Thirty-year simulation of environmental fate of 137Cs in the Abukuma River basin considering the characteristics of 137Cs behavior in land uses.

The Fukushima Daiichi Nuclear Power Plant accident caused a radioactive contamination of deposited radionuclides, including 137Cs, on the land surface. Cesium-137 deposited on the land surface was strongly adsorbed on soil particles and was then washed off through soil erosion. Trends of temporal variation of 137Cs wash-off varied greatly depending on land use. Therefore, it is important to reflect the characteristics of 137Cs migration processes in each land use to clarify the long-term fate of 137Cs. In this study, a 30-year simulation of environmental fate of 137Cs was conducted using a distributed radiocesium prediction model, taking into account the characteristics of the 137Cs behavior in each land use. Overall, in the Abukuma River basin, the 137Cs transported into the ocean for 30 years was estimated to correspond to 4.6 % of the initial deposition in the basin, and the effective half-life of 137Cs deposited in the basin was estimated to be 3.7 years shorter (by 11.6 %) than its physical half-life. These results suggested that 137Cs deposited from the accident could still remain for decades. Based on the analysis of the 137Cs behavior in land use, in 2011, the contribution of 137Cs export to the ocean from urban lands was estimated to correspond to 70 % of the total 137Cs export. Meanwhile, from 2012 to 2040, the contribution of 137Cs export from agricultural lands was estimated to correspond to 75 % of the total 137Cs export. The reduction ratios excluding radioactive decay of 137Cs remained in areas with and without human activities for 30 years after the accident, defined as the ratios of the total outflow to the initial deposition, were estimated to be 11.5 %-17.7 % and 0.4 %-1.4 %, respectively. These results suggested that human activities enhance the reduction of 137Cs remaining in land in the past and future.