The Fukushima nuclear accident in 2011 resulted in Cs contamination of large areas in northeast Japan. A watershed-scale Cs transport model was developed and applied to a forested catchment in Fukushima area. This model considers Cs wash-off from vegetation, movement through soils, and transport of dissolved and particulate Cs adsorbed to clay, silt and sand. Comparisons between measurements and simulations demonstrated that the model well reproduced Cs concentrations in the stream fed from the catchment. Simulations estimated that 0.57 TBq of Cs was exported from the catchment between June, 2011 and December, 2014. Transport largely occurred with eroded sediment particles at a ratio of 17:70:13 of clay, silt, and sand. The overall Cs reduction ratio by rainfall-runoff wash-off was about 1.6%. Appreciable Cs remained in the catchment at the end of 2014. The largest rate of Cs reduction by wash-off was simulated to occur in subwatersheds of the upper catchment. However, despite relatively low initial deposition, middle portions of the watershed exported proportionately more Cs by rainfall-runoff processes. Simulations indicated that much of the transported Cs originates from erosion over hillsides and river banks. These results suggested that areas where Cs accumulates with redeposited sediments can be targeted for decontamination and also provided insight into Cs transport at the watershed scale to assess risk management and decontamination planning efforts.
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