Geochemical Implications of Gas Leakage associated with Geologic

Critical Review pubs.acs.org/est

Geochemical Implications of Gas Leakage associated with Geologic CO2 StorageA Qualitative Review Omar R. Harvey, †,‡,* Nikolla P. Qafoku, and Christopher F. Brown †

†

Kirk J. Cantrell,

†

Giehyeon Lee,§ James E. Amonette,∥

†

Geosciences Group, Pacific Northwest National Laboratory, 902 Battelle Blvd, K6-81, Richland, Washington 99354, United States Department of Geography and Geology, The University of Southern Mississippi, 118 College Drive, #5051, Hattiesburg, Mississippi, 39406, United States § Department of Earth System Sciences, Yonsei University, 134 Shinchon-dong, Seodaemun-gu Seoul 120-749, Korea ∥ Chemical and Materials Sciences, Pacific Northwest National Laboratory, 902 Battelle Blvd, K8-96, Richland, Washington 99354, United States ‡

ABSTRACT: Gas leakage from deep storage reservoirs is a major risk factor associated with geologic carbon sequestration (GCS). A systematic understanding of how such leakage would impact the geochemistry of potable aquifers and the vadose zone is crucial to the maintenance of environmental quality and the widespread acceptance of GCS. This paper reviews the current literature and discusses current knowledge gaps on how elevated CO2 levels could influence geochemical processes (e.g., adsorption/desorption and dissolution/precipitation) in potable aquifers and the vadose zone. The review revealed that despite an increase in research and evidence for both beneficial and deleterious consequences of CO2 migration into potable aquifers and the vadose zone, significant knowledge gaps still exist. Primary among these knowledge gaps is the role/influence of pertinent geochemical factors such as redox condition, CO2 influx rate, gas stream composition, microbial activity, and mineralogy in CO2-induced reactions. Although these factors by no means represent an exhaustive list of knowledge gaps we believe that addressing them is pivotal in advancing current scientific knowledge on how leakage from GCS may impact the environment, improving predictions of CO2-induced geochemical changes in the subsurface, and facilitating science-based decision- and policy-making on risk associated with geologic carbon sequestration.

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leakage may be considered globally or locally.4,19 On a global scale, increased atmospheric CO2 concentration due to leakage of previously sequestered CO2 is of greatest concern. However, current trends in storage assurances suggest that a significant increase in atmospheric CO2, or any subsequent effects on climate change, due to leakage from geologic storage is unlikely. For example, the International Panel on Climate Change noted that with respect to global risk, the fraction of CO2 retained in appropriately selected and managed reservoirs is likely to exceed 99%.4 Such storage assurances are well within the 60−95% CO2 retentions suggested to make impermanent storage valuable for the mitigation of climate change. 4 Locally two gas leakage scenarios are of concern.4,19 The first is where there is a sudden, fast and short-lived release of gas as would occur in the case of well failure during injection or spontaneous blowouts.22−25 In general, this leakage scenario is

INTRODUCTION The capture and storage of CO2 in deep geologic formations (or geologic CO2 sequestration) is widely considered a feasible approach to reducing industrial loadings of greenhouse gases to the atmosphere.1−5 Although oil and gas reservoirs6−8 and unmineable coal seam formations9−11 have been identified as potential geologic repositories for CO2, deep (often saline) nonpotable aquifers are preferred and are the most widely studied. Reasons include ubiquity, availability of mature technology, high storage capacities, and potential for CO2 conversion to carbonate minerals.3,12−14 Estimates for CO2 storage capacity in a single deep nonpotable aquifer range from 10−2 to 104 Gt2,13−17 and would be sufficient for storing decades to centuries of future CO2 emissions.14 Despite apparent promise, a major risk factor and potential barrier to widespread deployment of geologic CO2 sequestration is leakage of gas from the storage aquifer.4,18−21 Shaffer18 suggested that gas leakage of 1% or less per thousand years from a storage reservoir, or continuous resequestration, would be required to maintain atmospheric CO2 concentrations close to those projected for alternative approaches (e.g., lowering global emissions by 2050 to 60% compared to 1990). Concerns with gas © 2012 American Chemical Society

Special Issue: Carbon Sequestration Received: Revised: Accepted: Published: 23

July 19, 2012 October 18, 2012 October 23, 2012 October 23, 2012 dx.doi.org/10.1021/es3029457 | Environ. Sci. Technol. 2013, 47, 23−36

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Critical Review

anticipated to be relatively rare (on the order of 1 every 105 wells year−1) with environmental damage confined to the vicinity of the accident.22,23 However, in cases where CO2 concentrations are high enough there is the potential for loss of animal and human life.19,26 The second local leakage scenario is where the leak is more gradual occurring from abandoned wells, along undetected faults, fractures or the injection-well lining.27 It is this type of leakage that is of greatest concern because, without rigorous monitoring, such diffusive leakage may go undetected for prolonged periods of time, and has the greatest potential to cause broad scale environmental impacts including decreases in domestic, agricultural, or industrial water quality and soil fertility.4,19,24 In this paper, the current literature on the geochemical implications associated with gas leakage from deep CO2 storage formations to near-surface environments is reviewed. Emphasis is placed on CO2-induced effects on dissolution/precipitation and adsorption/desorption reactions in the near subsurface, and potential beneficial or deleterious consequences on the geochemistry of potable aquifers or the vadose zone. Gaps in current knowledge and research needs related to geochemical implications of CO2 intrusion into the near subsurface are also identified and discussed.

