[M-AG34] Radioisotope migration: New Development of Environmental Dynamics Research on the 1FNPS Accident

[MAG34-P07] Variations in ¹³⁷Cs concentrations in river and coastal seawater during high-flow events

*Yoshifumi Wakiyama¹, Shun Satoh², Hyoe Takata¹, Pierre-Alexis Chaboche^1,3, Honoka Kurosawa² (1.Institute of Environmental Radioacitivity, Fukushima Universiy, 2.Graduate School of Symbiotic Systems Science and Technology, Fukushima University, 3.Japan Society for the Promotion of Science)

Keywords:Intensive rainfall, Cesium-137, Desorption

High-flow events reportedly result in substantial ¹³⁷Cs exportation via river to the ocean and increase ¹³⁷Cs concentrations in coastal seawater. Previous studies suggested dependency of terrestrial ¹³⁷Cs dynamics on catchment characteristics, such as land use and existence of dam reservoir. Various numerical calculations attempted to reproduce changes in dissolved ¹³⁷Cs concentrations in coastal seawater. However, data during high-flow events are still rare and it remains uncertainties for verifying these findings. This study presents results of contemporaneous sample collections during high-flow events on a river system and its coastal seawater to discuss ¹³⁷Cs transfer processes from terrestrial to marine environments. Study site was the Ukedo river system and its coastal sea. During 3^rd-19 ^th September 2023, water samples were collected for 13 times at two downstream points of the river system, on the mainstream (Ukedo river) and a tributary (Takase river), and for 8 times at seashore locating at ca. 500 m north from the river mouth. During the sample collection, total catchment mean rainfall was totaled 300 mm with intensive rainfalls on 4, 6 and 8 September. Collected water samples were filtrated to measure ¹³⁷Cs concentration in suspended solids (Bq/kg) and dissolved ¹³⁷Cs concentrations (Bq/L). ¹³⁷Cs concentrations in suspended solids in Ukedo and Takase river ranged from 7.0 to 67 kBq/kg and from 2.4 to 15 kBq/kg, respectively. The concentrations at peak water discharge phases in Takase river tended to be high when ratio of rainfall amount on downstream parts to that on whole catchments were high, but vice versa in the Ukedo river. This discrepancy can be attributed to the difference in spatial distribution of ¹³⁷Cs inventory between the two catchments. Dissolved ¹³⁷Cs concentrations in Ukedo and Takase rivers ranged from 52 to 70 mBq/L (5 samples measured out of 13) and from 8.4 to 37 mBq/L, respectively. At the seashore, ¹³⁷Cs concentrations in suspended solids and dissolved ¹³⁷Cs concentration ranged from 2.0 to 95 kBq/kg and from 6.7 to 410 mBq/L, respectively. Both concentrations appeared maximum in the sample collected 5 hours after a river water discharge peak which occurred with intensive rain on 8^th September, and decreased with time to reach the background levels in 10 days after the event. Higher dissolved ¹³⁷Cs concentration in seawater than in corresponding river water for the high-flow events indicated considerable desorption of ¹³⁷Cs from terrestrial suspended solids into coastal seawater.