Many-Years Dynamics and Seasonal Fluctuations of Dissolved Radiocesium in a Pond in the Vicinity of the Fukushima Daiichi NPP

Abstract The results of long-term (2015–2025) monitoring of dissolved 137 Cs activity concentration in water of Inkyozaka pond located in close vicinity of the Fukushima Daiichi NPP are presented. Regular seasonal variations of dissolved 137 Cs in pond water were observed concurrently with seasonal fluctuations of water temperature, which confirms their relation to the temperature dependence of the radiocesium desorption from sediments to solution. The activation energy of radiocesium desorption, obtained from the observational data, is 18.8 kJ/mol, which is in agreement with similar laboratory and field investigations. An empirical exponential model and a semiempirical diffusion model of long-term dynamics of dissolved 137 Cs in water of Inkyozaka pond were suggested. The modeling results qualitatively describe the seasonal fluctuations of the dissolved 137 Cs concentrations in the pond water.