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4.0 DISCUSSION 



The development of distinct disposal mounds on the NLDS seafloor through 

 controlled placement of dredged material serves several purposes: 



1) The short-term impacts associated with dredged material disposal are highly 

 localized, allowing the productivity of the rest of the seafloor to remain 

 unaffected. 



2) The individual sediment deposits can be monitored as independent bottom 

 features over the course of several years without being influenced by new 

 dredged material deposition. 



3) Dredged material mounds can be strategically placed on the seafloor to construct 

 artificial containment cells to be used as part of large-scale capping projects. 



4) Many small- to moderate-sized disposal mounds tend to maximize the capacity 

 of a disposal site and adhere to site-specific minimum depth requirements. 



The August 2000 survey over NLDS was conducted as part of a long-term monitoring 

 initiative for three capped disposal mounds developed within the confines of the site (NL-91 

 and D/S, Seawolf, and USCGA). Each mound was constructed under a separate set of 

 project conditions (sediment type, disposal volume, placement pattern, etc.) and, therefore, 

 each has a unique history and character. Both the Seawolf and USCGA Mounds have been 

 in place on the seafloor for over 5 years, allowing ample time for dredged material 

 consolidation and full benthic community recovery. The NL-91 and D/S Mound Complex 

 was originally constructed during the 1991-1992 disposal season. Monitoring data collected 

 in August 1992 showed benthic recolonization to be within normal parameters, but it was 

 recommended that cap material thickness be increased (SAIC 2001a). Supplemental capping 

 has been on-going over this Mound Complex. The results of the August 2000 survey effort 

 at NL-91 and D/S Mound Complex, Seawolf Mound and USCGA Mound are discussed 

 below in relation to the monitoring objectives. 



4.1 NL-91 and D/S Mound Complex 



One objective of the August 2000 bathymetric survey over the NL-91 and D/S Mound 

 Complex was to detect any changes in seafloor topography since September 1 997, when the 

 last bathymetric survey was conducted. Since September 1997, a total reported barge 

 volume of approximately 30,000 m^ of supplemental CDM was placed over the mound 

 complex. The depth difference calculations between the September 1 997 and August 2000 

 bathymetric surveys were successful in detecting several small areas of supplemental CDM 

 accumulation up to 0.5 m thick in the immediate vicinity of the former NDA-91 and D/S 



Monitoring Cruise at the New London Disposal Site, August 2000 



