Tag: Source Identification

PFAS Forensics

This article covers the basics of PFAS Forensics and other Environmental Forensics.

Look a crime scene looking person examining soil! Must be PFAS forensics – even if I used the photo from an article titled: “How did we fall for the Junk Science of Forensics”

Depending on whom you speak with the term forensics can be a dirty word. It either conjures up images of flawed feature-comparison disciplines (hair-, bite-, or tool-mark comparisons for example) underlined by poor science or single source DNA which exonerates the falsely accused. PFAS forensics, when practiced well, more closely resembles DNA evidence for a criminal case and may play a critical role in ensuring those responsible for contamination pay for its remediation. The purpose of this article is to provide a gentle introduction to environmental forensics with an emphasis on PFAS. This is just one of several articles I have written on PFAS, it may help to refresh on PFAS terminology as well as PFAS analytic strategies. I am acting in my own capacity, my views are my own and do not represent my employer. Nothing here constitutes legal or professional advice.

About 99.9% of DNA between humans is the same however, scientists have characterized DNA regions that are different between people who are not closely related. Forensic labs look at 20 DNA regions that vary between individuals, called short tandem repeats (STRs), to create a DNA “fingerprint.” The chance that two people who aren’t closely related have the same DNA profile is 1 in 1,000,000,000,000,000,000. Paraphrased from Mary May and figure by Rebecca Clements

Introduction to PFAS Forensics

PFAS forensics is not a bunch of EPA investigators playing “good cop bad cop” with a chemical manufacturer or waste manager tied to a chair with a bright spotlight shining in their face. PFAS forensics is the subset of environmental forensics which deals with PFAS. Environmental forensics more generally is really the logical outgrowth and terminus of one branch in fate and contaminant transport. While fate and contaminant transport in general describes how chemicals move and transform throughout the environment, when this is reframed through the lens of forensics the emphasis shifts from more describing mobility and persistence to assist in assessing exposure more to source identification/attribution as well as developing legal along with scientific defensibility of conclusions. With billions on the line in multidistrict litigation you can bet the science will be heavily scrutinized.

PFAS forensics is not the Reid Technique aka “good cop bad cop” instead it is really a different focus for traditional fate and transport

Basic PFAS and Environmental Forensics

Environmental forensics, and by extension PFAS forensics, typically has a few routine questions:

  • Who is responsible for the PFAS?
  • When and how was the PFAS released?
  • In cases with more than one responsible party: How much has each party contributed?
  • Were the releases progressive or sudden?

Environmental forensics uses several techniques to answer these questions to tease out information about how the contaminant (PFAS in this case) got to the place where it was detected. In many cases multiple releases of the same or related chemical are from the same site which may require contribution allocation for each identified release – this is especially important where more than one responsible party is identified. Beyond obvious applications in litigation, PFAS forensics could potentially lead to cost savings by better characterizing the sites and understanding related risks from them, better tailoring remediation or treatment, and assisting in insurance claims. In some cases, environmental forensics has also lead to completely new discoveries. For example, an unknown natural perchlorate source, unrelated to nitrate from the Atacama Desert or other known sources, was discovered using isotropic fingerprinting during a forensic investigation launched after routine perchlorate monitoring discovered contamination north of San Diego. While a PFAS forensic study may be solved using only one technique it is important to build multiple independent lines of evidence consisting of at least two distinct techniques to check your work. The independent evidence should obviously point to the same conclusion. Cases where one line of evidence is enough tend to include straightforward reported releases or dumping.

Common Non-Intrusive Environmental and PFAS Forensic Techniques

When performing document review, it’s critical to pay attention to chemicals which while may not pose health risks may be linked to the original health contaminant as an additive or may influence the fate and transport of the contaminant of concern. Site features such as underground storage tanks, https://www.epa.gov/ust ditches, loading docks, transformers/electrical components (especially for PCBs) are also important.

