Watershed Delineation and Ground Water Discharge to a Coastal Embayment

Abstract

AbstractNonpoint source management strategies for watersheds to eutrophic coastal waters require an adequate characterization of subsurface hydrogeology to understand the sources and fate of ground water and contaminant flow. An investigation was conducted to define the watershed to Waquoit Bay, a shallow eutrophic coastal embayment at the margin of the sand and gravel aquifer of Cape Cod, Massachusetts. This investigation included: (1) the development of a subregional water table map over 56 square kilometers; (2) delineation of a watershed and subwatersheds by plotting reverse ground water flowpaths; (3) a water budget based on a mass balance between recharge rates over riverine watershed segments and measured stream flows; and (4) a series of well points driven beneath the bed of the embayment and a transect of deep geologic borings perpendicular to the shore to characterize submarine ground water and hydrogeologic conditions at depth. The principal aquifer thins from the apex of the watershed, where it is approximately 46 m thick, to approximately 11 m thick as it nears Waquoit Bay. The detailed water table map shows extremely high hydraulic gradients to the rivers and comparably smaller gradients near the bay, indicating the thinning aquifer forces ground water to discharge into the rivers and causes them to function as significant surface drains for the aquifer. A classic sharp salt water interface was found in the upper 11.3 m thick High K unit, while the interface in the lower 34 m thick Low K unit was estimated to extend 700 m from the shoreline beneath the saline water of the bay. Eighty‐nine percent (89%) of the bay water budget is from ground water flow (55% channeled into rivers and 34% direct ground water discharge), while 11% is from atmospheric deposition. The Low K unit is an insignificant source of fresh water to the embayment. The conceptual hydro‐geologic model of the watershed is consistent with the water table configuration and the mass balance water budget and was confirmed by subsurface characterization of the near shore hydrogeology. This type of watershed characterization provides a firm basis for the development of nonpoint source management options for coastal embayments.