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Publication Abstract

Changes in Shallow Groundwater Quality in the Chicago Region in the Past 50 Years. 2008  Illinois State Water Survey, Champaign, IL,  ISWS IEM 2008-01    Full Text Available

Population and infrastructure are growing rapidly in the Chicago, Illinois, metropolitan area. Population is projected to increase from 8 million in 2005 to more than 10 million by 2030. Most of the growth is occurring in counties west and south of Chicago, such as Kane, McHenry, and Will, where populations may double in that period. Demand for water is also increasing substantially. Shallow bedrock and overlying sand and gravel aquifers are expected to be the main water sources to meet the increased demand in the Chicago region.

Unfortunately, shallow aquifers are vulnerable to surface contamination, and there are many potential sources of contamination in urban and suburban areas, including landfills, sewage treatment plants, industrial effluents, atmospheric deposition, septic fields, gasoline storage tanks, and road runoff. The list of potential contaminants is enormous, including various organic compounds, toxic metals, chloride (Cl-), sulfate, and nitrogen (nitrate, ammonium).

Chloride is a particularly useful indicator of aquifer contamination. Although not a primary threat to human health, elevated levels of Cl- make water non-potable and thus there is a secondary drinking water standard of 250 milligrams per liter (mg/L). Chloride is a common contaminant that behaves conservatively in the environment (i.e., does not react and thus is transported at the same velocity as water) and has numerous sources in urban areas.

Where Cl- concentrations exceed background (prior to 1950) levels of 15 mg/L in shallow groundwater in Illinois (Panno et al., 2006), human contamination is almost always to blame. Human sources of Cl- include road-salt runoff, septic and sewage effluent, and landfill leachate. Road salt, which was applied in earnest beginning in the 1960s, has been linked to groundwater degradation in many areas that have snowy climates. Once in groundwater, Cl- and other contaminants can persist for long periods due to slow travel times.

With the likelihood that shallow aquifer use will increase in coming decades, it is critical to determine if the water quality of these aquifers is being degraded. Kelly and Wilson (2008) examined historical shallow groundwater quality data to characterize temporal and spatial changes in water quality in the Chicago metropolitan area. This document summarizes the trends in Cl- concentrations from that report.

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