Acidic Brine Research Articles

Reactive Flow Channelization in Fractured Cement-implications for Wellbore Integrity

Successful long-term geological storage of carbon dioxide (CO2) to reduce increasing atmospheric levels is dependent on seal integrity. If wellbores that penetrate a sealing caprock have permeable fractures in or along a cemented annulus, the fractures provide a potentially fast path for leakage. Because the leaking fluids are reactive toward cement, understanding how reaction, transport, and flow are coupled along a cement fracture is critical to predicting leakage evolution over time. Core flow experiments conducted in our laboratory show channelized flow of acidic brine in fractured cement cores. By staining the initial flow path of an unreactive fluid along the core and comparing it with the final flow path after the acid is introduced we can show that reactive flow is localized to a fraction (16-40%) of the original flow path. This provides experimental evidence that reactive feedbacks can significantly alter the flow along the fracture and needs to be better understood to ensure safety of long-term geological CO2 storage.

Cement CO2-alteration propagation at the well–caprock–reservoir interface and influence of diffusion

A geochemical model of wellbore cement–caprock–reservoir interfaces is developed. The model predicts the mechanisms of chemical alteration of cement due to interaction with the acid brine obtained after the injection of supercritical CO2 into the reservoir rock. Numerical outputs indicate that, at the well–reservoir interface, intrusion into the cement phase of acid brine accompanied by aqueous CO2 quasi-instantaneously transforms Portlandite and CSH into Calcite and hydrated Si-gel, leading to a slight but moderate porosity decrease, from 28% to ∼20%. Conversely, at the well–caprock interface, Portlandite first leaches and the Ca2+ ions released from it migrate towards the caprock, where they eventually combine with slowly diffusing aqueous CO2 to form Calcite, up to the porosity collapse of the interface. This can significantly affect the sealing behaviour of the geological storage. We can finally deduce a time-space characterisation of the system enhancing the main chemical mechanisms in view of a future export to a geomechanical model.

Arhodomonas recens sp. nov., a halophilic alkane-utilizing hydrogen-oxidizing bacterium from the brines of flotation enrichment of potassium minerals

A halophilic nonpigmented rod-shaped (0.8–1.0 × 2.0–2.5 μm), gram-negative bacterium with a single polar flagellum (strain RS91) was isolated from acidic brines of flotation enrichment of potassium minerals (Silvinit Co., Solikamsk, Russia). The strain grew in the media with 2 to 25% NaCl (optimum at 10–12%), 20–45°C (optimum at 37°C), and pH 5.5–8.5 (optimum 6.5–7.5). It was an aerobe or facultative anaerobe incapable of fermentation. The strain was characterized by the absence of growth on glucose, fructose, and citrate, extensive aerobic growth on n-hexadecane and in the mineral medium with H2 + O2 + CO2 in the gas phase, anaerobic nitrate reduction with acetate or hydrogen (under H2 + CO2 + N2), and variable fatty acid composition. The DNA G+C content was 68.2 mol %. Phylogenetic analysis based on 16S rRNA gene sequencing revealed that while strain RS91 was most closely related to Arhodomonas aquaeolei HA-1T (98.3%) and Nitrococcus mobilis (98.1%), it was only remotely related to the halophilic phototroph Halorhodospira halophila (90.6%). Based on the combination of its phenotypic and genotypic characteristics, the organism was classified as a new species of the genus Arhodomonas, family Ectothiorhodospiraceae with the proposed name Arhodomonas recens sp. nov. The type strain is RS91T (= IEGM 796T = VKPM B-11280T).

Hydrogen Permeation and Corrosion Fatigue Crack Growth Rates of X65 Pipeline Steel Exposed to Acid Brines Containing Thiosulfate or Hydrogen Sulfide

Corrosion fatigue crack growth rates were obtained for X65 pipeline steel in acid brines containing thiosulfate (S2O32−) or hydrogen sulfide (H2S). Samples were exposed for 72 h at the open-circuit potential to allow bulk hydrogen charging. The corrosion fatigue crack growth rate increased with partial pressure of H2S and correlated with the steady-state flux of hydrogen permeation during corrosion in the same solutions. The rate of hydrogen absorption increased with increasing S2O32− concentration to a maximum at 10−3 M S2O32−, owing to a competition between increased surface concentration of H2S from S2O32− reduction and increased rate of iron sulfide film formation. Corrosion fatigue behavior in S2O32−-containing acidified brines is the same as in solutions with low partial pressures of H2S, in accordance with predictions from the hydrogen permeation results. This suggests that, for corrosion fatigue studies, S2O32− solutions are possible candidates for replacement of H2S gas, as long as the H2S partial pressure of interest is low. Also, the expected concentration of hydrogen inside the metal specimen at the end of the exposure period correlates with the rate of corrosion fatigue crack growth, suggesting that hydrogen charged during the pre-charging period controls corrosion fatigue crack growth.

