Radium and radon radioisotopes in regional groundwater, intertidal groundwater, and seawater in the Adelaide Coastal Waters Study area: Implications for the evaluation of submarine groundwater discharge

The input of groundwater-borne nutrients to Adelaide's (South Australia) coastal zone is not well known but could contribute to the ongoing decline of seagrass in the area. As a component of the Adelaide Coastal Waters Study (ACWS), the potential for using the radium quartet (223Ra, 224Ra, 226Ra and 228Ra) and 222Rn to evaluate submarine groundwater discharge (SGD) was evaluated. Potential isotopic signatures for SGD were assessed by sampling groundwater from three regional aquifers potentially contributing SGD to the ACWS area. In addition, intertidal groundwater was sampled at two sand beach sites. In general, the regional groundwaters were enriched in long-lived Ra isotopes (226Ra and 228Ra) and in 222Rn relative to intertidal groundwater. Radium activity (but not 222Rn activity) was positively correlated to salinity in groundwater from one of the regional aquifers and in intertidal groundwater. Radium isotope ratios (223Ra/226Ra, 224Ra/226Ra and 228Ra/226Ra) were less variable than individual Ra isotope activities within potential SGD sources. Recirculated seawater (estimated from the intertidal groundwater samples with seawater-like salinities) also had distinctly higher Ra isotope ratios than the regional groundwaters. The activities for all radioisotopes were relatively low in seawater. The activity of the short-lived 223Ra and 224Ra were highest at the shoreline and declined exponentially with distance offshore. In contrast, 228Ra and 226Ra activities had a weak linear declining trend with distance offshore. Rn-222 activity was at or near background in all seawater samples. The pattern of enrichment in short-lived Ra isotopes and the lack of 222Rn in seawater suggest that seawater recirculation is the main contributor to SGD in the ACWS area. Preliminary modeling of the offshore flux of 228Ra and 226Ra suggest that the SGD flux to the ACWS area ranges between 0.2 and 3·10−3 m3 (m of shoreline)−1 s−1.