11 



3.0 RESULTS 



3.1 Bathymetry 



The results of the June 1991 bathymetric survey (Figure 3-1) depict bottom 

 topographic features present at the New London Disposal Site. In general, the ambient 

 bottom slopes from a depth of approximately 16 m in the northern portion of the disposal site 

 to a depth of 23 m to the south. In an east/west direction the disposal site has topographic 

 relief of about 2.5 m. In the extreme southwest corner, there is a steep slope of 

 approximately 5 m. The six relic disposal mounds at NLON are clearly visible from 

 deflections in the bottom contours: NL-RELIC, NL-I, NL-II, NL-III, NL-85, and NL-88 

 (Figure 3-1; SAIC 1993). The NL-88 mound was first identified in the 1988 NLON master 

 survey, approximately 150 m west of NL-85. Minimum depths of the inactive mounds have 

 remained unchanged since the 1990 bathymetric survey. NL-TR was also present in the 



1990 survey and is located roughly 150 m northwest of NL-II. The June 1991 survey showed 

 a distinct mound at NL-TR in comparison to 1990 (Figures 3-2 and 3-3). This mound has a 

 diameter of approximately 150 m and minimum water depth of 14.5 m (Figure 3-3). 



The depth differences between the 1990 and 1991 surveys (Figure 3-4) showed a 

 maximum change in height of 2.4 m at the NL-TR region with small peaks of 0.8 m and 1 m 

 located approximately 230 m west of the disposal point. The NL-TR mound, as shown by 

 the depth difference plot was 140-160 m in diameter with a height of 2.4 m. The depth 

 profile of lanes 15 and 16 (the bathymetric survey lanes transecting the maximum height of 

 NL-TR; Figure 3-5) showed changes in mound height of 2.4 m (lane 15) and 3 m (lane 16). 

 A small mound that developed adjacent to the NL-TR mound and immediately to the west 

 had a maximum height of 0.6 m and a diameter of 110 m. The combined radius of this 

 mound plus the NL-TR mound was approximately 150 meters. Depth differences on the 

 order of 20 cm (i.e., approaching the limits of detection in this comparison of the 1990 and 



1991 surveys) were present 400 m west, 200 m south, and 150 m to the north of the 

 1990-1991 disposal site center. 



Volume calculations based on the depth difference between the 1990-1991 surveys 

 indicated that 10,500 m 3 ± 320 m 3 (95% C.I.) of sediment have accumulated in the vicinity 

 of the NL-TR disposal mound between the June-July 1990 survey and the present. During 

 this period, disposal logs showed an estimated deposition volume of 31,475 m 3 . Sources of 

 discrepancy between the measured and estimated deposition volumes are presented in the 

 discussion. 



Monitoring Cruise at the New London Disposal Site, June 1991 



