DULUTH – Cars spraying slush cruised down Interstate 35 just a few dozen feet above the Lakewalk, where Katie Cassidy stood casting a green plastic bucket into Lake Superior on an unseasonably warm February day.

As the winter snow starts to melt, the road salt and other chemicals deployed to treat icy roads are slowly trickling into lakes, rivers and streams. Researchers are out taking regular samples of Duluth's waterways in the hopes of finding out: Is the world's largest freshwater lake at risk of becoming too salty?

"We can't continue to put salt in a freshwater environment like Lake Superior and expect it to stay the same," said Chris Cheney, maintenance operations director for the Minnesota Department of Transportation's First District, which covers the northeastern part of the state.

MnDOT is teaming up with the University of Minnesota Duluth's Natural Resources Research Institute (NRRI) to test the environmental effects of an alternative to road salt — potassium acetate, the liquid de-icer often used on airport runways. The research team is also working with the Minnesota Department of Natural Resources to figure out where the most road salt is washing into Lake Superior's tributaries.

Road salt is the No. 1 source of chloride — a chemical ion that does not degrade or evaporate over time — in Minnesota's bodies of water. For the past decade or so, scientists have ramped up warnings that rising chloride levels could threaten aquatic life in the state and turn tap water salty.

Researchers see potassium acetate as a potential problem-solver. It's biodegradable, it doesn't corrode infrastructure, and it works in colder temperatures than salt does.

It's also seven times more expensive. And scientists don't yet fully understand how potassium acetate runoff could affect Minnesota's waterways.

"It's a balancing act," Cheney said. "We are surrounded by the purest sources of fresh water in the United States. This is the place where we really need to be thoughtful about what we're doing to our environment."

"At the same time," he added, "we have some of the most challenging winter road conditions in the country."

Contaminated by chloride

On that sunny February day, Cassidy, a 22-year-old graduate student, gathered bottles of lake water to take back to the lab. Later that day, a pair of NRRI researchers headed out to take samples from five sites along Duluth's Tischer Creek, a stream that feeds into Lake Superior.

The team is looking at a handful of creeks and stormwater drains that flow down the city's hillside to gauge how chloride is getting into waterways and if there might be parts of town where officials should focus on mitigating salt use.

Given the lake's sheer size, it's unlikely that road salt runoff could ever completely contaminate the Great Lake, said Chan Lan Chun, the lead NRRI researcher on the project.

That doesn't mean chloride can't do damage. Increasing concentrations of the substance — especially in areas closer to Superior's shore, where Duluth pumps its drinking water — could cause "a cascade of biological effects," Chun said.

Some freshwater plants and animals can tolerate only very low amounts of salt. At higher salt levels, water can become unusable for drinking or irrigation.

And once salt gets into water, it's difficult to remove. "Prevention is better than remediation," said Chun, who added that one teaspoon can contaminate five gallons of water.

Across Minnesota, 50 lakes, streams and rivers fail to meet the state Pollution Control Agency's water-quality standards because they contain too much chloride. Duluth's Miller Creek, a trout stream, has been considered contaminated by chloride since 2010.

MnDOT alone used more than 200,000 tons of salt last year. Counties, cities, businesses and homeowners applied additional heaps to keep their roads and parking lots clear through Minnesota's icy winters.

"I don't want my kids or grandkids down the road not being able to drink water because we've contaminated it," Cheney said. "At the same time, I want them to be safe on the highways when they're driving."

Substituting road salt

Enter potassium acetate, the vinegar-like chemical MnDOT has been using to treat a few stretches of Duluth roads for the past three years.

When a winter storm hits, the state agency has been using the substance to de-ice lanes on the Blatnik and Bong bridges, under the I-35 tunnels near downtown, and for a stretch of state highway farther up the hill.

Experts across the country have experimented with a variety of road salt substitutes, from beet juice to cheese brine. MnDOT has started to use potassium acetate on these routes, and others when it gets particularly chilly, because it works in colder climates. Salt starts to lose its effectiveness at 15 degrees, but this new alternative can work well in subzero temperatures.

"I think it's going to prove to be a very great tool in our toolbox," Cheney said.

Now the question is how often it makes sense for MnDOT to use that tool.

On average, it costs about $10 for MnDOT to apply road salt on one lane mile, a stark contrast to the $75 to $90 per-mile price tag of potassium acetate.

At lower temperatures, the need for repeated applications of salt can narrow that gap a bit, but for most of the winter, potassium acetate is much more expensive, Cheney said.

Is footing the bill worth it if potassium acetate can keep Minnesota waters clean and safe?

Before deciding that, officials need to know more about how higher concentrations of the biodegradable substance could affect aquatic life.

Too much potassium acetate could cause bacteria to grow and eat all the oxygen that freshwater plants and animals need. Chun's team is studying the areas where MnDOT is testing the chemical to assess whether it may have any harmful environmental impacts.

In the lab, Cassidy started using equipment to measure the amounts of carbon, oxygen, nitrogen, metals, E. coli and other substances in the lake water. NRRI will continue taking samples year-round through next winter. Then they'll be able to use data to give MnDOT a better idea of what using potassium acetate on roads will do to the environment.

"The stuff we already use — salt — is pretty bad," Cassidy said.

Officials could also start to argue that potassium acetate will save the state money on road and bridge repairs because it doesn't cause the same levels of corrosion.

"I would say our work has been watched pretty closely around the country," Cheney said.

He considers Duluth a good test site. Traffic winds through the city's hilly roads even on the coldest and snowiest winter days, and its freshwater resources are abundant.

"If it works here," he said, "it will work anywhere in North America."

Katie Galioto • 612-673-4478