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A coal ash wastewater leak at a northern Minnesota power plant earlier this week was more than five times larger than original estimates, according to Duluth-based Minnesota Power and the Minnesota Pollution Control Agency. 

The utility initially reported about a million gallons of wastewater leaked out of an underground pipe late Tuesday morning at its Boswell Energy Center in Cohasset near Grand Rapids. 

Now Minnesota Power and the MPCA say about 5.5 million gallons escaped. 

Preliminary modeling shows increased levels of sulfate and boron in the area where the wastewater entered a creek that flows into Blackwater Lake, an impoundment on the Mississippi River. 

Sulfate levels are below the federal drinking water standard of 250 milligrams per liter, but above Minnesota’s wild rice standard, which limits sulfate to 10 milligrams per liter to protect the culturally important plant. 

Sampling did not detect mercury, arsenic and selenium, the utility said. 

Ongoing sampling and monitoring will help determine how widespread or severe the impacts are, and what tools and technology will be most effective to minimize impacts. The MPCA has hired a contractor to conduct sampling independent of Minnesota Power. 

“The MPCA remains focused on evaluating concentrations and extent of potential pollutants released and the impacts to soils, Blackwater Creek and Lake, aquatic life and wild rice beds,” the agency said in a statement. 

The MPCA and Minnesota Department of Health say they are evaluating receptors and drinking water intakes downstream of the spill and will notify cities of what they find. 

Work is expected to increase over the weekend to clean up the site in conjunction with the MPCA and the U.S. Environmental Protection Agency. Minnesota Power plans to excavate soil near the spill location to remove contaminants. 

The utility is also coordinating with the Leech Lake Band of Ojibwe to assess the area for tribal or cultural resources before excavating. 

The leak occurred from an underground pipeline that transfers wastewater from an inactive settling pond that stored ash from coal combustion. The utility used the water in the plant for dust and temperature control. 

The spill occurred on land, but a portion of the wastewater flowed to Blackwater Creek and into Blackwater Lake. Of the estimated 5.5 million gallons, Minnesota Power said in a statement that “an unknown but potentially significant portion of that amount entrained within the upland soils.”

The utility added that the updated spill volume does not change the spill mitigation and response activities underway. 

The Boswell power plant is Minnesota Power’s largest electricity generator. The utility plans to cease coal operations at the power plant’s Unit 3 by 2030, and at Unit 4 by 2035, to comply with the state’s requirement for 100 percent carbon free electricity by 2040.

In the wake of the incident, the Sierra Club has called for the utility to retire the plant sooner, by 2030.

Correction (July 19, 2024): A previous version of this story misidentified the river near the impoundment where wastewater flowed. The story has been updated.

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Dan Kraker

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Minnesota Power first said the leak Tuesday at the Boswell Energy Center in Cohasset was about 1 million gallons. On Friday, the utility and state officials say it was actually 5.5 million gallons. 

Ash wastewater spill at northern Minnesota coal plant more than five times larger than first reported

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