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The sound of water gurgling through storm sewers is the promise of a spring that's been a long time coming.
But it's also the sound of a toxic legacy that for decades has been quietly building in lakes and streams around the Twin Cities -- road salt.
The fish, bugs and other wildlife that live in the lakes pay a price for winter traffic safety when the snow melts. This winter, the Pollution Control Agency (PCA) started a four-year project to figure out which Twin Cities' lakes hold too much chloride, a primary ingredient in salt, and what it will take to keep urban waters healthy.
But the far more difficult task will be changing long-held beliefs about what it means to be a good citizen in a northern city. After all, most people in Minnesota, from homeowners to city officials, feel pretty strongly about keeping the sidewalks and roads clear and safe in the winter -- even if it means putting down a lot more salt than is necessary.
"Folks who grow up in Minnesota have a clear idea of that expectation and responsibility," said Kristen Nelson, a sociology professor at the University of Minnesota who studies environmental behavior. "It takes on important symbolic connotations."
Shingle Creek, which flows under highways and behind shopping malls in the western suburbs, is a case in point. It's in one of the few watersheds where the salt load has been calculated. In theory, to get it back to healthy levels, the nine communities along its banks would have to cut salt use by a whopping 71 percent, according to a recent analysis by the Shingle Creek Watershed District.
"It won't be possible to reach that," said Diane Spector, water quality manager for the watershed district. "When your tradeoff is public safety, it's very difficult."
But it wasn't impossible for Prior Lake. That city, which has won national recognition for its success in reducing road salt use, puts down 60 percent less than it did just a few years ago, saving $50,000 to $60,000 per year.
"This has been a great balance," said Steve Albrecht, public works director for Prior Lake. "We can save taxpayer dollars and take care of the lakes" without reducing safety.
350,000 tons used each winter
Road salt use has been rising steadily for more than 20 years. Now, in an average winter, some 350,000 tons of salt are dropped on roads, sidewalks and parking lots in the Twin Cities metropolitan area, according to a recent study by engineers at the University of Minnesota. In part, that's because there are more roads. But it's also because 20 years ago, public works departments switched from a sand and salt mixture to pure salt. Sand clogged storm drains and caused its own pollution problems, while salt alone was more effective.
"We hit a stage where everyone thought more was better," said Brooke Asleson, head of the PCA's Twin Cities chloride-reduction project. "But you reach a point where you can keep adding more, but are not getting more. You are just throwing money down the storm drains."
And into the water. Where, surprisingly, it just stays.
Eric Novotny, one of the researchers on the university chloride study, found that 78 percent of the salt applied to roads stays in the water. Unlike some other pollutants, it does not flow to the Mississippi and down to the Gulf of Mexico. Instead, the denser, salty water sinks to the bottom of lakes and into groundwater, accumulating year after year.
It's not harmful to people, though in high enough concentrations it can make water taste bad. But lakes and streams can be severely damaged by high levels. Too much salt in the water dehydrates insects and other microscopic life at the bottom of the food chain, and some species of fish. It also affects plants that live in and alongside the water.
Brownie Lake, a small, deep lake in Minneapolis near Cedar Lake Parkway and Interstate 394, contains so much chloride that its top and bottom layers no longer mix, which is vital to a healthy lake. In part, it doesn't turn over because the lake is so deep relative to its surface area, and pollutants of all kinds flow down its steep banks. But salt has been collecting in its depths for years. Two meters below the surface, the water in Brownie contains virtually no oxygen, said Rachael Crabb, a water quality expert for the Minneapolis Parks Department, indicating there's not much life at the bottom.
"Whatever chloride has come into Brownie from 394 is still there, and it's going to say there," she said.
Prior Lake wanted to avoid that fate for the 14 lakes in the city, said Albrecht. Since 2000 the chloride levels in its lakes has doubled.
"If we didn't do something, eventually, we would get to a critical level," he said. "And there is no way to turn back." In recent years, the city bought new, high-tech salt trucks, and began making its own brine to apply to roads before a snowstorm. It also uses GPS units to precisely regulate exactly how much salt the trucks put down in any location.
Since 2007, the amount of salt used per mile per snowstorm has been cut in half, Albrecht said. It took awhile to educate Prior Lake citizens about the project, and to convince them that roads were still safe, even if they were not cleared down to bare pavement. It worked, he said.
"Our community is completely behind us," he said.
Prior Lake is not alone. The Minnesota Department of Transportation, by far the state's largest user of road salt, uses similar technologies. It also runs a three-week boot camp every year for its staff of "snow fighters" at Camp Ripley in Little Falls, where snow truck drivers learn the science of salt application.
"Our salt use has stayed steady or gone down," said Steven Lund, maintenance engineer for MnDOT's Twin Cities' region.
But the PCA's Asleson said many cities are hesitant to invest in expensive new equipment and training unless they know how much chloride is polluting their water, and whether the investments will pay off 10 or 20 years down the road.
That's why the PCA launched its Twin Cities Metropolitan Area Chloride Project two years ago, which is now reaching the second, critical phase. In the next four years the effort will include both education and training for public and private snow plow operators, plus studies to identify lakes at risk and what it will take to keep them healthy. So far, Asleson said, the PCA has found 16 lakes in the state with chloride above healthy levels -- and 12 of them are in the metropolitan area. Ten were identified in 2010.
After each watershed district figures out the scope of the problem, then comes the daunting task of persuading people that when it comes to road salt, public safety is not the only responsibility, said the university's Nelson. "If we have the benefit of a natural landscape, we have to think about our responsibility for protecting that public space," she said.
Josephine Marcotty • 612-673-7394