Long-Term Evolution of Uranium Mobility within Sulfated Mill Tailings in Arid Regions: A Reactive Transport Study

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.

Assessment of 226Ra and U colloidal transport in a mining environment

The colloidal transport of trace (Fe, Al, Ba, Pb, Sr, U) and ultra-trace (226Ra) elements was studied in a mining environment. An original approach combining 0.45 μm filtered water sampling, the Diffusive Gradient in Thin films (DGT) technique, mineralogical characterization, and geochemical modelling was developed and tested at 17 sampling points. DGT was used for the truly dissolved fraction of the elements of interest, while the 0.45 μm filtration includes both colloidal and truly dissolved fractions (together referred to as total dissolved fraction). Results indicated a colloidal fraction for Al (up to 50%), Ba (up to 86%), and Fe (up to 99%) explained by the presence of submicrometric grains of kaolinite, barite, and ferrihydrite, respectively. Furthermore, the total dissolved 226Ra concentration in the water samples reached up to 10-25 Bq/L (1.2-3.0 10-12 mol/L) at 3 sampling points, while the truly dissolved aqueous 226Ra concentrations were in the mBq/L range. Such high total dissolved concentrations are explained by retention on colloidal barite, accounting for 95% of the total dissolved 226Ra concentration. The distribution of 226Ra between the truly dissolved and colloidal fractions was accurately reproduced using a (Rax,Ba1-x)SO4 solid solution, with values of the Guggenheim parameter a0 close to ideality. 226Ra sorption on ferrihydrite and kaolinite, other minerals well known for their retention properties, could not explain the measured colloidal fractions despite their predominance. This illustrates the key role of barite in such environments. The measured concentrations of total dissolved U were very low at all the sampling points (<4.5 10-10 mol/L) and the colloidal fraction of U accounted for less than 65%. U sorption on ferrihydrite could account for the colloidal fraction. This original approach can be applied to other trace and ultra-trace elements to complement when necessary classical environmental surveys usually performed by filtration on 0.45 μm.

A reactive transport model designed to predict the environmental footprint of an 'in-situ recovery' uranium exploitation

Worldwide, most uranium production relies on the 'in situ recovery' (ISR) extraction technique. This consists of dissolving the ore using a leaching solution (acid or alkaline) directly within the deposit through a series of injection and extraction wells. Due to the nature of the injected ISR solutions, the water quality of the aquifer could be affected. Reactive transport modeling is a powerful tool for predicting fluid flow and geochemical reactions in ISR reservoirs. In this study we present a coupled 3D environmental geochemical model (EGM) (based on the HYTEC reactive transport software), capable of predicting the physico-chemical conditions in an acid-leaching ISR uranium mine and its environmental footprint on the aquifer in the years following the closure of the production block. The model was validated at the KATCO mine (Kazakhstan) on two different and independent production blocks, over 10 years after their closure. The model shows that incorporating two main geochemical processes, (1) cationic sorption on clay surfaces (smectite-beidellite) and (2) precipitation of gypsum (CaSO₄.2H₂O), successfully reproduces the measured well data (pH, acidity and SO₄) over short- and long-term time scales. Clay surface sites remain mostly saturated in protons during the production phase. Simulations show that sorbed protons on the clay surfaces maintains the acid conditions for a longer period of time. The environmental impact model was also compared to a pre-existing model specifically developed for production simulation purposes: differences are observed as expected for the uranium production, but also for the impact distances, due to differences in the considered reactive mineralogical paragenesis. Thus, the choice of geochemical model should be made with due regard for the desired objectives. This work will assist the mine operator by providing a tool capable of assessing both the short- and long-term environmental footprints of the ISR production operation conditions and of identifying the best remediation strategy.

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

The formation of alkaline earth(II)triscarbonatouranyl(VI) (Ae_nUO₂(CO₃)₃^(4-2n)-) species that have been evidenced both in laboratory and in-field studies, is important from slightly acidic pH up to near degraded cementitious in carbonated waters. They are also showing distinctive luminescence properties with a hypsochromic shift relative to UO₂²⁺. The conditions of pH, activities of alkaline earth(II) free ions (mostly Mg²⁺ and Ca²⁺) and carbonate ions (HCO₃^-) can be predicted from the thermodynamic functions and constants. The predictive validity of the activity of major alkaline ions (mostly Na⁺) is determined from the models used to describe the ionic strength comportment of these species, particularly using coefficients from the specific ion interaction theory (SIT). The stability domains of these species are better defined as a function of the activity of the constituents, and applied to natural waters. In this work, using recently obtained complete thermodynamic data and SIT coefficients, we will draw the stability domains of the Ae_nUO₂(CO₃)₃^(4-2n)- species in combinations of activities of H⁺, HCO₃^-, Mg²⁺, Ca²⁺, and Na⁺ for a wide selection of water compositions from the literature. Water samples were collected near a French mining legacy-site (Site du Bosc, Lodève, France). After determining the major ion compositions, we will verify that the luminescence signal of uranium is in agreement with the predicted speciation in the stability domains.

Editorial for Special Issue “Formation of Sulfate Minerals in Natural and Industrial Environments”

Sulfate is abundant in the environment and, as a result, sulfate-containing minerals constitute a large and important focus of research [...]