Although depths
are shown in the figures, these are not reliable for estimating actual layer thicknesses or

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

14

water depth. First, the depths are estimated using 1500 m/s as the speed of sound; the
speed of sound will be higher in sand and the well-consolidated, homogeneous BBC
(approximately 1700 m/s; Hamilton 1971). In addition, one lane was digitized to compare
the subbottom depths with bathymetry, and the scale printed on the subbottom cross section
was found to be inaccurate.

2.3.3 Sidescan Data Collection and Analysis

A Marine Sonics PC Scan side-scan sonar system was used for side-scan sonar data
collection. This system is a single frequency (300 kHz) system that collects digital data
directly to a PC-compatible computer. Bottom coverage was 100% for each pass. Several
N-S passes were made with the cell on one channel, and the best image was processed
using Adobe Photoshop® to enhance the visual resolution and add annotations. The image
was not geo-registered, so that the aspect ratio of the cell (width vs. length) has been
compressed in the vertical (N - S) direction because of survey vessel speed.

2.4 Videosled Survey

On 3 December 1997, CR Environmental, Inc. (CR) performed a towed video sled
survey at the CAD in Boston Harbor to demonstrate the effectiveness of using high
resolution underwater video in detecting the coverage of the sand cap. The CR
Underwater Video Sled is equipped with a high resolution Sony Hi8 video camera, and two
250 watt Deep Sea Power lights with variable light output. The system has a 100 meter
cable, portable monitor and VCR, and isolated power for shock protection.

The operations were performed from CR Environmental’s 32-foot survey vessel
Cyprinodon equipped with a hydraulic winch, A-frame and a large enclosed pilot house for
survey equipment. A Northstar DGPS and the Coastal Oceanographics’ HY PACK
navigation software package were utilized to provide the boundaries of the confined
disposal cell and a pre-programmed set of tracklines. The layback of the video sled to the
navigation antennae was estimated to be approximately 50 ft. By examining the real-time
video display, the vessel captain adjusted the towing speed and the pay out of the tow cable
to achieve the best towing angle and bottom coverage. Two north-south transects were
made through the disposal cell. Operations were performed at slack low tide and water
visibility was estimated as 5 to 10 ft.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

15

3.0 RESULTS

Results from the monitoring surveys are summarized in this section. First, the
surface topography and texture of the cell was measured using side-scan sonar (Section
3.1) and bathymetry (Section 3.2). The video data were useful in corroborating the
surface sediment topography and recolonization, and are incorporated throughout the
presentation of results. Next, the information gathered that documented layer thicknesses
within the cell, including core (Section 3.3) and subbottom (Section 3.4) results, is
presented.

3.1 Side-scan Sonar Results

The side-scan sonar data over the cell area showed distinct acoustic regions of the
cell (Figure 3-1). The southern end of the cell consisted of a uniform low reflectance area
interpreted as a flat surface. Strong backscatter from the central and northern portions of
the cell indicated a harder bottom, and a rough, uneven topography. Throughout the
central portion of the cell, clamshell bucket markings were noted clearly, with dark
shadows indicating strong changes in topography that we termed artificial “sand waves”
for further discussion. Two of the three dredge cuts (NAE 1997), aligned east-west,
showed individual ridges caused by clamshell bucket passes. The third most northern cut
was partially obscured because of the sand that was disposed in the northern part of the cell
after dredging (Section 1.2). Spud marks from the dredge were noted along the northern
edge.

Although the eastern edge of the pit was in shadow, there was a bright spot in the
lower southeast corner indicating a topographic feature, consistent with bathymetric results
(Section 3.2). This hard acoustic return (dark crescent) suggested that it was present above
the ambient sediments to rise above the edge shadow, and hard enough for a strong
acoustic return. Further interpretation of this feature is discussed below.

The final distinctive feature of the side-scan sonar results was the scalloped edges of
the cell. The shape of the cell walls still appeared to reflect much of the original clamshell
markings from the excavation. Although sequential bathymetric surveys indicated some
changes in the cell wall orientation, the sidescan results indicated that the BBC was firm
enough to retain much of the original dredged topography.

3.2 Bathymetric Results

Weeks Marine provided two bathymetric datasets for evaluation of the survey data,
including the survey conducted after the excavation of the cell prior to any disposal
(predisposal survey; 29 June 1997), and the final postcap survey conducted at the end of
the project (25 July 1997). The surveys were conducted using different survey parameters:
the predisposal survey was conducted over a large area with east-west lanes, at 50 ft lane

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

pate

_ Striations —
~~) Se & <

SS

i ‘Sand Cap SS

ie 2 se

Figure 3-1. Side-scan sonar image of surface of cell.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

17

spacing (Figure 1-4). The postcap survey was conducted at higher spatial resolution (25 ft
lane spacing), but over a smaller area. The October bathymetric survey collected by SAIC
was conducted using a 15 m lane spacing (approximately 45 ft), and over both east-west
and north-south lanes. Because of the different survey parameters, calculating differences
in depth by the normal procedure of subtracting equal grids over replicate areas was not
reliable, so therefore, qualitative differences between the surveys are discussed below. In
addition, comparing the depths collected around the cell (that presumably did not change)
showed variation in depth of +2 ft among all of the surveys. This variability is due to
different survey parameters, different vessels (with potentially different depths of the
transducer), and potential error due to sea state. These errors would be much reduced, and
allow for electronic survey comparison, if the surveys were conducted using exact replicate
survey parameters and equipment. All three surveys have been corrected to MLW for
comparison purposes.

