Long-term 137 Cs dynamics after decontamination of an urban pond in Fukushima Prefecture, Japan

137Cs tends to accumulate in ponds and dam reservoirs because of inputs from their catchments. Ponds contaminated by the Fukushima Daiichi Nuclear Power Plant accident have since been decontaminated. However, little attention has been paid to 137Cs accumulation after the decontamination measures, such as bottom sediment removal, especially for urban ponds with reportedly high 137Cs concentrations in their urban catchments. This study examined long-term 137Cs accumulation after decontamination to assess the influence of urban areas. Between 2019 and 2022, bottom sediment was collected together with inflow, pond, and outflow water at an urban pond located at Koriyama City, Fukushima Prefecture, Japan. The mean 137Cs inventory in the bottom sediment did not change significantly after decontamination, ranging from 271 to 337 kBq/m2, whereas the variability increased. A significantly positive correlation of the 137Cs concentration with delta 15N, an indicator of the source contribution to bottom sediment, was determined in the bottom sediment in 2018 but not after 2019. The correlation between the specific surface area and 137Cs concentration in the bottom sediment was significantly positive after 2020. These results suggested higher 137Cs concentrations at sites where particles from land hosting human activities were deposited immediately after decontamination; thereafter, 137Cs concentrations were higher at sites of fine-particle deposition. The annual mean 137Cs concentration in pond water was stable from 2019 to 2022. The mean-normalized 137Cs concentrations in the suspended solids and dissolved fractions in the inflow water were 0.041 m2/kg and 21.2 x 10-5 m- 1, respectively. These values were higher than those reported for other aquatic environments with low-urban catchment. However, the 137Cs load from inflows was unlikely to have been enough to increase the 137Cs inventory in the bottom sediment, despite the high 137Cs concentration in suspended solids. The mechanism of the persistent 137Cs contamination in urban areas merits further investigation.