Solute transport in unsaturated porous media plays a crucial role in environmental processes affecting soils, aquifers, and carbon capture and storage operations. Natural porous media are characterized by various degrees of structural heterogeneity in the pore size distribution, spatial arrangements and spatial correlations. The impact of this pore-scale heterogeneity on the spreading of a solute plume, its mixing with other solutes, and the resulting reaction rates, is not well understood for unsaturated flow. Since these processes take place at pore scale, direct pore scale experimental measurements are needed to gain comprehensive understanding of them. The aim of UnsatPorMix is thus to elucidate the impact of structural pore-scale heterogeneity on solute spreading/mixing and reaction rates during unsaturated flow, through the combination of micromodel experiments and numerical model simulations. In the first stage of UnsatPorMix, experiments in micromodels with varying degrees of heterogeneity will provide unprecedented results on the phenomenology of pore-scale mechanisms and their effect on solute spreading and mixing. In the second stage, the experimental measurements of phase distribution and solute concentrations, combined to numerically-computed pore scale velocities, will be used to design and validate a pore-scale model for solute transport in these porous media. This model will allow obtaining a large representative numerical data set, enabling statistical analysis and the derivation of quantitative relations between structural heterogeneity and solute transport/mixing. UnsatPorMix will make a significant contribution to the modelling of, and risk assessment for, the various subsurface phenomena and applications cited above. During UnsatPorMix, the applicant will acquire a set of invaluable experimental skills and modeling expertise which will enable him to become an independent researcher and expert in flow and transport in unsaturated porous media.