Minnesota Power, an ALLETE Company

Current Status and Background

On July 16, 2024, staff at Boswell discovered an underground elbow from a pipeline transporting air scrubber wastewater from the Unit 4 Pond to the plant had failed, releasing approximately 5.5 million gallons of wastewater. The wastewater entered soils surrounding the pipe elbow and eventually flowed through the subsurface soils and overland to the north bay of Blackwater Lake. The pipeline was immediately shut down on discovery of the spill, and all required regulatory agencies were notified.

The primary parameters of concern in the wastewater are sulfate and boron, based on historical and spill-related analytical sampling. No heavy metals, PCBs, PAHs, or other parameters have been detected at significant levels.

Since the release in July 2024, numerous investigations and remedial activities have been completed. Minnesota Power collaborated with regulatory agencies and tribal organizations to plan and execute each phase of work, as well as to obtain all applicable permits and approvals.

What are the potential impacts from the spill?

The spill contained primarily sulfate and boron, which were captured in the wastewater via air scrubbing operations in Boswell’s Unit 4 from 1980 to 2014. Unit 4 upgraded the wet scrubbing operation to a semi-dry NID scrubber in 2014 and discontinued use of Pond 4, which initiated decommissioning activities.

Sulfate

There are two regulatory standards for sulfate: a federal secondary drinking water standard from the U.S. Environmental Protection Agency (EPA) of 250 mg/L, and a Minnesota specific standard of 10 mg/L meant to protect wild rice. According to the EPA, secondary drinking water standards are not based on threats to human health. Rather, they are set to avoid secondary impacts to drinking water, such as odors or bad tastes.

The highest sulfate concentrations measured in the north bay of Blackwater Lake ranged from approximately 300-400 mg/L, but decreased significantly -- typically under 10 mg/L -- where Blackwater Lake enters the Mississippi River. There are no drinking water wells or intakes in the north bay area of Blackwater Lake.

Sulfate levels in the north bay of Blackwater Lake were above wild rice-based sulfate water quality standards, with occasional, short-lived concentrations above 10 mg/L observed south of the old Highway 6 bridge. No immediate impacts were observed to the existing low- and moderate-density wild rice stands upstream (or downstream) of the bridge in 2024; however, Minnesota Power will conduct field observations; water, sediment, and porewater sampling; and laboratory ecotoxicity testing to determine short- and long-term impacts of the spill to wild rice. Methodologies, data interpretation, and decisions on additional testing, mitigation, or restoration activities will be determined in coordination with the Minnesota Pollution Control Agency, state Department of Natural Resources, and the Leech Lake Band of Ojibwe.

Another potential concern from the spill was increased mercury methylation due to increased sulfate loading, which could increase the amount of sulfate-reducing bacteria in the spill area sediments. While mercury concentrations in both the natural lake water and the spill water were very low – at or around 0.5 parts per trillion – Minnesota Power is taking steps to measure the impact of the spill on mercury bioaccumulation through field sampling of tissue from young-of-year perch in 2026.

Boron

Boron was also released during the spill event. However, the higher concentrations of boron were limited to the area adjacent to the spill site, where there are no drinking water wells or intakes. Levels of boron were well below the most conservative drinking water standards where Blackwater Lake enters the Mississippi River. Minnesota Power continues to monitor boron levels and will take appropriate mitigation actions if needed.

Are operations at the Boswell facility impacted?

Wastewater from the Unit 4 Pond was being transported via pipeline to the facility for evaporation as part of the pond decommissioning effort. Because the pipeline was used for pond decommissioning, and not active operations at the energy-generating facility, there were and are no impacts to the day-to-day operations of Boswell Energy Center. Minnesota Power continues to conduct additional piping integrity evaluations to help prevent spills.

Where can I find more information?

Customers and community members can learn more from these additional resources:

• Protecting wild rice waters | Minnesota Pollution Control Agency
• Sulfate In Well Water - MN Dept. of Health
• Secondary Drinking Water Standards: Guidance for Nuisance Chemicals | US EPA
• Boron and Drinking Water