The U.S. Environmental Protection Agency (EPA) has proposed the sixth Unregulated Contaminant Monitoring Rule (UCMR 6), requiring public water systems to monitor for 30 unregulated contaminants between 2028 and 2030 to better understand their occurrence in drinking water systems nationwide. While the proposal expands monitoring for several emerging contaminants, EPA opted not to include microplastics, citing the lack of a validated analytical method capable of producing nationally consistent monitoring data.

Under the proposed rule, community water systems and non-transient non-community water systems serving more than 3,300 people would be required to monitor for all 30 contaminants. EPA would also select a representative sample of smaller systems serving 3,300 or fewer people to participate. The contaminant list includes seven ultrashort organofluorine compounds, including several PFAS, three pesticide metabolites, 13 semivolatile organic compounds and seven purgeable organic compounds. The agency said the resulting occurrence data will help determine whether additional contaminants warrant future regulation under the Safe Drinking Water Act.

Microplastics were among the most anticipated contaminants during development of UCMR 6. EPA acknowledged receiving significant public interest, including a petition from the governors of seven states requesting that microplastics be included in the monitoring program. However, the agency concluded that available methods are not yet mature enough to support a national occurrence study.

According to EPA, no validated EPA or consensus-based analytical method currently exists that can consistently detect and characterize microplastics in drinking water across laboratories. The agency noted that existing methods vary widely in the sizes and types of plastic particles they detect, making it difficult to generate reliable national occurrence data that could support future regulatory decisions.

Although microplastics are absent from UCMR 6, EPA emphasized that the contaminant remains a priority. Earlier this year, the agency proposed adding microplastics as a contaminant group to the draft Sixth Contaminant Candidate List (CCL 6), a move intended to support additional research on occurrence, health effects, analytical methods and treatment technologies. EPA said information gathered through the CCL process could ultimately pave the way for future monitoring requirements once standardized testing methods become available.

EPA also said it will continue collaborating with federal agencies, researchers and standards organizations to develop validated analytical methods and improve understanding of which microplastic particles may pose the greatest public health concern. The agency identified these efforts as a necessary step before nationwide monitoring can be implemented through a future UCMR cycle.

If finalized, UCMR 6 would replace UCMR 5, with monitoring taking place from January 2028 through December 2030. EPA estimates the program will cost approximately $33.7 million annually nationwide. As with previous UCMR cycles, EPA will pay laboratory analysis and shipping costs for systems serving 10,000 or fewer people, reducing the financial burden on small utilities. Public comments on the proposed rule are due by Aug. 31, 2026.