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

Abstract

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 Sccorrect (m2 kg−1), was also calculated based on granulometry. Sediment quantity-weighted mean values of Sc and Sccorrect, 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 Sccorrect 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 Sccorrect 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.