Chemical impacts of subsurface CO2 and brine on shallow groundwater quality.

Leakage from geologic CO2 sequestration (GCS) sites to overlying shallow drinking water aquifers is a tangible risk. A primary purpose of this study is to assess the potential impacts of CO2 leakage into a fresh-water aquifer with associated CO2-water-sediment interactions. The study site is the Ogallala aquifer overlying an active demonstration-scale GCS site in north Texas, USA. Using the results of combined batch experiments and reactive transport simulations, we discuss the effects of salinity on potential trace metal release and the potential for groundwater quality recovery after leakage ceases. RESULTS: suggest that trace metals are released from sediment due to impure carbonate mineral dissolution and cation exchange with exposure to aqueous CO2. Concentrations of Mn, Zn and Sr might exceed the U.S. Environmental Protection Agency's (EPA) limits. After CO2 leakage stops, most cation concentrations decrease to levels observed before leakage quickly, suggesting that water quality may not be a long-term concern. However, saline water that co-leaks with CO2 may increase salinity of a shallow aquifer and induce more trace metals release from the sediment. In most cases, pH is sensitive to even small increases of CO2, suggesting that pH may be a sufficiently sensitive parameter for detecting CO2 leakage.

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.