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Correspondence/Rebuttal
Response to Comment on “Geochemical Implications of Gas Leakage associated with Geologic CO2 Storage—A Qualitative Review” Omar R. Harvey, Nikolla P. Qafoku, Kirk Jason Cantrell, Giehyeon Lee, James E. Amonette, and Christopher F. Brown Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es401090n • Publication Date (Web): 18 Mar 2013 Downloaded from http://pubs.acs.org on March 19, 2013
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Response to Comment on “Geochemical Implications of Gas Leakage associated with Geologic CO2 Storage—A Qualitative Review” Omar R. Harvey,1,* Nikolla P. Qafoku,2 Kirk J. Cantrell,2 Giehyeon Lee,3 James E. Amonette,2 and Christopher F. Brown2 1,*
Department of Geography and Geology, The University of Southern Mississippi, 118 College
Drive, #5051, Hattiesburg, Mississippi, 39406, United States 2
Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, Washington 99354,
United States 3
Department of Earth System Sciences, Yonsei University, 134 Shinchon-dong, Seodaemun-gu
Seoul 120-749, Korea In a recent article Harvey et al.1 reviewed the published literature on how elevated CO2 levels may impact geochemical processes under conditions typical 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. Harvey et al.1 noted that; there is a significant amount of new scientific evidence to suggest that CO2 intrusion into potable aquifers or the vadose zone may have both beneficial and deleterious outcomes and; despite an increase in research effort, significant knowledge gaps still exist. From the perspective of beneficial outcomes, Harvey et al.1 pointed to evidence that CO2 intrusion may result in the immobilization of some contaminants by influencing chemical speciation, incorporation into stable mineral phases, or enhancing the precipitation of suitable 1 ACS Paragon Plus Environment
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sorbents. They also noted evidence to indicate that CO2 intrusion may be deleterious due to CO2induced dissolution of contaminant-bearing minerals, or desorption of contaminants from sorption sites. From the perspective of knowledge gaps, Harvey et al.1 identified and discussed several areas of research that they believe required a significant amount of experimental effort. These included; a systematic understanding of how CO2 influences pH, Eh and their coupled effects on precipitation/dissolution and adsorption/ desorption reactions in CO2-impacted systems; how microbes are impacted by or may impact pertinent geochemical processes in CO2 impacted systems; how mineral heterogeneity and distribution influences geochemical outcome; and, how geochemical processes are influenced by gas stream characteristics (e.g. composition and rate of intrusion). Wieger et al.2 agreed that the rate of CO2 intrusion is important, but argue that its significance is not related to its potential effect on aqueous CO2 concentration (or geochemical reactions) but rather to its effect on the size of the impact volume. According to Wieger et al.2, geochemical reactions are impacted only by the maximum solubility of the CO2 as controlled by the depth of aquifer. For such an argument to be valid, CO2 saturation of the soil or ground water must be instantaneous (no kinetic consideration to CO2 dissolution). However, we know that-as is typical of geochemical processes-there are kinetic considerations to the dissolution of CO2. That is, the time it takes for a system to reach equilibrium depends on the rate of mass transfer between the equilibrium phases. In the case of CO2 dissolving into a given aquifer (equilibrium =saturation =maximum solubility), the rate at which CO2 enters the aqueous phase is a function of how fast CO2 gas is entering the aquifer. Slower rate of CO2 gas intrusion into the aquifer results in longer time to reach saturation. 2 ACS Paragon Plus Environment
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Harvey et al.1 noted that the rate of CO2 transfer to the aqueous phase will directly impact the rate of change in major reaction drivers (pH and Eh). They argue that for a given aquifer, the strong dependence of surface chemistry and dissolution/precipitation rates of soil/aquifer minerals on pH and Eh dictates that gas intrusion rate (influencing the rate of CO2 transfer to the aqueous phase and subsequently pH and Eh) would be crucial in predicting contaminant release, aqueous speciation of contaminants, how rapidly potential sorbents may form and how rapidly a system may become supersaturated with respect to different minerals. Such an argument is consistent with observations from modeling efforts in both CO2-impacted aquifers and the vadose zone which show that for a given system changes in aqueous CO2 activity over time could induce changes in both the rate of and type of reaction.3,4,5 (vong et al, Zheng et al. altevogt and Jaffe) In addition to their view on CO2 intrusion rate, Wieger et al.2 discussed several other comments in reference to Harvey et al.1 These comments pertained to; possible assumptions made by Harvey et al.1; the relevance of considering mineral trapping in shallow aquifers; and, the role of sediment mineralogy. With respect to possible assumptions Wieger et al.2 suggested that Harvey et al.1 assumed that CO2 will intrude into thick aquifers at low leakage rates. It is however, unclear how Wieger et al. 2 reached this conclusion. In fact, to the contrary in the section titled “Carbon dioxide in subsurface environments” Harvey et al.1 discussed the importance of a confining layer in the physical containment of CO2 gas within confined aquifers and then noted that it is the partitioning of CO2 in to the aqueous phase that drives current thoughts on how CO2 leakage from deep storage reservoirs would impact the geochemistry of near-surface environments. Hence, their focusing of the review on aqueous phase CO2.
