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ABSTRACT
Downward migration of 137Cs in soils was studied in three mixed deciduous forests c.a. 40 km northwest of the Fukushima Daiichi Nuclear power plant (FDNPP), Japan. We selected three different types of forest regarding to environmental condition such as slope inclinations and snow coverage conditions at the time of deposition. We examined temporal changes in the vertical distribution of 137Cs from litter layers to 10 cm soil depth for two years (2.3 to 4.3 years after the FDNPP accident in 2011). At all three study sites, the 137Cs in the litter layer had largely migrated to surface soil by 2013. After 2014, about 80% of the 137Cs in forest soils (litter layer to 10 cm soil depth) remained within 0–5 cm soil layer. The vertical distribution had not changed substantially since 2014, suggesting that changes to the downward migration rates of 137Cs in soils drastically decreased with time. In addition, small amounts of migrating 137Cs could not be detected by the present method because there was a large spatial variation in the distribution of soil 137Cs. The results showed similar patterns of soil 137Cs distribution among the three study sites although there were differences in the environmental conditions.
Acknowledgements
We sincerely thank the Iwaki District Forest Office for supporting our field survey. We also thank Prof. K. Shizuma at the Graduate School of Engineering, Hiroshima University for the many useful suggestions and support for sample measurements. This study was supported by JSPS KAKENHI [grant number 15J03548], and Phoenix Leader Education Program (Hiroshima Initiative) for Renaissance from Radiation Disaster, Organization of the Leading Graduate Education Program. The radioactivity measurements were performed at the radiation research facility of Faculty of Engineering, Hiroshima University.
Disclosure statement
No potential conflict of interest was reported by the authors.