Long-term dynamics of 137Cs accumulation at an urban pond

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.