A groundbreaking new research study commissioned by AMWA and the American Water Works Association and published in the May-June 2026 issue of AWWA Water Science, found no statistically significant relationship between the level of disinfectant residual in drinking water supplied by public water systems, and concentrations of Legionella identified in water sampled from a cohort of federal office buildings. The study, completed by Corona Environmental Consulting, will be used by AMWA and AWWA to inform upcoming revisions to the Microbial and Disinfection Byproducts (MDBP) Rules that EPA is expected to propose next year.
The study, “Legionella Occurrence in Buildings: Implications of Public Water System Disinfectant and Residual,” took advantage of baseline drinking water quality tests performed by the U.S. General Services Administration (GSA) beginning in 2024, and which covered roughly 1,250 federally-owned building and 6,000 leased facilities. By cross-referencing the Legionella sampling results from a subset of GSA-operated buildings with disinfectant residual data from public water systems serving those facilities, study’s authors at Corona Environmental Consulting concluded that there is no consistent association between distribution system disinfectant residual levels and Legionella occurrence in GSA buildings.
The study was completed in the context of a 2023 National Drinking Water Advisory Council (NDWAC) working group recommendation that EPA establish a federal numeric minimum requirement for disinfectant residuals in public water system distribution systems – at levels as high as 0.7 mg/L for total chlorine for chloraminating systems. AMWA and AWWA subsequently recommended that EPA establish a minimum numeric level of 0.2 mg/L for chlorine and total chlorine secondary disinfectant residuals to ensure disinfection nationwide, and have expressed practical concerns about higher mandated minimum residuals. The newly published study was the first to attempt to establish whether higher minimum residuals could be relied upon to reduce Legionella exposure, so the finding of no statistically significant relationship is notable. As AMWA increases its engagement on the upcoming MDBP rule revisions in the coming months, it will ensure that EPA staff and leadership have access to this new research.
The full report is available for the public to review and download online free of charge through an Open Access license, via AWWA Water Science. The study can also be downloaded from the Publications section of amwa.net.