Abstract <p>This study discussed the migration of cesium (Cs) ions through porous media under unsaturated conditions in the shallow subsurface for the disposal of low-level radioactive waste. In the flow experiments, the column was packed with sand particles and placed horizontally, considering that the water saturation was distributed as uniformly as possible in the flow direction. From X-ray computed tomography observation, assuming that the unsaturated column had two kinds of flow paths, one of which easily flowed (saturated parts) and the other did not (unsaturated parts), this study combined the numerical solutions of two advection–diffusion equations and estimated the retardation coefficient for Cs ions in the column. The retardation coefficients were estimated to be smaller than those obtained in the conventional evaluation. To correct such differences, this study presented an approximate equation between the retardation coefficient and water saturation.</p> Graphical abstract <p></p>