Perchlorate Regulatory Update: What Drinking Water Professionals Need to Do Next

Perchlorate is back in the regulatory spotlight due to a court ordered timeline, mandating U.S. EPA move forward on previously withdrawn rulemaking. In this post, we provide a refresher on perchlorate in drinking water, an update on what’s transpired since the initial court order, and what drinking water professionals can do to prepare.

Perchlorate in Drinking Water: What the Data Show

In the early 2000s, the first Unregulated Contaminant Monitoring Rule (UCMR 1) required many public drinking water systems to sample for perchlorate with a reporting limit of 4 µg/L. Roughly 4% of sampled public water systems detected perchlorate above that level. For more information on which systems were impacted, UCMR 1 occurrence data are publicly available through EPA’s UCMR Archival Data Finder and downloadable text files.

UCMR 1 findings, combined with subsequent occurrence and health effects work, set the stage for today’s proposed federal drinking water standard. Here are a few of the more notable studies and reports:

Toxicological Profile for Perchlorates, Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services, 2005

Perchlorate in Water Supplies: Sources, Exposures, and Health Effects, National Library of Medicine, 2016

Health Implications of Perchlorate Ingestion, National Research Council, 2005

Structural Malformations in the Neonatal Rat Brain Accompany Developmental Exposure to Ammonium Perchlorate, EPA, Science Inventory, 2003

EPA’s Proposal: Three Possible MCLs

In January 2026, EPA proposed a National Primary Drinking Water Regulation (NPDWR) for perchlorate with a health based Maximum Contaminant Level Goal (MCLG) of 20 µg/L (0.02 mg/L). MCLGs are non-enforceable, health-based goals for contaminants in drinking water and represent the level at which no known or anticipated adverse health effects would occur. MCLGs do not take treatment technology limits or cost into account.

The consent decree in NRDC v. Regan requires EPA to regulate perchlorate but does not dictate the level at which the Maximum Contaminant Level (MCL) must be set. Rather than selecting a single enforceable MCL, the agency proposed three options: 20, 40, and 80 µg/L. EPA explained that it is proposing these three levels so it can fully evaluate public input on health benefits, technical feasibility, and compliance costs before proposing a final MCL.

For reference, it’s not unheard of for EPA to propose alternatives for comment before issuing a final rule. The clearest example is the 2000 arsenic rule in which EPA proposed an MCL of 5 µg/L but specifically requested comment on alternatives of 3, 10, and 20 µg/L before finalizing 10 µg/L. More recently, the EPA PFAS proposal included individual MCLs for PFOA/PFOS plus a Hazard Index for mixtures but explicitly requested comment on alternative compliance and averaging approaches.

Proposed Sampling Requirements: Initial and Ongoing Monitoring

EPA’s proposal would require all community water systems and non-transient, non-community systems to monitor for perchlorate, using the existing Standardized Monitoring Framework for inorganic contaminants as the backbone.

Initial Monitoring requirements include:

• Groundwater systems serving more than 10,000 people and all surface water systems must conduct quarterly monitoring at each entry point to the distribution system over 12 months.
• Groundwater systems serving 10,000 or fewer people must collect two samples in the 12 months before the compliance date, with the second sample collected 5–7 months after the first.
• States may allow recent perchlorate data (up to 6 years old) to satisfy initial monitoring requirements.

Ongoing compliance monitoring (after the compliance date) is contingent on the results at each sampling point:

• If all Initial Monitoring results are at or below 4.0 µg/L, monitoring frequency may drop to once every 9 years.
• If any results are above 4.0 µg/L but at or below the MCL, annual monitoring is required for surface water systems and once every 3 years for groundwater systems.
• If any result exceeds the MCL, quarterly monitoring would be required until conditions for reduced frequency are met.

States may require more frequent sampling where source water conditions are variable. In addition, water systems may seek waivers or further reductions in line with existing SDWA practice for other inorganic contaminants.

What Commentators Are Saying About the Proposed Limits

Commentary from public health and policy observers on the perchlorate MCLs reflects concern about perchlorate risks and debate over where the final line should be drawn. Environmental and health advocates generally argue that perchlorate’s thyroid and developmental effects justify adopting the lowest proposed level (20 µg/L) or an even more protective standard.

Water sector and technical commentators, by contrast, often focus on implementation feasibility and costs. Trade and engineering articles, such as this one from C&EN, acknowledge that a federal standard is now inevitable but question whether a 20 µg/L MCL is necessary nationwide, given regional variation in occurrence and the cost impacts on smaller systems.

Legal and policy analysts, such as Harvard Law and some legal firms have pointed out that EPA previously questioned whether national perchlorate regulation is cost effective, framing the new proposal as “court driven” and predicting that the choice among 20, 40, and 80 µg/L will be central in public comments and potential litigation. They point out that the court order effectively removed the EPA’s typically allowed discretion in such matters.

Timeline and Predictions

EPA’s proposed perchlorate rule is open for public comment until March 9, 2026. Under the consent decree, the agency must finalize the NPDWR for perchlorate by May 21, 2027. The short timeline gives utilities, states, and other stakeholders a relatively short window to review the  proposal, assess impacts, and submit data driven comments.

As for where EPA may land, no one knows for sure. A few commentators speculate that EPA is most likely to settle on 20 µg/L, arguing that SDWA’s requirement to set the MCL as close as feasible to the 20 µg/L MCLG will push the agency toward the lowest proposed value. However, EPA has not indicated which option it prefers, and official materials emphasize that the agency is genuinely seeking comment on all three alternatives. For planning purposes, many technical advisors suggest modeling compliance strategies against the most stringent option while recognizing that the final rule could still shift.

Treatment Options for Perchlorate in Drinking Water

While the final rule and compliance deadline are likely at least a couple of years away, it’s never too early to start evaluating treatment needs, pilot options, and funding strategies. Multiple proven technologies can reduce perchlorate to levels at or below the proposed MCL options, though costs and operational complexity vary by system size and water quality.

Common approaches include:

• Selective ion exchange (IX): Strong base anion exchange resins designed for perchlorate can provide highly efficient removal, particularly at lower concentrations. However, systems must manage regenerant brine or use single use resins with appropriate disposal.
• Tailored granular activated carbon (GAC): “Tailored” GAC combines carbon with quaternary ammonium surfactants to create a perchlorate selective sorbent, making this an attractive option where GAC is already used for other contaminants.
• Membrane processes (RO/NF): Reverse osmosis and nanofiltration reject perchlorate along with many other ions and contaminants, offering robust removal but with higher energy needs and concentrate management requirements.
• Biological treatment: Fixed bed or fluidized bed bioreactors use specialized microbes to reduce perchlorate to chloride and oxygen, permanently destroying the contaminant; biological polishing of IX regenerant streams is also an emerging strategy.

How Pace® Can Help

With EPA’s perchlorate proposal moving forward and a final rule due in 2027, now is the time for utilities and drinking water professionals to understand their potential exposure and compliance obligations. Pace® can help design and implement a targeted sampling program that leverages existing monitoring data, aligns with EPA’s proposed schedule, and supports a defensible risk and cost assessment.

If sampling identifies elevated perchlorate levels, selecting an effective treatment approach often requires a site-specific evaluation of perchlorate concentrations, co-contaminants (such as nitrate), existing infrastructure, brine or residuals management, and life-cycle costs. Pace® has extensive experience designing feasibility studies and pilot projects for drinking water treatment, and we can help you compare options and plan your next steps.

To discuss a perchlorate sampling strategy tailored to your system—whether you’re planning initial occurrence screening, moving into treatability studies, optimizing treatment, or preparing comments on the proposed rule—contact Pace® today.