Task A. Understanding Groundwater Quality Changes in Case of CO2 Intrusion.

 

Contact:	Jens Birkholzer and Chin-Fu Tsang
Key Personnel:	John Apps Yingqi Zhang Liange Zhang Tianfu Xu

 

Proper siting and management of geological storage projects will ensure that the risks of carbon sequestration to human health and the environment are low. However, there are realistic scenarios under which CO₂ could migrate from the deep storage formation(s) to shallower aquifers, and very little research has been done regarding the potential impact of such leakage. In this project, the consequences of CO₂ leakage on groundwater quality are further evaluated to provide sound scientific information to regulators and the public. Injection of high-pressure CO₂ could impact shallow aquifers through several processes (see below figure): (1) CO₂ gas migrating from depth could reach a USDW (underground source of drinking water) and dissolve in the water, with an increased acidity that could enhance the solubility of inorganic hazardous constituents. (2) In deep storage formations, the enhanced solvent properties of CO₂ likely lead to the leaching of organic compounds from organic material in (e.g., benzene and toluene). Subsequent transport of the contaminated CO₂ from depth and intrusion into a USDW could result in the contamination by these hazardous organic compounds. (3) Contaminants such as H₂S, a byproduct of coal gasification, could be co-injected with CO₂. H₂S would preferentially partition into the formation brine, but H₂S-bearing CO₂ could also leak into USDWs and adversely affect water quality. H₂S could furthermore interact with organic matter in the CO₂ reservoir.

Schematic of potential impact of CO₂ storage on groundwater quality

The following three subtasks are conducted during this three-year project to evaluate the potential hydrochemical impact of CO₂ storage projects on USDWs.

Sub-Task A1: CO₂-Related Dissolution of Heavy Metals and Other Constituents in USDWs
In this ongoing sub-task, we investigate the water quality changes upon intrusion of CO₂ into potable groundwater.  The intruding CO₂ would lower groundwater pH and thereby enhance the solubility of hazardous inorganic constituents (including heavy metals).  How and to what extent groundwater quality would be affected depends largely on the initial abundance and distribution of these constituents in the aquifers, as well as on the aquifer mineralogy and the oxidation state.  Using the USGS NWIS (National Water Quality Information System) data base, we have conducted a systematic evaluation of more than 38,000 groundwater quality analyses from aquifers throughout the United States that report non-zero concentrations of selected hazardous constituents.  The results of the evaluation are employed to set up an equilibrium geochemical model of the aquifer chemistry in order to estimate the distribution of each heavy metal between the aqueous phase and adsorption and ion exchange sites, and in solid solution in primary and secondary minerals.  Important qualitative conclusions can be drawn immediately from this evaluation regarding the geochemical vulnerability of the groundwaters. For quantitative evaluation, we use the equilibrium geochemical model as a starting point for reactive geochemical transport simulations that predict the impact of CO₂ intrusion into a fresh-water aquifer and the related changes to the host rock mineralogy and water chemistry.  To validate and support the numerical results, preliminary laboratory experiments are currently conducted, where CO₂ is injected into vessels initially filled with rock fragments and water and periodic aqueous samples are taken to measure changes in the concentrations of the heavy metals.  Our findings will help to understand (1) which aquifer systems and regions of the country might be vulnerable in case of CO₂ intrusion, and (2) which inorganic constituents might adversely affect water quality and to what extent.

Sub-Task A2: Leaching of Organic Compounds
In this future task, we will review and evaluate the distribution coefficients of selected organic compounds (e.g., benzene, toluene) and other potentially hazardous compounds to be identified following further review. The conditions assumed are representative of a deep saline aquifer. We will establish whether the solvent properties of CO₂ to hazardous organic compounds constitute a potential risk to USDW contamination. For example, we will investigate if solved organic compounds may be transported with leaking CO₂ or may be left behind, e.g., at phase change from supercritical to gaseous CO₂.

Sub-Task A3: Impact of Co-Injection of H₂S
In this future task, we will model the injection and transport of CO₂ containing co-injected contaminants into a deep saline aquifer. We will investigate the progressive partitioning of H₂S into the brine during transport and the extent to which H₂S is removed from the CO₂ phase with time and distance. The role of H₂S in inducing precipitation of hazardous metals and sulfides, or their leaching due to sulfide complexation, will be evaluated. This study may be expanded to include other co-injected hazardous trace constituents. The reactive transport code TOUGHREACT will be used for this purpose. Depending on H₂S partitioning results, we will study the impact of H₂S-bearing CO₂ migrating into USDWs.