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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



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



Marine Chemistry 109(3-4): 318-336



The input of groundwater-bome 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 (Ra-223, Ra-224, Ra-226 and Ra-228) and Rn-222 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 (Ra-226 and Ra-228) and in Rn-222 relative to intertidal groundwater. Radium activity (but not Rn-222 activity) was positively correlated to salinity in groundwater from one of the regional aquifers and in intertidal groundwater. Radium isotope ratios (Ra-223/Ra-226, Ra-224/Ra-226 and Ra-224/Ra-226) 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 Ra-223 and Ra-224 were highest at the shoreline and declined exponentially with distance offshore. hi contrast, Ra-228 and Ra-226 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 Rn-222 in seawater suggest that seawater recirculation is the main contributor to SGD in the ACWS area. Preliminary modeling of the offshore flux of Ra-228 and Ra-226 suggest that the SGD flux to the ACWS area ranges between 0.2 and 3 center dot 10(-3) m(3) (m of shorefine)(-1) s(-1).

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Accession: 033078749

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DOI: 10.1016/j.marchem.2007.08.010


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