The self-cleaning capacity of surface waters after radioactive fallout. Evidence from European waters after Chernobyl, 1986-1988

Over 160 water samples from 5 lakes in Switzerland were analysed between 1986 and 1988 for caesium-134 and caesium-137 following the release of radioactivity from the Chernobyl nuclear reactor accident. Caesium was extracted by impregnated ion exchange resins and counted in a high purity germanium well detector. Sediments were also examined. A model describing radionuclide washout from aquifers is proposed. Initial rates of decrease of caesium-137 in rivers was approximately 0.125 per day. Whole lake removal rates were relatively fast. Residence times of caesium-137 varied between 5 and 21 months. Horizontal boundaries in Zurich lake appeared to act first as sinks for caesium-137 from the upper water column and later as source to the deeper parts of the lake. Rates of adsorption and desorption associated with settling particles were small compared with other processes. There was evidence for remobilization of caesium from sediments in anoxic hypolimnetic waters in Lugano lake. Dangers to drinking water resources were smaller than previous estimates. There are 50 references.