  • Historical document review which can include historical documents (deeds, spill, or ownership records), environmental permits, operational or monitoring data, shipping manifests, as well as sales or invoices can be extremely helpful. These can also assist in recreating production processes if no records exist.
  • Photographic review is very valuable. Systematic coverage of the US has occurred since the 1930s and changes overtime can be very instructive. The Aerial Photography Field Office of the USDA is particularly valuable and infrared photography can also assist in differentiating vegetation changes.
  • Map review which can denote things such as waste or chemical storage, loading/unloading areas, warehouses etc… Sanborn maps (fire insurance maps) are particularly useful.
  • Interviews with current and past employees, neighbors, or other relevant parties can provide clues to focusing the search or turn up undocumented knowledge.
  • Market forensics started due to diffuse releases, for example in consumer products. Essentially sales and other relevant product information (eg formulation) is aggregated to estimate total loading. Market forensics has been successfully used on a local scale to determine fatty alcohol (detergent) contamination in Luray, Virginia (home of the caverns). I believe this to be most accurate for PFAS on a global scale as some emissions inventories have shown. In most instances on local scales it is closer to bite mark analysis (forensic odontology) than DNA analysis.

A whole review industry has sprung up from historical review in the form of EPA Phase 1 Environmental Site Assessments compliant with the All Appropriate Inquires Rule from 40 CRF 312. This can be considered defensive due diligence when performed prior to a real estate transaction.

A document review conducted according to the All Appropriate Inquires rule can serve as liability protection and should be the first step in any forensic investigation. Image from Digital WarRoom

Advanced Environmental and PFAS Forensic Techniques

Non-intrusive techniques alone may be enough to end investigations into PFAS sources however, sometimes more advanced techniques are required.  These advanced techniques should only be undertaken after integrating any existing knowledge body related to a specific site because advanced techniques are often costly.

Chemical Fingerprinting

Chemical fingerprinting is where specific sources are associated with characteristic distributions typically developed through comparison source samples or a priori knowledge developed using non-intrusive techniques. Chemical fingerprinting has successfully been used for PFAS source identification several times.

Specific chemicals can develop a “fingerprint” for specific sites and introduces some challenges to that technique in the form of unknown precursors
This fantastic image from Per- and polyfluoroalkyl substances in Chinese and German river water – Point source- and country-specific fingerprints including unknown precursors does an excellent job showing how testing for specific chemicals can develop a “fingerprint” for specific sites and introduces some challenges to that technique in the form of unknown precursors. Manufacturing sites are plotted and labeled in the left panel, their specific chemical fingerprint as percent of total load is given in the center panel, and the TOP assay was used to show that some of the targeted endpoints were from untargeted unknown precursors

Isomer Analysis

Isomer analysis, isomers are compounds with the same formula but differing chemical structures. PFAS are generally produced through either electrochemical fluorination (ECF) or telomerization. ECF products contain approximately 70–80% linear structures and 20–30% branched isomers; telomerization yields relatively pure linear or isopropyl branched isomers. This technique also has been used several times for PFAS source identification.

Isomers elute at different points during HPLC. Branched forms of PFAS elute after linear PFAS and the more branched it is the slower it typically elutes.
Isomers elute at different points during HPLC. Branched forms of PFAS elute after linear PFAS and the more branched it is the slower it typically elutes. Image from Eurofins

Chiral Analysis

Chiral analysis is a very unique subset of isomer analysis. The easiest way to think about chiral isomers, or enantiomers as they’re also known, is your right and left hand. Your hands are the same “isomer” except for an asymmetry so if you line your hands on top of each other palms down they do not match however, palms together they do.  This is the same concept behind chirality. Manufactured compounds (such as PFAS) are racemic mixtures which means they contain the same or near equal enantiomer ratios. Since enzymes are always chiral, biotransformation is normally enantioselective. Chiral analysis can show if biotic uptake is due to precursors or direct source and has been used for this purpose. While I am not aware of any studies, this kind of analysis may also be able to differentiate weathering and precursor transformation for example between manufactured PFOA against PFOA degraded from fluorotelomer alcohols.

PFOA enantiomers which can be helpful in elucidating precursor loading
PFOA enantiomers which can be helpful in elucidating precursor loading

Isotropic Analysis

While isomers have the same chemical formula but distinct structures; isotopes are elements that have the same atomic number but different atomic mass. Isotopic Analysis means techniques measuring the mass of different stable isotopes, typically carbon, hydrogen, oxygen, nitrogen, and chlorine although there are many other possibilities. Unless something is ionized its protons must balance charge with electrons however, neutrons, which carry no charge, affect the weight allowing for this type of analysis. There are three main Isotopic Analyses which fall into this category: bulk, compound specific, and positional specific.

