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4.0 DISCUSSION 



Since the October 1988 monitoring survey at the New London Disposal Site, 

 additional cap material has been released at the NL-TR mound, and additional dredged 

 material has been released at the 1988 buoy location to the south. The precision bathymetric 

 survey detected material deposited since the last survey in 1988 that exceeded the resolution 

 of the bathymetric system (greater than 20 cm). The REMOTS® survey detected the areal 

 extent of new material below the resolution of the bathymetric survey. This survey was also 

 used to determine the status of recolonization at the inactive NL-85 mound. Near-bottom 

 dissolved oxygen concentrations were assessed relative to REMOTS® benthic analyses at and 

 near the disposal site. 



4.1 NL-TR Disposal Mound 



The NL-TR mound, located in the northeastern region of New London near the 1989 

 and 1990 buoy locations, has an area of 550 m x 400 m and had gained up to 2 m of 

 material since October 1988. These dimensions were obtained by comparing the 1988 and 

 1990 bathymetric surveys (Figure 3-4) and were confirmed by the distribution of dredged 

 material in REMOTS® photographs (Figure 3-7). 



Bathymetric profiling at NL-TR did not show a single, conical mound but rather a 

 series of small peaks overlapping at their flanks and forming an irregularly shaped region of 

 new material (Figure 3-4). This complex topography was the result of capping operations at 

 NL-TR. Because dredged material was disposed over a wide region (six points instead of 

 one), the predicted mound radius of 250-300 m underestimated the footprint of dredged 

 material. The volume of dredged material at NL-TR based on barge logs was approximately 

 100,300 m 3 . This represented the total amount of material deposited since the 1988 survey 

 and included both the original sediment from the Thames Shipyard and Repair Company 

 dredging operations and the cap material deposited since October 1988. The volume 

 calculated by comparing the NL-TR region of the 1990 and 1988 bathymetric surveys was 

 46,700 m 3 , or about a 53% reduction from the volume recorded in the barge logs 

 (100,300 m 3 ). 



Some of this volume difference (53,600 m 3 ) can be attributed to the wide distribution 

 of disposal points and the significant volume of material deposited along the margins of the 

 six mounds in layers less than the 20 cm acoustic detection limit. In addition, loss of 

 interstitial water as the material settles on the bottom, and consolidation of dredged material 

 and ambient bottom will have contributed to the volume difference. These estimates are in 

 relatively good agreement with the results of Tavolaro (1984) who calculated an average of 

 41 % reduction in volume between barge volume estimates and measured disposal mound 

 volumes. 



Monitoring Cruise at the New London Disposal Site, June- July 1990 



