138 



4.3 Capping of the Dow/Stonington Disposal Mound 



The NL-91 and D/S mound complex reflects a complicated history. The products 

 of seven disposal sequences are reflected in the results of four separate monitoring surveys. 

 These results show that a low mound complex was formed in 1991-1992 from mixed 

 deposits rather than a distinct capped mound. This mound complex had a thin layer of 

 sand covering the central region that was subsequently covered with additional cap material 

 in 1997-1998 (Figure 3-30). While the intended capping process was not completed as 

 originally planned in 1992, the presence of the sand layer and addition of subsequent cap 

 material in 1997-1998 has provided sufficient interim isolation of the material intended for 

 capping. Throughout the survey period this mound complex supported a healthy benthic 

 community that progressed rapidly from early colonizers to a more mature state. There 

 was no evidence of adverse environmental impact from the sediments and the surface of 

 the mound responded physically and biologically the same as other mounds and the 

 reference areas. Further, additional sediment was sent to this location from 1998-2000, 

 and the results of follow-up surveys will be presented in a future report. 



Dredged material disposal activities at NLDS were confined to the period between 

 mid-October 1991 through mid- January 1992. Within those 90 days, the NDA buoy was 

 moored at four different locations, the D/S buoy was removed from the site during capping 

 operations and never recovered, and the U.S. Navy established a 300-m wide corridor for 

 submarines transiting through NLDS. In the midst of an unusually active buoy 

 management cycle, an apparent error in navigation information during disposal resulted in 

 much of the CDM being placed somewhat to the east of the buoy, leading to the formation 

 of the irregularly-shaped NL-91 and D/S mound complex (Figure 3-9). 



The depth difference map generated using survey data collected before and after 

 CDM disposal indicated that the bulk of the CDM was placed east of the NDA and D/S 

 buoys (Figure 3-8). REMOTS® sediment-profile photographs were further assessed to 

 determine if thickness of material < 20 cm could be mapped to further delineate the 

 distribution of dredged material (see Section 4.3.2 and Figure 3-11). 



4.3.1 Benthic Recolonization 



The area surrounding the NL-91 and D/S mound complex consistently showed 

 evidence of rapid benthic recolonization (Stage I progressing to Stage II with Stage III) 

 similar to results expected from sediments with a low potential for adverse biological 

 effects (Figure 3-24). In 1992, after the initial placement of material, the mounds 

 supported active benthic colonization and were more advanced than predicted. Median 

 OSI values ranging from +2.0 to +8.0 (average +4.9) were comparable to those of the 



Monitoring Cruise at the New London Disposal Site, 1992 - 1998 



