Experimental field study of basic mechanisms underlying dissolved 137Cs seasonal variations in ponds heavily contaminated after the Fukushima Daiichi nuclear power plant accident

Presented are results of field study of dissolved 137Cs seasonal variations in two ponds of the close vicinity of the Fukushima Daiichi nuclear plant contaminated after the accident in March 2011. Two mechanisms were investigated which can possibly explain regular seasonal variations of dissolved 137Cs concentrations in ponds: temperature dependence of 137Cs desorption from sediments to solution, and ion-exchange remobilization of 137Cs by cations of ammonium generated by decomposition of organic matter. A process-level model of dissolved 137Cs seasonal variations accounting for these two mechanisms was proposed and a correspondent equation describing 137Cs seasonality was suggested. The activation energy of 137Cs desorption from sediments for the two ponds is close to each other, being 30 kJ/mol and 24.6 kJ/mol and close to those obtained in laboratory experiments. Unlike Fukushima rivers, shallow and fast flowing, and hence with negligible ammonium concentration, in the stagnated waters of the studied ponds the role of ammonium in 137Cs mobilization seems to be comparable with that of water temperature, or even be prevalent.