Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant

137Cs activity concentration in the water of four ponds, Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), that are within 10 km of the Fukushima Dai-ichi nuclear power plant were observed from April 2015 to August 2016. 137Cs inventories in soils surrounding SU, FS, IZ, and KM were 6.4, 2.9, 2.1, and 0.9 MBq m−2, respectively. 137Cs inventories in the bottom sediments of SU, FS, IZ, and KM were 13, 8.9, 1.6, and 1.1 MBq m−2, respectively. Higher 137Cs inventories in bottom sediment than those of soil in SU and FS suggest that 137Cs was delivered to and accumulated in these ponds. Mean total 137Cs activity concentrations in SU, FS, IZ, and KM were 41, 13, 9.5, and 1.4 Bq L−1, respectively. Particulate 137Cs concentration accounted for 71-90% of total 137Cs in the water samples, on average. The mean distribution coefficient, Kd, in SU, FS, IZ, and KM was 1.3 × 105, 2.1 × 105, 1.7 × 105, and 6.2 × 105 L kg−1, respectively. These Kd values were higher than the Kd values observed in the Chernobyl area by 1-2 orders of magnitude. Although no significant decreasing trends were found, dissolved 137Cs activity concentration tended to be low during winter in all four ponds. Dissolved 137Cs activity concentrations were proportional to K+ and DOC concentrations in all the ponds. The results from principal component analysis performed for 137Cs activity concentration and water chemistry data sets suggested that there were different mechanisms behind variability of dissolved 137Cs activity concentrations for each pond. Continuous monitoring is required to reveal temporal trends in 137Cs activity concentrations of these waters and controlling factors of such in closed water systems in Fukushima.