Mineral-water Reactions Research Articles

Multi-decadal geochemical evolution of drainage from underground coal mines in the Appalachian basin, USA

Coal mine drainage (CMD) in Appalachia is a widespread source of dissolved metals, SO4, and acidity that can degrade aquatic habitats and water supplies for decades following mine closure and flooding. In the bituminous coalfield of Pennsylvania, the Irwin Coal Basin (ICB) contains a series of partly to completely flooded, abandoned underground mines separated by leaky barriers within the Pittsburgh coal seam. CMD originated throughout the basin from minepool aquifers that formed after mine closures dating from 1910 to 1957. Historical and recent water quality data for eight CMD sites across the ICB, plus mineralogy and cation-exchange capacity of overburden lithologies, were analyzed to quantify important reactants and evaluate spatial and temporal water-quality trends. As overburden thickness and residence time increase along a ~ 50-km flowpath northeast to southwest in the basin, CMD becomes more alkaline, and Na concentrations increase. Since the 1970s, all eight ICB discharges have become less acidic, with exponential decreases in acidity, SO4, and Fe concentrations; only two CMD remain net-acidic (acidic pH at equilibrium). Exponential decay models that include a steady-state asymptote consistent with background groundwater chemistry and siderite equilibrium describe the early-stage, rapid contaminant concentration decay immediately after the “first flush” (initial flooding) and the progressive evolution toward late-stage background conditions. A geochemical evolution PHREEQC model indicates that spatial and temporal trends in pH, net-acidity, SO4, Fe, and major cations could be explained by the continuous dilution of first flush water by ambient groundwater combined with sustained water-mineral reactions involving pyrite and carbonates (calcite, dolomite, siderite) plus cation-exchange by clays (illite, chlorite, mixed-layer illite/smectite). These data and model results indicate that 1) cation-exchange reactions enhance calcite dissolution and alkalinity production, resulting in the evolution of CMD to Na-SO4-HCO3 type waters, and 2) siderite equilibrium could maintain dissolved Fe >16 mg/L over the next 40 years.

Mineral–water reactions in Earth’s mantle: Predictions from Born theory and ab initio molecular dynamics

Recent studies present compelling evidence that a free aqueous fluid phase exists within the upper mantle. Fluid may be present at depths as great as the transition zone (410–660 km) and possibly beyond. The chemical reactivity of such deep fluids can be predicted from the Born model of solvation. To use the Born model, we need to know the dielectric constant of water under mantle conditions. We have used ab initio molecular dynamics simulations to determine the dielectric constant of water up to a pressure of 30 GPa and a temperature of 3000 K. Increased temperature lowers the dielectric constant and decreases ion solvation, but pressure overcomes this effect. The resulting high dielectric constant suggests that aqueous mantle fluids are highly reactive for ion solvation and mineral dissolution. We tested this by using the Helgeson–Kirkham–Flowers equation of state to estimate free energies of several mineral-solution and ion solvation reactions under mantle conditions. The results support previous estimates of carbonate solubility in the mantle. We also find that mantle fluids may play a key role in transporting ore metals: we evaluated the solubility of chalcopyrite and the complexation of Cu and Fe by Cl under mantle conditions and find that metal complexation is as significant as in ore-forming fluids in the crust. At reasonable conditions of pH and fH2, chalcopyrite is highly soluble. We tentatively hypothesize that exsolved fluids from subducted slabs may extract and mobilize primary sulfides in the mantle, implying potentially deep sources for porphyry copper deposits.

Hydrochemical Characterization of Groundwaters’ Fluid Flow through the Upper Mesozoic Carbonate Geothermal Reservoirs in the Geneva Basin: An Evolution more than 15,000 Years Long

Groundwaters circulating in Upper Mesozoic carbonates are of great interest for geothermal heat production and storage applications in the Geneva area. This study aims at providing new insights and proposing new interpretations about the mineral-water reactions and the fluid-flow paths mechanisms across the Geneva Basin (GB). Data from previous studies are combined and improved by new ones collected from cold and hot springs and geothermal exploration wells in 2018 and 2020 in the framework of the GEothermies program and HEATSTORE project. Major ions, trace elements, and the isotopes of Oxygen, Hydrogen, Sulfur, Strontium, and Carbo have been analysed and the results show that the sampled waters have a meteoric origin, the carbonate aquifers act as preferential host rocks for geothermal waters, and partial contribution from the Cenozoic sediments can be observed in some samples. The Jura Mountains and the Saleve Ridge are the main catchment areas and an evolution from a pure Ca-HCO3 footprint for the cold springs, to a Na > Ca-HCO3 and a Na-Cl composutions, is observed at the two geothermal wells. The residence time is in the order of a few years for the cold springs and reaches up to 15–20,000 years for the deep wells.

