Frequent Questions about PFAS Methods for NPDES Permits
As National Pollutant Discharge Elimination System (NPDES) permitting authorities and pretreatment control authorities implement effluent monitoring conditions in NPDES permits and Industrial User permits for the discharge of per- and polyfluoroalkyl substances (PFAS) in wastewater, stakeholders have raised questions about which analytical method to use and the best practices for analyzing these pollutants and collecting suitable samples. These questions have arisen in part because the EPA has recently made many advancements in the development of analytical methods for PFAS in wastewater. To address these concerns, the EPA provides the following questions and answers with respect to the EPA’s recommendation for implementation of these methods in NPDES permits.
On this page:
- Using PFAS Methods in NPDES Permits
- Sampling Considerations When Using PFAS Methods
- Related Information
Using PFAS Methods in NPDES Permits
1. What methods may be used to analyze PFAS in a 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:
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 has completed the multi-laboratory validation of EPA Method 1633 (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. Method 1633 contains the final quality control (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 (six wastewaters, three surface waters, three groundwaters, and three landfill leachates), nine solid matrices (three soils, three sediments, and three biosolids), and three aquatic tissues. EPA Method 1633A (Method 1633A) includes minor changes to the final version of Method 1633 that was validated, made in response to comments from the study participants and others.
Thus, EPA is recommending for the NPDES permit program and the National Pretreatment Program use Method 1633 or 1633A currently as the best analytical methods currently available for monitoring of effluent for PFAS. This recommendation updates the recommendation in the December 5, 2022 EPA memorandum from EPA Assistant Administrator Radhika Fox, "Addressing PFAS Discharges in NPDES Permits and Through the Pretreatment Program and Monitoring Programs," from the then best available method “draft Method 1633” to the currently best available methods, Method 1633 or 1633A.
2. When did the EPA publish the final multi-lab validated Method 1633?
The EPA released the final version of Method 1633 and the last volumes of the multi-laboratory study report on the Clean Water Act (CWA) Methods website on Wednesday, January 31, 2024. In response to comments from laboratories and others, the EPA developed Method 1633A. The changes between 1633 and 1633A are minor (mostly clarifications) and can be reviewed in the “Version History” section of the Method 1633A on Page ii.
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. As noted in answer #2 in this section, the EPA subsequently issued Method 1633A, which supersedes all the earlier versions.
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 analysis of wastewater.
If a permittee needs to have wastewater samples analyzed for a PFAS compound included in EPA Method 1633, the EPA recommends identifying a laboratory that has experience successfully conducting the method. To date, the DoD and other organizations have accredited dozens of laboratories to run Method 1633 (any of the four draft versions or the January 2024 final version, depending on when the accreditation occurred). All of the versions of the method 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 permittees and 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 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 described 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. However, now that Method 1633 has been released, there is little practical value in accepting the use of “modified” drinking water methods for NPDES compliance monitoring.
6. Can a modified Method 537.1 or modified Method 533 be used in a NPDES Permit?
Please note the concerns explained in answers #1 and #5 in this section. These methods have not been validated for wastewater matrices. The EPA recommends that a permittee obtain a copy of the laboratory’s SOP to confirm that the laboratory’s analytical operations employ all the relevant quality control elements discussed in 40 CFR 136.7 before selecting a laboratory for analysis of samples.
As noted in answer #1 in this section, the EPA recommends using Method 1633 or 1633A 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 EPA Method 1621 (Method 1621) and has finalized the method. The EPA released the final method and the multi-laboratory study report on the CWA 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. The method-defined parameter AOF 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?
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 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 discussed 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 sorbent is extracted with an organic solvent, and 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 relative to the measured PFAAs analytes. 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 when exposed to a strong magnetic field. 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 TOF method?
As noted in the table above, there are strengths and weaknesses to both TOF analysis and AOF analysis. The primary interferent in organic fluorine analysis is inorganic fluorine (primarily flouride), 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.
