I saw that before each snowstorm this winter they sprayed something on the roads to prevent freezing. What is it and how does it affect our water after it mixes with snow and runs off?

Answered by Flint Hall Hydrologist for DEQ Dept. of Environmental Quality

Deicers are applied to road surfaces, sidewalks, and parking lots for winter-time safety. Deicers work by reducing the freezing-point of water thereby melting ice, or, when sprayed on the road as a liquid (brine) before a snow storm, preventing ice from forming at all. A wide variety of chemicals are used; Sodium Chloride (NaCl – rock salt), Calcium Chloride (CaCl2) and Magnesium Chloride (MgCl2) are most commonly used in Idaho. Sodium Chloride is the raw material from salt mines while magnesium and calcium chloride are materials processed from salt brine or sea water. Each are most effective at different temperatures, and each have their environmental drawbacks, all due to their general chemistry determined by their position on the periodic table.

All three of these ice melt chemical chemicals can cause damage to trees and grass, and result in harmful impacts to soils and water quality of streams due to their chloride content. High chloride concentrations impact how plant and animal cells use water and other nutrients. De-icer compounds with sodium can cause more damage to living things and to soils then those with calcium or magnesium. An additional concern is that sodium chloride – rock salt – is a natural product that also contains very low levels of other metals. These impacts can be concentrated where water runs off from roadways and parking lots into soils and into surface water. High chloride content can also be damaging to concrete road surfaces and sidewalks and can increase corrosion of metals on our vehicles, additionally, de-icers with magnesium are more damaging to concrete and metal surfaces.

Use of de-icers on roadways is a balance between lives saved and the costs; in tax dollars, and in the impact to the environment, and to our vehicles and roadway. Idaho Department of Transportation has worked with Idaho’s Department of Environmental Quality to understand the impacts and to identify the best combination of de-icers and how to apply them to balance safety and environmental impacts.

Try this at home!

The melting & freezing point of water is the same temperature!


2 quart plastic bags 2 cups of water
1 Digital Thermometer ¾ cup of salt

Put 1 cup of water in each bag. Stir the salt all into 1 bag. Place them both in your freezer. Check every ½ hour. When ice starts to form in each bag record the temperature and the time. Experiment can be repeated with different amounts of salt.

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