To demonstrate the potential for geologic storage of CO₂ in saline aquifers, the Frio-I Brine Pilot was conducted, during which 1600 tons of CO₂ were injected into a high-permeability sandstone and the resulting subsurface plume of CO₂ was monitored using a variety of hydrogeological, geophysical, and geochemical techniques. Fluid samples were obtained before CO₂ injection for baseline geochemical characterization, during the CO₂ injection to track its breakthrough at a nearby observation well, and after injection to investigate changes in fluid composition and potential leakage into an overlying zone. Following CO₂ breakthrough at the observation well, brine samples showed sharp drops in pH, pronounced increases in HCO₃^- and aqueous Fe, and significant shifts in the isotopic compositions of H₂O and dissolved inorganic carbon. Based on a calibrated 1-D radial flow model, reactive transport modeling was performed for the Frio-I Brine Pilot. A simple kinetic model of Fe release from the solid to aqueous phase was developed, which can reproduce the observed increases in aqueous Fe concentration. Brine samples collected after half a year had lower Fe concentrations due to carbonate precipitation, and this trend can be also captured by our modeling. The paper provides a method for estimating potential mobile Fe inventory, and its bounding concentration in the storage formation from limited observation data. Long-term simulations show that the CO₂ plume gradually spreads outward due to capillary forces, and the gas saturation gradually decreases due to its dissolution and precipitation of carbonates. The gas phase is predicted to disappear after 500 years. Elevated aqueous CO₂ concentrations remain for a longer time, but eventually decrease due to carbonate precipitation. For the Frio-I Brine Pilot, all injected CO₂ could ultimately be sequestered as carbonate minerals. ?? 2010 Elsevier B.V.