USE OF RADIUM ISOTOPES TO DELINEATE SUBMARINE GROUNDWATER DISCHARGE FROM DIFFERENT SOURCES AND TO ESTIMATE FLUXES

Submarine groundwater discharge (SGD) is being acknowledged as an important flux of materials to estuaries and the coastal ocean Several components of SGD are recognized: (1) fresh water flowing directly from an aquifer to the estuary or ocean, (2) mixtures of freshwater and seawater cycling through surficial unconfined aquifers, and (3) inputs of freshwater-seawater mixtures from deeper semiconfined aquifers. These components may interact and mix before they enter the ocean. Ideally we would like to know the total flux of SGD to the system and the fraction of this flux attributable to each of these three components. Naturally-occurring radionuclide tracers, especially the four radium isotopes, offer promise in realizing this goal.

In August 2000 a SGD intercomparison experiment was organized on a small embayment on the northeast Gulf of Mexico to compare estimates of SGD obtained from seepage meters, geochemical tracers, and hydrologic modeling. Ra isotope samples were collected from seepage meters, piezometers, and surface waters.

Samples collected within the nearshore SGD experimental area were unusually enriched in all four radium isotopes. These enrichments must be due to radium input from SGD because there is practically no surface water flow into this area and the sediments are primarily quartz sand. Samples collected from seepage meters were about a factor of 2-3 higher in radium activity compared to the overlying waters. Samples from piezometers 1-4 meters below the sea bed were 1-2 orders of magnitude higher than surface waters.

The two long-lived Ra isotopes, Ra-228 and Ra-226, 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 endmember mixing model can describe the Ra distribution in all samples.

The short lived Ra isotopes, Ra-223 and Ra-224, were used to establish mixing rates for the 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 Ra-226 gradient established the Ra-226 flux. This flux must be balanced by Ra input from SGD. The average flux of SGD within 200 m of shore based on the Ra-226 budget was 10 cubic cm per square cm per day. This flux agreed well with other estimates based on seepage meters and Rn-222.