Road salt

An Econ-brand salt spreader spreading salt on a road during the 2010 UK winter.

Road salt (also known as de-icing salt or snow salt) is a salt used mainly as an anti-slip agent in winter road conditions, but also to prevent dust and snow build-up on roads. Various kinds of salts are used as road salt, but calcium chloride and sodium chloride (rock salt) are among the most common. The more expensive magnesium chloride is generally considered safer, but is not as widely used because of its cost and effect on structural integrity. When used in its solid form, road salt is often pre-wet to accelerate the ice-melting process.

Spreading

Road salt and brine are generally spread using a winter service vehicle called a salt spreader. Salt spreaders are typically added to trucks, loaders, or in the case of brine, tankers. The salt is stored in the large hopper on the rear of the vehicle, with a wire mesh over the top to prevent foreign objects from entering the spreading mechanism and hence becoming jammed. The salt is generally spread across the roadway by an impeller, attached by a hydraulic drive system to a small onboard engine. However, until the 1970s, it was often spread manually either by workers shoveling salt from trucks or by smaller wheelbarrow-like vehicles, the latter still being used today for personal use. Some older spreading mechanisms still require it to be manually loaded into the impeller from the hopper.

Mechanics

Salt for use of melting ice and snow works through a phenomenon called freezing-point depression, the lowering of a substances freezing point after the addition of solutes. When road salt is added to roads, aside from providing better friction for vehicles on the road, it also dissolves in the water of the ice, resulting in a lower freezing point. As long as the temperature is above this freezing point, this in turn results in the ice melting. Because of this, ordinary rock salt is only effective down to a range of −6 to −10 °C (21 to 14 °F). At colder temperatures, it can have the opposite effect. Road salt is sometimes used even in colder conditions, if milder weather is expected. In very cold and dry weather, the road surface becomes rough and the need for de-icing is reduced. However, during extreme cold and rain, the roads can become extremely difficult to pass and, in some cases, roads may need to be closed to traffic.

Types of salt

Storage tank for road salt in Puente de los Hocinos, Spain.

Sodium chloride

Sodium chloride is by far the most common kind of road salt. This is mainly due to its widespread use and low cost, and thanks to its large industrial infrastructure, it is used in many industrial and consumer applications. While it is common and inexpensive, its effective temperature range usually does not fall below −6 to −10 °C (21 to 14 °F), and under these temperatures, it is often counter-productive. When used in large quantities, it can also disrupt local ecosystems by heightening the salinity of bodies of water and the soil. Further, rock salt's abrasive nature erodes concrete or asphalt if used heavily.

Calcium chloride

Calcium chloride is less common compared to sodium chloride. While it is slightly more expensive, it can cover a far larger area and melts ice almost three times quicker. It has recently started rising in popularity since it is not as environmentally damaging as sodium chloride, and also because of its heightened effectiveness at clearing ice.

Magnesium chloride

Magnesium chloride is more expensive by far than the road salts in common use today. It has a very low environmental impact, and is quite effective at clearing ice. However, it has been discovered that magnesium chloride causes far more damage to concrete surfaces compared to the other salts, and its use as a de-icing chemical has largely been discontinued. It is still widespread as a highly effective dust clearer in warmer weather.

Enviromental impact

The widespread use of road salt has significant environmental and infrastructural repercussions. While effective and relatively inexpensive, this practice incurs hidden costs because of its corrosive nature, leading to approximately $5 billion in annual repairs across the United States, according to the country's Environmental Protection Agency.

One of the primary environmental concerns is the contamination of water sources. Road salt can infiltrate surface and ground water, elevating sodium and chloride levels in drinking water reservoirs and wells; one teaspoon of road salt can permanently pollute five gallons of water. Elevated sodium levels pose health risks for individuals with hypertension, and high chloride concentrations are toxic to aquatic life.

The accumulation of salt in roadside soils adversely affects vegetation by increasing soil salinity, which can hinder plant growth and lead to the death of sensitive species. This degradation of plant life not only disrupts local ecosystems but also contributes to soil erosion. Additionally, wildlife attracted to the salt (such as deer and moose) can be endangered, as they may ingest harmful amounts or be drawn to roadways, increasing the likelihood of vehicle collisions. The term "Salt Belt" refers to regions with heavy road salt usage, predominantly in the northeastern United States. In these areas, the cumulative effects of salt application are more pronounced, leading to higher concentrations of salt in the environment and exacerbating the associated negative impacts.

Alternatives to traditional road salt are being explored to mitigate its environmental and infrastructural damage. While magnesium chloride and calcium chloride are considered less harmful to the environment, they are more expensive and may require higher application rates. Other strategies that help reduce salt usage and discharge into waterways include spraying road surfaces with brine in anticipation of snowfall, as well as mixing salt with other substances such as sand to improve traction, blue dyes to aid in identification of salted areas, and biodegradables like beet juice, pickle juice, and molasses. Innovative solutions, such as porous pavements, have also been developed to reduce ice accumulation and minimize the need for de-icing agents.

See also

• Deicing