Response of 137Cs dynamics in dam lakes to temperature and weather conditions

This study examined the influence of dam lakes on ¹³⁷Cs dynamics in downstream river systems based on monthly observations of inflow/discharge water over 10 years at the Matsugabo Dam and Yokokawa Dam, which were affected by the Fukushima Daiichi nuclear power plant accident in 2011. Dissolved ¹³⁷Cs concentrations in the discharge water of both dams showed seasonal fluctuation, but the phase of the fluctuation lagged that of water temperature, and the relationship between the distribution coefficient of ¹³⁷Cs and temperature in the dam discharge water was poorly reproduced by the van't Hoff equation. These discrepancies indicate that dissolved ¹³⁷Cs concentrations in the dam discharge are not determined predominantly by in situ ion exchange between solid and aqueous phases. In autumn 2019, dissolved ¹³⁷Cs concentrations in the discharge of both dams decreased markedly, and the role of the Yokokawa Dam lake shifted from that of a source to a sink of dissolved ¹³⁷Cs for the following three years. This occurred because Typhoon Hagibis brought intensive rainfall and caused watershed erosion, which resulted in accumulation of large amounts of sediment and changed the ¹³⁷Cs balance between the water and sediment in the dam lake. When dam water storage decreased, notable increase in dissolved ¹³⁷Cs concentration was observed in the discharge water. Multiple partial regression indicated that water level decline is more influential relative to water temperature rise regarding the increase in dissolved ¹³⁷Cs concentrations in lake water, suggesting that water level management could mitigate the elevation of ¹³⁷Cs concentration in discharge water.