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Sedimentary rocks with high natural CO 2 concentrations provide invaluable analogues for the long-term engineered storage of CO 2. Some previous studies have reported high trace metal concentrations in sandstone aquifers exposed to CO 2, a cause for concern should stored CO 2 leak into underground sources of drinking water. However, the intensively studied Jurassic sandstone aquifer in the San Rafael–Green River (Utah, USA) area has trace metal concentrations that are within US Environmental Protection Agency's limits for drinking water. Exceptions are As which is plausibly introduced into the aquifer by saline brines external to the aquifer, and salinity which largely is. This shows that CO 2 in aquifers does not inevitably cause trace metal contamination. CO 2–water–rock batch experiments elucidated the controls on the trace metal concentrations. After the addition of CO 2, the experiments reproduce …