Sensitivity thresholds of groundwater parameters for detecting CO2 leakage at a geologic carbon sequestration site

Geologic carbon sequestration (GCS) projects in the USA are required to monitor groundwater quality for geochemical changes above the injection area that may be a result of CO₂ or brine leakage from the storage reservoir. Should CO₂ migrate into the groundwater around the compliance wells monitoring the shallower hydrologic units, each compliance parameter could react differently depending on its sensitivity to CO₂. Statistically determined limits (SDLs) for detection of CO₂ leakage into groundwater were calculated using background water quality data from the Illinois Basin Decatur Project (IBDP) sequestration site and prediction and tolerance intervals for specific compliance parameters. If the parameter concentrations varied outside of these ranges during the injection and post injection periods of a GCS project, then additional actions would be required to determine the reason for the changes in groundwater concentrations. Geochemical modeling can simulate the amount of CO₂ needed to alter water quality parameters a statistically significant amount. This information can then inform GCS operators and regulators as to which compliance parameters are relevant (sensitive) to CO₂ leakage for a given setting. For the system studied in here, Fe, Ca, K, Mg, CO₂, and pH were sensitive to CO₂ addition while Al, Cl, Na, and Si were not.