SO4 H2O Research Articles

Groundwater–rock interactions and mixing in fault–controlled karstic aquifers: A structural, hydrogeochemical and multi-isotopic review of the Pontina Plain (Central Italy)

Karstic aquifers represent crucial water resources and are categorized as either stratigraphically or fault–controlled. This study investigates groundwater–rock interactions and mixing processes within one of the largest fault–controlled karstic aquifers in Central Italy, adjacent to the Pontina plain, which is a highly populated area where agricultural activities and climate change challenge the groundwater assessment of a complex aquifer. We conducted structural, hydrogeochemical, and multi-isotopic screening of ten selected springs with different degrees of mineralization (ranging from Ca–HCO3 to Na–Cl hydrofacies), incorporating new analyses and modeling of δ34S(SO4), δ18O(SO4), 87Sr/86Sr, and δ11B. Additionally, the reinterpretation of a seismic section provides a more detailed framework extending to depths of approximately 5–7 km that allows the identification of the geometry of normal faults, which act as pathways for upwelling fluids. Our findings reveal that hydrogeochemical compositions result from multiple interactions between karstic water and deeper fluids that have interacted with different rocks. Concentration (Na/Li) and isotope (SO4–H2O) geothermometers, coupled with geochemical modeling and trace element analysis, enabled the estimation of a water temperature equilibrium of approximately 95.5 °C, with Triassic evaporites generally corresponding to a depth of approximately 3 km and a temperature of 40 °C with magmatic rocks at approximately 1 km depth, which is likely associated with ongoing tectonics and the Quaternary tectonically controlled Volsci Volcanic Field. To obtain the latter estimate, we used a new geothermometer activity based on the equilibrium between analcime and pollucite. Furthermore, this multidisciplinary approach enhances the understanding of groundwater behavior in fault–controlled karstic aquifers, where mantle-derived CO2 dissolved in groundwater is the driving force behind water–rock interactions. Given the potential for further variations in mixing, which may worsen water quality and increase aquifer vulnerability, periodic monitoring of these processes is essential in a human-impacted environment amidst ongoing climate change.

Tombstoneite, a new mineral from Tombstone, Arizona, USA, with a pinwheel-like Te6+O3(Te4+O3)3 cluster

AbstractThe new mineral tombstoneite (IMA2021-053), (Ca0.5Pb0.5)Pb3Cu2+6Te6+2O6(Te4+O3)6(Se4+O3)2(SO4)2⋅3H2O, occurs at the Grand Central mine in the Tombstone district, Cochise County, Arizona, USA, in cavities in quartz matrix in association with jarosite and rodalquilarite. Tombstoneite crystals are green pseudohexagonal tablets, up to 100 μm across and 20 μm thick. The mineral has a pale green streak and adamantine lustre. It is brittle with irregular fracture and a Mohs hardness of ~2½. It has one perfect cleavage on {001}. The calculated density is 5.680 g cm–3. Optically, the mineral is uniaxial (–) and exhibits pleochroism: O = green, E = light yellow green; O > E. The Raman spectrum exhibits bands consistent with Te6+O6, Te4+O3, Se4+O3 and SO4. Electron microprobe analysis provided the empirical formula (Ca0.51Pb0.49)Σ1.00Pb3.00Cu2+5.85Te6+2.00O6(Te4+1.00O3)6(Se4+0.69Te4+0.24S0.07O3)2(S1.00O4)2⋅3H2O. Tombstoneite is trigonal, P321, a = 9.1377(9), c = 12.2797(9) Å, V = 887.96(18) Å3 and Z = 1. The structure of tombstoneite (R1 = 0.0432 for 1205 I > 2σI) contains thick heteropolyhedral layers comprising Te6+O6 octahedra, Jahn-Teller distorted Cu2+O5 pyramids, Te4+O3 pyramids and Se4+O3 pyramids. Pb2+ cations without stereoactive 6s2 lone-pair electrons are hosted in pockets in the heteropolyhedral layer. Pb2+ cations, possibly with stereoactive 6s2 lone-pair electrons, are located in the interlayer region along with SO4 tetrahedra and H2O groups. Within the heteropolyhedral layer, the Te6+O6 octahedra and the Te4+O3 pyramids form finite Te6+O3(Te4+O3)3 clusters with a pinwheel-like configuration. This is the first known finite complex including both Te4+ and Te6+ polyhedra in any natural or synthetic tellurium oxysalt structure.

