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

Abstract 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 (137Cs) 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 137Cs inventory due to the Fukushima Dai-ichi Nuclear Power Plant accident. The results of our study showed that dissolved 137Cs concentration was significantly higher in the output water than that in the main input water, and that the effective ecological half-life of dissolved 137Cs in the output water was longer than in the main input water. We quantitatively evaluated the mass balance of dissolved 137Cs in the reservoir to elucidate how much dissolved 137Cs from the rivers and production from reservoir sediments contribute to 137Cs in the reservoir output. The annual output of dissolved 137Cs was significantly higher than the total input of dissolved 137Cs, with approximately 32%–40% of the dissolved 137Cs in the output water presumably being produced from reservoir sediments. Consequently, the estimated dissolved 137Cs fluxes from reservoir sediments to overlying water were 0.57–1.3 × 104 Bq m−2 y−1. This implies that approximately 0.04%–0.09% of 137Cs accumulated in the sediments was released through elution to the overlying water each year. Reservoir sediments containing high 137Cs levels may thus become even more important as sources of bioavailable dissolved 137Cs in the future.