The predisposal baseline bathymetric survey showed ambient water depths ranging
from 5 to 40 ft in the area around where the cell was excavated (Figure 1-4). Water
depths in the cell itself prior to disposal ranged from 56-64 ft MLW. The cell walls were
fairly irregular, showing scalloping along the edges remnant from the clamshell
excavation. The deepest part of the cell was in the southwestern corner, and there were
several topographically higher areas along the edges and in the center of the cell.

The postcap survey conducted after completion of the project (Figure 3-2a) showed
steep, almost vertical, western and eastern cell walls (lanes were not extended to the
southern and northern ends of the cell). Depths ranged between 46-50 ft, with the
shallowest depths in the center of the cell consistent with the areas of dredge-induced sand
waves shown by the side-scan sonar data. The southern end of the cell was relatively flat
(49-50 ft). The average thicknesses of dredged and cap material, therefore, was
approximately 10 ft, with a range of thicknesses over the variable topography of the cell
floor of approximately 6-14 ft.

The survey conducted in October by SAIC resulted in somewhat less steep cell
walls, and a more irregular topography on the floor of the cell (Figure 3-2b). Overall, the
bottom of the cell appeared to increase in depth by 1-4 ft (i.e., the bottom of the cell was
deeper) over most of the cell during the period between the July and October postcap
surveys. Some areas north and south of the sandy peaks from the final postdisposal survey
appeared to remain relatively constant between the two surveys (49-51 ft, blue areas).
Considering the approximate 1-2 ft of error between the two surveys, the maximum
consolidation that could be confidently estimated was at least 2 ft in the far southwestern
corner of the cell and in the center of the cell below the thick sandy peaks.

Conservatively, this resulted in an overall consolidation of 10-20% since the final postcap
survey, concentrated in the dredged material as sand is relatively incompressible.
Including the consolidation prior to capping, the total consolidation of the dredged material
ranged from 20-40% during the four months past the initial capping. This value was

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

18

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MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

19

within a reasonable range, as prior studies of dredged material have shown volume
reductions of up to 50% for fine-grained materials attributable to consolidation (e.g.,
Poindexter-Rollings 1990).

Viewing the data from a three dimensional perspective, the bathymetric data
suggested that the slope of the cell walls had become less steep over the 10 weeks between
surveys (Figure 3-3). Several areas appeared to show slumping of the wall itself (note far
northwestern corner). Sequential side-scan sonar datasets collected during the disposal
phase also suggested the outline of the cell wall changed through time (ENSR 1997a).
Because the clamshell dredge that was used to create the cell left a sawtooth pattern along
the cell walls, it is likely that the dredging process weakened parts of the wall and material
sloughed or calved from the walls into the cell. Because Boston Blue Clay is relatively
firm (e.g., CDM 1991), this process will likely not continue indefinitely, but until a stable
slope for BBC material has been reached. The force of ship propellers along the piers may
have also tended to weaken the unsupported walls, especially along the eastern wall of the
cell.

An anomalous topographic peak in the southeastern corner of the cell was noted
several feet higher than was measured in the previous postcap survey in July (Figure 3-2).
The location of the peak was consistent with the hard reflector seen in the side-scan sonar
data, and was also seen in the subbottom results (Section 3.4). As discussed below, this
deposit may be material that ended up overlying the uncapped dredged material, possibly a
remnant of material fallen from the cell wall.

Two N-S transects of raw depth soundings from the October survey (not gridded
data) were extracted from the data to produce cross sections of bathymetry. Lanes 6
(central cell) and 7 (eastern cell) were selected (Figure 3-4). The data were plotted to
show the transition from the rough surface associated with the re-dredged sand cap to the
smooth surface noted in side-scan sonar data. The results showed that the rough, uneven
surface slopes down to the smooth surface, and the transition is approximately '4 of the
distance from the southern end of the cell (Figure 3-5). Note that north and south are
reversed between Lanes 6 and 7, and are plotted relative to the direction of the boat transit.
The difference in topographic expression was investigated further with vibracore data.

3.3. Vibracore Results

A total of 12 cores at six stations were recovered, with core lengths ranging from
45-241 cm (Table 2-1; Figure 3-4). In all, a total of 14 cores were attempted; two core
liners became stuck (Cores CAD-6C, 7A; Figure 3-4). Cores collected from CAD-2A and
2B appeared to be outside of the cell. The recovery of 72-105 cm of apparent dredged
material, however, suggested that the cores were actually collected in the cell. One of the
CAD-2 cores showed evidence of BBC on the top drive motor, indicating the core was

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

Postcap Bathymetry (25 July)

Figure 3-3. 3D view of postcap cell bathymetry in a) July (Weeks Marine); and
b) October 1997 (SAIC); view towards the southern end of the cell.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

42.3798| = \ ie

42.3796—

42.3794-

42.3792-

42.37905

Depth (ft, MLV)

42.3788-

N

42.3/86—

42.3784—

OE C‘(
0 15 30 45 60
Meters

42.3782» i 3 :
71.0448 -71.0444 ~—-71.0440

I
-71.0436

Figure 3-4. Postcap bathymetry of cell collected in October 1997 (NAD83) showing locations of cores
and three cross sections.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

bo
bo

Depth (ft MLW)

Depth (ft MLW)

Lane 6

Relative Distance Along NS Lane

South

Relative Distance Along NS Lane

Figure 3-5. Bathymetric transects along north-south lanes 6 and 7 (see Figure 3-4). Note that

south is to the right on lane 6 and to the left on lane 7.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

collected very near the cell wall. Core recovery was greater in the cores collected
throughout most of the day in the southern area and along the edges where there was little
to no sand. The outside of the barrels of the last two cores which were not recovered
because of stuck core barrels (6C and 7A) returned covered with sand, indicating that core
recovery was hampered by thick sand intervals.