throughout this paper, reactions in the deep storage reservoir are not the primary focus of this review. Gaus45 recently reviewed the literature pertinent to CO2-rock interactions in deep geologic formations suitable for CO2 sequestration. It is the CO2 that would migrate diffusively (in the case of a leak) from the storage aquifer, through various leakage paths (e.g., fractures, faults or well-bores) and into overlying potable aquifers or the vadose zone that is of primary concern in this review. Celia and Nordbotten27 provide a good overview of these leakage pathways and their significance. In contrast to deep storage reservoirs, temperature and pressure conditions in near surface environments, where most potable water aquifers can be found, are below the critical point of CO2. Under these subcritical conditions, CO2 exists predominantly in the gaseous phase. In the event that CO2 migrates from deep storage, it is the lower pressure and temperature conditions (associated with near surface environments) that drive the transition of scCO2 to CO2 gas. The fate of the leaked CO2 gas will depend largely on the physical and chemical characteristics of the receiving subsurface environment. From a physical perspective, the presence of a restricting layerin the near subsurface (e.g., a confined aquifer or thick aquitard)reduces the risk of CO2 migrating back into the atmosphere. Otherwise, a significant amount of the leaked CO2 may eventually diffuse/migrate into the atmosphere.46,47 Klusman47 estimate that about 170 tons of CO2 is lost annually through leakage from deep storage to the atmosphere, at a CO2EOR site in Rangely, Colorado. If a confined aquifer or thick aquitard was present above the injection zone and reservoir caprock at Rangely, losses of CO2 to the atmosphere would likely be significantly less due to physical containment of the gas, as well as continued dissolution of CO2 into the groundwater. From a chemical perspective, the partitioning of the CO2 into the aqueous phase is crucial in determining the fate and impact of leaked CO2 in the near subsurface. In fact, it is the effect of the aqueous phase CO2 on aqueous phase pH, and subsequent rock/ mineral-solution interactions that drives current thoughts on how the leakage of CO2 from deep storage reservoirs would impact the geochemistry of near-surface environments. In the following sections, we synthesize the current literature on how the partitioning of CO2 into aqueous solution and associated changes in aqueous pH may effect beneficial or deleterious geochemical changes in potable aquifers and the vadose zone. Partitioning of CO2 into Solution and Changes in Aqueous Phase pH. The dissolution of CO2 in water to form carbonic acid, and its subsequent dissociation, is known to cause a decrease in pH as a result of aqueous phase proton enrichment:

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CARBON DIOXIDE IN SUBSURFACE ENVIRONMENTS In deep geologic formations, suitable for CO2 sequestration, the temperature and pressure conditions typically exceed those of the critical point of CO2 (31.1 °C/7.38 MPa). Carbon dioxide in storage aquifers will therefore exist primarily as a supercritical fluid.19,27,28 Several mechanisms for the sequestration of this supercritical CO2 (scCO2) in deep aquifers include geologic trapping through physical containment by geologic features, solubility trapping through dissolution in the formation water or residual oil, mineral trapping through the formation of carbonate minerals, hydrodynamic trapping due to differences in viscosity between the CO2 plume and formation water, and capillary trapping where CO2 is held in formation due to capillary forces.3,29 In uneconomical coal-bed formations, sorption of CO2 to the coal surface is the primary sequestration mechanism identified.30−32 It is difficult to ascertain how much CO2 is sequestered by a specific mechanism; however, estimates of coal-bed storage capacity suggest accessibility to sorption sites of less than 60% for CO2 sequestration.30,32 For deep aquifers/reservoirs, geologic trapping of CO2 is expected to be dominant during the early years with solubility, hydrodynamic and mineral trapping becoming increasingly significant with time.4,19,33 Several studies34−37 suggest that even after centuries of operation, CO2 sequestration via mineral trapping might be extremely small (less than 0.5%) in sedimentary formations. Within 2 km of the surface, scCO2 will be less dense than the formation water.28 Any CO2 not sequestered via solubility or mineral trapping would therefore be expected to form a buoyant scCO2 plume within the storage reservoir. The distribution of CO2 between the supercritical plume and aqueous phase (CO2 sequestered via solubility trapping) will depend on reservoir temperature and pressure, as well as the chemical composition of the formation water.27,28,38,39 The reactivity of both phases toward geologic and man-made materials (e.g., cement and steel used in well construction) has been extensively studied under conditions consistent with deep storage formations.40−44 Although, results from some of these studies will be mentioned

CO2 (g) + H 2O ↔ H 2CO3 ↔ HCO3−(aq) + H+(aq) (1)

