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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 137Cs 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 137Cs, suspended 137Cs, and dissolved 137Cs were positive, indicating that the reservoir acted as a source of 137Cs. The source of 137Cs from the reservoir was considered to be the sediment, with suspended 137Cs due to resuspension and dissolved 137Cs due to desorption. The dissolved 137Cs activity concentration in the outflow water was positively correlated with the water temperature, suggesting that the desorption of dissolved 137Cs from the sediment can be attributed to the exchange with cations such as NH4+ generated by biological activities. The dissolved 137Cs 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 137Cs 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 137Cs 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. [ABSTRACT FROM AUTHOR]

The 2011 Great East Japan earthquake and tsunami led to dispersal of radionuclides into the surrounding area from the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant. Before agricultural activities can safely resume after this accident, it is necessary to evaluate the dynamics of radiocesium in irrigation water in the area. We measured the concentrations of total and dissolved radiocesium in irrigation water and analyzed radiocesium concentrations in rice and soil samples, the soil to brown rice transfer factor (TF) in two decontaminated paddy fields located within the zone where paddy rice culture was prohibited, and radiocesium/water balance in a paddy field in 2014. Our key findings were as follows: (1) about 85% of the radiocesium in the irrigation water did not drain away but accumulated in the paddy field; (2) in comparison with the amount of the radiocesium deposited in the fields, the amount of radiocesium that accumulated during irrigation was approximately 0.076%; (3) the outflow of total radiocesium from paddy field was 13.0% of the inflow to a paddy field (irrigation water and fallout); (4) the soil to brown rice TF was 0.0015–0.0068 in the decontaminated paddy fields where soil improvement was performed to increase the content of exchangeable potassium to 200 (mg K)/kg soil before the conventional application; and (5) the concentration of radiocesium in brown rice was about 2% of the standard limit in Japan (100 Bq/kg), and the impact on brown rice from radiocesium in the irrigation water was limited. [ABSTRACT FROM AUTHOR]