Reservoir sediments as a long-term source of dissolved radiocaesium in water system; a mass balance case study of an artificial reservoir in Fukushima, Japan.

Because of their large mobility and high bioavailability, it is necessary to elucidate the origins and dynamics of dissolved radionuclides in river and reservoir systems to assess the transfer of those radionuclides from water to crops and aquatic organisms. Elution from contaminated reservoir sediments, a potential source of dissolved radionuclides, presents a long-term concern, particularly for long-lived radionuclides. In this study, we systematically investigated caesium-137 (Cs) concentrations using a time-series suite of input and output water samples collected from 2014 to 2019 from the Ogaki Dam Reservoir, which has a catchment with a high Cs inventory due to the Fukushima Dai-ichi Nuclear Power Plant accident. The results of our study showed that dissolved Cs concentration was significantly higher in the output water than that in the main input water, and that the effective ecological half-life of dissolved Cs in the output water was longer than in the main input water. We quantitatively evaluated the mass balance of dissolved Cs in the reservoir to elucidate how much dissolved Cs from the rivers and production from reservoir sediments contribute to Cs in the reservoir output. The annual output of dissolved Cs was significantly higher than the total input of dissolved Cs, with approximately 32%-40% of the dissolved Cs in the output water presumably being produced from reservoir sediments. Consequently, the estimated dissolved Cs fluxes from reservoir sediments to overlying water were 0.57-1.3 × 10 Bq m y. This implies that approximately 0.04%-0.09% of Cs accumulated in the sediments was released through elution to the overlying water each year. Reservoir sediments containing high Cs levels may thus become even more important as sources of bioavailable dissolved Cs in the future.