There were three distinct units recovered in the cores. The most common was a
black silty clay/clayey silt with a strong hydrocarbon odor. The texture ranged from very
watery silt to highly consolidated, low water content firm clay. The second end member
unit was a brown medium to coarse sand. Finally, several cores penetrated into a
continuous interval of well consolidated BBC. When the core penetrated to refusal into the
BBC material, we assumed the core had penetrated into the bottom of the cell. Comparing
total dredged material thicknesses as recovered in the cores with bathymetry suggested that
core recovery was hampered by loss or compaction of dredged material during the coring
process. In Table 2-1, a fourth unit was described as “Mixed,” which was commonly
medium to coarse sand with a component of black watery silt material, apparently from the
dredged material unit. These units were described in order to estimate the potential
magnitude of mixing between sand and dredged material.

The dredged material unit was mottled with BBC in many cores, and in one core
(CAD-SA), a solid 15 cm interval of BBC was recovered above the dredged material. The
presence of BBC in the dredged material was consistent with reports from on-site
inspectors. In addition to BBC, discrete sandier intervals were also noted in the dredged
material. Data from cores collected prior to capping indicated that the dredged material
consisted of up to 30% sand. For classification purposes, if the sand was a minor
component of the dredged material, and disseminated throughout an interval, it was
classified as dredged material. If the unit was dominated by medium-coarse sand but was
infused with black watery silt material, it was classified as mixed.

The two replicate cores collected at CAD-2, CAD-3, and CAD-4 recovered no
sand, and all but CAD-4B were cored to refusal in BBC. These results indicated that no
sand was placed in the southern section of the cell. Cores CAD-1A and 1B (Figure 3-6)
were collected near the sand/mud boundary, and the results indicated that, near the sand
cap boundary, limited mixing of sand and mud resulted in interleaved layers of sand and
mud in these cores. The subbottom data also indicated that a wedge of sand near the edge
of the sand cap may have been intermixed with the more fluid mud, hampering acoustic
differentiation between these lithologies (Section 3.4).

Cap thicknesses among the four cores where sand cap was clearly recovered at the
top of the core (CAD-5A, 5B, 6A, and 6B) ranged from 20-28 cm. In cores CAD-5A and
5B, the boundary between the sand cap and the underlying material was sharp and clearly
delineated due to the presence of consolidated clays below the sand (Figure 3-6). The
stratigraphy was uncertain at core CAD-6A (45 cm of recovery), because material was lost

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

oll

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MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

ZS)

from the bottom of the core during operations. Recovery was better for core CAD-6B,
where 20 cm of sand overlay 60 cm of mixed sand and dredged material, indicating a
maximum recovered thickness of mixed material of almost 2 ft.

The loss of 6C and 7A with evidence of thick sand so close to 6A and 6B indicated
a high variability of sand thicknesses. Coring data indicated that mixing of sand and mud
of up to 2 out of 3 ft occurred in areas of highly variable sand thickness. This mixing was
probably enhanced, and possibly caused, by the force applied to the sand during postcap
dredging operations. The consolidation state of the dredged material prior to capping, as
shown in Cores CAD-5A and 5B, however, contributed to the presence of a clear
sand/dredged material interface. The potential for increasing the consolidation state of the
material prior to capping is discussed further in Section 4.0.

3.4 Subbottom Results

Two north-south lanes (Lanes 5 and 7) were selected to show the results of
subbottom data (Figure 3-4). Lane 5, through the central portion of the cell, was
representative of most of the subbottom results (Figure 3-7). The view is a cross section
through the sediment, showing acoustic reflectors below the sediment/water interface
where changes in lithology (acoustic impedance) occurred. As discussed in the Methods,
the depth markings on the subbottom records do not accurately represent actual depths or
thicknesses of the cell lithologies.

Outside of the celi area, a series of horizontal reflectors throughout the harbor
resulted from the natural geology of Boston Harbor. These sediments are a combination of
BBC and glacial till (material left after a glacier melts) that were deposited in a nearshore
marine environment that existed in the Boston area during an interglacial period about
18,000 years ago (CDM 1991 and references therein).

