INCREASING CHLORIDE IN SURFACE WATERS AND GROUNDWATER OF THE CHICAGO REGION

Chloride (Cl^-) is a common ion found in all natural waters at a wide range of concentrations. Its concentration in precipitation in mid-continental regions is almost always less than 1 mg/L, and often less than 0.1 mg/L. In pre-development Illinois, Cl^- concentrations in surface waters and shallow groundwater (< 100 m) were probably less than 10 mg/L, except in areas where deep basin brines discharged at or near the land surface.

Humans have been actively removing large amounts of Cl^- from geologic storage, especially in the last century. Because Cl^- salts are very soluble, Cl^- exposed at the Earth’s surface invariably ends up in water supplies. Chloride concentrations have been increasing in surface waters and groundwater in urban regions of the northern United States and Canada since the 1960s, including the Chicago region. We have estimated annual fluxes for Cl^- in Illinois from various sources, with the predominant ones being road salt (~ 471,000 metric tons), fertilizer (373,000 metric tons), treated wastewater (315,000 metric tons), livestock (139,000 metric tons), and water conditioning salt (135,000 metric tons). Release of Cl^- to water bodies is greatest in the Chicago region, and elevated levels are observed in the Illinois River far downstream of Chicago. A majority of monitored surface water bodies in the Chicago region have increasing Cl^- concentrations, with concentrations of several hundred mg/L not uncommon. Chloride concentrations have also been increasing in shallow aquifers since the 1960s, particularly in the counties west and south of Chicago. About 43% of public supply wells in these counties have increases greater than 1 mg/L/yr and 15% have increases greater than 4 mg/L/yr.

Increasing Cl^- levels in water bodies of Illinois has important societal costs. Chloride is a significant corrosive agent, and elevated levels may lead to increased costs in drinking water supply treatment, wastewater treatment, and power generation due to increased corrosion to equipment. In rivers, lakes, and wetlands, elevated Cl^- levels may be toxic to aquatic invertebrates and amphibians, increase the mobility of some toxic metals, and decrease the biodiversity of vegetation.