Hydrogeochemical Characteristics and Geothermometry of Hot Springs in the Tensile Tectonic Region Leizhou Peninsula and Hainan Island in South China

Hydrochemistry and isotopes are studied for characterizations of geothermal waters, groundwaters, and surface waters in Leizhou Peninsula (LZ) and the northern part of Hainan Island (HN) across the tensile-tectonic Qiongzhou Strait, southern China. Water chemistry results showed that most geothermal waters are of alkaline and HCO3-Na types, while shallow groundwater samples were significantly different with HCO3-Ca, HCO3-Mg, and HNO3-Ca types. Stable isotope data indicated that the source of LZ geothermal waters is of atmospheric precipitation, while HN geothermal water has a slight “oxygen drift”. Cation geothermometers and silica geothermometers could appropriately predict reservoir temperatures. Based on multicomponent mineral equilibria and quart geothermometer without steam loss, reservoir temperatures were estimated to be in the range of 65.0°C to 165.0°C at an average circulation depth of 2023 m, substantiated with data from a thousand-meter-depth borehole. The saturation indexes (SI) simulated by PHREEQC and Na-K-Mg triangle diagram revealed that the chemical components in the geothermal waters are possibly derived from water-rock interactions at high temperatures. Four simulation paths were established to calculate the mass flux of minerals by PHREEQC program. The hydrothermal circulation process of the volcanic rocks on both sides of the Qiongzhou Strait was similar, but the water-mineral reactions of Leizhou Peninsula were more intense. During the high-temperature hydrothermal deep circulation in the central part of Hainan Island, the molar transfer was low and the degree of mineral reaction is weak. Finally, the geothermal genetic mechanism for the study area of the extensional geostructure was proposed. The findings mark the characteristics for the tensile-tectonic region in slight “oxygen shift”, low values of 87Sr/86Sr for possible deep source from the mantle, and higher reservoir temperatures at shallower circulation depths.

Uptake of trace elements into authigenic carbonate at a brine seep in the northern Gulf of Mexico

Hypersaline seafloor environments formed by brine seepage represent role model examples of extreme marine ecosystems; they may for example help to identify potential habitats for life on other planets. Trace and rare earth elements (REEs) are crucial components enabling microbial life at brine and hydrocarbon seeps. However, the behavior of trace elements at brine seeps is currently poorly understood. Because authigenic carbonates forming at brine seeps archive information on the composition of seepage fluids, we have analyzed carbonates collected from a brine seep site (Mississippi Canyon 709, MC709 for short) of the northern Gulf of Mexico (GoM). The dominance of low-magnesium (Mg) calcite among the carbonate minerals, moderately low δ13Ccarb values (average: −26.1‰, VPDB), as well as the presence of oil in carbonates suggest concomitant seepage of brine fluids and crude oil at this site. High δ18Ocarb values (up to +4.5‰, VPDB) suggest the involvement of 18O-rich fluids during carbonate precipitation, likely resulting from mineral-water reactions in deeper sediments. Significant enrichments of barium (Ba), strontium (Sr), and lithium (Li) in the samples suggest that deep ascending brine fluids contributed to carbonate formation. Enrichments in molybdenum (Mo), arsenic (As), and antimony (Sb) and a correlation between Mo, As, and Sb enrichment factors (EF) and the authigenic iron fraction (Fe/Al ratios) suggest that iron particulate shuttle processes were involved in scavenging of dissolved Mo, As, and Sb from seawater. This notion is strengthened by the co-variation of MoEF and UEF and the shale-normalized REE patterns of the brine-seep carbonates. This study allows to constrain the enrichment mechanisms of trace and REE elements at brine seeps, providing the foundation for an improved understanding of the distribution and behavior of trace metals, as well as their role in microbial metabolisms in the local hypersaline environment.

