[M-AG44] Dynamics of radionuclides emitted from Fukushima Dai-ichi Nuclear Power Plant in the environment

[MAG44-P12] Accumulation and redistribution of ¹³⁷Cs on floodplains along the Abukuma River

★Invited Papers

*Yoshifumi Wakiyama¹, Alexei Konoplev¹, Valentin Golosov², Maxim Ivanov², Mikhail Komissarov³ (1.Institute of Environmental Radioacitivity, Fukushima Universiy, 2.Faculty of Geography, M.V. Lomonosov Moscow State University, 3.Ufa Institute of Biology UFRC, Russian Academy of Science)

Keywords:Abukuma River, floodplains, sediment dynamics

A portion of sediment-borne ¹³⁷Cs in river water accumulates on floodplains during high-flow events and becomes a source of secondary contaminations in subsequent events. This study aims to elucidate processes of ¹³⁷Cs accumulation / redistribution on floodplains and presents the ¹³⁷Cs inventory and depth distribution in deposited sediments. The sediment cores were taken in 2018 and 2019 at six floodplain cross-sections, located along the Abukuma River in Nihonmatsu (NIH), Fukushima (KUR), Date (FUS), Marumori (MAR), Kakuda (KAK), and river mouth area in Iwanuma (Mouth). The mean ¹³⁷Cs inventory obtained by sampling campaigns in October 2018 at NIH, KUR, FUS, MAR, KAK, and Mouth were 150 kBq m^-2 (n = 2), 600 kBq m^-2 (n = 3), 490 kBq m^-2 (n = 2), 270 kBq m^-2 (n = 2), 90 kBq m^-2 (n = 2), and 40 kBq m^-2 (n = 1), respectively. These values were 1.2-9.1 times higher than those estimated by 6^th airborne survey at each point, suggesting secondary accumulation of ¹³⁷Cs. The ¹³⁷Cs inventory and maximum ¹³⁷Cs concentrations in sediment depth profile tended to be high at the sampling points, located on the highest floodplain level above the river channel within each floodplain cross-section. The mean ¹³⁷Cs inventory obtained by sampling campaigns in October and November 2019 at NIH, KUR, FUS, MAR, KAK, and Mouth were 180 kBq m^-2 (n = 2), 540 kBq m^-2 (n =2 ), 410 kBq m^-2 (n = 1), 120 kBq m^-2 (n = 2), 420 kBq m^-2 (n = 3), and 46 kBq m^-2 (n = 1), respectively. Although ¹³⁷Cs inventory on the floodplains are not so different, depth distributions of ¹³⁷Cs in the deposited sediment changed from those of 2018. These results suggest that an extreme flood triggered by Typhoon Hagibis in middle of October 2019 resulted in a substantial redistribution of ¹³⁷Cs at Abukuma fluvial system.