My car is coated in long smears of salt. If I brush the car door, I’m left with a large, white, and somewhat tasty smudge on my clothes. While I’m always pleased to get home on winter roads in one piece, I wonder how this plume of salt is affecting the world that I’m rushing through.
Farmers and gardeners are careful to keep salt out of the soil because it creates droughty conditions for plants. Excess salt also kills bacteria that bind the soil particles together, causing the soil to break apart and erode easily. When the Romans sacked Carthage in 149 BC, they plowed salt into the fields of the vanquished.
Above concentrations of 20 parts per million (PPM), the taste of salt in drinking water is noticeable. Recently, a house along I-89 in New Hampshire was abandoned after highway salt contaminated the drinking water.
White pine seedlings cannot tolerate salt concentrations above 67.5 PPM in the soil. In late winter, along the interstates, the clouds of brine lifted by speeding traffic leave a telltale zone of brown where the salt has desiccated and killed the pine needles.
In salt, the chloride anion (Cl-) causes more damage to metal and plants than does the sodium cation (Na+). Chloride is involved in the rusting of steel in bridges and also in some of the more damaging biological effects. An Ontario study reported chloride levels of 1,050 PPM on a highway median and 890 PPM 30 feet from the edge of the road. For reference, 650 PPM is a typical chloride concentration in a salt marsh.
Chloride levels in snow collected on Montreal streets run about 3,000-5,000 PPM. Highway runoff can reach as high as 19,000 PPM. During a thaw, the first snow to melt has the highest concentration of salt. Brine is denser than fresh water and can flow into wetlands near a highway and settle to the bottom. Wetlands and ponds in Montreal have been measured at 4,000 PPM. In rural lakes, the levels were between 150 to 300 PPM. In the Adirondacks, one vernal pool, located more than 500 feet from a two-lane state highway, had salt levels six times higher than normal.
Freshwater species exhibit a range of sensitivity to chloride levels. Some fish can tolerate levels as high as 30,000 PPM. On the other hand, in one study, seven days of exposure to salty water at 3,345 PPM killed half the populations of each of 17 species of freshwater crustaceans, amphibians, and fish. Rainbow trout eggs and embryos are killed at 990 PPM. More importantly, benthic macroinvertebrates, the underwater insects that feed on plant debris and form a key link in the food web of streams and ponds, are sensitive to chloride levels as low as 220 PPM.
A little salt in the wild isn’t entirely bad. Salt is a necessary mineral for all living cells. Sodium chloride, however, is hard to come by in the New England forest. It appears that rather than using poison, or thorns, some trees protect themselves from herbivores simply by being under-salted. Porcupines often have to travel far and feed on numerous trees to find the right mix of digestible carbohydrates, potassium, and sodium. Deep in the woods, lean-to cabins and ax handles, smeared with salty sweat, become attractive vitamin sources for porcupines.
In winter, moose will often restore their needed electrolytes from roadside puddles of brine. Roads are not safe places for mammals to dawdle after dark, and while drinking salty water, moose seem even more vulnerable to vehicles and humans than usual.
Seed-eating birds consume small pieces of gravel to grind food in their muscular gizzards. There is evidence of winter finches exhibiting abnormal behavior after eating one small chunk of road salt, and death after two.
Among amphibians, the effects of salt are still being investigated. Nancy Karraker, a Ph.D. candidate at the State University of New York, studied over 50 vernal pools along a two-lane highway. The highway itself depressed populations of wood frogs and spotted salamanders, presumably from road kill. She discovered that the survival of the embryos and larvae of wood frogs and spotted salamanders decreased with increasing salinity. Higher salinity was also associated with an increase in the number of deformities.
Karraker notes that roads influence over 20 percent of the land area of the United States. Alternative de-icing methods are being more widely examined and used by highway departments. Getting the timing right on applications of salt is critical. It doesn’t make sense to plow salt off the road. Still, there is a lot of wasted salt out there, and walking down my road, I often notice large piles of translucent crystals. Certainly the murk on my windshield is not the only downside of road salt.