Hydrogeochemistry of carbonate groundwaters of an urban area

A comparison of the groundwater chemical data from the shallow dolomite aquifer of Northeastern Illinois Metropolitan Area (NIMA) from August 1972 through April 1973 with data collected 12–40 yr earlier indicates that concentrations of all ions have increased, the greatest increase occurring in the south‐eastern part of the area where the overburden is thinnest. The largest increases are 843, 142, and 91% in Cl−, SO42−, and Na+, respectively. However, the spatial distribution of the groundwater chemical types has remained the same with time. The groundwater was found to be apparently supersaturated with respect to calcite and dolomite; however, the two saturation indices were reduced by 20 and 30%, respectively, after considering the effect of ion pairings. The chemical character of the groundwater is controlled, among other factors, by the change of carbon dioxide content of the water along the groundwater flow path. A model for the solution of dolomite based on the standard equations for carbonate equilibrium is presented. The application of the model to the study area indicates that a variety of geologic conditions between the dolomite aquifer and the overlying glacial drift prevail in the area.