29_ 



5.0 CONCLUSION 



The reconnaissance plan view and REMOTS® sediment-profile surveys identified 

 areas of historic dredged material at BLDS. The identification of dredged material by 

 these surveys was accomplished with a low level of effort by focusing on areas targeted for 

 the presence of dredged material in the side-scan sonar survey of BLDS. However, 

 dredged material was also found in an area devoid of side-scan sonar targets, and some 

 areas of side-scan sonar targets did not yield the expected fine-grained dredged material. 

 Side-scan sonar has a limited ability to locate relic dredged material. This technology is 

 unable to detect dredged material that does not present a distinct surface acoustic signature. 

 Even when a distinct surface feature is present, the disposal pattern may be due to rock or 

 construction debris which is impenetrable to REMOTS®. Based on the results of all 

 surveys, dredged material at the site was distributed in patches over both large (side-scan 

 sonar) and small (REMOTS®) horizontal spatial scales. 



Sediments which can support the level of recolonization observed at BLDS 

 generally should not require remediation action, only periodic monitoring (Germano et al. 

 1994). Twenty years after the cessation of disposal activity, the benthos of BLDS has a 

 Stage II and Stage III benthic community. The presence of these communities and the 

 evidence for extensive biogenic reworking of dredged material suggest a healthy benthic 

 habitat. Chemical analysis of the archived sediment samples is not recommended at this 

 time for the purpose of evaluating remediation. If the site is designated for additional 

 disposal, these archived sediments might be analyzed for evaluating baseline conditions. 



The depositional environment characterized at BLDS beyond the 50 m contour 

 shows no evidence to preclude its use for future dredged material disposal. Any 

 resuspension and transport that may occur is most likely episodic and does not occur 

 frequently. 



Monitoring Cruise at the Historic Boston Lightship Disposal Site, August 1994 



