A comparative study of riverine 137Cs dynamics during high-flow events at three contaminated river catchments in Fukushima

This study presents the temporal variations in riverine ¹³⁷Cs concentrations and fluxes to the ocean during high-flow events in three coastal river catchments contaminated by the Fukushima Daiichi Nuclear Power Plant accident. River water samples were collected at points downstream in the Niida, Ukedo, and Takase Rivers during three high-flow events that occurred in 2019–2020. Variations in both the dissolved ¹³⁷Cs concentration and ¹³⁷Cs concentration in suspended solids appeared to reflect the spatial pattern of the ¹³⁷Cs inventory in the catchments, rather than variations in physico-chemical properties. Negative relationships between the ¹³⁷Cs concentration and δ¹⁵N in suspended sediment were found in all rivers during the intense rainfall events, suggesting an increased contribution of sediment from forested areas to the elevated ¹³⁷Cs concentration. The ¹³⁷Cs flux ranged from 0.33 to 18 GBq, depending on the rainfall erosivity. The particulate ¹³⁷Cs fluxes from the Ukedo River were relatively low compared with the other two rivers and were attributed to the effect of the Ogaki Dam reservoir upstream. The ratio of ¹³⁷Cs desorbed in seawater to ¹³⁷Cs in suspended solids ranged from 2.8% to 6.6% and tended to be higher with a higher fraction of exchangeable ¹³⁷Cs. The estimated potential release of ¹³⁷Cs from suspended solids to the ocean was 0.048–0.57 GBq, or 0.8–6.2 times higher than the direct flux of dissolved ¹³⁷Cs from the river. Episodic sampling during high-flow events demonstrated that the particulate ¹³⁷Cs flux depends on catchment characteristics and controls ¹³⁷Cs transfer to the ocean.