TRANSFORMATION OF RADIOCESIUM SPECIATION IN PONDS AT THE VICINITY OF FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT AND ITS DYNAMICS IN “SEDIMENTS-WATER” SYSTEM

The paper is concerned with the results of 137Cs monitoring in the irrigation ponds of the Okuma town in the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) exclusion zone. The 137Cs activity concentrations in the ponds appeared to be higher than those in the rivers and dam reservoirs in the region. The study has revealed a trend for a decline in 137Cs activity concentrations, both particulate and dissolved. The rate of particulate 137Cs decline was much higher than that of dissolved. The total distri- bution coefficient Kd (137Cs) in the suspended sediment-water system in the studied ponds was decreasing in time with the rate constant of 0.12-0.18 year-1. Assuming that the decrease in Kd is associated with decomposition of hot glassy particles, the time scale of 137Cs leaching from them in these water bodies was estimated to be 5-8 years. These estimates are consistent with the findings of recent laboratory experiments on the subject. With respect to seasonal variations, the highest levels of dissolved 137Cs in the studied ponds were observed from June to October as a function of specific pond and monitoring year. Based on data about 137Cs speciation in the bottom sediment top layer of the ponds and its distri- bution in the sediment-water system, the exchangeable radiocesium interception potential RIPex(K) for the ponds sediments was estimated to be 1650-2250 mg-eq/kg, which is within the range of values measured by laboratory studies.