A loader fills a waiting truck with road salt from an enormous stockpile on Jones Island in the Port of Milwaukee.
Keeping roads on a low-salt diet
Cautious drivers, new de-icers, carefully timed applications and calibrated equipment give road crews a chance to keep roads clearer and safer with less salt.
Theresa J. Lins
In Wisconsin, winter means snow, and in the last two winters, lots of it. To deal with snow and ice, municipalities use road salt to keep streets and highways safer for driving. But salt use comes with both a financial and environmental price tag.
Statewide, the total cost of winter operations (snow removal and salting) was $86.3 million in 2007-8, about double the cost of an average winter reflecting the severe weather that dumped 60 percent more snow over the average of the previous five years according to the Wisconsin Department of Transportation (DOT).
"Road salt use is a sleeping giant," said Roger Bannerman, DNR water resources management specialist. "The potential for chloride to damage our water systems is more inevitable than climate change."
According to Bannerman, all the numbers related to chloride levels show increases. Monitoring by the Environmental Protection Agency (EPA) shows Lake Michigan's average chloride level – currently at about 12 milligrams per liter – is increasing by about 0.1 milligrams per liter every year, largely due to road salt and other human activities.
High salt concentrations can harm fish by drawing moisture from their bodies altering their electrolyte balances. It can also cause long-term problems like reproductive failure and increased disease susceptibility. And in small, enclosed water bodies like ponds or lakes, salt can settle and impede water circulation and transport of needed oxygen to bottom-dwelling organisms.
In the state's two largest cities, Madison and Milwaukee, the amount of salt used is carefully monitored. Municipal road crews prepare for winter by training personnel on salt usage and tightly calibrate spreading rates from trucks to control how much salt is dispensed.
Even with this close monitoring, University of Wisconsin-Milwaukee researchers have found chloride levels in local waterways are increasing by about 2 milligrams per liter every year. This amount of chloride is still within acceptable levels set by the EPA for human consumption. But higher chloride levels may contribute to the invasive species problem in Lake Michigan by making the water more hospitable to saltwater species that thrive in a slightly briny or brackish mix.
Bannerman contends that more information on what will happen in 50 years is needed. "We need to put numbers on the impact. When will chloride levels reach the point that indigenous species may not survive anymore?"
In Madison, chloride levels are generally increasing in the city's groundwater aquifers. Between 1975 and 2004, increases of 246 percent, 551 percent, and 282 percent respectively occurred in three of Madison's wells. Another well, now abandoned, had chloride levels greater than 100 milligrams per liter (mg/L), close to the groundwater preventative action limit (PAL) of 125mg/L. Similarly high values have been found in some area springs and shallow wells.
The story is no better for plant species harmed by road salt. Exposures to wind and toxic contaminants deposited by traffic make roadsides a dry and harsh environment for plants. Salt adds to that stress. High chloride concentrations can interfere with moisture absorption from soil and cause browning or burning of leaves. High sodium concentrations may affect plant growth by altering soil structure, permeability and aeration. The harm varies depending on the amount of salt, type of soil, total precipitation, distance from the roadway, wind direction and plant species. In short, the impacts are highly site-specific.
DOT spent $26.6 million on salt statewide in 2007-8 for 644,485 tons of road salt; the highest usage on record. Counties in the southern half of the state, where the winter was significantly more severe than average, had the greatest increase in salt use compared with previous winters.
Milwaukee County uses about 40,000 tons of salt annually to clear 2,400 lane-miles of interstate, state and county highways. Spread uniformly, that translates into more than 16 tons on every one-mile-long, one-lane-wide section of road each year. The City of Milwaukee uses about 55,000 tons of salt annually to maintain 7,000 lane miles of main roads and side streets.
All of this salt has to go somewhere when it melts; that's something the traveling public does not often think about, but various state and local officials monitor a lot. Milwaukee follows a practice of plowing as much snow and ice as possible before applying salt. A "sensible salting" policy is dictated by ice events, said Wanda Booker, a city sanitation services manager.
The City of Madison has been cutting its salt use each winter since the 1970s and approved recommendations to regulate the amount of salt applied by commercial snow removal operators on private parking lots. Reports indicate the private parking lots may be receiving twice as much salt as is applied to city streets, but there is no formal monitoring of private snow removal contractors.
Al Schumacher, of the City of Madison's Department of Public Works, wants to start a certification program which would train and monitor salt use by commercial operators. Since 1973 the city has only salted major arteries, connectors, schools, hospitals and bus routes. They use abrasives on minor routes. All Madison salt trucks have computerized settings so the amount of salt distributed is accurate and limited to 300 lbs. per two lane-miles of roadway.
Schumacher also sends his truck operators to training by the City of Madison's Engineering Department. Commercial snow operators attend on a voluntary basis. "It's a big selling point to our people to learn about the negative impacts of salt," he said.
"Everything has to depend upon education to reduce road salt," said Bannerman. "We have to ‘educate, educate, educate' because we can't be looking over people's shoulders all of the time."
If salt is so harmful, why isn't more sand used? In other parts of the state, especially the hilly areas in southwestern Wisconsin, sand is applied to the roadways and critical locations like hills, curves and intersections, but it is not always an adequate substitute for salt.
Increased traffic traveling at faster speeds brings demand and expectations to keep roads clear. Sand helps provide traction, but it does not melt snow or ice. Also it is easily blown off of roadways and can blow or flow into nearby bodies of water contributing to sedimentation and carrying other pollutants absorbed from the roadways.
