Semi-Empirical Diffusional Model of Radionuclide Wash-Off from Contaminated Watersheds and Its Testing Using Monitoring Data for Fukushima and Chernobyl Rivers

The purpose of the work is to analyze long-term radiocesium wash-off from the catchments contaminated due to the Chernobyl accident in 1986 and Fukushima Dai-ichi NPP (FDNPP) accident in 2011. A semiempirical diffusional model for radionuclide wash-off is proposed enabling estimation of 137 Cs dissolved and particulate wash-off coefficients for Chernobyl and Fukushima contaminated catchments, explaining the differences in these parameters and predicting their long-term trend. The model is based on the premise that the erosion of catchment topsoil is the source of sediments in the rivers, and the radionuclide concentration in the topsoil can be described by a simple diffusion equation. The normalized coefficients of particulate 137 Cs wash-off for the Fukushima contaminated catchments are slightly lower than those for Chernobyl. The normalized coefficients of the dissolved 137 Cs wash-off from Fukushima catchments are at least an order of magnitude lower those for Chernobyl, mainly due to an order of magnitude difference in the 137 Cs distribution coefficients for Fukushima and Chernobyl rivers. The proposed semiempirical diffusional model for radionuclide wash-off satisfactorily describes the temporal trends in the variations of the 137 Cs wash-off coefficients, both in the Chernobyl and Fukushima, and can be used for predicting 137 Cs wash-off after a nuclear accident.