Spatial and Temporal Changes of 137 Cs Concentrations in River Waters and Correlation with the Radiocesium Inventory in Fukushima and Adjacent Areas

This study compiled previously reported data on accident-derived 137Cs concentrations in rivers of Fukushima and adjacent areas during 2011–2019 to investigate the factors controlling spatial and temporal changes in 137Cs concentrations in river waters. Geomorphological analyses of the contributions of 137Cs sources, which were located at different distances from the sampling points in the catchment areas, revealed that the dissolved 137Cs concentrations in river water were strongly correlated with the mean 137Cs inventories of the entire catchment area. However, 137Cs in suspended solids was found to have relatively weak correlations with the mean 137Cs inventory. These results suggest that dissolved 137Cs can originate from a larger area of the catchment even under baseflow conditions, whereas 137Cs in suspended solids did not show such behavior. The dissolved 137Cs concentration normalized with the mean 137Cs inventories in the catchment area, which indicated that the transportability of the dissolved 137Cs from the catchment, was relatively high in the Abukuma and Tone Rivers. The normalized dissolved 137Cs concentrations have declined exponentially since the accident with the effective decreasing coefficient of 0.28–0.39 year−1 (0.26 year−1 for all rivers) for 10 years, whereas a significant decrease was not observed in Tone River.