Sampling Considerations When Using PFAS Methods
1. What materials should or should not be used to collect wastewater for PFAS analysis?
PFAS have surfactant properties, meaning they will “stick” to various surfaces, which can complicate collection of valid samples from aqueous sources. As a result, project planners must take greater care in developing the sample collection process than is typically necessary for many other classes of compounds. The first and most obvious precaution is to avoid the use of any tubing or other material made of poly-tetrafluoroethylene (PTFE), fluoroethylene-propylene (FEP), polyvinylidene difluoride (PVDF), or ethylene tetrafluoroethylene (ETFE). Any direct contact with these substances should be avoided whenever possible. Some exceptions may be unavoidable when no alternative is available, but equipment blanks are critical in such cases. Sample contact with any additional surfaces is also discouraged. For instance, using a composite sampler will make the sample contact tubing and other surfaces that could remove PFAS from the sample. Ideally, samples should go directly from the sampling point into the container.
2. What are some things that samplers collecting wastewater for PFAS analysis should avoid bringing in the field when sampling?
It is unlikely that simply having PFAS-containing materials such as those listed below in the vicinity of the samples will cause part-per-trillion level contamination1,2, but avoidance is an effective low-effort strategy when practical.
Avoid as many of the materials below when practical. If the use of one or more of these products cannot be avoided, then do not allow any direct contact with the samples. One option is to use of a two-person sampling team, where one person collects and handles the samples, while the second person takes notes and handles recordkeeping tasks, but does not touch the sample containers (a variation of the “clean hands, dirty hands” concept).
PFAS have been used in many common consumer products and, as a result, the sampling team should take greater care to avoid contamination of the sample than is typically necessary for most other classes of compounds. There are many materials the sampling team may use during the collection of samples that may contain PFAS and they should be avoided when practical, even if they do not have any direct contact with the sample itself. These materials include:
- Personal protective equipment (PPE) made of PTFE or other fluoropolymer
- Waterproof or stain-resistant clothing, unwashed new clothing, and clothing washed with fabric softeners
- Shoes or boots containing PTFE materials or fluoropolymer coatings
- Latex gloves (use nitrile gloves instead)
- Plastic clipboards, notebook binders, or hardcover spiral notebooks
- Adhesive notes
- Recycled paper products such as paper towels and notebook paper
- “Waterproof” paper products, including labels and logbooks
- Laboratory wipes with fluoropolymer coatings
- Permanent markers (use ballpoint pens instead)
3. Can glass containers be used to collect wastewater for PFAS analysis?
It is critical to prohibit the use of any glass containers or implements because PFAS stick to glass surfaces. This prohibition applies to the actual sample containers sent to the laboratory as well as any implements such as pipettes, stirring rods, etc. It also rules out the common practice of collecting a volume of wastewater in a large glass container and then pouring aliquots into smaller containers for various analyses. The samplers should use the sampling containers specified in the method they are using.
4. What other materials, if any, should be prohibited from the field when sampling wastewater for PFAS analysis?
See answers #2 and #3 in this section. Other groups of frequently prohibited materials include cosmetics and lotions, sunscreens (some “all natural” products are acceptable), fast food wrappers made of oil- or water-resistant paper, aluminum foil, and reusable ice packs (e.g., “blue ice”). If good field sampling practices are applied, these materials should never contact the sample. Cosmetics and lotions are easy to eliminate. While sunscreens are important protection for field personnel involved in many sample collection efforts, collecting a small number of wastewater samples for compliance monitoring purposes may not involve as much sun exposure as more involved sample collection projects. Thus, samplers might omit using sunscreens until the PFAS samples are collected and packaged and then apply sunscreen before collecting samples for other contaminants at the same discharge. Alternatively, some sunscreen products do not contain PFAS and may be used by field personnel. Here again, if the use of one or more of these products cannot be avoided, then do not allow any direct contact with the sample.
Food should never be consumed in the sample collection area, and thoroughly washing exposed skin with soap and water will minimize the risk of transferring PFAS from all these potential sources to the samples. At least one recent EPA study found little or no risk of PFAS contamination from foil, and there is little need to use aluminum foil when collecting wastewater or other aqueous samples. The DoD-funded method validation study of Method 1633 evaluated the use of reusable ice packs to ship the study samples and found no evidence of contamination from the ice packs. Moreover, bagged water ice is generally readily available and is the preferred approach for cooling samples during shipping because it conforms better to the sample containers than rigid plastic ice packs.
