Temporal Changes in ¹³⁷Cs Concentrations in the Surface Soil of Flood Channel at Abukuma River Tributaries

Abstract

We investigated temporal changes in ¹³⁷Cs concentrations in the surface soil of flood channels at a primary tributary (site name, Sakai) and a secondary tributary (site name, Nekoya) of Abukuma River from 2012 to 2014. In this study, we employed a real-time kinematic global positioning system (RTK-GPS) to collect samples from pinpoint locations, and to survey the microtopographical change at the monitoring sites. The flood channel of Nekoya was considered to have been completely inundated about 11 times, while Sakai had been partially inundated a few times during the survey period. ¹³⁷Cs concentrations in the surface sediments (0–5 cm) were decreased faster than those in the watershed by inundation (s) . Specifically, ¹³⁷Cs concentrations (Bq m^-2) decreased by 72 ± 21％ at Nekoya from 2012 to 2014, and by 59 ± 34％ at Sakai from 2013 to 2014. However, ¹³⁷Cs concentrations in the flood channel of Nekoya increased at a deeper horizon up to 30 cm. This result suggested that sediment with higher ¹³⁷Cs concentration was buried by that with lower ¹³⁷Cs concentration eroded during flood events.