We performed an injection of CO2-saturated water in a shallow aquifer following a 'push-pull' test protocol. A specific protocol was designed to measure in situ fluid pH and redox potential with careful sampling. We found increases of dissolved calcium, magnesium, alkalinity, iron and manganese, and other trace elements. Concentrations of Fe resulting from reactivity were estimated using measured concentrations of Fe corrected by a calculated fluid dynamics coefficient. Thermodynamic equilibrium calculations suggested that ferrihydrite Fe(OH)3 dissolution is the main source of iron release. The kinetic rate of Fe(OH)3 dissolution estimated by a surface protonation model indicates that the reaction order is two. Since laboratory experimental results show a reaction order of zero, we propose that the mechanism of ferrihydrite dissolution proceeds by a more complex mechanism under natural conditions.