Frequent Questions about PFAS Methods for NPDES Permits
As permitting authorities focus more on controlling the discharge of per- and polyfluorinated alkyl substances (PFAS) in wastewater, questions arise about the best practices for analyzing for these pollutants. To understand the current and upcoming regulatory and analytical landscape, the EPA has developed a list of questions and answers in response to public inquiries. The EPA released the final methods and the multi-laboratory study reports for both Method 1633 and Method 1621 on the Clean Water Act Methods website on Wednesday, January 31, 2024.
1. What methods may be used to analyze PFAS in a National Pollutant Discharge Elimination System (NPDES) permit or pretreatment permits for industrial users that discharge to publicly owned treatment works (POTWs)?
Currently, there are no EPA-approved methods in 40 CFR Part 136 for analyzing PFAS. However, in the NPDES regulations, 40 CFR 122.21(e)(3)(ii) provides that:
"(B) In the case of pollutants or pollutant parameters for which there are no approved methods under 40 CFR part 136 or methods are not otherwise required under 40 CFR chapter I, subchapter N or O, monitoring shall be conducted according to a test procedure specified in the permit for such pollutants or pollutant parameters."
Therefore, when applying or reapplying for an NPDES permit, the applicant or permittee may select ”any suitable method.” The permitting authority must then evaluate the method’s suitability for use and specify the appropriate procedure in the permit.
Pretreatment permits for industrial users that discharge to POTWs are also required to use a method approved at 40 CFR Part 136, as specified at 40 CFR 403.12(b)(5)(v), 403.12(e)(2)(iii), 403.12(g)(3) and (5), and 403.12(h). As with NDPES permits, pretreatment permits may use methods other than those specified in Part 136 under certain circumstances:
“Where 40 CFR part 136 does not contain sampling or analytical techniques for the pollutant in question, or where the Administrator determines that the part 136 sampling and analytical techniques are inappropriate for the pollutant in question, sampling and analysis shall be performed by using validated analytical methods or any other applicable sampling and analytical procedures, including procedures suggested by the POTW or other parties, approved by the Administrator.” [40 CFR 403.12(b)(5)(v) and 403.12(g)(5)]
The EPA recommends using EPA Method 1633 for PFAS contaminants, as outlined in the December 5, 2022 memorandum from EPA Assistant Administrator Radhika Fox, “Addressing PFAS Discharges in NPDES Permits and Through the Pretreatment Program and Monitoring Programs.”
The EPA has completed the multi-laboratory validation of Method 1633 in collaboration with the Department of Defense (DoD). This PFAS method aids regulatory entities that participate in the NPDES Program by providing them more options for analyzing PFAS and providing them a fully validated method that has been tested in a wide variety of wastewaters and is compliant with the required quality assurance and control procedures for chemical analysis of NPDES monitoring samples, as specified in 40 CFR 136.7. EPA Method 1633 contains the final QC criteria generated from the multi-laboratory validation study. The Office of Water is unaware of any other PFAS method that has been validated using 10 laboratories, 15 challenging aqueous matrices (6 wastewaters, 3 surface waters, 3 groundwaters, and 3 landfill leachates), 9 solid matrices (3 soils, 3 sediments, and 3 biosolids), and 3 aquatic tissues.
The EPA has not yet proposed or promulgated the method in Part 136 and the method is not required nationally for use in the NPDES program. However, under 40 CFR 122.21(e)(3)(ii), permit authorities can require one of the draft versions of the method in individual NPDES permits and the EPA is encouraging its use for that purpose.
2. When will the EPA publish the final multi-lab validated Method 1633?
The EPA released the final method and the multi-laboratory study report on the Clean Water Act Methods website on Wednesday, January 31, 2024.
3. Why did the EPA publish four draft versions of Method 1633?
The EPA decided to release multiple revisions of the draft method in response to stakeholder requests for the agency to update the method incrementally with the multi-laboratory data as soon as practicable. The final version of Method 1633 supersedes all of the earlier draft versions, however, it does not substantively change the analytical procedure.
4. Which laboratories should be used to analyze PFAS?
The EPA does not endorse products or services, including laboratory services. However, the Agency offers some considerations for permittees that are seeking a laboratory for PFAS wastewater analysis.
If attempting to analyze a PFAS compound included in EPA Method 1633, the EPA recommends using the final version of Method 1633 and identifying a laboratory that has experience successfully conducting the method. To date, DOD and other organizations have accredited dozens of laboratories to run Draft Method 1633 (all 4 versions, depending on when the accreditation occurred). The method was written to comply with all 40 CFR Part 136.7 method quality control procedures, which are required for NPDES analysis. Leveraging the EPA’s and DoD’s efforts may save permitting authorities some effort.
5. Many laboratories say they are running "Modified EPA Method 537.1," or "Modified EPA Method 533." What does that mean?