Phase equilibria in the sea-water Na, K, Mg//Cl, SO4–H2O quinary system at 0 °C

The phase equilibria in the quinary Na, K, Mg//Cl, SO4–H2O sea-water system at 0 °C were determined and a total phase equilibria diagram for the system was constructed by means of the translation method consistent with available literature data. There are seven points, 21 curves and 22 fields saturated, respectively, with 4, 3 and 2 equilibrium solid phases, for the quinary composition of the system. The constructed total phase equilibria diagram of the system was fragmented into divariant co-crystallization fields of two different solid phases and trivariant crystallization volumes of each of the solid phases at equilibrium. The structures of nine Janecke dry-salt phase diagrams were obtained by fragmentation of the total phase equilibria diagram into crystallization volumes. The comparison with the available literature data shows that, the obtained crystallization volume for halite agrees well with the available experimental data.

Geothermometry and geochemistry of groundwater in the Continental Intercalaire aquifer, southeastern Algeria: Insights from cations, silica and SO4–H2O isotope geothermometers

Characterization of geochemistry and geothermometry of the groundwater from the Continental Intercalaire (CI) aquifer, one of the largest aquifers in the world, stretching over one million km2 surface area shared between Algeria, Tunisia and Libya, was conducted in southeastern Algeria nearby the border with Tunisia. Thirty-two water samples were collected from boreholes to analyze the physicochemical parameters and stable isotopes (δ18O(H2O), δ18O(SO4), δ 34S(SO4)) and determine the origin of water mineralization of the CI aquifer. Water temperature was assessed using several geothermometers, such as cations, silica and SO4–H2O stable isotopes. The CI aquifer displayed a discharge temperature varying from 45 to 65.1 °C due to water circulation at deeper depths ranging from 1000 to 2200 m. The Results show that CI water contains high total dissolved solids (TDS) and neutral to slightly alkaline pH. The most frequent water types were Na–Cl–SO4 and Ca–SO4 indicating the geological formation nature of CI aquifer which is mainly composed of evaporites. The application of Na–K and Na–K–Ca geothermometers yielded unreliable temperatures. However, Na–Li geothermometer resulted in relatively plausible temperatures ranging from 74.69 to 147.83 °C. Quartz and CaSO4–H2O isotope geothermometer are the most suitable methods for temperature estimation, due to their attainment of equilibrium resulting in temperatures ranging from 62 to 93 °C, and validation by the multiple mineral equilibrium approach. The present study demonstrated a successful application of CaSO4–H2O isotope geothermometer in a low enthalpy (<100 °C) geothermal system. The results corroborate previous findings on the same CI aquifer at the Tunisian side and on carbonate-evaporite geothermal reservoirs. Furthermore, geothermal potential of the CI aquifer has been highlighted suggesting its use as a source of renewable energy which could be applied in heating greenhouses and/or generating electricity.

Phase Equilibria in Quinary Na, K, Mg//Cl, SO4–H2O System at 25 °C

The phase equilibria in the quinary Na, K, Mg//Cl, SO4–H2O system at 25 °C were determined, and a total phase equilibria diagram of the system was constructed by means of the translation method. St...