Sand and other abrasives are also wasted according to a recent newsletter by the Salt Institute, a nonprofit industry trade association. Excess sand can be expensive to clean up and corrosion from sand-salt mixtures can damage vehicles as abrasive particles hit and rust metal or crack windshields.
All sand piles trap moisture and some salt needs to be added so the moist sand won't freeze and will readily spread on the road. A mixture with about 2-4 percent salt by weight is recommended. Unfortunately, some plow operators mistakenly believe more is better when it comes to applying salt and sand mixtures.
In fact, salt and abrasives do different things and can oppose each other said Salt Institute newsletter author Don Walker. A common belief is that salt will anchor the sand, and/or sand will anchor the salt to the road. In fact, Walker explained, sand and dry salt particles are separate. As long as they remain dry, wind and traffic will quickly move both of them off the pavement.
Some salt may become brine, dissolved by moisture in the sand or from melting ice on the pavement. In theory, a small amount of moisture will help embed the sand in the surface of the snow and then refreeze to create a sandpaper effect. But this rarely occurs.
Research on friction on pavement treated with abrasives shows that there is little benefit when traffic is present. Traffic causes the abrasive to be quickly carried or blown off the road. If there is melting, it is not likely that the abrasive will float and stay on the surface. More likely it settles, or gets pounded into the melting snow mixture. When that happens, it is no longer "anchored" to the surface and provides little value for traffic safety.
That's why weather and road conditions are carefully monitored during snowfalls to determine when and where it is most effective to apply abrasives. Sand is most effective when it is too cold for de-icing chemicals to work and in low volume traffic where it stays on the road surface.
The most popular alternatives to common salt are other chloride salts. Calcium chloride and magnesium chloride melt snow and ice more quickly at lower temperatures, but they are far more expensive than sodium chloride. The search continues for more environmentally- friendly alternatives. Urea, a fertilizer, can melt snow but adds nutrients to surface water and hastens oxygen depletion in receiving waters. Calcium magnesium acetate (CMA) can also reduce available oxygen. Recent research on CMA and a few other liquid de-icers indicate they are more toxic to fish than salt brine. Several municipalities around the state are trying beet juice mixed with salt to reduce the amount of chloride being used. This organic alternative has its own downsides. "It's expensive and messy," said Madison's Schumacher.
The practice of dumping snow removed from streets directly into waterways has largely been halted statewide because this snow contains so many contaminants including salt, nutrients, oil, sand, silt, litter, heavy metals and toxic chemicals. Spring melt water from large snow piles can also deliver large doses of pollutants directly to water bodies through nearby storm drains if the waters are not pre-treated. State and municipal ordinances recommend disposing of snow on land where contaminants and debris can be contained, collected or gradually released. The best disposal sites are on lands that drain to detention basins, which capture melt water pollutants that otherwise might reach storm sewers, surface waters or soak into groundwater.
Playgrounds, ballparks and parking lots are poor places for snow dumping as they are heavily used by people after the snow is gone. Snow piles have to be monitored and cleared of debris after the spring thaw and before any potential flooding.
In the City of Milwaukee, excess snow is piled by the Port of Milwaukee to melt into the ground. The city sells permits to commercial snow removal companies to allow them to dump excess snow on this site as well.
DOT's Road Weather Information System provides maintenance crews with accurate information about current weather conditions and detailed weather forecasts. Those forecasts are particularly valuable in helping counties decide when to apply anti-icing chemicals, which saves materials, cuts costs and can improve safety. The system includes automated reports from 59 weather and pavement sensors along state highways, detailed reports from a forecasting service, a winter storm warning service for county highway departments, and more than 500 mobile infrared pavement temperature sensors mounted to patrol trucks around the state. These systems are installed as part of new and renovated road building projects.
Pre-wetting and anti-icing to improve road safety – County highway departments and city snow plowing crews strive to work economically by using techniques like anti-icing and pre-wetting pavement, which can lower the amount of salting compounds that need to be applied. Liquid anti-icing chemicals sprayed on pavement before a forecasted storm or frost event are effective in reducing or preventing snow and ice from bonding with the pavement surface, which can make cleanup more efficient. At least one anti-icing application was made in 52 counties around the state during the 2007-8 snow season. The use of salt brine for pre-wetting, another economical technique, is on the rise as well – counties used a record 968,000 gallons in 2007-8. Pre-wetting salt and sand with a liquid de-icing chemical can significantly improve the amount of material that stays on the road. In 2007- 8, 90 percent of Wisconsin's counties pre-wetted their salt; 44 counties used salt brine for pre-wetting, which was an economical choice at about 15 cents per gallon.
In the City of Milwaukee, Department of Public Works crews use brine on bridges and overpasses before a predicted snow event. "The city started experimenting with salt brine in 2005, but the temperature doesn't cooperate with us every time. It usually works best at 25º F," said Booker.
Policies on when and how much road salt will be applied are regularly reviewed and updated with the aim to reduce use and find more environmentally safe, effective alternatives. As DNR's Bannerman asserts, there is no room for trade-offs; the driving public's safety is top priority.
"But we are thinking too short term on this issue," he says. Increased expectations for clear roads in the winter have contributed to the increased use of road salt over the years. Trucks want to make it to the market on time, people do not want to spend more time driving to work, and people need to get to the store. All of these expectations are consistent with promoting a healthy economy, but they challenge our efforts to reduce road salt use. Keeping our roads safe for driving by using more and more road salt is not sustainable. At some point the benefits of driving during severe weather conditions might be outweighed by the damage to our lakes, streams and groundwater," he said. Both in terms of our salt use and our driving habits, part of the solution is to slow down.
Theresa J. Lins writes from Milwaukee.