HCO3−

Changes in pH and the production of will influence or control the dissolution of minerals and the subsequent release of chemical elements and contaminants into the aqueous phase, as well as precipitation reactions and formation of neophases. In addition, these changes may significantly and even dramatically affect the extent and rate of chemical, biological, and hydrological processes and reactions, which may control contaminant mobility in the subsurface. Experimental and modeling studies suggest that CO2 intrusion into the vadose zone or potable aquifers may induce a decrease in aqueous pH on the order of 1−3 units.48−56 For a given system, the magnitude of the decrease in pH will likely be a function of the solubility of CO2 in the aqueous solution, and the buffering 24

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solutions and potable groundwater are undersaturated (saturation index, SI < 0) with respect to most carbonate minerals (except calcite).60,61 Even in cases where soil solutions or groundwater are supersaturated, some carbonates will not precipitate due to kinetic limitations. Magnesite and dolomite are well-known examples.62−64 Saldi et al.63 estimated that for SI = 1 (10 times saturation) the growth rate of magnesite at 25 °C was at least 6 orders of magnitude lower than that of calcite under the same conditions. They further estimated that it would take at least 340 000 years to precipitate a 1 mm layer of magnesite from bulk solution. Jimenez-Lopez et al.65 found a similar situation for the precipitation kinetics of siderite (FeCO3) compared to calcite. They found that for a given SI the precipitation rate of FeCO3 at 25 °C was about 8 orders of magnitude lower than that of calcite under the same conditions. The higher ion surface-charge densities of Fe2+ and Mg2+ relative to Ca2+ have been used by Saldi et al.,63 Jimenez-Lopez et al.65 (and references therein) to explain the markedly different precipitation rates between calcite (which forms readily at low temperatures) and other carbonates. The smaller ionic radius of Fe2+(0.064 nm) and Mg2+(0.065 nm), compared to Ca2+ (0.074 nm), means that a higher activation energy is required to initiate dehydration of Fe2+ and Mg2+, and subsequently the precipitation of siderite, magnesite or dolomite. Activation energies of 38, 129, and 159 kJ mol−1 have been reported for the formation of calcite, siderite, and magnesite, respectively at 25 °C.63,65 The preceding discussion on thermodynamic and kinetic limitations to carbonate formation may raise some questions about the validity of mineral trapping as an important mechanism in CO2 resequestration in near surface environments. There is however evidence to suggest that some level of discussion is warranted. First, while some carbonates do not form readily under conditions consistent with near surface environments, precursors of these minerals are known to form readily under these conditions. For example, there is an increasing number of studies to suggest that nesquehonite (MgCO3•3H2O) forms readily in low temperature CO2-rich solutions that are supersaturated with respect to magnesite.66−68Jimenez-Lopez et al.65 also noted a series of metastable precursors that formed readily and eventually lead to the formation of well-crystallized siderite. It is therefore plausible that these precursors could play a significant role in mineral trapping in near surface environments impacted by CO2. A second reason to consider mineral trapping mechanisms in near-surface environments is the evidence that microorganisms are capable of overcoming the thermodynamic and kinetic barriers to produce carbonates in systems where they would not be predicted on the basis of temperature or saturation index.69−71Roberts et al.71 observed and demonstrated that methanogens were able to mediate the formation of dolomite in a shallow freshwater basalt aquifer on time scale of weeks to months. They suggested that the methanogens overcame the thermodynamic barrier by releasing Mg2+ and Ca2+ from the basalt. This was consistent with the findings of Kenward et al.,69 who showed that methanogenesis increased the dolomite saturation state of their solutions by 2 orders of magnitude. Roberts et al.,71 Kenward et al.,69 and other researchers72,73 all suggest that carbonate-forming microorganisms overcome the kinetic barrier by using their cells as a seeding/nucleation surface. The presence of a suitable seeding surface is known to increase carbonate precipitation rates by up to 3 fold.74The high pCO2 in

capacity of the system. The solubility of CO2 in aqueous solution is known to decrease with increasing temperature and solution ionic strength, but increase with pressure.38,57 Hence, for systems of a given buffering capacity, decrease in the pH at the lower end of the range (1−3 pH units) would be favored under conditions of lower CO2 solubility (e.g., warm, shallow potable aquifers or more saline arid soils). On the other hand, ceteris paribus, a greater magnitude of pH change would be expected in cooler/ deeper aquifers and nonsaline soils. For systems with similar CO2 solubility, a decrease in pH closer to 1 pH units would be typical of well buffered systems, where CO2-induced dissolution of reactive carbonates (eq 2), feldspars (eq 3), and/or the precipitation/dissolution of clays (eqs 3 and 4) would provide enough buffering capacity (via HCO3− alkalinity) to resist drastic changes in pH. CaCO3 + CO2 (g) + H 2O → Ca 2 + + 2HCO3−

(2)

2NaAlSi3O8 + 11H 2O + 2CO2 (g) → Al 2Si 2O5(OH)4 + 2Na + + 2HCO3− + 4H4SiO4 (3)

Al 2Si 2O5(OH)4 + 5H 2O + 6CO2 (g) → 2Al3 + + 6HCO3− + 2H4SiO4

(4)

