[M-AG33] Dynamics of radionuclides emitted from Fukuchima Dai-ichi Nuclear Power Plant in the environment

[MAG33-08] Variations in ¹³⁷Cs activity concentration in suspended sediment of the Niida river

*Yoshifumi Wakiyama¹, Yuichi Onda², Varentine Golosov^3,4, Alexei Konoplev¹, Yasunori Igarashi¹, Tsugiko Takase¹ (1.Institute of Environmental Radioacitivity, Fukushima Universiy, 2.Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 3.Institute of Ecology and Environment, Kazan Federal University, 4.Faculty of Geography, Lomonosov Moscow State University)

Keywords:Cesium-137, Typhoon, temporal trend

The upstream portions of the Niida river, flowing through the northern part of Hamadoori area, were exposed to radiological contamination of ¹³⁷Cs from the Fukushima accident. Downstream portions of the Niida are composed of urban and agricultural areas. Therefore understanding ¹³⁷Cs transfer throughout the river continuum is an important issue for mitigating radiological risk to residents. This study aims to understand ¹³⁷Cs dynamics in the Niida river basin and present ongoing observations of ¹³⁷Cs activity concentrations in suspended sediments of the river. Samples have been collected at Warabidaira on the Hiso river, Notegamikita n the upstream reach and Sakekawabashi in the lower reach of the Niida river since July 2014. At each location time-integrated suspended sediment samplers were installed. The mean ¹³⁷Cs activity concentrations during July to December 2014 at Warabidaira, Notagamikita and Sakegawabashi were 28.3, 13.4, and 17.5 kBq kg^-1, respectively. Those concentrations during May to October 2017 decreased to 11.9, 6.8, and 5.9 kBq kg^-1, respectively. These decreasing trends can be expressed by exponential equations of elapsed time since the accident. The difference between observed ¹³⁷Cs activity concentrations and ¹³⁷Cs activity concentrations estimated by the equations appears to be proportional to Fe₂O₃ contents in the suspended sediments. Furthermore, ¹³⁷Cs activity concentrations in suspended sediments sampled at Notegamikita during typhoons of August 2016 and October 2017 were higher as water levels increased, compared to times when water levels were decreasing. By contrast, maximum ¹³⁷Cs activity concentration was found in samples taken at the Niida-bashi point on downstream area in time of peak discharge.These results suggest that the difference in sources of suspended sediment are reflected by the ¹³⁷Cs activity concentrations of suspended sediments. However, further analyses of particle size distributions and carbon contents of the suspended sediments can help interpretation of sediment .