Assessing the suitability of lithium-related geothermometers for estimating the temperature of felsic rock reservoirs

Lithium-related geothermometers are relatively rarely used because they are based on empirical relations derived from poorly known water-mineral equilibrium reactions and do not always give valid estimations. In this study, we evaluated the applicability of Na-Li and Li-Mg geothermometers based on hydrochemical data for geothermal waters from nine geothermal systems in felsic reservoirs in China. The results indicate that the Na-Li geothermometer provided by Michard (1990), which is similar to those given by Fouillac and Michard (1981) and Verma and Santoyo (1997) with Cl <0.3 mol/L, is suitable for fully equilibrated waters from felsic reservoirs and their estimates better represent the temperatures of the deep rather than the shallow reservoirs. The Na-Li ion-exchange equilibrium in thermal waters cannot be reached without a prior Na-K equilibrium, which ensures that the Na concentration remains constant. Also, variations in the Na-Li ratios may be significantly unpredictable such that the Michard (1990) Na-Li geothermometer may coincidentally give valid temperature estimates for thermal waters from the low-temperature geothermal systems. Furthermore, the Li in the thermal waters may originate from water-mineral reactions or mixing with magmatic water or both. In reservoirs consisting of basalt and felsic rocks, variations in hydrothermal alteration may be the key factor that explains the large differences in Na-Li equilibria in the thermal waters. Additionally, small measurement errors could bring about large deviations in the temperature estimations due to the large difference between Na and Li concentrations in the thermal waters from the felsic reservoirs. Unlike the Li concentration, which remains constant once the thermal waters leave the deep reservoirs, the concentration of Mg in the thermal waters is sensitive to the temperature in the shallow reservoirs. Hence, we discourage the use of the Li-Mg geothermometer for estimating felsic reservoir temperatures.

Acid mine drainage sources and hydrogeochemistry at the Yatani mine, Yamagata, Japan: A geochemical and isotopic study

This paper describes the geochemistry of groundwater and its flow system in the closed Yatani mine in southern Yamagata Prefecture, Japan. The mine is located in a sulfide deposit containing pyrite and has been generating acid mine drainage (AMD). The study was intended to elucidate the formation of AMD and its flow patterns using geological, hydrological, geochemical, and isotopic techniques. The results indicate that AMD is formed by the interaction of groundwater with sulfide minerals, sand slime, and tailings back-filled into excavated mine areas. Groundwater recharge areas were identified on the mountain slope at an elevation of ~900 m. The formation of AMD in the drifts and shaft was more extensive than that in the deeper drainage levels. Principal component analysis was applied to the hydrogeochemical data to identify the causes of AMD formation. The first, second, and third principal components reveal that the increased ion concentrations in mine drainage are a result of water–mineral reactions in excavated mine areas, the contribution of groundwater in deep reductive environments, and isotopic fractionation during precipitation, respectively. A promising method of reducing AMD formation is to prevent contact between dissolved oxygen and sulfide minerals by increasing the drainage level or by filling the shallow underground excavated area with cementitious materials.

Empirical Kinetics and Their Role in Elucidating the Utility of Transition-State Theory to Mineral–Water Reactions

Transition-state theory (TST) is a successful theory for understanding many different types of reactions, but its application to mineral–water systems has not been successful, especially as the system approaches saturation with respect to a rate-limiting phase. A number of investigators have proposed alternate frameworks for using the kinetic rate data to construct models of dissolution, including Truesdale (Aquat Geochem, 2015; this issue). This alternate approach has been resisted, in spite of self-evident discrepancies between TST expectations and the data. The failure of TST under certain circumstances is a result of the presence of metastable intermediaries or reaction layers that form on the surface of reacting solids, and these phenomena are not anticipated by the current theory. Therefore, alternate approaches, such as the shrinking object model advocated by Truesdale, represent a potentially important avenue for advancing the science of dissolution kinetics.