Poorly buffered systems (e.g., sandy soils/aquifers) are devoid of sufficient quantities of alkalinity-producing minerals and therefore lack the ability to resist changes in pH. A major concern for systems with high CO2 solubility, and/or low buffering is that any geochemical change due to CO2 intrusion is likely to be more apparent and the risk for pH-induced perturbation to environmental quality is more significant and prolonged compared to well buffered systems.13,52,54

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BENEFICIAL CO2-INDUCED INTERACTIONS IN THE NEAR SURFACE ENVIRONMENTS The migration of CO2 from deep geologic storage into near surface environments could be considered beneficial if it resulted in resequestration (the trapping of CO2 that was previously stored in deep storage) and/or reduced mobility and bioavailability of contaminants. CO2 Resequestration. Estimates of up to 96% retention of leaked CO2 over a 1000 year period have been reported in the vadose zone.46 Processes identified for such CO2 resequestration in the subsurface include the accumulation of the CO2 gas at the water table (due to CO2 being denser than soil air), permeability trapping (due to anisotropy favoring the horizontal flow of CO2) and solubility trapping (due to dissolution of CO2 into infiltrating or residual soil water). The mineralogical trapping of CO2 in carbonate minerals provides another potential mechanism for CO2 resequestration in near surface environments. From an environmental and sequestration perspective, this would be the most desirable outcome for CO2 intrusion into near surface environments, because of the immobility of carbonate minerals compared to other CO2 trapping mechanisms. Several studies have demonstrated mineralogical trapping of CO2 under conditions associated with deep geologic CO2 storage, but such assessments have not been widely considered in near surface environments due to thermodynamic and kinetic limitations.40,46,58,59 Thermodynamic limitations to carbonate precipitation in the near surface environments are due to the fact that many soil 25



Critical Review

Table 1. Summary of Modeling and Experimental Studies Examining the Impact of CO2 Intrusion on the Geochemistry of Potable Aquifers and the Vadose Zone key findings on CO2 effects

descriptiona Modeling Studies Altevogt and Jaffe48

•2-D variably saturated hypothetical vadose zone exposed to CO2 for up to 40 days •variables: pH, O2(aq), organic matter, NO3−, NH4+, Mn(II), Mn(IV), Fe(II), Fe(III), SO42−, H2S, H2CO3

•potential mobilization of toxic metals, due to acidification of the vadose zone •decrease in organic matter degradation rates, and oxidized species, due to CO2 displacing O2

Vong et al.51

•2-/3-D glauconitic-sandstone aquifer (with trace metal - bearing sulfides added) exposed to CO2 over 10 years •variables: pH, Cd, Pb, Zn

•predicted acidification increased dissolution of metal- bearing sulfides and subsequently aqueous concentration of trace metals

Wang and Jaffe52

•2-D buffered/unbuffered hypothetical potable aquifers with galena (PbS) as source of Pb and exposed to CO2 for 8 years •variables: pH, total carbonate, Pb

•increased aqueous Pb from acidic dissolution of galena

Zheng et al.55

•3-D Eastern Coastal Plain model aquifer exposed to CO2 for 100 years

•acidification increased galena and arsenian pyrite dissolution resulting in increased aqueous Pb and As •aqueous As regulated by suitable sorbents

•variables: pH, Pb, As Jacquemet et.al.89

Zheng et al.90

•similar to Vong et al.51 (without sulfides) but with SOx and NOx (as impurities in CO2 gas stream) •variables: pH, Fe, Mn, SOx(aq), NOx(aq)

•yearlong CO2-nanopure water-sediment study with sediments from aquifers, which overlay potential geologic carbon storage sites in the U.S. •variables: pH, Li, Mg, Ca, Rb, Sr, Co, Se, Ba, U, As, Al, V, Cr, Mn, Fe, Ni, Cu, Zn, Mo, Cd, B

Lu et al.50

Wei et al.53

•sediments from representative potable aquifers within the US Gulf Coast region. Background electrolyte (∼1 mM NaCl) used as groundwater medium. Exposed to CO2 for 2 weeks, preceded by Ar purging (2 weeks) •variables: pH, Ca, Al, Zn, Mg, Fe, Cs, Na, B, Ni, K, Co, Rb, Si, Cu, As, Mn, Mo, Cr, Sr, U, Ba, V, Cl, SO4, Br, F, NO3

•variably saturated soils exposed to CO2 (at 25 bar) for 3 days •variables: pH, Mg, K, Al, Ti, V, Cr, Mn, Fe, Co, Cu, Rb, Sr, Mo, Cs, Ba, Pb, Th, U, Zn, Cd

