Implications of recently derived thermodynamic data and specific ionic interaction theory parameters for (Mg/Ca)nUO2(CO3)3(4-2n)- complexes on the predominance of the Mg2+-Ca2+-UO22+-OH--CO32- systems, and application to natural and legacy-mine waters

Modeling of hydrogeochemical processes influencing uranium migration in anthropized arid environments with application to the Teloua aquifer

Sandstone-hosted uranium is mined in the Sahel regions of Niger. The Teloua aquifer is located beneath the ore-processing facilities of one such former mine, COMINAK. The pores of the sandstone bedrock are partially filled by tosudite, a clay with sorption capacities. The local groundwater presents a strong oxidizing signature and very low water recharge. This study aims to determine the geochemical baseline of anthropogenic activity for uranium under such extreme conditions. The major and trace elements of both the contaminated and the pristine local groundwaters were sampled and analyzed to develop geochemical and reactive transport models. Kd distribution coefficients were calculated a posteriori from the mechanistic simulations. The entire water chemistry, with large variations in calcium, carbonate and sulfate concentrations, had to be taken into account to properly simulate the speciation and migration of U(VI) in the aquifer locally affected by the mining activities. U(VI) sorption significantly decreases during the propagation of the contaminant plume, due to the formation of CanUO2(CO3)3(4-2n)- complexes that were clearly demonstrated by TRLFS acquisition. The sorption of UO2(CO3)n(2-2n) can play a key role in the immobilization of U(VI). The mitigating factors for U(VI) are sorption on clay and the dispersion/dilution of the contaminated source terms within the groundwater, in which the strong ternary complexes are less important. There should be an efficient immobilization of fixed anthropic uranium by natural attenuation once the contaminant source terms have become depleted.

Modeling subsurface contaminant transport from a former open-pit uranium mine in fractured granites (La Ribière, France): Reducing uncertainties with geophysics

The long-term management of tailings from former uranium (U) mines requires an in-depth understanding of the hydrogeological processes and water flow paths. In France, most of the legacy U mines are located in fractured crystalline (plutonic) rocks, where the intrinsic subsurface heterogeneity adds to the uncertainties about the former extraction and milling activities and the state of the mine when production was ceased. U ores were mainly processed by sulfuric acid leaching, leading to high-sulfate-content mill tailings now contained in several tailing storage facilities (TSFs). The La Ribière site, located in western central France, is a former open-pit and underground U mine, closed in 1992 and used to store mill tailings. This site is being used as a test case to establish a workflow in order to explain and predict water flow and subsurface contaminant transport. A conceptual model of water flow and sulfate transport, at the scale of the La Ribière watershed, is first developed based on available information and hydrogeochemical monitoring. Recent geophysical investigations allows refining this model. Electrical Resistivity Tomography (ERT) proves to be efficient at localizing the extent of the highly conductive sulfate plume inherited from the U-mill tailings, but also at imaging the weathering profile. Magnetic Resonance Sounding (MRS), despite the limited signal intensity due to the low porosity in crystalline rocks, gives some insight into the porosity values, the depth of the fractured layer and the location of the low-porosity ore-processing muds. Based on this conceptual model, a 3D flow and non-reactive transport model with the METIS code is developed and calibrated. This model allows predicting the evolution of the sulfate plume, but will also be used in future investigations, to build reactive transport models with simplified hydrogeology for U and other reactive contaminants.

Predominance of the alkaline earth(II) triscarbonatoactinyl(VI) complexes in different geochemical contexts: Review of existing data and estimation of potentially unidentified species

From the available thermodynamic data in the literature, a review of the impact of the formation of complexes between triscarbonatoactinyl(VI) and alkaline earth(II) (Ae) is estimated under varying conditions. First, after analyzing the literature data and using the ascertained thermodynamic data available from the commissioned reviews from the Nuclear Energy Agency (Organization for the Economic Cooperation and Development) Thermochemical DataBank Project on actinides (An) U, Np, and Pu, and from recently determined AenUO2(CO3)3(4-2n)- thermodynamic functions, the formation of AenAnO2(CO3)3(4-2n)- complexes for Pu(VI) and Np(VI) are estimated using linear free energy relationships (LFERs). The data are in good agreement with the sole determination of AePuO2(CO3)32- from Jo et al. (Dalton Trans. 49, 11605), which gives a relative confidence in the LFERs, and allows the application to actual situations. From existing uranium data, first, the impact of the origin of the data on the calculated predominance is addressed under 0.1 M NaCl and atmospheric CO2(g); second, the influence of ionic strength and salinity on predominance is estimated; and finally, the influence of temperature up to 50 °C on the solubility of uraninite in a deep geological radioactive waste storage or disposal site is calculated. For neptunium and plutonium, the impact of the potential log10β°(AenAnO2(CO3)3(4-2n)-) on Pourbaix diagrams of Pu and Np in Mg-Ca-CO3 media are estimated from Jo et al. (Dalton Trans. 49, 11605) and LFERs. Finally, the application to the speciation of Pu and Np in seawater is proposed.