THE flow of ground water directly into the coastal ocean has been studied previously by in situ measurements, seep meters and diffusion gradient models¹. Although these techniques provide ample evidence that such flows occur, they do not provide a means of quantifying the groundwater flux on a regional scale. Here I report large enrichments of ²²⁶Ra in coastal waters of the South Atlantic Bight, and demonstrate that groundwater discharge is the main source of the ²²⁶Ra surplus. Using ²²⁶Ra data for brackish ground waters with estimates of residence times of nearshore waters, I conclude that the groundwater flux to these coastal waters must be about 40% of the river-water flux during the study period. Besides Ra, other metals, nutrients and organic compounds are expected to be enriched in brackish ground waters, so these findings require an upward revision of terrestrial fluxes of dissolved materials to these coastal waters, and perhaps a re-evaluation of such fluxes to the global ocean. These fluxes may be sensitive to hydrological factors, groundwater usage, dredging and sea-level change.