Field Studies Smyth et al.76

•increased aqueous Fe and Mn mineral dissolution • lower pH and increased aqueous Fe and Mn with SOx and NOx, (2%)of H2S, NOx, and SOx in an intruding gas stream is unlikely where regulations prohibit their co-injection and/or alternative capture technologies to postcombustion (e.g., precombustion or oxy-combustion) are employed. In such cases any co-injected gas present in the intruding gas stream would have been derived from the preinjection separation/stripping activities. For example, NH3

pyrite, galena, uraninite, sphalerite, and various solid solution assemblages). The intrusion of CO2 gas into these aquifers, or the vadose zone, is likely to induce changes in redox condition due to CO2-induced reactions resulting in the redistribution of oxidized and reduced aqueous species, O2 depletion/displacement with the CO2 gas,48,95,96 or changes in biological activity.97,98 The extent and direction of Eh change will control the extent and rate of mineral dissolution, precipitation of neophases, as well as sorption/desorption of contaminants onto pertinent sorbent such as organic matter, clays, and metal (hydr)oxides. In reality, pH and Eh in the subsurface will be interrelated. This is reflected in the fact that most redox reactions involve protons (oxidation releases H+ and reduction consumes H+) and will induce a simultaneous change in solution pH. A change in pH will also impact Eh due to pH effects on dissolution and subsequently the distribution of reduced versus oxidized species in solution. The impact of CO2 on chemical speciation and the dissolution/precipitation characteristics of minerals in the subsurface will therefore be a combined effect of Eh and pH. With respect to coupled changes in pH and Eh, one of two scenarios are plausible in the event of CO2 intrusion into the vadose zone or a potable aquifer. Scenario 1 is where a decline in pH is accompanied by a decline in Eh (e.g., due to the displacement of O2 by CO2) and scenario 2 is where a decline in pH is accompanied by an increase in Eh (e.g., due to an increase in the aqueous activity of oxidized to reduced species). Figure 1 suggests that while a CO2-induced change in pH of 1−2 units49,55 and Eh of 0.1 V, would have little to no impact on Fe geochemistry (irrespective of scenario), similar changes in pH and Eh would favor a shift in As speciation from HAsO2 to HAsS2 (scenario 1; reduced groundwater) or HAsO42‑ to H2AsO4− (scenario 1/2; oxidized groundwater). Such shifts in the chemical speciation of As have significant implication for its mobility, reactivity and toxicity in the environment.99,100Shifts in speciation, resulting in changes in the mobility, reactivity or toxicity of other redox-sentive contaminants (such as Cr and U) are also plausible outcomes of CO2-induced changes in the pH and Eh of the vadose zone or potable aquifers. We contend that, along with pH, it is crucial that Eh be routinely considered as a major variable in studies seeking to decipher how the geochemistry of aquifers and the vadose zone would likely be impacted by CO2 migration from deep storage reservoirs. Why is CO2 Intrusion Rate Important? The flux of the migrating gas stream into a potable aquifer or the vadose zone will influence the rate at which the system pH and Eh changes. The rate at which the pH and Eh of the system changes will directly impact mineral dissolution/precipitation kinetics,101,102 the nature and properties of the neophases precipitated,103 as well as the kinetics of (or tendency for) adsorption/desorption reactions (to occur).104The strong dependence of the dissolution and precipitation rate, for many minerals, on pH and Eh dictates that gas streamflow rate would be crucial in predicting contaminant release, how rapidly potential sorbents may form or, how rapidly a system may become supersaturated with respect to carbonate minerals. Similarly, the often strong dependence of sorbent surface chemistry, and aqueous speciation of contaminants, on pH and Eh also dictates that CO2 intrusion rate will impact the fate and transport of contaminants in CO2-impacted aquifers or the vadose zone. One criticism of current experimental studies is that the rate of CO2 intrusion used is unrealistically high and would overwhelm any chemical buffering provided by the minerals; as a consequence, these studies yield unrealistic mineral dissolution 29



Critical Review

undergo methanotrophic oxidation (to produce CO2) under oxic conditions. In the event of gas leakage at a CO2 sequestration site, the methanotrophic transformation of native hydrocarbons would increase CO2 concentration in a migrating gas stream and enhance its impact on geochemical processes in the subsurface. From the perspective of CO2 sequestration, many microorganisms likely to be encountered in near-surface environments above geologic sequestration sites (e.g., sulfate-reducers, nitratereducers, iron-reducers, urea-degraders, and methanogens) are capable of biogenic carbonate formation.71−73,122−125 For example, the formation of carbonate minerals by urea-degraders has been extensively studied.122−125 In a recent study122 it was shown that, under conditions consistent with CO2 storage conditions, urea-degraders incorporated up to 37% of total carbon into carbonate minerals. Since the high pressure conditions of deep storage formations could significantly inhibit cellular activity126 a higher incorporation of CO2 into carbonates might be expected in near surface environments. Roberts et al.71 found that iron-bearing dolomite formed after 3 months of suspending sterilized Columbia River basalt fragments into a fresh water aquifer (to a depth of 7 m) was associated with the colonization of the basalt by a consortium of methanogens and fermenters. Dolomite formation was suggested to occur via local microbially enhanced weathering of the basalt coupled with methanogenesis (with dissolved CO2 as a source of carbon) and crystal nucleation on the cell walls of the microorganism (eq 7).

