The US Environmental Protection Agency signed the twenty-second Clean Water Act Methods Update Rule on 6 December 2024 and published the proposal in the Federal Register on 21 January 2025. The public comment period closed on 24 March 2025. As of mid-July 2026, the rule remains in proposed form. No effective date has been announced.

That gap matters for laboratories and permit holders. The methods MUR 22 would codify are not new to the technical community - EPA Method 1633A has been in wide use for state PFAS monitoring since it was finalized on 5 December 2024 - but until the rule is promulgated the methods are not nationally required for Clean Water Act compliance monitoring. Discharge permits issued under the National Pollutant Discharge Elimination System therefore continue to reference the older tables in 40 CFR Part 136, and any Method 1633A data submitted in support of a permit currently qualifies as approved-method-equivalent only where EPA or the state has already granted an alternate-test-procedure approval or otherwise incorporated the method by reference.

The result is a two-track compliance landscape. Environmental laboratories accredited under state programs are running Method 1633A on wastewater, groundwater, biosolids, and fish tissue every day. The regulatory rule that would formalise it into the federal effluent-testing table is stuck in the same place it sat a year ago.

What MUR 22 proposes

Three new EPA methods enter Part 136 under the proposal.

Method 1633A measures 40 per- and polyfluoroalkyl substances by liquid chromatography-tandem mass spectrometry across wastewater, surface water, groundwater, soil, biosolids, sediment, landfill leachate, and fish tissue. Method 1633 (the non-A revision released in early 2024) is superseded by 1633A within the proposal.

Method 1621 measures adsorbable organic fluorine, a total-fluorine screening technique that captures PFAS chemistries not covered by the 40-compound target list in 1633A. It is intended as a complement to the targeted method, not a replacement.

Method 1628 measures all 209 polychlorinated biphenyl congeners individually by high-resolution gas chromatography-mass spectrometry. If finalized, Method 1628 replaces the older approach in which laboratories reported PCBs as one of seven Aroclor commercial mixtures.

Alongside those three EPA methods, MUR 22 proposes to codify voluntary-consensus-standard methods developed by ASTM International and Standard Methods for the Examination of Water and Wastewater: ASTM D8421 for the same 40 PFAS compounds as Method 1633A, Standard Method 4500-PAA for peracetic acid, and Standard Method 4500-H2O2 for hydrogen peroxide. The peracetic acid and hydrogen peroxide additions cover disinfection-chemistry residuals that have appeared in more discharge permits as chlorine-free treatment gains ground.

The rule also simplifies sample-collection requirements for acrolein and acrylonitrile - two volatile organics whose current specification carries preservation steps EPA has judged unnecessary - and makes a series of table corrections that consolidate deprecated method citations. Those pieces are administrative, but they are the ones most likely to affect a routine NPDES lab’s day-to-day work when the rule takes effect.

Aroclor withdrawal is the biggest shift

For laboratories that report PCBs, the Method 1628 provision is the substantive change in MUR 22. Under existing 40 CFR Part 136 procedures, PCB monitoring for effluent has been performed by resolving seven Aroclor commercial mixtures using gas chromatography with electron-capture detection. That approach was calibrated against Aroclor formulations that stopped being manufactured in the late 1970s; laboratories reconstitute reference standards to match those historical mixtures.

Method 1628 replaces that framework with direct measurement of the 209 individual congeners. The methodology is well established in research and forensic contexts. Transferring it into routine NPDES compliance monitoring, though, means every laboratory doing PCB effluent work will need to demonstrate proficiency on a congener-resolved method, and every permit that specifies “Aroclor 1242” or similar will need to be re-expressed in congener terms during the next renewal cycle. Neither step happens quickly across thousands of permits.

PFAS: the methods are in use, the rule is not final

The state of play for PFAS is different. Method 1633A was released in its December 2024 revision alongside MUR 22, and laboratories have been running it for state PFAS Clean Water Act monitoring programs, Department of Defense site work, and NPDES permits that already incorporate it by state action. The analytical supply chain has adapted to that demand, with reference-standard availability, calibration-transfer procedures, and column and consumable supply now stable enough that method transfer between laboratories is routine rather than pioneering.

What MUR 22 would add is the federal requirement. Once promulgated, 1633A becomes the reference method that a permit writer cites without needing to make a case for equivalency, and permit-driven demand for PFAS data would rise beyond its current volume. In its unpromulgated state, permit writers cite it where they choose to and defend that choice case by case. That is workable for the well-funded utility labs and third-party contractors already running the method. It is less workable for smaller dischargers whose permits still reference older, less-sensitive PFAS chemistry.

What operators can do while the rule sits

The pattern with MUR rules is that they typically finalize with a compliance date one to two years after promulgation, and that compliance date usually aligns with the following NPDES permit renewal cycle for a given facility. On that pattern, laboratories and permittees should expect to have transition time once MUR 22 is finalized - but they should not wait for a signal to start.

For laboratories, the practical near-term work is proficiency-testing coverage for Method 1633A and Method 1628 (if PCB work is in scope), staff training on congener-resolved PCB interpretation, and a review of instrument time budgets. PFAS work on LC-MS/MS is time-intensive; adding congener-resolved PCB analysis to a laboratory that has run Aroclor screens for decades is a step change in chromatographic method work and data review.

For permit holders, the useful preparation is inventorying which of their current permit conditions cite Aroclor mixtures or older PFAS surrogates, and flagging those for the next renewal negotiation. Permit writers are already thinking in Method 1633A terms even where the rule has not been finalized federally, and coming into renewal talks with the newer methods in hand is more productive than trying to defend historical Aroclor-based reporting once MUR 22 takes effect.

The rule remains proposed. When it moves, the changes are administratively deep even where the science is not new. The advantage goes to laboratories and dischargers who prepared during the pause rather than after it.