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[MAG34-P01] Temperature Dependence of Dissolved 137Cs Concentration in River Water: Differences among Forested River, Agricultural River, and Dam Discharge Water
Keywords:dissolved 137Cs, river water, dam discharge water
In many rivers, the concentration of dissolved 137Cs varies seasonally as a function of water temperature. Recent studies have suggested that this dependence indicates a shift in the thermodynamic equilibrium of 137Cs between the solid and aqueous phases in response to temperature change. However, the thermodynamic behavior of 137Cs has only been studied for a limited number of rivers, and the essential mechanism of the formation of the dissolved 137Cs concentration with temperature has not yet been elucidated. In this study, we investigated the relationship between dissolved 137Cs concentration and water temperature for agricultural rivers (flowing through agricultural areas), forested rivers and dam discharge waters based on monthly observations in the Fukushima Daiichi nuclear power plant accident affected area in 2014–2019.
When 137Cs can be considered to be in equilibrium between the solid and aqueous phases, the distribution coefficient of 137Cs and temperature follow the van't Hoff equation. If the ratio of the concentration of 137Cs in suspended solids to the dissolved form is taken as the distribution coefficient of 137Cs, the van't Hoff plots (logarithm of the distribution coefficient and the reciprocal of the absolute temperature on both axes) in two agricultural rivers were well approximated by the van't Hoff equation line. The standard enthalpies of the reaction derived from the regression equation were close to previously reported values (-19 kJ mol-1). On the other hand, the van't Hoff plots in two forested rivers and two dam discharges were poorly approximated by the linear regression equation. The high proportion of organic matter in the suspended solids in these waters suggests that dissolved 137Cs concentrations in forested rivers and dam discharge waters are not determined by ion exchange between suspended solids and the aqueous phases, but by other mechanisms such as microbial decomposition of litter and microbial growth within the dam lake.
When 137Cs can be considered to be in equilibrium between the solid and aqueous phases, the distribution coefficient of 137Cs and temperature follow the van't Hoff equation. If the ratio of the concentration of 137Cs in suspended solids to the dissolved form is taken as the distribution coefficient of 137Cs, the van't Hoff plots (logarithm of the distribution coefficient and the reciprocal of the absolute temperature on both axes) in two agricultural rivers were well approximated by the van't Hoff equation line. The standard enthalpies of the reaction derived from the regression equation were close to previously reported values (-19 kJ mol-1). On the other hand, the van't Hoff plots in two forested rivers and two dam discharges were poorly approximated by the linear regression equation. The high proportion of organic matter in the suspended solids in these waters suggests that dissolved 137Cs concentrations in forested rivers and dam discharge waters are not determined by ion exchange between suspended solids and the aqueous phases, but by other mechanisms such as microbial decomposition of litter and microbial growth within the dam lake.