and some volatile organic compounds are known to be produced during amine-based gas stripping/separation activities.107 At the time of this review only the modeling efforts of Jacquemet et al.108 and Gislason et al.15 were found to consider the potential effects of co-injected gases on near surface environments. Jacquemet et al.108 predicted that presence of co-injected SO2 (∼1.5%) and NO (∼0.5%), could lower pH by an additional 1 unit (compared to CO2 alone) and increase the release of metals from aquifer minerals. Such a finding is consistent with the acidification effects associated with dissolution of NOx and SOx in water,109−111 but ignores any potential beneficial consequences of NOx and SOx. For example, there is some evidence to suggest that aqueous SOx and NOx species may serve as an important oxidant in microbially mediated mineral precipitation.112−116 Gislason et al.15 also noted that the presence of SO4 (from SO2 oxidation) and F (from HF in flue gas) may serve to enhance mineralogical trapping through the complexation of Al3+ and the enhance dissolution of Mg2+, Ca2+, and Fe2+ bearing alumino-silicates. Accounting for such beneficial outcomes, in terms of the potential for mineralogical trapping of CO2 or contaminant immobilization, is important and highlights the need for significant experimental research to validate current modeling efforts as well as assess both beneficial and deleterious outcomes of other co-injected gases (including those from stripping activities). Studies of gases in the subsurface suggest that geologic formation suitable for CO2 storage are likely to contain native gases that have different partitioning/solubility characteristics than CO2. Kharaka et al.39 found that gas samples collected from around 1500 m in the Frio formation, Texas contained 95 ± 3% CH4 with the bulk of the remaining gas comprised of N2, C2, and higher hydrocarbon gases. High concentrations of CH4 and H2 have also been observed in basaltic aquifers down to 1200 m.117 Any leakage of injected gas from the CO2 storage reservoir will therefore occur in conjunction with these native reservoir gases. Considering that native aquifer gases will exhibit very different geochemical behavior than CO2, accounting for their potential impact on geochemical changes associated with leakage of CO2 from deep storage aquifers is also important. To our knowledge, none of the current studies have attempted to address this issue. Establishing research looking at native reservoir gases and how their combined effect with CO2 is likely to influence subsurface geochemistry is pivotal. The establishment of research that considers native gases, their transformation and subsequent impact on potable aquifer and vadose zone geochemistry is therefore very important. Why is Microbial Activity Important? In the event of gas leakage from geologic storage, microbial activity will impact subsurface geochemistry largely through its influence on the fate of the gases comprising the intruding gas stream, changes in pH and Eh, and subsequently, changes in dissolution/precipitation or adsorption/desorption reactions. Several recent studies have considered microbial activity in the storage reservoir,97,118,119 but to our knowledge, only the modeling effort of Onstott98 and a preliminary assessment by Jones et al.120 have considered how CO2 leakage might influence microbial life in the near subsurface. However, several additional studies support the need for systematic assessments of the role of microorganisms and microbial activity on the geochemical outcomes of gas migration in potable aquifers or the vadose zone. For example, Klusman47,121 found that most of the CH4 (and other hydrocarbons) in soils above exploited oil fields, will

3HCO3− + 4H 2 + Ca 2 + + 0.9Mg 2 + + 0.1Fe 2 + → Ca1.0Mg 0.9Fe0.1(CO3)2 + CH4 + 3H 2O + H+

(7)

Interestingly, there is also ample evidence in the literature to suggest that the CH4 produced in eq 3 could be reoxidized via anaerobic methane oxidation (AMO) by reducing bacteria (including iron-, nitrate- and sulfate reducers), resulting in further biogenic carbonate formation (eq 8).112−116 CH4 + SO4 2 − + M2 + → MCO3 + HS− + H 2O + H+ (8)

where M2+, could be any carbonate forming divalent metal (e.g., Ca2+, Mg2+, or Fe2+). If M2+ was Cd2+ or Pb2+, biogenic carbonate formation could also potentially serve to enhance natural attenuation of toxic metals through the formation of CdCO3 and PbCO3. Theoretically, eq 7 suggests that mineral trapping alone could account for 66% of dissolved inorganic carbon. In cases where eq 8 is thermodynamically and kinetically favorable, and if the system is not limited by oxidant (e.g., SO42‑, NO3−, or Fe3+) or dissolved metal (e.g., Ca2+, Mg2+, or Fe2+) concentrations, the number would be closer to 100%. It is therefore plausible that at geologic sequestration sites where biogenic carbonate formation is favorable, mineral trapping could be very important in determining the fate of leaked CO2 in the near surface environments. At potential geologic sequestration sites (e.g., enhanced CO2-EOR operations, exploited oil reservoirs or basaltic formations) where anaerobic subsurface microbial ecosystems have been shown to be comprised of significant quantities of methanogens and reducing bacteria,117,127,128 microbial activity coupled to biogenic carbonate formation should be considered. From a contaminant mobilization perspective, there is also some evidence to suggest that microbial metabolism of migrating gases may result in enhanced 30



Critical Review

Figure 2. Conceptual framework for assessing geochemical impact of CO2 on near surface environments.

