Groundwater Disinfection Halts Deadly Legionnaires' Outbreak in Minnesota

University of Minnesota researchers discover simple water treatment fix to prevent future community-wide epidemics.

Apr. 10, 2026 at 9:43am

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A ghostly, translucent X-ray photograph revealing the internal structure of a water pipe fitting, conceptually illustrating the hidden risks within a community's water system that can enable the spread of Legionnaires' disease.An X-ray view exposes the hidden vulnerabilities within a community's water infrastructure that can enable the spread of deadly Legionnaires' disease.Grand Rapids Today

Researchers at the University of Minnesota Twin Cities have published a groundbreaking study in Emerging Infectious Diseases that reveals how introducing chloramine disinfection to untreated groundwater successfully halted a deadly Legionnaires' disease outbreak in Grand Rapids, Minnesota. This is the first time scientists have proven that strategic water treatment can stop a community-wide Legionnaires' epidemic, raising urgent questions about the safety of groundwater systems across the country.

Why it matters

Legionnaires' disease is a severe and often fatal form of pneumonia caused by the Legionella bacteria, and it has become the most prevalent waterborne disease in developed countries. The University of Minnesota study directly connects a Legionnaires' outbreak to an entire community water system for the first time, underscoring the critical need to monitor and treat groundwater supplies that many smaller towns and rural areas rely on.

The details

In 2023-2024, Grand Rapids faced a Legionnaires' disease crisis with 34 confirmed cases and 2 fatalities. Researchers discovered alarming levels of Legionella throughout the community's water infrastructure, linked to high levels of assimilable organic carbon (AOC) - a nutrient that fuels bacterial growth. To combat the outbreak, the city introduced chloramine disinfection, which caused Legionella levels to plummet to undetectable and halted new cases.

  • In 2023 and 2024, Grand Rapids faced a Legionnaires' disease outbreak with 34 confirmed cases and 2 fatalities.
  • The University of Minnesota study was published in April 2026 in Emerging Infectious Diseases.

The players

Molly Bledsoe

An environmental engineering alumnus and lead author of the University of Minnesota study.

Tim LaPara

A professor in the University of Minnesota's Department of Civil, Environmental, and Geo-Engineering.

Maya Adelgren

A graduate student who co-authored the University of Minnesota study.

Apoorva Goel

A post-doctoral researcher who co-authored the University of Minnesota study.

Raymond Hozalski

The project co-leader of the University of Minnesota study.

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What they’re saying

“This study directly connects a Legionnaires' disease outbreak to a community-wide water system for one of the first times.”

— Molly Bledsoe, Environmental engineering alumnus and lead author

“Our findings expose a significant gap in public health safety.”

— Tim LaPara, Professor, University of Minnesota Department of Civil, Environmental, and Geo-Engineering

What’s next

The University of Minnesota research team is now pushing to investigate whether other groundwater-supplied systems across the country face similar risks of Legionnaires' disease outbreaks linked to untreated water sources.

The takeaway

This study challenges communities to rethink assumptions about the safety of groundwater and highlights the critical importance of monitoring and treating water supplies, even in smaller towns and rural areas, to prevent future deadly Legionnaires' disease outbreaks.