The potential for the release of metals into groundwater following the injection of carbon dioxide (CO2) into the subsurface during carbon sequestration projects remains an open research question. Changing the chemical composition of even the relatively deep formation brines during CO2 injection and storage may be of concern because of the recognized risks associated with the limited potential for leakage of CO2-impacted brine to the surface. Geochemical modeling allows for proactive evaluation of site geochemistry before CO2 injection takes place to predict whether the release of metals from iron oxides may occur in the reservoir. Geochemical modeling can also help evaluate potential changes in shallow aquifers were CO2 leakage to occur near the surface. In this study, we created three batch-reaction models that simulate chemical changes in groundwater resulting from the introduction of CO2 at two carbon sequestration sites operated by the Midwest Geological Sequestration Consortium (MGSC). In each of these models, we input the chemical composition of groundwater samples into React??, and equilibrated them with selected mineral phases and CO 2 at reservoir pressure and temperature. The model then simulated the kinetic reactions with other mineral phases over a period of up to 100 years. For two of the simulations, the water was also at equilibrium with iron oxide surface complexes. The first model simulated a recently completed enhanced oil recovery (EOR) project in south-central Illinois in which the MGSC injected into, and then produced CO2, from a sandstone oil reservoir. The MGSC afterwards periodically measured the brine chemistry from several wells in the reservoir for approximately two years. The sandstone contains a relatively small amount of iron oxide, and the batch simulation for the injection process showed detectable changes in several aqueous species that were attributable to changes in surface complexation sites. After using the batch reaction configuration to match measured geochemical changes due to CO2 injection, we modeled potential changes in groundwater chemistry at the Illinois Basin - Decatur Project (IBDP) site in Decatur, Illinois, USA. At the IBDP, the MGSC will inject 1 million tonnes of CO2 over the course of three years at a depth of about 2 km below the surface into the Mt. Simon Formation. Sections of the Mt. Simon Formation contain up to 10 percent iron oxide, and therefore surface complexes on iron oxides should play a major role in controlling brine chemistry. The batch simulation of this system showed a significant decrease in pH after the injection of CO2 with corresponding changes in brine chemistry resulting from both mineral precipitation/dissolution reactions and changes in the chemistry on iron oxide surfaces. To ensure the safety of shallow drinking water sources, there are several shallow monitoring wells at the IBDP that the MGSC samples regularly to determine baseline chemical concentrations. Knowing what geochemical parameters are most sensitive to CO2 disturbances allows us to focus monitoring efforts. Modeling a major influx of CO2 into the shallow groundwater allowed us to determine that were an introduction of CO2 to occur, the only immediate effect will be dolomite dissolution and calcite precipitation. ?? 2011 Published by Elsevier Ltd.