27 



In the central and northern portions of the cell, there was a marked change where 

 the surface became highly irregular as the sound was reflected from the dredge marks 

 shown by side-scan sonar data. Although the surface was very rough and uneven, the 

 amplimde (strength) of the surface reflector was still strong, indicating a relatively hard 

 surface consistent with sand. Below the surface, there was a series of discontinuous 

 internal reflectors indicating a more heterogeneous deposit. Throughout this layer there 

 were u-shaped reflectors that indicated refraction off of irregular deposits. The bottom of 

 the cell was not a continuous reflector, most likely because most or all of the sound was 

 lost in the sand-capped area. This is because at each acoustic boundary where sound was 

 reflected or scattered within the sand deposit itself, less energy was left to continue 

 downward penetration. 



The subbottom data indicated that approximately 25 % of the cell was uncapped. 

 There was a transition zone between the sand-capped area, and the fine-grained, uncapped 

 area in the south along the N-S cross section (Figure 3-7). Comparing results from cores 

 CAD-IA and IB near the boundary and the bathymetric transects, it was apparent that 

 there is an interval of transition where sand and mud was interleaved. The reflector along 

 the peak of sand at the southern end of the capped section appeared to dip down towards 

 the south, and was overlain by the mud in that transition. Because the subbottom cross 

 section shown are uncorrected for speed of sound, the transition area between the capped 

 and uncapped portion of the cell should not be interpreted as the actual stratigraphy, as the 

 speed of sound in the sand (and BBC) is faster than in the fine-grained, heterogeneous 

 dredged material. Without digitizing and correcting for speed of sound, the data can still 

 be used to make qualitative conclusions about the interval between the capped and 

 uncapped areas. Because of the weight of the thick sand layer in the central portion of the 

 cell, in combination with the force applied to the central sand cap by the postcap dredging 

 operation, the boundary was most likely characterized by deformed layers of sand and 

 mud. 



The farthest eastern N-S lane (Figure 3-4), Lane 7, showed two interesting features 

 (Figure 3-8). First, the cell bottom was a relatively continuous reflector below the cap and 

 dredged material. This indicates that the material was more homogeneous, so that sound 

 could penetrate further to the bottom of the cell. In the sand capped area, however, there 

 was still no sand/mud reflector. Results from cores CAD-6B showed a thick mixed 

 boundary between the sand and mud. These data suggested that the method of delineating 

 cap thickness using subbottom will not be effective if there is a mixed zone that is greater 

 than the depth resolution (wavelength) of the system. If there is a consistent sand/dredged 

 material boundary (as in CAD-5A and 5B, Figure 3-6), subbottom will be more effective 

 in determining cap thickness. 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



