A University of Minnesota study shows chloride concentrations in Twin Cities metro waters are significantly higher than natural levels. The culprit: road salt—sodium chloride. Every winter metro government and commercial users apply almost 350,000 tons of road salt for de-icing. About 70 percent of it washes off into nearby lakes, wetlands, soils, and shallow aquifers where it is retained, according to research led by Heinz Stefan, professor in the Department of Civil Engineering at St. Anthony Falls Laboratory. The rest of the salt drains through creeks and storm sewers into the Mississippi River.
"No one has asked the question of where the road salt has gone when winter is over. Our study, funded by the Local Road Research Board, has been concerned with that issue in particular," said Stefan.
The research team tracked movement of chloride throughout the Twin Cities' watershed, distinguishing chloride applied by humans from chloride of geological or natural origin. Human-induced chloride concentrations (salinity) in 39 metro-area lakes increased from 1984 to 2005, following a similar upward trend in the amount of road salt purchased by state and local agencies.
If continued, this trend would produce a doubling of current salinity in some metro lakes in about 50 years. Compare this with a near zero concentration in the 1950s, when road salt application began.
"If we keep on doing this for another 50 years, we may have a significant problem [with water quality]," Stefan said. "Certainly if ground water becomes saline, when we use that water we may have to treat it, at significant cost, by reverse osmosis, to remove that salt."
In their 2010 draft report, the Minnesota Pollution Control Agency listed 11 metro-area streams as impaired by chloride. Minnehaha, Nine Mile, Bevens, Shingle, and Battle creeks are just a few of those with chloride concentration levels above the chronic standard of 230 milligrams per liter.
Continuous levels of chloride concentration as low as 230 mg/L (equivalent to roughly 1 teaspoon of salt in 5 gallons of water) have been shown to be harmful to aquatic life. Salinity exceeding this standard can interfere with plant growth and reduce the diversity of organisms in rivers, according to a recent U.S. Geological Survey study. Road salt also mobilizes heavy metals. Soil along major highways contains lead and cadmium, which come from motor vehicles. When road salt spreads onto roadsides, the chloride releases these heavy metals from the soil and they become water soluble and can run into nearby lakes, rivers, and ground water.
Keeping roads free of ice is essential for safe winter travel, but using less salt can reduce the impact on watersheds. Sodium chloride is ineffective at temperatures below 15 F, when other, more expensive melting agents such as magnesium or calcium chloride work better. For temperatures at or above 15 degrees, road and sidewalk applications need only be 1 to 3 cups of rock salt per 1,000 square feet.
The University of Minnesota began using these guidelines in 2006. Prior to its reduction program, the university used an average of 775 tons of salt per year on its Twin Cities campus. From 2006 to 2009, the university used a yearly average of 462 tons, about a 40 percent reduction. The program saved the university more than $50,000 in road salt costs in the first year alone. Minnesota Department of Transportation road crews have also improved the efficiency of their road salt applications by putting down a brine mixture in many areas before a storm to prevent ice from bonding to the pavement.
"If road salt application rates are reduced in future winters, it is projected that the lakes will respond with noticeably lower chloride concentrations within five to 10 years," said Stefan.
There is good news ahead, as the DNR is using Clean Water Legacy funding to support three research projects that aim to better assess and reduce pollutants entering lakes, streams, and wetlands. This research includes the evaluation of new technologies to reduce road salt use in winter.
"The Clean Water Legacy staff will use information from the road salt study to better understand how aquatic organisms are impaired where chloride is one of the potential stressors," said Nick Proulx, DNR Clean Water Legacy specialist.
Learn more about Stefan's research. Learn more about road salt application training.
Maia Homstad, freelance writer
