Using excess radon-222 to assess submarine groundwater discharge in the Delaware River and Bay estuary

A submarine groundwater discharge site was identified during the investigation of a zone of persistent excess radon in the Delaware River and Bay Estuary. The water column within and surrounding this zone, the excess radon maximum (ERM), was sampled during seven cruises performed between October 1997 and April 2001. The ERM was located approximately 82 km upstream from the mouth of the Delaware Bay and contained water column radon activity in excess of 1.5 dpm L−1; in six of seven sampling events, water column radon activity at the ERM exceeded 2.0 dpm L−1. A radon budget model developed to assess the contribution of multiple radon sources to the ERM proved that simple molecular diffusion from sediments cannot sustain the ERM; an additional source of radon is required to sustain the excess radon activity in the ERM. Submarine groundwater discharge at a rate of 14 to 29 m3 s−1 from two aquifers that subcrop on the Delmarva Peninsula immediately west of the ERM can support the excess radon in the ERM. The geometry and hydrology of these aquifers makes them likely sources of the groundwater discharge that results in the ERM. Seasonal variability of the excess radon signal at the ERM was attributed to dynamic dilution of a constant excess radon signal by a variable fluvial flux.; The biogeochemical impact of this groundwater flux was assessed by investigation of the potential nitrate loading from groundwater transport. Groundwater nitrate concentrations in the two source aquifers are elevated and submarine groundwater discharge can transport dissolved nutrients along with the radon-222 that serves as a groundwater tracer. The potential groundwater nitrate flux was determined to be up to 5 moles N s−1, a flux that is similar to other estuarine nitrate fluxes, including atmospheric deposition and fluvial inputs. A preliminary investigation of the stable isotope composition of nitrogen in water column nitrate did not reveal persistent isotopic perturbations in the ERM, but was limited by a lack of information about the isotopic composition of the groundwater nitrate and by present limitations of methods for direct sampling of groundwater discharge.