Elements are tracked by protons, unless an atom is ionized, the charge must balance so protons equal electrons; the neutrons may vary in stable compounds which is the basis for Isotopic Analysis
Elements are tracked by protons, unless an atom is ionized, the charge must balance so protons equal electrons; the neutrons may vary in stable compounds which is the basis for Isotopic Analysis

Bulk Stable Isotropic Analysis (BSIA)

Bulk Stable Isotropic Analysis is a specific type of isotropic analysis that measures the total concentrations of stable elements in a sample and can be correlated to the specific element in the releasing source. While I do not know any direct applications to PFAS yet, this technique has been used before to combat pharmaceutical counterfeiting.

Root and within plant uptake may be fractionating processes
The assumption that root water uptake and within-plant water transport are non-fractionating is slowly crumbling. Several studies such as this one from Javier de la Casa have reported offsets between plant and source water stable isotope composition. Without proper accounting, these isotopic offsets can result in the erroneous source water attribution and uptake overestimations by vegetation. This image shows a shift in soil water excess against line conditioned excess the green dots are where both LC-excess and SW-excess plus the corresponding standard error were negative.

Compound specific isotopic analysis (CSIA)

Compound specific isotopic analysis (CSIA) is isotropic analysis for specific compounds (for example PFOA) within a sample and is generally much more powerful a method. This is in the process of being applied to PFAS, through Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) project ER24-4239.

Isotope signatures can allow different contaminant sources to be distinguished at the same site
While CSIA is not yet proven for PFAS, analogies can be drawn to chlorinated solvents. Since several manufacturing processes and raw materials are used to make chlorinated solvents each solvent tends to have different δ13C, δ37Cl and δ2H signatures for specific VOCs which can allow distinguishing different contaminant sources at the same site, identifying a contaminant from an off-site source located upgradient, as well as establishing links between observed compounds and emitting sources. Image courtesy of Terus Environmental

Position specific isotopic analysis (PSIA)

Position specific isotopic analysis (PSIA) provides stable isotope ratios at each carbon position where a C–F bond is present. This has been done with PFAS in the lab and theoretically is the most powerful isotopic analysis method available.

Position specific isotopic analysis (PSIA) is isotopic analysis at each bond
Figure showing PSIA for fipronil, a PFAS insecticide, which is blamed for spreading colony collapse disorder among bee colonies (along with neonicotinoid insecticides). Image courtesy of Cornelia Rasmussen and David Hoffman

Advanced Statistical Techniques

Advanced statistical techniques such as principal component analysis can also assist in forensic studies. PCA is designed to reduce a dataset to its principal components by taking an original dataset and finding a new smaller set of variables that still describes the most variance in the original dataset – that is to say to take the chemical inputs and reduce it to source inputs. PCA relies on eigen values and eigen vectors to reduce the dimensionality of a dataset.

This image shows two sources derived from several dimensions (chemical ratios)
A principal component analysis for PFAS profiles. The score plot is shown to the left and the loading plot is shown to the right. In the loading plot, PFCA are colored purple and pink, PFSA are colored green, and PFOS precursors are colored yellow. In this study PCA was used to aid in PFAS source identification.
Karl Pearson is a founding father of mathematical statistics
PCA was discovered in 1901 by Karl Pearson. Pearson was a brilliant but misguided statistician who founded mathematical statistics along with the world’s first statistical department but is also the poster child for scientific racism. Image from the American Statistical Association. I wonder why they didn’t decide to call themselves the American Statistical Society.

Dendrochronology

Dendrochronology, or dating tree rings, may provide inference on when PFAS releases first occurred, particularly for birch or spruce although I do not think this has been used in this way yet; most information revolves around phytoremediation. Dendrochronology may not yet meet the Daubert Criteria (some states still use the Frye Standard but all federal courts use Daubert) but there is some evidence for it.

