Capping survey at the New London disposal site, February 3, 1989

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Capping Survey at the Jan 1490
New London Disposal Site 7
February 3, 1989

Disposal Area
Monitoring System
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Contribution 71
January 1990

US Army Corps
of Engineers

New England Division

CAPPING SURVEY AT THE
NEW LONDON DISPOSAL SITE
FEBRUARY 3, 1989

CONTRIBUTION #71

JANUARY 1990
Report No.
SAIC-89/7554 &C76

Contract No. DACW33-86-D-0004
Work Order No. 18

Submitted to:

Regulatory Branch
New England Division
U.S. Army Corps of Engineers
424 Trapelo Road
Waltham, MA 02254-9149

Submitted by:

IMI

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

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TABLES:

Table 1

Table 2

FIGURES:

Figure 1

Figure 2

Figure 3

Figure 4

LIST OF TABLES AND FIGURES

Proposed Capping Plan for the Thames River Shipyard
Disposal Project.

Actual Capping Operation for the Thames River
Shipyard Disposal Project.

Post-capping bathymetric contour plot of the Thames
River Shipyard Disposal Location, February 1989.

Post-disposal bathymetric contour plot of the Thames
River Shipyard Disposal Location, October 1988.

Depth difference contour plot of the Thames River
Shipyard Disposal Location, showing the distribution
of cap material, February 1989 - October 1988.

Depth difference contour plot of the Thames River
Shipyard Disposal Location, showing the distribution
of contaminated dredged material, October - August
1988.

CAPPING SURVEY AT THE NEW LONDON DISPOSAL SITE
FEBRUARY 3, 1989

1.0 INTRODUCTION

The New London Disposal Site covers a one square nautical
mile area located approximately two nautical miles south of the
mouth of the Thames River, Connecticut. This site, centered at
latitude 41 16.1'N and longitude 72 04.6'W, has been monitored since
1977 by the New England Division (NED) of the Army Corps of
Engineers. This study focuses on a subsection of the New London
Disposal Site which received an estimated 13,000 cubic meters of
contaminated dredged material (at buoy location 41 16.425'N and 72
04.320'W) from the Thames River Shipyard in October 1988. A
precision bathymetric survey and a REMOTS® sediment profile survey
were performed at this location after disposal of the contaminated
material to document the distribution of dredged material. The
results were used to develop a capping plan intended to provide
complete coverage of the contaminated sediment with clean material
(SAIC, 1988).

From October 1988 through January 1989, clean sediment
was deposited at six disposal points over the observed distribution
of contaminated dredged material (Table 1). The number of scow
loads of cap material to be deposited at each disposal point was
planned to achieve a desired cap thickness of 50-100 cm over the
contaminated material. After completion of the capping operation,
on February 3, 1989 a bathymetric survey was performed to delimit
the distribution and thickness of capping material over the
previously disposed contaminated dredged material. From the results
of this survey, the capping operation and the potential need for
additional cap material at the location were assessed.

2.0 METHODS

On February 3, 1989, a precision bathymetric survey was
conducted at 25 m lane spacing over an 800 X 800 m area centered at
the coordinates of the disposal buoy. The precision navigation
required for the survey was provided by the SAIC Integrated
Navigation and Data Acquisition System (INDAS). This system uses
a Hewlett-Packard 9920 series computer to collect position, depth,
time, and date information for subsequent analysis as well as for
providing real-time navigation for the helmsman. Positions were
determined to an accuracy of + 3 m from ranges provided by a Del
Norte Trisponder System. Shore stations for this system were
established in Connecticut at known benchmarks at Millstone Point

and New London Lighthouse. A detailed description of the navigation
system and its operation can be found in DAMOS Contribution #60
(SAIC, 1989).

Depths were determined to a resolution of 3.0 cm (0.1 ft)
using an Odum DF3200 Echotrac Survey Recorder with a narrow beam 208
kHz transducer. The speed of sound used in depth calculations was
determined from water temperature and salinity data measured by an
Applied Microsystems CTD probe, model STD-12. A complete
description of this instrument and its operations are given in DAMOS
Contribution #66 (SAIC, 1990). The speed of sound determined from
CTD casts and the transducer depth were entered into the fathometer
to adjust the depth values being transmitted to the computer.
During analysis, raw bathymetric data were standardized to Mean Low
Water by correcting for changes in tidal height occurring during the
survey. A detailed discussion of the bathymetric analysis technique
is given in DAMOS Contribution #60 (SAIC, 1989).

