53 



5.0 CONCLUSIONS 



The information obtained from the bathymetric survey showed an increase in mound 

 development at the NL-TR region. The original disposal simulation model was generated 

 based on the formation of one mound. However, the results of the depth difference 

 calculations indicated the presence of a second, smaller mound that developed around the 

 disposal buoy, adjacent to the NL-TR mound. The combined heights of these two mounds 

 were more than the predicted value of the disposal simulation model. The predicted mound 

 radius was approximately equal to the combined radii of the two mounds. The footprint of 

 recently deposited dredged material as determined from REMOTS® photographs coincided 

 with small deposits detected acoustically. However, the REMOTS® survey also showed 

 evidence of dredged material deposition beyond that detected from the bathymetric survey. 



The increase in mound height between the 1990 and 1991 surveys detected at the NL- 

 TR region was primarily over locations "B" and "D". An additional 20 cm of new sediment 

 was added to disposal locations "A", "C", and "F". The cumulative cap thickness 

 (comparing the 1988 with the 1991 survey) indicates that there is at least 50 cm of cap over 

 the NL-TR mound except at capping point "C". The effect of consolidation, however, 

 indicates that the actual cap is thicker than observed, and may be approaching 50 cm at point 

 "C". As a conservative precaution, this area should receive additional cap sediment when it 

 is available. 



Analysis of REMOTS® parameters indicated that significant differences existed 

 between reference areas and on-site stations for boundary roughness and the OSI. Benthic 

 recolonization at the active disposal area had progressed, for the majority of stations, beyond 

 the Stage I pioneering assemblages. Stage II and Stage HI organisms were dominant 

 throughout the area. This finding, together with data from DO and RPD depths, indicates a 

 recovery well within expected recolonization rates. 



Results of the sediment grain size analyses were in agreement with the visual estimate 

 of grain size range and major mode as determined from REMOTS®. Results of the metal 

 and PAH analyses indicate relatively low levels for the reference areas, and therefore, no 

 immediate need for further testing beyond the collection of baseline information. 



Near-bottom DO concentrations were similar at the reference areas and disposal site, 

 and results indicated a well-oxygenated water column throughout the surveyed area. The DO 

 concentrations were similar to those measured in previous surveys. The presence of Stage HI 

 taxa and fairly well-developed apparent RPD depths for this early a survey, in conjunction 

 with the measured DO levels, suggest that low oxygen stress had not affected benthic 

 recolonization on the disposal site, at least within several weeks prior to the survey. 



Monitoring Cruise at the New London Disposal Site, June 1991 



