Land use types control solid wash-off rate and entrainment coefficient of Fukushima-derived ¹³⁷Cs, and their time...

Understanding solid ¹³⁷Cs wash-off in sediment sources is important for predicting radiological risks in zones contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Yoshimura et al. (2015) studied solid ¹³⁷Cs wash-off using soil erosion plots representing different land uses in Fukushima. However, temporal trends of ¹³⁷Cs activity concentration in sediments remained unclear owing to the short duration of their observations. This study is a follow-up to that of Yoshimura et al. (2015) and provides additional observations that test the dependency of temporal variations in ¹³⁷Cs activity concentration in sediment and solid wash-off processes of ¹³⁷Cs on land use types. Eight soil erosion plots were observed, and data from 2011 to 2014 were analyzed. A normalized ¹³⁷Cs solid entrainment coefficient, defined as Sc (m² kg^-1), was calculated by dividing ¹³⁷Cs activity concentration in sediment by initial ¹³⁷Cs deposition. A particle size-corrected Sc, defined as Sc_correct (m² kg^-1), was also calculated based on granulometry. Sediment quantity-weighted mean values of Sc and Sc_correct, ranged from 0.0072 to 0.084 m² kg^-1 and 0.0052-0.078 m² kg^-1, respectively. Annual wash-off rates of solid ¹³⁷Cs were 0.0029-12% year^-1. There was no significant decreasing trend in Sc or Sc_correct on most of the plots due to its huge variability. However, on an uncultivated farmland after the removal of surface vegetation, marked surface erosion including formation of rill network was found, and Sc_correct significantly decreased as the cumulative sediment discharge increased. Our follow-up observations suggest that temporal changes in ¹³⁷Cs activity concentration in sediment at the sediment source should be controlled by soil erosion processes and their intensity.