119 



Fine to very fine sand characterized the sediment at most stations in the Northern 

 Region, as the major modal grain size was 4 to 3 phi (very fine sand) in most photographs 

 (Table 3-8). At Station N9 fine sediments, either gray clay or brown clayey silt (>4 phi), 

 were predominant. In contrast, one replicate at Station N6 contained gravel with overlying 

 shells. Station 10 had two photographs with a coarser grain size of fine sand (3 to 2 phi). 

 Surface sand overlying fine-grained sediment (sand over mud stratigraphy) was noted in 

 one-fourth of the photographs from the region. 



Despite the occasional appearance of an overlying sand layer, all of the stations in 

 the Northern Region had relatively low boundary roughness values, with a replicate- 

 averaged mean of 0.9 cm. The coarse grain size in replicates at Station N6 and N10, in 

 addition to the predominance of shell lag and disturbed tube mats on the sediment surface, 

 indicated potential bottom current scouring. Although some stations were indeterminant or 

 had biogenic activity, boundary roughness was primarily due to physical forces. 



The replicate-averaged apparent RPD ranged from to 6.2 cm (3.0 average; Figure 

 3-52; Table 3-8). At Station N9, two replicates had an indeterminate RPD and one 

 replicate had no RPD visible due to the presence of gray clay. Seven replicate images 

 collected throughout the region had a visible redox rebound ranging from 4 cm to 7 cm 

 depth, suggesting a recent reduction in RPD depth. 



The biological assemblage at the Northern Region stations showed a dominance of 

 Stage II organisms (amphipods) with some Stage III organisms present (Table 3-8). The 

 Stage II organisms settle and create dense tube mats on the sediment surface, filtering 

 particles from currents they create at the top of the tubes. This high density of tubes and 

 filtering activity may serve to exclude Stage I organisms. Only a few replicates were 

 suspected of having retro-Stage II conditions. Stage III organisms, indicated by sub- 

 surface feeding voids, were present in 13 replicates of the Northern Region stations 

 (Figure 3-53). 



Median OSI values ranged from + 1 to +11 over the Northern Region, with an 

 overall average of +7.4 (Table 3-8). The majority of the stations had OSI values > +6. 

 The lowest OSI was detected at Station N9, where gray clay was present from the nearby 

 Seawolf disposal mound. The highest possible OSI value, +11, was calculated for four 

 replicate images and assigned to Station N8. Neither low dissolved oxygen conditions nor 

 methane were observed in any of the photographs collected in September 1997. 



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