What we know and still not know about oceanic salts

Abstract The term oceanic salts concerns the salts, which can be formed from solutions containing the major ions of seawater, that is the hexary system Na+, K+, Mg++, Ca++, Cl–, SO4 ––H2O. Besides in geochemistry of evaporites and the potash extraction the solubility equilibria and properties of aqueous solutions containing these ions are of importance in many fields of science and technology in a wide range of temperature and composition. After a short introduction into the types of subsystems and the main features of their solubility diagrams the known salt phases, their properties and stability are listed and discussed with a focus on recent work and discoveries. The status of solubility data evaluation of the multi-component system is discussed from the view point of the data itself and the possibilities and limitations of applying thermodynamic modelling. This is demonstrated with the data compilation of Usdowski and Dietzel, the Pitzer models of Harvie, Möller and Weare as well as the model developed in the THEREDA project. Future work should be directed to improve accuracy of solubility data in multi-component solutions combined with modelling and to consider kinetics and mechanistic aspects in crystallization of phases like anhydrite or polyhalite.

Effect of sulfidogenesis cycling on the biogeochemical process in arsenic-enriched aquifers in the Lanyang Plain of Taiwan: Evidence from a sulfur isotope study

This study evaluated the biogeochemical interactions between arsenic (As) and sulfur (S) in groundwater to understand the natural and anthropogenic influences of S redox processes on As mobilization in the Lanyang Plain, Taiwan. Cl− and the sulfate isotopic composition δ34S[SO4] were selected as conservative tracers. River water and saline seawater were considered as end members in the binary mixing model. Thirty-two groundwater samples were divided into four types of groundwater (I, pyrite-oxidation; II, iron- and sulfate-reducing; III, sulfate-reducing; and IV, anthropogenic and others). The binary mixing model coupled with discriminant analysis was applied to yield a classification with 97% correctness, indicating that the DO/ORP values and δ34S[SO4] and Fe2+ concentrations are effective redox-sensitive indicators. Type I groundwater is mostly located in a mountainous recharge area where pyrite oxidation is the major geochemical process. A high 18O enrichment factor ε[SO4–H2O] and high 34S enrichment factor ε34S[FeS2–SO4] indicate that disproportionation and dissimilatory sulfate reduction are both involved in Type II and Type III groundwater. The process of bacterial sulfate reduction may coprecipitate and sequester As, a mechanism that is unlikely to occur in Type II groundwater. The presence of high As and Fe2+ concentrations and enriched δ34S[SO4] in Type II groundwater suggest that biogeochemical reactions occurred under anaerobic conditions. The reductive dissolution of As-bearing Fe oxyhydroxides together with microbial disproportionation of sulfur explains the substantial correlations among the high As concentration and enriched δ34S[SO4] and Fe2+ concentrations in the iron- and sulfate-reducing zone (Type II). The As concentration in Type III groundwater (sulfate-reducing) is lower than that in Type II groundwater because of bacterial sulfate reduction and coprecipitation with As. Furthermore, the dissolution of sulfate minerals is not the major source of aqueous SO4, based on the sulfur isotopic composition of solid-phase sulfate (δ34S[sulfate mineral]). Finally, the anthropogenic influence of fertilizers with wide S isotopic ranges is proposed as the plausible sulfate budget in Type IV groundwater. The results of this study suggest that the As release into groundwater involves the reductive dissolution of As-bearing Fe-hydroxides, bacterial sulfate reduction, and microbial disproportionation in the Lanyang Plain.

The crystal structure of arangasite, Al2F(PO4)(SO4)·9H2O determined using low-temperature synchrotron data

AbstractThe crystal structure of the fibrous mineral arangasite, Al2F(PO4)(SO4)·9H2O from the Alyaskitovoje deposit, Eastern Yakutiya, Russia, was solved using low-temperature single-crystal data from synchrotron radiation and refined against F2 to R = 9.8%. Arangasite crystallizes in the monoclinic space group P2/a, with unit-cell parameters a = 7.073(1), b = 9.634(2), c = 10.827(2) Å, β = 100.40(1)°, V = 725.7(7) Å3 and Z = 2. The positions of all the independent H atoms were obtained by difference- Fourier techniques and refined in an isotropic approximation. The arangasite crystal structure is built from one-dimensional chains of Al octahedra and PO4 tetrahedra sharing vertices, quasi-isolated SO4 tetrahedra and H2O molecules. All O atoms are involved in the system of H bonding, acting as donors and/or acceptors. Hydrogen bonding serves as the only mechanism providing linkage between the main structural fragments, thus maintaining the framework. Chains of corner-sharing Al octahedra and P tetrahedra in the arangasite structure are topologically identical to the chains built from (Fe, Al) octahedra and P tetrahedra in the crystal structure of destinezite, Fe2(OH)(PO4)(SO4)·6H2O. It has been shown that in spite of very similar chemical formulae, arangasite and sanjuanite, Al2(OH)(PO4)(SO4)·9H2O, are not isotypic.

