Assessing national HAA9 occurrence and impacts of a potential HAA9 regulation

The US Environmental Protection Agency is tasked with proposing revisions to Microbial and Disinfectant and Disinfection Byproducts rules (DBPR) by 2025. Disinfection byproducts are currently regulated by the Stage 2 DBPR, inclusive of total trihalomethane and five haloacetic acids (HAA5). EPA's Fourth Unregulated Contaminant Monitoring Rule (UCMR4) included monitoring for HAA5, six brominated haloacetic acids (HAA6Br), and nine haloacetic acids (HAA9). UCMR4 and state-managed compliance data were utilized to assess the potential impacts of regulating HAA9. HAA9 modeling techniques were validated and applied for greater representation of smaller systems. Outcomes suggest a HAA9 maximum contaminant level (MCL) equivalent to the HAA5 MCL by number of public water systems (PWSs) implicated would be between 72–77 μg/L. An additional 395 PWSs (2%) not currently implicated by Stage 2 DBPR would have compliance challenges if HAA9 is regulated with an MCL equal to the 60 μg/L HAA5 MCL.