Abstract: The origin of groundwater recharge and subsequent flow paths are often difficult to establish in fractured, multi-lithological, and highly compartmentalized aquifers such as the Troodos Fractured Aquifer (TFA). As the conjunctive use of stable isotopes and hydrogeochemical data provides additional information, we established a monitoring network for stable isotopes in precipitation in Cyprus. The local meteoric water line, altitude effect and seasonal variation of stable isotopes in precipitation are derived from monitoring data. Stable isotopes and hydrogeochemical data are combined to model water-rock interactions and groundwater evolution along a complete ophiolite sequence. As a result a generic hydrogeologic description for the observed water types is developed. Isotope hydrology was applied in conjunction with hydrogeochemical modelling in Kargiotis Watershed, a major north-south transect of the TFA. PHREEQC was used for hydrogeochemical modelling to establish generic descriptions for observed water types. Mean precipitation-weighted values from 16 monitoring stations were used to calculate the Local Meteoric Water Line (LMWL), which was found to be equal to δ2H = (6.58 ± 0.13)*δ18O + (12.64 ± 0.91). A general decrease of 1.22‰ for δ2H and 0.20‰ for δ18O in precipitation was calculated per 100 m altitude. A generic groundwater evolution path was established: 1. Na/MgClHCO3, 2. MgHCO3, 3. Ca/MgHCO3, 4. Ca/MgNaHCO3, 4a. MgNa/CaHCO3/Cl, 5. NaMg/CaHCO3/Cl, 6. NaHCO3, 7. Na/MgHCO3SO4, 8. NaSO4Cl/HCO3. Hydrogeologic descriptions, consisting of groundwater origin, flow path and possible active water-rock processes, have been realised for the observed water types. The first two water types occur in serpentine and ultramafic-gabbro springs. Type 3 waters represent early stages of recharge and/or short flow paths, in gabbro whereas types 4 and 5 are typical for further percolating waters in gabbro and diabase. Water types 6 and 7 occur both in diabase and in the basal group and represent the regional flow. Water type 8 is the end member of regional, upwelling groundwater in the basal group. The presented descriptions and methods have practical applications in groundwater exploration, characterization, and protection. The methodology can be applied in other complex aquifer systems.

Abstract: Complex pyrite textures associated with large changes in isotopic and trace element compositions are routinely assumed to be indicative of multi-faceted processes involving multiple fluid and sulfur sources. We propose that the features of ore-stage pyrite from roll front deposits across the world, revealed in exquisite detail via high-resolution trace element mapping by LA-ICP-MS, reflect the dynamic internal evolution of the biogeochemical processes responsible for sulfate reduction, rather than externally driven changes in fluid or sulfur sources through time. Upon percolation of oxidizing fluids into the reduced host-sandstones, roll front systems become self-organized, with a systematic reset of their activity cycle after each translation stage of the redox interface down dip of the aquifer. Dominantly reducing conditions at the redox interface favor the formation of biogenic framboidal pyrite (δ34S from −30.5 to −12.5‰) by bacterial sulfate reduction and the genesis of the U mineralization. As the oxidation front advances, oxidation of reduced sulfur minerals induces an increased supply of sulfate and metals in solution to the bacterial sulfate reduction zone that has similarly advanced down the flow gradient. Hence, this stage is marked by increased rates of the bacterial sulfate reduction associated with the crystallization of variably As-Co-Ni-Mo-enriched concentric pyrite (up to 10,000′s of ppm total trace contents) with moderately negative δ34S values (from −13.7 to −7.5‰). A final stage of pyrite cement with low trace element contents and heavier δ34S signature (from −6.9 to +18.8‰) marks the end of the roll front activity cycle and the transition from an open to a predominantly closed system behavior (negligible advection of fresh sulfate). Blocky pyrite cement is formed using the remaining sulfate, which now becomes quickly heavy according to a Rayleigh isotope fractionation process. This ends the cycle by depleting the nutrient supplies for the sulfate-reducing bacteria and cementing pore spaces within the host sandstone, effectively restricting fluid infiltration. This internally-driven roll front activity cycle results in systematic, large S isotope and trace element fractionation. Ultimately, the long-time evolution of the basin and fluid sources control the metal endowment and evolution of the system; these events, however, are unlikely to be preserved by the roll front, as a direct result of its hydrodynamic nature.

