30 



but could not be distinguished from UDM by standard bathymetric survey techniques 

 (Morris et al. 1996). 



The returns from the subbottom profiler were able to detect the UDM/CDM 

 interface during the July 1994 survey based on slight differences in acoustical signature. 

 The subbottom data were used to quantify the thickness of cap material deposited northwest 

 of the NHAV buoy and develop isopach maps of Layer 1 (assumed to be CDM), Layer 2 

 (assumed to be UDM), and total material thickness for the larger 2553 m x 525 m area, as 

 well as for the 1600 m x 525 m area directly over the NHAV 93 mound. 



The thickness of Layer 1 , measured from the seafloor to the first strong reflector, is 

 assumed to be the thickness of the cap material (Figure 3-8). The cap thickness over the 

 NHAV 93 mound is approximately 0.75 m along the margins and up to 1.25 m over the 

 center. An overlay plot of the X-Star data on the apparent cap thickness from March 1994 

 bathymetry confirms that the northwest quadrant is acmally covered with 0.5 m to 1.5 m 

 of cap material (Figure 3-9). 



The thickness of Layer 2 (assumed to be dredged material) over ambient bottom 

 was also calculated in the subbottom analysis (Figure 3-10). Distinct patterns in material 

 accumulation were detected, corresponding to historic disposal mounds developed within 

 the survey area. A maximum height of 6.0 m of material is visible at the clearly defined 

 CLIS 87 and CLIS 88 mound complex. CLIS 89 is apparent near the northern limit of the 

 analysis area with a dredged material height of 4.0 m at the apex. The NHAV 93 mound 

 displays an average subcap dredged material height of 2.75 m over its wide deposition 

 area. 



The total thickness of material deposited over die ambient bottom varied throughout 

 the surveyed area (Figure 3-11). The CLIS 87 and CLIS 88 mound complex displays a 

 maximum thickness of 6.5 m, while the CLIS 86 and CLIS 89 mounds show a total 

 accumulation of 3.5 m and 4.5 m, respectively. The majority of the NHAV 93 mound 

 shows few distinct topographic feamres, and a total thickness of 3.0 m to 3.75 m. The 

 remainder of the surrounding mounds display maximum heights ranging from 1.0 m to 

 2.5 m over ambient Long Island Sound sediments. 



3.1.3 REMOTS® Sediment Profiling 



The REMOTS® sediment-profiling photography survey over the NHAV 93 mound 

 was conducted to delineate the CDM dredged material footprint, as well as to assess the 

 benthic recolonization rate of the surface sediments. Fresh dredged material (CDM) was 



Monitoring Cruise at the Central Long Island Sound Disposal Site, July 1994 