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With respect to mineral trapping, Wieger et al.2 argue that estimated timescale of mineral trapping and the low weathering rates of feldspars and clays precluded them from being important contributors to CO2 trapping in shallow aquifers. However, it is important to note that the timescale estimates and weathering rates being referenced by Wieger et al.2 ignores the contribution of microbial activity, the formation of metastable carbonate minerals and the effect of CO2 on weathering rates. Harvey et al.1 discussed several reasons why these factors should be considered. These include evidence from recent experimental studies which show that under high CO2 concentrations and conditions consistent with shallow aquifers; metastable carbonate minerals form readily, many microorganisms are capable of overcoming the thermodynamic and kinetic barrier to carbonate formation; and weathering rates may increase by several orders of magnitude. With respect to sediment mineralogy, Wieger et al.2 noted that geochemical reactions in shallow aquifers depend significantly on the composition and anion/cation exchange capacity of the sediment. As evidence they pointed to the fact that in aquifers of low buffering capacity (calcite-free) CO2-induced decrease in pH could increase the mobility of trace metals through enhanced dissolution of trace-metal bearing minerals or ion exchange involving H+ and Ca2+. They argue that the distribution of such parameters should be determined intensively. The argument put forth by Wieger et al.2 is very much a restating of that discussed by Harvey et al.1 in the sections on “contaminant immobilization” and “Why is sediment mineralogy important?” Literature Cited 1. Harvey, O. R.;Qafoku, N. P.; Cantrell, K. J.; Lee, G.; Amonette, J. E.; Brown, C. F. Geochemical Implications of Gas Leakage associated with Geologic CO2 Storage—A Qualitative Review. Environ. Sci. Technol. 2013, 47 (1), 23-36
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2. Wiegers, C. E.; Schafer, D.; Dethlefsen, F; Dahmke, A. Comment on “Geochemical Implications of Gas Leakage associated with Geologic CO2 Storage—A Qualitative Review”. Environ. Sci. Technol. 2013, In press 3. Vong, C. Q.; Jacquemet, N.; Picot-Colbeaux, G.; Lions, J.; Rohmer, J.; Bouc, O. Reactive transport modeling for impact assessment of a CO2 intrusion on trace elements mobility within fresh groundwater and its natural attenuation for potential remediation. Energy Procedia 2011, 4, 3171−3178. 4. Zheng, L. G.; Apps, J. A.; Zhang, Y. Q.; Xu, T. F.; Birkholzer, J. T. On mobilization of lead and arsenic in groundwater in response to CO2 leakage from deep geological storage. Chem. Geol. 2009, 268 (3−4), 281−297. 5. Altevogt, A. S.; Jaffe, P. R., Modeling the effects of gas phase CO2 intrusion on the biogeochemistry of variably saturated soils. Water Resour. Res. 2005, 41, (9).
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