Geochemical modeling of changes in shallow groundwater chemistry observed during the MSU-ZERT CO2 injection experiment

Abstract

a b s t r a c tA field experiment involving the release of carbon dioxide (CO 2 ) into a shallow aquifer was conducted near Bozeman, Montana, during the summer of 2008, to investigate the potential groundwater quality impacts in the case of leakage of CO 2 from deep geological storage. As an essential part of the Montana State University Zero Emission Research and Technology (MSU-ZERT) field program, food-grade CO 2 was injected over a 30 day period into a horizontal perforated pipe a few feet below the water table of a shallow aquifer. The impact of elevated CO 2 concentrations on groundwater quality was investigated by analyzing water samples taken before, during, and following CO 2 injection, from observation wells located in the vicinity of the injection pipe, and from two distant monitoring wells. Field measurements and laboratory analyses showed rapid and systematic changes in pH, alkalinity, and conductance, as well as increases in the aqueous concentrations of naturally occurring major and trace element species.The geochemical data were evaluated using principal component analysis (PCA) to (1) understand potential correlations between aqueous species, and (2) to identify minerals controlling the chemical composition of the groundwater prior to CO 2 injection. These evaluations were used to assess possible geochemical processes responsible for the observed increases in the concentrations of dissolved constituents, and to simulate these processes using a multicomponent reaction path model. Reasonable agreement between observed and modeled data suggests that (1) calcite dissolution was the primary pH buffer, yielding increased Ca +2 concentrations in the groundwater, (2) increases in the concentrations of most major and trace metal cations except Fe could be a result of Ca +2 -driven exchange reactions, (3) the release of anions from adsorption sites due to competitive adsorption of carbonate could explain the observed trends of most anions, and (4) the dissolution of reactive Fe minerals (presumed ferrihydrite and fougerite, from thermodynamic analyses) could explain increases in total Fe concentration.

Developments since 2005 in understanding potential environmental impacts of CO2 leakage from geological storage

“…It is also being used to interpret observed groundwater changes at CO 2 injection field experiments Yang et al, 2013a;Yang et al, 2014b;Zheng et al, 2012). For these studies, the most commonly applied models are TOUGHREACT (Xu et al, 2004) and PHREEQC (Parkhurst and Appelo, 1999).…”

“…In other cases, complex mineralogical characterization of geological formations has been obtained from core analyses (e.g. XRD, SEM-EDS) (Keating et al, 2010;Keating et al, 2013b;Viswanathan et al, 2013;Zheng et al, 2012;Zheng et al, 2009). Such complex mineralogy allows one to predict impacts on water composition and the precipitation / dissolution of the main minerals (Humez et al, 2011;Zheng et al, 2012;Zheng et al, 2009).…”

“…Sulphides and carbonates are mainly considered in models (Keating et al, 2010;Navarre-Sitchler et al, 2013;Siirila et al, 2012;Wang and Jaffe, 2004) but sorption on clays and oxides may also affect trace element mobility as they can be a sink or a source (Humez et al, 2013;Viswanathan et al, 2013;Zheng et al, 2012;Zheng et al, 2009). Although sorption processes (ion exchange and complexation surfaces) are not systematically implemented in all codes, they can significantly reduce metal mobility and thus should be considered.…”

Evaluation of Potential Changes in Groundwater Quality in Response to CO2 Leakage from Deep Geologic Storage

“…The purpose of their study was to understand the resulting pH changes and to evaluate the detectability of the acidic plume in the aquifer; the issue of mobilization of hazardous trace elements was not addressed. Zheng et al (2009) recently reported on a field experiment with CO 2 injection into a shallow aquifer in Montana. Preliminary groundwater analyses suggest that the dissolution of CO 2 leads to the mobilization of trace metals.…”

Assessment of CO2 Injection in Fractured Calcite Rock Clogged by Pseudomonas putida Biofilm

“…However, possible accidental leakage of CO 2 could degrade the water quality of overlying shallow aquifers [1][2][3][4][5]. Although numerous studies published in the literature have focused on the impact of pH decrease associated with CO 2 leakage on mobilization of naturally occurring hazardous trace metals and ions in aquifers due to processes involving desorption and dissolution [1,6,7], the effect of CO 2 leakage on bacteria and biofilms in shallow overlying aquifers has received very little attention [8][9][10].…”