706 



(Figure 3-42). One replicate at 50NE showed evidence of low apparent DO, but no 

 methane was evident in any replicate. 



3.3.2.2 September 1997 Survey 



In September 1996, the NDA-96 buoy was deployed approximately 80 m west of the 

 NL-94 mound. A total volume of 3,400 m 3 of supplemental cap material was placed over 

 the mound at the buoy, adding to the pre-existing mound (Figures 1-3 and 3-44). In 

 September 1997, 15 stations at the NL-94 mound were sampled with the REMOTS® 

 sediment-profile camera, duplicating the survey conducted in 1995. Three or more replicate 

 images were obtained at each station to document the placement of the new material and 

 monitor benthic recovery. 



Dredged material was detected in all replicates at all stations. The measured 

 thickness of dredged material ranged from 6.0 to 16.2 cm (> 13.5 cm average; Table 3-10). 

 Dredged material layer thickness was equal to, or exceeded, the camera penetration in all 

 replicates images collected in September 1997. In 1995, dredged material was not detected 

 at stations 100NE, 100NW, and 100W. Thin layers of dredged material over ambient 

 sediments were noted at Stations 100SE, 100SW, and 150NW. The presence of dredged 

 material greater than camera penetration at these stations during the 1997 survey was 

 consistent with the placement of material at the NDA 96 buoy (Figure 3-44). 



The REMOTS® images characterized the sediment as a mix of fine to very fine sand 

 (3 to 4 phi) with some variability (Table 3-7). As a result, the major modal grain size for 

 the entire mound was 4 to 3 phi (very fine sand) at most stations. Stations 100NW, 100W, 

 50NW, and 50SE displayed a finer grain size, with a mix of silt-clay predominating (>4 

 phi). Coarser-grained sediment consisting of fine to medium sand (2 phi) occurred in at 

 least one replicate at Stations CTR, 50SW, and 100E. Surface sand overlying fine-grained 

 sediment (sand-over-mud stratigraphy) was noted in 28 of the 46 photographs collected. 



The boundary roughness values for the NL-94 mound ranged from 0.6 to 4.2 cm, 

 with an average of 1.4 cm (Table 3-7). Shell lag or disturbed amphipod tube mats were 

 visible on the sediment surface within most replicate images. One replicate image at each 

 of Stations 100NE, 50NE, and 100SW was identified as winnowed. Armoring of the 

 sediment surface by shell lag (current scouring), visible in the images at Stations 150SE, 

 50NW, 50S, and 100E, may protect the mound from further current-induced winnowing 

 (Figure 3-45). In contrast to the 1995 survey results, boundary roughness was primarily 

 attributed to physical forces, although some surface disturbances were indeterminate or 

 caused by biogenic activity. 



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



