Changes in the chemistry of shallow groundwater related to the 2008 injection of CO2 at the ZERT field site, Bozeman, Montana

Abstract

Approximately 300 kg/day of food-grade CO 2 was injected through a perforated pipe placed horizontally 2-2.3 m deep during July 9-August 7, 2008 at the MSU-ZERT field test to evaluate atmospheric and near-surface monitoring and detection techniques applicable to the subsurface storage and potential leakage of CO 2 . As part of this multidisciplinary research project, 80 samples of water were collected from 10 shallow monitoring wells (1.5 or 3.0 m deep) installed 1-6 m from the injection pipe, at the southwestern end of the slotted section (zone VI), and from two distant monitoring wells. The samples were collected before, during, and following CO 2 injection. The main objective of study was to investigate changes in the concentrations of major, minor, and trace inorganic and organic compounds during and following CO 2 injection. The ultimate goals were (1) to better understand the potential of groundwater quality impacts related to CO 2 leakage from deep storage operations, (2) to develop geochemical tools that could provide early detection of CO 2 intrusion into underground sources of drinking water (USDW), and (3) to test the predictive capabilities of geochemical codes against field data. Field determinations showed rapid and systematic changes in pH (7.0-5.6), alkalinity (400-1,330 mg/l as HCO 3 ), and electrical conductance (600-1,800 lS/cm) following CO 2 injection in samples collected from the 1.5 m-deep wells. Laboratory results show major increases in the concentrations of Ca (90-240 mg/l), Mg (25-70 mg/l), Fe (5-1,200 ppb), and Mn (5-1,400 ppb) following CO 2 injection. These chemical changes could provide early detection of CO 2 leakage into shallow groundwater from deep storage operations. Dissolution of observed carbonate minerals and desorptionion exchange resulting from lowered pH values following CO 2 injection are the likely geochemical processes responsible for the observed increases in the concentrations of solutes; concentrations generally decreased temporarily following four significant precipitation events. The DOC values obtained are 5 ± 2 mg/l, and the variations do not correlate with CO 2 injection. CO 2 injection, however, is responsible for detection of BTEX (e.g. benzene, 0-0.8 ppb), mobilization of metals, the lowered pH values, and increases in the concentrations of other solutes in groundwater. The trace metal and BTEX concentrations are all significantly below the maximum contaminant levels (MCLs). Sequential leaching of core samples is being carried out to investigate the source of metals and other solutes.

Potential Impacts of Leakage from Deep CO2 Geosequestration on Overlying Freshwater Aquifers

“…This discrepancy may be explained by a stable, but slowly declining pH trend in our data (Figure 2) in contrast with a pH rebound that may have caused reprecipitation of the elements in the Lu et al data. Aluminum and the oxyanion-forming trace metals (e.g., As, Se, Sb, Mo, V, and Cr) decreased in our study and in the Lu et al batch experiment but increased at the MSU-ZERT site, consistent with the behavior of oxyanion-forming metals, which are immobilized in moderately acidic, oxidizing aqueous systems (34,24,27). The coupling of CO 2 plume modeling with laboratory experiments under a range of redox conditions should provide a robust tool for predicting the areal extent and geochemical impact of leakage (20).…”

Modeling Studies on the Transport of Benzene and H2S in CO2-Water Systems

“…116 In the first part of an EPA-funded project on potential effects of carbon capture and geologic storage (CCS) on groundwater resources (Birkholzer et al, 2008), we evaluated the potential for mobilization of trace elements in response to CO 2 intrusion via systematic reactive-transport simulations involving hypothetical release scenarios Apps et al, 2010). Other researchers have investigated this issue by laboratory experiments (e.g., Carroll, 2009;McGrath et al, 2007;Smyth et al, 2009) as well as modeling work integrated with controlled field experiments of CO 2 release in freshwater aquifers (e.g., Kharaka et al, 2010;Ambats et al, 2009). …”

“…Other researchers have investigated this issue by laboratory experiments (Carroll, 2009;McGrath et al, 2007;Smyth et al, 2009) as well as modeling work integrated with controlled field experiments of CO 2 release in freshwater aquifers (Ambats et al, 2009;Kharaka et al, 2010).…”

Determination of free CO2 in emergent groundwaters using a commercial beverage carbonation meter

“…Processes contributing to CO 2 in groundwater include terrestrial respiration, the oxidation of buried organic matter in shallow sediments (Pentecost, 2005), acid neutralization reactions with carbonate minerals (Choi et al, 1998), and migration from magma bodies (De Gregorio et al, 2011;Nordstrom et al, 2005;Ray et al, 2009). The upward migration of fossil fuel-derived CO 2 from proposed deep subsurface geologic disposal sites may also introduce exogenous CO 2 to groundwater aquifers in the future (Kharaka et al, 2010).…”