In my last post I presented some data showing increasing chloride concentrations in groundwater in the Chicago region. One of the interesting things was that concentrations and rates of increase were lower in Cook and Lake Counties, compared to counties west or south of Chicago. We believe there are several reasons for this.
First, most of the major roads in Cook and Lake Counties are curbed. Thus storm water runoff is diverted to the combined sewer systems, eventually winding up in Chicago’s wastewater treatment plants, where it is treated and discharged to canals and streams. This limits the recharge of contaminated water to aquifers. On the other hand, there are fewer impermeable surfaces and a lower percentage of curbed roads in Chicago’s collar counties, which results in more recharge of contaminated water to the shallow groundwater system. For example, only about 10 % of roadways in Will County and 6.5 % of county road miles in Kane County are curbed (information from P. Killinger, Will Co. Dept. of Highways, and P. Holcomb, Kane County, Div. of Transportation).
Another possible explanation for the greater increase in chloride concentrations in the western and southern counties is geological. Namely, unconsolidated sand and gravel deposits are generally thicker and closer to the surface, especially McHenry and Kane Counties, compared to Cook and Lake Counties. Areas where aquifer material (usually sand and gravel) is within 50 feet of the surface in northeastern Illinois are plotted in gray in figure (b) below, along with most recent chloride concentrations:
Wells with the greatest chloride concentrations (and rates of change) are often found in areas where aquifers are within 50 feet of the surface. While geological factors probably play some role, land use is still the most important factor. In Kane and Will Counties, for example, most of the wells with low chloride concentrations are in the western and southern sections, respectively, where there is less urban and suburban development. In figure (a) above, the gray areas are municipalities.
It appears that because most roadways in Cook and Lake Counties are curbed, saline runoff is being channeled to storm water retention and not recharging aquifers. Results from monitoring wells near I-94 indicate that where curbing is absent in the city of Chicago, chloride concentrations in shallow groundwater can reach extremely high levels (> 3500 mg/L). This is an important point, as recently there has developed a consensus among water resource managers that a goal of storm water management should be to maximize infiltration viz-a-viz runoff. If the quality of the recharge to groundwater is poor, then fixing one problem (storm water runoff) may produce another (decreasing groundwater quality).
Based on trends in chloride concentrations, many areas in the Chicago region have not yet seen maximum concentrations. With recharge rates giving average groundwater travel times of 0.2 to 1.0 m/yr, the maximum distance traveled since 1960 for the bulk of recharge from the surface is less than 150 feet. Clearly there are areas where there have been significantly more rapid travel times, as evidenced by elevated concentrations in wells deeper than 150 feet. However, even if all sources of pollution were stopped immediately, peak concentrations of surface-derived dissolved contaminants will almost surely be considerably higher in the future than they are now.
There are no easy solutions to the problems of road salt runoff. Many public officials are aware of the issues and have taken steps to reduce application rates, but there is still a strong societal desire to keep roads clear following snow events. This and the fact that road salt is much more inexpensive than alternative deicing agents means that large amounts of road salt will continue to be applied in the future. Its high solubility means that it will continue to find its way into water resources, complicating effective water resource management.
