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4.0 DISCUSSION 



4.1 The Sedimentary Environment of BLDS 



An understanding of the sedimentary environment at BLDS is necessary to 

 determine if there is evidence to preclude its suitability for dredged material disposal. For 

 example, the preferred locations for silty dredged material, such Boston Harbor sediment, 

 have been in low-energy environments where sediment resuspension and transport is 

 minimal. 



BLDS is located between the 40 m and 70 m depth contours on the western slope of 

 Stellwagen Basin in Massachusetts Bay. The seafloor of Massachusetts Bay can be divided 

 into three major sedimentary environments: 1) areas of erosion or nondeposition; 2) areas 

 of sediment reworking, a combination of erosion and deposition; and 3) areas of sediment 

 deposition (Knebel 1993). The distribution of these sedimentary environments reflects the 

 dominant processes in operation. Areas in the bay subjected to high-energy (erosional) 

 conditions tend to be characterized by coarse glacial drift and bedrock outcrops while areas 

 of lower energy (depositional) are characterized by fine-grained sandy muds (Knebel 

 1993). Through a compilation of sonographic, photographic, and direct sediment 

 sampling, Knebel mapped these three sedimentary environments from the Massachusetts 

 Bay coastline to the 50 m depth contour of the continental shelf. Knebel' s maps indicated 

 that erosional and sediment reworking environments comprise the majority of the shelf 

 bottom in the Bay. Depositional environments are found primarily in Boston Harbor and 

 at the 50 m contour of the Basin slope. The transects used in the present survey are all just 

 beyond the area characterized by Knebel (1993). 



Based on Knebel' s (1993) sedimentary environment maps, the majority of the 

 Massachusetts Bay Inner Shelf region is composed of erosional and sediment reworking 

 environments. However, deeper depositional areas appear along the 50 m depth contour 

 (Figure 4-1). This depositional zone begins within the southern portion of BLDS (Figure 

 1-2), but was not mapped in the initial study of Massachusetts Bay. The REMOTS® and 

 plan view photographs provided some insight about the nature of the sedimentary 

 environment within BLDS and support the extension of the depositional regime mapped by 

 Knebel north and eastward into the disposal site. 



Grain sizes at BLDS below the 50 m depth contour ranged from very fine sands (3 phi) to 

 silt plus clay (>4 phi) with a major modal size of silt plus clay. Sediments in depositional 

 environments of Massachusetts Bay are composed primarily of sandy muds and muddy 

 sands and have average concentrations of 0.5% gravel, 45.8% sand, and 53.7% silt plus 



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



