34 



The effectiveness of the capping operation at NL-TR was assessed by comparing the 

 depths in the October 1988 (postdisposal, precapping) survey with the same region after 

 capping in February 1989 and July 1990 (Figures 4-1 and 4-2). The original postdisposal 

 survey recommended that cap material be deposited at locations A-F (SAIC 1988; Table 1-1) 

 to fully cover the material associated with dredging activities at the Thames Shipyard and 

 Repair Company. The postcapping survey in February 1989 was not able to detect a 

 sufficiently thick layer (between 50 and 100 cm; SAIC 1990a; Figure 4-1) of new material at 

 stations "A", "B", and "F". Despite deposition of an additional 39,483 m 3 of cap material 

 between 13 March and 17 June 1990, the July survey results indicate that locations "A" and 

 "F" do not display the minimum 50 cm of cap material over the eastern one third of the 

 project area (Figure 4-2). The depth difference values must be considered minimum 

 thicknesses as the ambient and disposed material under the cap will have consolidated after 

 placement, reducing the measured difference. 



The postcapping survey found that cap material had formed seven distinct mounds 

 (SAIC 1990a) rather than a uniform cover over the area. No disposal buoys were deployed 

 to mark the individual locations. Only the LORAN-C navigation system was used to guide 

 all disposal barges to designated discharge points. LORAN-C utilizes a series of highly 

 repeatable time delay signals (TDs) to calculate positions. As the capping operation 

 proceeded, barges discharged at the same location several times, eventually forming the 

 irregular bottom topography. If a wider variety of discharge points was utilized, it is 

 possible that the cap material would have been spread more evenly over the area. 



Benthic recolonization at NL-TR was predicted to be at Stage I in the central portion 

 of the mound, while the margins were expected to exhibit Stage II, III, and Stage I on II and 

 III. For the most part this prediction was correct, although there were more Stage II than 

 Stage I assemblages present in the center of the mound (Figure 3-16). Stage II represents a 

 transitional sere between Stages I and III and is associated with recovery of a disturbed 

 benthic habitat (Rhoads and Germano 1986). REMOTS® photographs taken at the margins 

 of the mound showed the presence of Stage II and Stage III assemblages, as predicted. 

 Organism-Sediment Indices were generally high (+7 or greater) across the mound, indicating 

 a lack of severe benthic disturbance (Figure 3-17). Among the assemblages present at the 

 western edge of NL-TR were large beds of blue mussels (Figures 3-7 and 3-10). These 

 indicate relative stability in the area and correspondingly are found in stations that do not 

 have recent dredged material. These mussels would be excellent candidate species for future 

 bioaccumulation studies (Germano et al. 1994). Analyzing the tissue contaminant level of 

 these suspension feeders would help address some of the concerns about the effectiveness of 

 capping operations for isolating contaminants. 



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



