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

(137)Cs 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. (137)Cs inventories in soils surrounding SU, FS, IZ, and KM were 6.4, 2.9, 2.1, and 0.9 MBq m(-2), respectively. (137)Cs 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 (137)Cs inventories in bottom sediment than those of soil in SU and FS suggest that (137)Cs was delivered to and accumulated in these ponds. Mean total (137)Cs activity concentrations in SU, FS, IZ, and KM were 41, 13, 9.5, and 1.4 Bq L(-1), respectively. Particulate (137)Cs concentration accounted for 71-90% of total (137)Cs in the water samples, on average. The mean distribution coefficient, Kd, in SU, FS, IZ, and KM was 1.3 × 10(5), 2.1 × 10(5), 1.7 × 10(5), and 6.2 × 10(5) 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 (137)Cs activity concentration tended to be low during winter in all four ponds. Dissolved (137)Cs activity concentrations were proportional to K(+) and DOC concentrations in all the ponds. The results from principal component analysis performed for (137)Cs activity concentration and water chemistry data sets suggested that there were different mechanisms behind variability of dissolved (137)Cs activity concentrations for each pond. Continuous monitoring is required to reveal temporal trends in (137)Cs activity concentrations of these waters and controlling factors of such in closed water systems in Fukushima.