3.0 RESULTS

The contour plot from analysis of the 1989 bathymetric
data revealed a highly variable topography of old disposal mounds
resulting from dredged material disposed in previous years, as well
-as from cap material disposed at the buoy location (Figure 1). The
clearly visible north-south contours running along the western
border of the survey area delimited the eastern flanks of the old
disposal mound NL-RELIC. The NL-III mound was seen along the
southern border of the survey area. The western flanks of the NL-II
mound were also shown on the eastern limits of the survey area.
Changes in topography resulting from capping material deposition
were indicated by comparison of this contour plot with the results
of the October 1988 post-disposal survey (Figure 2).

Most of the capping material was evident in the area to
the southeast of the buoy. At roughly 150 m south of the buoy, the
cap layer showed a minimum depth of 14.8 m, in comparison to the
1988 minimum depth of 15.5 m at this location. Another topographic
high of 15.6 m was detected 140 m southeast of the buoy in the 1989
contour plot, 0.6 m higher than the 16.2 m shown at this location
in the 1988 contour plot. While this capping layer was relatively
steep-sided in parts, it showed several isolated topographic highs.
This reflected both the areal extent of the underlying disposal
mound as well as a spatial variation in cap thickness ranging from
10 to 70 cm. It should be noted that in 1988 the western part of
NL-II showed a minimum elevation of 15.5 m, whereas in 1989 the very
western part of the mound showed a minimum elevation of 15.2 nm.
This indicated an extension of the cap layer to the edge of NL-II
with a thickness of approximately 30 cm in this area.

To determine cap thickness a depth difference plot was
prepared by subtracting the depth matrix of the October 1988
bathymetric survey from that of the February 1989 survey (Figure
3). The cap material clearly showed a roughly circular and
continuous distribution varying in thickness from 10 to 80 cm. This
depth difference comparison resulted in a calculation of 28,270 m
of cap material detected at the site. According to scow log records
an estimated 59,517 m° of cap material was deposited between October
1988 and January 1989 (Table 2).

4.0 DISCUSSION AND CONCLUSIONS

The objective of the February 1989 bathymetric survey at
the New London Disposal Site was to assess the distribution of
capping material and verify coverage of the previously disposed
contaminated dredged material. Changes in depth from 10 to 80 cm
were determined by comparison of the bathymetric surveys from 1988
and 1989 (Figure 3). Several isolated topographic highs were
identified in the distribution of cap material, indicating a notable
variation in cap thickness. The areas of greater thickness within
the cap layer probably represent disposal efforts to locate scows
at the six recommended disposal points within the area.

Examination of the distribution of cap thickness in
relation to the six recommended disposal points revealed that cap
material was clearly deposited at points "D" and "E". These were
the locations of the two thickest parts of the cap layer, having
cap material accumulations of 80 cm (Figure 3). Cap thicknesses of
50 to 60 cm were also established to the southwest of these two
disposal points. However, point "A" was intended to have received
the most scow loads of cap material because of the contaminated
dredged material thickness at this point (Table 1). The depth
difference contour plot indicated only 20 cm of cap material here.
At disposal points "B", "Cc", and "F" only 10 cm of cap material
appeared to have been deposited (Figure 3).

The erratic topography of the cap layer generally reflects
the spatial distribution of the recommended disposal points (Figure
3). However, detected cap thicknesses surrounding these locations
suggests a consistent shift in cap material deposition to the
southeast. The largest volumes of cap material were reportedly
disposed of at points "A" and "B" (Table 2). However as already
noted, the two thickest parts of the cap layer actually detected
are located at points "D" and "E". These points are offset to the
southeast of points "A" and "B". Similarly at point "F" little cap
material was indicated in the depth difference contour (Figure 3),
yet a localized cap layer of 40 cm was detected just south of this

disposal point. Similar topographic highs in the cap material
distribution were indicated south of disposal points "D" and "E"
(Figure 3). This suggests that some offset was consistently

3

affecting the positioning accuracy of cap material deposition during
the capping operation. Most likely this was due to a consistent
Loran error, resulting in the cap layer effectively covering the
majority of the contaminated dredged material with the exception of
the northern and western borders of its distribution.

