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The injection of circulating fluids (usually water) is the predominant means of heat extraction in an enhanced geothermal system (EGS); however, the additional injection of circulating fluids can cause a water-rock reaction with a dry thermal rock mass, resulting in the generation of a large number of secondary minerals. These secondary minerals cause fracture closure in artificial reservoirs and severely impede the sustainable use of these geothermal systems. Therefore, this study combined laboratory tests with a PHREEQC hydrogeochemical simulation. First, the changes and mechanisms of mineral and aqueous chemical fractions in the artificial reservoir fractures were analyzed after using groundwater from the sandstone aquifer at the Frontier Observatory for Geothermal Energy Research (FORGE) site as a heat transfer fluid in the FORGE project. Second, the dissolution patterns and precipitation of minerals commonly found in artificial fractures were simulated in four different environments with the aid of PHREEQC after completing transconversion. Finally, the effects of different fluids were evaluated for the unblocking of typical minerals. The results show that as a heat transfer fluid, groundwater causes the dissolution of K-feldspar, albite, and illite, partial decomposition of kaolinite and proto-silicate, rapid production of anorthite, decomposition of dolomite, sodization of Ca-montmorillonite, and dolomitization of black mica. Mud acid was more effective in unblocking fissure closures caused by anorthite, chlorite, and kaolinite. NaOH was more effective in unblocking fissure closures caused by illite and Ca-montmorillonite. Groundwater is not suitable as a blockage remover and can easily produce secondary mineral blockages (calcite and aragonite). The effect of ultrapure water is relatively stable; it can be used as a heat exchange fluid or blockage remover, but when used as a remover, it behaves primarily as a physical-mechanical action process.

Reactive transport modeling has established itself as a key player to analyze sophisticated hydro-geochemical interactions that occur over spatiotemporal scales on par with subsurface applications. In this paper, we benchmarked a new reactive transport package—porousMedia4Foam at the continuum scale considering complex and well-established cases available in the reactive transport modeling community. porousMedia4Foam was born by the successful coupling of two open-source packages—OpenFOAM® and PHREEQC. The flow governing equations, transport of species and the evolution of porous media properties are handled by OpenFOAM®, whereas the geochemistry is exclusively handled by PHREEQC. We further demonstrated the capability of using porousMedia4Foam to investigate reactive transport processes considering unstructured meshes. As porousMedia4Foam is an open-source package with included benefits to account for reactive transport processes occurring at various scales—pore, hybrid, and Darcy scales, we believe that porousMedia4Foam opens a new dimension to analyze reactive transport physics for various intriguing subsurface applications. [ABSTRACT FROM AUTHOR]

Arsenic contamination of groundwaters in South and Southeast Asia is a major threat to public health. In order to better understand the geochemical controls on the mobility of arsenic in a heavily arsenic-affected aquifer in northern Kandal Province, Cambodia, key changes in inorganic aqueous geochemistry have been monitored at high vertical and lateral resolution along dominant groundwater flow paths along two distinct transects. The two transects are characterized by differing geochemical, hydrological and lithological conditions. Arsenic concentrations in groundwater are highly heterogenous, and are broadly positively associated with iron and negatively associated with sulfate and dissolved oxygen. The observed correlations are generally consistent with arsenic mobilization by reductive-dissolution of iron (hydr)oxides. Key redox zones, as identified using groupings of the PHREEQC model equilibrium electron activity of major redox couples (notably ammonium/nitrite; ammonium/nitrate; nitrite/nitrate; dissolved oxygen/water) have been identified and vary with depth, site and season. Mineral saturation is also characterized. Seasonal changes in groundwater chemistry were observed in areas which were (i) sandy and of high permeability; (ii) in close proximity to rivers; and/or (iii) in close proximity to ponds. Such changes are attributed to monsoonal-driven surface-groundwater interactions and are consistent with the separate provenance of recharge sources as identified using stable isotope mixing models.

Abstract Injectivity decline due to mineral scale deposition in near wellbore region of water injection wells is one of the main challenging issues and have been widely reported in the literature. One of the main mechanisms of injectivity loss is incompatibility between injected and formation waters that may result in inorganic scale precipitation and subsequent deposition in porous media. Reliable reactive flow models to predict type and amount of scale along with permeability decline estimation allow planning and risk management of water flood projects. In this paper, we present a coupled geochemical and hydrodynamic model to simulate the scale precipitation and deposition of mineral scales in multicomponent aqueous systems which is able to predict type of scale and permeability decline in porous medium. Geochemical software was used to simulate reactions between injected and formation brine suspicions to risk of composite scale formation and hydrodynamic flow model was applied to predict subsequent permeability decline during water injection. High quality match of permeability decline in following of robust static modeling of mineral scale formation in multicomponent aqueous systems was achieved. Highlights • A geochemical package was used to identify type and amount of composite scale formation during water injection. • Coupling geochemical and hydrodynamic models allowed prediction of permeability decline. • Laboratory data was matched successfully with proposed model. [ABSTRACT FROM AUTHOR]

