Water Treatment Overhaul Stops Legionnaires' Outbreak

University of Minnesota study shows disinfection of groundwater curbed deadly disease in Grand Rapids, Minnesota

Published on Feb. 20, 2026

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Researchers at the University of Minnesota Twin Cities published a study in Emerging Infectious Diseases showing how strategic changes in water treatment effectively stopped a deadly outbreak of Legionnaires' Disease in Grand Rapids, Minnesota in 2023 and 2024. The city implemented chloramine disinfection after finding concerning levels of the Legionella bacteria in its community water system, leading to a dramatic drop in cases.

Why it matters

Legionnaires' disease has become the most common waterborne disease in the developed world, with outbreaks often linked to cooling towers and water heaters. This study is one of the first to directly connect a Legionnaires' outbreak to an entire community water system, highlighting a critical gap in public health safety, especially in smaller or rural towns that rely on undisinfected groundwater.

The details

In 2023 and 2024, Grand Rapids, Minnesota had 34 confirmed cases of Legionnaires' disease and two fatalities. Researchers found concerning levels of Legionella bacteria in buildings throughout the community. To combat the spread, the city implemented chloramine disinfection, which caused Legionella levels to drop so low they could no longer be detected. Since the new treatment system was put in place, the city has had zero new cases of Legionnaires' disease.

  • In 2023 and 2024, Grand Rapids, Minnesota had 34 confirmed cases of Legionnaires' disease and two fatalities.
  • The city implemented chloramine disinfection to combat the outbreak.

The players

Molly Bledsoe

A University of Minnesota environmental engineering alumnus and lead author on the paper.

Tim LaPara

A professor in the University of Minnesota Department of Civil, Environmental, and Geo- Engineering and one of the senior authors on the paper.

Grand Rapids, Minnesota

The city that experienced the Legionnaires' disease outbreak and implemented the water treatment changes.

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What they’re saying

“This study is one of the first to directly connect a Legionnaires' disease outbreak to an entire community water system.”

— Molly Bledsoe, University of Minnesota environmental engineering alumnus and lead author (Emerging Infectious Diseases)

“Our findings highlight a critical gap in public health safety. Many smaller or rural towns rely on undisinfected groundwater, assuming it is naturally protected.”

— Tim LaPara, Professor, University of Minnesota Department of Civil, Environmental, and Geo- Engineering (Emerging Infectious Diseases)

What’s next

The research team is hoping to continue this study to explore whether other groundwater-supplied community water systems also contain elevated levels of assimilable organic carbon (AOC), a type of nutrient for bacteria that may have fueled the Grand Rapids outbreak.

The takeaway

This case highlights the importance of proper water treatment, even in smaller or rural communities that rely on groundwater. By implementing chloramine disinfection, Grand Rapids was able to effectively stop a deadly Legionnaires' disease outbreak, demonstrating how strategic changes to water infrastructure can protect public health.