Contrasting Conditions of Coastal Groundwater Discharge in Two Long Island Estuaries

The first phase of an investigation of coastal groundwater systems was performed along the north shore of Long Island, New York in May of 2008 to constrain nutrient delivery to Northport Harbor and Manhasset Bay via groundwater. The embayments are bounded by steep moraines and are underlain by fine-grained sediments of glacial Lake Connecticut, which are over 100 m thick in some areas. Beach sand and gravel bound these deposits along shorelines. Piezometer sampling at two sites showed an unusually thick (up to 7 m) intertidal zone of recirculating brackish groundwater. Results of continuous resistivity profiling surveys were consistent with the existence of low-salinity groundwater in shore-parallel bands of varying width along the shorelines of both bays; piezometer sampling indicated that these likely discharge zones also varied in thickness of low-salinity groundwater. Discharge rates measured by Lee-type seepage meters were as high as 32 cm/day, and were modulated by the tide. These observations indicate that the high tidal ranges (~3 m at these sites), and the steepness of onshore topography and hydraulic gradients, which could be enhanced by the presence of bulkheads along some shorelines, are important variables in determining the nature of coastal groundwater discharge zones in settings such as these. The extensive mixing and offshore transport observed in the discharge zones is likely to enhance biogeochemical reactions, possibly including denitrification.