Abstract In the Tamagawa geothermal area of Akita Prefecture, northern Japan, Obuki spring discharges a large amount of thermal water (∼9000 L/min), which is chloride-rich and acidic (pH 1.2). We have investigated changes in the physico-chemical nature and fractionation mechanisms of rare earth elements (REEs) including Y and actinides (Th and U) in the Shibukuro and Tama rivers into which acid thermal water from the Obuki spring discharges. The geochemical behavior of these elements is shown to be mainly controlled by pH-dependent sorption onto ferric and/or aluminum oxy-hydroxides (HFO and HAO). HFO is formed around pH > 3. In the upstream region, where pH is less than 4, dissolved Th is removed due to precipitation of Th(SO 4) 2(s). Whereas, under pH condition greater than 4, remaining dissolved Th is nearly completely sorbed onto suspended HFO, where hydroxyl species of Th are thermodynamically significant. Then, the Th(SO 4) 2(s) and HFO-sorbed Th are nearly completely removed in an upstream man-made lake. Dissolved U is also removed mainly by sorption onto HFO, where hydroxyl species of UO 2 are predominant. A considerable portion of U is also trapped in the sediments of this lake. On the other hand, removal of REEs is nearly negligible until the lake. This is the first fractionation among REEs and actinides caused by pH-dependent sorption, and the order of removal from river water is Th > U ≫ REEs. The remaining dissolved U and REEs in the river waters are transported farther downstream. At pH values of >6, suspended HAO is effectively formed, and the second fractionation among REEs and U occurs. During the second fractionation, REEs are removed by HAO, whereas U largely remains as a dissolved species in river water under higher pH conditions. Lighter REEs, such as La and Ce, tend to remain in the river waters compared to the other REEs including Y. The predominant chemical species for UO 2 during the second fractionation are carbonate complex species, and relative proportions of hydroxyl species drastically decreases, resulting in prevention of U sorption onto HAO. [ABSTRACT FROM AUTHOR]

Storage of CO 2 into geological formations, such as deep saline aquifers and depleted oil/gas reservoirs, is being investigated to help reduce releases to the atmosphere. A critical issue is the long-term efficacy of engineered (man-made) hydrated cement seals that surround or plug boreholes. In order to investigate the impact of CO 2 interactions with hydrated cement, the present study focuses on the alteration processes that were observed in a hydrated cement core (based on Type-G Portland cement) after laboratory experimental reaction with a CO 2 -saturated brine for 6 months. The carbonated cement block consisted of an orange outer zone, an alteration rim, and the intact cement core. The PHREEQC geochemical code was used to simulate the experimental conditions and data using 1D reactive transport modeling. The first phases to dissolve were portlandite, CS and ettringite providing calcium to solution. Three CaCO 3 polymorphs were predicted to precipitate, but the less stable of these (aragonite, vaterite) were transformed to calcite over time. According to simulations the final product after 1000 years of reaction will be 68% calcite, 18% Friedel's salt and 14% Al-Fe-Si hydro/oxides. As the reaction front moved within the sample, porosity decreased due to the formation of amorphous phases and carbonates. The proposed model is a useful tool in describing longer-term hydrated cement alteration behavior during CO 2 geological storage and to compare their results with in situ observations from field experiments. [ABSTRACT FROM AUTHOR]

Arsenic contamination of groundwaters in South and Southeast Asia is a major threat to public health. In order to better understand the geochemical controls on the mobility of arsenic in a heavily arsenic-affected aquifer in northern Kandal Province, Cambodia, key changes in inorganic aqueous geochemistry have been monitored at high vertical and lateral resolution along dominant groundwater flow paths along two distinct transects. The two transects are characterized by differing geochemical, hydrological and lithological conditions. Arsenic concentrations in groundwater are highly heterogenous, and are broadly positively associated with iron and negatively associated with sulfate and dissolved oxygen. The observed correlations are generally consistent with arsenic mobilization by reductive-dissolution of iron (hydr)oxides. Key redox zones, as identified using groupings of the PHREEQC model equilibrium electron activity of major redox couples (notably ammonium/nitrite; ammonium/nitrate; nitrite/nitrate; dissolved oxygen/water) have been identified and vary with depth, site and season. Mineral saturation is also characterized. Seasonal changes in groundwater chemistry were observed in areas which were (i) sandy and of high permeability; (ii) in close proximity to rivers; and/or (iii) in close proximity to ponds. Such changes are attributed to monsoonal-driven surface-groundwater interactions and are consistent with the separate provenance of recharge sources as identified using stable isotope mixing models. [ABSTRACT FROM AUTHOR]

Geologic storage of CO 2 is one option for avoiding CO 2 emissions from a large-scale point source such as a thermal power plant and a gas refinery. The alteration of well materials by CO 2 under reservoir conditions requires characterization because the wells are the main possible leakage pathways for CO 2 from a geological reservoir. This paper presents a numerical modeling of interaction experiments involving a composite well sample formed from steel casing surrounded by Portland cement, itself surrounded by sandstone and CO 2 -saturated brine at 10 MPa and 50 °C during a period of up to 8 weeks, as reported by Mito et al. (2015). A reactive-transport model was developed to simulate diffusion of the CO 2 -saturated brine in the well sample and the resulting successive dissolution/precipitation reactions in the sandstone, cement and steel. The observed changes in mineralogy (which primarily consist of dissolution of portlandite and Ca-rich CSH phases and precipitation of calcite, amorphous silica and zeolite) and the associated evolution in brine composition were reproduced by the model. A buffering role of sandstone on the cement degradation was evidenced, thus avoiding the re-dissolution of calcite usually observed in experiments with direct interaction between cement and CO 2 -saturated brine. Interestingly, the model results also noted a possible perturbation in the measured pH and Ca content due to CO 2 outgassing during solution sampling. The Si behavior control linked with the uncertainty in zeolite stability is also discussed. [ABSTRACT FROM AUTHOR]

