[M-AG39] Radioisotope migration: New development for dynamics study of radionuclides from 1F NPP accident

[MAG39-04] ¹³⁷Cs of decontamination effect and temporal changes in an urban pond

*Honoka Kurosawa¹, Kenji Nanba^2,3, Toshihiro Wada², Yoshifumi Wakiyama² (1.Major of Environmental Radioactivity, Graduate school of Symbiotic System Science and Technology, Fukushima University, 2.Institute of Environmental Radioactivity, Fukushima University, 3.Faculty of Symbiotic System Science, Fukushima University)

Keywords:Fukushima Dai-ichi Nuclear Power Plant accident, 137Cs, pond, decontamination, δ15N

There are more than 4,000 ponds in Fukushima Prefecture, and many ponds were contaminated by radiocesium (¹³⁴Cs+¹³⁷Cs) due to the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. The decontamination and/or countermeasures against radioactive materials were considered for ponds heavily contaminated by radiocesium, higher than 8 kBq/kg in bottom sediment as a criteria based on the Act on Special Measures Concerning the Handling of Radioactive Materials Contamination. The bottom sediment removal, one of the countermeasures, is an effective way to decrease the radiocesium concentration in bottom sediment and water body. The previous studies have been shown that ¹³⁷Cs accumulate in ponds and dam reservoirs due to its secondary inflow from catchments. Therefore, it is necessary to understand how and how much ¹³⁷Cs accumulates after decontamination. However, there are few studies on the effects of decontamination and the temporal changes in the pond. The objectives of this study were to reveal the effect of decontamination on the ¹³⁷Cs and water quality and to elucidate accumulation process of ¹³⁷Cs after decontamination, based on long-term investigation on an urban pond.
The investigated pond is located in Koriyama City. The water was taken from the Asaka- canal and the Minami River, and flowed into the pond through culvert and open culvert. The amount of ¹³⁷Cs deposited around the investigated pond was 0.170 MBq/m². The investigated pond was decontaminated by dredging of bottom sediment in 2017. The bottom sediment, pond water, inflow water, and outflow water were collected in 2015, 2018-2021, and analyzed for ¹³⁷Cs concentration, particle size distribution, nitrogen and carbon stable isotope ratio, and water quality.
Compared between 2015 and 2018, the ¹³⁷Cs inventory and the ¹³⁷Cs concentration in the top layer of bottom sediment at 7 points were decreased by 78 % and 72 %, respectively. Since the ¹³⁷Cs inventory and the ¹³⁷Cs concentration in the bottom sediment were decreased, it was said that the decontamination had an effect. After decontamination, there was no change in the average ¹³⁷Cs inventory, however, the ¹³⁷Cs inventory was increased due to the deposition of fine particles at points where the water flow was stagnant. There was a significant positive correlation between δ¹⁵N and ¹³⁷Cs concentrations in 0-1 cm of bottom sediment after decontamination. It was suggested that ¹³⁷Cs concentrations tend to be higher at points where the contribution of particles from urban areas was larger, since δ¹⁵N is reported to increase with human activity. The total ¹³⁷Cs concentration (dissolved ¹³⁷Cs concentration + particulate ¹³⁷Cs concentration) in pond water before and after decontamination was decreased by 33 %. The ¹³⁷Cs concentration in SS of pond water was significantly decreased by decontamination, however, the ¹³⁷Cs concentration in SS of pond water was remained at 8 kBq/kgDW after decontamination. The ¹³⁷Cs concentration in SS of the inflow water were exceeded 8 kBq/kgDW under high-flow condition. The ¹³⁷Cs concentration in SS of the inflow water at the investigated pond was higher than that in soil of the cultivated farmland and paddy fields. It was suggested that the infow of SS with high ¹³⁷Cs concentrations from urban areas is a factor in remain high concentrations of ¹³⁷Cs in SS of pond water and bottom sediment. The total ¹³⁷Cs concentration in the outflow water was higher than that in the inflow and pond water, suggesting that ¹³⁷Cs tended to be discharged. The δ¹⁵N of the outflow water was between the bottom sediment and the ponded water, suggesting that the bottom sediment and the ponded water were mixing and flowing out. In the investigated pond, the ¹³⁷Cs concentration after decontamination was higher than that in the pond located in forests and farmlands, reflecting the fact that the catchment area in the investigated pond was an urban area.