Long-term ¹³⁷Cs 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 δ¹⁵N, 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 × 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.