Experimental method for determining mixed-phase precipitation kinetics from synthetic geothermal brine

A batch reactor design for determining precipitation kinetics from an experimental (i.e. laboratory made) geothermal brine is presented. The purpose of the method is to improve techniques for predicting scaling risks in basin-hosted, enhanced geothermal systems (EGS). The method is applied to determining kinetic rate laws for alkali-earth metal sulfate precipitation in the Na–Ca–Ba–Sr–Cl–SO4–H2O system, which are a set of components commonly found in sedimentary basin brines, worldwide. The method of integral rates was used to determine alkali-earth metal precipitation kinetic rate laws for solutions reacting over 4h at 75°C and 150°C. At 75°C, the rate law for total sulfate precipitation is rate=0.0137*[SO42−]2. At 150°C, the rate law for total sulfate precipitation is rate=0.011*[SO42−]2. All precipitants are found to be barite, with minor amounts of calcium and strontium incorporated into the barite structure. If only barite precipitation is considered, the rate coefficients become 0.045 and 0.025 at 75°C and 150°C, respectively. The precipitants’ specific reactive surface areas are estimated for each time step and temperature and are qualitatively observed to decrease as the reactions progress. The (1) the reaction order, (2) the rate coefficient and (3) the change in specific reactive surface area over time are the three parameters required to develop reactive transport models of mineral precipitation in a geothermal power plant. All three of these parameters can be determined with the method employed in this study. Results from these experiments confirm previous results that precipitation kinetics are dependent on both starting concentrations and background salinities. Experiments designed to predict scaling risks in real geothermal systems require adjustments to the method presented herein, specifically (1) experimental fluids must more closely approximate real geothermal brines, (2) a larger reactor is required to produce a greater precipitant mass and (3) full quantitative assessment of the precipitants’ specific reactive surface area are required to reduce the error of these measurements.

Predicting the solubility of SrSO4 in Na–Ca–Mg–Sr–Cl–SO4–H2O system at elevated temperatures and pressures

Precipitation of strontium sulfate (or SrSO4) has already been distinguished as one of the most costly and critical problems which may occur in process industries and oilfield operations. Costs due to scaling and remedial actions that need to be taken afterward are generally high owing to low solubility of SrSO4 in aqueous solutions. Therefore, a thorough understanding of the SrSO4 thermodynamic behavior under various operating conditions is vital to predict or even avoid the overall damage caused by scaling. The primary aim of this work is to develop a model based on Least Squares Support Vector Machine (LSSVM) and Coupled Simulated Annealing (CSA) referred to as CSA-LSSVM to predict strontium sulfate solubility as a function of pressure, temperature and ionic compositions. In this context, we have employed almost 1641 experimental data regarding strontium sulfate solubility to build a comprehensive model and to evaluate its reliability. The results show that the proposed model has a better performance in comparison with pre-existing empirical correlations used for predicting SrSO4 solubility and it is also in well accordance with experimental measurements. A comparison was also drawn between predictions of Pitzer ion interaction model with CSA-LSSVM model forecasts in terms of sensitivity analysis and single point solubility predictions. The whole dataset was analyzed at the end to diagnose the possible outliers and to investigate the reliability of recorded measurements. Based on the results obtained from this study, developed model could successfully be used in predicting SrSO4 solubility in aqueous Na–Ca–Mg–Sr–Cl–SO4–H2O system over temperature ranges from 2 to 253.5°C, and pressures from 1 to 568.51atm.

