An Evaluation of Temporal Changes in Shallow Groundwater Quality in Northeastern Illinois Using Historical Data

The rapid increase in population and developed land in the Chicago, Illinois, metropolitan area has placed a heavy demand on water resources. Water sources most likely to be developed in this region during the next few decades are shallow aquifers. Since shallow aquifers are vulner-able to surface-derived contaminants, the increase in developed land may be escalating the rate at which groundwater quality is being degraded. A statistical study of historical groundwater quality data was undertaken to determine if urbanization activities have affected shallow ground-water quality. Of the major ions, chloride (Cl-) concentrations have shown the largest increases in the region, due primarily to road salt runoff. In the majority of shallow public supply wells in the western and southern collar counties of DuPage, Kane, McHenry, and Will, Cl- concentra-tions have been increasing since the 1960s. About 43 percent of wells in these counties have rate increases greater than 1 milligram per liter per year (mg/L/yr) and 15 percent have increases greater than 4 mg/L/yr. Approximately 24 percent of samples collected from public supply wells in the Chicago area in the 1990s had Cl- concentrations greater than 100 mg/L (35 percent in the western and southern collar counties), compared with median values of less than 10 mg/L prior to 1960. The greater increase in Cl- concentrations in the western and southern collar counties compared to Cook and Lake Counties is likely due to both natural and anthropogenic factors, including the presence of more significant and shallower sand and gravel deposits and less curbing of major highways and streets.