An Outbreak That Revealed a Hidden Vulnerability
When 34 residents of Grand Rapids, Minnesota, developed Legionnaires' disease between 2023 and 2024, two of them fatally, the outbreak prompted an investigation that would reshape understanding of how waterborne disease can emerge in communities that believe their water supply is safe. The culprit was Legionella pneumophila, the bacterium responsible for a severe form of pneumonia that is particularly dangerous for elderly and immunocompromised individuals.
What made the Grand Rapids outbreak unusual was not the pathogen itself — Legionella outbreaks occur regularly across the United States, typically associated with cooling towers, hot tubs, and large building plumbing systems. What was unusual was the source: the city's groundwater supply, which had been distributed without disinfection based on the widespread assumption that groundwater drawn from underground aquifers is naturally protected from microbial contamination.
Researchers from the University of Minnesota Twin Cities, publishing their findings in the journal Emerging Infectious Diseases, have now documented what happened when the city implemented chloramine disinfection of its water supply — and the results carry implications for communities across the country that rely on similar untreated groundwater systems.
The Investigation: Finding the Bacterial Fuel
The research team, led by Molly Bledsoe and including Tim LaPara, Maya Adelgren, Apoorva Goel, and Raymond Hozalski, conducted a comprehensive analysis of Grand Rapids' water system to understand not just what was causing the outbreak but why it was happening in this particular system at this particular time.
Their investigation identified elevated levels of assimilable organic carbon (AOC) in the groundwater — a finding that proved critical to understanding the outbreak mechanism. AOC is a measure of the organic compounds dissolved in water that bacteria can use as food. While groundwater typically contains lower levels of organic nutrients than surface water, the Grand Rapids supply had sufficient AOC to support robust Legionella growth within the distribution system's pipes and infrastructure.
This discovery challenged a fundamental assumption in water treatment. As Professor Tim LaPara noted, many smaller or rural towns rely on undisinfected groundwater, assuming it is naturally protected. That assumption holds when the groundwater is low in nutrients that support bacterial growth. But when organic carbon levels are elevated — whether due to geological conditions, infiltration from surface sources, or changes in aquifer chemistry — the water can support pathogen growth as readily as surface water would.
The Intervention: Chloramine Disinfection
Based on the investigation's findings, Grand Rapids implemented chloramine disinfection of its water supply. Chloramine, a compound formed by combining chlorine with ammonia, is widely used in municipal water systems for its ability to maintain residual disinfection throughout distribution networks. Unlike free chlorine, which dissipates relatively quickly, chloramine persists longer in pipes, providing sustained antimicrobial protection that reaches the most distant endpoints of the distribution system.
The results were unambiguous. Following implementation of chloramine treatment, Legionella levels in the water system became undetectable. More importantly, zero new cases of Legionnaires' disease were reported after the disinfection system was activated. The outbreak was stopped entirely through a single, strategic change in water treatment practice.
This represents what the researchers describe as the first documented evidence of a Legionnaires' disease outbreak being halted through disinfection of previously untreated groundwater. While chloramine treatment of surface water is standard practice in most municipalities, applying it to groundwater systems that were previously considered safe enough to distribute without treatment is a relatively novel intervention.
The Scope of the Problem
The implications of the Grand Rapids case extend far beyond a single Minnesota city. Across the United States, thousands of communities — predominantly smaller and rural ones — distribute groundwater without disinfection. These systems serve millions of people who, like the residents of Grand Rapids, may be drinking water that supports bacterial growth without their knowledge.
The prevalence of elevated AOC in groundwater systems nationally is not well characterized, precisely because testing for it has not been a standard part of water quality monitoring for many utilities. The Grand Rapids experience suggests that this gap in monitoring may be exposing communities to preventable disease risk.
Legionnaires' disease itself is likely underdiagnosed. The symptoms — fever, cough, shortness of breath, muscle aches — overlap with many common respiratory illnesses, and specialized laboratory testing is required for definitive diagnosis. Public health authorities have noted a steady increase in reported Legionnaires' cases over the past two decades, but it remains unclear how much of this increase reflects genuine rising incidence versus improved detection and reporting.
Recommendations for Water Utilities
The research team's findings translate into several actionable recommendations for water utilities across the country. First and foremost, communities relying on undisinfected groundwater should evaluate their supplies for AOC levels and other indicators of bacterial growth potential. This testing is relatively inexpensive compared to the cost of an outbreak investigation and the human toll of preventable disease.
For systems where elevated AOC is identified, chloramine disinfection represents a proven intervention with a strong track record in municipal water treatment. The infrastructure required is well understood and available from multiple vendors, making implementation feasible even for smaller utilities with limited engineering resources.
Beyond disinfection, the researchers recommend ongoing monitoring of water quality parameters that indicate bacterial growth potential, including AOC, heterotrophic plate counts, and residual disinfectant levels throughout the distribution system. This continuous surveillance approach can detect emerging problems before they manifest as disease outbreaks.
A Wake-Up Call for Water Safety Assumptions
Perhaps the most important lesson from the Grand Rapids outbreak is the danger of untested assumptions. The belief that groundwater is inherently safe has been a cornerstone of water management practice in many communities for generations. In most cases, this assumption has been reasonable — groundwater filtered through deep geological formations is genuinely lower in microbial contamination than surface water.
But reasonable assumptions are not the same as verified safety. The Grand Rapids outbreak demonstrates that conditions can exist — or develop — in groundwater systems that support dangerous pathogen growth, and that the consequences of discovering this through a disease outbreak rather than proactive testing are measured in hospitalizations and deaths.
The University of Minnesota team's work provides both the evidence and the roadmap for addressing this vulnerability. Strategic disinfection implementation, guided by AOC monitoring and other water quality indicators, can effectively prevent disease transmission through water infrastructure. The question now is whether communities and water utilities across the country will act on this evidence before their own outbreaks force the issue.
This article is based on reporting by Medical Xpress. Read the original article.
