Controlling snow and ice requires balancing three, not quite equal, goals: meet the public's need to travel safely, avoid harmful environmental impacts, and keep costs in line.
Over the past decade or so, research and experimentation have produced knowledge that makes it easier to succeed at all three. Using the right materials and methods can not only increase traffic safety, but also reduce pollution and save money.
SALT'S SEVERE TOLL
The traditional methods of snow and ice control consist of spreading deicing salts to promote melting, and sand or other abrasive materials to improve traction. But these actions have the potential to harm the environment in several ways.
Most deicing salts are chlorides. As these salts dissolve, chlorides carried with the water contaminate soil, groundwater, and wetlands. These chlorides have toxic effects on plant and animal life.
According to Connie Fortin, founder of Fortin Consulting Inc., a Hamel, Minn.-based firm dedicated to wetlands protection, federal standards limit the safe chloride level for groundwater to 230 million gallons/liter—about the concentration of a teaspoon in a bucket of water. Chlorides don't break down, but rather accumulate over time, and there's no practical way to remove them.
Besides their toxicity, salts promote the corrosion of steel in bridges, both in exposed structural members and in embedded reinforcement if they enter through cracked or damaged pavement.
“Bridge decks are the first surfaces to freeze, so we treat them aggressively against icing in the fall and spring as well as in the winter,” says Mike Kennedy, director of winter operations for the Minneapolis Department of Public Works. “To limit damage, we do a lot of preventive maintenance on decks during the summer, flushing and sealing to protect them.”
Salt solution carried in by vehicles is just as hard on embedded steel in concrete parking decks and garages, and it promotes corrosion of vehicles, though this has become less of an issue with the use of more plastics and better undercoating in cars.
Though not as corrosive as salt, sand presents unique challenges of its own.
Because it reduces traction on dry pavement, it must be cleaned up as soon as possible. Also, traffic moving at high speeds over sand throws fine particles into the air, increasing particulate matter pollution. Unless it's removed, sand can clog storm sewers, fill in wetlands, and accumulate at the bottom of ponds and streams, affecting the ecosystem and harming plant and animal life.
Kennedy's department uses straight salt for arterial streets, and a mixture of salt and sand for residential streets. “We use the sand to improve traction because we don't get to treat residential areas until after snowpack,” he says. “But the cost of cleaning it up afterward is high.”