37 



4.2 NL-88 and NL-85 Disposal Mounds 



The 1990 bathymetric survey detected a disposal mound at the 1988 buoy location. 

 This disposal mound, NL-88, was 1.2 m high at the apex of the mound with dimensions of 

 175 X 125 m. The mound is centered about 150 m west of the NL-85 mound and was first 

 detected in the 1988 survey (Figure 3-1) (SAIC 1990d). Since that time it has received a 

 relatively small amount of dredged material (21,200 m 3 ). The sediment volume calculated 

 by subtracting the 1990 and 1988 bathymetric surveys was 11,560 m 3 , or about a 45% 

 reduction of the volume reported by disposal logs within the vicinity of the NL-88 mound. 

 This was in relatively good agreement with the results of Tavolaro (1984), who showed that 

 depth difference estimates of disposal volume were approximately 41 % lower than barge 

 volume estimates, principally due to compaction, loss of interstitial water, and material in 

 layers too thin to be detected acoustically. As predicted, the mound height at NL-85 

 (immediately to the west of NL-88) was unchanged since the 1988 survey. 



Although dredged material was present at all NL-85 stations, the spatial coverage of 

 the REMOTS® survey was not sufficient to characterize the areal extent of new material in 

 the disposal area, particularly material deposited at NL-88. The 13-station cross-shaped 

 REMOTS® sampling grid was centered on the NL-85 mound, and only the western portion of 

 the survey crossed over the NL-88 mound (Figure 1-2). Additional dredged material, below 

 the resolution detectable with bathymetry, may also have been present to the west, south, and 

 north of the NL-88 mound. 



Benthic recolonization at NL-85 was found to be primarily at Stage II and Stage HI 

 (as predicted; Figure 3-16). However, at the center of NL-85, Stage I assemblages were 

 present, indicating that this area had probably experienced recent disturbance. The depth 

 difference plot of the 1988 and 1990 bathymetric surveys does indicate that there has been 

 40 cm of material added to the center of the NL-85 mound since 1988. The northern and 

 eastern arms of the NL-85 REMOTS® survey have mussel beds. This indicates sediment 

 stability and is supported by the lack of sediment accumulation and the absence of barge 

 release points in the area (Figure 4-3). The plot of barge disposal points shows that most of 

 the material intended for the 1988 buoy location was released within 150 m north and west of 

 the NL-88 buoy location. 



The plot of barge disposal points also displays 21 positions scattered to the southwest 

 of the NL-TR and NL-88 disposal locations (Figure 4-3). This plot was generated from the 

 LORAN-C TDs provided by the disposal logs. The X-ray (X) and Yankee (Y) secondary 

 stations of the LORAN-C 9960 chain were customarily used for navigation at the New 

 London Disposal Site. However, these 21 points correspond to the plotted Whiskey (W) and 

 Yankee (Y) LORAN-C TDs as reported in the disposal logs of the M/V Nancy G. 

 According to these same disposal logs, cap material was discharged in close proximity to 

 Buoy 90 on the NL-TR disposal area. We believe that poor signal geometry of the Whiskey 



Monitoring Cruise at the New London Disposal Site, June-July 1990 



