Factors controlling the dissolved 137Cs seasonal fluctuations in the Abukuma River under the influence of the Fukushima Nuclear Power Plant accident

The 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released large amounts of radioactive materials into the environment. River systems play an important role in the terrestrial redistribution of FDNPP-derived ¹³⁷Cs in association with water and sediment movement. We examined the seasonal fluctuations in dissolved and particulate ¹³⁷Cs activity concentrations and clarified the biological and physicochemical factors controlling ¹³⁷Cs in the Abukuma River’s middle course in the region affected by the FDNPP accident. The results showed the water temperature and K+ concentration dominated the seasonality of the dissolved ¹³⁷Cs activity concentration. We concluded that the ¹³⁷Cs in organic matter is not a source of dissolved ¹³⁷Cs in river water. The study also revealed the temperature dependence of Kd in riverine environments from a Van ’t Hoff equation. The standard reaction enthalpy of ¹³⁷Cs in the Abukuma River was calculated to be approximately −19.3 kJ/mol. This was the first study to clearly reveal the mechanisms by which the dissolved ¹³⁷Cs activity concentration and Kd are influenced by chemical and thermodynamic processes in the middle course of a large river, and it is expected to lead to an improved model of ¹³⁷Cs dynamics in rivers.