[M-AG38] Ten years after the Fukushima Dai-ichi NPP Accident : Dynamics of radionuclides in the environment

[MAG38-P03] The ¹³⁷Cs dynamics in a pond of urban area after decontamination

*Honoka Kurosawa¹, Misaki Usuki¹, Shota Moritaka¹, Kenji Nanba², Toshihiro Wada², Yoshifumi Wakiyama² (1.Graduate School of Symbiotic Systems Science and Technology, Fukushima University, 2.Institute of Environmental Radioactivity, Fukushima University)

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

More than 4,000 irrigation ponds in Fukushima Prefecture have been contaminated by radiocaesium released from the Fukushima Dai-ichi Nuclear Power Plant accident. The contaminated pond would cause problems such as increase of air dose rates around the pond by water drawdown and prevention of the restart/reconstruction of farming and agricultural activities. As a countermeasure, the decontamination has been conducted for ponds with more than 8,000 Bq/kgDW of radiocaesium (¹³⁴Cs+¹³⁷Cs) concentration in its bottom sediment. This study aims to evaluate the effect of decontamination and presents investigations on ¹³⁷Cs concentration in bottom sediment, pond water, inflow water, and outflow water at a decontaminated pond locate in urban area.
Study site is the Sakabuta pond located in urban area of Koriyama city. The decontamination was carried out in 2017. Bottom sediment was collected in 2015, 2018 and 2020. Bottom sediment was basically collected at 2 points around cage for commercial aquaculture of carp and additionally at 5 points in the whole pond once a year. The depth of 0-20 cm, 20-30 cm, and 30 cm or more was cut for 1 cm, 2 cm, 5 cm interval, respectively. Pond water was collected in 2015, 2018, and 2020, simultaneous to bottom sediment sampling. Inflow and outflow water was collected in 2020. The ¹³⁷Cs concentration in bottom sediment, pond, inflow, and outflow water were measured with the Ge semiconductor detector.
The ¹³⁷Cs concentrations in bottom sediment and pond water were compared between before and after decontamination. The ¹³⁷Cs concentrations in the top layer of bottom sediment at 2 points around cage were decreased from 36,100 and 25,500 Bq/kgDW to 15,800 and 15,300 Bq/kgDW. The ¹³⁷Cs concentration in pond water was decreased from 1.40 Bq/L to 0.950 Bq/L. It was said that the decontamination was effective because the ¹³⁷Cs concentrations in the top layer of the bottom sediment and pond water were significantly decreased by decontamination. However, the ¹³⁷Cs concentration in the top layer of bottom sediment was 8,000 Bq/kgDW or more even after decontamination. The ¹³⁷Cs concentrations in the top layer of bottom sediment at 5 points in the whole pond were 16,700-44,300 Bq/kgDW in 2015, 3,630-12,200 Bq/kgDW in 2018, and 6,800-13,500 Bq/kgDW in 2019.The layer of more than 8,000 Bq/kgDW of radiocesium concentration in the bottom sediment of 2019 was thicker than that of 2018. In 2019, Hagibis passed through about a month before the bottom sediment collection and increase of ¹³⁷Cs in 2019 was attributed to suspended solid input due to the typhoon. The sedimentary environments were compared between the points with more and less than 8,000 Bq/kgDW of raciocesium concentration. At the point with less than 8,000 Bq/kgDW of radiocesium concentration, it was suggested that new particle was not deposited after decontamination. However, at the point with more than 8,000 Bq/kgDW of radiocesium concentration, it was suggested that new particle was deposited after decontamination. The ¹³⁷Cs concentration in suspended solids of pond water was more than 8,000 Bq/kgDW even after the decontamination. Furthermore, the ¹³⁷Cs concentration in suspended solids of inflow water sometimes exceeded 8,000 Bq/kgDW. These results suggested that a large amount of soil particle would flow into the pond during the rainfall events, resulting in high concentration of ¹³⁷Cs in the top layer of bottom sediment.