Being primarily a groundwater guy, my initial interest in road salt runoff was whether we could detect it in aquifers. The Illinois State Water Survey (ISWS) has a groundwater quality database going back over 100 years, and since road salting started taking off around 1960, I figured we might be able to see increases in chloride and total dissolved solids (TDS) concentrations in well samples. Short answer: Yes we do.
Consider this figure which lumps all the samples we have in our database from shallow wells (200 feet or less) in the 6-county Chicago region (Cook, Lake, DuPage, Kane, McHenry, Will).
(This is a box-and-whisker diagram, which is a graphical representation of basic statistical values. The horizontal line in the middle of the “box” is the median value, the top and bottom of the box are the 75th and 25th percentile values, the “whiskers” are the 90th and 10th percentile values, and the dots are 95th and 5th percentile values.) Chloride (Cl-) concentrations have been increasing in shallow groundwater since the 1960s, and the increasing trends are statistically significant. And you can see that the shallowest wells (< 100 feet) have been increasing the fastest, which makes sense for a surface-derived contaminant such as road salt.
If we split the data out by county, we get a more complicated picture:
Chloride concentrations have increased significantly for the collar counties to the west and south of Chicago, most obviously for DuPage. The increases for McHenry and Will Counties weren’t significant (statistically speaking) until the 1970s, and for Kane County not until the 1980s. On the other hand, the increase in chloride concentrations was not significant in Cook County until the 1990s, and there were no significant changes in Lake County.
These findings at first puzzled me, because I figured that the older, more urban counties, especially Cook, should have the greatest chloride concentrations and most rapid increases. There are several potential explanations that I’ll explain more fully in my next post, but these include both infrastructure effects (i.e., curbed streets and combined sewers) and differences in geology.
One more figure for this post:
This figure shows changes in chloride concentrations for individual wells over long periods of time. These are all public supply wells in the Chicago region, selected somewhat randomly to show the different behaviors that are observed. The legend gives the depth in feet for each well. As you can see, chloride has been increasing in most of the wells, with the exception of Cook and Lake Counties. About 55% of the almost 250 wells that we analyzed had increasing trends that were statistically significant, only 3% had significant decreasing trends (the rest showed no significant trend). In DuPage, Kane, McHenry, and Will Counties, more than 60% of the wells (71% in Kane) had significant increasing trends. We estimated the increase in chloride concentrations to be at least 4 mg/L per year in about 12% of the wells.
Next post: I’ll discuss some of the results in more detail and talk about why the results are important and what are options are.
[For more details, we published a paper and a report on these results a few years back.]