[M-AG39] Radioisotope migration: New development for dynamics study of radionuclides from 1F NPP accident

[MAG39-P01] Riverine ¹³⁷Cs dynamics during high-flow events in the Niida River basin

*Yoshifumi Wakiyama¹, Takuya Niida², Hyoe Takata¹, Keisuke Taniguchi³, Kazuki Fujita⁴, Alexei Konoplev¹, Yuichi Onda⁵ (1.Institute of Environmental Radioacitivity, Fukushima Universiy, 2.KANSO TECHNOS, CO. LTD., 3.National Institute of Technology, Tsuyama College, 4.Fukushima Prefectural Centre for Environmental Creation, 5.Center for Research in Isotope and Environmental Dynamics, University of Tsukuba)

Keywords:137Cs, Catchment, Desorption, Typhoon

Majority of terrestrial ¹³⁷Cs is transported from river basins to ocean during high-flow events triggered by rainstorms. Recent studies have revealed substantial impacts of riverine ¹³⁷Cs flux on its behavior in ocean during typhoon events, underlying importance of better understanding of riverine ¹³⁷Cs dynamics. A comparative study on riverine ¹³⁷Cs dynamics in the coastal contaminated catchments suggested dependency of ¹³⁷Cs dynamics on rainfall properties and on catchment characteristics, such as spatial pattern of ¹³⁷Cs inventory and land use composition. Further inter-catchment comparison will lead us to achieve better understanding of ¹³⁷Cs dynamics. This study presents temporal variations in riverine ¹³⁷Cs concentration at four sites and quantified ¹³⁷Cs flux at one site in the Niida river basin during high-flow events occurred in 2016 and 2017.
Study sites were Sakegawa-bashi (SK), Haramachi (HR), Notegami-kita (NT) and Warabi-daira (WR) in the Niida River basins. Respective catchment area on SK, HR, NT, WR were 29, 103, 206, 259 km². Respective mean ¹³⁷Cs inventory on SK, HR, NT, WR were 1420, 790, 850, 740 kBq m^-2. River water samples were collected for 5-6 times during typhoon events in 16-17 August 2016 and 22-23 October 2017. Dissolved ¹³⁷Cs concentration (Csdis: Bq L^-1) and ¹³⁷Cs concentration in suspended sediment (CsSS: Bq kg^-1) were measured. Based on the concentrations and hydrological data, ¹³⁷Cs fluxes were calculated for HR. SS samples collected at HR in peak water discharge phase were further analyzed for desorption with seawater to estimate potential quantity of ¹³⁷Cs desorbed from SSs into ocean.
Mean CsSS ranged from 5900 to 21000 Bq kg^-1, and mean Csdis from 11 to 34 Bq L^-1. The concentrations depended on catchment mean ¹³⁷Cs inventory. Temporal variations in CsSS appeared to differ between upstream and downstream sites; peaked in peak water discharge phase at SK and HR whereas declined throughout events at NT and WR. These tendencies suggest reflections of spatial patterns of ¹³⁷Cs inventory in the catchments to the variations. Although we have not found any explainable feature for variations in Csdis by inter-catchment and -event comparisons, it declined and elevated in all events without monotonous trends. ¹³⁷Cs fluxes at HR in the events in 2016 and 207 were 60 and 55 GBq, respectively. They accounted for 0.034 and 0.031% of total deposited ¹³⁷Cs in the catchment. Desorption ratio of ¹³⁷Cs, obtained by 1-day shaking experiment of SS in seawater, were 3.6% for 2016 and 5.3% for 2017. By multiplying these ratios by correspondent particulate ¹³⁷Cs fluxes, potential quantity of ¹³⁷Cs desorbed from SSs into ocean in 2016 and 2017 events were estimated as 0.21 and 0.28 GBq, respectively. These quantities 15 and 3.6 times higher than correspondent dissolved ¹³⁷Cs fluxes. Further analyses, such as evaluation of decontamination impacts and inter-catchment comparisons of ¹³⁷Cs fluxes, are required for better understanding.