PFAS environmental investigations: how to decide on PFAS analysis and/or analytes

PFAS present unique challenges when investigating their presence in the environment (soil, groundwater, surface water, etc.).

For other common contaminants, the presence of environmental impacts can be determined well before samples are submitted to a laboratory for analysis. For example, petroleum-impacted soil near the phreatic (groundwater) surface is typically stained a greyish color and will present strong odors, while groundwater may have a sheen present; chlorinated solvents can be observed either by odors or via field screening with a photoionization detector (PID); and metals-impacted soil can be field-screened with an x-ray fluorescence (XRF) spectrometer to estimate metals concentrations prior to laboratory analysis.

PFAS impacts, however, are not typically evident by field observations and currently, there are no readily available technologies for screening samples in the field. Laboratory analysis is relied on to determine the presence or absence of PFAS and to subsequently make decisions regarding risk communications and remedial options.

There are also analytical challenges associated with PFAS including, but not limited to:

• Out of thousands of different PFAS, commercial laboratories are only able to accurately detect up to approximately 40 compounds.
• Detection limits are typically required in parts-per-trillion (ppt) for both soil and groundwater, one to two orders of magnitude lower than the parts-per-billion (ppb) and parts-per-million (ppm) levels, respectively, that are typically of interest for other common contaminants.
• Many PFAS including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) have structural isomers (the same chemical formula but different molecular shapes), which can be difficult to distinguish from each other using existing analytical methods.
• Some PFAS are known to transform into other PFAS over time – these precursors are polyfluorinated compounds, the vast majority of which are not target analytes for existing analytical methods

Other analytical challenges include how to select a laboratory method and which PFAS or isomers to analyze for an initial investigation.

On April 10, 2024, the US Environmental Protection Agency (USEPA) finalized the National Primary Drinking Water Regulation (NPDWR), which is the first nationwide drinking water regulation that establishes enforceable maximum contaminant levels (MCLs) for six PFAS. The NPDWR includes PFOA and PFOS as individual contaminants, with MCLs of 4 ppt and MCL goals of zero ppt for each. Perfluorohexane sulfonic acid (PFHxS), perfluorononanoic acid (PFNA), and hexafluoropropylene oxide dimer acid (HFPO-DA) and its ammonium salt, commonly known as “GenX” chemicals, were incorporated in the final rule version as individual contaminants with MCLs and MCL goals of 10 ppt each.

No enforceable federal groundwater or soil standards exist yet for any PFAS.

In advance of the federal MCLs, many US states opted to set their own regulatory PFAS thresholds for drinking water, groundwater, soil, and surface water.

USEPA has developed various analytical techniques for quantitating PFAS in drinking water, groundwater, wastewater, surface water, soils, sediments, biota, and biosolids. The analytical techniques for PFAS can be separated into three broad groups:

• Targeted analyses
• Isomer analysis
• Non-targeted analyses including total fluorine (organic fluorine) analysis and total oxidizable precursor methods

In 2022, on behalf of the Norwegian Environment Agency, Ramboll collected and compiled information available for analytical methods for PFAS in products and the environment. The report gave insights into established analytical methods available for the primary matrices in commercial and consumer products, and environmental matrices, along with cost considerations.

Targeted methods are well-established methods to quantitate specific target PFAS in various matrices. Targeted methods can use liquid chromatography (LC), coupled with several different types of mass spectrometry (MS) to quantitatively measure PFAS concentrations.

Targeted analysis can accurately quantify individual PFAS, and most methods have a target quantitation limit typically in the range of 1-10 ppt for drinking water, groundwater, wastewater, surface water, soils, sediments, biota, and biosolids.

However, this analytical technique requires laboratories to obtain isotopically labeled analytical standards for each compound of interest and is therefore limited to quantifying only the PFAS specific to that method. Targeted analysis does not provide an indication of other PFAS that may exist in the sampled media.

A summary of the most common commercially available targeted PFAS analyses is provided in Tables 1 and 2, and below.

USEPA method 537.1: This is the most common method currently used to quantitate PFAS in drinking water. USEPA method 537, published in 2009 to quantify 14 PFAS analytes, was updated to 537.1 in 2018 to include 18 target PFAS analytes (as further detailed in Table 1).

