By Jeremiah Duncan, Ph.D., GZA GeoEnvironmental

Per- and polyfluoroalkyl substances, or PFAS, are well known as contaminants at this point. However, it is less well-known and may be surprising to both regulators and owners that many PFAS are not currently detectable but do transform in the environment to detectable and regulated PFAS. In fact, there were many more of these compounds, known as “precursors,” used commercially than the ones you have probably heard of (e.g., PFOA and PFOS). In this blog, we’ll discuss what these precursors are, and how to anticipate the regulatory concerns they might raise.

There are tens of thousands of PFAS, but only about 1,400 of them have seen any form of commercial production; of those, about 256 compounds were sold in large quantities on the market for any sort of industrial use, making them the most likely to be encountered in the environment. Of all the known PFAS, the most common analytical techniques test for up to 40, and only a handful of compounds are regulated.

In short, the regulated compounds are barely the tip of the PFAS iceberg. Many of those other PFAS that aren’t regulated can, with time, transform into compounds that are regulated. While the strength of the carbon-fluorine bond has given PFAS the nickname “forever chemicals,” those aren’t the only bonds in these compounds. The weaker bonds in precursors can break down over time, until what remains is primarily the original portion of the molecule with fluorinated remains, and you are left with a molecule like PFOA or PFOS. For this reason, PFOA, PFOS, and related molecules with longer or shorter fluorinated carbons chains are often termed “terminal” compounds.

Imagine an arrow—with fletching, a shaft, and an arrowhead—embedded in a tree. Over time, the fletching may drop away, the shaft may snap or degrade, but the arrowhead will remain in the tree. PFAS compounds can act much the same way, with the terminal PFAS (e.g., PFOS) as the arrowhead, in your water and soil.

Because most of these precursors cannot currently be measured in environmental samples, they are the “unknown unknowns” of the PFAS world—and they may be a source of detectable, regulated compounds. The good news is that, while we cannot currently test for them, these precursors aren’t completely invisible, if you know how to look. The Total Oxidizable Precursor (TOP) Assay, for example, uses an oxidant and heat to transform the precursors in a sample, and then test for the terminal PFAS. Knowledge is power. Knowing whether there may be precursors slowly transforming into regulated compounds can anticipate future problems on a site, inform remediation plans, and in some cases, be used to demonstrate that the responsibility for the contamination lies elsewhere.

For developers and owners, it’s key to take the following steps:

• Determine, as much as possible, what the history of your site is, focusing on what was manufactured or used on site and whether it might include any form of PFAS. 
• Work with your due diligence team to find records of all PFAS that might have been used, not just regulated ones.
• Remember that precursors can transform over a lengthy timeframe, and they may not degrade completely. Speak with a project manager about whether a TOP assay may be advisable.
• Stay abreast of regulatory changes, especially ones that regulate PFAS as a class, instead of individual compounds.

 We often talk about PFAS in environmental or hydrological terms, but it’s important to remember the key role chemistry plays with these compounds. By knowing the chemistry of what was used on your site, you can plan more effectively for regulatory risk.

The Author:

Jeremiah Duncan, Ph.D. Dr. Duncan is a Senior Chemist with the Environmental Remediation and Environmental Site Investigation Groups at GZA GeoEnvironmental, Inc. He provides specialized expertise in analytical and environmental chemistry, based on more than 25 years of consulting and academic experience. He also held an AAAS fellowship at the U.S. EPA in Washington, DC working on regulation of emerging contaminants and the environmental implications and applications of nanomaterials. He currently serves on a working group with the National Ground Water Association to co-author a white paper on PFAS forensics. Contact him at Jeremiah.Duncan@GZA.com or 603-707-3204.