The chemical composition of groundwater results from the reaction of mineral dissolution and precipitation. We can use the thermodynamic approach to predict water composition under conditions where water and newly formed minerals are in equilibrium. Although some minerals exist in a state of equilibrium with water, other minerals are always unstable. In the latter case, we can evaluate the extent of the overall irreversible mass transfer between minerals and water to quantify the mineral surface area participating in the water–rock interaction. This parameter is fundamental to basic and applied research in areas such as the geological sequestration of CO2 and the safe geological storage of waste.