Estimation on the origin and behavior of radioactive Cs in Natsui river basin

In this study, we conducted a survey of radioactive Cs contained in soil, sediment, vegetation, and lakes and river water in the Natsui river basin, which was extensively contaminated by radioactive fallout from the nuclear accident. This study focuses on the runoff behavior, pollution level/scale and accumulation in soils/sediments of radioactive 134Cs and 137Cs. Half-life ratios of 134Cs and 137Cs were used to quantitatively evaluate the impact from the nuclear accident source. In addition, a model was created to reproduce and forecast the concentration of Cs and to predict long-term changes in water contamination levels in the Kodama-dam lake and its river inflow. In this study, a relatively high Cs concentration was detected in the lake and pond sediments with the Fukushima nuclear power plant estimated to be the source of more than 90% of the Cs contamination. From the results it was estimated that the Cs concentration in the Lake of Kodama dam was initially a few Bq/L, with an expected reduction to about 0.1 Bq/L in 10 years.