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 14850 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. Environmental implications Even if environmental concerns (ex. wastewater treatment plant) have been integrated in the management plant of the majority of U-mine sites by the operators, in some cases, the mining activities have led to the dissemination of radioactive materials (ex, dust, particles transported through water) in nearby areas. Contaminated sites could cause potential radiological risks to the environment and living organisms. For this reason, this work proposes U-Pb isotopic tracing allowing the efficient identification of these materials in the environment. The identification of these materials can help to address an effective management and remediation strategies for these sites.