Clearing roadways: A little salt goes a long way

by Mary Caperton Morton
Thursday, January 5, 2012

Although winter in the Northern Hemisphere does not technically begin for another month, snowfalls and icy conditions are already making driving hazardous. When winter weather strikes, most states spread salt to clear roadways. However, more and more studies are showing that salt has lasting environmental repercussions, which may force a winter roadway maintenance overhaul. But if not salt, then what?

Of all the chemicals we use in agriculture, industry and in our everyday lives, sodium chloride seems like it would be less dangerous than most. It’s an essential nutrient; we put it on our food and get sick if we don’t get enough. Salt is also everywhere; it’s found in abundant mineral deposits around the world. And seawater is about 3.5 percent salt (roughly 35 grams of sodium chloride and other salts per liter of water). But every winter, more than 18 million metric tons of salt are spread on icy roads throughout the United States, and studies have found as much as 70 percent of that salt finds its way into streams, lakes and groundwater, wreaking havoc on aquatic ecosystems and compromising drinking water supplies.

“Of all the environmental problems out there, salt isn’t something most people worry about,” says Robert Jackson, an environmental scientist at Duke University in Durham, N.C. “When it’s on an icy road, you’re focused on driving and then once the ice disappears, the salt does too, and it’s out of mind.” Though the salt might have dissolved, it has not disappeared: Much of it ends up in waterways, where it lingers and accumulates year after year. “In the White Mountains of New Hampshire, I’ve seen streams with a salt content higher than 100 milligrams per liter,” says Sujay Kaushal, a biogeochemist at the University of Maryland Center for Environmental Science in Solomons. “That’s as salty as the Hudson River estuary.”

Other studies have shown salt contents of urban waterways can be even higher. One study looked at year-round salinity of 13 lakes in the Twin Cities metropolitan area of Minnesota, which applies 320,000 metric tons of road salt to melt an average of 144 centimeters of snow a year. All the lakes tested had alarmingly high salinity levels during the winter, as high as 1,018 milligrams per liter, more than four times the chronic pollution level allowed by the state’s pollution control agency.

The study — conducted by Andrew Sander, a water resources engineering student, Heinz Stefan, a water resources engineer, and other researchers at the University of Minnesota at Minneapolis — also found significantly high levels of salt in the lakes during the summer too. “Some of the salt gets flushed out of the lakes by summer rains,” Sander says, “but a lot of it stays in the lakes and accumulates year after year.”

“Long-term buildup of salt has physical, chemical and biological consequences,” Jackson says. Salty water is denser than freshwater so it sinks to the lake bottom, creating a chemocline that can prevent vertical mixing of water and lead to anoxic conditions near the lake bottom — especially toxic for bottom-dwellers such as whitefish and carp. Increased salinity has also been shown to trigger a release of heavy metals like mercury and lead from lake sediments, further contaminating the water. Salt also affects freshwater organisms at the cellular level, Jackson says, interfering with the transport of water and chemicals across cell membranes. The effects can be lethal to most freshwater organisms in salt concentrations higher than 250 milligrams per liter.

Long-term modeling suggests the salt situation will only get worse. “If winter salt use continues at its current rate, not accounting for more road building, we estimate that salt concentrations in most urban and suburban streams will exceed 250 milligrams per liter within the next century,” Jackson says. Not only would such levels be lethal to aquatic life, but they would make the water unfit for human consumption.

As pressing as the salt problem is, it’s not an easy one to solve. After all, “the driving public has come to expect bare roads, storm or no storm,” says Joshua Katz, a hydrogeologist at the Maine Department of Transportation in Augusta. Public expectations weren’t always so high, he says. Salt was first applied to icy roads in New Hampshire in 1938 and was not commonly used until after World War II. But once President Lyndon Johnson established a bare-roads policy for the interstate roads system in the 1960s, there was no going back. Katz stresses that public safety has to come first, but he says he’s not about to sling more salt than necessary. He is heading a broad analysis for the Maine DOT, looking at the whole spectrum of problems with road salt. “Winter roadway maintenance is an evolving art,” he says.

Canada is leading the way in testing the degree to which winter roadway maintenance can evolve. Environment Canada, the Canadian environmental protection agency, has threatened to reclassify road salt as a toxic substance. But instead of declaring road salt toxic outright, which would lead to severe restrictions on its use, Environment Canada released a set of guidelines in 2004 called the Code of Practice for the Environmental Management of Road Salts, which is designed to help road authorities manage how they use road salts to reduce environmental harm while still maintaining road safety. All road authorities that spread more than 500 metric tons of salt a year are highly encouraged, though not yet obligated, to follow the code. “Environment Canada is collecting data and reviewing practices by road authorities around Canada. In 2010, they will determine how effective the code has been,” says Peter Noehammer, director of Transportation Services for the city of Toronto, Ontario. The threat looms that if the code cannot change how salt is used, then salt will be listed as a toxic substance and become heavily regulated, he says.

