Impacts of road salt on water resources in the Chicago region

Over 270,000 tons of road salt, primarily as halite (NaCl), is applied to roads in the Chicago region during an average winter. The "bare pavement" policy that exists for major roads allows traffic to flow at close to normal volumes during snow events. However, most of the chloride eventually is transported to water bodies. Recent research has indicated that chloride concentrations have been increasing in groundwater and surface waters in the Chicago region since the 1960's, when road salt began to be applied in earnest. More than half of shallow public supply wells in the region have increasing trends in chloride concentrations, especially in counties west of Chicago. In surface water bodies, large spikes are seen during the winter and early spring, and are observable in the Illinois River well downstream of Chicago. While there are undeniable safety and economic benefits to applying road salt to roads in the Chicago region, there are many potential detriments as well. Chloride is a significant corrosive agent to steel in road beds and bridges. Elevated chloride levels in potable aquifers may lead to increased water supply treatment costs due to increased corrosion to equipment and the need to remove chloride to meet the drinking water standard. In surface waters, elevated chloride levels may be toxic to aquatic invertebrates and amphibians, increase the mobility of some toxic metals, and decrease plant diversity. Record amounts of road salt were applied in the U.S. in two of the past three winters. Currently, alternative deicing agents are significantly more expensive. Assuming that the "bare pavement" policy will continue, projected increases in population and roads in the Chicago region will almost certainly lead to increased chloride contamination of water resources. Planners will have some difficult decisions with respect to deicing policies in the next few decades. They must also be aware of the interconnectedness of water resources. For example, policies aimed at directing storm water runoff away from discharge to surface water bodies and into groundwater recharge have been promoted. While this may protect surface water bodies from further degradation, it will increase chloride concentrations in shallow aquifers.