Cation release rates to catchment runoff from chemical weathering were assessed using an integrated catchment model that included the soil's unsaturated, saturated and riparian zones. In-situ mineral dissolution rates were calculated in these zones as a function of pH, aluminum and dissolved organic carbon (DOC) concentrations along a hillslope in Northern Sweden where soil water was monitored over nine years. Three independent sets of mineral dissolution equations of varying complexity were used: PROFILE , Transition-State Theory (TST), and the Palandri & Kharaka database. Normalization of the rate-coefficients was necessary to compare the equations, as published rate-coefficients gave base cation release rates differing by several orders of magnitude. After normalizing the TST- and Palandri & Kharaka -rate coefficients to match the base cation release rates calculated from the PROFILE -equations, calculated Ca 2 + and Mg 2 + release rates are consistent with mass balance calculations, whereas those of Na + and K + are overestimated. Our calculations further indicate that a significant proportion of base cations are released from the organic soils in the near-stream zone, in part due to its finer texture. Of the three sets of rate equations, the base cation release rates calculated from the normalized TST-equations were more variable than those calculated using the other two sets of equations, both spatially and temporally, due to its higher sensitivity to pH. In contrast, the normalized Palandri & Kharaka -equations were more sensitive to variations in soil temperature. [ABSTRACT FROM AUTHOR]

Arsenic (As) mobilization and contamination of groundwater affects millions of people worldwide. Progress in developing effective in-situ remediation schemes requires the incorporation of data from laboratory experiments and field samples into calibrated geochemical models. In an oxidizing aquifer where leaching of high pH industrial waste from unlined surface impoundments led to mobilization of naturally occurring As up to 2 mg L −1 , sequential extractions of solid phase As as well as, batch sediment microcosm experiments were conducted to understand As partitioning and solid-phase sorptive and buffering capacity. These data were combined with field data to create a series of geochemical models of the system with modeling programs PHREEQC and FITEQL. Different surface complexation modeling approaches, including component additivity (CA), generalized composite (GC), and a hybrid method were developed, compared and fitted to data from batch acidification experiments to simulate potential remediation scenarios. Several parameters strongly influence the concentration of dissolved As including pH, presence of competing ions (particularly phosphate) and the number of available sorption sites on the aquifer solids. Lowering the pH of groundwater to 7 was found to have a variable, but limited impact (<63%) on decreasing the concentration of dissolved As. The models indicate that in addition to lowering pH, decreasing the concentration of dissolved phosphate and/or increasing the number of available sorption sites could significantly decrease the As solubility to levels below 10 μg L −1 . The hybrid and GC modeling results fit the experimental data well (NRMSE<10%) with reasonable effort and can be implemented in further studies for validation. [ABSTRACT FROM AUTHOR]

In order to understand and mitigate the deterioration of water quality in the aquifer system underlying Guadalajara metropolitan area, an investigation was performed developing geochemical evolution models for assessment of groundwater chemical processes. The models helped not only to conceptualize the groundwater geochemistry, but also to evaluate the relative influence of anthropogenic inputs and natural sources of salinity to the groundwater. Mixing processes, ion exchange, water–rock–water interactions and nitrate pollution and denitrification were identified and confirmed using mass-balance models constraint by information on hydrogeology, groundwater chemistry, lithology and stability of geochemical phases. The water–rock interactions in the volcanic setting produced a dominant Na HCO 3 water type, followed by Na Mg Ca HCO 3 and Na Ca HCO 3 . For geochemical evolution modeling, flow sections were selected representing recharge and non-recharge processes and a variety of mixing conditions. Recharge processes are dominated by dissolution of soil CO 2 gas, calcite, gypsum, albite and biotite, and Ca/Na exchange. Non-recharge processes show that the production of carbonic acid and Ca/Na exchange are decreasing, while other minerals such as halite and amorphous SiO 2 are precipitated. The origin of nitrate pollution in groundwater are fertilizers in rural plots and wastewater and waste disposal in the urban area. This investigation may help water authorities to adequately address and manage groundwater contamination. [ABSTRACT FROM AUTHOR]

AOD (Argon Oxygen Decarburization) slag is a by-product of the stainless steel refining process. The leaching toxicity of chromium from AOD slag cannot be ignored in the recycling process of the AOD slag. To assess the leaching toxicity of the AOD slag, batch leaching tests have been performed. PHREEQC simulations combined with FactSage were carried out based on the detailed mineralogical analysis and petrophysical data. Moreover, Pourbaix diagram of the Cr–H 2 O system was protracted by HSC 5.0 software to explore the chromium speciation in leachates. It was found that AOD slag leachate is an alkaline and reductive solution. The Pourbaix diagram of the Cr–H 2 O system indicated that trivalent chromium, such as Cr ( OH ) 4 − , is the major chromium species in the experimental Eh-pH region considered. However, toxic hexavalent chromium was released with maximum concentrations of 30 µg L −1 and 18 µg L −1 at L/S 10 and 100, respectively, during the earlier leaching stage. It concluded that the AOD slag possessed a certain leaching toxicity. After 10 d of leaching, trivalent chromium was the dominant species in the leachates, which corresponded to the results of PHREEQC simulation. Leaching toxicity of AOD slag is based on the chromium speciation and its transformation. Great attention should be focused on such factors as aging, crystal form of chromium-enriched minerals, and electrochemical characteristics of the leachates. [ABSTRACT FROM AUTHOR]