mobilization of some contaminants.129,130 Considering multifold effects of the impact of migrating gases on subsurface microbial populations and subsequently microbial-mediated processes is therefore important. Why Is Sediment Mineralogy Important? Geology and subsequently mineralogy at geologic sequestration sites will vary on an intra- and intersite basis. In the event of a CO2 gas stream intruding into a potable aquifer or the vadose zone, it is the mineralogy of the system that will dictate how well the pH or Eh is buffered, the type and amount of contaminants likely to be mobilized, and what neophases/sorbents are likely to precipitate. A systematic understanding of the minerals present, their relative distributions and their chemical and physical behavior is crucial to assessing the impact of CO2 on near surface environments. In many of the studies reviewed, the significance of mineral type to outcomes of CO2-intrusion was apparent. However, the question on how geochemical outcome is influenced by mineral distribution/heterogeneity has not been answered. In the studies by Lu et al.50 and Smyth et al.,76 the dissolution of carbonate minerals and adsorption/desorption to hydrous oxides, organic matter, and clay minerals were highlighted as controlling the total aqueous concentration of various metals; but no information was provided on the distribution of the pertinent minerals or their origin. This information is relevant to answering the question of whether the minerals were precipitated before or after the introduction of CO2. Kharaka et al.56 also attributed observed increases in aqueous metal concentrations at the ZERT field site to CO2-induced dissolution of carbonates, iron oxides and/or desorption-proton exchange reactions possibly from clay minerals but, again, the distribution and origin of the minerals responsible for the observed changes were unknown. Knowledge of mineral distribution/heterogeneity in aquifers or the vadose zone may also be crucial in interpreting and designing laboratory experiments with material from field sites.

For example, Little and Jackson49 found that CO2 intrusion had no effect on aqueous Cd concentration in calcite-rich sediments but induced a 1000% increase in Cd for the calcite-deficient sediments. In contrast, concentrations of Al, V, and Cr increased after CO2 intrusion in chalcopyrite-containing sediments but decreased in other sediments. The lack of an effect on Cd concentrations in the calcite-rich sediments of Little and Jackson49 is attributable to CO2-induced dissolution of calcite which would serve to buffer the pH and thereby prevent dissolution of Cd-bearing minerals. Similarly, the oxidative dissolution of chalcopyrite would decrease solution pH thereby enhancing dissolution of Al, V, or Cr-bearing minerals and increasing aqueous concentrations of Al, V, and Cr. Thus, from the perspective of experimental design, knowledge of mineralogy is crucial in matching laboratory conditions (as closely as possible) to those that could be expected under field conditions. We believe that the lack of matching laboratory (e.g., solution and redox) conditions to those observed in the field is one area where current laboratory studies could be improved. For example, Little and Jackson49 noted that all their experiments were conducted in deionized water under oxidized conditions. However, the presence of sulfide minerals in some of their sediments was more representative of sediments originating from a reduced environment. Hence despite Little and Jackson49 pointing to pH as the primary factor responsible for the discrepancies between their study and that of Lu et al.,50 it is more plausible that a change in redox conditions was the factor controlling metal release (especially since pH values between the two systems were comparable). Rather than the oxic conditions of Little and Jackson,49 conditions existing in the Lu et al.50 experiments were likely anoxic (an Ar purge was used prior to CO2 introduction). This difference in redox condition is a plausible reason why, in contrast to Little and Jackson,49 Lu et al.50 observed no significant changes in the aqueous concen31



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of gas migration from geologic storage. Instead, they should seek to qualify such value. For example, although a given natural analog may have little value in predicting short-term geochemical consequences of gas leakage associated with GCS, it does not negate the fact that it could provide valuable information on potential flow paths, leakage size, reaction kinetics, equilibration time and equilibrium mineral phases. Such information could be important in terms of policy and strategic decisions on remediation approaches in the event of significant gas-leakageinduced deterioration in potable water or soil quality. We believe that questions and concerns with natural analogs or comparable sites would be best addressed using a set of rigorous protocols that clearly defines what constitutes an analog or comparable site, for what reasons, and what stage in the life cycle of the gas leak are the use of these sites appropriate?

