Modeling watershed-scale ¹³⁷Cs transport in a forested catchment affected by the Fukushima Dai-ichi Nuclear Power Plant accident

The Fukushima nuclear accident in 2011 resulted in ¹³⁷Cs contamination of large areas in northeast Japan. A watershed-scale ¹³⁷Cs transport model was developed and applied to a forested catchment in Fukushima area. This model considers ¹³⁷Cs wash-off from vegetation, movement through soils, and transport of dissolved and particulate ¹³⁷Cs adsorbed to clay, silt and sand. Comparisons between measurements and simulations demonstrated that the model well reproduced ¹³⁷Cs concentrations in the stream fed from the catchment. Simulations estimated that 0.57 TBq of ¹³⁷Cs 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 ¹³⁷Cs reduction ratio by rainfall-runoff wash-off was about 1.6%. Appreciable ¹³⁷Cs remained in the catchment at the end of 2014. The largest rate of ¹³⁷Cs 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 ¹³⁷Cs by rainfall-runoff processes. Simulations indicated that much of the transported ¹³⁷Cs originates from erosion over hillsides and river banks. These results suggested that areas where ¹³⁷Cs accumulates with redeposited sediments can be targeted for decontamination and also provided insight into ¹³⁷Cs transport at the watershed scale to assess risk management and decontamination planning efforts. - Highlights: • A watershed-scale ¹³⁷Cs transport model was developed. • The model reproduced ¹³⁷Cs transport in the Kuchibuto River catchment. • The model estimated ¹³⁷Cs load and the ratio of particulate ¹³⁷Cs transported by different solid particles from the catchment. • An appreciable amount of ¹³⁷Cs still remained in the catchment at the end of 2014. • Spatial distribution of ¹³⁷Cs transport in the catchment was identified.