Toronto receives an average of 130 centimeters of snow a year and uses about 114,000 metric tons of salt, although Noehammer says those numbers can fluctuate quite a bit from one winter to the next. “Our approach to reducing road salt applications is three-pronged: equipment, training and alternate deicing chemicals,” he says.

First, in winter 2001-2002, the city installed computerized controllers on salt-spreading trucks to regulate the amount of salt sprayed from the machine. “We have figured out exactly how much salt is needed depending on driving speed, snow conditions and road width,” he says. Trucks have also been fitted with tanks and hoses to spread liquid salt brine along with the solid granules. “This wets the salt kernels so they stick to the roadway and don’t bounce off,” Noehammer says. “We’ve found pre-wetting reduces application rates by as much as 15 percent.” And when ice storms are predicted to strike, the city also pre-wets certain high-risk roadways like bridges, intersections and expressways with salt brine to prevent black ice from forming instead of trying to melt it after the fact, which usually requires more salt.

Training has also played a big role in reducing Toronto’s salt usage. “Many drivers have been operating snowplows for years, long before anybody worried about salt,” Noehammer says. “We teach the drivers the latest techniques for how to apply the right amount of deicing product at the right time for maximum efficiency.” Each fall, right before snow season begins, snowplow and salt truck operators from all over Canada attend a two-day snow school to learn the latest techniques in winter roadway maintenance. Drivers even have the chance to climb in a salt truck simulator to test their knowledge about treating roads in a variety of conditions. A similar gathering, the American Public Works Association Snow and Ice Conference, is held each fall in Estes Park, Colo., and features a competitive “Snow Roadeo,” where snowplow and salt truck drivers are judged for safety, accuracy and efficiency as they maneuver over icy obstacle courses.

Still, if salt is so bad for the environment, why aren’t more places using alternatives? The problem, Katz says, is that sodium chloride is “really the best thing out there. It’s cheap and it does the job extremely well.”

Nonetheless, some regions are trying to find alternatives. Toronto deploys a few alternate chemical-deicing agents, but only under special circumstances. “In temperatures below minus 20 degrees Celsius, the effectiveness of sodium chloride is significantly reduced,” Noehammer says. In those cases, magnesium chloride or a mixture of natural-based compounds (byproducts of Canada’s sugar beet refining industry) is used. “Many of the alternative chemical deicers biodegrade well and aren’t red flags in terms of toxicology,” Jackson notes. “Most contain cations like calcium, magnesium and potassium that are better for the environment than sodium.”

In the last couple of decades, a number of European chemical companies began developing alternate compounds, initially for use on airport runways. In the late 1990s, Finland began a vigorous campaign to reduce sodium chloride use as much as possible. To that end, in 1998, the Finnish Environment Institute (SYKE) and the Finnish Road Administration set about testing a number of biodegradable deicing compounds to determine which was the most effective. In 2004, SYKE released a report concluding that potassium formate was the best option. The main drawback is its price: Potassium formate is more than 15 times more expensive than sodium chloride.

“Potassium formate is more expensive because it has to be industrially manufactured, whereas salt is a mining product,” says Timo Nissinen, technical development manager of deicers for Kemira, a European-based chemical company that sells potassium formate, among other deicing products. However, once you start taking into account the “real cost” of using road salt, it may not actually be as cheap as we think, Stefan says. “Salt is highly corrosive to cars and bridges. It may turn out that it might be more cost-effective to use alternative chemicals if it would mean fewer repairs,” he says.

Additionally, the cost of road salt may ultimately go beyond bridge and car repairs, warns Kaushal, who has worked closely with Bill Stack of Baltimore’s Department of Public Works on how salt runoff affects the city’s water supply. “That’s really our primary concern,” Stack says, “protecting the streams that supply freshwater for the 2 million people who live in and around Baltimore.” Salt cannot be filtered out of water; only time-consuming and costly desalinization methods can remove it, so if Baltimore’s surrounding streams get too salty, providing freshwater to the city could get expensive.

In much of the eastern half of the United States, salt use is still prevalent, but a few places avoid using chemical deicers altogether, opting, instead, to use sand or gravel to increase friction on icy roadways. “Minnesota used to spread a mixture of salt and sand but they stopped because they didn’t know what to do with the sand left on the roads in the spring,” Stefan says. Around Albuquerque, N.M., where snow and freezing events are comparatively rare, locally mined red pumice is used. In the mountains in Oregon, the Oregon Department of Transportation uses both sand and magnesium chloride, a deicing liquid that the department says is safer for cars and the environment than salt. In other locations, fly ash left over from coal-fired power plants has been suggested, but the toxicity of such ash is still under debate.

No matter how many alternate options are offered, salt will probably always be the best and cheapest roadway deicer, Katz says. “Salt isn’t going anywhere anytime soon.” He points out that while transportation agencies have an obligation to reduce their salt usage as much as possible, the driving public needs to take some responsibility for their winter road safety as well. “Personally, I’m a firm believer that if we were willing to drive slower or stay home after snowstorms, we could use a lot less chemicals on the roads. I know it’s a big cultural shift, but maybe it’s time we make it.”