Road Salt: Is it worth it?
by Judy HelgenSalt from your saltshaker seems natural, innocuous, even non-toxic, but, in reality, it kills plants, damages soils and harms aquatic life. The United States extracts and consumes more salt than any nation in the world. More than half, or 44 billion pounds, is used for de-icing roads. In Minnesota, de-icing roads is a major project: over 200,000 tons of salt are used each winter, the equivalent of 400 million one-pound containers of household salt every year!
Most of Minnesota's road salt originates from Cargill's Avery Island salt mine in coastal Louisiana, an ancient dome (200 million years old) of pure white salt going down 40,000 feet. 2.5 million tons of salt are mined from Avery every year.
The salt is hoisted up and run down a long, covered conveyor belt to the barge-loading area. "Here is where your YPS is mixed with the salt to prevent hardening" said Rod Etie, an engineer at the mine, as he pointed to a green shack below the conveyer belt, its wooden sides stained blue from the chemical.
"When we didn't add YPS to the salt, it would get so hard in three days you had to break it apart with a pick," said Rod. A toxic additive to all road salt, YPS, or ferrocyanide, is not put into salt blocks that are made for cattle. And it can harm the environment: when it gets into water and sunlight, it generates lethal cyanide.
The salt is dumped into Cargill's barges that bring grain to the southern U.S. from the midwest, then it is shipped up the Mississippi River to Port Cargill near Savage, and other locations in Minnesota.
Salt in the environment
Sodium, whose name is derived from sodanum, meaning headache remedy, is no remedy for plants: it inhibits the growth of roots and leaves and impedes water uptake and movement. Plants can live without sodium, but they must have calcium. Excess sodium in soil wreaks havoc with calcium, yanking it from cell walls and membranes. On top of this, photosynthesis is inhibited by too much chloride from salt.
"Salt can blow off a road 200-300 feet and damage the trees," said Paul Walvatne, forester for Minnesota's Department of Transportation (MNDOT). "It gets thrown up into the air, and just keeps wafting, and if it settles down on sensitive trees like lindens, there can be problems," he said. Pine trees near highways take a huge toll; yellowing trees can be seen along the roadways. "For high-traffic areas, we're trying the ironclads, trees that are more tolerant of the salt, like honey locust," said Walvatne.
No surprise that wetland plants are also hammered by road salt: "The number of plants we define as sensitive, like blue-flag iris and several marsh grasses, declined with increasing amounts of chloride," said Mark Gernes, research scientist from the Minnesota Pollution Control Agency (MPCA).
Road salt flows off roads into ditches, wetlands and streams; or, it can get into the groundwater and percolate through the soil into waterways. "We're not required to measure chloride, so we don't," said Keith Cherryholmes of the MPCA. "The sad part is we have streams in the Metropolitan area with high chloride concentrations, and they are impaired (meaning polluted)."
"The more chloride that runs into the water of urban streams, the fewer mayflies we find," said Kathy Lee, researcher at the U.S. Geological Survey. Studies of wetlands by MPCA (by J. Helgen) had fewer species of aquatic invertebrates when chloride was high.
"In the urban streams we've studied, there's a negative relationship between the amount of chloride and the number of species of fish we find," said aquatic ecologist Phil Talmage of the Minnesota Department of Natural Resources. Where there is more chloride, fewer kinds of fish survive. Chloride damages functions of fish gills essential for survival; it harms their ability to take in life-supporting oxygen from the water and to maintain chemical balance. "We saw a very significant relationship between the amount of paved ("impervious") surface and the amounts of sodium and chloride in streams," said Talmage.
One stream heavily polluted with salt runoff is Shingle Creek, which flows through the northern Twin Cities metropolitan area to the Mississippi River. "We're aiming to reduce the amount of chloride getting into Shingle Creek by 71%, long term," said Tim Larson, hydrologist at MPCA, "but we need to balance environmental needs with public safety."
There's hope, because MNDOT can reduce the amount of salt by applying wet salt (brine). "The idea is to melt that hair-thin layer between the road surface and the snow. It works best to get there immediately before a storm," he added, "When they use dry rock salt, a lot of it just bounces and blows off the road."
Curiously, we don't really know if de-icing with massive quantities of salt actually makes driving safer. It's a perception we all have, but it's hard to pin down. "It's a strange thing that happens, there are more fatal crashes in the summer, but more accidents in winter" said Allan Rodgers, transportation research analyst. Establishing whether road salting actually reduces accidents will be difficult, because plowing and sanding are also factors.
The U.S. Environmental Protection Agency (EPA) and MPCA have standards for the allowable amount of chloride in surface waters (230 mg/L, the chronic toxicity standard); the standard for industrial use water is 100 mg chloride. According to Tim Larson, the 100 mg standard is not being used for the cleanup target for Shingle Creek. They are using the chronic (230 mg/L) standard.
"MNDOT asked MPCA to relax the 230 mg chronic standard for chloride because of road salt" said Dave Maschwitz, the research scientist at MPCA who oversees revisions of water quality rules. "In spite of what they would prefer, MPCA is not going to relax that standard," he said, "but there is a proposal right now to relax the industrial water standard from 100 to 250 mg," he said. Canada has declared road salt as toxic based on its data on environmental impacts, MN has made no such declaration.
Salt is accumulating in the groundwater, streams, soils, and wetlands in Minnesota. "It just builds up and builds up," said Keith Cherryholmes of MPCA. In Massachusetts and New York, road salt has intruded into some drinking water wells. Salt spray can cause power outages in power stations near highways.
MNDOT is researching ways to reduce the use of road salt, which is a good idea, not just because of the damage to the environment, but also because salt will get more expensive in the near future.
At 1600 feet down in the Avery Island salt mine, it's a warm 90 degrees; the temperature will continue to climb the deeper they go towards the earth's heated core. Someday, possibly as soon as 50 years from now, the expense of cooling the air and the greater hoisting distance will make the cost of road salt prohibitive. "We're looking for alternatives, like Mexico," said Bob Vasek of MNDOT.
The hidden costs of road de-icing, assessed in 1976 by EPA for the United States, have not, apparently, been updated since then. Included in EPA's estimate 30 years ago were the costs of purchasing the salt, its application, damage to highway structures, vehicles, utilities and vegetation (trees), and the impacts on drinking water supplies. No estimates were made on costs for impairments to fish, wildlife or soils.
It costs $6-8 million just to buy the salt for Minnesota roads each year, and it may go as high as $12 million. Do the benefits of applying 400 million pounds of salt every year to our highways truly outweigh the costs?
Judy Helgen can be reached at (651) 636-6544.