Haglöf increment borer - a dendrochronology tool
Careful analysis on core parts can assist in developing timelines, while unproven for PFAS it has been used for petroleum and chlorinated solvents cases. Image from Britannica

Some PFAS Forensics Considerations

PFAS forensics is a rapidly evolving science with many unknowns. However, it can draw many parallels to petroleum forensics which has a very deep body of knowledge around it because of the environmental liability associated with petroleum. For example, both are made of complex chemical mixtures enabling chemical fingerprinting for ready source identification. Despite a varied chemical composition, there are a few constants with PFAS. For example, Perfluoroalkyl acids (PFAAs) are strong acids that are anionic at environmentally-relevant pH. PFAAs are extremely persistent in the environment and do not degrade or transform under typical environmental conditions polyfluoroalkyl substances include compounds that have the potential to transform into PFAAs and are known as precursors. Carbon chain length and functional moiety control sorption and therefore transport. High sorption capacity implies slower environmental transport and vice versa.

Major sorption potential of various types of PFAS with the defining characteristics listed on the sides
This image I made helps to show major sorption potential of various types of PFAS with the defining characteristics listed on the sides.

Some specifics that make PFAS forensics unique are:

  • With no known natural formation there is a source responsible somewhere.
  • Formulation can help establish dates. PFAS are entirely anthropogenic first starting in 1938 with several types not produced till much later. Presence of ether-ified PFAS, such as HFPO-DA (GenX Chemicals), for instance implies a 2000s source at the earliest.
  • Broad industrial use means many sources exist and differentiation may be important.
  • PFAS typically had industrial formulations to meet performance standards eg being able to extinguish a fire in a set time and not chemical composition standards so the chemical composition varies through time. In other words PFAS formulation depends on production year. Commercial formulation and usage profiles can identify sources.
  • Individual PFAS measured are only a fraction of total PFAS.
  • Lowered measured concentrations lead to increased uncertainty.
  • Due to long range transport capabilities, PFAS are ubiquitous global contaminants. For instance, rainwater has been found to exceed the new PFAS drinking water regulation.
  • Transport properties can additional sources. For example, if PFOA is ahead of PFHxA it can indicated separate releases since the longer chained PFAS has lower transport capabilities.
Showing more mobile lighter PFAS leading plumes
This image I made helps to show how an example plume could look.
  • PFAS are generally resistant to weathering and transformation (hence the term “forever chemicals”) however, precursors may occur. For example fluorotelomer alcohols may transform into perfluoroalkyl carboxylic acids.
  • Ratios of precursors (such as FTOHs) to terminal end products (PFAAs) should increase (ie fewer FTOHs and more PFAAs) with time and distance from sources. There should be a precursor gradient in other words.
  • PFAS tend to accumulate at interfaces for example between bulk liquids and L- or D-NAPLs, soil/water, soil/water and the atmosphere and so forth. In fact, this tendency has lead to foam fractionation as a potential treatment method.
  • PFAS tend to accumulate in muscle as opposed to adipose (fat) tissue unlike most contaminants.
  • Linear compared to branched PFAS should increase with distance from point source because most Br-PFAS have a higher water solubility and faster biological elimination from organisms which also results in faster biotransformation closer to the source.
  • Due to biotransformation, specific environmental PFAS concentrations should not be used to predict biotic uptake (take total organofluorine into account).
  • Low water concentrations compared to biota (hence implied high bioaccumulation factors) can indicate that water is not the only source.
  • Age-dating may be possible using specific diagnostic ratios.
Summary of major patterns discussed in the article
Image from L. Trozzolo, TRC, and C. Higgins, Colorado School of Mines which summarizes some of the patterns discussed above.

Conclusion and Key Take Aways

Practically, it is impossible to remediate global PFAS contamination however, managing PFAS hotspots through forensic investigations may significantly reduce global contamination’s extent. There are many parallels which can be drawn between petroleum forensic investigations and PFAS as well as many well-developed techniques to assist in source identification however, it is still a rapidly evolving field. The goal of PFAS forensics, and really environmental forensics, is ultimately to develop legal as well as scientific defensible conclusions related to source attribution. To that end, multiple independent lines of evidence should be used to validate and strengthen conclusions. If you learned something or enjoyed this article (I know I separated those but for me they tend to be correlated!) take a look at some other articles I have written on PFAS.