Distribution, cycling and mean residence time of Ra-226, Pb-210 and Po-210 in the Tagus estuary

Results for dissolved and particulate Ra-226, Pb-210 and Po-210 in the Tagus river, estuary and coastal sea system show different distribution and chemical behaviour patterns for these radionuclides in the three aquatic environments. Ra-226 from riverborne particles dissolves in the estuary and contributes to increased concentrations of dissolved Ra-226 in estuarine water. In the estuary, dissolved Pb-210 and Po-210 from river discharge and atmospheric deposition are scavenged by suspended matter, which in turn becomes enriched in these nuclides in comparison with riverborne particles. As a result of these processes, the estuarine water flowing into the coastal sea contains enhanced concentrations of dissolved Ra-226, but is depleted in dissolved Pb-210 and Po-210. Under average river flow conditions, mass balance calculations for dissolved Po-210 and Pb-210 in the estuary allowed their mean residence times to be estimated as 18 and 30 days, respectively. Due to the rapid sorption of these radionuclides on to settling particles, bottom sediments in the estuary represent a sink for Pb-210 and Po-210 from both natural sources and industrial waste releases. Results also suggest that partial re-dissolution of these radionuclides from bottom sediments and intertidal mudflats is likely to occur in the mid- and low-estuary zones. Nevertheless, box-model computations indicate that the discharge of Pb-210 and Po-210 into the coastal sea takes place mainly with the transport of sediment, whereas the discharge in the dissolved fraction can only account for one third of the activities entering the estuary in the soluble phase. Implications of these results to the cycling of radionuclides in phosphate waste releases into estuarine environments are discussed. (C) 1997 Elsevier Science B.V.