The city has been fighting a losing battle to reduce rock salt use since 1977, when they set a target of no more than 2,846 tons each winter. Since then, average annual use has increased by 265%. Missing the goal by so much means we have a complex problem--poorly understood by the public. The myths below persist because the effects of salt are largely invisible.
I like my fries with salt, so it can’t be toxic
Salt--a combination of sodium and chloride--has profound effects on life, especially small aquatic critters like the water flea (daphnia)--which is food for many fish.
Road salt has analogous lethal effects on small aquatic organisms. Scientists found that water fleas started to show acute mortality at chloride levels of 600-1100 milligrams per liter (mg/L). They all died starting at 1900 mg/L. The lethal dose for 17 species of fish, amphibians and crustaceans ranged from 1,440-6,031 mg/L (seven days exposure).
Sampling in February of 2010 by the Health Department found runoff from East Towne private roads and parking reaching 4850 mg/L, and from nearby city streets reaching 5890 mg/L.
Odana Pond (above) receives runoff from the Westgate area and the beltline. From there, water flows towards Lake Wingra. At Odana Pond in early 2008, 2009, and 2010, chloride levels reached 1600, 1200, and over 900 mg/L.
Chloride levels in Lake Wingra have increased at least 16 times above 1940 levels (from about 5 to over 80 mg/L). Besides poisoning aquatic life, salt can have indirect effects. It can reduce the turnover of waters in lakes, leading to oxygen starvation of fish, and it can increase the release of toxic heavy metals from sediments. Elevated salt levels tip the balance in favor of invasive species.
According the Public Health Department, “Over half of the city’s drinking water wells have sodium levels that are trending higher. Three wells are above the United States Environmental Protection Agency’s drinking water guidelines for individuals with a restricted sodium diet.
Ferrocyanide is added to road salt to prevent caking. When dissolved in the presence of light, it produces free cyanide--a potent poison. Rock salt also contains traces of toxic heavy metals.
If salt is a problem--then it’s limited to early spring, or to areas near roads
All Madison’s lakes show steadily increasing levels of salt.
Trees up to 150 feet from a highway can be damaged--sometimes a salty mist from tires can harm trees a mile away. Salt splashes beyond streets, killing grass and gardens. Resulting bare ground erodes, releasing sediment and nutrients to the lakes. Salt degrades soil quality and its ability to absorb rainwater, leading to increased flooding.
Scientists estimate that 55% the salt goes into groundwater. During summer, lakes and streams get much of their water from groundwater seepage. So elevated levels of salt in streams are now persisting into summer and fall. Peaks of chloride during spring thaw are now more harmful, because they add to lingering levels already present.
Salt is a cost-effective way to remove ice
Salt is effective at melting ice, and it seems cost-effective, because the many indirect costs aren’t paid by those who spread.
Salt is very corrosive...
A 2006 report from Madison’s Commission on the Environment summarized the annual costs to US infrastructure:
- Corrosion of concrete reinforcing rods in roads, bridges, and parking structures weakens them. It’s hard to detect and expensive to repair.
- Corrosion costs estimated at $3.5 to $7 billion
- Corrosion protection increases the cost of auto manufacturing by nearly $4 billion.
- Corrosion protection costs estimated at $8.3 billion for highway bridges, and $109 billion for epoxy coating.
- Everywhere, you can see salt damage to gardens and grass near pavement (below). Many species of urban trees are sensitive to salt.
Unquestionably, salt melts ice, and drivers experience better conditions. But this “experience” isn’t the same as safer roads. When more salt is spread, driving speeds increase, resulting in more accidents, according to a study in Canada. There are better ways to increase winter safety-- lower speed limits, a legal requirement for snow tires, and driving less when roads are bad.
We have to shift responsibility for safety back to drivers--driving behavior counts most, not what the city does.
All we need is a “technical fix”
Whenever I discuss salt with my friends, they say “I hear the city is trying beet juice” or some other miracle de-icer. So far, all these magic solutions have failed to be as cheap or effective as salt--or they have unwanted side effects. Some of the proposed substitutes could overfertilize our waterways, or use up the oxygen in waterways, suffocating fish.
To their credit, the Streets Department has frequently tried new de-icers or new techniques, especially since 1992. Despite 19 years of such improvements, salt use still increased by 265%. Increased traffic, increased miles of pavement, and increased public demand has outpaced their good efforts. So it’s abundantly clear that “technical fixes” at the Streets Department aren’t going to solve the salt problem.
The one area where better techniques CAN work is with private business and the shopping malls. They spread an amount roughly equal to the 10,439 tons the city spreads in an average year. There’s evidence that private business is overusing salt, and that better equipment, incentives, and training would help. A proposed bill in New Hampshire would provide limited liability from slips and falls to businesses who have a certified salt program.
What citizens can do
Citizens can help reduce salt damage by shoveling their sidewalks frequently, then applying salt and sand in small amounts, only when ice persists. If grains of salt remain after the pavement dries, then you’ve applied too much. The correct amount is four pounds per 1000 square feet--or about 4 coffee mugs on a sidewalk 184 feet long (x 65” wide). A handheld Scotts applicator can help you spread salt evenly. Salt does not melt ice when applied at temperatures below 15 F.
Deicing of residential sidewalks and driveways contributes only a small amount, compared to the city, county, and business. The larger role of citizens comes through the influence they have in the workplace, or the pressure they put on government agencies. Government overuses salt because citizens demand it. So teaching citizens correct use of salt at home can have benefits by decreasing demand.
Our current “love affair” with salt isn’t sustainable. It’s a complex problem, because the damage is mostly invisible and because so many stakeholders are involved. Right now, concerns for “highway safety” seem to trump all other concerns. We can only gain traction on salt overuse when we learn to balance real highway safety against other equally important concerns, such as environmental health, purity of drinking water, and maintenance of urban structures.
References and additional information
Thanks for the great post. I appreciate all the research you've done.
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