29 



Finally, the southern end of Lane 7 showed an apparent mound of material in the 

 southeast corner of the cell overlying the uncapped mud with a clear acoustic reflector in 

 between. This mound was consistent with the topographic peak that showed up in both the 

 bathymetric and side-scan sonar data. This finding has important implications. The 

 subbottom data were consistent with the bathymetric data that suggested material had 

 settled on top of the dredged material after capping was completed. The fact that the 

 material appeared to have settled on top of the uncapped material suggested that the 

 bearing strength of the material had increased from July to October, and was potentially in 

 an advanced state of consolidation that would increase the success of final capping of the 

 material. Further geotechnical testing of the uncapped material by investigators at MIT 

 will be continuing to address the issues of consolidation and bearing strength. 



3.5 Video Results 



The videosled survey that was conducted following the monitoring survey 

 confirmed the acoustic data in two north-south transects. The video data provided clear 

 evidence of sand in the north and central portions of the cell, showing tunicate-covered 

 sand waves with dramatic topography (Figure 3-9). Audio data provided additional 

 evidence as the sled was dragged and scraped through the sand. The transition from the 

 sand cap to the uncapped dredged material was apparent primarily from the additional 

 resuspended material associated with the videosled movement through the far southern end 

 of the cell, but also from the change in sound. In general, water clarity was good 

 throughout the cell, contrasting with the suspended sediment present in the ambient Boston 

 Harbor sediments above the edge of the cell. During the second pass of the sled, the 

 tracks from the first pass were noted in both the sand and mud areas, indicating a relatively 

 consolidated surface of the dredged material. 



In addition to the surface coverage of sand and mud, the video captured images of 

 ex situ material deposited on top of the dredged/cap material deposit, including entrapped 

 debris and blocks of high-reflectance BBC (Figure 3-9). The blocks of BBC probably 

 were remnant from material loosened from the cell walls, similar to the evidence provided 

 by the acoustic data. The presence of debris indicated that material had become entrapped 

 below the ambient current of Boston Harbor and settled on top of the cap. Inferences of 

 the physical environment also could be drawn from the benthic community (mnicates, 

 lobsters, etc.) that inhabited the cell at the time of the video survey. Tunicates, especially, 

 tend to attach to hard bottoms (associated with ship fouling), and are filter feeders. The 

 presence of mnicates on the sand cap indicated that the sand provided sufficiently hard 

 substrate, within a fairly quiescent environment. Finally, video data collected along the 

 cell walls confirmed that the steep walls were scalloped, primarily from the dredging 

 process (Figure 3-10). The BBC walls also had begun to be colonized by a variety of 

 burrowing organisms. 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



