Using Ra isotopes to examine transport processes controlling benthic fluxes into a shallow estuarine lagoon

Measurements of the benthic flux of four naturally occurring radium isotopes in a shallow lagoon in the Bega River estuary has provided information on the types and rates of transport processes operating in the lagoon sediments. The measurement techniques included Ra mass budgets of the lagoon, Ra fluxes into benthic chambers, and modelling of the pore water and solid phase Ra profiles in a sediment core. The sediment profile of Pb-210, and the solid phase and pore-water profiles of the longer-lived Ra isotopes, Ra-228 (half-life 5.7 years) and Ra-226 (half-life 1600 years), indicate bioturbation to a depth of 10 cm. A diffusion-bioturbation model has been used to assess the relative importance of molecular diffusion and bioturbation as transport processes controlling the benthic Aux of Ra. The flux of the shortest-lived isotope, Ra-224 (half-lift: 3.7 days), is not significantly enhanced by bioturbation, and its Bur is consistent with diffusion-controlled release. However bioturbation enhances the Ra-228 Bur by a factor of more than two over the flux due to molecular diffusion alone. Modelled pore-water profiles and Bur calculations are consistent with a bioturbation time scale between 0.5 and 2 years. The measured benthic flux of Ra-226 is much greater than can be accounted for by the modelled profile, and may be due to slow Ra-226 desorption from the sediment, variable sediment accumulation rates, or groundwater flow. Based on Ra-226 pore-water and flux measurements at the time of this study, groundwater how has an upper limit of 0.3 cm d(-1). Copyright (C) 2000 Elsevier Science Ltd.