SUBMARINE GROUNDWATER DISCHARGE TO GREAT SOUTH BAY FROM LONG ISLAND AND FIRE ISLAND, NEW YORK

An investigation of submarine aquifers adjacent to Fire Island National Seashore and Long Island is being conducted to assess the importance of groundwater discharge as a potential non-point source of nitrogen delivery to Great South Bay. Over 200 km of electrical resistivity data were collected in Great South Bay in 2008. A continuous resistivity profiling system was used to image the fresh-saline groundwater interface in sediments beneath the bay. In addition, intensive groundwater sampling was performed at sites along the shore of Patchogue Bay that were representative of the developed Long Island shoreline. A shore-perpendicular transect of piezometer samples was collected in this bay, with sample sites extending 30 m out from the beach, and piezometers penetrating to a maximum depth of 9 m below the sediment surface. The samples revealed a zone of reduced-salinity groundwater up to 8 m thick that extended >30 m offshore, confined by buried salt marsh peat in the shallow subsurface. Seepage meters deployed at the Patchogue site revealed discharge of up to16.2 cm per day at low tide, at a distance of 4.5 m from shore. Submarine groundwater within 30 m of the shore, both fresh and saline, was high in ammonium and methane (up to 1300 and 62 micromoles/L, respectively), and generally low in nitrate, nitrite, and dissolved oxygen. Dissolved gas data from submarine groundwater samples indicate loss, possibly by denitrification, of up to 40 percent of the land-derived nitrate load in groundwater prior to discharge. Two sites on Fire Island, a 50-km-long barrier island on the south side of Great South Bay, were sampled in April and May of 2009. At one site (Kismet, NY) reduced-salinity groundwater was only present within about 7 m of the shoreline, indicating the absence of offshore confining units. The second site (Watch Hill, NY) showed similar conditions to Patchogue, with reduced-salinity groundwater approximately 6 m thick extending up to 70 m into the bay beneath a buried peat layer. Dissolved oxygen was generally low at both sites; chemical analysis of the Fire Island samples is ongoing. Based on these results, the onshore and offshore shallow hydrostratigraphy of the Great South Bay shorelines appears to exert primary control on the width and chemistry of the submarine groundwater flow and discharge zones.