Around near-surface underground, an unsaturated zone is formed in such porous layer composed of mainly silicate and silica sand. In the unsaturated zone in which pores are not completely filled with groundwater but also air, the migration of RNs is influenced by the presence of air phase. So far, though many studies have been reported about the unsaturated zone, little studies on the relation of the unsaturated zone and the migration of RNs have been carried out. In this study, the change of the migration of tracer ions in the unsaturated zone was examined by using a bed column, and analyzed by applying advection dispersion equation to the experimental results. As tracer ions, cesium (Cs) and strontium (Sr) were chosen in this study. These elements are the most important elements for the decontamination of Eastern Japan area caused by Fukushima the Daiichi Nuclear Power Plant accident. The experimental results showed the facilitation of the migration of tracer ions in the unsaturated zone. This causes the decrease in the sorption of tracer ions due to the inhibition of air phase. That is, the increase in the retardation effect by the clogging with air phase in flow paths was not remarkable. In the assessment of RNs migration under the unsaturated condition, these competing effects should be reflected adequately.

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