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

[MAG38-P04] Variations in riverine ¹³⁷Cs concentration and its exportation to the ocean during high-flow events at three rivers in Hamadori area

Takuya Niida¹, *Yoshifumi Wakiyama², Hyoe Takata², Kazuki Fujita³, Keisuke Taniguchi⁴, Alexei Konoplev² (1.Graduate School of Symbiotic System and Technology, Fukushima Universiy, 2.Institute of Environmental Radioactivity, Fukushima Universiy, 3.Fukushima Prefectural Centre for Environmental Creation, 4.Center for Research in Isotope and Environmental Dynamics, University of Tsukuba)

Keywords:River, Radiocesium, Suspended solid

A considerable amount of ¹³⁷Cs deposited on terrestrial is exported to the ocean during high-flow events. Recent studies have reported increases of dissolved ¹³⁷Cs concentration in near-shore water after high-flow events assumedly due to desorption of ¹³⁷Cs from suspended solids. Existing literature on riverine ¹³⁷Cs dynamics during high-flow events have focused on upstream area, but little attention has been paid to downstream area. It remains uncertainties in transfer process of terrestrial ¹³⁷Cs to the ocean. This study aims to elucidate riverine ¹³⁷Cs dynamics and its influence on the ocean environment and presents analyses of river water samples taken at three rivers in the Hamadori area during high-flow events. River water samples were taken at 5 times at Haramachi site (206 km², 853 kBq m^-2) in the Niida river catchment, 3 times at Kiyobashi site (147 km², 2360 kBq m^-2) in the Ukedo river catchment, and 3 times at Takase site (262 km², 701 kBq m^-2) in the Takase river catchment during high-flow events occurred in the period of 2016-2020. Five to eight samples were taken per event and particulate and dissolved ¹³⁷Cs concentrations were measured. When sufficient amount of suspended solid was obtained, ¹³⁷Cs speciation and ¹³⁷Cs desorption by seawater were further analyzed. Mean ¹³⁷Cs concentration in suspended solid in the Niida, Ukedo, Takase rivers were 5100 Bq kg^-1 (n = 32, CV = 40%), 23000 Bq kg^-1 (n = 20, CV = 97%), and 4600 Bq kg^-1 (n = 20, CV=46%), respectively. Mean dissolved ¹³⁷Cs concentration at the Niida, Ukedo, Takase rivers were 16 mBq L^-1 (n = 32, CV = 53%), 73 mBq L^-1 (n = 20, CV = 55%), and 15 mBq L^-1 (n =20, CV = 40%), respectively. Concentration of ¹³⁷Cs in suspended solid was positively correlated with specific water discharge rate for three rivers. Dissolved ¹³⁷Cs concentration also showed positive correlation with specific water discharge rate, except for Takase river. Exportation of ¹³⁷Cs increased with increase of total rainfall amount and ranged from 6.6 to 60 GBq from the Niida river catchment, from 1.9 to 8.8 GBq from the Ukedo river catchment, and from 2.8 to 13 GBq from the Takase river catchment. The Ukedo river catchment showed low ratio of ¹³⁷Cs exportation to the total ¹³⁷Cs deposition in the catchment and low ratio of particulate ¹³⁷Cs exportation to the total ¹³⁷Cs exportation. They infered influences of the Ogaki dam located at midstream in the catchment. Ratio of ¹³⁷Cs desorbed by seawater to ¹³⁷Cs in suspended solid ranged from 2.8 to 6.6% and tended to be high for suspended solid with high exchangeable ¹³⁷Cs ratio. By assuming the ratio of desorption, potential release of ¹³⁷Cs from suspended solid to the ocean was estimated. The estimates for corresponded events ranged from 0.19 to 2.8 GBq. These values were 0.8-15 times higher than dissolved ¹³⁷Cs exportation. These results suggest that secondary release of ¹³⁷Cs from terrestrial suspended solid can be one of factor determining dissolved ¹³⁷Cs concentrations in the near-shore sea.