We use 228Ra and 226Ra to determine the mass balance of dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) in the North Inlet salt marsh-estuarine system. While this system has only minor freshwater inputs of nutrients or radium, it is an extremely productive ecosystem. In addition, there are significant exports of these dissolved species to the coastal ocean. Saline groundwater in this estuarine system contains nutrient and radium concentrations more than an order of magnitude greater than surface waters. Using a radium mass balance, we estimate the groundwater discharge necessary to support the export of radium to the coastal ocean and the corresponding flux of nutrients from the groundwater. From these calculations, we show that the underlying aquifer supplies nutrients sufficient to support the net primary productivity of the salt marsh ecosystem and to account for the known export of nutrients from the marsh. We conclude that the major nutrient source to the North Inlet, South Carolina, salt marsh is the saline aquifer lying just beneath the surface of the marsh. Furthermore, extrapolation of the nutrient export to include other South Carolina marshes suggests that nutrient fluxes from salt marshes to the coastal ocean rival riverine nutrient fluxes for the region. ¿ 2000 American Geophysical Union