72 



200N, and 300N, where the layer of sand may have been incorporated into the sediment or 

 was obscured by decaying amphipod tube mats. 



The range of replicate-averaged boundary roughness values over the 1995 REMOTS® 

 stations ranged from 0.7 cm to 4.8 cm, with the highest values at 200S (4.8 cm) caused by the 

 presence of pebbles and shell lag (Table 3-2). Boundary roughness in the majority of the 

 replicate images that were analyzed was less than 2 cm and often attributed to biogenic 

 activity. 



Replicate averaged RPD values over the NL-91 and D/S mound complex ranged from 

 nearly 1.0 cm to 3.5 cm, with an overall average of 2.2 cm (Figure 3-22; Table 3-2). The 

 shallowest RPDs were concentrated along the southern (100S, 200S) and eastern (100E, 300E, 

 400E, 500E) legs of the sample grid. The center station (CTR) displayed a relatively deep 

 RPD of 3.0 cm, however, Station 300N demonstrated the deepest replicate averaged RPD 

 value of 3.48 cm. 



The presence of an RPD rebound layer was noted at several stations (CTR, 100S, 

 100E, 200E, 300E, 400E, and 500E). This rebound layer is the result of the RPD becoming 

 shallower within the surface sediment several days to weeks before the REMOTS® sediment- 

 profile photography survey. The reduction in RPD depth is often related to a decrease in 

 bottom water DO concentrations, in association with a seasonal increase in oxygen demand 

 (biological and chemical) within the surface sediments. 



Stage III benthic communities were observed in at least one replicate photograph 

 from all REMOTS® stations over the NL-91 and D/S mound complex. The dominant 

 biological assemblage was Stage II on III as the August 1995 survey results indicated by 

 the presence of decaying or disturbed amphipod tube mats (Figure 3-23 A). At Stations 

 300E, 300N, and 400E one or two replicates contained only evidence of Stage II organisms 

 (amphipods). Stage I on III was noted in replicate images from stations nearer the center 

 of the mound (100E) and historic dredged material off the disposal mound (200N, 100W, 

 200S; Figures 3-23B and 3-24). 



Organism Sediment Index values at the NL-91 and D/S mound complex ranged 

 'from +6 to + 10, with an overall average of +8.0 (Figure 3-22; Table 3-2). The lowest 

 OSI value ( + 6) was calculated for Station 400E primarily due to a shallow RPD depth in 

 one replicate image, although the area surrounding 400E is still considered quite healthy. 

 The highest OSI of + 10 was generated for Station 200E, reflecting a Stage II on III 

 successional stage and deep RPD depths in all three replicate images. There was no visible 

 evidence of low apparent DO levels in the sediment at any of the stations, although 

 methane gas bubbles were observed in one replicate image at Station 100S. 



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



