Key Points

• On April 2, 2026, the EPA added microplastics to its Sixth Contaminant Candidate List for identifying which contaminants it may monitor under the Unregulated Contaminant Monitoring Rule 6.
• While listing microplastics imposes no regulatory requirements, it signals that the EPA is considering whether it should regulate microplastics under federal laws like the Safe Drinking Water Act.
• This federal action follows California, which already requires that certain large water suppliers monitor microplastics in drinking water.

On April 2, 2026, the U.S. Environmental Protection Agency and Department of Health & Human Services released a draft Sixth Contaminant Candidate List, or “CCL 6,” for public comment. On this latest list, the EPA and HHS have included microplastics and certain pharmaceuticals, PFAS compounds, and disinfectants, among others.

The Contaminant Candidate List is a product of the Safe Drinking Water Act, which requires that the EPA publish every five years a list of unregulated contaminants that may warrant future regulation. The EPA may then select any of the contaminant candidates for monitoring under the Unregulated Contaminant Monitoring Rule 6, or “UCMR 6.” Neither placing a contaminant on CCL 6 nor subjecting a contaminant to UCMR 6 is regulatory, meaning their presence in the water supply would not trigger notification and treatment requirements under the Safe Drinking Water Act. Instead, UCMR 6 would only allow the EPA to monitor and better understand a contaminant’s prevalence in water supplies. With more data, the EPA can better prioritize research and data collection and unlock related funding opportunities. From there, the EPA can better determine whether to regulate the contaminant under the Safe Drinking Water Act.

If the EPA includes microplastics in UCMR 6, then public water systems must sample, analyze, and report monitoring results during the 2027–2031 compliance period. Whether and when UCMR 6 will lead to enforceable limits remains uncertain. Contaminants may—and often do—progress through the CCL and UCMR processes without resulting in enforceable regulatory standards. Even when they do, as discussed in our September 2025 alert on PFAS drinking water standards, those standards can be repealed.

There is an added layer of uncertainty with how these federal efforts affect California water suppliers. California is the only state that requires statewide microplastics monitoring. Under Senate Bill 1422, the State Water Resources Control Board has defined “microplastics” and directed large public water systems to test for and disclose microplastics data under its statewide monitoring program. Phase One required monitoring for microplastics in untreated water from the state’s largest systems between 2023 and 2025. Phase Two will require monitoring in treated drinking water from the state’s largest systems beginning Fall 2026.

The timing of California’s Phase Two monitoring may coincide with the EPA’s UCMR 6 monitoring. But for many California water suppliers, how this federal rule may affect existing monitoring requirements is uncertain. The EPA, for instance, may require testing methodologies or reporting requirements that differ from California’s requirements. Whether UCMR 6 is consistent with, redundant to, or divergent from California’s program will depend on the final federal rule.

Public comments on the draft CCL 6 are due by June 5, 2026. And the EPA is expected to finalize the rule in November 2026.

If you have any questions about whether or how these updates may affect you, please contact Hanson Bridgett’s Environmental and Water lawyers.