EXECUTIVE SUMMARY 



As part of the overall Boston Harbor Navigation and Improvement Project 

 (BHNIP), shipping berths 11 and 12 at Conley Terminal in South Boston were deepened to 

 MLLW -40 and -45, respectively, in June- July 1997. In phase 1 of the BHNIP, fine- 

 grained maintenance sediment, classified as unsuitable for open ocean-disposal, was 

 dredged and placed into an in-chaimel confined aquatic disposal (CAD) cell in Boston 

 Harbor. The cell was excavated into the existing federal channel, below the BHNIP 

 channel depth of -40 ft MLLW. Following placement of maintenance material into the 

 cell, sufficient sand to cover the dredged material with a minimum of a 3 ft capping layer 

 was placed using split-hull scows. 



A monitoring survey was conducted by SAIC in October 1997 to assess the stams of 

 capped CAD cell. Survey methods included one day of vibracoring, and one day of 

 acoustic surveying including bathymetry, subbottom, and side-scan sonar. Results of the 

 survey indicated that most of the CAD cell was covered with a highly variable thickness of 

 sand, while the southern end had little to no cap material. This distribution was consistent 

 with the positioning of the split-hull scows used to dispose the sand. Sand disposal was 

 permitted during the outgoing (southerly) tidal cycle. Prior to the initiation of the project, 

 preliminary modeling of sand transport due to Boston Harbor tidal currents predicted that 

 sand would be transported to the south, so no barge was placed directly over the southern 

 end of the cell. The results suggested that the sand remained in the convective state during 

 placement, so that all the cap material was placed directly below each positioned barge. 



The monitoring results also suggested that postcap operations designed to level the 

 sand served to enhance mixing of the cap and underlying dredged material, and resulted in 

 uneven sand coverage. A final videosled survey was conducted by C. R. Environmental in 

 December 1997. These data confirmed the presence of a thick layer of sand covered with 

 tunicates and other organisms in most of the cell, and the flat, fme-grained uncapped mud 

 surface to the south. Overall, the maintenance material was successfully placed in the cell, 

 and capped with sand in all locations where capping barges were located. The results of 

 nionitoring of Phase I provided guidance for operational and monitoring modifications for 

 Phase 2 of the BHNIP in 1998-99. 