Temperature‐dependent Li isotope ratios in Appalachian Plateau and Gulf Coast Sedimentary Basin saline water

AbstractLithium (Li) concentrations of produced water from unconventional (horizontally drilled and hydraulically fractured shale) and conventional gas wells in Devonian reservoirs in the Appalachian Plateau region of western Pennsylvania range from 0.6 to 17 mmol kg−1, and Li isotope ratios, expressed as in δ7Li, range from +8.2 to +15‰. Li concentrations are as high as 40 mmol kg−1 in produced waters from Plio‐Pleistocene through Jurassic‐aged reservoirs in the Gulf Coast Sedimentary Basin analyzed for this study, and δ7Li values range from about +4.2 to +16.6‰. Because of charge‐balance constraints and rock buffering, Li concentrations in saline waters from sedimentary basins throughout the world (including this study) are generally positively correlated with chloride (Cl), the dominant anion in these fluids. Li concentrations also vary with depth, although the extent of depth dependence differs among sedimentary basins. In general, Li concentrations are higher than expected from seawater or evaporation of seawater and therefore require water–mineral reactions that remove lithium from the minerals. Li isotope ratios in these produced waters vary inversely with temperature. However, calculations of temperature‐dependent fractionation of δ7Li between average shale δ7Li (−0.7‰) and water result in δ7Liwater that is more positive than that of most produced waters. This suggests that aqueous δ7Li may reflect transport of water from depth and/or reaction with rocks having δ7Li lighter than average shale.

Reactive transport modeling of the Dixie Valley geothermal area: Insights on flow and geothermometry

A 2D reactive transport model of the Dixie Valley geothermal area in Nevada, USA was developed to assess fluid flow pathways and fluid rock interaction processes. The model includes two major normal faults and the incorporation of a dual continuum domain to simulate the presence of a small-scale thermal spring being fed by a highly permeable but narrow fracture zone. Simulations were performed incorporating fluid flow, heat conduction and advection, and kinetic mineral-water reactions. Various solute geothermometry methods were applied to simulated spring compositions, to compare estimated reservoir temperatures with “true” modeled reservoir temperatures, for a fluid ascending the simulated fracture and cooling on its way to the surface. Under the modeled conditions (cooling but no mixing or boiling), the classical Na–K(–Ca) geothermometers performed best because these are least affected by mineral precipitation upon cooling. Geothermometry based on computed mineral saturation indices and the quartz geothermometer were more sensitive to re-equilibration upon cooling, but showed good results for fluid velocities above ca. 0.1m/d and a reactive fracture surface area 1–2 orders of magnitude lower than the corresponding geometric surface area. This suggests that such upflow rates and relatively low reactive fracture surface areas are likely present in many geothermal fields. The simulations also suggest that the presence of small-scale fracture systems having an elevated permeability of 10−12 to 10−10m2 can significantly alter the shallow fluid flow regime of geothermal systems. For the Dixie Valley case, the model implies that such elevated permeabilities lead to a shallow (less than 1km) convection cell where superficial water infiltrates along the range front normal fault and connects the small-scale geothermal spring through basin filling sediments. Furthermore, we conclude that a fracture permeability on the order of 10−12m2 may lead to near surface temperature >100°C whereas a permeability of 10−10m2 is not realistic because this permeability led to extreme upflow velocities and to a short-circuit of the regional fault zone.

CO2–water–mineral reactions during CO2 leakage: Geochemical and isotopic monitoring of a CO2 injection field test

A shallow CO2 injection experiment was performed at the CO2FieldLab site near Svelvik, Norway in Sept. 2011. The gas was injected through a 45° inclined well to a target depth of 20m. Its aim was to test various geophysical and geochemical monitoring tools for integrated near-surface leakage monitoring. A total of 1.67t of CO2 was injected over a period of six days. Groundwater level was at 60cm-depth and a complex salinity stratification with brackish water overlying fresh- and saltwater was observed at the beginning of the experiment. Four water sampling boreholes were located at each corner of a square configuration of 8m with the nominal injection at the center. From baseline conditions to the post-CO2 injection phase, pH, temperature and electrical conductivity of the groundwater were measured, collecting water samples at three different depths (5, 10 and 15m) for analyses of major, minor, and trace elements (Ca, Na, SO4, Cl, Mg, Al, Ba, Mn, Ni, Co, B, Li), and isotopes (δ11B, δ7Li, δ34SSO4, δ18OSO4, 87Sr/86Sr, δ18OH2O, δ2HH2O). Significant changes in chemical and isotope signatures of water over the duration of the experiment indicated two processes: 1) Binary mixing with seawater and rainwater as the possible end-members due to a near-seashore location, and 2) CO2–water–rock interactions enhanced by dissolved CO2 causing acid conditions and favoring rock dissolution. The relative contribution of those processes was quantified by a mass balance model and equilibrium calculations. This comprehensive geochemical and isotope approach allowed discriminating reactive mechanisms from non-reactive (mixing) processes associated with CO2 leaks, within an aquifer with a strong conductivity stratification and heterogeneous mineralogy and this approach seems promising for investigating CO2 leaks at field scale.

