ESTIMATION OF ECOLOGICAL HALF-LIVES OF CAESIUM-137 IN LAKES CONTAMINATED BY CHERNOBYL FALLOUT

摘要

The concept of ecological half-life describes the decrease in radionuclide concentration in natural ecosystems resulting from all processes, including a wide range of physical, chemical and biological processes as well as physical decay. Knowledge of ecological half-lives is important from a radiation protection point of view, since such values can be used for a rapid estimate of recovery times in contaminated areas. Within the scope of the IAEA co-ordinated research programme Validation of Environmental Model Predictions (VAMP), sets of empirical data from seven European lakes contaminated by Chernobyl fallout have beenrnused to determine the ecological half-lives of radiocaesium for water and different fish species by curve fitting of observed data. For radiocaesium in lake waters, two distinct ecological half-lives were observed: (1) an initial short half-life of a few months due to transfer to the sediments, as well as a short water retention period during spring in several lakes; and (2) the subsquent ecological half-life of radiocaesium in lake waters, which was longer and was related both to the lakes' water retention period and to the characteristics of the secondary load from the catchment. The ecological half-life in fish varied with both species and lake, ranging from 1 year for small perch from Usselmeer in the Netherlands to 4.6 years for pike from Iso Valkjarvi in Finland. The influence of temperature on the biological half-life in fish was reflected in the values of ecological half-lives, with more southern, warmer lakes showing a faster rate of recovery. Ecological half-lives are not constant but vary with both lake characteristics and fish species. However, the VAMP studies have shown that simple predictive models can account for a wide range of different processes by utilizing the concept of ecological half-life.