HISTORICAL CHANGES IN SHALLOW GROUNDWATER QUALITY IN THE CHICAGO METROPOLITAN AREA

The Chicago, IL, metropolitan area is one of the most rapidly expanding metropolitan areas in the USA, with a projected population increase of greater than 20% 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 aquifers, are being used at their legislated and sustainable limits, respectively. The main 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. Several hundred shallow (< 200 ft) municipal wells have been periodically sampled in northeastern Illinois in the last 20 years, and some of these have been sampled for decades before that. Concentrations of several of the major ions, especially chloride, and total dissolved solids have increased in the last 20 years, especially in the shallowest wells (< 100 ft). Samples from private wells show similar trends. Linear regressions were performed on 184 municipal wells that have been sampled three or more times over at least an 8 year period. Over 80% of these wells had positive trends for chloride concentrations, with rates varying between 0.1 and 21.1 mg/L/yr, with a median value of 1.2 mg/L/yr. The increase in chloride concentrations began in the 1960s, when road salt began being used in large amounts. Prior to 1960, about one-third of samples had chloride concentrations < 10 mg/L and less than 10% had concentrations > 40 mg/L. By the 1990s, 80% of samples were > 10 mg/L, almost 50% > 40 mg/L, and 16% > 100 mg/L. The increases in chloride concentrations are most pronounced in the counties west of Chicago, where land use changes are most rapid and aquifer material is closer to the land surface.