23 



least 200 m north, 400 m west, and 300 m south of the disposal site center (Figure 3-11). 

 Recently deposited dredged material was also present at Station 400E. Fresh dredged 

 material at the center of the disposal site and 400W consisted of coarse-grained sands and 

 gravel overlying fine-grained mud (Figure 3-12). The surface of Station 200S consisted of 

 shellhash and mud clasts formed from the deposition of fresh dredged material. Station 300S 

 showed a large difference in optical reflectance between the sands (high reflectance) and 

 underlying mud (low reflectance; Figure 3-13). This area also showed the presence of sand 

 ripples and washing of fine sediments from the sediment surface layer of the mound by 

 bottom currents. At Station 100SE, dredged material, consisting of gravel and cobble, 

 limited penetration of the bottom by the REMOTS® camera (Figure 3-7). Prism penetration 

 depth was lowest at those stations having active bedforms and a thick surface layer of coarser 

 grained sands (Figure 3-11). At the majority of the disposal site stations, the thickness of the 

 dredged material layer exceeded the penetration depth of the camera. Dredged material was 

 not apparent at the reference stations. 



A thin layer of sand deposited in the spring of 1990, as a component of the clean 

 material cap, was readily visible in REMOTS® photographs taken at stations 200 m 

 northeast, 200 m southeast, and 300 m east of the NL-TR survey center (Figure 3-14). The 

 depth of the sand overlaying fine-grained cap material varied from 1.9 cm to 5.3 cm at 

 stations 100SE and 200SE (Figure 3-11). Patches of densely aggregated mussels were 

 present on the NL-TR survey area at 400W and at the WREF area, Station 300N. Scoured 

 shell lag deposits, characteristic of the WREF area, were also present at the western and 

 southwestern portions of the NL-TR survey area, and at stations west and north of the 

 NLON reference center. Hydroids were also abundant throughout the western section of the 

 survey area and within 300 m north and south of the NL-TR survey center. 



3.2.4 Apparent RPD Depth 



The frequency distribution for the mean apparent Redox Potential Discontinuity 

 (RPD) depth (i.e., the boundary between the oxygenated sediment and hypoxic or anoxic 

 sediment underlying it) indicated that the majority of disposal site stations ranged from 1.5 to 

 2.0 cm (Figure 3-15). The mean RPD depths of the combined reference stations ranged 

 from 1.0 to 2.0 cm. The mean apparent RPD depths for the reference areas and disposal site 

 were not different (null hypothesis accepted, p=0.05, Mann-Whitney U-test). The coarse 

 sediments and shell deposits over much of the WREF area restricted the penetration depth of 

 the camera, permitting RPD measurements at only 7 of the 13 stations (Figure 3-16). The 

 RPD depths measured in the WREF area were much shallower in comparison to the NE-REF 

 and NLON-REF areas, lowering the combined RPD values for the reference areas. 

 Compared to the overall NL-TR region surveyed in June- July 1990, the RPD values 

 measured in this survey were lower at both the disposal site and reference areas. 



Monitoring Cruise at the New London Disposal Site, June 1991 



