Here’s an a propos chart on this, the first day of dealing with actual below-freezing precipitation on Twin Cities streets. That said, I can’t pretend to really understand what’s going on here, other than that the trend is worsening.
First, I should say that chloride is salt, and salt in this case refers to the act of salting our roads to make them easier to drive around in the wintertime. There was a great article recently in Minnpost that referenced a study about the effects of road salt on lakes, and this chart is from that report.
OK here it is. A chart showing both the amount of impervious surface (pavement / roads / parking lots) and the amount of chloride in Minnesota lakes over time, with a lot of info:
And here’s the close-up of Minnesota, showing the amount of chloride over time:
And finally an explanation of what you’re looking at here, FWIW, from the study:
(A) Distribution of impervious land cover within a 500-m buffer of all lakes >4 ha in the lower 48 United States (n = 149,350). Black squares represent the median impervious land cover percentage in each state. Thick horizontal black lines denote the interquartile range of the distribution, and thin black lines extend to 1.5 times the interquartile range. The vertical dashed line is shown at impervious land surface = 1%. Circles represent lakes included in this study, colored by slope (yellow, negative slope; purple, zero slope; red, positive slope). Due to the frequency of zero values on the x axis, circles are spread out within the gray rectangle. Percentages following y axis labels represent the percent of lakes in that state with greater than 1% impervious land cover within a 500-m buffer. In states with >10 lakes present in the dataset, an asterisk denotes that the sampling distribution in our dataset was significantly different from statewide distribution (Mann–Whitney test, P < 0.05), and ^ denotes that the sampling distribution was not significantly different from statewide distribution. (B) Chloride trends, as represented by linear regression model fits, are shown for four states with relatively large sample sizes (New York, Minnesota, Wisconsin, and Rhode Island). The dotted gray line demarcates the EPA’s aquatic life criterion of 230 mg L−1.
The Minnpost article, by Greg Breining, suggests that road salt is something to which we need to pay much more attention.
As recently as 2014, when biologist Rick Relyea began studying the effects of salt-laden runoff at Rensselaer Polytechnic Institute, “the world of science didn’t pay very much attention to the impacts of road salt on water,” he says. “Now we’re paying much more attention.”
Recent research is showing that in many waterways, chloride is on a persistent upward trend, with potential to harm aquatic communities and even impair drinking water.
Dugan’s big-picture look at North American lakes squares with Asleson’s finer-grained analysis of Minnesota’s Twin Cities, in which 19 lakes currently exceed the water-quality standard for chloride. And chloride concentrations were increasing in most Twin Cities lakes.
“When you have a watershed area that has a road density of 18 percent or greater [in the entire watershed], that’s where you’re most likely to see water quality problems because of winter deicing salt,” says Asleson.
With great amounts of both winter and lakes, this is certainly a central issue here in Minnesota. Keep that in mind as you’re compplaning about ice on the roads!