Impact of elevated dissolved CO_2 on aquifer water quality

Abstract

Carbon capture and storage (CCS), specifically by means of geologic sequestration (GS), is a developing technology to reduce CO2 emissions to the atmosphere. This technology involves separating CO2 from flue gas and transporting the CO2 to underground storage locations that are isolated from the atmosphere. These storage locations are typically permeable and porous geologic formations that are not useful for any other purpose, such as drinking water. Geologic carbon sequestration operated at full-scale will require extensive performance monitoring, including potable groundwater monitoring. However, researchers and regulators do not fully understand what impact elevated CO2 levels would have on groundwater quality in the event that CO2 should leak into an overlying aquifer. The focus of the current study was to thoroughly characterize the properties of a typical Gulf Coast potable aquifer for purposes of performing a controlled CO2 release experiment and to construct coupled geochemical and transport models capable of predicting impacts from CO2 migration into a drinking water aquifer. The aquifer is a methanogenic environment composed primarily of quartz and feldspars, with minor or trace amounts of pyrite, mica, illite, smectite, and kaolinite. The formation water is dominantly Na-HCO3, consistent with the theory and PHREEQC modeling results that suggest aquifer freshening and ion exchange have played dominant roles in determining the present-day dissolved major ion composition. This study also presents the design and implementation of a closed loop pumping and injection system designed to simulate CO2 leakage into a test site aquifer. Process monitoring results indicated that the test was performed with minimal variation in key process parameters, including temperature, pressure and injectate pH. In situ instrumentation deployed in monitoring wells allowed continuous readings of groundwater pH and conductivity, which were critical parameters for evaluating the aquifer response to carbonation and acidification. Successful modeling simulation of the pH response using results from the aquifer testing program suggested that the test was implemented and monitored appropriately and that that future data interpretations and modeling of the field experiment were not compromised by test design. Test results showed that no constituent was mobilized in excess of US EPA maximum contaminant levels, but that many constituents (primarily major and minor cations) were released in a pulse-like response at levels above their baseline concentrations. Dissolution of trace carbonate and pyrite in the aquifer are hypothesized to have triggered cation exchange reactions, a dominant geochemical process affecting major and minor cation behavior in the aquifer. Overall, the test has shown that the migration of carbon dioxide into a drinking water aquifer can mobilize ions into solution, but at levels that may not exceed EPA MCLs under the field conditions tested for this specific system. Data presented here are potentially applicable to assessments across the Gulf Coast, where the potential for deep geologic carbon sequestration and continued reliance upon groundwater resources are high.