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 distributi on and chemical behaviour patterns for these radionuclides in the thre e aquatic environments. Ra-226 from riverborne particles dissolves in the estuary and contributes to increased concentrations of dissolved R a-226 in estuarine water. In the estuary, dissolved Pb-210 and Po-210 from river discharge and atmospheric deposition are scavenged by suspe nded matter, which in turn becomes enriched in these nuclides in compa rison with riverborne particles. As a result of these processes, the e stuarine water flowing into the coastal sea contains enhanced concentr ations of dissolved Ra-226, but is depleted in dissolved Pb-210 and Po -210. Under average river flow conditions, mass balance calculations f or dissolved Po-210 and Pb-210 in the estuary allowed their mean resid ence times to be estimated as 18 and 30 days, respectively. Due to the rapid sorption of these radionuclides on to settling particles, botto m sediments in the estuary represent a sink for Pb-210 and Po-210 from both natural sources and industrial waste releases. Results also sugg est that partial re-dissolution of these radionuclides from bottom sed iments 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 dissolve d fraction can only account for one third of the activities entering t he estuary in the soluble phase. Implications of these results to the cycling of radionuclides in phosphate waste releases into estuarine en vironments are discussed. (C) 1997 Elsevier Science B.V.