Environmental geochemistry of the Hycroft Mine: a case study on the limitation of Sobek-style acid-generation predictions

The Hycroft Mine is an open pit gold mine situated along the eastern edge of the Black Rock Desert in NW Nevada, a semi-arid region where annual evaporation significantly exceeds precipitation. The Hycroft deposit is hosted in volcanic breccias and conglomerates of the Tertiary Kamma Mountain volcanic group that have been faulted by north-trending structures that acted as conduits for hydrothermal fluids. An acid-leaching event resulted in the development of vuggy, quartz alunite-altered zones produced by the removal of most minerals, including those capable of buffering acid. Accessory minerals associated with the acid leach zones are rare, except native sulphur which is locally abundant. The complex sulphur mineralogy of the Hycroft deposit, coupled with the deficiency of acid-neutralizing minerals, limits the application of traditional Sobek-style Acid-Base Accounting (ABA) methods that rely upon sulphur speciation data to predict acid-generation potential. A comparison of results from geochemical predictive tests including the modified Sobek ABA method, Net Acid Generation (NAG) test and kinetic humidity cell test (HCT) illustrates the complexity of interpreting ABA data for material containing native sulphur and low solubility sulphate minerals such as alunite. Comparing ABA data to HCT results shows the predictions of acid generation from the ABA test are not consistent with those from the HCT, which is considered a more definitive test for the prediction of acid and metals mobility. The results of this study indicate the ABA test does not provide a reliable prediction of acid generation for samples containing native sulphur and low solubility sulphate minerals. A reasonably good correlation is, however, observed between the HCT and NAG results, suggesting the NAG test is a more effective tool for predicting the geochemical behavior of the Hycroft deposit, and potentially also other deposits with complex sulphur mineralogy. The presence of native sulphur in the acid leach material presents additional challenges with respect to acid-generation prediction due to the unique properties of this mineral. Although acid generation is predicted from the NAG test for samples containing native sulphur, the actual amount of acid and metal release generated by this mechanism in the HCT is limited in comparison to that associated with natural pyrite oxidation, which is the main contributor to acid rock drainage. The potential for acid generation from this mechanism is further limited by the semi-arid site conditions that are unlikely to produce conditions similar to the humidity cell tests that are designed to accelerate the weathering process and enhance mineral-water reaction rates. This study demonstrates the importance of considering the site-specific mineralogical characteristics of the deposit in the selection of the most appropriate geochemical test methods to use in the prediction of potential acid generation.

THE ORIGIN OF HIGH SILICON CONTENT IN POTENTIALLY MEDICINAL GROUNDWATER OF GRAN CANARIA (CANARY ISLANDS, SPAIN). MODELLING OF CHEMICAL WATER-ROCK INTERACTIONS

