Geohydrology and potential water-supply development on Bumkin, Gallops, Georges, Grape, Lovell, and Peddocks Islands, eastern Massachusetts

Abstract

An investigation of the geohydrology and of the potential for water-supply development on several of the Boston Harbor Islands, eastern Massachusetts, was conducted to evaluate the possibility of developing a permanent small-capacity water supply to support recreational activities, such as camping, hiking, and swimming. The Boston Harbor Islands, including Bumkin, Gallops, Georges, Grape, Lovell, and Peddocks Islands are part of a larger group of glacially deposited drumlins, which are composed of thick, dense, homogeneous till in their core that are overlain by a thin layer of stratified-beach deposits. The surficial materials over-lie a weathered zone of the metasedimentary Cambridge Argillite in the Boston Harbor area and were deposited by continental ice sheets that covered New England twice during the late Pleistocene Epoch, and by near-shore processes in the Holocene Epoch. The thickness of these materials range from less than 1 to about 300 feet where present. The till was deposited by glacial ice and is characterized as an unsorted matrix of sand, silt, and clay with variable amounts of stones and large boulders. The stratified deposits primarily consist of sorted and layered sand and gravel that accumulated and formed the beaches and tombolos of the harbor islands. These deposits overlie the till at altitudes generally less than 10 feet above sea level. A cross-sectional, ground-water-flow model was developed to estimate depth to the water table for a hypothetical drumlin-island flow system, which was assumed to be representative of the drumlin islands in Boston Harbor. Areas were identified in each island flow system with the greatest potential for small-capacity water-supply development based on the model-calculated depth to water and surficial geology of the islands. Model-calculated depth to water estimates were used because of the lack of available hydrologic data for the islands. Model results indicate that the simulated depth to water is less than 20 feet within 240 feet from the shore of the hypothetical drumlin-island flow system. This area on the topographic maps of the six Boston Harbor Islands roughly coincides with the high transmissivity zones of stratified-beach deposits and weathered till on the lower slopes of the drumlins where ground-water discharge and surface and subsurface runoff occurs.