Potential Chemical Impacts of Subsurface CO2: An Integrated Experimental and Numerical Assessment for a Case Study of the Ogallala Aquifer.

Leakage from geologic CO2 sequestration (GCS) reservoirs to overlying underground sources of drinking water (USDW) is a tangible risk. This study is an integrated assessment that combines column experiments and reactive transport simulations of sediments sampled from the Ogallala aquifer above an active commercial‐scale GCS site (the Farnsworth Unit in northern Texas). Experimental and simulation results suggest that carbonate mineral (calcite and dolomite) dissolution is the most significant reaction following CO2 intrusion, and is also the dominant source of trace metal release. Cation exchange is another key mechanism controlling trace metal release by cation interference. Most of the trace metals, including Ba, Sr, As, Pb, and Zn, show a short‐term release and quickly drop to the baseline values, suggesting low risk to the overlying USDW quality. Other trace metals, such as Mn and U, exhibit a tangible increase of their concentrations in the beginning, and drop to a higher level compared to the baseline, which may become a potential concern of long‐term USDW quality change with CO2 introduction. This study provides a comprehensive example of combining laboratory experiments and simulations for assessment of CO2‐sediment interactions with combined release mechanisms in shallow groundwater aquifers. Data presented here provides useful insights for quantitative risk assessment and effective public education regarding CO2 geological sequestration, and trace metal reactive transport studies in shallow groundwater aquifers. Plain Language Summary: Geologic CO2 sequestration (GCS) is an applicable option for reducing greenhouse gas emissions from point sources by injecting CO2 into deep geologic formations. The key concern of GCS is the potential leakage into shallow groundwater aquifers, because CO2 may cause water pH reduction and metal release from sediments. To study CO2‐water‐sediment interactions and potential metal release mechanisms from the sediments, this study combines laboratory column experiments and reactive transport simulations of shallow groundwater aquifer sand samples above an active GCS site. Results of experiments and simulations suggest that dissolution of carbonate minerals is the most significant reaction in CO2‐rich conditions, leading to metal release into aqueous phase. Cation exchange is another mechanism controlling metal release. Most of the toxic trace metals show a short‐term release and quickly drop to a lower value, suggesting low risk to groundwater quality. Metals such as manganese (Mn) and uranium (U) show a stable release with high CO2 level, suggesting a possible risk of drinkable groundwater quality with CO2 leakage. This study provides useful information for risk assessment and effective public education for GCS, as well as reactive transport research of trace metals in shallow groundwater aquifers. Key Points: CO2‐water‐sediment interactions in the largest aquifer in the USA is assessed by combined column experiments and numerical simulationsCarbonate mineral impurity and cation exchange are key mechanisms of cation release in CO2‐rich conditionsManganese and uranium exhibit a stable release during the experiments, which might impact groundwater quality with CO2 intrusion [ABSTRACT FROM AUTHOR]