Reaction Paths of Thiosulfate during Corrosion of Carbon Steel in Acidified Brines

Corrosion tests with gaseous H2S require special facilities with safety features, because H2S is a toxic and flammable gas. The possibility of replacing H2S with thiosulfate (S2O32−), a non-toxic anion, for studying stress corrosion cracking of stainless and carbon steels in H2S solutions was first proposed by Tsujikawa in 1993. H2S production was detected in presence of carbon steel corroding in acidified thiosulfate-containing solutions. In this paper, the kinetics of H2S evolution are used to estimate the range of partial pressure of H2S that can be simulated with thiosulfate solutions. It was determined that acid brines containing 10−4 M and 10−3 M S2O32− could be used for replacing continuous bubbling of dilute H2S/N2 mixtures in tests of degradation of carbon steels, with H2S partial pressures ranging between 0.03 and 0.56 kPa. The kinetics of H2S production were compared with the amount of sulfur in side reactions, like formation of iron sulfide films and elemental sulfur.

Giant uranium deposits formed from exceptionally uranium-rich acidic brines

The Athabasca Basin, Canada, is home to some of the world’s largest uranium deposits. Analysis of preserved ore-forming fluids and experimental measurement of uranium solubility in analogous solutions show that the giant deposits could have formed relatively rapidly from extremely uranium-rich brines under acidic conditions.

Comparisons of the four Miller Range nakhlites, MIL 03346, 090030, 090032 and 090136: Textural and compositional observations of primary and secondary mineral assemblages

Abstract– Petrological and geochemical analyses of Miller Range (MIL) 03346 indicate that this meteorite originated from the same augitic cumulate layer(s) as the nakhlite Martian meteorites, but underwent rapid cooling prior to complete crystallization. As with the other nakhlites, MIL 03346 contains a secondary alteration assemblage, in this case consisting of iddingsite‐like alteration veins in olivine phenocrysts, Fe‐oxide alteration veins associated with the mesostasis, and Ca‐ and K,Fe‐sulfate veins. We compared the textural and mineralogical compositions of MIL 090030, 090032, and 090136 with MIL 03346, focusing on the composition and Raman spectra of the alteration assemblages. These observations indicate that the meteorites are paired, and that the preterrestrial olivine‐bound alteration assemblages were produced by weakly acidic brine. Although these alteration assemblages resemble similar assemblages in Nakhla, the absence of siderite and halite in the Miller Range nakhlites indicates that the parental alteration brine was comparatively HCO3− depleted, and less concentrated, than that which altered Nakhla. This indicates that the Miller Range nakhlite alteration brine experienced a separate evolutionary pathway to that which altered Nakhla, and therefore represents a separate branch of the Lafayette‐Nakhla evaporation sequence. Thin‐sections cut from the internal portions of these meteorites (away from any fusion crust or terrestrially exposed edge), contain little Ca‐sulfate (identified as gypsum), and no jarosite, whereas thin‐sections with terrestrially exposed edges have much higher sulfate abundances. These observations suggest that at least the majority of sulfate within the Miller Range nakhlites is terrestrially derived.

The Effect of CO2-Saturated Brine on the Conductivity of Wellbore-Cement Fractures

Summary The efficiency of carbon-capture and -storage (CCS) projects is directly related to the long-term sealing ability of cemented sections in wellbores penetrating CO2-storage reservoirs. The microfractures inside the wellbore cement and/or microannuli are possible pathways for CO2 leakage to the surface and/or freshwater-aquifer leakage and could jeopardize safe and long-term containment of CO2 in the subsurface. This paper presents an experimental study that investigates the changes inside the cement internal structure when exposed to acidic brine through an artificial fracture. A 30-day-long flow-through experiment was conducted using a 1 × 12-in. cement core and CO2-saturated brine as a permeant at a flow rate of 2 mL/min in a coreflooding apparatus with 10 and 600 psi of injection and net overburden pressure, respectively [low-pressure (LP) experiment]. The same experiment was repeated with the same flow rate but with 1,800 and 600 psi of injection and net overburden pressures, respectively, for 10 days in order to account for the effects of pressure on the degradation process of cement [high-pressure (HP) experiment]. High-resolution X-ray computed tomography (CT) was used to image several subvolumes extracted from the flow-through cores. The images were processed and thresholded, followed by calculation of porosity. Total porosity was observed to decrease from 26 to 22% after 30 days of exposure in the LP experiment. The HP experiment did not cause any significant change in total porosity, possibly because of the short duration of the experiment.

