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 Cs-137 in association with water and sediment movement. We examined the seasonal fluctuations in dissolved and particulate Cs-137 activity concentrations and clarified the biological and physicochemical factors controlling Cs-137 seasonality in the Abukuma River's middle course in the region affected by the FDNPP accident. The results showed seasonal fluctuations in the dissolved Cs-137 activity concentration, with an increase in summer and a decrease in winter, although there was no clear seasonal change in the particulate Cs-137 activity concentration. Water temperature and K+ concentration dominated the seasonality of the dissolved Cs-137 activity concentration. The volumetric activity concentration of the organically bound Cs-137 was found to be several orders of magnitude lower than that of the dissolved Cs-137. Therefore, we concluded that the Cs-137 in organic matter is not a source of dissolved Cs-137 in river water. The study also revealed the temperature dependence of K-d in riverine environments from a Van't Hoff equation. The standard reaction enthalpy of Cs-137 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 Cs-137 activity concentration and K-d 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 Cs-137 dynamics in rivers.