Underground sources of drinking water chemistry changes in response to potential CO₂ leakage

The purpose of this study was to quantify changes to underground sources of drinking water (USDW) quality in response to potential CO₂ leakage from geologic CO₂ sequestration (GCS) reservoirs. We developed a framework of combined laboratory experiments and reactive transport simulations and used this framework to evaluate the Ogallala aquifer overlying the Farnsworth Unit (FWU), an active GCS site, as a case study. Using chemical reaction parameters obtained from laboratory experiments and numerical simulations, site-specific mechanisms of CO₂-water-sediment interactions at the USDW aquifer were interpreted. Long-term risks of potential CO₂ leakage were then evaluated with field-scale numerical models using the regional hydrogeological characteristics and reaction parameters obtained from our experiments and simulations. Results suggest that carbonate mineral impurity and cation exchange are key mechanisms for interactions between CO₂ and the aquifer sediment. Additionally, for a large leakage rate of 0.1 % injection from one leaky well, the leakage plume might impact an area of 300 m in diameter and significantly affect the local water quality by changing pH and cation concentrations (e.g., Zn, Ba and Sr). After leakage ceases, the zone of impacted fluids would not migrate significantly in subsequent decades due to a low regional groundwater flowrate (for this case study). The relatively small area of impact might not be detected in a monitoring well given the broader spacing in a typical field scenario. Effective early leakage detection may require additional tools, e.g., borehole CO₂ movement, four-dimensional seismicity, CO₂ soil flux, samples from deeper aquifers, etc., to ensure effective leakage detection and long-term safety of GCS projects.