Groundwater of Gran Canaria (Canary Island, Spain) have been appreciated and used as an element of health tourism since the 19th Century. This activity was abandoned in the second half of 20th Century when springs disappeared due to groundwater drawdown. The chemistry of groundwater from 19 intakes in volcanic rocks of the north part of Gran Canaria was studied by applying geochemical modelling for quantifying processes responsible for high Si concentrations. Studied groundwater has temperature of 16.3°C–25.5°C, pH of 4.40–7.40, and usually HCO3-(Cl)-Mg-Ca-Na hydrochemical types. At near-neutral pH, fresh groundwater usually has 0.1-0.3 mM of Si. In studied groundwater Si concentrations are 0.42 to 1.82 mM, and show positive correlation with ionic strength and temperature. Volcanic bedrocks consist of, generally, easily reactive silicate minerals. Weathering is not supported by low rainfall; however, it shall be intensified by high influx of salts from marine aerosols and lithogenic carbon dioxide into groundwater. Geochemical modelling has found water-mineral reactions which reflect properly diversity of bedrock mineralogy. Based on those chemical reactions, contributions of particular silicate minerals to the pool of silicon dissolved in groundwater were calculated. Understanding the processes responsible for water chemistry might help in proper management and protection of groundwater. The Si-rich waters might be found in numerous places of Gran Canaria in all volcanic rocks. Silicic acid is the only form of silicon which is biologically available, and is regarded as a component which provides balneotherapeutic benefits. Many studies have showed beneficial and essential aspects of silicon in humans. Studied groundwater from Gran Canaria has an unexploited balneotherapeutic potential, and due to very high Si contents they seem to be ideal for testing the health benefits of such waters to humans. Hydrogeochemical methods, including geochemical modelling, provide effective tools for protection of such medicinal waters.

Predictive modeling of CO2 sequestration in deep saline sandstone reservoirs: Impacts of geochemical kinetics

One idea for mitigating the increase in fossil-fuel generated CO2 in the atmosphere is to inject CO2 into subsurface saline sandstone reservoirs. To decide whether to try such sequestration at a globally significant scale will require the ability to predict the fate of injected CO2. Thus, models are needed to predict the rates and extents of subsurface rock–water–gas interactions. Several reactive transport models for CO2 sequestration created in the last decade predicted sequestration in sandstone reservoirs of ∼17 to ∼90kg CO2 m−3. To build confidence in such models, a baseline problem including rock+water chemistry is proposed as the basis for future modeling so that both the models and the parameterizations can be compared systematically. In addition, a reactive diffusion model is used to investigate the fate of injected supercritical CO2 fluid in the proposed baseline reservoir+brine system. In the baseline problem, injected CO2 is redistributed from the supercritical (SC) free phase by dissolution into pore brine and by formation of carbonates in the sandstone. The numerical transport model incorporates a full kinetic description of mineral–water reactions under the assumption that transport is by diffusion only. Sensitivity tests were also run to understand which mineral kinetics reactions are important for CO2 trapping.The diffusion transport model shows that for the first ∼20years (20a) after CO2 diffusion initiates, CO2 is mostly consumed by dissolution into the brine to form CO2,aq (solubility trapping). From 20 to 200a, both solubility and mineral trapping are important as calcite precipitation is driven by dissolution of oligoclase. From 200 to 1000a, mineral trapping is the most important sequestration mechanism, as smectite dissolves and calcite precipitates. Beyond 2000a most trapping is due to formation of aqueous HCO3-. Ninety-seven percent of the maximum CO2 sequestration, 34.5kg CO2 per m3 of sandstone, is attained by 4000a even though the system does not achieve chemical equilibrium until ∼25,000a. This maximum represents about 20% CO2 dissolved as CO2,aq, 50% dissolved as HCO3,aq-, and 30% precipitated as calcite. The extent of sequestration as HCO3- at equilibrium can be calculated from equilibrium thermodynamics and is roughly equivalent to the amount of Na+ in the initial sandstone in a soluble mineral (here, oligoclase). Similarly, the extent of trapping in calcite is determined by the amount of Ca2+ in the initial oligoclase and smectite. Sensitivity analyses show that the rate of CO2 sequestration is sensitive to the mineral–water reaction kinetic constants between approximately 10 and 4000a. The sensitivity of CO2 sequestration to the rate constants decreases in magnitude respectively from oligoclase to albite to smectite.

An Equation of State for thermodynamic equilibrium of gas mixtures and brines to allow simulation of the effects of impurities in subsurface CO2 storage

