Contaminant Accumulation in the Boston Harbor-Massachusetts Bay Sedimentary System

Figure 3. Distribution of sedimentary environments in the Boston Harbor-Massachusetts Bay sedimentary system. The map shows areas of sediment deposition (typically fine-grained muddy sands and mud in red), two areas characteristic of sediment erosion or non-deposition (areas of boulders or bedrock in blue; areas of gravel and medium to coarse sand in green), and areas of sediment reworking (typically patches of gravelly sand, sand, and mud in yellow). (See Knebel and Circe,1995; Knebel and others, 1996.)

Contaminants discharged into Boston Harbor are sequestered in the sediments of Boston Harbor, Massachusetts Bay, and Cape Cod Bay. Contaminant levels in sediments decrease offshore from Boston Harbor. Silver is a key element in tracing the distribution of sewage-derived particles. Because of silver's use in photography, sewage particles contain silver at concentrations often 1,000 times higher than found in natural, uncontaminated sediments. The concentration of silver in the surficial sediments and the total amount of silver in the sediments are highest in Boston Harbor and lowest in the western Gulf of Maine (fig. 4). Elevated concentrations of silver (and other contaminants) occur in the small patches of fine sediments near the future outfall site and in the depositional areas of Massachusetts and Cape Cod Bay. The relatively high amounts of silver in Cape Cod Bay suggest preferential accumulation of sewage-associated contaminants in this region.

Figure 4. Concentration of silver in surficial sediments (in micrograms per gram of dry sediment). Silver is a key element in tracing the distribution of sewage particles in the coastal ocean. (Bothner and others, 1993.)