Chloride and Boron behavior in fluids of Los Humeros geothermal field (Mexico): A model based on the existence of deep acid brine

Geothermal field Los Humeros, Mexico, is characterized by a high steam fraction in the well fluids, by extremely high B concentrations in separated water (grams per liter, with a magmatic B signature, δ11B ± 2σ = −0.8 ± 1.6‰), by the absence of correlation between B and Cl concentrations and by positive correlation between B content in separated water and fluid enthalpy. Such behavior is consistent with the existence of moderately acid brine (pH 3–5) at depth with a high B concentration (500–600 mg/kg). In this case a 3-level model can be suggested for Los Humeros: (1) immature, moderately acid brine at depth which is boiling at a temperature ∼350 °C producing the HCl-bearing vapor with a high B content; (2) partial condensation of this vapor at the upper level accompanying water–rock interaction and neutralization; (3) formation of a shallow water-dominated aquifer above a lithologic low-permeability boundary as has been proposed by other authors. A thermochemical computer code has been used to model boiling of an aqueous fluid at 350 °C with 0.1 M of NaCl, 0.05 M of H3BO3 and variable amount of HCl (0.001–0.1 M), then a partial condensation of the produced vapor at 250 °C and then separation of the steam–water mixture at 150 °C. Results of simulation are in a qualitative agreement with the observed data.

Experimental study on a single cement-fracture using CO2 rich brine

The efficiency of Carbon Capture and Storage (CCS) projects is directly related to the long term sealing efficiency of barrier systems and of wellbore cement in wellbores penetrating storage reservoirs. The microfractures inside the wellbore cement provide possible pathways for CO{sub 2} leakage to the surface and/or fresh water aquifers, impairing the long-term containment of CO{sub 2} in the subsurface. The purpose of this experimental study is to understand the dynamic alteration process in the cement caused by the acidic brine. The first experiment, at ambient temperature and pressure, was conducted by flowing CO{sub 2}-rich brine through 1 in. by 2 in. (25.4 mm by 50.8 mm) cement cores for 4 and 8 weeks durations. The second experiment was a 4 weeks long flow-through experiment conducted at ambient conditions using a 1 in. by 12 in.(25.4 mm by 304.8 mm) cement core and CO{sub 2}-rich brine with a core flooding system under 600 psi (4.13 MPa) confining stress. Post-experiment material analysis from both experiments confirmed leaching of Ca{sup 2+} from reacted cement, as reported in literature. However for the first time, porosity of the reacted regions was semi-quantified applying micro-CT images.

Coupled reactive flow and transport modeling of CO 2 sequestration in the Mt. Simon sandstone formation, Midwest U.S.A.

Multi-phase reactive flow and transport modeling is an effective tool for monitoring, verification, and accounting of CO 2 sequestration in deep geological formations. In the current study, modeling is performed to simulate large scale CO 2 injection (a million tons per year for 100 years) into Mt. Simon sandstone, a major candidate saline reservoir in the Midwest of USA. The long term fate of CO 2 was simulated by extending the modeling period to 10,000 years. The results indicate that most of the injected CO 2 remains within a radius of 3300 m lateral distribution. Four major trapping mechanisms and their spatial and temporal variations are evaluated in our simulations: hydrodynamic, solubility, residual, and mineral trapping. A strongly acidified zone (pH 3–5) forms in the areas affected by the injected CO 2 (0–3300 m), and consequently causes extensive mineral precipitation and dissolution. The predicted long-term fate of CO 2 is closely linked to the geochemical reactions conceptualized in the models. In our model, the replenishing upstream water continues to dissolve CO 2 long after the injection, which results in total dissolution of hydrodynamically trapped CO 2 at the end of 10,000 years. In contrast, most previous models neglected the regional flow after injection and hence artificially limited the extent of geochemical reactions as if in a batch system. Consequently, a supercritical CO 2 plume (hydrodynamic trapping) would persist after 10,000 years. The continued supply of acidified water from interaction between replenishing water and CO 2 also results in extensive dissolution of feldspars and precipitation of secondary clay minerals, to a much more extent than what predicted in models without including regional flow. However, the prediction of complete dissolution of feldspars in 10,000 years can also result from the artifact that the linear rate laws are used in our model (as well as all previous work), which overestimates the rates of feldspar dissolution near equilibrium. Nevertheless, our simulations indicate the prolonged existence of an acidic brine plume, which suggests long-term risk assessment should transfer from the primary risk of CO 2 leakage to secondary risk of acidic plume leakage after all CO 2 is dissolved.

