Using the radium quartet for evaluating groundwater input and water exchange in salt marshes

The fluxes of Ra-226 (half-life = 1600 years) and Ra-228 (half-life = 5.7 years) from the North Inlet salt marsh to the sea are much larger than can be supported by decay of their Th parents in the surface marsh sediments. These fluxes are sustained almost entirely by groundwater flow through the marsh. An average groundwater how of approximately 10 cm(3) cm(-2) day(-1) is indicated if the groundwater activities we have measured are representative. The fluxes of Ra-223 (half-life = 11.4 day) and Ra-224 (half-life = 3.6 day) are factors of 22, and ten more than those expected from the Aux of Ra-226. Groundwater also sustains most of the flux of the short-lived isotopes. The measured Ra activity ratio pattern in the marsh creeks matches the groundwater signature but is distinct from the pattern of the parent thorium isotopes in the sediment. We present a model to explain the anomalous distribution pattern of these isotopes. Despite their large throughput, the inventories of desorbable (226)R and Ra-228 in the top 15 cm sediment layer are very low. Nevertheless, the activities of Ra-226 and Ra-228 in the porewaters are large, indicating a low distribution coefficient (similar to 10) for radium and a short retention time (similar to 10 days) in the surface sediment layer. We surmise that groundwater flow may be a significant source of radium isotopes in the waters of shallow estuaries and coastal margins. This source must be recognized while considering mass balance of any tracer, be it radium, nutrients, other metals, or delta(18)O.