EXECUTIVE SUMMARY 



Our knowledge of the physical factors that control the deposition of dredged material 

 suggested that in deep water most deposits will cover a relatively large area with only minor 

 vertical relief. From this argument and barge log volume calculations, it was presumed that 

 material disposed in MBDS at the "MDA" buoy since November 1988 would not provide a 

 vertical signature large enough to be observed with precision bathymetric equipment. This 

 study, conducted from 13 to 17 August 1990, set out to test the supposition through 

 bathymetric and REMOTS® surveys. The thickness and extent of dredged material 

 surrounding the "MDA" disposal buoy were mapped and compared to data collected in 1988 

 and 1987. Against expectation, the bathymetric survey did indeed detect a mound measuring 

 0.8 m in height and 420 m in diameter. 



The site boundaries for the interim Massachusetts Bay Disposal Site (MBDS) were 

 established in 1977, but the area has been used for the disposal of dredged material at least 

 since the 1960s. The disposal area during this study was a 2 nmi diameter circle centered at 

 42° 25.700' N and 70° 34.000' W. The MBDS received a great deal of public and private 

 scrutiny during consideration as a permanent Ocean Dredged Material Disposal Site (as part 

 of the final site designation by the EPA in 1993, the disposal site center was moved 

 approximately 0.95 nmi southwest). Since the last survey in November 1988, an estimated 

 260,300 m 3 of dredged material has been deposited at this site. The MBDS is expected to 

 receive large volumes of material over the next several years due to the major construction 

 projects underway in the Boston area. 



The August 1990 bathymetric data around the "MDA" disposal buoy was compared 

 to bathymetric data collected over the same area in 1988 and 1987. From 1987 to 1990, the 

 dredged material had formed a mound 1 m high and 450 m in diameter. The portion of the 

 deposit formed between 1988 and 1990 was 0.8 m high and 420 m in diameter. This 

 demonstrated the successful formation of a well-defined dredged material mound at MBDS. 

 The ability to form well-defined dredged material mounds is essential, if capping operations 

 are planned to isolate contaminated dredged material at MBDS, should the need arise in the 

 future. 



"Fresh" dredged material, as indicated by chaotic sedimentary fabrics and anomalous 

 grain size distributions, was detected in REMOTS® sediment-profile photographs out to 

 800 m west, 500 m south, 400 m east, and 500 m north of the center of the disposal site. 

 These results showed an area of the seafloor affected by disposal activity 83% larger than 

 that indicated by bathymetry. The REMOTS® photographs also indicated a steady recovery 

 in the benthic ecosystem since the 1989 REMOTS® survey as indicated by an increase in 

 Stage III taxa. 