Navajo Sandstone–brine–CO2 interaction: implications for geological carbon sequestration

The injection of CO2 into deep saline aquifers is being considered as an option for greenhouse gas mitigation. However, the response of an aquifer to the injected CO2 is largely unknown. Experiments involving the reaction of Navajo Sandstone with acidic brine were conducted at 200°C and 25 or 30 MPa to evaluate the extent of fluid–rock interactions. The first experiment examined sandstone interaction with CO2-impregnated brine; the second experiment examined sandstone dissolution in CO2-free acidic brine; the third one is carried out in a mixed-flow reactor and designed to measure sandstone dissolution rates based on time-series Si concentrations. The solution chemistry data indicate that the SiO2(aq) increases gradually and pH increases slowly with reaction progress. Silicate minerals in the sandstone display textures (dissolution features, secondary mineralization), indicating that these phases are reacting strongly with the fluid. Dissolution of feldspars and conversion of smectite to illite are likely to be the two reactions that contribute to the release of SiO2(aq). The product minerals present at the end of the experiments are illite, illite/smectite, allophane, and carbonate minerals (for the CO2-charged system). Dissolved CO2 is likely to acidify the brine and to provide a source of carbon for the precipitation of carbonate minerals. Mineral trapping through the precipitation of carbonate minerals is favored thermodynamically and was observed in the experiments. The chemical reactions likely increase the bulk porosity of the sandstone due to dissolution of silicate minerals. However, allophane and illite/smectite fill voids in sandstone grains. There is no evidence for the removal of clay coatings due to chemical reactions. It is uncertain whether the mechanical forces near an injection well would mobilize the smectite and allophane and clog pore throats. Trace amounts of metals, including Cu, Zn, and Ba, were mobilized.

Contemporaneous deposition of phyllosilicates and sulfates: Using Australian acidic saline lake deposits to describe geochemical variability on Mars

Studies of the origin of the Martian sulfate and phyllosilicate deposits have led to the hypothesis that there was a marked, global‐scale change in the Mars environment from circum‐neutral pH aqueous alteration in the Noachian to an acidic evaporitic system in the late Noachian to Hesperian. However, terrestrial studies suggest that two different geochemical systems need not be invoked to explain such geochemical variation. Western Australian acidic playa lakes have large pH differences separated vertically and laterally by only a few tens of meters, demonstrating how highly variable chemistries can coexist over short distances in natural environments. We suggest diverse and variable Martian aqueous environments where the coetaneous formation of phyllosilicates and sulfates at the Australian sites are analogs for regions where phyllosilicates and sulfates coexist on Mars. In these systems, Fe and alkali earth phyllosilicates represent deep facies associated with upwelling neutral to alkaline groundwater, whereas aluminous phyllosilicates and sulfates represent near‐surface evaporitic facies formed from more acidic brines.

Application of Cationic Surfactant-Based Fluids for Acid Diversion

Summary This paper examines the use of surfactant gels during matrix acid treatments and describes field trials of these fluids. Unlike available viscoelastic surfactants used today in the field, this surfactant is cationic. If used in live acids, the fluid has a relatively low viscosity when pumped. Once the acid is spent, however, the surfactant molecules increase its viscosity significantly. To enhance diversion further, the acidic fluids or brines can be foamed with this surfactant. Rheological measurements were conducted on Hastelloy®- fitted rotational viscometers at temperatures ranging from 70 to 300°F. The effects of surfactant concentration, shear rate, temperature, and acid additives on the apparent viscosity of various surfactant-based fluids were investigated in detail. The surfactant was stable thermally and hydrolytically with most acid additives. While it was compatible (i.e., still formed a viscosifying gel), some additives adversely affected the apparent viscosity of surfactant solutions at a given temperature. The apparent viscosity of surfactant solutions increased with salt concentration and can be predicted by use of the Carreau-Yasuda model. Coreflood tests indicated that the surfactant delayed acid breakthrough in calcite cores. Acceptable corrosion rates were obtained when this surfactant was added to the acid. The performance of this surfactant was validated with field treatments. The surfactant was used in more than 100 matrix acid treatments (oil producers and water injectors). It was used to increase the viscosity of acids in situ and enhance the stability of foams used for diversion. All wells responded positively to these treatments, and no operational problems were encountered. Downhole gauges confirmed the ability of surfactant-based fluids to divert the acid into various zones.

