Understanding the behaviour of contaminants in soils and their transport in water is crucial. This study focuses on the mobility of uranium (U) in a wetland contaminated 70 years ago by the first uranium mining activities in France. Combining laboratory and field studies, we used a multi-scale approach to connect molecular processes to operational parameters. Eight core samples were collected from an area of approximately 1000 m². Laboratory desorption-based studies reveal a small fraction of labile U in the contaminated soil, which was further corroborated by DET (Diffusive Equilibrium in Thin films) /DGT (Diffusive Gradients in Thin films) passive sampler data monitored over time. The smart-Kd approach, combined with a comprehensive system characterization, identifies this labile fraction as U(VI) primarily adsorbed on 2:1 clay minerals, which is in line with X-ray Absorption Spectroscopy (XAS) spectroscopy. XAS data also unveil an inert U fraction composed of UO₂ particles, that were also confirmed by electron microscopy, and adsorbed U(IV) monomers. This study highlights the necessity of integrated approaches in investigating contaminant mobility to provide reliable operational parameters for incorporation into reactive transport models.