Acid rock drainage (ARD) is a major environmental problem that poses significant environmental risks during and after mining activities. A new methodology for environmental risk assessment based on probability bounds and a geochemical speciation model (PHREEQC) is presented. The methodology provides conservative and non-conservative ways of estimating risk of heavy metals posed to selected endpoints probabilistically, while propagating data and parameter uncertainties throughout the risk assessment steps. The methodology is demonstrated at a minesite located in British Columbia, Canada. The result of the methodology for the case study minesite shows the fate-and-transport of heavy metals is well simulated in the mine environment. In addition, the results of risk characterization for the case study show that there is risk due to transport of heavy metals into the environment. [ABSTRACT FROM AUTHOR]

Groundwater with elevated dissolved-solids concentrations-containing large concentrations of chloride, sodium, sulfate, and calcium-is present in the Mud Lake area of Eastern Idaho. The source of these solutes is unknown; however, an understanding of the geochemical sources and processes controlling their presence in groundwater in the Mud Lake area is needed to better understand the geochemical sources and processes controlling the water quality of groundwater at the Idaho National Laboratory. The geochemical sources and processes controlling the water quality of groundwater in the Mud Lake area were determined by investigating the geology, hydrology, land use, and groundwater geochemistry in the Mud Lake area, proposing sources for solutes, and testing the proposed sources through geochemical modeling with PHREEQC. Modeling indicated that sources of water to the eastern Snake River Plain aquifer were groundwater from the Beaverhead Mountains and the Camas Creek drainage basin; surface water from Medicine Lodge and Camas Creeks, Mud Lake, and irrigation water; and upward flow of geothermal water from beneath the aquifer. Mixing of groundwater with surface water or other groundwater occurred throughout the aquifer. Carbonate reactions, silicate weathering, and dissolution of evaporite minerals and fertilizer explain most of the changes in chemistry in the aquifer. Redox reactions, cation exchange, and evaporation were locally important. The source of large concentrations of chloride, sodium, sulfate, and calcium was evaporite deposits in the unsaturated zone associated with Pleistocene Lake Terreton. Large amounts of chloride, sodium, sulfate, and calcium are added to groundwater from irrigation water infiltrating through lake bed sediments containing evaporite deposits and the resultant dissolution of gypsum, halite, sylvite, and bischofite. [ABSTRACT FROM AUTHOR]

The use of the subsurface for CO storage, geothermal energy generation, and nuclear waste disposal will greatly increase the interaction between clay(stone) and concrete. The development of models describing the mineralogical transformations at this interface is complicated, because contrasting geochemical conditions (Eh, pH, solution composition, etc.) induce steep concentration gradients and a high mineral reactivity. Due to the complexity of the problem, analytical solutions are not available to verify code accuracy, rendering code intercomparisons as the most efficient method for assessing code capabilities and for building confidence in the used model. A benchmark problem was established for tackling this issue. We summarize three scenarios with increasing geochemical complexity in this paper. The processes considered in the simulations are diffusion-controlled transport in saturated media under isothermal conditions, cation exchange reactions, and both local equilibrium and kinetically controlled mineral dissolution-precipitation reactions. No update of the pore diffusion coefficient as a function of porosity changes was considered. Seven international teams participated in this benchmarking exercise. The reactive transport codes used (TOUGHREACT, PHREEQC, with two different ways of handling transport, CRUNCH, HYTEC, ORCHESTRA, MIN3P-THCm) gave very similar patterns in terms of predicted solute concentrations and mineral distributions. Some differences linked to the considered activity models were observed, but they do not bias the general system evolution. The benchmarking exercise thus demonstrates that a reactive transport modelling specification for long-term performance assessment can be consistently addressed by multiple simulators. [ABSTRACT FROM AUTHOR]

Assessment of redox state, pH, environmental isotope ratios (δ 18 O, δ 2 H) coupled with PHREEQC speciation modeling investigations were conducted to understand trace element geochemical controls in basement complex aquifer in Blantyre, Malawi. Groundwater in the area is typical Ca–Mg–Na–HCO 3 type suggesting more of carbonate weathering and significance of carbon dioxide with dissolution of evaporites, silicate weathering and cation exchange being part of the processes contributing to groundwater mineralization. The significance of pH and redox status of groundwater was observed. The groundwater redox state was mostly O 2 -controlled with few exceptions where mixed (oxic–anoxic) O 2 –Mn(IV) and O 2 –Fe(III)/SO 4 controlled redox states were modeled. More so, some of the main trace element species modeled with PHREEQC varied with respect to pH. For instance vanadium(III) and vanadium(IV) decreased with increase in field pH contrasting the trend observed for vanadium(V). The isotopic composition of the sampled groundwater varied between −5.89‰ and −3.32‰ for δ 18 O and −36.98‰ and −20.42‰ for δ 2 H. The δ 2 H/δ 18 O and δ 18 O/Cl − ratios revealed that groundwater is of meteoric origin through vertical recharge and mixing processes. The d -excess value approximated the y -intercept of GMWL of 10 ( d -excess = 9.269, SD = 1.240) implying that influence of secondary evaporative processes on isotopic signature of the study area is minimal. Thus, there is evidence to suggest that groundwater chemistry in the studied aquifer is influenced by inherent processes with contribution from human activities and furthermore, the water originates from rainwater recharge. With such results, more studies are recommended to further constrain the processes involved in mineralization through isotopic fractionation investigations. [ABSTRACT FROM AUTHOR]