Geochemical characteristics of naturally acid and alkaline saline lakes in southern Western Australia

Abundant shallow saline lakes on the Archean Yilgarn Craton in southern Western Australia exhibit a rare spectrum of geochemical conditions. Here the field geochemistry over three seasons (pH, salinity, and temperature), as well as major ions, trace elements, and H, O, and S stable isotopes of surface waters and shallow groundwaters from 59 ephemeral lakes in southern Western Australia (WA) are reported. Approximately 40% of the lakes and 84% of the measured groundwaters in WA are extremely acidic (pH 28%. The fluids are typically Na–Cl to Na–Mg–Cl–SO4 brines with variable yet locally high amounts of Ca, K, Al, Fe, Si, and Br. The acid brine fluid compositions are unusual. For example, in some fluids the amount of Al Ca, the amount of Br > K, and comparison of total S to SO 2 4 values suggest the presence of other uncommon S-bearing species. Trends in d 18 O and d 2 H illustrate the separation between surface lake water and shallow groundwaters, and indicate the contribution of meteoric waters to the lakes. The chemical and isotopic compositions of these fluids indicate a spatially and temporally dynamic, yet regionally consistent, history of brine evolution that is fundamentally different from most other terrestrial closed basin brines. The WA lake brines do not evolve from surface evaporation of dilute inflow waters, but rather are fed by highly evolved regionally acid saline groundwaters. The lake waters then diversify with locally varying surface and near-surface processes such as meteoric dilution by flooding, evapoconcentration, mineral precipitation and dissolution, and fluid mixing. The WA lake waters and groundwaters are somewhat similar to those in an entirely different geologic setting in northeastern Victoria, illustrating the potential for different geochemical pathways to lead to the formation of similar lacustrine acid brines. Although these types of environments are rare in modern settings, ancient ephemeral acid saline lake deposits have been recognized in the geologic record on Earth and on Mars, indicating that natural evolution of acid saline waters may be more ubiquitous than previously recognized.

Influence of hydrogeology, microbiology and landscape history on the geochemistry of acid hypersaline waters, N.W. Victoria

Abstract There is now evidence that naturally occurring acid–water is more abundant than previously thought and that it has been important in the geologic past. Understanding the processes leading to the formation of such systems is required to appreciate the role of acid systems in geologic processes and to develop indicators for recognizing the geologic/environmental importance of these systems in the past. This paper characterizes the hydrogeology, hydrogeochemistry, microbial biogeochemistry and landscape attributes of acid–groundwater surface water systems in Australia with an emphasis on a well studied playa-lake system, Lake Tyrrell, Murray Valley. A model for the origin of these acid brines is presented and the importance of acid-brine producing systems is speculated upon. Data include porewater and groundwater geochemical measurements (collected during a 10 day field campaign) and results from geochemical modeling and graphics (e.g., Piper diagrams and x–y plots of seawater evaporation trajectories). Key characteristics of the system are (1) aquifer materials have low acid buffering capacities, (2) saline groundwater flowing onto playa surfaces is an oxic, H2SO4 solution, (3) authigenic minerals include combinations of jarosite [KFe3(SO4)2(OH)6], alunite [KAl3(SO4)2(OH)6] and Fe oxides that can form as evaporite minerals, (4) a source for solutes can be marine aerosols and (5) the formation of ironstones. Groundwater acidification by various processes including sulfide oxidation and ferrolysis, and at many different times, are the unique aspects for evolution of these acid brines and they can be considered another end member of the Eugster–Jones–Hardie model for the evolution of brines in closed basins. Acid–hypersaline groundwater and playa systems such as Lake Tyrrell may be an example of expected changes in the hydrogeochemistry of terrestrial water during late-stage continental denudation under arid conditions. Historically these systems may have been integral to the formation of opal, bauxite, some low temperature ore deposits, of authigenic K-feldspars, and continental redbeds. Natural acid saline systems, such as those in Australia, may also be representative of acid saline systems on Mars.

