Since the 1950s, chloride (salt) levels in shallow groundwater have increased significantly in Cook and surrounding counties, indicating that the quality of groundwater resources needed to meet future growing demand is deteriorating, according to Illinois State Water Survey (ISWS) researchers.
Walt Kelly, groundwater geochemist, and Steve Wilson, associate hydrologist, analyzed ISWS data from 4,600 private and public well samples collected between 1906 and 2005 in Cook, DuPage, Kane, Lake, McHenry, and Will Counties.
Chloride is a common contaminant of lakes, rivers, and groundwater in urbanized areas from sewage waste, landfills, and, of most importance in the Chicago region, road-salt runoff during the winter. Although chloride in drinking water is not a health hazard, it is a useful indicator of water contamination.
"It's like a canary in a coal mine, a first indicator that other contaminants may follow," Kelly said.
Median chloride levels for all six counties steadily increased from 6 milligrams per liter (mg/L) prior to 1950 to nearly 20 mg/L in samples from 1990 to 2005, with many samples having much higher concentrations. The greatest increases in chloride contamination were found in the western counties surrounding Chicago—DuPage, Kane, and McHenry Counties, and to a lesser extent, Will County.
In DuPageCounty, the median value of chloride increased from 4 mg/L prior to 1950 to 101 mg/L in 1990-2005.
In Kane, McHenry, and Will Counties, most wells with low chloride concentrations are located in areas where there is less urban development. As residential and road development expands in these counties, shallow aquifers where groundwater is stored are more likely to become contaminated by human activities.
Groundwater chloride levels in 1990-2005 samples from Cook (median 25 mg/L) and Lake (4 mg/L) Counties were not as high as expected, given the large population and industrial development in that region. Kelly explained that thicker clay deposits closer to Lake Michigan may be helping to protect groundwater sources from surface contamination.
Another possible protective factor is the curbing of roads, which diverts storm water runoff into streams and rivers. In curbed areas, less rain water and snow melt from roadways is absorbed into the soil to replenish groundwater supplies.
Some counties surrounding Chicago have more road curbing than others; for example, only 6.5 percent of county road miles in Kane County are curbed, compared to more than 60 percent in DuPage County. Still, thick clay deposits are less prevalent in DuPage County than in areas closer to the lake, so more contaminants can reach groundwater sources.
Since groundwater travels slowly, most areas in the Chicago region have probably not yet seen maximum concentrations of chloride.
"Even if all sources of pollution were stopped immediately, it is likely that peak groundwater concentrations of chloride and other surface-derived dissolved contaminants will be higher in the future than they are now," Kelly said. "Water quality of shallow aquifers has degraded and will likely continue to degrade given the ongoing and projected development in the Chicago metropolitan area."
Related research has also shown that the increasing chloride levels in surface lakes, rivers, and streams in the Chicago region may pose a threat to aquatic life, Kelly said.