Vertical distributions of radiocesium in Japanese forest soils following the Fukushima Daiichi Nuclear Power Plant accident: A meta-analysis

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (F-l/t), the migration center (X-c) and the relaxation depth (alpha) using 99 soil inventory data sets. F-l/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, F-l/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only X-c increased significantly with time according to generalized mixed model analysis (p < 0.01). However, X-c and alpha in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high Cs-137 retention in the organic layer even in the fifth year after the accident. The vertical migration of Cs-137 in the mineral soil layer in the FDNPP zone appears to be due to low input of Cs-137 from organic to surface mineral soil layer after the second year. These results indicate that Cs-137 retention capacity of the organic layer can affect the apparent vertical migration of Cs-137 in the underlying mineral soil layer.