DEEP-WATER CAPPING DEMONSTRATION PROJECT AT THE MASSACHUSETTS BAY DISPOSAL SITE

摘要

A deep-water capping demonstration project was conducted at the 90-m deep Massachusetts Bay Disposal Site (MBDS). Although capping has been successful at shallower-water sites (<65 m depth), it previously had not been proven effective at deeper sites. In order to accomplish the goals of the capping demonstration without actually using sediment with unacceptable contaminant levels, sediment deemed suitable for unconfined open water disposal from Cohasset Harbor, Cohasset, Massachusetts was used to represent the sediment needing capping. Sediment representing the cap originated from the deepening dredging project in Chelsea River as part of the Boston Harbor Navigation Improvement Project (BHNIP). Cohasset Harbor and Chelsea River sediments were visually and physically distinct. A series of monitoring surveys was completed over the Cohasset Harbor Capping Project (CHCP) mound during the different phases of capped mound formation, including a baseline survey, a pre-cap survey and a post-cap survey. The capped mound was developed in 86 m of water over a 0.64 km~2 area of featureless seafloor located near the southern boundary of MBDS, away from the active region of the disposal site. From December 1998 through February 1999 a total estimated barge volume of 41,250 m~3 of sandy silt and gravel from Cohasset Harbor was deposited, creating a mound 0.4 m high, with a detectable footprint approximately 600 m wide. The mound was capped in spring 2000 with 170,000 m~3 of sediment, primarily from Chelsea River. A post-cap survey indicated that the majority of cap had accumulated within a 100 m radius of the CHCP disposal buoy. However, the cap apron was over 800 m wide, somewhat irregular in shape, and covered all but a small area of the Cohasset Harbor sediment. The results of the single beam bathymetry, side-scan sonar, sediment-profile imaging, and sediment sampling (surface grabs and cores) surveys used to document the development of the CHCP mound agreed well throughout the demonstration project. These data indicate that a layered deposit consisting of distinct sediment strata was developed on the MBDS seafloor.