River–aquifer interfaces are essential for ecosystem functioning in terms of nutrient exchange and biological habitat, but are greatly threatened world-wide. This study examined geochemical aspects of river–aquifer interaction in one regulated and one unregulated boreal river in Northern Sweden to determine whether the geochemical functioning of the hyporheic zone is affected by hydrological alterations, e.g. regulated river discharge and river–aquifer connectivity. In the unregulated Kalix River, the hyporheic pore water was well-oxygenated with orthogonal fluxes (≈ 0.6–0.7 m d − 1 ) and acted as a sink for Fe, Mn, Al, NH 4 , and Ca, with fractional losses of 95%, 92%, 45%, 31%, and 15%, respectively. A corresponding elevation in the concentrations of these elements in the hyporheic sediment was observed, with higher saturation indices of Fe-, Mn-, and Al-bearing secondary minerals in hyporheic waters. In the regulated Lule River, hydraulic connectivity at the river–aquifer interface was altered by the presence of a clogging layer (0.04 m d − 1 ). In addition, the river discharge oscillated daily, severely reducing exchange flows across the riverbed (< 0.01 m d − 1 ). As a result, the hyporheic pore water was suboxic, with elevated concentrations of filtered Fe and Mn (fractional increases of ≈ 3700% and ≈ 2500%, respectively) and other solutes (NH 4 , Si, S, Ca). A conceptual model revealed functional differences between geochemical features of the hyporheic zone of regulated and unregulated rivers. Overall, the results showed that hyporheic processes are altered along regulated rivers, with resulting impacts on the geochemistry of riverine, riparian and related marine ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the development, verification and application of an efficient interface, denoted as iCP, which couples two standalone simulation programs: the general purpose Finite Element framework COMSOL Multiphysics® and the geochemical simulator PHREEQC. The main goal of the interface is to maximize the synergies between the aforementioned codes, providing a numerical platform that can efficiently simulate a wide number of multiphysics problems coupled with geochemistry. iCP is written in Java and uses the IPhreeqc C++ dynamic library and the COMSOL Java-API. Given the large computational requirements of the aforementioned coupled models, special emphasis has been placed on numerical robustness and efficiency. To this end, the geochemical reactions are solved in parallel by balancing the computational load over multiple threads. First, a benchmark exercise is used to test the reliability of iCP regarding flow and reactive transport. Then, a large scale thermo-hydro-chemical (THC) problem is solved to show the code capabilities. The results of the verification exercise are successfully compared with those obtained using PHREEQC and the application case demonstrates the scalability of a large scale model, at least up to 32 threads. [Copyright &y& Elsevier]

The Vado Carranza dump, located in the Mexicali Valley, Baja California, northwest Mexico, was operated for more than 15 years receiving about 30 tons/day of solid wastes. The disposed wastes were periodically burned in open air. The presence of a shallow aquifer in the area makes the groundwater vulnerable to contamination processes. The purpose of this study was the evaluation of heavy metals content (Cu, Cd, Ni, Pb and Zn) in soil and groundwater in the vicinity of this dump. The results indicate high content of metals in soil, mainly at a superficial level, with the highest concentrations in the areas where burning of wastes occurred. Elevated concentrations of cadmium and copper were detected in groundwater with the highest concentrations occurring in monitoring wells located in the north side of the dump, downward of groundwater flow. Although the high content of metals in soil can be attributed to the burning of waste, other sources of pollution could be the agricultural irrigation in the vicinity of the dump. The program PHREEQC was used to model the geochemical evolution of groundwater. Results suggest that evaporation of the contaminated waters circulating below the landfill is one of the key processes that explain the increased concentration of contaminants in groundwater and its seasonal variations. As groundwater flows away from the dump, evaporation can concentrate the chemicals making the water more toxic. These results are important because they illustrate processes that are likely to occur in landfills located in other desert areas of the world. [ABSTRACT FROM AUTHOR]

Abstract: We present results from forward geochemical modelling, coupled to fluid flow, of CO2 injection in a depleted gas field in the UK sector of the North Sea (a candidate storage site). For the reservoir a long-term porosity decrease after shut-in is predicted. Similarly for the caprock a reduction in porosity is predicted (for the average mineralogical composition), suggesting an increase in containment integrity. We consider the risks posed by local composition variations, in the form of possibly through-going calcite pathways (prone to dissolution). Our analysis shows that 1) the dissolution process is transport limited; 2) in the region above the gas water contact the transport process is diffusive, resulting in negligible calcite dissolution; 3) in the region below the contact it would take more than 200,000 years to dissolve a calcite streak through the primary caprock and the resultant flow path would then facilitate the transfer of brine, not continuous phase CO2. The long timescale emerging from this analysis is not very site specific. [Copyright &y& Elsevier]

Abstract: Waters with low salinity and high sodium adsorption ratios (SARs) present a challenge to irrigation because they degrade soil structure and infiltration capacity. In the Powder River Basin of Wyoming, such low salinity (electrical conductivity, EC 2.1mScm−1) and high-SAR (54) waters are co-produced with coal-bed methane and some are used for subsurface drip irrigation (SDI). The SDI system studied mixes sulfuric acid with irrigation water and applies water year-round via drip tubing buried 92cm deep. After six years of irrigation, SAR values between 0 and 30cm depth (0.5–1.2) are only slightly increased over non-irrigated soils (0.1–0.5). Only 8–15% of added Na has accumulated above the drip tubing. Sodicity has increased in soil surrounding the drip tubing, and geochemical simulations show that two pathways can generate sodic conditions. In soil between 45-cm depth and the drip tubing, Na from the irrigation water accumulates as evapotranspiration concentrates solutes. SAR values >12, measured by 1:1 water–soil extracts, are caused by concentration of solutes by factors up to 13. Low-EC (<0.7mScm−1) is caused by rain and snowmelt flushing the soil and displacing ions in soil solution. Soil below the drip tubing experiences lower solute concentration factors (1–1.65) due to excess irrigation water and also contains relatively abundant native gypsum (2.4±1.7wt.%). Geochemical simulations show gypsum dissolution decreases soil-water SAR to <7 and increases the EC to around 4.1mScm−1, thus limiting negative impacts from sodicity. With sustained irrigation, however, downward flow of excess irrigation water depletes gypsum, increasing soil-water SAR to >14 and decreasing EC in soil water to 3.2mScm−1. Increased sodicity in the subsurface, rather than the surface, indicates that deep SDI can be a viable means of irrigating with sodic waters. [Copyright &y& Elsevier]

