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Abstract. 14 years after the nuclear reactor accident in Chernobyl, activity concentrations of lake water are measured as 0.06 mBq/1. 2 mBq/1: and 60 mBq/1 in Lake Constance. Lake Lugano and Lake Vorsee. These values can be explained by differences in limnological character of these lakes. To describe and predict the data of lake water contamination we use (1) a simple exponential "transfer function" model for the runoff of activity from contaminated catchments, and assume that runoff water determines lake water contamination. In a second approach (2) lake water residence times and l3, Cs uptake by sediments are taken into account together with runoff. With their blind predictions both models describe the 137 Cs contamination in lake water of the three lakes quite well Based on the second approach, a fish model is used to blindly predict the fish contamination in Lake Constance and Lake Vorsee with a deviation smaller than factor 10 from measured values. Attempts to use these models also for a number of other lakes in Europe and in the CIS countries near to the site of Chernobyl are promising with respect to a generalisation of our results to predict ni Cs contamination of freshwater lakes for radiation protection purposes in case of emergency.

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