EPA Methods 537.1 and 533 are drinking water methods and were only evaluated by the EPA for use in analyzing drinking water samples. These two drinking water methods rely on solid-phase extraction (SPE) procedures to separate the PFAS analytes from the sample matrix. Although SPE can work well for wastewater analyses, the applications of SPE described in Methods 533 and 537.1 may require adjustments to handle the suspended solids in a typical wastewater discharge. The drinking water methods also do not contain many of the mandatory cleanup steps in Method 1633 that are needed to analyze many wastewater samples.
Many commercial laboratories currently offer PFAS analysis for wastewater. Some label their methods as "Modified Method 537.1," or "Modified Method 533." The term "modified" means that the analytical procedure has been changed from what is allowed in the published method. Drinking water methods (such as 537.1 and 533) do not allow for modification, so the label is misleading. If a laboratory labels the method as modified, the laboratory is using an in-house standard operating procedure (SOP) that is based on Method 537.1 or 533. Sometimes the SOP only loosely parallels the analytical procedures of those methods. Therefore, it is critical to obtain a copy of their SOP and performance data, so that an informed decision can be made about its utility for PFAS monitoring.
6. Can a modified Method 537.1 or modified Method 533 be used in a NPDES Permit?
Perhaps, but please note the concerns explained in answers #1 and #5. These methods have not been validated for wastewater matrices. The EPA recommends that, before selecting a laboratory for analysis of samples, a permittee obtain a copy of the laboratory’s standard operating procedure (SOP) to confirm that the laboratory’s analytical operations employ all the relevant quality control elements discussed in 40 CFR 136.7.
The EPA recommends using EPA Method 1633 for PFAS contaminants, as outlined in the December 5, 2022 memorandum from EPA Assistant Administrator Radhika Fox, “Addressing PFAS Discharges in NPDES Permits and Through the Pretreatment Program and Monitoring Programs.” If the permitting authority decides to use an in-house method (e.g., modified 537.1 or 533), then the data generated may not be as consistent with the data generated by Method 1633 nationwide.
7. What is Adsorbable Organic Fluorine (AOF)?
Organofluorines cover a broad class of chemicals and include pharmaceuticals, pesticides, and PFAS. An organofluorine contains at least one carbon-fluorine bond. Naturally occurring organofluorines are rare. Known sources of naturally occurring organofluorines are produced by semitropical and tropical plants and volcanic activity. AOF is a method-defined parameter (MDP) for the measurement of organofluorines that adsorb to a carbon sorption media. An MDP is a parameter defined solely by the method used to determine the analyte. The EPA completed a multi-laboratory validation study of Method 1621 and has finalized the method. The EPA released the final method and the multi-laboratory study report on the Clean Water Act Methods website on Wednesday, January 31, 2024.
8. What is the purpose of a method for AOF?
There are thousands of organofluorine compounds, which include PFAS, as well as fluorinated pharmaceuticals and pesticides. This method-defined parameter is particularly useful in the context of PFAS analysis. Targeted methods typically require a matching analytical standard for each compound of interest. In the case of PFAS, analytical standards are not available for every compound. An AOF method can help provide context for what targeted analyses might miss, especially if used on a sample where PFAS contamination is suspected. Most of the naturally occurring organofluorine molecules only contain one fluorine, so they may contribute less to overall organofluorine concentrations compared to PFAS compounds. Stakeholder interest and demand for aggregate methods like AOF is increasing and we anticipate commercial lab availability to increase now that the AOF method has been validated and finalized.
9. What does the AOF method (Method 1621) measure?
EPA Method 1621 is used to determine concentrations of organofluorines in wastewaters and surface waters. This method utilizes granular activated carbon (GAC) adsorption, where aqueous samples are passed through GAC columns to adsorb the organofluorine compounds from both PFAS and non-PFAS fluorinated sources (such as fluorinated pesticides and pharmaceuticals), that can be retained on at least 80 mg of GAC. The single-laboratory validation study tested 22 individual PFAS, one PFAS mixture containing 33 PFAS, one pharmaceutical, and one pesticide, all of which were detected as AOF.
10. What are the limitations of the AOF method (Method 1621)?
Method 1621 has a higher detection limit than other more selective targeted methods. Because AOF relies on adsorption to GAC, low recoveries are possible for organofluorine compounds containing less than four carbons that are poorly retained on GAC, or for compounds containing more than eight carbons that tend to adsorb to other surfaces (e.g., sample bottles or instrument tubing). The worst performing PFAS compound tested still showed greater than 40% recovery, which is adequate for a method-defined parameter that can detect so many different fluorinated chemicals. Additional considerations for this method include potential interferences from the sample matrix, including inorganic fluoride, chloride, dissolved organic carbon, and contamination of the GAC or the instrument – all of which are addressed in the single and multi-laboratory validation study reports.
