While lakes have been impacted by road salt runoff in the Chicago region (see my previous 2 posts), the impact on rivers and streams has probably been even more dramatic. Various agencies, including the USGS, Illinois EPA, and Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), have been monitoring was quality in many rivers and canals since at least the 1970s. Almost all of the rivers and streams monitored have had significant increases in chloride (Cl-) and sodium (Na) concentrations since that time.

While lakes have been impacted by road salt runoff in the Chicago region (see my previous 2 posts), the impact on rivers and streams has probably been even more dramatic. Various agencies, including the USGS, Illinois EPA, and Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), have been monitoring was quality in many rivers and canals since at least the 1970s. Almost all of the rivers and streams monitored have had significant increases in chloride (Cl^-) and sodium (Na) concentrations since that time.

Here are 4 sites being monitored by the MWRDGC, showing the change in Cl^- concentrations since the mid-70s:

(CSSC stands for Chicago Sanitary & Ship Canal. The red lines are regressions on 5-point medians.)

Of the 42 MWRDGC river stations that have been monitored regularly between 1975 and 2008, 36 and 32 had significant increasing trends for Cl^- and Na, respectively. The median increase for Cl^- at the stations with increasing trends was 2.7 mg/L per year, with several streams having rates in excess of 10 mg/L/yr. And there’s some evidence that rates are increasing with time. Consider this plot, where the dashed line is a regression between 1975 and 2008, and the solid line is between 2001 and 2008.

There are obvious seasonal controls on Cl^- and Na. Most storm water and snowmelt in the Chicago area is funneled to wastewater treatment plants, then discharged to rivers and canals. This rapid transfer of saline snow melt produces very high Cl^- and Na concentrations in the winter. In snowy winters, large applications of NaCl produce Cl^- and Na concentrations and loads in streams and rivers considerably higher than in winters with low snowfalls:

The largest increases in Cl^- and Na concentrations are in the Fox River watershed, to the west of Chicago. The average increase in median Cl^- values for the four USGS stations in the Fox River between 1981–1985 and 2001–2005 was 116 percent, and 147 percent for two stations on Poplar Creek. The Fox River watershed is primarily rural, but there has been increased urbanization in the watershed, primarily in Kane and McHenry Counties, in recent years. As urban and residential infrastructure increases in this region, the increasing impact of road salt runoff is becoming apparent.

Next post: road salt and groundwater.