USEPA method 537.1 modified (sometimes also referred to as USEPA method 537 modified): A modification of USEPA method 537.1 by the commercial laboratories for the quantification of at least 18, but up to 70, targeted PFAS analytes for all matrices (other than drinking water) as further detailed in Tables 1 and 2. This method uses an isotope dilution technique that involves quantitation of targeted PFAS using a labeled isotope of the compound. On January 31, 2024, USEPA method 1633 was finalized, which is the first USEPA method specifically designated for non-drinking water environmental media. For analysis of media other than groundwater, given the emergence of USEPA method 1633, many commercial laboratories are no longer offering USEPA method 537.1 modified (unless specifically requested).

USEPA method 533: Published in 2018, this method focuses on the determination of PFAS with a chain of fewer than 12 carbon atoms, in drinking water matrices only. This method builds upon the USEPA method 537.1 with notable differences. It has a target analyte list of 25 PFAS, including 14 of the 18 listed in USEPA method 537.1, with more short chain PFAS. This method also includes several odd-chain PFAS, fluorotelomersulfonic compounds, and novel perfluoroether carboxylates and sulfonates (Table 2). This method incorporates isotope dilution in the method, which can minimize sample matrix interference and improve data quality.

USEPA method 8327: Published in 2019 to quantitate 24 PFAS (16 PFAAs and eight other PFAS) in non-drinking water matrices, this method expands the analyte list with the addition of 10 different analytes in the list compared to USEPA method 537.1. However, it does not include four analytes that are on the method 537.1 list (Table 2). This method uses external standard calibration instead of isotope dilution calibration to quantify the analytes, with resulting elevated quantitation limits, and is rarely used by commercial laboratories.

USEPA method 1633: This is the new USEPA method finalized on January 31, 2024 for the quantification of 40 target PFAS analytes (see Table 2) in eight different media (wastewater, surface water, groundwater, soil, biosolids, sediments, landfill, and biota) as shown in Table 1. Many commercial laboratories participated in the validation study of this method, and have already started offering this method for commercial availability.

In addition to the methods listed in Table 1, there are also targeted methods published by the American Society for Testing and Materials (ASTM). These standard test methods include ASTM D7979 for the determination of PFAS substances in water, sludge, and wastewater, ASTM D7968 for the determination of PFAS substances in soil, and ASTM D8535 for the determination of PFAS substances in soil/biosolids matrices. These ASTM methods are not commonly offered by most commercial laboratories; however, many commercial laboratories can offer these methods upon request.

Isomers of certain PFAS can be present in the environment in linear or branched forms, with an example shown in Figure 1 below.

When analyzing a sample (using all the USEPA methods described above), unless directed otherwise, laboratories will report only the total sum of a PFAS compound including all isomers. For example, the linear and branched PFOA would be integrated as a single compound to quantify the total concentration of PFOA.

However, for certain laboratories, it is possible to request that the laboratory report the estimated concentrations of each isomer. For this analysis, the laboratory uses a quantitative or qualitative reference standard containing mixtures of available linear and branched isomers and compares the instrument response from the reference standard against the response from suspected isomers in the samples. This analysis can be especially useful for forensics purposes when seeking to determine unique signatures from a given potential source of contamination.

A summary of two common commercially available non-targeted PFAS analyses is provided below.

Total organic fluorine (TOF) analysis quantifies the concentration of total organic fluorine in a sample and may be used as a surrogate for total PFAS present (including non-targeted PFAS compounds). It should be noted that this method also detects organic fluorine in other organic compounds (such as pharmaceuticals and herbicides) so the TOF results cannot necessarily be interpreted to equal the total PFAS concentration. However, this method can be used as a screening technique to identify whether the sample contains any organic fluorine or not (i.e., if no organic fluorine is detected, then by default, no PFAS is present at levels detectable by the analysis).

This method involves the removal of inorganic fluorine during sample preparation, followed by combustion ion chromatography (CIC) to measure the total fluorine content of a sample, including non-target precursors, other complex PFAS, or fluorine from any organic compound.

TOF does not identify nor quantify individual PFAS, nor does it provide information about the compound that originally contained the fluorine being measured.

In January 2024, USEPA finalized USEPA method 1621 related to the determination of adsorbable organic fluorine (AOF) in aqueous matrices. This method has completed the multi-laboratory validation study and is currently offered by many commercial laboratories. This method can measure the concentration of organofluorines but cannot identify the sources of organofluorines in the sample. The organofluorine measured in the sample may be associated with PFAS or other non-PFAS fluorinated compounds such as fluorinated agrochemicals and pharmaceuticals. The result is simply reported as the concentration of fluoride (F^-) in the sample.

