Navigating the United States perchlorate rulemaking: compliance strategies for the May 2027 final rule deadline

A court-driven mandate is forcing the EPA to regulate perchlorate, reshaping monitoring, treatment and compliance planning for public water systems.

Misty Guard

In January 2026, the United States Environmental Protection Agency (USEPA) issued a rulemaking that could significantly impact how public water systems monitor and treat drinking water for perchlorate, a chemical compound used in rocket propellants, fireworks, and munitions.

While perchlorate has been on the regulatory radar for decades, this specific action is driven by a complex legal history. Understanding the nuances of this rulemaking is critical for water system operators, municipal engineers and the contractors and manufacturers who support them, as it outlines specific requirements for monitoring, reporting and treatment technologies. For those seeking clarity, call us at Regulosity. Our team specializes in detailed and personalized assessments to help you navigate and engage with the complexities of the regulatory framework.

The regulatory context

In 2020, USEPA determined that regulating perchlorate did not present a "meaningful opportunity for health risk reduction," and withdrew its previous determination to regulate the contaminant. However, in 2023, the United States Court of Appeals for the District of Columbia Circuit ruled in Natural Resources Defense Council (NRDC) v. Regan that the agency could not withdraw a regulatory determination once it had been finalized.

USEPA is legally required to move forward with a regulation, despite the agency's 2020 determination that the costs of the regulation may outweigh the quantifiable benefits.

Consequently, USEPA is legally required to move forward with a regulation, despite the agency's 2020 determination that the costs of the regulation may outweigh the quantifiable benefits. USEPA is operating under a consent decree that mandates the issuance of a final rule by May 21, 2027. This means that regardless of the economic analysis, a compliance framework is being constructed that water systems must be prepared to implement.

Proposed contaminant levels and compliance metrics

USEPA is proposing a Maximum Contaminant Level Goal (MCLG) of 20 micrograms per liter (µg/L). For compliance purposes, the agency is proposing to set the enforceable Maximum Contaminant Level (MCL) at 20 µg/L. However, the agency is also requesting public comments on alternative MCLs of 40 and 80 µg/L. The selection of the final MCL will dictate the extent of treatment upgrades required for affected water systems.

Monitoring requirements and "binning"

The rulemaking applies to all community water systems and non-transient non-community water systems. To minimize the administrative and financial burden on water systems where perchlorate is not present, USEPA has designed a "binning" approach for compliance monitoring.

Under this framework, water systems will conduct initial monitoring prior to the compliance date specified in the final rule. For groundwater systems serving more than 10,000 people and all surface water systems, this involves quarterly sampling over 12 months. Smaller groundwater systems would sample twice within a year. Additionally, states may permit water systems to utilize valid historical data collected after January 1, 2021, to meet initial monitoring requirements.

The results of this initial monitoring determine the long-term compliance schedule:

• Reduced monitoring: If all initial samples at an entry point are at or below a threshold of 4.0 µg/L, the system qualifies for automatic reduced monitoring. These water systems would only be required to test once every nine years. The 4.0 µg/L threshold was selected based on the Minimum Reporting Level (MRL) established during the first Unregulated Contaminant Monitoring Rule (UCMR 1).
• Standard monitoring: Water systems with results above 4.0 µg/L but below the MCL would monitor annually (for surface water) or triennially (for groundwater).
• Increased monitoring: Water systems exceeding the MCL would be required to monitor quarterly.

Treatment technologies and mechanical solutions

For water systems that exceed the MCL, the installation of treatment technologies will be necessary. USEPA has identified "Best Available Technologies" (BATs) that are feasible for large and medium water systems. These include:

1. Ion Exchange: A process widely used in water treatment where contaminants are swapped for other ions on a resin bed.
2. Biological Treatment: Using bacteria to break down perchlorate into harmless components.
3. Reverse Osmosis: A membrane filtration technology that physically removes the contaminant.

For small water systems serving 10,000 people or fewer, the rulemaking identifies specific "Small System Compliance Technologies" (SSCTs). Notably, this includes point-of-use reverse osmosis units.

Public notification and transparency

The rulemaking designates a perchlorate violation as a Tier 1 consumer communication notification event. This is the most urgent category of public notice, requiring water systems to alert customers within 24 hours of learning of a violation. Furthermore, community water systems will be required to include perchlorate data in their annual Consumer Confidence Reports.

Implementation timeline

USEPA is currently accepting public comments on the rulemaking, with the comment period closing on March 9, 2026. Following the review of these comments, the agency is obligated by the consent decree to sign a final rule by May 21, 2027. The proposed compliance date for water systems is three years after the publication of the final rule.

Conclusion

While USEPA has acknowledged that the benefits of this specific regulation may not justify the costs at a national level, the legal mandate from the D.C. Circuit Court ensures that a regulation is forthcoming. For the plumbing and mechanical industry, the focus must now shift to readiness. By understanding the "binning" approach and the approved treatment technologies, plumbing and mechanical industry professionals can help water systems navigate this new compliance requirement efficiently.