[M-AG34] Radioisotope migration: Development from Environmental Dynamics Research on the F1NPS Accident

[MAG34-P01] Variations in ¹³⁷Cs concentrations in river water and coastal seawater in a high-flow event on Abukuma river catchment

*Yoshifumi Wakiyama¹, Hyoe Takata¹, Honoka Kurosawa², Nobuhiro Suzuki² (1.Institute of Environmental Radioacitivity, Fukushima Universiy, 2.Graduate School of Symbiotic Systems Science and Technology, Fukushima University)

Keywords:Typhoon, speciation, suspended solids, desorption

A substantial ¹³⁷Cs exportation through rivers during high-flow events reportedly increase ¹³⁷Cs concentration in coastal seawater. Accumulation of observation cases in rivers and coastal seas during high-flow events will contribute to a better understanding of the dynamics of ¹³⁷Cs on terrestrial and marine environments. In this study, we report observation results during a high-flow event on the Abukuma River catchment. The results were compared with those observed in Ukedo river in 2023 to discuss relevant ¹³⁷Cs dynamics. Water samples were collected for five times from August 11 to 23, 2024, at Iwanuma point downstream of the Abukuma River and at the beach near the mouth of the river. During this period, an average of 217 mm of rainfall was observed in the basin, with a peak on August 16, as Typhoon No. 7 approached. The average ¹³⁷Cs concentration of suspended solids in river water at the Iwanuma site was 1120 Bq/kg, and the average dissolved form ¹³⁷Cs concentration was 20 mBq/L, with the lowest value on August 17, the peak flow day. The total discharge of ¹³⁷Cs during this period was estimated to be 82 GBq. Mean ¹³⁷Cs concentration in suspended solids in coastal seawater was 380 Bq/kg, higher than in the river. Mean dissolved ¹³⁷Cs concentration in coastal seawater was 43 mBq/L, lower than in the river. Particulate ¹³⁷Cs concentration increased after the peak of ¹³⁷Cs discharge from the river. Dissolved ¹³⁷Cs concentration remained lower than before the typhoon and was not significant above the concentration of dissolved ¹³⁷Cs in the coastal seawater. On the other hand, the September 2023 observation of the Ukedo River showed that the dissolved ¹³⁷Cs concentration in coastal seawater increased by about 50 times the background after 190 GBq of ¹³⁷Cs exportation, indicating a difference in response in the coastal waters of the two rivers. To examine the difference between the two rivers, we investigated speciation of ¹³⁷Cs in suspended solids by sequential extraction. In the Abukuma River, the exchange fraction was 1.6%, the organic fraction was 1.2%, and in the Kakedo River, the exchange fraction was 4.6% and the organic fraction was 3.6%. These results suggest that relatively small ¹³⁷Cs desorption from suspended solids in the Abukuma River prevented increase of dissolved ¹³⁷Cs concentration in coastal seawater.