Uranium speciation and transport behavior in the wetland of the former extraction mine of Rophin (France)
Résumé
Introduction
In the vicinity ofold uranium mines, wetlands prove to be specific zones with significant amounts
of 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 concentrations
up to 3.6 g.kg-1. In this context, the main challenge is to describe the behavior ofU and relevant
decay products in the wetland using a predictive model that combines chemical speciation and
transport.
Methods
The overall adopted scientific approach is to propose a mechanistic description ofthe mobility of
these elements, from the molecular scale (speciation) to in situ behavior (lability), by coupling
field and laboratory experiments with modelling. Batch desorption and column experiments were
carried out with in situ soil samples to assess the mobile fraction part of U and its distribution
coefficients (Kd). Labile behavior of U was examined directly on site by using the DGT/DET
techniques 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.
Results
Observations by SEM, XANES and EXAFS confirmed the U transportin particulate form during
the mining activities and the establishment of reducing conditions over time, implying the
presence of both U(IV) and U(VI). Labile quantity and Kd values of U were confirmed to be
homogeneous at the scale of the Rophin wetland and consistent with in situ labile behavior
deduced from the DGT/DET approach, considering a kinetically controlled interaction.
Conclusions
Thanks to the combination of different techniques, a better understanding of what happened in
the wetland over 70 years can be achieved. All these data should now allow us to define the key
parameters to be integrated in the reactive part ofthe transport model ofthe Rophin site