Identification of Secondary Metabolites in Citrus Fruit Using Gas Chromatography and Mass Spectroscopy

This experiment targets undergraduate students in an analytical or organic instructional context. Using a simple extraction, this protocol allows students to quantify and qualify monoterpenes in essential oils from citrus fruit peels. The procedures involve cooling down the peels by immersing them into icy water. After a few minutes, the chilled peels are pulped in a simple kitchen blender using acidic brine to hydrolyze the undesired fatty acids. Essential oils are extracted from the emulsion using methylene chloride and are then injected in a gas chromatograph coupled with a mass spectrometer. Among the fruit tested—limes, grapefruits, and oranges—all showed a high concentration of (R)-limonene, a monoterpenoid commonly found in these fruits. Students are invited to quantify (R)-limonene in the extracts following an accurate 5-point standard calibration curve. For students, this experiment may be a first contact with the analysis of plant extracts as well as an introduction to the biochemistry of monote...

GRS evidence and the possibility of paleooceans on Mars

The Gamma Ray Spectrometer (Mars Odyssey spacecraft) has revealed elemental distributions of potassium (K), thorium (Th), and iron (Fe) on Mars that require fractionation of K (and possibly Th and Fe) consistent with aqueous activity. This includes weathering, evolution of soils, and transport, sorting, and deposition, as well as with the location of first-order geomorphological demarcations identified as possible paleoocean boundaries. The element abundances occur in patterns consistent with weathering in situ and possible presence of relict or exhumed paleosols, deposition of weathered materials (salts and clastic minerals), and weathering/transport under neutral to acidic brines. The abundances are explained by hydrogeology consistent with the possibly overlapping alternatives of paleooceans and/or heterogeneous rock compositions from diverse provenances (e.g., differing igneous compositions).

<i>Lactobacillus pentosus</i> DSM 16366 starter added to brine as freeze-dried and as culture in the nutritive media for Spanish style green olive production

Lactobacillus pentosus DSM 16366, a strain originally isolated from olive fermentation, was used as a starter culture for “Azeiteira” the preparation of Spanish style green olives. Inoculum was added to the fermentors as a freezedried starter culture or as a culture in the nutritive media. Lactic acid fermentation induction produced a more rapid acidification of brines and reduced the survival period of Enterobacteriaceae compared with the uninoculated process. The best results were obtained using the nutritive media as a culture carrier rather than the freeze-dried starter.

Biogeochemistry of a Hyperacidic and Ultraconcentrated Pyrite Leachate in San Telmo mine (Iberian Pyrite Belt, Spain)

This work describes recent research carried out in an extremely acidic (pH 0.61–0.82) and hypersaline (e.g., 134 g/L SO4 2-, 74 g/L Fe, 7.5 g/L Al, 3 g/L Mg, 2 g/L Cu, 1 g/L Zn) leachate which seeps from a pyrite pile in San Telmo mine (Huelva, SW Spain) and forms evaporative pools of ultra-concentrated water in which attractive crystals of Zn-rich melanterite (FeIISO4 7H2O) are formed. Geochemical modeling with the Pitzer method indicates that the acidic brine was near saturation with respect to melanterite (SIMel = 0 ± 0.2). The microbiological investigation has revealed a surprisingly high biomass (1.4 × 106 cells mL−1) and an exotic ecosystem composed of acidophilic, Fe-oxidizing archaea (mainly Ferroplasma spp., representing 52% of the microbial population), and minor numbers of acidophilic bacteria (including Leptospirillum spp. (3.2%), Acidithiobacillus spp. (1.6%), and Alphaproteobacteria (2.8%)). The microbial production of FeIII allows the oxidative dissolution of pyrite and other sulphides, which results in additional inputs of FeII, SO4 2- and acidity to the system. The surfaces of the pyrite crystals show a typical etch-pitted texture, as well as blobs of elemental sulphur, which are both compatible with this indirect, microbially mediated oxidation mechanism. The composition of the acidic leachate seems to result from the combination of several processes which include: (1) formation of melanterite within the pile during relatively dry seasons, (2) subsequent dissolution of melanterite during rainy episodes, (3) microbial oxidation of FeII, (4) sulphide oxidation mediated by FeIII, (5) dissolution of chlorite and other aluminosilicates present in the pile, and (6) cooling and/or evaporation of seepage from the pile and consequent melanterite precipitation.