Unveiling the origins and transport processes of radioactive pollutants downstream from a former U-mine site using isotopic tracers and U-238 series disequilibrium

High U concentrations (reaching up to 14,850 mg ⋅ kg^-1), were determined in soils and sediments of a wetland downstream of a former U mine in France. This study aims to identify the origin of radioactive contaminants in the wetland by employing Pb isotope fingerprinting, (²³⁴U/²³⁸U) disequilibrium, SEM, and SIMS observations. Additionally, information about U and ²²⁶Ra transport processes was studied using U-238 series disequilibrium. The results of Pb fingerprinting highlighted inherited material inputs of different U-mines with mainly two types of U-ores: i) pitchblende (UO₂), and ii) parsonsite (Pb₂(UO₂)(PO₄)₂). Moreover, significant disequilibrium of (²³⁰Th/²³⁸U) and (²²⁶Ra/²³⁰Th) activity ratios highlighted the mobility of ²³⁸U and ²²⁶Ra in the wetland, primarily driven by the water table fluctuations. Finally, this work uncovered a limitation of Pb isotope fingerprinting in the case of parsonsite materials, as the high natural Pb content of this mineral may hide the uranogenic Pb signature in the samples.