Soil and water

Minnesota’s cleaned lakes and rivers show a way forward

LeMay Lake was doomed.

Harmful levels of nutrients have leaked into the small, shallow lake for decades, swept away by stormwater draining from highways, homes, and sidewalks built over the past century. In 2014, to everyone’s surprise, nutrient pollution forced the state to add LeMay in the town of Eagan to its dastardly list of degraded water.

But only seven years later, the water in the lake is clearer. The nutrients are under control.

Like LeMay, dozens of other polluted lakes and rivers across the state have been made healthy over the past decade. This number is paltry compared to the thousands of waters that were added to the state’s list of polluted and degraded waters during this period, but it shows that municipalities, lake associations and other managers of the water have the tools to bring their lakes back to life.

It takes time, money and willpower, said Eric Macbeth, manager of Eagan’s water resources.

“One size doesn’t fit all,” he said. “We had to find specific projects to remove phosphorus from the lake. With so much waterproof surface, we knew it would be a challenge.

Since the passage of the Clean Water Act in the 1970s, Minnesota and the rest of the country have made huge strides in reducing water pollution. Gone are the days when raw sewage was dumped directly into the Mississippi River and companies had carte blanche to dump whatever they wanted into the Port of Duluth. But Minnesota and the Upper Midwest have never brought nutrient pollution under control.

Farmers, whose main source of nutrient pollution from fertilizers, are largely exempt from the Clean Water Act. And in urban areas, so much land has been paved that streams, rivers and lakes collect much more stormwater than decades ago. This runoff brings nutrients from all the dead leaves, cut grass, yard waste and animal droppings it touches.

Nutrients, especially phosphorus, become a problem when there is too much of it. They are the reason why so many ponds and shallow bays in the Twin Cities and their surrounding suburbs turn a brilliant, sickly green in the summer. They cause algae blooms that make the water too dirty to dip a toe in and cause epidemics of cyanobacteria that can poison dogs and hospitalize swimmers. When algae die, they suck oxygen from a lake, stressing or even killing fish and insects.

Nearly 750 of Minnesota’s waters have unhealthy nutrient levels, according to the Minnesota Pollution Control Agency’s proposed altered water list for 2022. 891 others have too much bacteria and 444 are too cloudy from mud, sediment and water. erosion, caused by runoff problems similar to nutrient pollution.

But the state may be turning a corner. In the past two years, 53 lakes and rivers have been taken off the list, many of them like LeMay, because cities have been successful in reducing nutrient pollution. This is the largest number ever removed at one time, said Miranda Nichols, wastewater list coordinator for the Pollution Control Agency.

“Restorations get very complicated very quickly,” she said. “Sometimes it can be as simple as replacing a culvert or removing an obstacle to a stream – the fruit at your fingertips. But every write-off and every project is completely different.”

The most significant improvements have been the elimination of phosphorus pollution.

The changes won’t happen overnight, but progress is happening, said Wayne Cords, head of the watersheds section for the Pollution Control Agency.

“Water is truly a reflection of the land that surrounds it,” he said. “It depends on the science advancing and helping us better determine where the sources are. You have to stop the source.”

The town of Eagan discovered that phosphorus entered Lake LeMay from all of the stormwater that flowed through its gutters. An in-depth study led to one site in particular where rainwater was pouring into the lake.

At this site, the city installed a filter using a mixture of sand and iron developed by the University of Minnesota for approximately $ 450,000. The phosphorus and iron molecules bind together, so when stormwater flows through the sand, the phosphorus attaches to the iron and is trapped before it reaches the lake.

The city also spent about $ 80,000 to sprinkle the lake with a mixture of aluminum hydroxide that has been used in lake management for decades, said Macbeth, the water resources manager. Like iron, phosphorus binds to alum, causing it to sink out of surface waters and trap it at the bottom of the lake where it cannot harm.

“So we found plans to prevent it from going externally and to reduce it internally,” he said.

Sanitary standards would allow a lake the size of LeMay to have phosphorus levels of around 60 parts per billion. Before the filters were added and processing started, LeMay was reaching almost 90 parts per billion. Two-thirds have since been removed, leaving the lake with less than 27 parts per billion.

The city also just completed a $ 600,000 underground vault that will divert much of the excess stormwater from the lake, which should further reduce nutrient pollution.

“When you change a capital infrastructure, it can easily be over a million dollars the city has invested,” Macbeth said. “But we’re pretty optimistic. We’re going to take this lake off the list. I didn’t even expect to imagine that during my time it could have such an improvement.”

Nutritional treatments can become trickier in rural areas, but they are possible.

Wright County removed 25 pounds of phosphorus per year to reach Waverly Lake by installing ponds to temporarily store rainwater at two nearby farms. The total cost was approximately $ 45,000.

The town of Waverly has also restored the shorelines and farmers have added cover crops to limit erosion, said Dan Nadeau, senior resource curator for the Wright Soil and Water Conservation District.

“It’s the icing on the cake,” he said. “Farmers lose money when their topsoil goes into the lake, so it’s about finding these winners.”