Groundwater Seepage along a Barrier Island

Resistivity and water level measurements were made on a barrier island on the south shore of Long Island, New York to examine the distribution of fresh groundwater and the potential for recirculation of saline groundwater. The depth to the base of the freshwater lens was overpredicted by calculations of the static-equilibrium depth to a sharp interface apparently because of the sensitivity of the calculation to the low water-table elevations which are in turn sensitive to variations in sea level because of the existence of a transition zone between fresh and saline groundwater. Mixing and recirculation of saline groundwater at the base of the lens produced a transition zone up to 9.65 m thick. Measurements also support model forecasts of a mean bay level several centimeters above sea level, augmented by atmospheric forcing and wave setup. A time lag of about 8 hours between the response of the ocean level to longshore winds and the corresponding response of the bay level can result in a difference in elevation between the bay and the ocean that is up to four times that produced by other agents such as Stokes transport and density differences. In the presence of differential hydraulic head, bay and ocean water may be exchanged via groundwater flow between the base of the freshwater lens under the barrier beach and a deeper clay layer.