26 



clay (Knebel 1993). Following this definition the areas sampled may be described as 

 depositional environments. 



Defining an area as a depositional environment does not preclude the potential for 

 resuspension of fine-grained particles, but merely implies that the combined result of the 

 dominating dynamic processes is net sedimentation. The deficiency of silt/clay particles in 

 the upper centimeter compared to deeper sediments is at least in part due to episodic 

 resuspension. The smaller fine-grained particles are more easily entrained in bottom 

 currents and are winnowed from the surface sediments, leaving behind coarser grained 

 material. Although periodic resuspension may occur during the stormy winter months 

 (Knebel 1993), there is no evidence of extensive sediment movement that might limit the 

 future use of this site for dredged material disposal. 



4.2 Presence of Dredged Material 



The primary purpose for this survey was to examine a historic unconfined disposal 

 site and attempt to determine if any deleterious environmental conditions still persist nearly 

 twenty years after disposal activity had ceased. The combined use of side-scan sonar with 

 REMOTS® sediment-profile and plan view photography allowed the 1994 reconnaissance 

 effort to be focused on areas where dredged material, and the need for remediation, was 

 more likely. 



Disposal of dredged material at BLDS was widespread with material deposited in 

 localized patches throughout the site. The highest concentrations of dredged material, 

 located by side-scan sonar, encompassed the former DG buoy location and extended south 

 below the 50 m depth contour (Figure 1-2). REMOTS® photographs collected from the 

 transects surrounding the DG buoy location and to the south confirmed that the side-scan 

 records were indicative of dredged material deposits (Figure 3-1). However, the presence 

 of relic dredged material at transect C6, where no dredged material was detected in the 

 side-scan survey, indicates that limitations exist in the use of side-scan sonar as the only 

 tool for locating relic dredged material. Side-scan sonar, by detecting changes in the 

 acoustic signature of the surface sediments, displays patterns that are characteristic of 

 dredged material disposal. When dredged material is spread over a wide area or buried 

 beneath the surface, side-scan sonar may not detect it. Dredged material in the regions 

 surrounding the DG buoy was light gray, high-reflectance, fine-grained Boston Blue Clay 

 (Figures 3-2 and 3-3). This material may be part of the 2.3 million m 3 of Boston Harbor 

 dredged material deposited in the 1960s and 1970s (Normandeau Associates 1994). 



Even though fine-grained dredged material was not observed in the REMOTS® 

 images from transect C4, which was also located in an area of concentrated dredged 



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



