Modeling and Interpretation of Laboratory and Field Data Showing CO2-induced Groundwater Changes

One of the issues related to the risk assessment of CO2 sequestration is the potential impact of leaking CO2 on shallow groundwater quality. Numerous studies have been conducted using laboratory tests, natural analogues, and numerical models to evaluate the changes in groundwater chemistry induced by elevated CO2 concentrations. In this study, a controlled-release field experiment, integrated with laboratory tests and reactive transport modeling, has been conducted to simulate the release and impacts of CO2 at a geologic storage site. The field test involved a dipole system -- the groundwater was pumped from one well, saturated with CO2 at the pressure corresponding to the average hydraulic pressure of the test formation, and then re-injected into the same formation using a second well. Groundwater quality data were collected in four monitoring wells. At the same time, a series of lab-scale sequential leaching experiments were carried out with sediments from the field site to investigate the effects of the pH decrease and increased dissolved carbonate concentrations resulting from CO2 influx into groundwater. The lab tests showed similar metal release concentrations from sediments exposed to synthetic groundwater solutions saturated with CO2 at aquifer pressures and non-pressurized solutions at pH 5, indicating that the pH drop was the trigger for much of the initial metal release. Field concentration data revealed that a pH drop of ~ 3 units led to a rapid pulse-like release of dissolved cations during injection followed by slowly-rising cation concentrations almost immediately after the pumping and injection period. This phenomenon can be explained by a conceptual model that considers two metal source terms: (a) a fast-reacting but limited pool of reactive minerals that respond quickly to changes in pH, i.e. the dissolution of a limited amount of calcite and the subsequent Ca-driven cation exchange, which leads to the pulse-like initial response, and (b) a slow-reacting but essentially unlimited mineral pool (dissolution of plagioclase and the accompanied cation exchange reactions) to yield rising concentrations upon decreased groundwater velocities after pumping and injection stopped. The release trends of alkali and alkaline earth metals observed in the lab-scale sequential leaching tests are generally comparable with those observed in the field, and can be successfully interpreted with the same model concept. However, faster reaction rates for key minerals are required to match lab data (likely caused by different mixing conditions in laboratory tests). According to our knowledge, this is the only field test of this kind that demonstrates rising metal concentrations after the pumping and injection period. This suggests that the dissolution of slowly-reacting minerals and the ambient groundwater flow rate are two important parameters to be considered when evaluating potential impacts of CO2 leakage on groundwater quality.