The City of Madison, Wisconsin began spreading road salt on its streets in 1959. In the next 15 years ( by 1974), the chloride concentration in Lake Wingra tripled, and by 2014 had increased nearly seven fold from 18mg/L in 1959 to nearly 120 mg/L in 2014, according to the 2014 Road Salt Report by Public Health Madison & Dane County.
Chloride concentrations are measured in milligrams per liter (mg/L).
The 2014 road salt report states that “Lake Wingra chloride concentrations have been increasing at the rate of about 2 mg/L, per year since 1962. The rest of the Yahara Lakes have seen average annual chloride increases of about 1 mg/L.”
Chloride concentrations in Lakes Monona, Mendota, Waubesa, and Kegonsa lag behind the concentrations in Lake Wingra (Figure 2 in the Road Salt Report—2014 ). The lower concentrations in these other lakes is perhaps a dilution effect, explained by the fact that they are larger and deeper and thus contain more water.
According to the 2014 Road Salt Report, the best estimate of annual road salt usage in Madison in 2014 was about 15,000 tons; Dane County applied an additional 50,000 tons of road salt in 2014.
Chlorides Also Found in Natural Springs
Public Health Madison & Dane County has monitored three city springs since 2003 (Figure 4 in the report) and documented that chloride concentrations have increased since monitoring began. Researchers found that the rate of increase in these shallow groundwater reserves in slower rate than that found in our lakes.
Two of the monitored springs are in the Lake Wingra watershed: Big Spring in the Arboretum on the south shore of Lake Wingra, and Nakoma Spring (aka Duck Pond Springs), also on Arboretum property and on the north shore of Lake Wingra. The third monitored site is East Spring which is the headwaters of the East Branch of Starkweather Creek in a heavily urbanized area.
The chloride source for the Nakoma or Duck Pond Springs is likely explained by its location at the base of an urban neighborhood with its grid of steep city streets, the busy Nakoma Road and the adjacent sidewalk. Nakoma Road is a heavily traveled auto commuter artery and a bus route, both of which receive priority road salting and sanding by city crews. The sidewalk is also maintained in the winter by city snow removal crews.
Arboretum Big Spring
The source of the chloride found in the Big Spring is harder to explain. It’s groundwater recharge area is likely—at least partially—within a wooded portion of the Arboretum. The spring is also about ¼ mile from the nearest road—Arboretum Drive. Responsibility for winter maintenance of Arboretum Drive is split between crews from the Town of Madison and from the University. Neither unit strives for a “bare pavement” and instead they plow early and often and both are sparing in their use of a sand/salt mix which is applied only hills, curves, and intersections.
Perhaps airborne salt spray and/or salt-laden snow melt/storm water runoff from the Beltline Highway that infiltrates into the shallow groundwater is the source. More research is needed.
History of Road Salt Use in Wisconsin
In 1956 the Wisconsin Department of Transportation (WIDOT) adopted a “bare-pavement policy” which required “continuous snow plowing all through a storm and salt application rates averaging 400-1200 pounds per lane mile. (Road Salt Report—2014, page 2). In 1959 the City of Madison followed suit and began applying road salt (Figure 1 in 2014 Road Salt Report).
In the mid-1970’s a variety of factors caused state and city officials to rethink the bare pavement policy. First was the high labor costs of continuous plowing; secondly, increased fuel costs caused by the oil crisis of 1973 played a role in changing policies; and thirdly the environmental impacts of high salt use were becoming obvious. For example, chloride concentrations in Lake Wingra had tripled in the 15 years since road salt was first applied in 1959.
As a result, both the state and the city adopted various policies to reduce the use of road salt as a deicer. In 1973 the City of Madison adopted a Wingra Watershed salt reduction plan (50% of normal application rates); then in 1977 the 50% reduction plan was extended city-wide. Both attempts were short-lived. In the face of strong public outcry, by 1980 the city was back to the pursuit of bare pavement and the widespread application of road salt. That action signaled “the end of effective means of curtailing the use of road salt.” (Road Salt Report—2014, p 3). The data in the report show that after 1980, chloride concentrations in our surface and groundwater supplies have steadily increased.
What Environmental Impacts from Road Salt Can We Expect in the Future?
Although the chloride levels in surface and groundwater are not to the point where they constitute a threat to public health—the chronic toxicity criterion is 395 mg/L—continued application of road salt and surges from storm water/snowmelt, according to the 2014 report, could pose a threat to human health, fish, and other aquatic organisms.
After all these years of road salt application, chlorides have built up to such a level in the soil that their discharge into surface and ground water is a year-round event. Even if we stopped applying road salt now, it would take a number of years for the residual chlorides in the soil to be depleted.
The widespread and chronic presence of chlorides in our surface and groundwater, and in the soil make it difficult, if not impossible, to restore or even to just maintain natural plant and animal communities.
What Would it Take to Change Winter Driving Expectations?
Any gains made in reducing salt application rates through efficiencies and increased effectiveness are offset by additional road, sidewalk, and parking lot construction as the city expands. Because the public has come to expect “summer driving conditions in the winter,” it will be difficult for agencies to change their road salt application policies.
As long as public expectations insist on bare pavement in the winter, road salt use will continue.
As long as cars are the primary way to get around, then public expectations for winter driving conditions will remain unrealistic.
As long as there are insufficient transit alternatives such as more bike routes and additional bus service-for example, there is no public transit to either the Dane County Regional Airport or to the Dane County Expo Center–public demand for snow and ice-free roads will continue.