Utility of radium isotopes for evaluating the input and transport of groundwater‐derived nitrogen to a Cape Cod estuary

Abstract

Because of rapid increases in population, anthropogenic sources of nitrogen have adversely impacted the water quality of coastal ponds on Cape Cod. A major source of “new” nitrogen to these estuaries is groundwater, which intercepts septic tank fields in its flow path to the coastline. Many attempts have been made to quantify this process; however, groundwater discharge is often patchy in nature and is therefore difficult to study by use of traditional techniques such as seepage meters. In Waquoit Bay, MA, we tested an approach based on radium, which is naturally enriched in aquifer fluids and has four isotopes with half‐lives ranging from 4 d to 1600 yr. Groundwater entering the bay was low in salinity and contained several orders of magnitude greater radium and dissolved inorganic nitrogen (DIN) relative to ambient bay water. Using a mass‐balance approach for radium, we calculated a submarine groundwater flux of ~37,000 m3 d−1, which compared well with aquifer recharge rates calculated from rainfall. From the DIN content of the groundwater, we estimated that ~2100 mol N d−1 was directly input to the estuary. However, this nitrogen flux was small in comparison with literature values for DIN fluxes from the heavily populated subestuaries. Furthermore, our results suggest that groundwater flux of DIN was assimilated by plant biomass during the summer but may be exported from the embayment to coastal waters during the winter months.