34 



5.0 CONCLUSIONS 



Dredged material deposited since the November 1988 bathymetric and January 1989 

 REMOTS® surveys formed a distinct mound, centered slightly east of the buoy with 

 gradually sloping sides, and a maximum thickness of 0.8 m at the apex. Based on changes 

 in bathymetry, the diameter of the mound was estimated to be approximately 420 m, while 

 results from REMOTS® photographs extended the detected dredged material to 500 m north 

 and south, 400 m east, and 800 m west of the disposal site center. Depth difference between 

 the 1988 and 1990 surveys indicated a maximum change in depth of 1 m and an average 

 diameter of 450 m. The mound is well within the disposal site boundaries. These results 

 indicate that when there has been tight operational control during disposal operations, a 

 distinct dredged material mound can be formed at MBDS which is detectable by bathymetry 

 and REMOTS® sediment-profile photography. The formation of a well-defined mound 

 supports the use of capping at MBDS as an effective management option for proposed 

 projects in the Boston Harbor area. 



The benthic communities surrounding the "MDA" buoy were similar to those in 

 January 1989. Despite ongoing disposal activity, the percentage of disposal site stations 

 containing Stage EQ organisms increased since the 1989 survey. The higher OSI values also 

 indicated a steady recovery of the benthic infauna. 



The bathymetric volume calculation accounted for 50.9% of the corrected (41%) 

 barge log estimates. Comparison of the depth difference volume estimates and barge log 

 volume estimates resulted in a discrepancy. This discrepancy is probably due to the 

 consolidation of basement sediments and the need for improved techniques for measuring 

 barge log volume. Combined bathymetric and REMOTS® analyses (average camera 

 penetration depth of 10 cm) accounted for a conservative estimate of 65.5% of the corrected 

 barge log volume (Table 4-1). A more reasonable assumption is that the dredged material 

 layers were at least as thick as the maximum camera penetration depth (20 cm). With this 

 thickness, the bathymetry and REMOTS® measurements accounted for 95.5% of the 

 corrected barge log volume. These results support past oceanographic studies (SAIC 1987a, 

 SAIC 1988) which indicated that deposited dredged material was contained within the 

 disposal site boundaries and also indicated that capping of dredged material would be 

 successful at this site. Continued monitoring at MBDS by the DAMOS Program is 

 recommended to ensure protection of nearby resources such as Stellwagen Bank. 



Monitoring Cruise at the Massachusetts Bay Disposal Site, August 1990 