Comprehensive understanding and prediction of chemical and reactive processes during and following injection of CO2 in depleted gas reservoirs and saline aquifers is important for the assessment of the performance and impacts of planned and existing Carbon Capture and Storage (CCS) projects. Over the last decade significant improvements have been made in numerical modelling of the complex, coupled processes involved. Among the many remaining issues where progress is still called for, is the consistent simulation of impacts of gas mixtures. In particular the presence of ‘impurities’ or ‘co-contaminants’ in the injected CO2 stream that are retained from the original flue-gases, such as H2S and SO2, have the potential, upon dissolution in the pore water, to alter aqueous and water–mineral reactions. Moreover, presence of these and other injected or in situ gases (e.g. CH4) affect CO2 solubility and thermodynamic properties of the fluid and gas phases, which, in turn, impact transport processes.As an important step towards evaluation of the impact of gas mixtures on these processes, a new Equation of State (EOS) has been developed which allows accurate and efficient modelling of thermodynamic equilibrium of gas mixtures and brines over a large range of pressure, temperature and salinity conditions. Presently the new EOS includes CO2, SO2, H2S, CH4 and N2. This model is based on equating the chemical potentials in the system, using the Peng–Robinson EOS to calculate the fugacity of the gas phase. The model performs favourably with respect to existing EOS's and experimental data for single gas systems and accurately reproduces available data sets for gas mixtures. Preliminary analysis shows, amongst others, that CO2 solubility is most sensitive to CH4 admixture and least sensitive to the presence of SO2 in the injected gas.

Characterization of deep weathering and nanoporosity development in shale--A neutron study

We used small-angle and ultra-small-angle neutron scattering (SANS/USANS) to characterize the evolution of nanoscale features in weathering Rose Hill shale within the Susquehanna/Shale Hills Observatory (SSHO). The SANS/USANS techniques, here referred to as neutron scattering (NS), characterize porosity comprised of features ranging from approximately 3 nm to several micrometers in dimension. NS was used to investigate shale chips sampled by gas-powered drilling (“saprock”) or by hand-augering (“regolith”) at ridgetop. At about 20 m depth, dissolution is inferred to have depleted the bedrock of ankerite and all the chips investigated with NS are from above the ankerite dissolution zone. NS documents that 5-6% of the total ankerite-free rock volume is comprised of isolated, intraparticle pores. At 5 m depth, an abrupt increase in porosity and surface area corresponds with onset of feldspar dissolution in the saprock and is attributed mainly to peri-glacial processes from 15 000 years ago. At tens of centimeters below the saprock-regolith interface, the porosity and surface area increase markedly as chlorite and illite begin to dissolve. These clay reactions contribute to the transformation of saprock to regolith. Throughout the regolith, intraparticle pores in chips connect to form larger interparticle pores and scattering changes from a mass fractal at depth to a surface fractal near the land surface. Pore geometry also changes from anisotropic at depth, perhaps related to pencil cleavage created in the rock by previous tectonic activity, to isotropic at the uppermost surface as clays weather. In the most weathered regolith, kaolinite and Fe-oxyhydroxides precipitate, blocking some connected pores. These precipitates, coupled with exposure of more quartz by clay weathering, contribute to the decreased mineral-pore interfacial area in the uppermost samples. These observations are consistent with conversion of bedrock to saprock to regolith at SSHO due to: (1) transport of reactants (e.g., water, O 2 ) into primary pores and fractures created by tectonic events and peri-glacial effects; (2) mineral-water reactions and particle loss that increase porosity and the access of water into the rock. From deep to shallow, mineral-water reactions may change from largely transport-limited where porosity was set largely by ancient tectonic activity to kinetic-limited where porosity is changing due to climate-driven processes.

Biogeochemical processes in a clay formation in situ experiment: Part E – Equilibrium controls on chemistry of pore water from the Opalinus Clay, Mont Terri Underground Research Laboratory, Switzerland