5. What are the sampling requirements for collecting wastewater that are compliant with EPA Method 1633A?
EPA Method 1633A requires the use of high-density polyethylene (HDPE) sample bottles with linerless HDPE or propylene caps. The nominal bottle size is 500 mL. To minimize subsampling in the laboratory, the method allows the use of smaller sample containers to collect samples that are high in solids or contain high levels of PFAS. Although U.S. commercial suppliers of sample containers routinely provide containers for other types of analyses that have been tested and lot-certified to be free of pollutants such as metals, pesticides, or semivolatile organics, at this time, they are less likely to provide HDPE bottles that have been lot-certified to be free of PFAS. Therefore, you may need to work with the laboratory that will perform the PFAS analyses to obtain suitable sample containers that have been tested for PFAS contamination.
The sampling team also will need the laboratory to provide a sufficient volume of PFAS-free reagent water to be used in the field to prepare field blanks and for equipment rinsing needs.
Regardless of the sample size, all PFAS methods involve processing the entire contents of the sample bottle and subsampling is to be avoided whenever possible. That also means that the sampler should not collect a bulk sample in a larger container and then pour aliquots into separate containers for various analyses when PFAS are among the contaminants to be analyzed. In addition, as noted in Method 1633A, because PFAS are known surfactants, EPA strongly discourages composite sampling for CWA compliance monitoring.
When collecting a grab sample for Method 1633A, do not lay the cap on any flat surface while collecting the sample because that could transfer PFAS from the surface to the sample. Fill the HDPE container to the shoulder to allow room for expansion during frozen storage (leaving headspace is essential, but do not pour out any of the collected sample – many PFAS are known to reside at the air-water interface and pouring off the top portion of the sample may introduce a low bias). If water from the source filled the container above the shoulder or overflowed, discard the sample and container, and collect a new sample in a new container. Carefully cap the container and firmly hand tighten the cap. If sample contacted the outside of the container, rinse the outside of the capped container with a small amount of PFAS-free water to minimize transfer of PFAS from the outside of the container to other samples or materials in the field and in the laboratory. Each sample container should be stored in a separate self-sealing plastic bag and placed in a cooler. Labels should not be affixed to container(s) or written on labels until sample has been collected. Samplers should avoid using permanent markers to write directly on the sample containers.
Method 1633A requires that a second smaller sample aliquot be collected for the determination of total suspended solids and for pre-screening analysis. The method also recommends that in the absence of historical data for the sample source, a second full-size sample be collected if re-extraction and reanalysis is required. To assess the risk of contaminating the samples during the collection process, prepare a field blank for the sampling location by transferring approximately 500 mL of the PFAS-free water obtained from the laboratory into a sample container of the same type and source used for the field samples. If any sample collection equipment cleaned and reused to collect additional samples, prepare an equipment blank by pouring PFAS-free water over the cleaned equipment and collect the rinsate. Frequency of field and/or equipment blanks is project/site dependent and should be documented in the sampling and analysis plan (SAP).
Because Method 1633A uses isotope dilution quantitation, separate matrix spike samples are not routinely required. However, if the SAP for a given site or project requires matrix spike and/or duplicate sample analyses, collect those QC samples in the same manner described above.
When collecting multiple samples from the same site, place bagged water ice in the cooler to keep the samples cool and add more bagged ice as additional samples are collected. Follow standard procedures for the final packing and shipping of the cooler and all associated sampling paperwork. Use sufficient water ice to maintain the sample temperature at or below 6 °C during transport for a period of at least 48 hours, to allow for shipping delays. Alternatively, samples for Method 1633A may be frozen prior to shipment.
6. What are the sampling requirements for collecting wastewater compliant with ASTM D8421-24?
ASTM D8421-24 uses a much smaller sample than does Method 1633A. Only 5 mL of sample is collected in a 15-mL conical polypropylene tube with a cap and graduations marked on the side. As with Method 1633A, the sampling team should work with the laboratory to obtain sample tubes that have been lot-tested and found free of PFAS, as well as a sufficient volume of PFAS-free reagent water to be used in the field to prepare field blanks and for equipment rinsing needs.
The method recommends that the sample tubes be labeled and pre-weighed at the laboratory before they are shipped to the sampler. This allows the laboratory to determine the actual sample size by re-weighing the tube and sample once they are delivered to the laboratory and determining the sample weight by subtraction. Therefore, the sampler must not add any additional labels to the tubes. Even writing on the labels will add weight to the tube. It is important to coordinate with the PFAS laboratory regarding the sample labeling practices being used in the field. The most common approach is to place the tubes with samples in self-sealing plastic bags and label the bags, rather than the tubes.
