TEMPORAL CHANGES IN SHALLOW GROUND-WATER QUALITY IN NORTHEASTERN ILLINOIS: PRELIMINARY RESULTS

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 ground water, 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.

This study is designed to determine temporal changes in water quality in these shallow unconfined aquifers using historical data. One source of data being evaluated is the Illinois Environmental Protection Agency's (IEPA) ground-water quality data base. IEPA has sampled approximately 250 shallow (< 200 ft) municipal wells in northeastern Illinois in the last 20 years. Forty-four of these have been sampled three or more times over at least an 8 year period. Initial analyses include linear regressions on the chloride data; chloride is a useful indicator of general water quality. Increases in chloride concentrations in urban areas are generally the result of anthropogenic inputs, usually road-salt runoff, sewage effluent, or brine-waste disposal.

Thirty-six of the 44 wells tested had positive trends 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 McHenry and DuPage Counties. It appears that the quality of many of the shallow aquifers in northeastern Illinois is being degraded.