IT'S AS MUCH A RITE OF WINTER AS ICE SKATES AND HOT CHOCOLATE: The storm clouds gather, the cold winds blow, and — often before the snow flies — the trucks are out on the roads, bouncing small chunks of salt on the pavement or spraying it with a brine solution. We've come to expect that traffic and commerce can flow unencumbered all winter long. Salt is the main tool states and municipalities use to ensure that's the case.

Salt — sodium chloride, the same stuff as ordinary table salt — causes water to freeze at a lower temperature. The technology used to spread it has become much more efficient in recent years, as computerized delivery systems ensure that drivers can spread it evenly. But, in an effort to keep streets totally clear, more and more municipalities have also begun to spread salt crystals or brine before any snow falls. In some cases, alternatives are in use, such as calcium chloride. But ordinary sodium chloride is much cheaper than alternatives, and it retains the widest use. Every winter some 13 million tons, more than half the salt produced in the United States, is spread on wintry roads. In the six-county Chicago area, the Illinois Department of Transportation alone uses 140,000 tons of sodium chloride in an average winter, and counties and municipalities cumulatively add even more.

If any other substance were scattered into the environment in such quantities, there would likely be a public outcry. But salt, for the most part, is taken for granted. It has caused great costs as cars and highway bridges corrode, but citizens and politicians have generally accepted those costs as part of the bill for clear winter roads. The environmental costs have generally been taken for granted too.

That's partly been the case because they have not been well understood. But these days a growing number of people — scientists and ordinary citizens — are raising an alarm about what salt is doing to animals and plants. Their concerns are particularly acute in places like Chicago Wilderness, where an extensive road network coexists with a rich array of biodiversity.

"Road salt is a problem," says Dr. Douglas Wilcox, a researcher with the U.S. Geological Survey (USGS) who has extensively studied salt impacts on vegetation in the Indiana Dunes area. "It's nasty stuff and nobody pays much attention to it."

"Even in high-quality wetlands that are far removed from roadways, concentrations of salt are starting to build," echoes Wayne Lampa, a Chicago-area ecologist who has studied salt in ecosystems. "I think some of our high-quality wetlands could be in jeopardy in another ten years or so. The prospect is pretty frightening."

Salt in the Air
Salt has been used to improve traction since the 1930s. But it was the construction of the interstate highway system in the 1950s and 1960s, and the concurrent boom in car ownership, that dramatically swelled the use of deicing salts. One of the regional sites where its effects became most apparent was the Morton Arboretum in Lisle, which was founded in 1922 by Joy Morton, who, ironically, made his fortune with the Morton Salt Company and was the son of the founder of Arbor Day. Morton Salt has sold road salt for most of its existence and still sells a lot today.

Interstate 88 was built along the Arboretum's southern edge in the 1950s. The Arboretum staff planted the highway margin with trees and shrubs intended to show off how roadside landscaping could look beautiful. It did, up to a point. But as the western suburbs boomed and traffic swelled, says soil scientist Pat Kelsey, who studied salt deposition at the Arboretum in the 1980s and 1990s, "there was a significant decline in that vegetation from the use of deicing salts. Close to the tollway you could see all this dieback."

It's well known why sodium and chloride ions — the constituent elements of common salt — harm plants. They interfere with the osmotic balance of cells, making it impossible for cells, and ultimately entire plants, to properly regulate the absorption of water. Plants exposed to excessive salt show symptoms of dehydration. Their uptake of water and nutrients is inhibited, growth slows, needles or leaves turn brown, "witch's brooms" form, branches die. When the soil becomes salty, the germination of seeds slows or stops entirely. Add enough salt to the soil, and even the soil structure changes; it becomes poorly aerated and alkaline and becomes much less hospitable to many plant species.

The dieback of trees and shrubs at the Arboretum prompted Kelsey and his colleagues to study how salt travels by air. Even pine trees planted more than 200 meters from the highway, it turned out, were suffering damage from salt. They were exposed primarily to the white rime that you've seen on expressways during cold, dry weather; tires turn it to dust that can disperse a long way.

The most immediate result of that study is visible today to drivers along both the North-South and East-West tollways as they pass the Arboretum; it is walled off by a high earthen berm intended to shield the Arboretum from both traffic noise and salt. The barrier works, up to a point. It keeps most salt confined in the highway corridor, though at least one study has shown that Chicago and other urban areas are effectively enveloped in a cloud of salt dust during the winter.

The other visible effect of the Arboretum study is along Lake Shore Drive. A desire to minimize salt's effects on roadside plantings and adjacent parts of Lincoln Park prompted Mayor Daley to reduce the winter speed limit to 40 miles per hour there in 1991. The mayor based his decision on Kelsey's discovery that speeds of 45 and higher put significantly more salt into the air than lower speeds.

