25 



from the bottom of the core during operations. Recovery was better for core CAD-6B, 

 where 20 cm of sand overlay 60 cm of mixed sand and dredged material, indicating a 

 maximum recovered thickness of mixed material of almost 2 ft. 



The loss of 6C and 7A with evidence of thick sand so close to 6A and 6B indicated 

 a high variability of sand thicknesses. Coring data indicated that mixing of sand and mud 

 of up to 2 out of 3 ft occurred in areas of highly variable sand thickness. This mixing was 

 probably enhanced, and possibly caused, by the force applied to the sand during postcap 

 dredging operations. The consolidation state of the dredged material prior to capping, as 

 shown in Cores CAD-5A and 5B, however, contributed to the presence of a clear 

 sand/dredged material interface. The potential for increasing the consolidation state of the 

 material prior to capping is discussed further in Section 4.0. 



3.4 Subbottom Results 



Two north-south lanes (Lanes 5 and 7) were selected to show the results of 

 subbottom data (Figure 3-4). Lane 5, through the central portion of the cell, was 

 representative of most of the subbottom results (Figure 3-7). The view is a cross section 

 through the sediment, showing acoustic reflectors below the sediment/water interface 

 where changes in lithology (acoustic impedance) occurred. As discussed in the Methods, 

 the depth markings on the subbottom records do not accurately represent acmal depths or 

 thicknesses of the cell lithologies. 



Outside of the cell area, a series of horizontal reflectors throughout the harbor 

 resulted from the namral geology of Boston Harbor. These sediments are a combination of 

 BBC and glacial till (material left after a glacier melts) that were deposited in a nearshore 

 marine environment that existed in the Boston area during an interglacial period about 

 18,000 years ago (CDM 1991 and references therein). 



In the area of the cell, much of the subbottom acoustic information reflective of the 

 namral geology of the material was lost, indicating no sound penetration to depths below 

 the dredged and cap material. This result is not surprising, as prior acoustic work over 

 dredged material has indicated that the acoustic signamre of dredged material is distinct 

 because of sound loss due to scattering and refraction, indicative of the heterogeneous 

 namre of the deposit (Bokuniewicz et al. 1976; Schock et al. 1992; Murray et al. 1995). 

 Two distinct acoustic regions were consistent with side-scan sonar data. In the southern 

 area of the cell, the top reflector was a strong, smooth reflector indicating a smooth surface 

 with little topography to scatter sound. The high amplitude of the reflector indicated either 

 a harder surface (not borne out by coring data), or a very flat surface. Below this surface 

 reflector, there was a very homogeneous layer as indicated by few internal reflectors, and 

 there is a clear reflector from the base of the cell. 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



