Road salt background and history
The Wisconsin Department of Transportation has used rock salt to remove ice since the 1950s. In 1956, they adopted a “bare pavement” policy for state highways.
This policy increased motorists’ expectations for road maintenance. The increased expectations persist today.
Salt use reduction efforts began at both the state and city level in 1973.
How road salt works
Vehicles driving on snow-covered roads leads to ice forming on the pavement. Plowing is usually not enough to prevent the layer of ice from forming.
De-icers are used to prevent or remove ice. De-icers work by getting lowering the freezing point of water. The de-icer ions get in the way of ice forming a solid structure, keeping it liquid for longer.
Road salt is usually made up of sodium and chloride. When road salt is applied, it melts and splits into two parts: sodium ions and chloride ions.
- The sodium ions become a part of the soil.
- Chloride ions end up washed away by rainwater into our lakes or groundwater. As a result, chloride concentration in our water resources continues to grow with each salt application.
The only ways to mitigate chloride are through rainwater diluting it in lakes and groundwater, or by salty water flowing out of lakes or groundwater.
Why chloride is harmful
Chloride in surface waters is toxic to aquatic life. Heavier, salt-laden water can also accumulate at the bottom of lakes and cause a condition known as "meromixis", where water near the bottom does not circulate during seasonal turnover.
The Wisconsin Department of Natural Resources (DNR) has set the chronic toxicity criterion for chloride to 395 mg/L. This maximum 4-day concentration ensures adequate protection of aquatic life if not exceeded more than once every 3 years.
Chloride affects the taste and drinkability of our water. The Environmental Protection Agency (EPA) has set a secondary standard for chloride in drinking water at 250 mg/L. Secondary standards are advisory for aesthetic considerations. There is no health-based standard for chloride in drinking water.
Chloride also increases the corrosiveness of water, which damages bridges, concrete roadways and structures, and automobiles. The results can be disastrous:
- In 2015, the National Highway Traffic and Safety Administration found 2008 and earlier vehicles could experience unexpected brake failure due to corrosion caused by road salt.
- The Algo Centre Mall in northern Ontario collapsed from corrosion to steel supports caused by salt water from a leaky roof top parking lot.
- Road salt was a contributing factor in the lead contamination of drinking water in Flint, Michigan.
- The former Highway 19 Bridge over the Yahara River grew noticeably more corroded over a six-year span. An accounting of the cost of damages caused by road salt would be extensive and difficult to estimate.
Why we use road salt
De-icers are important for public safety, and we don’t have great alternatives.
Even though road salt damages our infrastructure, water resources, and our environment, it is widely considered cheap and effective. Wisconsin experiences many freeze-thaw cycles throughout the winter season, which can cause dangerously slick roadways. Thus, some level of anti-icing/de-icing road maintenance is necessary to keep roads passable for emergency equipment and commerce.
Chloride salts of calcium and magnesium were some of the first alternative deicers used. While they both work at lower temperatures than sodium chloride (rock salt), they still contribute chlorides to the environment. Also, they are not as efficient as sodium chloride, and don’t lower the freezing point of water as much.
Calcium magnesium acetate (CMA) is another alternative that has seen limited use for a long time. It is much less corrosive than chloride salts, but it reduces oxygen levels in water. It also has a higher effective minimum temperature.
Other less corrosive alternatives have been developed. These are usually a combination of a chloride salt and an organic, phosphate, or metal constituent that add oxygen demand, nutrients, or metals to the environment along with a lower level of chloride than straight chloride salts.
Sodium chloride remains the most commonly used deicer. If deicers are necessary for safe roads, and if road salt is the best option, then reducing the harmful effects of deicing can only be achieved by reducing the amount applied.
