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 b ed sediments. Radium accumulates in bottom sediment porewater, and is then mixed with estuarine surface water, probably as a result of tidal action. It is shown experimentally that the enrichment of the short-l ived isotopes (Ra-224 and Ra-223) relative to Ra-226 in estuarine pore water can be explained by the repeated leaching of radium from bottom sediments by saline water, and the rapid regeneration of the short-liv ed isotope activity from their sediment-bound parent nuclides, The lea ching of radium from bottom sediments is apparently occurring on a tim e scale which is long (weeks-months) compared with the Ra-224 and Ra-2 23 half-lives, indicating that the amount of ion-exchangeable radium a dsorbed to the sediments is large compared with the amount dissolved i n porewater. By applying a simple 2-D steady-state multi-box model, Ra -224 and Ra-223 surface water and porewater concentrations have been u sed to estimate the daily flux of porewater crossing the sediment-wate r interface in the Bega estuary. This flux is found to be about 15% of the estuary volume.