In the area of the cell, much of the subbottom acoustic information reflective of the
natural geology of the material was lost, indicating no sound penetration to depths below
the dredged and cap material. This result is not surprising, as prior acoustic work over
dredged material has indicated that the acoustic signature of dredged material is distinct
because of sound loss due to scattering and refraction, indicative of the heterogeneous
nature of the deposit (Bokuniewicz et al. 1976; Schock et al. 1992; Murray et al. 1995).
Two distinct acoustic regions were consistent with side-scan sonar data. In the southern
area of the cell, the top reflector was a strong, smooth reflector indicating a smooth surface
with little topography to scatter sound. The high amplitude of the reflector indicated either
a harder surface (not borne out by coring data), or a very flat surface. Below this surface
reflector, there was a very homogeneous layer as indicated by few internal reflectors, and
there is a clear reflector from the base of the cell.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

CEs ay tee

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No.21162 Edgelech Wo.21412 EdgeTech No. 21662 Edgetech No.24912 Edgetech Wo.22162 Et

Time: 14:32:27 Time:14:33:10 Time314:33:52 Tiae344394:35 Tine:14:95:47
Date:410/10/1997 Ine Date:10/10/1997 Inc Date:10/10/1997 Ine Date:10/10/1997 In Date:10/10/1997
Lat:42°22.826'H ; Lat:42°22.792'H ; Lat:42°22. 755‘ : Lat:42°22,718"N 4 Lats42°22.682'H
Lon:71°2.644°9 Lon:71°2.650°H Lon:71°2.645'H Lon:74°2.646'9 Lon:71°2,645°8

Course:150 = File: 1 Course:176 Fille: 1 Course:i81 File: 1 Course:189 File: 1 Course:165 = Fi
Speed:2.7 Rec: 14800 © Speed:3.1 Rec: 15050 Speed:3.2 Rec: 15900 Speed:3.2 Rec: 15550 Speed:3.4 Rei

Figure 3-7. | Subbottom profile of Lane 5 (see Figure 3-4).

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

In the central and northern portions of the cell, there was a marked change where
the surface became highly irregular as the sound was reflected from the dredge marks
shown by side-scan sonar data. Although the surface was very rough and uneven, the
amplitude (strength) of the surface reflector was still strong, indicating a relatively hard
surface consistent with sand. Below the surface, there was a series of discontinuous
internal reflectors indicating a more heterogeneous deposit. Throughout this layer there
were u-shaped reflectors that indicated refraction off of irregular deposits. The bottom of
the cell was not a continuous reflector, most likely because most or all of the sound was
lost in the sand-capped area. This is because at each acoustic boundary where sound was
reflected or scattered within the sand deposit itself, less energy was left to continue
downward penetration.

The subbottom data indicated that approximately 25% of the cell was uncapped.
There was a transition zone between the sand-capped area, and the fine-grained, uncapped
area in the south along the N-S cross section (Figure 3-7). Comparing results from cores
CAD-1A and 1B near the boundary and the bathymetric transects, it was apparent that
there is an interval of transition where sand and mud was interleaved. The reflector along
the peak of sand at the southern end of the capped section appeared to dip down towards
the south, and was overlain by the mud in that transition. Because the subbottom cross
section shown are uncorrected for speed of sound, the transition area between the capped
and uncapped portion of the cell should not be interpreted as the actual stratigraphy, as the
speed of sound in the sand (and BBC) is faster than in the fine-grained, heterogeneous
dredged material. Without digitizing and correcting for speed of sound, the data can still
be used to make qualitative conclusions about the interval between the capped and
uncapped areas. Because of the weight of the thick sand layer in the central portion of the
cell, in combination with the force applied to the central sand cap by the postcap dredging
operation, the boundary was most likely characterized by deformed layers of sand and
mud.

The farthest eastern N-S lane (Figure 3-4), Lane 7, showed two interesting features
(Figure 3-8). First, the cell bottom was a relatively continuous reflector below the cap and
dredged material. This indicates that the material was more homogeneous, so that sound
could penetrate further to the bottom of the cell. In the sand capped area, however, there
was still no sand/mud reflector. Results from cores CAD-6B showed a thick mixed
boundary between the sand and mud. These data suggested that the method of delineating
cap thickness using subbottom will not be effective if there is a mixed zone that is greater
than the depth resolution (wavelength) of the system. If there is a consistent sand/dredged
material boundary (as in CAD-5A and 5B, Figure 3-6), subbottom will be more effective
in determining cap thickness.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

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Figure 3-8. | Subbottom profile of Lane 7 (see Figure 3-4).

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

Finally, the southern end of Lane 7 showed an apparent mound of material in the
southeast corner of the cell overlying the uncapped mud with a clear acoustic reflector in
between. This mound was consistent with the topographic peak that showed up in both the
bathymetric and side-scan sonar data. This finding has important implications. The
subbottom data were consistent with the bathymetric data that suggested material had
settled on top of the dredged material after capping was completed. The fact that the
material appeared to have settled on top of the uncapped material suggested that the
bearing strength of the material had increased from July to October, and was potentially in
an advanced state of consolidation that would increase the success of final capping of the
material. Further geotechnical testing of the uncapped material by investigators at MIT
will be continuing to address the issues of consolidation and bearing strength.

3.5 Video Results

The videosled survey that was conducted following the monitoring survey
confirmed the acoustic data in two north-south transects. The video data provided clear
evidence of sand in the north and central portions of the cell, showing tunicate-covered
sand waves with dramatic topography (Figure 3-9). Audio data provided additional
evidence as the sled was dragged and scraped through the sand. The transition from the
sand cap to the uncapped dredged material was apparent primarily from the additional
resuspended material associated with the videosled movement through the far southern end
of the cell, but also from the change in sound. In general, water clarity was good
throughout the cell, contrasting with the suspended sediment present in the ambient Boston
Harbor sediments above the edge of the cell. During the second pass of the sled, the
tracks from the first pass were noted in both the sand and mud areas, indicating a relatively
consolidated surface of the dredged material.