Total oxidizable precursors (TOP) are measured in the “TOP assay” analysis, which estimates the PFAS precursor content of a sample by converting non-target compounds to quantifiable compounds using a thermal hydroxyl radical oxidation process.

Through the TOP assay process, precursors (such as a non-target sulfonamide compound) are degraded to target PFAS end products (such as PFOA). TOP assay results can provide an indication of the total mass of PFAS present in a sample and can be useful when considering plume and source area behavior over time.

TOP assay requires the quantification of PFAS in a sample prior to and following the TOP assay digestion so that the difference in quantifiable PFAS mass can be estimated and is, therefore, a relatively expensive test.

For drinking water, USEPA method 537.1 is the most commonly utilized method. In general, USEPA method 533 for drinking water is used to analyze samples that are suspected to be impacted by short-chain PFAS compounds that cannot be measured by method 537.1. When both methods are used together, 29 PFAS compounds can be tested in drinking water.

For other non-drinking water environmental media (soil, groundwater, etc.), in many cases, the decision is not quite as simple. Based on Ramboll’s experience, the most common analysis currently used in the US is quickly becoming USEPA method 1633 as this method was just recently finalized in January 2024; USEPA method 537.1 modified, which was previously the most common method utilized for PFAS analysis of non-drinking water samples will gradually phase out of use. The transition expected by the laboratories is that USEPA method 1633 will fully replace USEPA method 537.1 modified in two years or less.

The TOP assay and TOF method are typically utilized only if required by a regulator, or to further characterize the nature and extent of PFAS on sites that have already had an initial investigation completed. Currently, USEPA method 1621 is offered by commercial laboratories and can be used to broadly screen contaminated sites for thousands of known and unknown PFAS compounds.

USEPA method 8327 is generally not used by commercial laboratories for PFAS due to the high detection limits.

In some locations in the US, regulators currently require a specific method to be utilized, so before proceeding with analysis, verify if any local requirements are applicable. For example, even before USEPA method 1633 was finalized, New York state was requiring this method for PFAS analysis in all non-drinking water environmental media.

We have heard from many of our clients about the concern for comparability between targeted PFAS analyses. In particular, the difference between USEPA method 1633 and USEPA method 537.1 modified. However, based on our discussions with multiple laboratories, the two methods have been demonstrated to generate data of comparable quality, and while USEPA method 537.1 modified supports slightly lower reporting limits for certain PFAS, overall limits are comparable between the two methods. In summary, we would expect that if split samples were collected using USEPA method 1633 and USEPA method 537.1 modified, the results would be nearly identical for the target PFAS compounds.

Also, in some cases, the cost of analysis may also be a consideration in method selection, given that the cost of PFAS analysis is higher than most other contaminants. The cost of analyzing samples for targeted PFAS is expensive because it requires an intensive extraction process, reference standards, and isotopes. To put this into context, below is a summary of the range of costs that were provided to Ramboll by three well-known laboratories in the US: Eurofins Scientific, SGS Analytics, and Pace Analytical Services.

If desired, when running one of the targeted analyses above, the laboratory can report fewer analytes than their full list. For example, if only a few PFAS are regulated in a particular state, it is possible to request that the laboratory only report the results for those regulated compounds, if this would be useful for decision-making purposes. In many cases, this results in lower analytical costs as the use of isotopically labeled standards is reduced and instrument time decreases. If it is later desired to know the concentrations of the other analytes and if all of the performance criteria for the additional compounds were met, the laboratory can typically provide data for the full list, potentially for a small fee.

Before collecting PFAS samples, significant thought should be put into how the samples will be collected and analyzed, as the analytical strategy is a key component in determining the nature and extent of PFAS.

Factors that should be taken into consideration include the environmental setting, the presence or absence of nearby sensitive receptors, the regulatory setting, cost of analysis, and the risk tolerance of the party conducting the work.

Typically, a phased approach makes strategic sense where the first round of data collection involves a narrow scope of work and, depending on the results, further testing is performed.

It is also recommended that, in certain cases, legal counsel be consulted to develop a strategy before proceeding with PFAS sampling.

In summary, environmental investigation of sites for PFAS can be challenging and expensive, making the selection of analytical methodology as part of the early strategic planning phase critical.

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