Prediction of the aqueous solubility of BaSO4 using pitzer ion interaction model and LSSVM algorithm

Deposition of barium sulfate (or BaSO4) has already been recognized as a devastating problem facing process industries and oilfield operations, mainly owing to its low solubility in aqueous solutions. Predicting and also preventing the overall damage caused by BaSO4 precipitation requires a profound knowledge of its solubility under different thermodynamic conditions. The main aim of this study is to develop a solubility prediction model based on a hybrid of least squares support vector nachines (LSSVM) and coupled simulated annealing (CSA) aiming to predict the solubility of barium sulfate over wide ranges of temperature, pressure and ionic compositions. Results indicate that predictions of the presented model are in well accordance with experimental measurements yielding overall correlation coefficient (R2) of 0.996 and total RMSE of 0.00077. Constructed model was also found to outperform a previously well-known correlation employed for predicting BaSO4 solubility in aqueous solutions. Solubility predictions based on Pitzer ion interaction model were also compared to CSA-LSSVM predictions in terms of single point estimations and sensitivity analysis. Results of this study suggest that CSA-LSSVM model could be implemented in assessing the solubility of BaSO4 in Na–Mg–Ca–K–Ba–Cl–SO4–H2O system over wide ranges of temperature from 0 to 279°C and pressures ranging from 1 to 1517 bars with an acceptable degree of accuracy.

Thermodynamic modelling of high salinary phosphate solutions. I. Binary systems

A comprehensive set of interaction parameters for the thermodynamic modelling of PO43−, HPO42− and H2PO4− in concentrated solutions in the Na–K–Cl–SO4–H2O system at 25°C was calculated by regression of experimental isopiestic and solubility data with the Pitzer formalism. The isopiestic method was used to generate additional sets of activity data, which complete and improve the statistical quality of data extracted from an extensive literature review. Thermodynamic properties of phosphate phases were calculated. In this first part, binary systems are presented.

Activity Coefficients of Fission Products in Highly Salinary Solutions of Na+, K+, Mg2+, Ca2+, Cl–, and SO42–: Cs+

Binary and ternary interaction parameters of the Pitzer formalism for the thermodynamic modeling of the fission product Cs+ in concentrated salt solutions of the hexary oceanic system Na–K–Mg–Ca–Cl–SO4–H2O at 25 °C were calculated from experimental isopiestic, potentiometric, and solubility data available in the literature. The obtained parameters were checked by comparing measured and predicted water activities and solubilities for binary, ternary, and quaternary systems.

Spatial variations in gas and stable isotope compositions of thermal fluids around Lake Van: Implications for crust–mantle dynamics in eastern Turkey

We investigate the helium (3He/4He) and carbon (δ13C) isotope compositions and relative abundance ratios (CO2/3He) of gas samples together with the stable isotope compositions of dissolved carbon and sulfur and the oxygen and hydrogen isotopic compositions of the associated water phase from a number of geothermal fields located around Lake Van in eastern Anatolia, Turkey. The mantle-derived helium component, which is likely transferred to the crust beneath eastern Turkey by recent magmatism, is found to constitute up to 96% (e.g. Nemrut Caldera) of the total He content in fluids. As regards the spatial distribution of He, samples collected from areas of Pliocene–Quaternary volcanics are characterized by a wide and generally higher range of R/RA ratios (0.93 to 7.76 RA) compared to those of non-volcanic regions (0.85 to 1.0 RA). CO2/3He ratios vary over a wide range (2.4×105–3.8×1013) but are mostly higher than that of the nominal upper mantle (~2×109). Oxygen–hydrogen isotope values of the waters are conformable with the Global Meteoric Water Line and indicate a local meteoric origin. Sulfate in waters is most probably derived from dissolution of marine carbonates and terrestrial evaporite units. Temperatures calculated by SO4–H2O isotope geothermometry lie between 40 and 199°C, and are in poor agreement with reservoir temperatures estimated from silica geothermometers. Discordant temperatures may be due to either the relatively slow rate of isotopic equilibrium between water and sulfate or mixing of geothermal water with sulfate-bearing shallow waters which may modify the δ18O value. The δ13C (CO2) values of gas samples are consistently lower than those of their water counterparts, consistent with loss of CO2 from waters by degassing. Mixing between mantle and various crustal C-sources appears to be the main control on the C-isotope composition. The principal origin of CO2 in all samples is crustal lithologies, mainly limestone (~85 to 98% of the total carbon inventory): thus, the crustal carbon flux is at least 10 times that from the mantle. There is a broad correlation between high 3He/4He values and thinner crust in the western part of the Lake Van region, where several historically-active volcanoes are located. This observation indicates that localized volcanic and magmatic activity exerts the primary control on the balance between mantle and crustally-derived volatiles in the region.

