Source and distribution of dissolved radium in the Bega River estuary, Southeastern Australia

Measurements of the activities of the four naturally occurring radium isotopes in the surface water and porewater of an estuary have yielded information on the release of radium from sediments and on the extent of surface water-porewater interaction in the estuary. Under low-flow conditions, the non-conservative behaviour of dissolved radium in the estuary is almost entirely due to the flux of radium from estuarine bed sediments. Radium accumulates in bottom sediment porewater, and is then mixed with estuarine surface water, probably as a result of tidal action. shown experimentally that the enrichment of the short-lived isotopes (224Ra and 223Ra) relative to 226Ra in estuarine porewater can be explained by the repeated leaching of radium from bottom sediments by saline water, and the rapid regeneration of the short-lived isotope activity from their sediment-bound parent nuclides. The leaching of radium from bottom sediments is apparently occurring on a time scale which is long (weeks-months) compared with the 224Ra and 223Ra half-lives, indicating that the amount of ion-exchangeable radium adsorbed to the sediments is large compared with the amount dissolved in porewater. lying a simple 2-D steady-state multi-box model, 224Ra and 223Ra surface water and porewater concentrations have been used to estimate the daily flux of porewater crossing the sediment-water interface in the Bega estuary. This flux is found to be about 15% of the estuary volume.