Road salt accumulates in metro-area waters, a new study finds

Road salt accumulates in metro-area waters, a new study finds

By Maia Homstad

Road salt used throughout the winter is making the state's lakes, rivers, and groundwater saltier, a situation that could harm aquatic life and drinking water, according to a University of Minnesota study. But additional training of snow plow drivers and more judicious use of road salt could help lessen the impact and save the state money.

The researchers studied 39 lakes, three major rivers, 10 tributaries, and numerous observation wells. Their results came as a shock: About 70 percent of the road salt being applied in the metro area is retained in the watershed. The University researchers recently reported their findings to the Local Road Research Board, which funded the study.

Nearly 350,000 short tons of road salt, in the form of sodium chloride, are applied for de-icing in the seven-county Twin Cities metro area every year.

Watch a video of Heinz Stefan and Eric Novotny discussing their research.

"Nobody has asked the question, 'Where does the road salt go after the winter is over and roads are safer?,'" says research team leader Heinz Stefan, a civil engineering professor at the University's St. Anthony Falls Laboratory. "Our study has been concerned with that question in particular."

Stefan, along with Eric Novotny, Andrew Sander, Dan Murphy, and Omid Mohseni, tracked the movement of chloride applied by humans throughout the metro area water system, distinguishing it from salt of geological or natural origin. They found that the salinities (the concentration of chloride ions in milligrams per liter, or mg/L) of 39 metro area lakes have increased over the past 22 years, following a similar trend in road salt purchases by the state of Minnesota.

Although there is much variation, the median salinity in metro-area lakes stands at about 87 mg/L. It is rising at about 1.5 mg/L per year, which puts it on course to double in 58 years. The Minnesota Pollution Control Agency has set a salinity standard of 250 mg/L (equivalent to a teaspoon of salt in five gallons of water) for surface waters designated important for aquatic life and recreation.

Some lakes already exceed that salinity, especially in March and April, when snow melt washes salt into lakes and rivers. Highest were the surface waters of Spring Lake--essentially a storm detention pond near I-394 in Minneapolis--which showed an average salinity of 505 mg/L, with an annual maximum of 1018 mg/L. On the low end, White Bear lake posted average and maximum salinities of 31 and 43 mg/L, respectively.

In contrast, metro-area lakes had near zero salinity in the 1950s, when road salt application began.

The long-term effects could be severe. Various studies have shown that continuous salinities of 250 mg/L can be harmful to aquatic life and can affect the taste of drinking water. In 2008 the Minnesota Pollution Control Agency listed five metro area streams as already impaired by chloride. Increases in sodium and chloride have been shown to decrease the biodiversity in wetland areas, altering the development of wood frogs, decreasing the number and types of fish available, and increasing mortality rates of organisms that rely on an aquatic system. Increases in sodium and chloride have also been shown to increase mobilization of heavy metals in the soil along major highways.

To help reduce the effects of rising salinities, winter road managers for counties and cities and at the Minnesota Department of Transportation recommend more judicious use of road salt by training snowplow drivers to get the most out of the amounts they apply. For example, using sodium chloride at pavement temperatures below 15 F is generally not effective. At higher temperatures, the managers suggest using liquid salt solutions (brines) at a rate of only one to three cups of salt per 1,000 square feet, and applying the salt prior to snowfall events. Training began at the University of Minnesota two years ago; since then the University has reduced use of road salt by 41 percent, and it saved more than $50,000 in the first year.

Other, less harmful chemicals that increase winter road safety are available, but they cost more, Stefan says.

A pdf of the report is available.