Abstract: A large number of radionuclides remain in uranium tailings, and U, 226Ra and 210Pb leach out with water chemistry, causing potential radioactive contamination to the surrounding environment. In this paper, uranium tailings from a uranium tailings pond in southern China were collected at different depths by means of borehole sampling, mixed and homogenised, and analysed for mineral and chemical composition, microscopic morphology, U, 226Ra and 210Pb fugacity, static leaching and dynamic leaching of U, 226Ra and 210Pb in uranium tailings at different pH conditions. The variation of U, 226Ra and 210Pb concentrations in the leachate under different pH conditions with time was obtained, and the leaching mechanism was analysed. The results showed that the uranium tailings were dominated by quartz, plagioclase and other minerals, of which SiO2 and Al2O3 accounted for 65.45% and 13.32% respectively, and U, 226Ra and 210Pb were mainly present in the residue form. The results of the static leaching experiments show that pH mainly influences the leaching of U, 226Ra and 210Pb by changing their chemical forms and the particle properties of the tailings, and that the lower the pH the more favourable the leaching. The results of dynamic leaching experiments during the experimental cycle showed that the leaching concentration and cumulative release of U, 226Ra and 210Pb in the leach solution were greater at lower pH conditions than at higher pH conditions, and the leaching of U, 226Ra and 210Pb at different pH conditions was mainly from the water-soluble and exchangeable states. The present research results are of great significance for the environmental risk management and control of radioactive contamination in existing uranium tailings ponds, and are conducive to ensuring the long-term safety, stability and sustainability of uranium mining sites.

Abstract: Permeable reactive barriers (PRBs) are used for groundwater remediation at contaminated sites worldwide. This technology has been efficient at appropriate sites for treating organic and inorganic contaminants using zero-valent iron (ZVI) as a reductant and as a reactive material. Continued development of the technology over the years suggests that a robust understanding of PRB performance and the mechanisms involved is still lacking. Conflicting information in the scientific literature downplays the critical role of ZVI corrosion in the remediation of various organic and inorganic pollutants. Additionally, there is a lack of information on how different mechanisms act in tandem to affect ZVI-groundwater systems through time. In this review paper, we describe the underlying mechanisms of PRB performance and remove isolated misconceptions. We discuss the primary mechanisms of ZVI transformation and aging in PRBs and the role of iron corrosion products. We review numerous sites to reinforce our understanding of the interactions between groundwater contaminants and ZVI and the authigenic minerals that form within PRBs. Our findings show that ZVI corrosion products and mineral precipitates play critical roles in the long-term performance of PRBs by influencing the reactivity of ZVI. Pore occlusion by mineral precipitates occurs at the influent side of PRBs and is enhanced by dissolved oxygen and groundwater rich in dissolved solids and high alkalinity, which negatively impacts hydraulic conductivity, allowing contaminants to potentially bypass the treatment zone. Further development of site characterization tools and models is needed to support effective PRB designs for groundwater remediation.

Abstract: The migration process of leached uranium in the in-situ leaching of uranium is considered a typical reactive transport problem. During this process, the lixiviant concentration and injection rate are important in-situ leaching process parameters (ISLPP) to efficiently recover uranium. However, several uncertain factors affect the outcomes of the ISLPP design. In addition, the repeated use of the reactive transport model (RTM) for investigating the acid in-situ leaching of uranium with the application of the Monte Carlo method leads to a substantial computational load. For this reason, a machine learning (ML)–based surrogate model was developed with the backpropagation neural network (BPNN) method to replace the RTM under the condition of uncertain parameters. Moreover, the simulated annealing optimisation model for ISLPP was created based on the proposed surrogate model. The optimal ISLPP was achieved that generated maximum profits from uranium recovery under different lixiviant prices, uranium prices and exploitation times. The optimal design framework of ISLPP based on the proposed ML algorithm was then applied in the Bayan-Uul sandstone-type uranium deposit in Inner Mongolia, China. From our analysis, it was demonstrated that the ML-based surrogate model exhibited great fitness with the RTM. The optimal results of the ISLPP indicated that the lixiviant concentration and injection rate could be adjusted based on the fluctuations in lixiviant price, uranium price and exploitation time. If the prices of sulphuric acid were high, a specific concentration of hydrogen peroxide could be injected into the injection well to promote the oxidation and dissolution of the uranium ore to increase the income from the uranium recovery. The optimisation model can also use the ISLPP scheme to boost the revenues from different lixiviant prices, uranium prices and exploitation times under the uncertainty of porosity, illustrating the applicability of the ML-based optimal design method of ISLPP in ISL mining. A general framework for developing surrogate models, as well as for conducting uncertainty analyses for a wide range of groundwater models was proposed here yielding valuable insights.