Abstract: Hydrochemical, multivariate statistical and inverse hydrogeochemical modeling techniques were used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the El Eulma Mio-Plio-Quaternary aquifer, East Algeria. Cluster analysis based on major ion contents defined three main chemical water types, reflecting different hydrochemical processes. The first, group 1, has low salinity (mean EC=937μS/cm) and abundance orders Ca2+ >Na+ ≈Mg2+ >K+ and . With increased water–rock interaction, waters in groups 2 and 3 become more saline, changing composition towards Cl–HCO3–Ca and Cl–Ca–Na types. The PHREEQC geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals, quartz, kaolinite and Ca-smectite; (3) ion exchange. [Copyright &y& Elsevier]

One of the uncertainties in the field of carbon dioxide capture and storage (CCS) is caused by the parameterization of geochemical models. The application of geochemical models contributes significantly to calculate the fate of the CO after its injection. The choice of the thermodynamic database used, the selection of the secondary mineral assemblage as well as the option to calculate pressure dependent equilibrium constants influence the CO trapping potential and trapping mechanism. Scenario analyses were conducted applying a geochemical batch equilibrium model for a virtual CO injection into a saline Keuper aquifer. The amount of CO which could be trapped in the formation water and in the form of carbonates was calculated using the model code PHREEQC. Thereby, four thermodynamic datasets were used to calculate the thermodynamic equilibria. Furthermore, the equilibrium constants were re-calculated with the code SUPCRT92, which also applied a pressure correction to the equilibrium constants. Varying the thermodynamic database caused a range of 61% in the amount of trapped CO calculated. Simultaneously, the assemblage of secondary minerals was varied, and the potential secondary minerals dawsonite and K-mica were included in several scenarios. The selection of the secondary mineral assemblage caused a range of 74% in the calculated amount of trapped CO. Correcting the equilibrium constants with respect to a pressure of 125 bars had an influence of 11% on the amount of trapped CO. This illustrates the need for incorporating sensitivity analyses into reaction pathway modeling. [ABSTRACT FROM AUTHOR]

Hydrochemical, multivariate statistical, and inverse geochemical modeling techniques were used to investigate the hydrochemical evolution within the Ain Azel aquifer, Algeria. Cluster analysis based on major ion contents defined 3 main chemical water types, reflecting different hydrochemical processes. The first group water, group 1, has low salinity (mean EC = 735 μS/cm). The second group waters are classified as Cl-HCO-alkaline earth type. The third group is made up of water samples, the cation composition of which is dominated by Ca and Mg with anion composition varying from dominantly Cl to dominantly HCO plus SO. The varifactors obtained from R-mode FA indicate that the parameters responsible for groundwater quality variations are mainly related to the presence and dissolution of some carbonate, silicate, and evaporite minerals in the aquifer. Inverse geochemical modeling along groundwater flow paths indicates the dominant processes are the consumption of CO, the dissolution of dolomite, gypsum, and halite, along with the precipitation of calcite, Ca-montmorillonite, illite, kaolinite, and quartz. [ABSTRACT FROM AUTHOR]

Abstract: We present a software tool for simulations of flow and multi-component solute transport in two- and three-dimensional domains in combination with comprehensive intra-phase and inter-phase geochemistry. The software uses IPhreeqc as a reaction engine to the multi-purpose, multidimensional finite element solver COMSOL Multiphysics® for flow and transport simulations. Here we used COMSOL to solve Richards’ equation for aqueous phase flow in variably saturated porous media. The coupling procedure presented is in principle applicable to any simulation of aqueous phase flow and solute transport in COMSOL. The coupling with IPhreeqc gives major advantages over COMSOL’s built-in reaction capabilities, i.e., the soil solution is speciated from its element composition according to thermodynamic mass action equations with ion activity corrections. State-of-the-art adsorption models such as surface complexation with diffuse double layer calculations are accessible. In addition, IPhreeqc provides a framework to integrate user-defined kinetic reactions with possible dependencies on solution speciation (i.e., pH, saturation indices, and ion activities), allowing for modelling of microbially mediated reactions. Extensive compilations of geochemical reactions and their parameterization are accessible through associated databases. [ABSTRACT FROM AUTHOR]

