Modelling the long-term behaviour of radioactive substances in fresh water systems: role of migration from catchment basins and of radionuclide exchange between water and sediment

The migration of radionuclides through catchment basins, by run-offer wash-off waters and rivers, the resuspension from sediments and the subsequent transport are important phenomena involving long-term contamination of water bodies. The main aim of the CEC Project "Analysis and modelling of the migration of radionuclides deposited in catchment basins of fresh water systems" is to investigate the various aspects of these phenomena in order to develop models assessing the behaviour of radionuclides in drainage areas of fresh water systems. In the present paper preliminary results of the research will be described: (a) The migration from catchment basins to water body may be quantified by using the so called "transfer functions" ("T.F." = amount of radionuclide (Bq s(-1)) flowing, per unit time, from the catchment to the water body following a pulse deposition on the catchment). A variety of such functions were evaluated using experimental data collected in various European rivers following the Chernobyl accident; (b) A literature survey of models predicting radionuclide migration from catchment to water bodies was carried out. The model review was intended to highlight the dominant processes involved in radionuclide migration from catchment areas to surface water bodies and to investigate the various possible approaches to drainage area modelling; (c) The long-term exchange of radionuclides between sediment and water was investigated.