Temperature variable chemical model of bromide–sulfate solution interaction parameters and solid–liquid equilibria in the Na–K–Ca–Br–SO4–H2O system

The experimental solubility data of bromide and sulfate minerals available in the literature are used to construct a chemical model that calculates solid–liquid equilibria in mixed systems NaBr–Na2SO4–H2O, KBr–K2SO4–H2O, and CaBr2–CaSO4–H2O from low to high solution concentration within the T=(0–100) °C temperature range. The solubility modeling approach based on fundamental Pitzer specific interaction equations is employed. The resulting model for mixed systems gives a very good agreement with bromide and sulfate salts equilibrium solubility data available in the literature. Temperature extrapolation of the mixed system models provides reasonable mineral solubilities at low (0 °C) and high temperature (up to 100 °C). Limitations of the mixed solutions models due to data insufficiencies at high temperature are discussed. The model for mixed system CaBr2–CaSO4–H2O at T=(0–50) °C was developed using pure electrolyte parameters of Ca–SO4 interactions and without including into a model aqueous species CaSO4 °(aq). It was showed that solution parameters fully account possible association reactions in low calcium sulfate molality regions of binary CaSO4–H2O and mixed solutions. The model presented here expands the previously published temperature dependent sodium–potassium–calcium–sulfate model by evaluating bromide–sulfate mixing solution parameters and inclusion of 5 bromide minerals precipitating within Na–K–Ca–Br–SO4–H2O system. The model predictions on the effect of bromide–sulfate mixing on the equilibrium deliquescence relative humidity (DRH) in NaBr–Na2SO4–H2O and KBr–K2SO4–H2O solutions are also given.

The effect of copper on the precipitation of scorodite (FeAsO4·2H2O) under hydrothermal conditions: Evidence for a hydrated copper containing ferric arsenate sulfate-short lived intermediate

The effect of copper sulfate on scorodite precipitation and its mechanism of formation at 150°C was investigated. Scorodite was determined to be the dominant phase formed under all conditions explored (0.61<Fe(III)/As(V)<1.87, 0.27–0.30M Fe(SO4)1.5, 0–0.3M CuSO4, 0–0.3M MgSO4, at 2.5h and 150°C). The produced scorodite was found to incorporate up to 5% SO4 and ⩽1% Cu or Mg in its structure. The precipitation of scorodite was stoichiometric, i.e. the Fe/As molar ratio in the solids was equal to one independent of the starting Fe/As ratio in the solution. The presence of excess ferric sulfate in the initial solution (Fe/As>1) was found to slow down the ordering of the H-bond structure in scorodite. Precipitation under equimolar concentrations (As=Fe=Cu=0.3M), short times and lower temperatures (30–70min and 90–130°C) revealed the formation of a Cu–Fe–AsO4–SO4–H2O short lived gelatinous intermediate that closely resembled the basic ferric arsenate sulfate (BFAS) type of phase, before ultimately converting fully to the most stable scorodite phase (96min and 138°C). This phase transition has been traced throughout the reaction via elemental (ICP-AES, XPS), structural (PXRD, TEM) and molecular (ATR-IR, Raman) analysis. ATR-IR investigation of an arsenic containing industrial residue produced during pressure leaching of a copper concentrate (1h and 150°C) found evidence of the formation of an arsenate mineral form resembling the intermediate basic ferric arsenate sulfate phase.