There are several Li-mica and spodumene deposits in Central Europe. The Cínovec deposit on the border of the Czech Republic and Germany is the largest known hard-rock Li deposit in Europe and is currently being explored. Weathering and mining of such deposits could release Li and F to the environment. Both elements are associated with potential health effects when ingested via drinking water, yet little is known about the aqueous geochemistry of streams and creeks around Li hard-rock deposits. In this study, we sampled surface waters (n = 47) near three Li-mica and one spodumene deposit to examine a potential public health risk resulting from the weathering of Li-minerals. At the Cínovec and Homolka (also in the Czech Republic) sites, several water samples have elevated Li contents relative to those in typical surface waters, but they are all below the U.S. EPA-recommended threshold of 0.7 mg/L. Three of the ten samples from Cínovec contain F in concentrations that are above World Health Organization's 1.5 mg/L drinking water limit, and thus these waters (max. concentration: 3.8 mg/L) may represent a public health hazard. Based on geochemical modeling and statistical analysis, the lithium in the water comes from Li-micas and the fluoride is derived from a combination of Li-micas and fluorite, depending on the specific site analyzed. Lithium seems to be scavenged by clay minerals. • Weathering of hard rock lithium rocks could release Li and F into the environment. • Lithium comes from Li-micas. • Li is likely scrubbed by secondary minerals in the streams. • Fluoride comes from a mixture of Li-micas and accessory fluorite. [ABSTRACT FROM AUTHOR]

PHREEQC is a geochemical computer code developed by the United States Geological Survey and used to model mineral solubilities, aqueous solutions, and water-rock interactions over a large range of conditions. This easy-to-use and open-source program is widely used in the fields of geochemistry, environmental science and engineering, petroleum industry, mining and chemical engineering and biogeochemistry. However, the quality of the modeling depends on the experience of the modeler, and the quality of thermodynamic data available for the code, which is addressed in this review. The authors of the PHREEQC program clearly stated that the choice of thermodynamic data files (TDFs) is the responsibility of the user. Thirteen, mostly "third party", TDFs are distributed along with the software release without detailed documentation or critical evaluation. The quality of the TDFs varies with a wide spectrum of the internal consistency of the thermodynamic data, comprehensiveness of the chemical components and species, the temperature-pressure coverage of log K values, activity coefficient models suitable for ionic strength, and traceability of the sources of data. In this review, we provide some basic documentation, evaluate and compare these TDFs, and attempt to outline the limits of their applicability. We also used the TDFs to model examples of river water, seawater, groundwater, oil-field brine, and selected mineral solubility data for some oxides, oxy-hydroxides, hydroxides, aluminosilicates, carbonates, and sulfates. These comparisons clearly demonstrate that, with the same PHREEQC computer code but a different TDF, significantly different modeling results can be produced. It is imperative that users specify which TDF they used when referring to their modeling results. [ABSTRACT FROM AUTHOR]

Abstract: The Illinois State Geologic Survey is conducting several ongoing CO2 sequestration projects that require geochemical models to gain an understanding of the processes occurring in the subsurface. The ISGS has collected brine and freshwater samples associated with an enhanced oil recovery project in the Loudon oil field. Geochemical modeling allows us to understand reactions with carbonate and silicate minerals in the reservoir, and the effects they have had on brine composition. For the Illinois Basin Decatur project, geochemical models should allow predictions of the reactions that will take place before CO2 injection begins. [Copyright &y& Elsevier]

The article reports on the hydrochemical appraisal of ice- and rock-glacier meltwater in the hyperarid Agua Negra drainage basin, Andes of Argentina. It is said that the Agua Negra drainage system holds several ice- and rock-glaciers, which are distributed from 4200 up to 6300 m a.s.l. It is also said that the geochemical study of meltwaters reveals that ice-glaciers deliver a HCO3- -Ca2+ solution. The site is said to be presumably strongly influenced by sublimation and dry deposition.

Abstract: Spatial and temporal variations in groundwater chemistry indicate that the use of low TDS lake water for irrigation, on land located just south of the City of Dixon, Solano County, California, is primarily responsible for improving groundwater quality with regards to salts. The stable isotopes of water further support this finding and suggest that TDS concentrations decrease as groundwater evolves to a more highly evaporated state. This seemingly contradictory finding was primarily attributed to infiltration of low TDS Lake Berryessa surface water, which has an isotopic signature indicative of an evaporated source and is used extensively for irrigation in the area, mixing with poorer quality locally recharged shallow groundwater. Geochemical modeling using the program PHREEQC further supports the anthropogenic aquifer freshening hypotheses through computed reductions in the saturation state of carbonate minerals in the vicinity of land irrigated by lake derived water, which is undersaturated with regards to modeled carbonates. Additionally, δ18O and δ2H were found to be useful in estimating climatic variables such as temperature and humidity, illustrating the potential for applying these models in hydrologic investigations within the area. It was however found that USDA NRCS soils data and measured water chemistry were not well correlated and thus the use of soils classifications to assess potential groundwater quality impacts was of limited utility. [Copyright &y& Elsevier]

Thermal water of Thermopylae and from other geothermal fields located in the southern part of the Sperchios basin (central Greece) are characterized by high salinity (total dissolved salts, or TDS, range from 1.2 to 30.3 g L−1) associated with a degassing of CO2. To determine the mineralization processes, geochemical and isotopic investigations (major elements, 18O, 2H and 13C) have been carried out upon 17 thermal waters from springs and boreholes. This study emphasizes that all the thermal waters result from the mixing of a seawater end-member, several fresh water components depending on the field location, and a mantle-derived CO2 rising upward through an E–W fault system. The seawater identified in the thermal mixture is likely to be evolved Aegean seawater (ASW). Once intruded into the basin sediments, the trapped seawater has its chemical content modified by both water–rock interactions and massive dissolution of the deep CO2 (pCO2 of 100.5 atm). The modelling performed with PHREEQC indicates that the anomalous major ion ratios measured in the so-called evolved ASW are explained by the dissolution of calcite and dolomitization process associated to precipitation of gypsum within the thermal aquifer. [ABSTRACT FROM AUTHOR]