11. How does Method 1621 compare to Method 1633?
Method 1621 is not as selective as Method 1633. Whereas Method 1633 precisely measures 40 specific PFAS in a variety of matrices, Method 1621 cannot distinguish which specific organofluorines are present, but estimates the aggregate concentration of numerous organofluorines in aqueous samples. Method 1621 can be a complementary analysis to targeted approaches like Method 1633 because aggregate concentration results can indicate the presence of additional organofluorines that may not be included in the targeted analyte list.
12. Are there different types of aggregate PFAS methods?
There are several different aggregate PFAS methods, each with their own strengths and weaknesses. The table below summarizes some of the common aggregate techniques referred to in literature as “total” organic fluorine methods. However, this description is often not accurate, given the limitations of each technique.
| Method | Brief Description |
|---|---|
| Total Organic Fluorine (TOF) | Most often calculated as the difference between total fluorine and inorganic fluorine measurements of a sample. Because organofluorines are typically present at levels at or below the inorganic fluorine concentration, the results are always limited by the concentration of inorganic fluorine present in the sample. The detection limits for this technique are orders of magnitude higher than those for the AOF or EOF techniques. |
| Adsorbable Organic Fluorine (AOF) | Typically utilizes GAC sorbent to adsorb organofluorine compounds from an aqueous sample. PFAS ranging in chain-length from C4 to C10 will be retained with typical sorbents used for this technique. Inorganic fluorine is removed with a wash solution. The sorbent is analyzed by combustion ion chromatography (CIC). Reported detection limits are significantly lower than TOF. Not applicable to non-aqueous matrices. |
| Extractable Organic Fluorine (EOF) | Typically utilizes weak anion exchange (WAX) solid-phase extraction. An aqueous sample is passed through cartridges containing sorbent. Therefore, the method is selective for only those organofluorines that can be extracted from the sample. In contrast to AOF, where the sorbent is combusted and analyzed, in EOF the sample extract is analyzed by CIC. Like AOF, organofluorine concentrations are dependent on the sorbent selectivity, which may result in poor recoveries for PFAS compounds with chain-lengths less than C6 or neutral PFAS. Reported detection limits are significantly lower than those achieved by TOF, but were higher than AOF at the time EPA started validating Method 1621. More recent literature indicates that EOF may now be as sensitive or more sensitive than AOF, but there is very little wastewater data available. |
| Total Oxidizable Precursor Assay (TOP Assay) | Measures only those PFAS that can be oxidized to form perfluoroalkyl acids (PFAAs) and cannot account for all possible PFAS in the sample. As such, this is a more selective technique than the previous aggregate methods listed in this table. Concentrations of PFAAs are compared both before and after oxidation of the sample by hydroxyl radicals. The difference in these concentrations indicates the total amount of chemical precursors to PFAAs. The specific identities of the precursors cannot be determined. |
| Particle-Induced Gamma Ray Emission (PIGE) | This is a surface analysis technique for measuring fluorine. This technique is most often applied to solid matrices and can only measure fluorine within the depth of gamma ray beam penetration. Currently, there is no streamlined procedure for separating inorganic and organic fluorine during PIGE analysis. Unlike other aggregate PFAS methods, the required instrumentation, which includes a proton beam accelerator system, is cost prohibitive and not commonly found in commercial laboratories. |
| Fluorine-19 Nuclear Magnetic Resonance (19F NMR) | This technique monitors the chemical shift associated with terminal CF3 group of a PFAS. This technique has increased selectivity by removing interferences from inorganic fluorine and organofluorine pharmaceuticals and pesticides. Like PIGE, the instrumentation required for 19F NMR is cost prohibitive and not commonly found in commercial laboratories. |
13. Why did the EPA develop an AOF method and not a “total” organic fluorine (TOF) method?
There are strengths and weaknesses to both TOF analysis and AOF analysis. The primary interferent in organic fluorine analysis is inorganic fluorine, and AOF is better suited to mitigate this interferent than TOF. A typical TOF analysis uses CIC to measure total fluorine and then analyzes for inorganic fluorine using only the ion chromatograph portion of the CIC. The organic fluorine is calculated by subtracting the inorganic fluorine from the total fluorine. A TOF method is likely to have reduced sensitivity in the presence of inorganic fluorine, such as the ppm background levels often found in drinking water, surface water, and wastewater. In contrast, adsorbing the organic fluorine on GAC allows about 99.9% of the inorganic fluorine to pass through so that it does not interfere with the measurement of AOF.
Method 1621 may not capture short-chain PFAS as accurately as a TOF method might, but Method 1621 for AOF is far more sensitive. The multi-laboratory validation study demonstrated that Method 1621 is capable of measuring AOF with a method detection limit of 1.5 parts per billion (ppb), where TOF procedures often have detection limits greater than 400 ppb. For these reasons TOF is more likely to return a non-detect result than Method 1621 for AOF when analyzing the same sample.