38_ 



The median of replicate OSI values ranged from +5 to +1 1 , with an overall average of 

 nearly +8 (Figure 3-14; Table 3-3). The Seawolf Mound median OSI values were greater 

 than the values of the ambient sediments observed at the reference areas, which varied 

 between +5 to +9 (+7 average; Table 3-2). 



3.3 USCGA Mound 



REMOTS® sediment-profile imaging was used to document the status of benthic 

 recolonization over the USCGA Mound five years after the development of the mound on the 

 NLDS seafloor. A complete set of REMOTS® image analysis results for the USCGA Mound 

 is provided in Appendix B3. 



The USCGA mound primarily consisted of sandy fine-grained sediments (grain size 

 major mode of >4 phi; Table 3-4). Consolidated clay or clayey mud was observed in five 

 images over the mound. All of the sediment that was observed in the images collected at this 

 mound was considered to be historic dredged material, having a thickness exceeding the 

 camera penetration depth (Table 3-4). Boundary roughness was low and uniform for most 

 stations, ranging from 0.7 to 2.6 cm (1 .4 cm average). Boundary roughness was primarily 

 due to biogenic activity (surface tubes). 



Over the USCGA Mound, the replicate-averaged RPD for each REMOTS® station 

 ranged from 1 .04 to 5.74 cm (Figure 3-15; Table 3-4). The average RPD, 3.80 cm, was 

 greater than the average RPD from the reference areas (2.66 cm). There was no evidence of 

 low dissolved oxygen (DO) conditions or methane bubbles observed in the USCGA Mound 

 sediment profile images obtained in August 2000. 



The successional status was advanced, with Stage II or Stage II on III communities 

 observed over the mound (Figure 3-17; Table 3-4). Stage III organisms were present at 10 of 

 13 stations. The various stages of the amphipod life cycle (juvenile, adult, and decaying tube 

 mats) were also apparent, and the mats appeared to be undisturbed by physical forces. 

 Similar to the Seawolf Mound, large tubes of the polychaete Chaetopterus sp were observed 

 in a few of the images over the surface of the mound (Figure 3-19). The presence of this 

 Stage III organism is indicative of advanced benthic recolonization over the USCGA Mound. 



The median of replicate OSI values ranged from +6 to +1 1, with an overall average of 

 +9 (Table 3-4). The lowest OSI values were observed at 50N (+6) and lOON (+7), but still 

 indicate healthy benthic conditions. The USCGA average OSI value was greater than both 

 the reference area average (+7) and the average observed in August 1995 (+6). Overall, 

 these results suggest the rapid benthic recolonization of this mound detected in the initial 

 survey continued without degradation over the past five years. 



Monitoring Cruise at the New London Disposal Site, August 2000 



