Mid- to long-term radiocesium wash-off from contaminated catchments at Chernobyl and Fukushima.

We analyzed mid- to long-term Cs wash-off from the catchments contaminated due to the Chernobyl accident in 1986 and the Fukushima Dai-ichi Nuclear Power Plant accident in 2011. A semi-empirical diffusional model for radionuclide wash-off is proposed to enable estimation of the dissolved and particulate Cs wash-off ratios for the Chernobyl and Fukushima contaminated catchments; the differences in the wash-off characteristics for these two regions are explained and their long-term trends predicted. The model is based on the premise that the catchment topsoil layer is the source of sediments in the rivers, and the radionuclide concentration in the topsoil can be described by a simple diffusion equation.