Uranium speciation and transport behavior in the wetland of the former extraction mine of Rophin (France)

International audienceIntroductionIn the vicinity ofold uranium mines, wetlands prove to be specific zones with significant amountsof accumulated U. At the center ofthe ancient mining district of Lachaux in France (45.994°N,3.596°E), the site of Rophin (within the ZATU: Uranium Working Zone = Long Term SocioEcological Research Tool ofCNRS) is characterized by awetland areawith high U concentrationsup to 3.6 g.kg-1. In this context, the main challenge is to describe the behavior ofU and relevantdecay products in the wetland using a predictive model that combines chemical speciation andtransport.MethodsThe overall adopted scientific approach is to propose a mechanistic description ofthe mobility ofthese elements, from the molecular scale (speciation) to in situ behavior (lability), by couplingfield and laboratory experiments with modelling. Batch desorption and column experiments werecarried out with in situ soil samples to assess the mobile fraction part of U and its distributioncoefficients (Kd). Labile behavior of U was examined directly on site by using the DGT/DETtechniques while U speciation was investigated by the combination ofseveral techniques (SEM,XAS). From laboratory data, predictive modelling (smart-Kd model) was implemented under sitespecific conditions.ResultsObservations by SEM, XANES and EXAFS confirmed the U transportin particulate form duringthe mining activities and the establishment of reducing conditions over time, implying thepresence of both U(IV) and U(VI). Labile quantity and Kd values of U were confirmed to behomogeneous at the scale of the Rophin wetland and consistent with in situ labile behaviordeduced from the DGT/DET approach, considering a kinetically controlled interaction.ConclusionsThanks to the combination of different techniques, a better understanding of what happened inthe wetland over 70 years can be achieved. All these data should now allow us to define the keyparameters to be integrated in the reactive part ofthe transport model ofthe Rophin sit