Downward migration of radiocesium in an abandoned paddy soil after the Fukushima Dai-ichi Nuclear Power Plant accident.

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, continuous monitoring of the detailed vertical distribution of radiocesium in soil is required to evaluate the fate of radiocesium and establish strategies for remediation and management of the contaminated land. It is especially important to investigate paddy soil because little knowledge has been accumulated for paddy soil and wetland rice is a major staple in Japan. Therefore, we monitored the vertical distribution of 137Cs in abandoned paddy soil in a planned evacuation zone from June 2011 to March 2016. The decontamination works (i.e., 5 cm of surface soil removal and re-covering with uncontaminated soil) were conducted by the government in 2015. As a result of monitoring, the 137Cs gradually migrated downward with time and the 137Cs concentration in the 0-10 cm soil was almost homogenous in October 2014, although it was non-cultivated. The liner relationship was obtained between the median depth, which is the thickness of a soil layer containing half of the total 137Cs inventory, and the time after the accident, indicating the migration rate was constant (1.3 cm y-1) before the decontamination works. After the decontamination works, the 137Cs concentration in the uppermost surface layer was reduced by 90%, however the total 137Cs inventory was reduced by only 50-70%. It was shown that the efficiency of 137Cs removal by the decontamination works decrease linearly over time in fields like the studied paddy, in which the homogenization of 137Cs concentration occurred. Conversely, the downward migration of 137Cs to subsurface layers deeper than 10 cm (i.e., plowpan layer) with low permeability rarely occurred. It is expected that these unique trends in distribution and migration of 137Cs would be found in abandoned paddy soils with properties similar to the studied soil, sandy loam but poorly drained because of the low permeable plowpan layer, although further validation is necessary.