In addition to the surface coverage of sand and mud, the video captured images of
ex situ material deposited on top of the dredged/cap material deposit, including entrapped
debris and blocks of high-reflectance BBC (Figure 3-9). The blocks of BBC probably
were remnant from material loosened from the cell walls, similar to the evidence provided
by the acoustic data. The presence of debris indicated that material had become entrapped
below the ambient current of Boston Harbor and settled on top of the cap. Inferences of
the physical environment also could be drawn from the benthic community (tunicates,
lobsters, etc.) that inhabited the cell at the time of the video survey. Tunicates, especially,
tend to attach to hard bottoms (associated with ship fouling), and are filter feeders. The
presence of tunicates on the sand cap indicated that the sand provided sufficiently hard
substrate, within a fairly quiescent environment. Finally, video data collected along the
cell walls confirmed that the steep walls were scalloped, primarily from the dredging
process (Figure 3-10). The BBC walls also had begun to be colonized by a variety of
burrowing organisms.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

30

Boston Blue Clay

Piece of Brick

Crab Tunicates

Debris

Figure 3-9. Video image of sand cap showing presence of
tunicates and other debris.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

31

Boston Blue Clay;*
Base of Cell Wall

Shrimp Holes

Hydroids

Shrimp Holes

Figure 3-10. Video image of Boston Blue Clay cell wall.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

4.0 DISCUSSION
4.1. Topography and Texture of the Sediment Surface of the CAD Cell

The presence of two distinct acoustic regions was consistent among all of the data
collected within the CAD cell. The acoustic data, in tandem with cores collected
throughout the cell, indicated that the northern and central portions of the cell were
covered with a coarse sand. Throughout the central portion of the cell, clamshell bucket
markings were noted clearly in the sidescan data, resulting in the appearance of sand waves
that confounded core recovery in the thicker layers of sand. The distribution of sand
across the surface of the cell also was clearly delineated in subbottom data. In that area of
the cell, the subbottom reflectors apparent in the ambient Boston Harbor sediments were
dissipated, indicating no effective sound penetration to depths below the dredged and cap
material. Below the surface of the central and northern portions of the cell, there was a
marked change in subbottom penetration, where there was a series of discontinuous
internal, u-shaped reflectors that indicated refraction from the heterogeneous sand wave
deposit. Clear evidence of sand on the surface was supported by video data, that showed
tunicate-covered sand waves with dramatic topography (Figure 3-9), as well as audio
evidence as the sled was dragged and scraped through the sand.

In the southern end of the cell, the acoustic and coring data indicated that no sand
was present, as all of the cores recovered in the southern end (except CAD-4B) recovered
dredged material to refusal in BBC. In the southern area of the cell, the sediment/water
interface was a high amplitude reflector in the subbottom data caused by the flat,
featureless fine-grained cell surface. Below this surface reflector, there was a very
homogeneous layer as indicated by few internal reflectors, and the base of the cell was
noted as a continuous subbottom reflector along the southern portion of the cell. In
combination, these results indicated that no sand was placed in the southern section of the
cell.

The lack of sand in the southernmost area of the CAD cell was attributed to the
placement of the split hull scows used for capping. Modeling conducted prior to capping
using tidal current data predicted that the sand would spread towards the south (down
current). Monitoring data suggested, however, that the majority of the sand, although
released slowly from the barge, was released convectively so that the areal coverage was
more limited than predicted. The distribution of barges over the cell indicated that no
barge was ever placed directly over the southern end, resulting in a lack of sand in this
area.

4.2 Thickness of the Sand Cap

The second major result of the monitoring survey was that the central and northern
areas of the disposal cell were covered in sand, but the thickness was unevenly distributed.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

33

Sand thickness appeared to be most variable in the area impacted by postcap dredging
operation. As noted in the side-scan sonar data, clamshell bucket operations resulted in a
topography similar to sand waves, so that the resulting cap thicknesses were highly
variable. Sand cap thicknesses among the four cores where sand cap was clearly recovered
at the top of the core (CAD-5A, 5B, 6A, and 6B) ranged from 20-80 cm of sand or mixed
sand and mud (Table 2-1). The inability to collect cores at 6C and 7A with evidence of
thick sand so close to 6A and 6B indicated a high variability of sand thickness. A mass
balance approach was used to approximate the variable thicknesses of the deposit.
Assuming that the total volume of sand was deposited over, conservatively, 75% of the
cell, the average thickness would be 4 ft. The minimum cap thickness measured was
approximately 1 ft (26 cm), so the range of cap thicknesses could vary from 1-7 ft.

4.3 Implications of Cap/Dredged Material Mixing

One of the lithological units recovered in the cores was described as mixed, which
had the appearance of the sand cap (medium to coarse sand), but with a component of
black watery silt material, apparently from the dredged material unit. Coring data
indicated that mixing of sand and mud of up to 2 ft occurred in areas of highly variable
sand thickness, indicating that mixing was enhanced, and possibly caused, by the force
applied to the sand during the postcap dredging operations.

The consolidation state of the dredged material prior to capping, as shown in cores
where sand overlay more consolidated clay, contributed to the presence of a clear
sand/dredged material interface (Figure 3-6). These data suggested that the more
consolidated the dredged material was prior to capping, the less mixing occurred.
Maximizing consolidation of the dredged material prior to capping, thereby increasing the
bearing strength of the dredged material and reducing the interval over which sand and
mud are mixed, therefore, has several advantages, including:

e Increasing the protection of benthic organisms by maximizing the “effective
cap” above the zone of advective flux of potentially contaminated pore waters
into sand (e.g., Murray et al. 1994b);

e Reducing the volume of sand that has to be placed to ensure 3 ft of coverage;

e Maximizing the efficiency of the subbottom profiling technique to detect the
cap/dredged material boundary, which has provided the widest spatial coverage
of cap confirmation data to date;

e Increasing the overall long-term stability of the deposit.

