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

Because of rapid increases in population, anthropogenic sources of nitrogen have adversely impacted the water quality of coastal ponds on Cape God. A major source of "new" nitrogen to these estuaries is groundwater, which int ercepts 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 tradi tional 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 magni tude greater radium and dissolved inorganic nitrogen (DIN) relative to ambi ent bay water. Using a mass-balance approach for radium, we calculated a su bmarine groundwater flux of similar to 37,000 m(3) d(-1), which compared we ll with aquifer recharge rates calculated from rainfall. From the DIN conte nt of the groundwater, we estimated that similar to 2100 mol N d(-1) was di rectly input to the estuary. However, this nitrogen flux was small in compa rison with literature values for DIN fluxes from the heavily populated sube stuaries. Furthermore, our results suggest that groundwater flux of DIN was assimilated by plant biomass during the summer but may be exported from th e embayment to coastal waters during the winter months.