Salt Waters of the Northern Apennine Foredeep Basin (Italy): Origin and Evolution

The salt waters from the Emilia-Romagna sector of the Northern Apennine Foredeep have been investigated using major and trace element and stable isotope (δ2H, δ18O, δ37Cl, δ81Br and 87Sr/86Sr ratio). Ca, Mg, Na, K, Sr, Li, B, I, Br and SO4 vs. Cl diagrams suggest the subaerial evaporation of seawater beyond gypsum and before halite precipitation as primary process to explain the brine’s salinity, whereas saline to brackish waters were formed by mixing of evaporated seawater and water of meteoric origin. A diagenetic end-member may be a third component for mud volcanoes and some brackish waters. Salinization by dissolution of (Triassic) evaporites has been detected only in samples from the Tuscan side of the Apennines and/or interacting with the Tuscan Nappe. In comparison with the seawater evaporation path, Ca–Sr enrichment and Na–K–Mg depletion of the foredeep waters reveal the presence of secondary processes such as dolomitization–chloritization, zeolitization–albitization and illitization. Sulfate concentration, formerly buffered by gypsum-anhydrite deposition, is heavily lowered by bacterial and locally by thermochemical reduction during burial diagenesis. From an isotopic point of view, data of the water molecule confirm mixing between seawater and meteoric end-members. Local 18O-shift up to +11‰ at Salsomaggiore is related to water–rock interaction at high temperature (≈150°C) as confirmed by chemical (Mg, Li, Ca distribution) and isotopic (SO4–H2O) geothermometers. 37Cl/35Cl and 81Br/79Br ratios corroborate the marine origin of the brines and evidence the diffusion of halogens from the deepest and most saline aquifers toward the surface. The 87Sr/86Sr ratio suggests a Miocene origin of Sr and rule out the hypothesis of a Triassic provenance of the dissolved components for the analyzed waters issuing from the Emilia-Romagna sector of the foredeep. Waters issuing from the Tuscan side of the Apennines and from the Marche sector of the foredeep show higher 87Sr/86Sr ratios because of the interaction with siliciclastic rocks.

Processes affecting δ 34S and δ 18O values of dissolved sulfate in alluvium along the Canadian River, central Oklahoma, USA

The δ 34S and δ 18O values for dissolved sulfate in groundwater are commonly used in aquifer studies to identify sulfate reservoirs and describe biogeochemical processes. The utility of these data, however, often is compromised by mixing of sulfate sources within reservoirs and isotope fractionation during sulfur redox cycling. Our study shows that, after all potential sulfate sources are identified and isotopically characterized, the δ 34S SO4 and δ 18O SO4 values differentiate processes such as sulfate-source mixing, sulfide oxidation, barite dissolution, and organosulfur decomposition. During bacterial reduction of sulfate, the values reflect kinetic sulfur isotope fractionation and exchange of oxygen isotopes between sulfate and water. Detailed analysis of the chemistry (Cl and SO 4 concentrations) and isotopic composition (δ 2H H2O and δ 18O H2O) of groundwater in an alluvial aquifer in Central Oklahoma, USA allowed the identification of five distinct end members that supply water to the aquifer (regional groundwater flowing into the study area, river water, leachate from a closed landfill that operated within the site, rain, and surface runoff). The δ 34S SO4 and δ 18O SO4 values in each end member differentiated three sources of sulfate: sulfate dissolved from Early to Late Permian rocks within the drainage basin (δ 34S SO4 = 8–12‰ and δ 18O SO4 = 10‰), iron sulfides oxidized by molecular oxygen during low water-table levels (δ 34S SO4 = − 16‰ and δ 18O SO4 = 10‰), and organosulfur compounds (predominately ester sulfates) from decomposition of vegetation on the surface and from landfill trash buried in the alluvium (δ 34S SO4 = 8‰ and δ 18O SO4 = 6‰). During bacterial reduction of these sulfate sources, similar isotope fractionation processes are recorded in the parallel trends of increasing δ 34S SO4 and δ 18O SO4 values. When extensive reduction occurs, the kinetic sulfur isotope fractionation (estimated by ε H2S–SO4 = − 23‰) results in the steady increase of δ 34S SO4 values to greater than 70‰. Equilibrium isotope fractionation during exchange of sulfate oxygen and water oxygen, a process not commonly observed in field-based studies, is documented in δ 18O SO4 values asymptotically approaching 21‰, the value predicted for conditions at the study site ( ε SO4–H2O = 27‰). These results show that recognition of all potential sulfate sources is a critical first step to resolving complexities in δ 34S SO4 and δ 18O SO4 data. The approach taken in this study can be used in other aquifer systems where the identification of multiple sulfate sources and sulfur redox cycling is important to understanding natural processes and anthropogenic influences.