The bathymetric results suggested that consolidation has continued throughout the
cell, in both the sand-capped and uncapped areas. The critical parameter of consolidation

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

34

was identified from the monitoring results; specifically, the time necessary to allow for
self-weight consolidation of dredged material.

4.4 Consolidation of the Maintenance Material

The overall depth of the cell appeared to increase by 1-4 ft during the 10-week
period between the bathymetric data collected immediately after capping in July 1997, and
in October 1997. The sequential bathymetric data were used to calculate an overall
consolidation of at least 2 ft in the far southwestern corner of the cell, and in the central
cell below the sand wave deposits. Conservatively, this resulted in an overall consolidation
of the dredged material from initial deposition to 10 weeks after capping of 20-40% .
Previous data collected on dredged material consolidation indicated that this value is
probably a minimum, considering volume reductions of up to 50% for fine-grained
materials have been attributed to consolidation (e.g., Poindexter-Rollings 1990). More
accurate estimates of the rate of consolidation within the CAD cells will be useful for the
second phase of the BHNIP (Section 5.0).

The acoustic data consistently indicated the presence of a large block of material in
the southeast corner of the cell overlying the uncapped dredged material (Figure 3-8).
Although no samples were collected from the area of this topographic anomaly, the most
logical conclusion is that a large block of BBC fell from the cell wall and settled on top of
the dredged material. Although this is highly conjectural, the inference can be drawn that
the bearing strength of the uncapped material had increased by the October survey to
support the overlying block of material. If this is the case, sometime during the 10-week
period between the completion of the dredging project and the October monitoring survey,
the material developed sufficient self-weight consolidation to optimize capping.

4.5 Implications of Erosion from the Unsupported Cell Walls

The side-scan sonar data from this report and from prior data collection efforts
(ENSR 1997a), as well as the bathymetric and subbottom data, suggested strongly that the
slopes of the unsupported cell walls have become less steep, and material has fallen from
the walls into the pit. The video captured images of the surface of the cell showing blocks
of BBC on top of the CAD cell cap (Figure 3-9). Because the clamshell dredging process
that was used to create the cell walls left a sawtooth pattern along the cell walls, it is likely
that the dredging process weakened parts of the wall resulting in sloughing or calving from
the weakened walls. Video data collected in the cell confirmed that the steep walls are
scalloped, potentially increasing the chance for erosion of the cell walls (Figure 3-10).

Boston Blue Clay has a high strength and is relatively firm (CDM 1991), indicating
that the erosional process will not continue indefinitely, and may be limited to areas that
were weakened during the dredging process by the clamshell bucket. In addition, spud
marks on the side-scan image were close to the edge of cell, potentially contributing to

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

35

sloughing. Because the walls remained unsupported (unfilled cell) at the time of this
survey, however, these erosion processes may continue, especially on the eastern side of
the pit because of the impact of vessel propeller wash.

For Phase 2 of the BHNIP, the cells will be filled up to ambient seafloor depth
(channel depth), so that this result does not affect the dredging and monitoring plans. For
the Phase 1 CAD cell, the interesting implication of the monitoring data is that material
will continue to settle into the cell, from both weakened cell walls and entrapped sediment
and debris. During Phase 2 surrounding areas will be deepened by 5 to 7 feet decreasing
unsupported cell wall height. This suggests a sedimentation rate that is more rapid than the
surrounding Boston Harbor seafloor, making the estimation of overall sand cap thickness
more difficult to assess as time progresses. Ultimately, dependent upon the sedimentation
rate and the rapidity of erosion of the remaining cell wall, the dredged material will be
covered to ambient depth with sediment. This very thick layer of sediment, be it sand,
BBC, or ambient fine-grained sediment, will provide ample containment for the dredged
material placed at the bottom of the cell.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

5.0

Sell

5.1.2

SUMMARY AND RECOMMENDATIONS
Summary
Sidescan

e The southern end of the cell had low reflectance relative to the rest of cell,
indicating a smooth cell bottom and homogenous grain size;

e There was an acoustic bright spot in the southeast corner indicating a
topographic peak lying above the shadow created by the edge of the pit;

e The central portion of the cell showed a strong topographic signature from
clamshell dredging, resulting in artificial sand waves throughout the center of
the cell;

e Rough texture of the surface of the northern end of the cell indicated sand cover,
but was less disturbed than central section.

Bathymetry

e Consolidation of at least 1-2 ft in some areas, especially in the southwestern
corner and in the center of the cell below the thickest sand layers, continued
between the final postcap survey conducted in late July, and the postcap
bathymetric survey conducted 10 weeks later in early October;

e Total consolidation of the dredged material since the time of placement was
estimated to be 20-40%;

e Evidence for material falling from the unsupported cell walls included the
irregular cell outline and flatter slope, as well as the anomalous topographic
peak in the southeast corner;

e The dredging process, as well as propeller action along the eastern side and
spudding, likely weakened parts of the cell wall, enhancing the potential for
material slumping;

Coring

e Cores indicated that the southern end of the cell was uncapped;

e The artificial sand waves in the central portion of the cell resulted in widely
varying sand thicknesses over short distances;

e A sharp cap/dredged material boundary was present over consolidated dredged
material;

e The estimated range of thickness of the sand in the capped area was estimated to
range from 1-7 ft;

e Potential mixing of up to 2 ft of the cap into the dredged material in one core
was probably enhanced by the postcap dredging operation, but may also be a
result of underconsolidated dredged material prior to capping;

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

37

e Loss or compaction of dredged material resulted in underestimated total unit
thicknesses above BBC.