trations of redox-sensitive metals (e.g., Fe, Cu, and U) or metals (e.g., Ni) that were likely contained in redox-sensitive minerals. Why Are Experimental and Assessment Protocols Crucial? A revisiting of current experimental approaches and the establishment of rigorous protocols is crucial to the alleviation of discrepancies between experimental studies, as well as the reconciliation of information from “analog” or “comparable” systems with leakage at geologic sequestration sites. The likelihood of experimental discrepancies can be minimized if experiments are conducted under protocols that were developed from a process-based understanding of geochemical reactions in the subsurface. For example, inconsistencies such as those between Lu et al. and Little and Jackson may be prevented by defining specific protocols/guidelines for redox condition and solution chemistry used in experiments geared toward understanding how gas intrusion is likely to impact the geochemistry of potable aquifers and the vadose zone. Such protocols could also provide a framework within which to compare findings from different studies and conduct research aimed at closing current knowledge gaps. One possible framework within which to assess the geochemical impact of CO2 on near-surface environment is presented in Figure 2. The framework in Figure 2 is centered on the master variables (pH and Eh). It is the magnitude and/or direction of change in pH and Eh, as controlled by the properties of the impacted system (e.g., mineralogy or initial solution composition) and the characteristics of the intruding gas stream (e.g., composition or influx rate), that would determine if gas leakage would have beneficial or deleterious impact on the geochemistry of the receiving potable aquifer or vadose zone. For example, gas-leakage-induced changes in system pH and Eh favoring precipitation reactions could be beneficial providing that such precipitation results in the formation of carbonate minerals thereby enhancing CO2 sequestration, the formation of sorbents with significant sorption capacity to immobilize potential contaminants from solution, and/or incorporation of contaminants into the structure of the neophase(s) thereby reducing their mobility. On the other hand, gas-leakage-induced changes in system pH and Eh favoring dissolution could be deleterious especially in the absence of a suitable sorbent. There is also a wealth of information from “analog” or “comparable” systems that needs to be reconciled, to ascertain their geochemical value in understanding the impacts of gas leakage from GCS. Although a detailed reconciliation of analog or comparable studies is really beyond the scope of this review, we do believe that a set of rigorous experimental and assessment protocols would be useful in the process. For example, it has been argued that natural CO2 analogs highlight the low risk for gasinduced contamination associated with GCS. As evidence for this low risk, proponents have pointed to the fact that water from these systems are often potable and are consumed with no adverse health effects.105 There is also evidence from studies at analog sites that have shown no, or only localized, deterioration in water quality due to intrusion of CO2 from natural sources.131−134 However, before conclusions on risk can be made, it is important to consider pertinent questions such as: How comparable are the gas stream characteristics (e.g., composition, flow rate or duration) at natural analogs to that expected at GCS? Are (or were) the characteristics of the receiving environment (e.g., mineralogy, solution composition, sediment exposure to gas) comparable? It is important to note that asking these questions should not negate the value of natural analogs in understanding the impact

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SUMMARY AND CONCLUSIONS Gas leakage from the storage reservoir is a major risk factor and potential barrier to the widespread acceptance of geologic CO2 sequestration. Different schools of thought exist, concerning how such leakages would impact the geochemistry of critical near surface environments such as potable aquifers and the vadose zone. On one hand it has been argued that the intrusion of CO2 into the near surface would have little to no negative impact on geochemistry. Proponents of this thought point to the fact that potable sources of CO2 rich groundwater exist across the world and have been consumed for centuries without adverse health effect. Another common school of thought is that CO2-induced acidification of groundwater will enhance the dissolution of contaminant-bearing soil and rock minerals, resulting in a degradation of environmental quality. In recent years a significant amount of scientific research has been conducted to assess the potential geochemical implications of CO2 migration into the vadose zone or potable aquifers. These studies include 1- to 3-D modeling efforts and laboratory experiments, as well as field and natural analog studies. We conducted a comprehensive review of the recently published literature on how elevated CO2 levels may impact geochemical processes under low-temperature, low-pressure conditions characteristic of near surface environments. Emphasis was placed on CO2-induced effects on dissolution/precipitation and adsorption/desorption reactions, and consequences for the geochemistry of the vadose zone and potable aquifers. The review revealed that, 1) there is a significant amount of new scientific evidence that suggests that CO2 intrusion into potable aquifers or the vadose zone may have both beneficial and deleterious outcomes, 2) despite an increase in recent efforts significant knowledge gaps still exist. From the perspective of beneficial outcomes there is strong evidence to suggest that CO2 intrusion may result in the immobilization of certain contaminants by influencing their chemical speciation, enhancing their incorporation into stable mineral phases, or enhancing the precipitation of suitable sorbents. On the other hand, there is also strong evidence to indicate that CO2 intrusion may be deleterious due to mobilization of some contaminants as a result of CO2-induced dissolution of contaminant-bearing minerals, or desorption of contaminants from sorption sites. From the perspective of knowledge gaps, we have identified and discussed several areas of research that we believe require a significant amount of effort (particularly experimental work). These include the development of a systematic understanding of how CO2 impacts both pH and Eh and, subsequently, their coupled effect on precipitation/dissolution and adsorption/ 32



Critical Review

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desorption reactions in CO2-impacted systems, how microbes are impacted by or may impact pertinent geochemical processes (e.g., precipitation/dissolution) in CO2 impacted systems, how mineral heterogeneity and distribution influences geochemical outcome, and how specific geochemical processes are influenced by gas stream characteristics (such as composition and flux characteristics). We are cognizant that the knowledge gaps discussed in this manuscript by no means represent an exhaustive list and could be expanded to include other relevant factors such as mixing/dispersion, contact area and residence time. However, we do believe that a systematic closing of these knowledge gaps would most efficiently advance scientific knowledge, improve predictability and risk assessment associated with CO2 leakage and facilitate better science-based decision- and policy-making to support GCS.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (601) 266-4529; fax: (601) 266-6219; e-mail: omar. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the National Risk Assessment Partnership (NRAP) through the U.S. DOE Office of Fossil Energy's Division of Crosscutting Research under contract DE-AC05-76RL01830. We are grateful to Drs. Hongbo Shao (PNNL), Wooyong Um (PNNL), Susan Carroll (LLNL), and Nic Spycher (LBNL) who reviewed an earlier version of the manuscript and provided valuable comments and suggestions. Comments and suggestions from the Associate Editor and anonymous reviewers also improved this manuscript.

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Geochemical Implications of Gas Leakage associated with Geologic

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