An idealized sequence of chemical changes in groundwater. As groundwater moves through rock its chemical composition normally changes. In general, the longer groundwater remains in contact with the aquifer rocks the greater the amount of material it will take into solution. Changes in composition also occur with increasing depth of travel, as bicarbonate anions, which dominate in many shallow groundwaters, give way to sulphate and then chloride anions, and calcium is exchanged for sodium.