5.1.4 Subbottom

e In the southern area of the cell, a sharp seafloor reflector indicated a smooth
surface with little topography to scatter sound;

e In the central and northern portions of the cell, the surface was highly irregular
and very rough, coincident with the sand cap and sand wave area;

e Below the surface in the sand-capped area, a series of discontinuous internal
reflectors indicated a more heterogeneous deposit, and most or all of the sound
was lost in the upper portion of the deposit;

e The data suggested that delineating cap thickness using the subbottom method
will not be effective if there is a mixed zone that is greater than the resolution of
the acoustic system;

e New material from the cell wall appeared to have settled on top of the uncapped
material, suggesting that the strength of the material has increased and is
potentially in a state of consolidation sufficient to support even large sized cap
materials with minimal mixing.

5.1.5 Video

e Video data confirmed the presence of thick sand waves in the central and
northern parts of the cells, and the transition from the sand cap to the uncapped
dredged material was apparent both in audio and video;

e Further evidence of consolidation of the uncapped material was suggested by the
presence of sled tracks noted during the second pass of the sled;

e The video captured images of ex situ material deposited on top of the
dredged/cap material deposit, including entrapped debris and BBC, indicating
that material had become entrapped below the ambient current above the cell;

e The presence of tunicates on the sand cap indicated that the sand provided
sufficiently hard substrate, within a fairly quiescent environment;

e The video data collected along the cell walls confirmed that the steep walls were
scalloped, primarily from the dredging process.

5.2 Recommendations

Results from all of the environmental monitoring surveys were discussed among the
project proponents and the BHNIP TAC. Recommendations to modify the requirements
for dredging and disposal operations, and potential alterations to the environmental
monitoring approach, were considered. The recommendations briefly described below
were summarized from the October monitoring survey report (Murray 1997), and
preliminary recommendations from the project proponents as summarized by the CZM

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

38

Independent Observer (ENSR 1997b). The recommendations were designed around the
primary concerns raised by the dataset, including lack of spatial coverage of sand, variable
thicknesses of sand, and potential mixing between sand and dredged material. The
recommendations are intended as practicable methods of improving the CAD capping
process for Phase 2 of the BHNIP and other projects.

e Operational Control During Capping

The primary source of both uneven spatial coverage and variable cap thicknesses
was the method of sand placement in the first CAD cell. For Phase 2, operations will be
designed to improve placement of the material, as well as increase the ability to diffuse the
sand while capping. In addition, physical disturbance of the sand cap after placement will
be minimized.

e Increase Bearing Capacity of Dredged Material Prior to Capping

Increasing the consolidation time for the fine-grained maintenance sediments will
increase the bearing capacity of the material, increasing the likelihood for a sharp
cap/dredged material boundary. The Phase 1 monitoring studies provided no clear
guidance for the time required for sufficient consolidation, as there was no hard evidence
that the consolidation time for Phase 1 (9 days) was insufficient. The time allowed for
consolidation partially will be governed by the potential risk of resuspension of this
material. Future geotechnical studies will be conducted to address this issue for further
projects.

The most cost-effective method to increase bearing capacity is to allow more time
for self-weight consolidation to take place. The subbottom data provided an estimate of
the maximum time necessary for the dredged material to be able to support an overlying
load, because of the apparent ability of the uncapped dredged material to support the mass
of material at the southeast corner of the cell. The topographic peak appeared sometime
between the last postcap survey in July and the October survey, providing a maximum
waiting period of 10 weeks. The actual waiting period may be shorter (perhaps much
shorter depending on when the block fell or could have been supported), and will also
depend on the geotechnical character of the actual dredged material as it is placed. This
waiting period will be established through careful monitoring of the first few cells.

Another method to increase the bearing capacity of the dredged material is to reduce
the water content of the material prior to disposal in the cell. Dewatering, however, is
expensive and time-consuming. Use of the environmental clamshell bucket increased the
water content of the material, ultimately decreasing the strength of the placed dredged
material. The final potential method for increasing the bearing strength of the material
prior to placement is to phase the capping process. By adding additional material to the

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

39

top of the dredged material (additional dredged material, cap material, BBC), the strength
of the material will increase and improve the ultimate success of final capping.

e Confirming Cap Coverage and Thickness

The basic methods used for this survey can be successful in determining cap spatial
coverage and thickness in tandem with improved operational methods. Modifications to
these procedures are summarized for the individual types of monitoring.

Bathymetry. Bathymetry proved to be a useful tool to monitor overall consolidation of the
project. The advantage of the Boston Harbor project is that the underlying BBC is
relatively incompressible, so that any difference in height of the material can be attributed
to consolidation of the dredged material itself. It is imperative, especially in small areas
like the CAD cells, to replicate survey parameters so that electronic depth differencing
between surveys can be conducted. Typically, consolidation will occur rapidly at first, and
then slow and become more gradual (e.g., Poindexter Rollings 1990). At the point the
consolidation curve begins to flatten is the optimum time to begin capping from a strength
perspective. Bathymetry can be used as a tool during dredging, therefore, to allow
qualitative analysis of the consolidation state of the material.

