Significant groundwater input to a coastal plain estuary: assessment from excess radon

A submarine groundwater discharge zone was identified through the investigation of persistent excess radon in the Delaware River and Bay Estuary. This zone, the excess radon maximum (ERM), was located approximately 82km upstream from the mouth of the Delaware Bay. Water column radon activity at the ERM was elevated relative to the radon activity observed outside of this zone and was as high as 2.73 dpm l 1. A radon budget model developed to assess the contribution of multiple radon sources to the ERM indicated that simple molecular diffusion from sediments could not support the observed excess radon activity; an additional source of radon is required to sustain the excess radon activity in the ERM. A groundwater flux of 14 to 29m3 s 1 from the two aquifers that subcrop on the Delmarva Peninsula immediately west of the ERM can support the excess radon observed in the water column. The stratigraphy and hydrology of these aquifers makes them likely sources of the groundwater discharge that results in the ERM. The calculated submarine groundwater discharge flux is equivalent to the surface water discharge of the second and third largest tributary rivers of the Delaware Estuary.