Influential factors of long-term and seasonal 137 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 ¹³⁷ 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 ¹³⁷ Cs concentration fluctuated seasonally with water temperature in all rivers, and the increase in dissolved ¹³⁷ 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 ¹³⁷ 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 ^-1 . 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 ¹³⁷ Cs concentrations in forested rivers is not only water temperature effect on ion exchange, but rather the input of ¹³⁷ Cs and K ⁺ (competing with ¹³⁷ Cs for exchange sites on mineral particles) into the water phase via litter leaching. Suspended solids concentrations in agricultural rivers correlated negatively with ¹³⁷ Cs concentrations in suspended solids, suggesting an increased proportion of coarse particles or the input of soils with low ¹³⁷ Cs concentration from decontaminated agricultural land. At some sites, ¹³⁷ 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 ¹³⁷ Cs.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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