[M-AG34] Radioisotope migration: New Development of Environmental Dynamics Research on the 1FNPS Accident

[MAG34-P03] Model Development for Elucidation of Cs-137 Adsorption and Desorption Process in River Bottom Sediment in Fukushima Prefecture

*Naoyuki Wada¹, Yuichi Onda², Jim T Smith³, Yasunori Igarashi⁴ (1.University of Tsukuba, 2.Center for Research in Radiation, Isotopes and Earth System Sciences, 3.University of Portsmouth, 4.Institute of Environmental Radioactivity)

Keywords:Cs-137, The Fukushima Daiichi Nuclear Power Plant, River-bottom sediment, desorption/adsorption model, Dissolved form, Distribution coefficient

The Fukushima Daiichi Nuclear Power Plant accident in March 2011 released large amounts of Cs-137 into the atmosphere, which was quickly deposited on land. Most of the Cs-137 deposited in the watershed is discharged in dissolved and suspended forms. Ion exchange between solid and liquid phases and sedimentation and resuspension of soil particles are involved in Cs-137 transport in rivers, so it is necessary to focus on river bottom sediment in addition to the suspended and dissolved forms. In particular, the suspended sediment associated with sediment erosion occurs only during rainfall, but focusing on the river bottom sediment will make it possible to elucidate the dynamics of Cs-137 during normal water conditions. In this study, we focus on the adsorption and desorption between the solid and liquid phases, and analyze them using a model to elucidate the formation factors of Cs-137 concentrations in river sediments. The data used in this study are river-bottom sediment data collected by the Ministry of the Environment from 2011 to 2019, and dissolved form data collected by Fukushima Prefecture and the Radiation and Isotope Earth System Research Center, University of Tsukuba. The target area was the Nakadori and Hamadori watersheds in Fukushima Prefecture, where both river-bottom sediment and dissolved forms were collected. The adsorption/desorption model was developed to account for the adsorption reaction rate from the liquid phase, the desorption reaction rate from the solid phase, and radiative decay. By substituting the time-series data of the dissolved state obtained in the field and the initial concentration of the river-bottom sediment measured in the field into the model, the time-series data of the Cs-137 concentration in the riverbed soil became two variables that indicate the reaction rate. The reaction rate of sorption and desorption is evaluated by expressing the measured river-bottom sediment Cs-137 concentration by the rive-bottom sediment Cs-137 concentration in this model. In fact, it was found that the change in the Cs-137 concentration in the river bottom sediment during the first year was caused by the desorption rate, suggesting a relationship with the deposition distribution of Cs-137 and the river water quality.