Hydrologic forcing of submarine groundwater discharge: Insight from a seasonal study of radium isotopes in a groundwater-dominated salt marsh estuary

A seasonal study of radium-derived submarine groundwater discharge (SGD) and associated nitrogen fluxes was carried out in a salt marsh estuary between 2001 and 2003 (Pamet River Estuary, Massachusetts). Twelve-hour time series of salinity and radium at the estuary inlet were used to determine the relative importance of fresh versus saline SGD, respectively. The distinct radium (Ra-228: Ra-226) isotopic signature of marsh peat pore water and aquifer-derived brackish groundwater was used to further partition the Ra-derived SGD estimate. Of these three groundwater sources, only the marsh-derived groundwater was constant across time. The ratio of brackish to fresh SGD was inversely correlated with water table elevation in the aquifer, suggesting that Ra-derived SGD was enhanced during dry periods. The various SGD fluxes were responsible for an average annual dissolved inorganic nitrogen (DIN) input of between 1.7 mol m(-2) yr(-1) and 7.1 mol m(-2) yr(-1) and a soluble reactive phosphate (SRP) flux of 0.13-0.54 mol m(-2) yr(-1). Approximately 30% of the SGD-derived DIN and SRP flux is exported to coastal waters (Cape Cod Bay), whereas 70% is retained by the salt marsh ecosystem.