T278 Impact of Winter De-Icing Chemicals on the Environment

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room M101AB

ROAD SALT AND THE SALINIZATION OF SURFACE WATERS AND GROUNDWATER OF THE CHICAGO REGION

Paper number: T278-3
Program: Technical Sessions
Time: 1:55 PM – 2:10 PM

Authors

Abstract

Road salt runoff has heavily impacted the water quality of surface water and groundwater in the Chicago, IL region. Almost all parts of the hydrologic cycle are seeing increases in chloride (Cl-), sodium (Na), and total dissolved solids (TDS) concentrations: lakes, streams and rivers, groundwater, and soil water. Most of the rivers and streams being monitored in the region have had significant increases in Cl- and Na concentrations since the mid-1970s. Conversely, there were significantly decreasing trends at most locations for the other major anions, i.e., bicarbonate and sulfate. The rates of the increase in Cl- are in excess of 10 mg/L/yr for several streams, and at some sites the rates of increase appear to be accelerating with time. Baseflow concentrations of Cl- appear to be > 100 mg/L in most streams and rivers in the region, compared to natural background concentrations of Cl- between <1 and 15 mg/L. Concentrations of Cl- and Na are significantly higher during the winter months as a result of direct runoff from freshly salted roadways. Chloride concentrations > 2,000 mg/L have been measured in some streams, which is likely harmful to aquatic biota. In recent years, Cl- and Na concentrations have increased most rapidly in the Fox River Basin west of the Chicago metropolitan area, a region undergoing significant urbanization.

Chloride and Na concentrations are also increasing in shallow aquifers in the Chicago region. Geological conditions (i.e., till thickness) and stormwater management practices appear to be the primary factors affecting groundwater contamination by road salt runoff. 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.