The chemistry of pore water (particularly pH and ionic strength) is an important property of clay rocks being considered as host rocks for long-term storage of radioactive waste. Pore waters in clay-rich rocks generally cannot be sampled directly. Instead, their chemistry must be found using laboratory-measured properties of core samples and geochemical modelling. Many such measurements have been made on samples from the Opalinus Clay from the Mont Terri Underground Research Laboratory (URL). Several boreholes in that URL yielded water samples against which pore water models have been calibrated. Following a first synthesis report published in 2003, this paper presents the evolution of the modelling approaches developed within Mont Terri URL scientific programs through the last decade (1997–2009). Models are compared to the composition of waters sampled during dedicated borehole experiments. Reanalysis of the models, parameters and database enabled the principal shortcomings of the previous modelling efforts to be overcome. The inability to model the K concentrations correctly with the measured cation exchange properties was found to be due to the use of an inappropriate selectivity coefficient for Na–K exchange; the inability to reproduce the measured carbonate chemistry and pH of the pore waters using mineral–water reactions alone was corrected by considering clay mineral equilibria. Re-examination of the measured Ca/Mg activity ratios and consideration of the mineralogical composition of the Opalinus Clay suggested that Ca/Mg cation exchange rather than dolomite saturation may control the ratio of these ions in solution. This re-examination also suggests that the Ca/Mg ratio decreases with increasing pore-water salinity. Several possible reasons for this are proposed. Moreover, it is demonstrated that feldspar equilibria must not be included in Opalinus Clay modelling because feldspars are present only in very small quantities in the formation and because Na/K ratios measured in pore water samples are inconsistent with feldspar saturation. The principal need to improve future modelling is additional or better data on rock properties, in particular: (i) a more detailed identification of phases in the Opalinus Clay that include redox-sensitive elements together with evaluation of their thermodynamic properties; (ii) an improved understanding of the distribution of celestite throughout the Opalinus Clay for Sr/SO4 concentrations control; (iii) improvements in analytic and thermodynamic data for Ca–Mg rock cation exchange and mineral chemical properties and (iv) the measurement of composition and stability constants of clay minerals actually present in the formation.

Groundwater origins and mixing pattern in the multilayer aquifer system of the Gafsa-south mining district: a chemical and isotopic approach

Major ion geochemistry and environmental isotopes were used to identify the origins and the mineralisation processes of groundwater flowing within the three aquifer levels of the multilayer system of the Gafsa-south mining district (Southwestern Tunisia). It has been demonstrated that groundwaters are characterised by a Ca–Mg–SO4 water type. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of dolomite. δ18O and δ2H values are much lower than the isotopic signature of regional precipitation and fall close to the meteoric water lines, indicating that groundwaters have not been significantly affected by evaporation or mineral–water reactions. The distribution of stable and radiogenic isotopes (δ18O, δ2H, δ13C and 14C) within the aquifer levels suggests that the deep confined aquifer receives a significant modern recharge at higher altitudes, while, the shallow unconfined aquifer has been mainly recharged under cooler paleoclimatic condition, likely during Late Pleistocene and Early Holocene humid periods. However, waters from the intermediate confined/unconfined aquifer have composite isotopic signatures, highlighting that they are derived from a mixture of the two first end-members.

Solubility and near-equilibrium dissolution rates of quartz in dilute NaCl solutions at 398–473 K under alkaline conditions

The dissolution–precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH) 3 solutions with pH 8.2–9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol −1 of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol −1 are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at Q 1 or Q 2 silicon sites, respectively. The dissolution mechanism varies with Δ G such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at Q 1 sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by Bickmore et al. (2008). However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated Q 1 reaction rate on pH, and a poorly-constrained pH dependence of the postulated Q 2 rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at Q 3 sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.

Geochemical Impacts to Groundwater from Geologic Carbon Sequestration: Controls on pH and Inorganic Carbon Concentrations from Reaction Path and Kinetic Modeling

Geologic carbon sequestration has the potential to cause long-term reductions in global emissions of carbon dioxide to the atmosphere. Safe and effective application of carbon sequestration technology requires an understanding of the potential risks to the quality of underground sources of drinking water. In particular, concern is warranted regarding the potential for CO(2) leakage through geological features and abandoned wells that may result in detrimental perturbations to subsurface geochemistry. Reaction path and kinetic models indicate that geochemical shifts caused by CO(2) leakage are closely linked to mineralogical properties of the receiving aquifer. CO(2) gas dissolution into groundwater and subsequent reaction with aquifer minerals will control the evolution of pH-bicarbonate envelopes. These parameters provide geochemical context for predicting how regulated contaminants associated with aquifer solids will respond via various mineral-water reaction processes. The distribution and abundance of carbonate, silicate, oxide, and phyllosilicate minerals are identified as key variables in controlling changes in groundwater geochemistry. Site-specific risk assessments may require characterization of aquifer geology, mineralogy, and groundwater chemistry prior to CO(2) injection. Model results also provide a frame of reference for developing indicative measurement, monitoring, and verification (MMV) protocols for groundwater protection.