Dynamics of radiocesium in an irrigation reservoir managed at low water level from the perspective of inflow and outflow water quality under non-flood conditions

Understanding the dynamics of radiocesium, including dissolved radiocesium with high bioavailability, in irrigation reservoirs located in nuclear disaster-affected areas is crucial. In this study, we estimated the 137 Cs supply flux from an irrigation reservoir managed at low water levels by examining the inflow and outflow water quality mainly under average flow conditions. The results showed that the annual supply fluxes of total 137 Cs, suspended 137 Cs, and dissolved 137 Cs were positive, indicating that the reservoir acted as a source of 137 Cs. The source of 137 Cs from the reservoir was considered to be the sediment, with suspended 137 Cs due to resuspension and dissolved 137 Cs due to desorption. The dissolved 137 Cs activity concentration in the outflow water was positively correlated with the water temperature, suggesting that the desorption of dissolved 137 Cs from the sediment can be attributed to the exchange with cations such as NH 4 + generated by biological activities. The dissolved 137 Cs activity concentration in the outflow water was equal to that in the inflow water in winter and exhibited seasonal variation with an increasing trend in summer. Furthermore, the carriers of suspended 137 Cs in the outflow water were considered to be fine mineral soil particles including clay, along with organic matter. Therefore, the removal of bottom sediments with high 137 Cs concentrations in reservoirs with low water levels would contribute to reducing the runoff of radiocesium, effectively restore freshwater resources, and allow for the resumption of agricultural water use.