EXECUTIVE SUMMARY 



The Boston Harbor Navigation 

 Improvement Project will generate an 

 estimated 2.2 x 10 6 m 3 of dredged 

 material. Approximately 500,000 m 3 of 

 this sediment is expected to be unsuitable 

 for unconfined open water disposal. One 

 alternative proposed was that the unsuitable 

 sediments be deposited at the existing 

 Massachusetts Bay Disposal Site (MBDS), 

 where they would be capped by the 

 remaining 1.7 x 10 6 m 3 of clean dredged 

 material. Successful disposal of 

 contaminated dredged material at open 

 ocean sites requires formation of a distinct 

 dredged material mound, careful placement 

 of capping materials, and bathymetric and 

 environmental monitoring to ensure that the 

 operation is successful initially and 

 effective over the long term. 



the ambient sediment. If the dredged 

 material forms a distinct, stable mound, 

 then the following conditions can be 

 satisfied: the sediment is being contained 

 at the site; the area over which capping 

 material must be placed is known; and the 

 capped mound can be monitored to verify 

 that the cap is isolating the unsuitable 

 sediments effectively. Based on past 

 disposal at MBDS, as well as deep water 

 sites (> 100 m) in Puget Sound, we can 

 predict that dredged material will form a 

 well-defined mound at these depths and that 

 capping can be a viable means of 

 containing unsuitable sediments at these 

 sites. 



MBDS is a disposal site approximately 

 17 nmi east-northeast of Boston Harbor in 

 water depths averaging 90 m. This site is 

 deeper than existing disposal sites in Long 

 Island Sound where capping operations 

 have occurred in a maximum of 

 approximately 25 m water depth. Several 

 concerns have been raised regarding 

 proposals to extend the depth of capped 

 disposal operations to deeper waters ( e.g., 

 Dolin and Pederson 1991). Monitoring of 

 disposal at MBDS over the past 7 years has 

 shown that dredged material released at the 

 site does form a distinct disposal mound 

 which can be detected by acoustic 

 bathymetry. The formation of a well- 

 defined disposal mound has been the 

 criterion on which capping decisions have 

 been made at shallower sites. 



Such a formation indicates that the 

 dredged material is stable and distinct from 



