79_ 



contaminated material to a thickness of 1.0 m was directly related to the lateral spread of 

 the mound during disposal. 



Dredging operations conducted in urbanized or industrialized areas may not produce 

 an abundance of CDM for use in capping operations. At CLIS, approximately 145,000 m^ 

 more UDM was deposited than at the New York Mud Dump, requiring the investment of 

 1,293,000 m^ less CDM. The systematic deposition of dredged material to form a lateral 

 containment ring proved to be a valuable and viable management strategy at CLIS. The 

 ring of mounds acted as an artificial containment ridge and facilitated the deposition of a 

 large volume of sediment in a relatively small area. In addition, the pattern of disposal 

 over the past seven to ten years allowed for the incorporation of historic mounds, 

 minimizing bottom coverage. The continued management strategy of containment cell 

 formation followed by central deposition will efficiently utilize the 8.375 km^ area of 

 seafloor to conduct disposal operations at CLIS well into the future (Morris et al. 1996). 



The comparison of the July 1994 survey to the March 1994 post-cap bathymetry 

 indicates little or no change in NHAV 93 mound topography occurred over the four month 

 period. Depth difference plots completed during the five previous bathymetric surveys 

 show that the majority of consolidation occurred during disposal and capping operations 

 (Morris et al. 1996). The long-term outlook for NHAV 93 suggests slow consolidation of 

 the CAD mound as a result of pore water extrusion and basement material compression 

 over time (Poindexter-RoUings 1990). 



There is sufficient agreement between the subbottom and bathymetric data 

 comparisons of the cap material thickness to state that cap thickness over the entire 

 NHAV 93 project area conforms to project requirements of 50 cm. The X-Star subbottom 

 system detected as much as 1.25 m of cap material in the northwest quadrant of the NHAV 

 93 mound. 



A depth difference plot comparing the March 1994 postcap bathymetric survey to 

 the November 1993 precap survey detected an apparent hole in the NHAV 93 cap over the 

 northwest quadrant of the mound (Morris et al. 1996). Disposal logs indicated the 

 deposition of approximately 76,000 m^ of CDM prior to the completion of the November 

 1993 precap survey. As a result, this capping material could not be discerned from the 

 UDM deposit during subsequent surveys that utilized conventional bathymetric data 

 processing techniques. However, the subbottom survey confirmed the presence of 0.5 m 

 to 1.25 m of CDM northwest of the NHAV buoy, indicating the NHAV 93 mound had 

 been completely capped. Through the use of sequential bathymetric and subbottom 

 profiling surveys, an average cap thickness of 0.75 m was detected over the surface of the 



NHAV 93 mound. 



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



