Linkage of 137Cs dynamics in river and coastal seawater during high-flow events

Previous studies indicated that high-flow events can result in substantial ¹³⁷Cs exportation via river to the ocean and increase ¹³⁷Cs concentrations in coastal seawater. Assessing response of marine ¹³⁷Cs behavior to terrestrial ¹³⁷Cs inflow will lead to a better understanding of ¹³⁷Cs transfer processes in terrestrial and marine environments. This study presents results of sample collections under various flow conditions on a river system and its coastal seawater to discuss the transfer processes in detail. The study was conducted in 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 8 times at seashore locating at 500 m north from the river mouth. In the sampling period, the 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 the peak river water discharge which occurred with intensive rain on 8^th September. Higher dissolved ¹³⁷Cs concentration in seawater than in corresponding river water for the high-flow event indicates considerable desorption of ¹³⁷Cs from terrestrial suspended solids into coastal seawater. Both ¹³⁷Cs concentrations in seawater decreased with time to reach the background levels in 10 days after the event despite of quite stable concentrations in rivers. These results provide important implications for quantifying ¹³⁷Cs transfer processes in terrestrial-marine environments.