Many irrigation ponds in Fukushima Prefecture were decontaminated due to the contamination of radiocesium released from Fukushima Daiichi Nuclear Power Plant. To evaluate the impact of decontamination on ¹³⁷Cs dynamics in an urban pond in Koriyama City, Fukushima Prefecture, Japan, temporal changes in ¹³⁷Cs concentrations in bottom sediments and pond water were investigated before and after bottom-sediment removal. Post-decontamination, ¹³⁷Cs inventories in bottom sediments decreased by 46–89%. ¹³⁷Cs inventories in bottom sediments were relatively high in fine sediments before decontamination, and were also high at points near the water inlet after decontamination. Following decontamination, the mean ¹³⁷Cs concentration in suspended solids (SS) and the mean dissolved ¹³⁷Cs concentration in pond water decreased by 52% and 5%, respectively. Even after decontamination, the normalized ¹³⁷Cs concentrations in SS and in water, which were calculated by dividing the ¹³⁷Cs concentrations by the mean ¹³⁷Cs inventories in each area, were higher than those in rivers, dam reservoirs, and ponds elsewhere in Fukushima. The positive correlations between δ¹⁵N values, an indicator of the source contribution to bottom sediments, and ¹³⁷Cs concentrations in the upper 5 cm of bottom sediments after decontamination implied that SS from urban areas gradually increased the ¹³⁷Cs inventories in the pond. The results underline the importance of secondary inputs of ¹³⁷Cs from highly urbanized catchments.

It is known that the semi-enclosed water area such as pond and dam reservoir is readily subject to 137Cs accumulation because of the secondary inflow from the catchment area. We present the long-term monitoring data of the 137Cs concentration in bottom sediment and pond water in an urban pond located in the central area of Koriyama City, Fukushima Prefecture to discuss the 137Cs dynamics of the urban pond. The pond was decontaminated by the bottom sediment removal in 2017. The bottom sediment core and pond water were collected in 2015 and 2018-2021. The inflow and outflow water were collected in 2020-2021. The river water around the pond was collected in 2021. The bottom sediment and water samples were measured for 137Cs concentration, particulate size distribution, and N and C stable isotopes. Compared between 2015 and 2018, the 137Cs inventory and 0-10 cm depth of 137Cs concentration in the bottom sediment at 7 points were decreased by 81 % (mean 1.50 to 0.28 MBq/m2) and 85 % (mean 31.5 to 4.8 kBq/kgDW), respectively. Although mean 137Cs inventory in bottom sediment did not drastically change during 2018-2021, its variability became wider. Points with increased 137Cs inventory in bottom sediment showed year-by-year increase in thickness of layer with concentrations higher than 8 kBq/kgDW, a criterion for considered decontamination. The 137Cs concentration in suspended solids (SS) in pond water was lowered after decontamination, although it still remained above 8 kBq/kgDW. The 137Cs concentrations in SS of inflow water were also high, exceeding 8 kBq/kgDW. The 137Cs concentration in SS of the river water around the pond was higher when it passed through the urban area, suggesting that the inflow of particles from urban origin maintained high 137Cs level in the pond.

Long-term changes in particulate and dissolved ¹³⁷Cs concentrations in rivers and dam reservoirs have been reconstructed using ¹³⁷Cs depth distributions in bottom sediments near Chornobyl and Fukushima. However, few studies have applied this method to pond water. This study tested the applicability of this reconstruction method in ponds within the Fukushima evacuation zone. Bottom sediments in ponds were collected in November 2019. Our reconstruction of a shallow closed pond disagreed with our observations. Even in ponds where the reconstruction agreed well with observations, underestimations may have occurred because the time constant for the decrease in ¹³⁷Cs concentration was lower than those previously reported. We conclude that the present reconstruction method may be applied to deeper Fukushima ponds with inflow and outflow, but underestimations of initial ¹³⁷Cs concentrations in water may lead to uncertainties. For realistic reconstruction, attention should be paid to pond characteristics, catchment conditions, and sediment core selection.