In the two-year update released Tuesday, 511 water bodies were added to the list, bringing the total to 3,643.
Powderhorn Lake, an urban pond in the heart of the Minneapolis landscape, was once the poster child for dirty lakes that couldn't be fixed.
For years, it was a scummy holding pond for trash, excess fertilizer and other nutrients running off from storm sewers, streets and lawns.
But today, thanks to stormwater treatment, neighborhood rain gardens and barley bales that suppress algae, Powderhorn Lake is one of 13 bodies of water to be removed from the long list of impaired waters in Minnesota.
"Never say never," said Glenn Skuta, water monitoring manager for the state Pollution Control Agency (PCA), which released its biennial inventory of polluted waters Tuesday.
To date, the PCA has assessed slightly more than half of the state's 81 watersheds, a list that includes thousands of lakes, streams and rivers.
In the two-year update released Tuesday, 511 water bodies were added to the list, bringing the total to 3,643. The additions include 148 due to mercury contamination in fish and 89 for excess nutrients. Ten, including five sections of the Le Sueur River, were added because of the chemical compound known as PCBs in fish. One, a section of Seven Mile Creek west of Mankato, was included for chlorpyrifos, a pesticide.
A total of 45 water bodies have been removed from the list over the years. Because the agency is constantly testing new bodies of water, entries accrue much faster than removals, say state officials. And improving water quality to the point where it meets state standards can take many years.
The list of 13 released Tuesday also includes Jewitts Creek in Meeker County, the receiving water body for the Litchfield wastewater treatment plant. It was originally listed in 1994 as impaired for ammonia, which can be toxic to some forms of aquatic life. Upgrades to the wastewater treatment facility solved the problem, officials said.
The Credit River in Scott County was originally listed as impaired for water clarity. A collaboration of citizens, the county, the city of Savage, the PCA and other groups cleaned up the river by installing rain gardens to filter urban runoff and stabilizing eroding slopes with vegetation.
But nine of the 13 were delisted either because they improved on their own for unknown reasons or because more comprehensive monitoring showed that they weren't polluted in the first place, Skuta said. "Sometimes the data is better and we don't know why," he said.
For Powderhorn Lake's improvement, the reasons are clear.
Years ago, the surface of the lake would be covered in trash after a big storm, said Rachel Crabb, water quality specialist with the Minneapolis Park and Recreation Board. Then the city installed treatment systems that use centrifugal force to separate trash, sediment and other pollutants from storm water.
Starting in 2004, the park system began treating the lake annually with bales of barley straw anchored near the shoreline, which suppresses algae growth.
By 2007, the water was clear enough to bring back aquatic plants. Today, there are two types of native plants growing in the lake and, so far, no invasives, said park officials.
In addition, about 100 rain gardens were installed in the neighborhood. The park also began managing the lake's fish population: Channel catfish were stocked to eat the sunfish that stirred up sediment on the bottom.
Now, the problem is keeping enough big fish in the lake to control the little fish -- because Powderhorn Lake has become one of the most popular fishing spots in the city, Crabb said.
Josephine Marcotty • 612-673-7394