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4.0 DISCUSSION 



4.1 Topography and Texture of the Sediment Surface of the CAD Cell 



The presence of two distinct acoustic regions was consistent among all of the data 

 collected within the CAD cell. The acoustic data, in tandem with cores collected 

 throughout the cell, indicated that the northern and central portions of the cell were 

 covered with a coarse sand. Throughout the central portion of the cell, clamshell bucket 

 markings were noted clearly in the sidescan data, resulting in the appearance of sand waves 

 that confounded core recovery in the thicker layers of sand. The distribution of sand 

 across the surface of the cell also was clearly delineated in subbottom data. In that area of 

 the cell, the subbottom reflectors apparent in the ambient Boston Harbor sediments were 

 dissipated, indicating no effective sound penetration to depths below the dredged and cap 

 material. Below the surface of the central and northern portions of the cell, there was a 

 marked change in subbottom penetration, where there was a series of discontinuous 

 internal, u-shaped reflectors that indicated refraction from the heterogeneous sand wave 

 deposit. Clear evidence of sand on the surface was supported by video data, that showed 

 tunicate-covered sand waves with dramatic topography (Figure 3-9), as well as audio 

 evidence as the sled was dragged and scraped through the sand. 



In the southern end of the cell, the acoustic and coring data indicated that no sand 

 was present, as all of the cores recovered in the southern end (except CAD-4B) recovered 

 dredged material to refusal in BBC. In the southern area of the cell, the sediment/water 

 interface was a high amplitude reflector in the subbottom data caused by the flat, 

 featureless fine-grained cell surface. Below this surface reflector, there was a very 

 homogeneous layer as indicated by few internal reflectors, and the base of the cell was 

 noted as a continuous subbottom reflector along the southern portion of the cell. In 

 combination, these results indicated that no sand was placed in the southern section of the 

 cell. 



The lack of sand in the southernmost area of the CAD cell was attributed to the 

 placement of the split hull scows used for capping. Modeling conducted prior to capping 

 using tidal current data predicted that the sand would spread towards the south (down 

 current). Monitoring data suggested, however, that the majority of the sand, although 

 released slowly from the barge, was released convectively so that the areal coverage was 

 more limited than predicted. The distribution of barges over the cell indicated that no 

 barge was ever placed directly over the southern end, resulting in a lack of sand in this 

 area. 



4.2 Thickness of the Sand Cap 



The second major result of the monitoring survey was that the central and northern 

 areas of the disposal cell were covered in sand, but the thickness was unevenly distributed. 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