Aqueous geochemistry and oxygen isotope compositions of acid mine drainage from the Río Tinto, SW Spain, highlight inconsistencies in current models

The Río Tinto river in SW Spain is a classic example of acid mine drainage and the focus of an increasing amount of research including environmental geochemistry, extremophile microbiology and Mars-analogue studies. Its 5000-year mining legacy has resulted in a wide range of point inputs including spoil heaps and tunnels draining underground workings. The variety of inputs and importance of the river as a research site make it an ideal location for investigating sulphide oxidation mechanisms at the field scale. Mass balance calculations showed that pyrite oxidation accounts for over 93% of the dissolved sulphate derived from sulphide oxidation in the Río Tinto point inputs. Oxygen isotopes in water and sulphate were analysed from a variety of drainage sources and displayed δ 18O (SO4–H2O) values from 3.9 to 13.6‰, indicating that different oxidation pathways occurred at different sites within the catchment. The most commonly used approach to interpreting field oxygen isotope data applies water and oxygen fractionation factors derived from laboratory experiments. We demonstrate that this approach cannot explain high δ 18O (SO4–H2O) values in a manner that is consistent with recent models of pyrite and sulphoxyanion oxidation. In the Río Tinto, high δ 18O (SO4–H2O) values (11.2–13.6‰) occur in concentrated (Fe = 172–829 mM), low pH (0.88–1.4), ferrous iron (68–91% of total Fe) waters and are most simply explained by a mechanism involving a dissolved sulphite intermediate, sulphite–water oxygen equilibrium exchange and finally sulphite oxidation to sulphate with O 2. In contrast, drainage from large waste blocks of acid volcanic tuff with pyritiferous veins also had low pH (1.7), but had a low δ 18O (SO4–H2O) value of 4.0‰ and high concentrations of ferric iron (Fe(III) = 185 mM, total Fe = 186 mM), suggesting a pathway where ferric iron is the primary oxidant, water is the primary source of oxygen in the sulphate and where sulphate is released directly from the pyrite surface. However, problems remain with the sulphite–water oxygen exchange model and recommendations are therefore made for future experiments to refine our understanding of oxygen isotopes in pyrite oxidation.

Application of selected geothermometers to exploration of low-enthalpy thermal water: the Sudetic Geothermal Region in Poland

The most important intakes of thermal waters within the Sudetic Geothermal Region occur in three separate hydrogeothermal systems: (1) Lądek, (2) Duszniki and (3) Cieplice. All these waters are of meteoric origin and circulate in crystalline rocks to different depths. Their outflow temperatures are between less than 20°C and to about 87°C. To evaluate the geothermal fields in the light of their prospectiveness, to further exploration of thermal energy resources, we took an effort to apply selected isotopic and chemical geothermometers to assess the maximum possible temperatures, which may be found in the reservoirs. The only chemical geothermometers which give a reliable range of reservoir temperatures are SiO2 (chalcedony), Na–Ka–Ca and partly Na–K ones. The oxygen isotopic geothermometer in the SO4–H2O system gives a real range of estimated reservoir temperatures only for deeply circulating waters in the Cieplice area. On the other hand, in the case of CO2 rich waters in the Duszniki area, where outflow temperatures do not exceed 30°C, application of chemical or isotopic temperature indicators always leads to erroneous results due to the lack of equilibrium in the thermodynamic system of water–rock interaction.