Using bathymetry to estimate cap thickness is problematic, however, because of further
consolidation of the dredged material after cap placement. Use of subbottom (below) will
allow a more comprehensive and accurate acoustic assessment of cap thickness (Murray et
al. 1994a).

Coring. Coring has the advantage of actual, visual evidence of the state of the cap. It is
limited, however, to measuring in only discrete points so that, with uneven coverage, it
may provide an inaccurate picture of the overall cap. Coring can be used to evaluate the
boundary between cap and dredged material, so that the ability of subbottom (below) to
detect the overall coverage of the cap can be evaluated. Vibracores are required for this
type of operation.

Subbottom. As discussed above, if the material is sufficiently consolidated to minimize
mixing between the sand and cap, subbottom is the most promising method to evaluate
both the spatial coverage and overall thickness of a sand cap. It becomes less useful if the
sand is unevenly placed so that sound is lost at the surface, and if the mixed interval
exceeds the depth resolution of the subbottom frequencies.

Sidescan. Sidescan data were essential in Phase 1 because of the unique topography of the
sand waves. If the sand cap is uniformly placed throughout the cell area, sidescan will
show the uniform deposit but will need groundtruth data (cores or video) to document the
lithology.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

oe sete oan ec en ee ee ee ee
int ee ESN IA Se ee a

Video. The use of the videosled data proved useful to visually document the status of the

cap. Because the area of the cells is small, video can provide relatively good coverage at a
reasonable cost.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

4]

6.0 REFERENCES

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large estuary, Long Island Sound.” Est. Coast. Mar. Sci., 4:523-536.

CDM. 1991. “Environmental consequences of utilizing Boston Blue Clay in landfill
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Submitted to Massachusetts Dept. of Public Works, Camp Dresser & McKee, Inc.,
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Demos, C. J. 1997. “The federal environmental review process.” Conference Proceedings
of Coastal Zone ’97, Vol. I., pp. 187-189.

ENSR. 1997a. “Summary report of independent observations Phase 1 - Boston Harbor
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ENSR. 1997b. “Summary of capping investigations.” BHNIP Independent Observer
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Hamilton, E. L. 1970. “Reflection coefficients and bottom losses at normal incidence
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Hamilton, E. L. 1971. Prediction of in-situ acoustic and elastic properties of marine
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LeBlanc, L. R., L. Mayer, M. Rufino, S. Schock, and J. King. 1992. “Marine sediment
classification using the chirp sonar.” J. Acoust. Soc. Am., 91(1):107-115.

Murray, P. M., M. B. Wiley, D. Pabst, and B. May. 1994a. “Subbottom profiling of a
sand capped disposal mound.” Jn: McNair, E.C., Jr. (ed.), Proceedings of the
Second International Conference on Dredging and Dredged Material Placement,
Vol. I. Lake Buena Vista, FL, Nov. 13-16, 1994. Amer. Soc. Civ. Eng., New
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42

Murray, P. M., D. Carey, and T. J. Fredette. 1994b. “Chemical flux of pore water through
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FL, Nov. 13-16, 1994. Amer. Soc. Civ. Eng., New York, pp.1008-1016.

Murray, P. M., J.T. Morris, A. J. Silva, and T. J. Fredette. 1995. “Swept-frequency
(chirp) subbottom analysis of a shallow-water dredged material deposit in Long
Island Sound.” SAIC Report No. 342. Submitted to U.S. Army Corps of
Engineers, New England Division, Waltham, MA.

Murray, P. M. 1997. “Postcap monitoring of Boston Harbor Navigation Improvement
Project (BHNIP) Phase 1: assessment of inner confluence CAD cell.” Submitted to:
U.S. Army Corps of Engineers, New England District, Waltham, MA. SAIC
Report No. 413, SAIC, Newport, RI.

NAE (U.S. Army Corps of Engineers, New England Division [now District]) and
Massport (Massachusetts Port Authority). 1995. “Boston Harbor, Massachusetts
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NAE (U.S. Army Corps of Engineers, New England District). 1997. “Bathymetric survey
results at disposal cell, Conley Terminal dredging project.” MPA Contract No.
3.288D. Prepared for the Massachusetts Port Authority, Boston, MA.

Pace, S. D., D. L. Dorson, and S. E. McDowell. 1998. “A surveillance system for
dredged material disposal.” Submitted to Ports 98, Long Beach, CA, March 1998.

Poindexter-Rollings, M. E. 1990. Methodology for Analysis of Subaqueous Sediment
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Station, Vicksburg, MS 110pp.

Schock, S. G., D. J. Kieth, D. L. Debruin, E. Dettmann, and G. Tracey. 1992. “Utilizing
a chirp sonar to accurately characterize newly deposited material at the Calcasieu
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IEEE, 26-29 October 1992, Newport, RI.

Shull, D. and FitzGerald, S. 1997. “Follow-up survey results of disposal cell 1
(8/28/97).” Report prepared for Massachusetts Institute of Technology Sea Grant

College Program.

Wentworth, 1922. A scale of grade and class terms of clastic sediments. J. Geol., 3:377-390.

MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL

APPENDIX

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