40 



5.0 CONCLUSIONS 



The results of the bathymetric survey at the New London Disposal Site showed two 

 regions of topographic change relative to the 1988 New London master survey. In the 

 southern region of the survey, the NL-88 disposal mound covered an area 175 x 125 m. 

 The dredged material volume estimated from subtracting the 1988 and 1990 surveys was 

 11,560 m 3 , compared to 21,200 m 3 estimated from barge disposal records for the same area. 

 A REMOTS® survey found dredged material distributed across the entire NL-85 mound; 

 however, the detectable change in mound height relative to the 1988 survey was 40 cm at the 

 center of the mound. Benthic recolonization at NL-85 was substantially as predicted (Stage 

 II and III). Future REMOTS® surveys should be extended to characterize adequately all 

 recent dredged material in the vicinity of both NL-88 and NL-85. 



The NL-TR mound was approximately 550 x 400 m with a volume of 46,700 m 3 . 

 Since October 1988, material has been deposited at several locations in an effort to cap the 

 mound; therefore, the resulting sediment footprint was more widespread than the predicted 

 radius of 250-300 m. The bathymetric analyses indicate that less than 50 cm of capping 

 material is present at locations "A" and "F". However, it is probable that consolidation and 

 interstitial water loss in the underlying dredged material may have decreased the apparent 

 thickness of the cap and given the appearance that the capping operation at NL-TR did not 

 cover the original mound adequately (Figure 4-2). Locations "A" and "F" (Table 1-2) 

 should receive more clean material as a precaution. To cover these points to an apparent cap 

 thickness of 50 cm, an estimated barge volume of 15,225 m 3 of dredged material is required. 

 In order to cover location "F" completely, disposal operations should be directed to the Buoy 

 89 position (41° 16.430' N, 72°04.320' W) and deliver an estimated 7,448 m 3 of new cap 

 material. Location "A" requires an estimated 2,682 m 3 of material at coordinates 

 41°16.423' N, 72°04.270' W, as well as an estimated 5,095 m 3 at coordinates 

 42°16.384' N, 72°04.310' W. 



Benthic recolonization at the margins of NL-TR was largely as predicted (Stage II , III 

 and Stage I on II and III). At the mound center some of the predicted Stage I seres were 

 present, but this region was more typically heterogenous, with Stage II, Stage HI, and 

 "Indeterminate" stations. This indicates a healthy recovery well within expected 

 recolonization rates and is typical of pulsed disturbance patterns with long intervals. 



Near-bottom dissolved oxygen concentrations were uniform at stations both on and off 

 the disposal site (Table 3-1). Little stratification of the water column was present, and 

 bottom waters were well oxygenated on the day of sampling (3 July 1990). The measured 

 oxygen concentrations were nearly identical to conditions observed during the July- August 

 1988 survey. High oxygen levels in the presence of Stage III taxa and well-developed 

 apparent RPD depths suggested that low oxygen stress had not affected benthic recolonization 

 on the disposal site, at least within several weeks prior to the survey. Similarly, there was 



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



