Future Carbon Capture and Storage (CCS) deployment is likely to use deep saline aquifers for CO2 storage. Small CO2 leakage ( 300 days with three goals: 1) understand how CO2 leakage affects shallow freshwater quality; 2) develop selection criteria for sequestration sites based on inorganic metal contamination caused by CO2 leaks; and 3) identify geochemical signatures for early-detection criteria. After exposure to CO2, water pH declines of 1-2 units were apparent in all aquifer samples. CO2 caused concentrations of the alkali and alkaline earths and some transition metals (Mn, Co, Ni, and Fe) to increase by more than 2 orders of magnitude. Potentially dangerous elements U and Ba continued to increase over the entire 300 days of monitoring in some samples. Mobility of solid phase metals, carbonate buffering capacity, and redox state of the shallow overlying aquifer all control the impact on groundwater quality and should be considered when selecting deep, underlying sequestration sites. Mn, Fe, Ca, and pH should be used as geochemical markers of a CO2 leak because they respond substantially within 2 weeks of exposure to CO2.