The southern Laramie Basin groundwater system was studied to explore how standard thermodynamic chemical calculations can produce misleading results with respect to water-rock interactions when groundwaters with different chemical compositions mix. Geological and hydrological data determined the direction of flow to be NW in the Casper aquifers, as well as in the Redbeds aquifers. Hydrological and structural data were used to identify wells having mixtures of groundwater. The chemical data revealed that waters in the different aquifers are distinguishable by their chemical compositions and that mixtures have intermediate compositions. The different chemistries of the mixed groundwater samples are identical to those predicted by geochemical evolution paths, including calcite precipitation, evolution toward equilibrium with gypsum, and Ca-Mg ion exchange. ¿ American Geophysical Union 1993