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I attended the American Water Works Association’s (AWWA’s) Annual Conference and Exposition (ACE) in Denver, Colorado, earlier this year. There were some excellent sessions on a wide variety of topics. However, the session that stood out the most to me covered Legionnaires' Disease and drinking water. Presenters include Dr. Mark Lechavellier, Dr. Chad Seidel, Sheldon Masters, and Julie Kennedy. The session covered some of the latest research into Legionella and drinking water, plus a case study regarding an outbreak in Grand Rapids, Minnesota, that occurred from 2023 to 2024.
Although there are over fifty Legionella species and about half can cause disease, Legionella pneumophila is the most important because it causes most Legionnaires' disease cases and occurs naturally in aquatic environments. The species can infect free-living Amoeba, where the amoeba can act as a host for Legionella. The 1989 drinking water Surface Water Treatment Rule (SWTR) set a treatment technique for Legionella and established a Maximum Contaminant Level Goal (MCLG) equal to zero. Groundwater systems have no such treatment technique requirements, but one-third of the outbreaks occur in groundwater systems. Partly due to better reporting and testing, Legionella cases rose 10-fold from 1990 to 2021. Legionnaires’ outbreaks often occur in buildings such as hotels, medical facilities, and offices. People catch Legionnaires’ disease by inhaling small droplets of water suspended in the air containing Legionella. Legionnaires’ disease does not spread from person to person.
Dr. Lechavallier reported on a Legionella occurrence study that involved water testing 57 utilities. Overall, 9,118 samples were collected, and 1.2% (109) were positive. Thirty-two percent of utilities had detections. A chlorine residual of at least 0.4 mg/L dramatically reduced the positive detection frequency. Dead-end water lines, low water usage, and tank sediment raised the risk of finding Legionella. Flushing can be effective in reducing Legionella levels. Optimum Corrosion Control Treatment (OCCT) reduces pipe corrosion, which reduces the habitat where Legionella can grow. The study concluded that while eliminating Legionella is impossible, testing and management are feasible. Communicating with commercial and industrial customers about Legionella can also be beneficial.
Unfortunately, Grand Rapids, Minnesota, experienced a severe Legionella outbreak from 2023 to 2024. Grand Rapids has a population of 11,000 and uses groundwater from five wells. The community did not disinfect and had three storage tanks (0.5 million-gallons each) and 81 miles of distribution system. Increased disease incidence began in mid-2023, but water was not believed to be the cause at that time. Legionella were not found in the water system, but the bacteria were found in the water in buildings with Legionnaires’ Disease cases. Over the next year, there were 34 total cases with 30 people hospitalized and two fatalities. After a complete assessment and expert assistance, including Dr. Seidel, chloramination started in June 2024. There have been no Legionella cases in Grand Rapids since then. Considerable effort also involved working with building owners in the town. Generally, buildings that maintain and actively implement water quality management plans do not have outbreaks.
EPA is in the process of updating its rules addressing Microbial Pathogens/Disinfection Byproducts (M/DBPs). It will be a few years before the update is finalized, and these rules will still not apply to groundwater systems. The updated rules are likely to improve the treatment technique requirements for Legionella at surface water systems. While we believe that Colorado is well-positioned to implement improvements due to our rules covering disinfectant residual, storage tanks, and cross connection control (backflow prevention), the occurrence data in this study are concerning. Additionally, even with solid operations by water utilities, important work involving water quality management plans within buildings is not part of our oversight. We encourage utilities to learn more about Legionella and consider partnering with their commercial and industrial customers, especially health facilities like hospitals and nursing homes, to take steps that could reduce the likelihood of a Legionnaires’ Disease outbreak in Colorado.
As always, thank you for keeping our drinking water safe.
➽ Ron Falco, P.E. Safe Drinking Water Program Manager
➽ Co-Author Chelsea Cotton, P.E. Lead Drinking Water Engineer
