Search is on for the road salt that's polluting Minnehaha Creek

Road salt is efficient at melting ice and improving traction on slippery streets and sidewalks, but it's a menace to clean water in nearby lakes and streams.

A team of researchers was tracking the amount of saline runoff into Minnehaha Creek last week, searching for weak spots in the ice to sample the season's earliest trickles of water below.

"What we want is to get that mixture of stream flow, and how the chloride or salt is getting into the system," said Yvette Christianson, water quality specialist for the Minnehaha Creek Watershed District.

She was standing below the 28th Avenue bridge over the creek in south Minneapolis, one of 10 spots along Minnehaha's 22-mile run through the west metro that is checked almost monthly for temperature, flow and various pollutants, including salt.

Most of the salt products used as winter de-icers contains chloride, said Christianson, and that's the problem. Chloride is denser than water and sinks to the bottom of lakes, streams and wetlands. Unlike other pollutants that might flush through a system or break down with seasonal sunlight and temperature changes, chloride does not degrade or evaporate and accumulates over time, she said.

"One teaspoon of salt can pollute five gallons of water," said Brooke Asleson, metro watershed project manager for the Minnesota Pollution Control Agency. At high concentrations, chloride is lethal to fish, bugs and vegetation, she said, but even at lower levels it can harm their ability to reproduce and thrive.

Especially vulnerable, said Asleson, are lakes and streams in the metro area, which uses about 365,000 tons of salt each winter on roads.

A 2009 University of Minnesota study estimated that 70 percent of the salt applied in the seven-county metro area remains in area lakes and wetlands and is not washed into rivers that leave the state.

Pollution control officials are working with Minnehaha Creek Watershed District and 10 other agencies on a three-year study of chloride in 74 lakes, 27 streams and eight storm sewers. After this spring's samplings, a final report will be compiled to better define the problem and offer solutions, Asleson said.

From the data collected since 2010, she said, it looks like 28 of the lakes have serious problems: year-round salt levels that violate state standards.

That number vastly underestimates the scope of the problem, Asleson said, since more than 800 of the metro area's 949 lakes have not been properly tested for chloride.

"If we want to continue to have lakes and streams and wetlands with healthy fish and bug communities, we really do need to pay attention to this situation," she said.

Granata wore waders and stepped carefully on the snow-covered ice to find open spots. He collected water samples in small bottles for a lab to analyze. He stuck a probe in the flowing water to measure dissolved oxygen, conductivity, temperature and other indicators of water quality. It's the beginning of the spring tests, he said — so early that some sections of the stream were still frozen solid, with runoff pooling on the ice surface.

Water in the creek is sparse in winter, since the dam that controls most of its flow from Lake Minnetonka is closed.

"We're sampling our 10 sites to build a picture of what's happening in the whole system," Granata said.

His meter showed that conductivity in the water was high, indicating that it probably contained fairly high concentrations of chloride that will be confirmed more precisely in lab results.

Asleson said that the Minnesota Department of Transportation, cities, counties and others in the winter road maintenance industry have been receptive to environmental concerns and are experimenting with ways to use less salt or different formulations of it without compromising road safety.

Urban waters are delicate to begin with, said Asleson, and lessening their salt intake will help keep them healthy.

"It doesn't take a lot of pollution to have a bigger impact on these streams, because they're already stressed," she said.

Tom Meersman • 612-673-7388