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DAMOS Contribution #115 - Executive Summary |
Baseline
surveys of the newly designated Massachusetts Bay Disposal Site (MBDS) were
conducted in September 1993 to delineate the topography and sediment
composition of the site for DAMOS management. This information will enable
management to optimize site usage and minimize adverse impact of future
disposal projects. MBDS was characterized acoustically through collection of
1) precision bathymetric, 2) side-scan sonar, and 3) Sediment Acoustic
Characterization System (SACS) data. In addition, sediments were sampled for
grain size analyses to calibrate the acoustic data, and for chemical analyses
to map the areal distribution of historically contaminated sediment
documented in a prior survey. MBDS
was relocated approximately 1 nmi southwest from the interim MBDS; the
interim site was used for the disposal of dredged material from 1977 to 1993.
During the 1992/1993 disposal season, the majority of the material disposed
at the interim MBDS originated from the Third Harbor Tunnel (THT) project in
Boston Harbor. Previous monitoring results obtained through the Disposal Area
Monitoring System (DAMOS) Program showed that THT material included fresh
blasted rock and large consolidated clasts of Boston blue clay. Results
of the 1993 baseline survey indicated that the new MBDS was composed of two
relatively distinct areas: 1) the newly incorporated southwestern area, and
2) the northeastern portion which overlaps the interim MBDS. The southwestern
area, where no documented disposal of dredged material has occurred, was
topographically featureless and sloped gradually towards the northeast.
Sediments in this area were composed predominantly of fine-grained silts and
clays. The northeastern region contained two major topographic features: the
most recent dredged material disposal mound and a large, shallow basin where,
historically, dredged material has been disposed. This shallow basin can be
enhanced by management for a potential capping site. The highest topographic
peak also was observed in this region, outside of the new site boundary, and
interpreted as a remnant glacial outcrop. Acoustic
data suggested that the sediment composition in the northwest region was more
heterogeneous than observed in the southwestern area, as would be expected
for an area where disposal activity has occurred. The presence of older
dredged material in the shallow basin was documented in the SACS data but was
not evident in the side-scan mosaic. These results indicated that the high
frequency side-scan sonar detected a thin layer of fine-grained sediment
deposited on top of dredged material in the basin. Sediment
samples were collected in a 600 m radial grid surrounding an area where
polycyclic aromatic hydrocarbons (PAHs) were found to be elevated in a 1989
survey. Results confirmed that the center station of the grid contained
higher levels of both metal and organic contaminants than the surrounding
sediments. Contaminant concentrations decreased with radial distance away
from this station and possibly indicated further contamination towards the
west as suggested by the previous data. The
integrated acoustic and chemical data proved to be useful in mapping not only
the high-reflectance THT material at the most recent dredged material mound,
but historical dredged material as well. Comparisons of the SACS and
side-scan data indicated that the surface of dredged material disposed less
than ten years ago was either covered by ambient sediments or sufficiently
reworked to appear uniform to side-scan acoustics. Both the acoustic and
chemical data will be used to manage future dredged material disposal and to
plan potential in situ remediation of historically contaminated sediment. |
Dated May 28, 1997