Salt marsh submarine groundwater discharge as traced by radium isotopes

Submarine groundwater discharge (SGD) generally occurs through permeable sediments where the hydraulic head of an aquifer is above sea level, and often includes a recirculated seawater component. In order to determine SGD to the Great Sippewissett Marsh, West Falmouth, MA, we measured the activities of four radium isotopes (226Ra, 228Ra, 223Ra, 224Ra) at the marsh inlet in July 1999 and 2001 and compared our data with Ra activities measured at the same location in 1983 and 1985. A radium-based approach for estimating SGD to the marsh yielded a summer average of 3900 m3 day−1. This flux was in good agreement with independent estimates from the literature, which ranged from 600 to 23,000 m3 day−1. Radium activities of the long-lived isotopes (226Ra, 228Ra) were on average more than two times higher in 1999 than during the three other time periods. These results suggest that drought conditions leading to enhanced seawater–sediment interactions may be an important mechanism in delivering certain dissolved substances to coastal waters.