6J_ 



5.0 CONCLUSIONS 



A comparison of sequential bathymetric surveys showed a subtle but detectable 

 change in topography at the NL-91 and D/S Mound Complex between September 1997 and 

 August 2000, attributed to the placement of approximately 30,000 m^ of supplemental CDM 

 since 1997. Accumulations of CDM up to 0.5 m thick were detected in the vicinity of the 

 former D/S and NL-91 buoy locations. Analysis of REMOTS® sediment profile images 

 confirmed that a surface depositional layer of recentiy placed CDM was present over most of 

 the mound complex. Older CDM dating back to 1992 was detected on the eastern arm of the 

 NL-91 and D/S Mound Complex station grid. 



There were no detectable changes in topography detected over the Seawolf Mound in 

 August 2000 compared to the previous bathymetric survey of July 1998. Sequential 

 bathymetric survey results obtained at this mound since its creation in 1996 indicate that 

 most of the consolidation of dredged material on the seafloor occurred during the first year 

 following the completion of capping (i.e., between 1996 and 1997). 



The REMOTS® sediment-profile photographs obtained in August 2000 showed 

 advanced benthic recolonization at all three of the bottom features surveyed (NL-91 and D/S, 

 Seawolf, and USCGA). The benthic community at all three mounds was dominated by a 

 combination of Stage II and Stage III successional seres. Redox depths (RPD values) were 

 consistently deep, indicating good oxygen penetration within the surface sediments. In 

 contrast to previous surveys, there was little evidence of recent physical disturbance of the 

 surface sediments at either NLDS or the reference areas. Intact amphipod tube mats and a 

 depositional layer of organic matter were visible at the sediment surface in the majority of 

 sediment profile photographs, reflecting quiescent (depositional) conditions in the weeks 

 leading up to the August 2000 survey. The amphipods (Stage II) appeared to be in a 

 transition fi-om inactive decaying mats to the reestablishment of active juvenile populations. 



The average median OSI value at each of the three mounds in August 2000 (NL-91 

 and D/S = +8.2, Seawolf = +8.3, and USCGA = +8.8) was greater than the average for the 

 reference areas (+6.9). Both the mound and reference area OSI values are generally 

 considered indicative of healthy or undisturbed benthic habitat quality existing at the time of 

 the August 2000 survey. Overall benthic habitat quality at each of the mounds was 

 comparable to that on the ambient seafloor in August 2000. 



Monitoring Cruise at the New London Disposal Site, August 2000 



