A COLLECTIVE MODEL FOR PREDICTING THE LONG-TERM BEHAVIOR OF RADIONUCLIDES IN RIVERS

Here I describe a collective model to predict the long term behaviour of Sr-90 in river catchments. The model is applied to 11 Italian river s contaminated by Sr-90 due to nuclear explosions in the atmosphere ov er past decades. The uncertainty at the 68% confidence level of the mo del, when used as a generic tool for evaluating the concentration of t he 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 d espite the large range of catchment characteristics. The model is base d on the assumption that the time behaviour of the Sr-90 (Bq s(-1)) tr ansported by water, following a single pulse of radionuclide depositio n, is the sum of some exponential components. In the present paper the components were supposed characterised by the following decay constan ts: 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 Sr-90 transfer coefficient from the catchment to the river, that, in the case of a pu lse deposition, is approximately equal to the ratio between the radion uclide concentration in water and the deposition, is estimated to be 0 .2 m(-1). (C) 1997 Elsevier Science B.V.