A collective model for predicting the long-term behaviour of radionuclides in rivers.

Here I describe a collective model to predict the long term behaviour of 90Sr in river catchments. The model is applied to 11 Italian rivers contaminated by 90Sr due to nuclear explosions in the atmosphere over past decades. The uncertainty at the 68% confidence level of the model, when used as a generic tool for evaluating the concentration of the radionuclide in water, is a factor 1.8 around the predicted values. The reliability of the model output is due to the mutual compensation effects of different phenomena occurring in the catchments that lead to 'collective' behaviours which are scantily variable and uncertain despite the large range of catchment characteristics. The model is based on the assumption that the time behaviour of the 90Sr (Bq s-1) transported by water, following a single pulse of radionuclide deposition, is the sum of some exponential components. In the present paper the components were supposed characterised by the following decay constants: lambda 1 = 2.3 x 10(-7) s-1, lambda 2 = 4.2 x 10(-9) S-1 and lambda 3 = 4.2 x 10(-10) S-1. The average value of 90Sr transfer coefficient from the catchment to the river, that, in the case of a pulse deposition, is approximately equal to the ratio between the radionuclide concentration in water and the deposition, is estimated to be 0.2 m-1.