Sources and Fluxes of Submarine Groundwater Discharge Delineated by Radium Isotopes

This paper reports the results derived from radium isotopes of a submarine groundwater discharge (SGD) intercomparison in the northeast Gulf of Mexico. Radium isotope samples were collected from seepage meters, piezometers, and surface and deep ocean waters. Samples collected within the near-shore SGD experimental area were highly enriched in all four radium isotopes; offshore samples were selectively enriched. Samples collected from seepage meters were about a factor of 2-3 higher in radium activity compared to the overlying waters. Samples from piezometers, which sampled 1-4 meters below the sea bed were 1-2 orders of magnitude higher in radium isotopes than surface waters. The two long-lived Ra isotopes, 228Ra and 226Ra, provide convincing evidence that there are two sources of SGD to the study area: shallow seepage from the surficial aquifer and input from a deeper aquifer. A three end-member mixing model can describe the Ra distribution in these samples. The short-lived radium isotopes, 223Ra and 224Ra, were used to establish mixing rates for the near-shore study area. Mixing was retarded within 3 km of shore due to a strong salinity gradient. The product of the mixing rate and the offshore 226Ra gradient established the 226Ra flux. This flux must be balanced by Ra input from SGD. The flux of SGD within 200 m of shore based on the 226Ra budget was 1.5 m3 min-1. This flux agreed well with other estimates based on seepage meters and 222Rn.