Boston Harbor Islands National Recreation Area - Water Quality

Groundwater Resources In 1996, the US Geological Survey conducted an investigation of the geohydrology and of the potential for water supply development for six of the Boston Harbor islands including Bumpkin, Gallops, Georges, Grape, Lovell and Peddocks islands (Masterson et al., 1996). The primary purpose of this study was to investigate the possibility of developing permanent, small-capacity water supplies capable of supporting recreational activities such as hiking, camping, and swimming. Masterson et al. (1996) found that the hydrology of each of the six islands studied was characterized by hydraulically independent freshwater-flow systems that consisted of about 10 to 30 feet of weathered till containing a dense substratum near the land surface that may perch water. The freshwater flow systems of each island is underlain by either bed rock or saltwater and they are also laterally separated from one another by the saline waters of Boston Harbor (Masterson et al., 1996).

Thus, the sole source of freshwater to each of the islands studied is the infiltration of precipitation into the soils of the island. The amount of precipitation reaching the saturated zone (recharge) would be expected to vary spatially and temporally in response to climatic (e.g. precipitation patterns, etc.) and biologic (e.g. evapotranspiration) factors, as well as to the local topography. The recharge rates would also be affected by local differences in the infiltration capacity and other hydraulic properties in the unsaturated zone. Using estimates of recharge rates from previous investigations in similar hydrogeologic settings, Masterson et al. (1996) were able to model approximate groundwater movements for the six islands. Masterson et al. (1996) determined that groundwater flow generally is radial from the center of the island towards the coast. Topographically high areas, such as the upper slopes and crests are typically the recharge areas with downward vertical gradients. Groundwater discharge would typically occur at the lower slope areas of the drumlins, the topographically low areas such as coastal marshes or ponds, or directly along the coast (Masterson et al., 1996).

Harbor Water Quality Contaminants can remain in the marine environment for decades and Boston Harbor is only now slowly recovering from many years of unregulated discharges through sewers, storm drains, and direct commercial or industrial discharge. In the mid-1980s, the harbor's sediments contained such high levels of contaminants that it was labeled the "dirtiest harbor in the nation." On average, the Inner Harbor contained the most contaminated sediments with concentrations in the Outer Harbor lessening from Dorchester and Winthrop bays to Quincy Bay. Historically, the lowest contamination levels were found in Hingham and Hull bays. Even though change is slow, several recent studies strongly suggest that, as contaminant inputs decline, water quality is improving and sediments are beginning to cleanse themselves (Pawlowski et al., 1996) Inner Harbor Group: The Inner Harbor group include those islands located within the Inner Harbor, Old Harbor and Dorchester Bay/Neponset Estuary areas of Boston Harbor. This part of Boston Harbor includes some of the larger islands within the national park area including Long Island (214 acres), Deer Island (210 acres), Thompson Island (157 acres), Spectacle Island (97 acres), and Moon Island (44 acres). Because of their size, close proximity, and, in some cases, easy access to the mainland, several of these islands have historically seen intensive land use. Deer Island is currently the site of MWRA's Deer Island Treatment Plant, which treats the wastewater for the metropolitan area before discharging the effluent into Massachusetts Bay.