[M-AG34] Radioisotope migration: New development for radioisotope migration study related to 1F NPP accident

[MAG34-03] Long-term changes in ¹³⁷Cs concentration in river-bottom sediment and the transport in rivers in Fukushima Prefecture

*Naoyuki Wada¹, Yuichi Onda², Xiang Gao¹, Chen Tang¹ (1.University of Tsukuba, 2.Center for Research in Isotope and Environmental Dynamics)

Keywords:Cs-137, The Fukushima Daiichi Nuclear Power Plant, River-bottom sediment, Suspended sediment, Electrical conductivity, distribution coefficient

The Fukushima Daiichi nuclear power plant accident in 2011 released a large amount of radioactive material into the atmosphere, which was promptly deposited on the ground as a result of rainfall. Most of the deposition area is covered by forests, but some of it is carried by rivers to downstream areas. The form of ¹³⁷Cs in rivers has mainly focused on the dissolved or suspended form, but there have been few studies on river-bottom sediments. Since the contamination of river-bottom sediment would last longer, it is important to clarify the transport of ¹³⁷Cs in rivers. The aim of this study was to use long-term monitoring data of river-bottom sediment and suspended sediment collected in Fukushima Prefecture since 2011, to clarify the changes and factors of ¹³⁷Cs in river-bottom sediment. The study sites were 75 watersheds in the Hamadori and Nakadori areas of Fukushima Prefecture, where river-bottom sediment is sampled by the Ministry of Environment. The results showed that the concentration of ¹³⁷Cs in river-bottom sediments reaches the same level as that in suspended sediments about one year after ¹³⁷Cs deposition, but then decreases to a level about two orders of magnitude lower. The distribution coefficient (Kd), as an index of ¹³⁷Cs migration between the solid and liquid phases, showed that the relationship between Kd and electrical conductivity became unclear with time in both river-bottom sediments and suspended sediments. In addition, we found the possibility that the level of ¹³⁷Cs concentration varies depending on the geological conditions. It seems that mineral composition also plays an important role in ¹³⁷Cs transport in the river, and the difference in mineral composition influences the formation of the ¹³⁷Cs concentration ratio between river sediment and suspended sediment.