Authorities are delaying a planned aluminum sulfate treatment of Lake Agnes until 2020, instead planning to concentrate on tagging carp in Lake Winona, they said at a Tuesday evening meeting.
The two lakes, plus Lake Henry, form a chain of lakes affected by the decades-old practice of pumping sewage into the lakes. The lakes' levels of phosphorus, as well as chloride, have landed them on the state's list of impaired waters, a black eye for a region that capitalizes on its recreational lakes, most of which are not impaired.
Officials plan to restore the lakes by using the alum treatment to bind to phosphorus pollution and sink it to the bottom of the lake, removing carp from Lake Winona because the fish stir up pollutants, possibly emptying the water from Lake Winona and upgrading the sewage treatment plant.
Alum is a safe, effective treatment that has worked in other lakes, said Joe Bischoff, a consultant for the Alexandria Lake Area Sanitary District, which has been tasked with restoring those three lakes.
"What we're applying is naturally occuring in the lakes anyway," he said. "We're just adding a little more."
Aluminum, he said, is the third most abundant element on earth and the most abundant metal in the earth's crust.
The process would involve a boat injecting it into the water through hoses, and technology would show operators where to go and how much to apply, given the contours and composition of the lake bottom. The aluminum sulfate would create a milky cloud that would settle quickly to the bottom, where it would remain.
While Bischoff said he wouldn't swim in the lake during the application, he has seen waterskiers cut across aluminum sulfate plumes in other lakes he has treated, and Lake Agnes would remain safe for recreation.
"You would have to physically eat a tablespoon of sediment every day" to exceed safe levels of consumption, he said.
The alum can become toxic if the acidity levels fall below 6, and it can kill macroinvertebrates, but acidity levels can be controlled and the macroinvertebrates will rebound after the treatment, Bischoff said. Macroinvertebrates include spineless organisms such as snails and crayfish.
The alum would need to be re-applied in about two years, and likely every couple of decades after that, he said. The cost for the first two treatments would come to approximately $318,000.
Once the lake water clears, aquatic plants will grow, which are healthy for the ecosystem but which may hinder boating, he said.
"It'll take some management once you get there," Bischoff said.
About 30 people attended the meeting, mostly sanitary district board members and lakeshore owners.
Speaking before the meeting, Craig McMillan, president of the Lake Henry Lake Association, said he has a major problem with the lake's chloride levels. A dozen years ago, a new landscaping project at his home burned up when he sprayed it with lake water.
"Our lakes are our biggest asset in Alexandria," he said. "We need to take care of them."