Northeastern Illinois is one of the most rapidly expanding metropolitan areas in the country, with a projected population increase of 25% by 2020. Water use increased about 27% from 1980 to 1992 and demand is expected to continue to grow as the population of the region increases. The principal sources of water in the area, Lake Michigan and deep bedrock groundwater, are being used at their legislated and sustainable limits, respectively. The only additional source of water to meet the anticipated increases in water demand are the shallow bedrock and overlying sand and gravel aquifers.

In this study, temporal changes in water quality in these shallow unconfined aquifers using historical data are being evaluated. The Illinois Environmental Protection Agency (IEPA) has sampled approximately 250 shallow (< 200 ft) municipal wells in northeastern Illinois in the last 20 years. Concentrations of most of the major ions and total dissolved solids have increased between 1980 and 1998, especially in the shallowest wells (< 100 ft). Samples from private wells show similar trends.

Linear regressions were performed for forty-four of the 250 municipal wells from the IEPA database that have been sampled three or more times over at least an 8 year period. Thirty-six of the 44 wells tested had positive trends for chloride concentrations and 29 of these had r² values > 0.500. For positive trends, rates varied between 0.1 and 6.4 mg/L-yr, with a median value of 2.3 mg/L-yr. Background chloride concentrations are usually < 10 mg/L; approximately 11% of all tested wells and 21% of the shallowest wells (< 100 ft) had chloride concentrations in excess of 100 mg/L at the last sampling event. Highest concentrations and greatest rates of change tend to occur in the outermost counties, where growth is most rapid and roads are generally uncurbed. It appears that the quality of many of the shallow aquifers in northeastern Illinois is degrading.