33 



Sand thickness appeared to be most variable in the area impacted by postcap dredging 

 operation. As noted in the side-scan sonar data, clamshell bucket operations resulted in a 

 topography similar to sand waves, so that the resulting cap thicknesses were highly 

 variable. Sand cap thicknesses among the four cores where sand cap was clearly recovered 

 at the top of the core (CAD-5A, 5B, 6A, and 6B) ranged from. 20-80 cm of sand or mixed 

 sand and mud (Table 2-1). The inability to collect cores at 6C and 7A with evidence of 

 thick sand so close to 6A and 6B indicated a high variability of sand thickness. A mass 

 balance approach was used to approximate the variable thicknesses of the deposit. 

 Assuming that the total volume of sand was deposited over, conservatively, 75% of the 

 cell, the average thickness would be 4 ft. The minimum cap thickness measured was 

 approximately 1 ft (26 cm), so the range of cap thicknesses could vary from 1-7 ft. 



4.3 Implications of Cap/Dredged Material Mixing 



One of the lithological units recovered in the cores was described as mixed, which 

 had the appearance of the sand cap (medium to coarse sand), but with a component of 

 black watery silt material, apparently from the dredged material unit. Coring data 

 indicated that mixing of sand and mud of up to 2 ft occurred in areas of highly variable 

 sand thickness, indicating that mixing was enhanced, and possibly caused, by the force 

 applied to the sand during the postcap dredging operations. 



The consolidation state of the dredged material prior to capping, as shown in cores 

 where sand overlay more consolidated clay, contributed to the presence of a clear 

 sand/dredged material interface (Figure 3-6). These data suggested that the more 

 consolidated the dredged material was prior to capping, the less mixing occurred. 

 Maximizing consolidation of the dredged material prior to capping, thereby increasing the 

 bearing strength of the dredged material and reducing the interval over which sand and 

 mud are mixed, therefore, has several advantages, including: 



• Increasing the protection of benthic organisms by maximizing the "effective 

 cap" above the zone of advective flux of potentially contaminated pore waters 

 into sand (e.g., Murray et al. 1994b); 



• Reducing the volume of sand that has to be placed to ensure 3 ft of coverage; 



• Maximizing the efficiency of the subbottom profiling technique to detect the 

 cap/dredged material boundary, which has provided the widest spatial coverage 

 of cap confirmation data to date; 



• Increasing the overall long-term stability of the deposit. 



The bathymetric results suggested that consolidation has continued throughout the 

 cell, in both the sand-capped and uncapped areas. The critical parameter of consolidation 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



