Road salt’s environmental cost weighed Balance sought in travel safety, wildlife welfare

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ORONO – Driving carefully in the tracks of a snowplow during a late-winter blizzard, it’s difficult to see the downside to salting Maine’s roads. But the salt and other chemicals that are used to melt snow and ice from highways don’t just disappear when a storm ends.

“Most people would think the business end of the snowplow was on the front. Well, I think it’s on the back,” said Nancy Karraker, a New York-based scientist, who visited the University of Maine in Orono Monday to share her research into how road salt affects amphibians.

Sometimes, the salt mixes with wet snow and is sprayed hundreds of feet into the woods by the pressure of car tires. On dry days after storms, leftover salt on the road can re-form into crystals, which are pulverized and sent into the air as dust when traffic passes.

And salt residue mixes with water and runs off roads throughout the year, carried by summer rainstorms as well as melting snow.

“There is a lot of salt that gets into the nooks and crannies of the road,” Stephen Norton, a professor at UMaine who has studied the impact of road salt on water chemistry, said Tuesday. “It literally drains out over the summer.”

Karraker saw the impact of salt in small seasonal wetlands called vernal pools located as far as 550 feet from roads. And Norton has seen changes in water chemistry that can be traced to salt, many miles downriver from a road crossing.

“You can see the effects all the way down to the ocean,” Norton said.

Road salt is composed primarily of sodium chloride, a substance that is chemically identical to table salt and has the characteristic of lowering the freezing point of water – it can melt ice.

However, dyes and other chemicals are frequently added to improve the road salt’s performance.

Americans have been using salt on their roads since 1938, when New Hampshire started the practice. But the amount of salt being added to the environment has been increasing since the interstate highway system was built in the 1950s and ’60s.

In Maine, 104,000 tons of salt have been used so far this winter, according to Brian Burne, an Augusta-based highway maintenance engineer for the Maine Department of Transportation.

Nationwide, more than 13 million tons are applied annually, according to the federal Environmental Protection Agency, but relatively minimal research into how all this salt is affecting the environment has been done.

“It’s nasty stuff, and nobody pays much attention to it,” Douglas Wilcox, a U.S. Geological Survey researcher, recently told a Midwestern magazine.

Scientists know that road salt can kill trees and that white pines are particularly sensitive. Sometimes, road salt puts such a strain on native species that hardier invasive plants and animals take over.

Researchers, including Norton, have learned that excess salt changes stream chemistry, causing certain minerals to leach out of soils.

In fact, at high enough concentrations, salt can increase the acidity of water, causing some of the same negative effects as acid rain.

Studies have shown that road salt attracts large ungulates such as deer and moose, causing collisions with vehicles. Other scientists have learned that some amphibians refuse to cross salted roads and, as a result, can be separated from their traditional breeding areas.

Karraker looked at how road salt affects populations of wood frogs and spotted salamanders – both species that live in Maine – in vernal pools in the Adirondacks.

She found that high salt concentrations affected the ability of both species to survive, with eggs and embryos dying in experiments when subjected to the high end of salt concentrations she saw in the wild.

Both species also seemed to suffer birth defects more frequently in the presence of high salt concentrations.

“They may live to hatch … But at some point, they don’t make it,” Karraker said.

Species such as these frogs and salamanders, which breed in vernal pools, are particularly susceptible to salt pollution since nearly all of the water in their pools comes from spring runoff.

As the summer goes on, vernal pools dry up, with the water becoming saltier and saltier over time – in fact, reaching peak saline levels during the critical midsummer metamorphosis.

“Typically [vernal pools] have no inlet and no outlet, so everything that comes in stays in,” Karraker said.

Scientists who study road salt’s effect on the ecology do not advocate leaving icy roads untreated, but they hope to learn more about how to prevent salt’s negative impacts without sacrificing public safety.

Some cities have addressed road salt problems by requiring lower speed limits in salted areas to limit the aerosol effect, or by declaring certain stretches of highway resource protection zones where less salt is used in hopes of reducing runoff, Karraker said.

Maine has never taken such actions, despite scattered complaints about dying trees and salt in residential wells.

“It tends to be more of a problem in urban areas because you’ve got a denser concentration of roads,” Joshua Katz, who has studied salt issues for the environmental office of Maine’s DOT, said Tuesday.

Maine has been trying to reduce the amount of salt used on roads in recent years – primarily to reduce costs, but also with environmental benefits, said Jerry Waldo, regional manager for the DOT’s Bangor office.

Nationwide, transportation officials are using less salt by shifting away from using it after roads have been plowed. Instead, salt is scattered just before a storm begins in the hope that it will prevent ice from forming, which reduces the need for more salt application.

Plow trucks are now equipped with thermometers to gauge when the road’s surface reaches prime salting temperature. One truck in the Bangor area has a new, European-made hopper that has allowed it to apply wet salt in an “oatmeal” consistency for optimum results with less salt.

In some parts of Maine, salt is mixed completely with water and then sprayed onto the road as a 23 percent saline brine before snow and ice start falling. Doing so uses between half and a quarter as much salt per mile as scattering dry salt, according to the DOT.

State road crews in Aroostook County and other selected areas in northern Maine are using a product called Ice-Be-Gone, a brown liquid solution of magnesium chloride, which is believed to make salt more effective at lower temperatures.

Elsewhere, calcium chloride is mixed with the salt to help it adhere to the pavement, Waldo explained.

Some worry that introducing additional chemicals into the environment without first researching their potential impacts could be as harmful as high salt levels.

For now, the state believes that using small amounts of additives and timing salt applications precisely while clearing roads can achieve safety with less salt – helping both the environment and the state budget.

“There’s an art to it, a science to it,” Waldo said.