Tsuji Hideki, Nishikiori Tatsuhiro, Ito Shoko, Ozaki Hirokazu, Watanabe Mirai, Sakai Masaru, Ishii Yumiko, Hayashi Seiji
National Institute for Environmental Studies, Fukushima Regional Collaborative Research Center, Japan; Fukushima Institute for Research, Education and Innovation, Japan.
National Institute for Environmental Studies, Fukushima Regional Collaborative Research Center, Japan; Agricultural Radiation Research Center, Tohoku Agriculture Research Center, National Agriculture and Food Research Organization, Japan.
Environ Pollut. 2025 Aug 1;378:126445. doi: 10.1016/j.envpol.2025.126445. Epub 2025 May 16.
This study examined the influence of dam lakes on Cs 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 Cs 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 Cs and temperature in the dam discharge water was poorly reproduced by the van't Hoff equation. These discrepancies indicate that dissolved Cs concentrations in the dam discharge are not determined predominantly by in situ ion exchange between solid and aqueous phases. In autumn 2019, dissolved Cs 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 Cs 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 Cs balance between the water and sediment in the dam lake. When dam water storage decreased, notable increase in dissolved Cs 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 Cs concentrations in lake water, suggesting that water level management could mitigate the elevation of Cs concentration in discharge water.
本研究基于2011年受福岛第一核电站事故影响的松ヶ尾坝和横川坝10年的月入流/排水量观测数据，研究了堰塞湖对下游河流系统中铯动态的影响。两座坝排放水中溶解态铯的浓度均呈现季节性波动，但其波动相位滞后于水温，且范特霍夫方程无法很好地再现坝排放水中铯的分配系数与温度之间的关系。这些差异表明，坝排放水中溶解态铯的浓度并非主要由固相和水相之间的原位离子交换决定。2019年秋季，两座坝排放水中溶解态铯的浓度显著下降，在随后的三年里，横川坝湖的作用从溶解态铯的源转变为汇。这是因为台风海贝斯带来了强降雨并导致流域侵蚀，从而导致大量沉积物堆积，并改变了坝湖水中与沉积物之间的铯平衡。当坝蓄水量减少时，排放水中溶解态铯的浓度显著增加。多元偏回归分析表明，就湖水溶解态铯浓度增加而言，水位下降比水温升高的影响更大，这表明水位管理可以减轻排放水中铯浓度的升高。