Salt in the Water
Salt doesn't just leave roads by air; much of it drains off and collects in ponds, lakes, and waterways. How it does so has been very clear at Prairie Crossing, a new conservation-oriented subdivision in northwest suburban Grayslake. Through the late 1990s, while the subdivision was being built, its roads were treated only lightly with salt. They were, at times, a bit slippery. In 2001, the Village of Grayslake took over certain maintenance duties and greatly increased salt use. As a result, chloride levels in the subdivision's lake are more than five times what they were, while grasses have visibly browned and died along the road shoulders.

	Salt levels (ppm) in Lake Leopold at Prairie Crossing. The village of Grayslake took over road maintenance in 2001. Source: Mike Sands, Prairie Crossing.

Mike Sands, the subdivision's environmental team leader, grew up in rural New England and rather fondly remembers how roads there might be snow-covered for days after a winter storm. These days, he says, drivers want streets cleared immediately, and concerns about liability ensure that municipalities strive for just that. The effects show in the lake at Prairie Crossing. "Chloride levels aren't high enough that any major species are going to disappear," he says, "but they are high enough that you're going to see subtle changes in algae populations, zooplankton, and eventually plant communities."

The effect of salt on plants is borne out by experience elsewhere. In the 1960s and early 1970s, an uncovered road-salt storage pile sat along the Indiana Toll Road immediately adjacent to Pinhook Bog, part of the Indiana Dunes National Lakeshore. Runoff from the salt pile entered the bog and caused a dieoff of bog plants such as tamarack trees, red maples, and sphagnum mosses, which were replaced by more salt-tolerant plants such as narrow-leaved cattails. "It's not a direct tie," says the USGS' Wilcox, "but salt gives an additional competitive advantage to the species that can tolerate it." Wilcox says it can take 20 years or more for precipitation to flush out high salt concentrations to levels native plants can tolerate. Long after the salt pile was removed, bog plants had only begun a very slow recovery in the affected area.

Salt levels in two types of wetlands, in parts per million. Source: Conservation Research Institute.

More recently, some of the highest levels of salt in the area have been recorded in artificial wetlands constructed as retention basins. The Conservation Research Institute conducted a study of such wetlands in the Chicago area for the Army Corps of Engineers, and found that salt concentrations often reached 650 parts per million (ppm). Relatively undisturbed natural wetlands in the region typically have concentrations of 8 to 20 ppm; the U.S. Environmental Protection Agency stipulates that drinking water supplies should not exceed 250 ppm.

"Six hundred and fifty ppm is up in the range of salt marshes," says Wayne Lampa. "Native plants just can't hack it, but halophytes [salt-loving plants] from the East or Gulf Coasts can."

Along many area roadways, salt-tolerant plants have become more common — annual saltmarsh aster, salt meadowgrass, narrow-leaved cattails, phragmites reeds, seaside goldenrod. Though no one has been able to point to a precise cause-and-effect relationship to show that salt runoff promotes the growth of these plants, it's clear that there is a connection. Salt changes water and soil conditions, and that affects which plants grow where.

"We have seen a generalized shift in wetland patterns, especially along highways, from a diverse mixture of wetland species to a mix of cattails and phragmites," says Dennis Dreher, an environmental engineer with the Northeastern Illinois Planning Commission. "That's pervasive throughout the region. Salt levels are leading to more monotypic vegetation."

Seaside goldenrod — a recent invader from the Atlantic coast — has replaced most other vegetation along many salted roadsides. Photo by Donald Bolak.

Salt All Around
More difficult to tease out are the effects of salt on aquatic animals. High salt levels are known to change the composition of invertebrate communities in lakes and springs, but most mammals and fishes are believed to tolerate raised salt levels fairly well. Still, little is known about the effects of particularly high levels of salt in a localized area, such as a vernal pool where frogs or salamanders — which absorb water through their skin — breed. Concentrated salt has been shown to inhibit amphibians from crossing forest roads in at least one study.

"Certainly where you have vernal pools, one would predict quite a bit of an effect, because it's such a limited system," says Richard Forman, an expert on road ecology at Harvard University. "And salted roadways are certainly truncating the route that's available to salamanders to move. My sense is that there are probably big effects but rather few of them have been studied. It's a clarion call for research."

Further research is beginning, partly through volunteer work. In McHenry County, people such as retired engineer Ed Ellinghausen of the Boone Creek Watershed Alliance are monitoring salt levels in rare fen communities and correlating them with levels of new development and road salt use. He's finding that a combination of road salt runoff and leaching from the septic systems of homeowners who use water softeners is often causing a tenfold increase in salt levels in area wetlands, raising concerns about the survival of rare plants.

The question residents of Chicago Wilderness and other heavily populated regions may have to ask themselves, then, is how much we are willing to sacrifice for convenience. Biodiversity and high-speed winter travel may not always coincide. JoAnn Seagren of Barrington Hills remembers when her town spread sand on the roads rather than salt. "I think people drove more carefully and slowly then," she says. "Now the roads are totally clear and people don't have to slow down."