Article pubs.acs.org/est
Potential Impacts of CO2 Leakage on Groundwater Chemistry from Laboratory Batch Experiments and Field Push−pull Tests Patrick J. Mickler,* Changbing Yang, Bridget R. Scanlon, Robert Reedy, and Jiemin Lu Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924, United States S Supporting Information *
ABSTRACT: Storage of CO2 in deep saline reservoirs has been proposed to mitigate anthropogenically forced climate change. If injected CO2 unexpectedly migrates upward in shallow groundwater resources, potable groundwater may be negatively affected. This study examines the effects of an increase in pCO2 (partial pressure of CO2) on groundwater chemistry in a siliclastic-dominated aquifer by comparing a laboratory batch experiment and a field single-well push−pull test on the same aquifer sediment and groundwater. Although the aquifer mineralogy is predominately siliclastic, carbonate dissolution is the primary geochemical reaction. In the batch experiment, Ca concentrations increase until calcite saturation is reached at ∼500 h. The concentrations of the elements Ca, Mg, Sr, Ba, Mn, and U are controlled by carbonate dissolution. Silicate dissolution controls Si and K concentrations and is ∼2 orders of magnitude slower than carbonate dissolution. Changing pH conditions through the experiment initially mobilize Mo, V, Zn, Se, and Cd; sorption reactions later remove these elements from solution and concentrations drop to preexperiment levels. The EPA’s primary and secondary MCL’s are not exceeded except for Mn, which exceeded the EPA’s secondary standard of 0.05 mg/L. Push−pull results also identify carbonate and silicate dissolution reactions ∼2 orders of magnitude slower than batch experiments.
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INTRODUCTION Capture of CO2 from point sources, including power plants, followed by compression to a supercritical liquid, and storage in deep saline reservoirs has been proposed to reduce atmospheric CO2 levels and mitigate anthropogenically forced climate change.1−3 Because supercritical CO2 is less dense than aqueous solutions in reservoir pore spaces, CO2 may migrate upward until a confining system, required for permitting, is encountered. Upward migration of CO2 to fresh-water aquifers is possible along discrete pathways, including faults and improperly constructed deep wells.3,4 CO2 dissolution in potable groundwater may adversely affect groundwater chemistry by increasing mineral dissolution as a result of a drop in pH with increased pCO2, and by desorption and ion exchange reactions, which may increase water salinity and mobilize hazardous elements.5,6 Experimental methods used to assess changes in shallow groundwater chemistry due to an unintentional CO2 releases include (1) numerical modeling of water-rock-high pCO2 interactions (2) laboratory-based batch experiments and (3) field-based push−pull tests. Numerical modeling has been used to theoretically equilibrate high-CO2 groundwater with reactive mineral phases to predict changes to groundwater chemistry.6−13 Generally, these studies have identified geochemical reactions that have impacts on aqueous geochemistry, including CO2 (and, to a lesser extent, H2S and SO2) dissolution, © 2013 American Chemical Society
carbonate, silicate, sulfide, oxy-hydroxides and sulfate dissolution, absorption/adsorption/desorption reactions, and mineral precipitation reactions.14 These studies may overestimate the dangers of an unintentional CO2 release by including reactive mineral phases such as galena and arsenopyrite, whose dissolution would release Pb and As into the groundwater. Selecting mineral suites that represent natural aquifer systems may be difficult. Laboratory batch experiments react aquifer sediments with water in equilibrium with a high-CO2 atmosphere. Periodic sampling of the aqueous solution is used to quantify changes in aqueous chemistry and identify specific geochemical reactions.15−19 A weakness of these studies is that collected samples are exposed to oxidizing conditions, whereas confined groundwater is reducing. Field-based push−pull tests involve equilibration of groundwater with 100% CO2 atmosphere and injection of that water into the aquifer from which it was pumped. Injected water is allowed to incubate for a short time and then is sampled to determine the effect of elevated CO2 on groundwater chemistry.20−26 A weakness of push−pull tests is Received: Revised: Accepted: Published: 10694
April 3, 2013 August 7, 2013 August 12, 2013 August 12, 2013 dx.doi.org/10.1021/es401455j | Environ. Sci. Technol. 2013, 47, 10694−10702
Environmental Science & Technology
Article
Table 1. Mineralogy Compositions of Aquifer Sediments at the Brackenridge Shallow Aquifer (Weight Percentage) sample
quartz
microcline
albite
illite
kaolinite
calcite
dolomite
Brackenridge
50
13
14
4
