Tracing groundwater discharge into the ocean via continuous radon-222 measurements

The direct discharge of groundwater into the coastal zone has received increased attention in the last few years as it is now recognized that this process represents an important pathway for material transport. Assessing these material fluxes is difficult, as there is no simple means to gauge the water flux. To meet this challenge, a working group established by the Scientific Committee on Oceanic Research (SCOR) and the Land-Ocean Interactions in the Coastal Zone (LOICZ) Project of the International Geosphere-Biosphere Program (IGBP) is conducting a series of groundwater discharge assessment intercomparison experiments. Two such experiments have been held to date, one along the Gulf of Mexico coast in Florida (August, 2000) and a second in a coastal plain environment south of Perth, Australia (November/December, 2000). A multi-disciplinary group of investigators made estimates of submarine groundwater discharge based on manual and automated seepage meter measurements, natural isotopic tracers, and hydrogeological modeling approaches. A continuous radon monitor measured radon concentrations in the shallow coastal zones during both experiments. A mass balance approach was used to calculate groundwater inputs into both systems based on these radon measurements. Results suggest good correspondence between geochemical tracers and seepage meters while hydrogeological modeling results appear lower, perhaps because of recirculated seawater, detected via tracers and seepage meters but not accounted for by modeling.