Depth difference calculations from the pre- and
post-disposal bathymetric surveys of contaminated material revealed
the distribution of material requiring capping (Figure 4). The

mapped distribution of dredged material indicated from the REMOTS®
survey of this area conducted in October 1988 confirmed this
distribution detected by bathymetric techniques, and extended it
roughly 50 m to the south, east and west (SAIC, 1988).
Superimposing this distribution of dredged material as detected by
the REMOTS® survey of October 1988 over the cap distribution
detected in 1989 again confirmed that most of the contaminated
material did receive some cap material (dashed line, Figure 3).
However, at disposal point "A", 70 cm of contaminated material
apparently received only 20 to 30 cm of cap material (Figures 3 &
4). In addition, at disposal point "F" 10 to 20 cm of contaminated
dredged material was apparently capped by only 10 cm of clean
material based on the 1989 bathymetric depth difference plot (Figure
3)

The depth difference plot from the pre- and post-disposal
bathymetric surveys of contaminated material indicated a small
deposit located roughly 150 m southwest of the buoy (near point "C",
Figure 4). Although this material fell outside the mapped
distribution of dredged material verified by the 1988 REMOTS®
survey, it was recommended that this deposit be capped in addition
to the main disposal mound because of the substantial amount of
material detected here in the depth difference comparison (SAIC,
1988). This location should have been covered by scow loads of cap
material deposited at point "Cc". However, the 1989-1988 depth
difference plot indicated little or no cap material at this
location. It should be noted that bathymetric surveys are limited
in their ability to detect thin layers of sediment typical of
disposal mound flank deposits. Thus, it is likely that the actual
borders of the cap layer extend further than indicated in the
bathymetric depth difference contour plot, covering locations such
as disposal point "Cc", However it is unlikely that such flank
layers would be thicker than the minimum change in depth detectable
by bathymetric techniques (10 cm).

It appears that additional capping material is required
at certain locations along the northern and western borders of the
disposal mound. It is recommended that additional material be
deposited at locations "A", "B", "Cc", and "F" in order to cover the
contaminated material with a sufficiently thick layer of clean
material (between 50 to 100 cm). However, because of discrepancies
between the recommended disposal point locations and the actual
distribution of cap material detected, it is recommended that

4

greater navigational control be employed during disposal of this
additional cap material. This could be achieved through either the
use of better navigational systems (e.g., shore-based microwave),
or the temporary deployment of a disposal buoy at each of the
locations requiring additional capping material.

5.0 REFERENCES

SAIC. 1989. Monitoring Cruise at the New London Disposal Site,
July 1986. U.S. Army Corps of Engineers, New England Division,
Waltham, MA. DAMOS Contribution #60 (SAIC Report # £SAIC-
86/7540&C60) .

SAIC. 1988. Bathymetry and REMOTS® Surveys at the New London
Disposal Site, October 1988. (SAIC Report # SAIC-88/7547&215).

SAIC. 1990. Monitoring Cruise at the New London Disposal Site,
July 1987. U.S. Army Corps of Engineers, New England Division,
Waltham, MA. DAMOS Contribution #66 (SAIC Report # £SAIC-
88/7511&C66) .

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TABLE 1

Proposed Capping Plan for the Thames River
Shipyard Disposal Project

# Scow LORAN-C
Station Loads Latitude Longitude Xray Yankee
A 13 41° 16.423 72° 04.270 26133.0 43976.5
B 12 41° 16.416 72° 04.176 26132.2 43976.3
c 7 41° 16.359 72° 04.373 26133.8 43976.2
D 6 41° 16.378 72° 04.235 26132.6 43976.1
E 6 41° 16.385 72° 04.137 26131.7 43976.0
F 6 41° 16.416 72° 04.325 26133.5 43976.5
TABLE 2

Actual Capping Operation For the Thames River Shipyard
Disposal Project Completed January 23, 1989

# Scow Total Volume Disposed
Station Loads According to Scow Logs (M°)
A 16 20,579
B 14 12,087
Cc 9 7,765
D 8 6,655
E 10 6,655
F 10 5,776

Total Volume Disposed According to Scow Logs - 59,517m°

Total Volume Detected in Bathymetric Depth Difference
Calculation - 28,270m°

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