Road salt runoff has heavily impacted the water quality of surface water and groundwater in the Chicago, IL, region. High salt contents in surface water and groundwater pose a significant threat to aquatic ecosystems and to infrastructure and industrial operations. Almost all of the rivers and streams monitored have had significant increases in chloride (Cl−) and sodium (Na) concentrations since the mid-1970s. Rates of Cl− increase for several of the streams are in excess of 10 mg/L/yr. Concentrations of Cl− and Na are significantly higher during the winter months as a result of direct runoff from freshly salted roadways. In recent years, Cl− and Na concentrations have increased most rapidly in the Fox River Basin west of the Chicago metropolitan area, where land use is rapidly changing from rural to urban. Chloride and Na concentrations are also increasing in shallow aquifers in the Chicago region. Surface waters currently have approximately equimolar concentrations of Cl− and Na, while groundwater impacted by road salt tends to have an excess of Cl− relative to Na, suggesting Na retardation in the subsurface, likely due to cation exchange. A rough estimate of inputs and outputs of Cl− in the Chicago region suggests that most of the road salt applied in a year is removed by surface discharge. However, about 14 percent of the road salt is retained in the subsurface, approximately 50,000 metric tons of NaCl annually, representing a long-term source of Cl− and Na and other associated ions.