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

This study presents the temporal variations in riverine Cs 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 and particulate Cs concentrations appeared to reflect the spatial pattern of the Cs inventory in the catchments, rather than variations in physico-chemical properties of water and suspended solid. Negative relationships between the Cs concentration and δN in suspended solid were found in all rivers during the intense rainfall events, suggesting an increased contribution of sediment from forested areas to the elevation of particulate Cs concentration. The Cs flux ranged from 0.33 to 19 GBq, depending on the rainfall erosivity. The particulate Cs 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 percentage of Cs desorbed in seawater relative to Cs in suspended solids ranged from 2.8% to 6.6% and tended to be higher with a higher fraction of exchangeable Cs. The estimated potential release of Cs desorbed from suspended solids to the ocean was 0.022-0.57 GBq, and its ratio to the direct flux of dissolved Cs was 0.12-6.2. Episodic sampling during high-flow events demonstrated that the particulate Cs flux depends on catchment characteristics and controls Cs transfer to the ocean.