The method recommends, and some labs may require, that all samples be collected and analyzed in duplicate, or even triplicate. If so, place all replicate tubes from the same sampling location in a single bag which is then labeled. If the sample is collected for NPDES monitoring, one cannot ignore a detect just because it is not present in a replicate sample. Inconsistent detects may be a product of a non-homogeneous sample, which may be more likely to occur with a 5-mL sample. If inconsistent results between replicate 5-mL samples is a common occurrence when using ASTM D8421-24, then using Method 1633A and its 500-mL sample size as an alternative may be more appropriate for these matrices. Here again, it is critical that the sampling team coordinate with the laboratory to determine the sample collection needs before going into the field and documenting the procedures to be used in the SAP or other planning document.
ASTM D8421-24 does not routinely employ isotope dilution quantitation, but it is an option available to the laboratory. If the laboratory is not using isotope dilution, then the SAP must specify that additional samples be collected for matrix spike and matrix spike duplicates (MS/MSD), typically at least once every 20 samples from a discharger, as mentioned at 40 CFR 136.7(c)(1)(v). The sampling team must make sure that there are enough sample tubes to collect those samples using the same procedures used for the field samples.
Place bagged water ice in the cooler with the sample containers to keep them cool and add more bagged ice as additional samples are collected. Standard operating procedures must be followed for the final packing and shipping of the cooler and all associated sampling paperwork.
Although the smaller sample size used for ASTM D8421-24 allows for the use of smaller coolers than would be required if using Method 1633A, bags containing the sample tubes still need to be packed carefully and the cooler filled with sufficient water ice to maintain the sample temperature at or below 6 °C during transport for a period of at least 48 hours, to allow for shipping delays.
7. Are there any challenges associated with the smaller sample volume (5-mL) collected when following ASTM D8421-24?
The collection of the 5-mL samples for ASTM D8421-24 presents additional challenges beyond just limiting PFAS contamination from other sources. At a minimum, the flow of the sample stream must be adjusted to be low enough to allow the sampler to partially fill the sample tube (only 5 mL in a 15-mL tube and excess sample volume must not be poured off), as well as low enough to not force the tube out of the sampler’s hand. The samplers should strive to collect a sample that has a representative amount of total suspended solids from the discharge. If the sample stream flow is too low, those suspended solids may not be collected. This may require using a sampling pole to partially insert the tube into the stream. It may take a few tries to only fill the tube roughly one-third full and each attempt will require starting with a new tube. Given these challenges, it may be useful to request more sample tubes from the laboratory than the sampling team initially anticipates needing.
Additionally, smaller sample sizes are more likely to generate inconsistent detects among replicates which can be a result of non-homogeneous samples. If inconsistent results between replicate 5-mL samples is a common occurrence when using ASTM D8421-24, using Method 1633 and its 500-mL sample size as an alternative may be more appropriate for these matrices (as discussed in answer #6 above).
8. How can a sampling procedure be assessed for PFAS-contamination?
To assess the risk of contaminating the samples during the collection process, EPA recommends preparing a field blank for the sampling location by transferring the approximate targeted sample volume (Method 1633: 500 mL; ASTM D8421-24: 5 mL) of the PFAS-free reagent water obtained from the laboratory into one or more sampling containers used for collecting wastewater samples. For ASTM D8421-24, the sample containers used for field blanks must be pre-weighed in the laboratory. In addition, if any sample collection equipment is cleaned and reused between samples, an equipment blank should be prepared by pouring PFAS-free water over the cleaned equipment and collecting the rinsate.
1 Alix E. Rodowa, Emerson Christie, Jane Sedlak, Graham F. Peaslee, Dorin Bogdan, Bill DiGuiseppi, and Jennifer A. Field, Environmental Science & Technology Letters 2020 7 (3), 156-163. DOI: 10.1021/acs.estlett.0c00036.
2 Elizabeth Denly, Jim Occhialini, Phil Bassignani, Michael Eberle, and Nidal Rabah, Remediation. 2019, 29: 65–76. DOI/10.1002/rem.21614