Before and after it snows, people often spread loads of salt all over the sidewalks and roads. I know the salt makes your winter shoes worn out more quickly. How does the salt the environment then?
Salt is applied on the roads in winter because the freezing point of salty water is lower than that of pure water. So scattering salt on ice or snow can help accelerate the melting process, opening up the roads to traffic that much sooner. It is estimated that over 50 kg of salt is used annually for every resident in the North America!
"Salt" can refer to any compound consisting of the cation from a base and the anion from an acid and which is readily dissociated in water. While sodium chloride (NaCl) is by far the most frequently used road salt in Canada, other inorganic salts used in Canada include calcium chloride (CaCl2), magnesium chloride (MgCl2) and potassium chloride (KCl). Sometimes sodium ferrocyanide (Na4Fe(CN)6·10H2O) is added as an anti-caking agent.
After it dissolves—and is split into sodium and chloride ions—it gets carried away via runoff and deposited into both surface water (streams, lakes and rivers) and the groundwater under our feet.
The biggest concern with road salt is how it affects water quality when the snow and ice melts, the salt is washed away into lakes and streams or seeped into groundwater supplies. ~70 % of the salt applied to roads stays within the region's watershed. Once it gets there, the chemicals is difficult and expensive to remove. The salt from the roads can cause salty groundwater. That's a health issue for people on restricted-sodium diets and a taste problem for everyone else. When salt migrates into lakes and streams, it can harm aquatic plants and animals. A heavy influx of sodium and chloride ions will disrupt the ability of freshwater organisms to regulate how fluid passes in and out of their bodies. An estimated 40 percent of the country's urban streams have chloride levels that exceed safe guidelines for aquatic life, largely because of road salt. Changes in the salinity of a pond or lake can also affect the way the water mixes as the seasons change, leading to the formation of salty pockets near the bottom and biological dead zones. Salt applied on roads can also erode the soil, and damage trees and vegetation .
With urbanization and increasing number of roads, he mounting piles of salt dumped on the road may be getting to be a bigger problem than ever. In 2004, Canada categorized road salt as a toxin and placed new guidelines on its use.
While salt can cause such problems, we still have to rely on it because it's cheap and applying it on icy road can reduce risks such as traffic accident. Although there are alternative chemicals available but the alternatives are much more expensive and the alternatives may still cause environmental problems once largely used because toxicology tells us every chemical is toxic; the dose plays a role. and they often require municipalities to invest in new spreading equipment. So far, there are no better solutions to get ice off the roads—except, perhaps, the shovel.
As we still have to rely on salt to de-ice the road, more environmental friendly way we can do include (1) cutting back salt usage by pre-wetting the salt, which allows for more controlled application and better sticking power. (2) Applying salt just before a storm hits, so that the snow can't adhere to the ground (this relies on accurate weather forecast!) (3) Apply salt only when the pavement temperature is below freezing but above -21 degreeC because sodium chloride doesn't work below that temperature.
References and more information, check
http://www.ec.gc.ca/sels-salts/
http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/contaminants/psl2-lsp2/road_salt_sels_voirie/road_salt_sels_voirie-eng.pdf
http://www.smithsonianmag.com/science-nature/what-happens-to-all-the-salt-we-dump-on-the-roads-180948079/?no-ist
http://www.slate.com/articles/health_and_science/the_green_lantern/2010/02/salting_the_earth.html
