Submarine groundwater flux to the coastal ocean creates estuarine conditions at and near the point of discharge, thereby altering local benthic habitats and ecology. At a sandflat at Cape Henlopen, Delaware, adjacent to the Atlantic Ocean, we have documented low salinity in sedimentary pore waters within 10 m of the beachface. These low salinity patches are correlated with dense assemblages (in thousands per square meter) of a deep tube-dwelling polychaete worm, Marenzelleria viridis, otherwise regarded as an estuarine endemic species of fresher, oligohaline conditions and typically found farther up the Delaware Bay. Size-frequency analysis indicates that recruitment of juvenile M. viridis occurs in the spring of the year, coinciding with the time of greatest extent of low-salinity patches. Worm and low salinity patches persist for a year or longer. Where present, M. viridis is a numerical and biomass dominant, defining a benthic community strikingly different from that in nearby, non-seep locations. Because of its ecological role as a feeder on sediment diatoms, M. viridis may provide an important trophic linkage between microalgal growth fueled by high nutrient loads associated with the discharging groundwaters and worm-feeding predators such as bottom fish or shorebirds common on the Cape Henlopen sandflat.