Distribution and migration of radionuclides in the Japanese terrestrial environment

In this paper, the concentrations and vertical distributions of fallout radionuclides in surface soils collected in Japan, and the transfer parameters, such as soil-to-crop transfer factor (TF) and distribution coefficient (Kd), are discussed. The concentrations of 137Cs and 90Sr in surface soils, from surface to 5 cm depth were 17 ± 17 Bq kg-1 and 3.2 ± 3.2 Bq kg-1, respectively. The concentrations in subsurface soil, from 5 to 20 cm depth were 7.8 ± 7.8 Bq kg-1 for 137Cs and 2.4 ± 2.3 Bq kg-1 for 90Sr. The effective removal constants from the surface soil, including the radiological decay constants, were estimated as 0.044 y-1 for 90Sr and 0.037 y-1 for 137Cs. Vertical distributions of fallout radionuclides, 137Cs, 237Np, 239,240Pu and 99Tc showed that more than 90% of each radionuclide was retained in the surface layer (upper 10 cm) which contains more organic material than the subsurface layer. Transfer factor (TF) is commonly used for estimating radionuclide transfer from soil to crops. TF values of 137Cs, 99Tc, Pu and I for brown rice and polished rice were discussed. Because the distribution of a radionuclide is not uniform in a crop, it is reasonable that TFs should be given by type of edible parts, i.e. rice, cereals other than rice, tubers, root vegetables, leafy vegetables, fruit and seeds. (author)