Influential factors of long-term and seasonal Cs change in agricultural and forested rivers: Temperature, water quality and an intense Typhoon Event.

In this study, the effect of temperature, water quality, and the impact of an intense typhoon event on change in Cs concentration in the water of agricultural and forested rivers near the Fukushima Daiichi nuclear power plant (Japan) was evaluated using monthly stationary observations obtained under baseflow conditions 2.8-10.6 years after the nuclear accident in 2011. The dissolved Cs concentration fluctuated seasonally with water temperature in all rivers, and the increase in dissolved Cs concentration for unit increase in temperature was higher in forested rivers than in agricultural rivers. The relationship between water temperature and the apparent distribution coefficient of Cs well followed the van 't Hoff equation in the two agricultural rivers, where the enthalpy of reaction was estimated as -15.6 and -19.6 kJ mol. The van 't Hoff equation was not well followed for a forested river, where the suspended solids mainly comprised organic matter, suggesting that the dominant process determining dissolved Cs concentrations in forested rivers is not only water temperature effect on ion exchange, but rather the input of Cs and K (competing with Cs for exchange sites on mineral particles) into the water phase via litter leaching. Suspended solids concentrations in agricultural rivers correlated negatively with Cs concentrations in suspended solids, suggesting an increased proportion of coarse particles or the input of soils with low Cs concentration from decontaminated agricultural land. At some sites, Cs concentrations in dissolved form and in suspended solids were reduced sharply in association with the passage of Typhoon Hagibis in October 2019, suggesting that Typhoon Hagibis caused large-scale surface erosion that removed the source of Cs.