Abstract: Mine managers and regulators use geochemical predictions of pit lake chemistry to assess whether open pit mines will have positive or negative environmental effects after closure. Sensitivity analyses are a useful approach to explore uncertainties in these predictions. This study investigates the significance of subaqueous water–rock reactions and surface adsorption reactions on a geochemical model of a proposed pit lake at the Martha Au Mine, New Zealand. Our hypothesis was that subaqueous pyrite oxidation would lower surface water pH over time because pyrite will be present in over 1/3 of the submerged wall rock area, and that surface adsorption reactions would lower trace metal concentrations. An initial geochemistry prediction was created in PHREEQC based on site hydrology, representative input water chemistry, physical limnology, and the precipitation of ferrihydrite, manganite, amorphous gibbsite, and barite. Modeling water–rock reactions required the surface area of the submerged wall rocks, the concentration (volume %) of the dominant minerals found in the wall rock (i.e. pyrite, adularia, albite, chlorite, illite, and kaolinite), reaction rates for each mineral, and the volumes of circulating lake layers. These variables determined the mass of each mineral that would react with lake water over a one-year period corresponding to annual turnover. To accommodate for uncertainty in the estimation of surface area, the surface area was increased by ×10, ×100, and ×1000 in three additional models. To model surface adsorption reactions, the mass of ferrihydrite precipitated annually was set equal to the mass of hydrous ferric oxide allowed to adsorb trace metals. Results of the baseline geochemical prediction without water–rock reactions showed the surface water pH dropped from 6.5 to 5.0 over 50 years. The sensitivity analysis on water–rock reactions produced nearly identical results to the initial model for surface areas of ×1, ×10, and ×100. Only the surface area ×1000 model lowered pH more than half a pH unit from the initial prediction. These data suggest that water–rock reactions will not have a significant effect on the pH of the proposed Martha lake owing to the surface area of submerged minerals. Surface adsorption reactions lowered the concentrations of some trace metals (As, Cu, and Pb), whereas other trace metals were unaffected (Cd, Ni, Zn), which emphasized the pH dependence of these reactions. In the absence of model calibration or observed water chemistry, sensitivity analyses are a useful tool to explore uncertainties in pit lake predictions. [Copyright &y& Elsevier]

The article presents abstracts of several studies on environmental geochemistry. Some of the abstracts presented include "Biogeochemical Cycling of Nutrients in Karstic Catchments, Southwestern China: Linkages to Changes of Eco-Environments," by Congqiang Liu and "Emerging Organic Contamination in China," by Guibin Jiang and colleagues.

porousMedia4Foam is a package for solving flow and transport in porous media using OpenFOAM® - a popular open-source numerical toolbox. We introduce and highlight the features of a new generation open-source hydro-geochemical module implemented within porousMedia4Foam , which relies on micro-continuum concept and which makes it possible to investigate hydro-geochemical processes occurring at multiple scales i.e. at the pore-scale, reservoir-scale and at the hybrid-scale. Geochemistry is handled by a third party package (e.g. PHREEQC) that is coupled to the flow and transport solver of OpenFOAM®. We conducted benchmarks across different scales to validate the accuracy of our simulator. We further looked at the evolution of mineral dissolution/precipitation in a fractured porous system. Application fields of this new package include the investigation of hydro-bio-geochemical processes in the critical zone, the modelling of contaminant transport in aquifers, as well as and the assessment of confinement performance for geological barriers. • We developped an open-source platform for multi-scale simulations of hydro-geochemical processes. • A unique formulation can handle pore-scale, hybrid-scale and continuum-scale simulations for hydro-geochemical processes. • The framework is verified using reference solutions both at the pore-scale and at continuum-scale using PHREEQC. • We investigate reactive transport in a fractured porous medium at the hybrid-scale. [ABSTRACT FROM AUTHOR]

Mixing incompatible brines is one of the leading causes of mineral precipitation in porous media. Deposition of the precipitated minerals in pore spaces leads to the porosity and permeability reduction. This study, in the first step, proposes a novel coupled approach for modeling mineral precipitation and deposition in porous media based on geochemistry and deep bed filtration theory. In the proposed approach, minerals precipitate as suspended particles modeled by PHREEQC geochemical package, and deposition of the precipitated minerals is modeled through the deep bed filtration theory. During the second step, results of the proposed approach are verified using the reported experimental data, which show the proposed approach is applicable and accurate with a high degree of confidence. In the third step, the impact of mineral scaling on deposition pattern and permeability decline is studied with respect to the porosity heterogeneity, the existence of fracture, and heterogeneity in the matching parameters of the model. The results of this study show that the porosity heterogeneity influences not only the deposition pattern of the precipitated minerals but also the amount of permeability decline. This paper demonstrates that deep invasion of the precipitated minerals through the porosity heterogeneity and fracture intensifies the permeability decline. It is also observed that heterogeneities in the modeling parameters (formation damage and filtration coefficients) do not affect the permeability decline of the porous media. [Display omitted] • Geochemical modeling of minerals precipitation. • Analyzing the impact of minerals deposition on permeability decline of porous media. • Increasing the porosity heterogeneity leads to a more intensive permeability decline. • Invasion of the reactive species through fracture intensifies the permeability decline. [ABSTRACT FROM AUTHOR]