Using multiple lines of evidence helps in the study and remediation of PFAS

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soil sampling
A lot can be learned from samples of PFAS in the soil, sediment and water taken on-site.

By Stefano Marconetto

Investigating and remediating soil and groundwater impacted by per-and polyfluoroalkyl substances, collectively known as PFAS, is a particularly challenging puzzle. Conventional techniques tend not to be very effective, calling for a multiple lines of evidence (MLE) approach.

Over the past 50 years, thousands of PFAS have been developed for different applications. Each has its own chemical and physical properties, which affect the way it migrates and is distributed in soil, sediment and water.

The huge range of PFAS-containing products, such as firefighting foam, carpets, waxes and breathable outerwear, and their use in several industrial processes, mean different dispersal patterns. Firefighting foams may spread PFAS into soil and sediment as well as surface and groundwater. Industrial uses may result in leaks, spills, air, wastewater or waste discharges, and PFAS from consumer products may end up in landfill leachate.

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PFAS may be spread far from point of origin by wind, sediment or water without breaking down.

Their persistence, wide use in many products, long-standing utilization of over half a century, and their long-range atmospheric transport and deposition mean that they can be almost anywhere. It is important to separate the effects of these background concentrations from those of the PFAS specific to the site being investigated.

These factors make PFAS a good example of a particularly challenging class of compounds to study and to remediate. The MLE approach used for PFAS is practical, customizable and scalable, and can be designed to fit the objectives and complexity of the project at hand.

The MLE approach can be used on a wide range of PFAS-impacted sites. Its purpose is to determine the source(s) through fingerprinting the range of PFAS that are found. This approach also helps separate PFAS impacts that are specific to the site, and those that come from off-site sources or background concentrations. This is particularly important, since PFAS have dispersed widely into the environment.

Golder’s MLE approach involves four steps:

1. Analyze PFAS present on the site – Even though PFAS are almost ubiquitous in the environment, their composition and release mechanisms are often different, depending on the source. This fact means we can learn a lot from samples of PFAS in the soil, sediment and water taken from the site. This includes studying the concentrations in the source zones, as well as along the primary flow paths, and how the composition stays the same or evolves along them.

Many site investigators consider their work to be complete if they assess the concentrations for the few PFAS that are currently being regulated. We find that we gain valuable insights from a more comprehensive approach. This includes studying the relative concentrations of all of the PFAS being analyzed, typically 10 to 30, depending on the laboratory and the site being analyzed.

Some of those insights come from producing radial or “star” plots for each sample, so that the concentration of each PFAS is represented by an arm of the star. When each star is placed on a map of the site to indicate where the sample was taken, it is easy to see whether the proportions of each PFAS are the same throughout the site.

If there are differences, it may indicate that some impacts are from different sources or may suggest preferential transport/partitioning of some PFAS versus others, based on chain length or other properties.

Radial plots, positioned on a site map, help with the fingerprinting process to determine the origin and cause, as well as with fate and transport assessment. We also find that these maps are readily understood by stakeholders, including property owners and political leaders. Therefore, they form a valuable tool for gaining support for site management and remediation.

Radar plots
Radial plots can be used to display relative PFAS concentrations at any sampling location and can assist with the identification and characterization of multiple PFAS sources and plumes. This example shows distinct PFAS signatures for three different sources.

2. Determine the total mass of PFAS on the site and analysis of its signature – While the radial plots provide part of the answer, the MLE approach means going broader. This is partly because of the limitations of current commercial laboratories in analyzing PFAS. Most laboratories are only able to test for about 30 PFAS, out of well over 3,000 that have been developed over the past 50 years.

This means that much of the PFAS chemistry on any given site is still a frustratingly large and impenetrable black box. We are not able to analyze all the compounds that may be present, including some compounds (precursors) that over time could biotransform into the PFAS that are being regulated.

However, what we can do is assess the total PFAS mass at any given location. This not only provides insights into the size of the contamination problem, but the analysis of its signature also helps us gain insights into whether specific PFAS precursors are present. This becomes a valuable line of evidence for characterization of PFAS impacts and fingerprinting.

3. Study PFAS linear and branched isomer composition – Going beyond the study of PFAS concentrations and mass, our MLE approach can include analysis of their chemical structure. Some PFAS compounds have a linear molecule. Others have a branched molecule that looks like a letter “L” or a “T”. It’s the same compound, but with different variants. Each variant has its own properties and so may behave differently in the environment. This includes how easily the compound migrates by means of air dispersal, groundwater or surface water.

Therefore, we use chromatograms of the PFAS analysis to assess the linear versus branches ratio for selected PFAS. This, along with the other analytical tools in the MLE toolbox, helps us distinguish sources, and understand the PFAS transport and fate pattern.

PFAS diagram

PFAS diagram
In certain circumstances, additional lines of evidence such as the use of TOP Assay results, branched vs. linear isomer compositions and local geochemistry can assist with distinguishing PFAS sources and assessing fate and transport. By providing a better understanding of the PFAS issues at the site, this approach can help to better scope follow-up investigations, monitoring programs, risk assessments, risk management or remediation.

4. Analyze the general chemistry of the site – A fourth aspect of our MLE approach is to take a broader look at the site’s chemistry. We find that this often provides insights into the way PFAS will behave. This stage includes studying the pH, specific conductivity, dissolved oxygen, temperature, oxidation-reduction potential, as well as analyzing other contaminants that may be present.

The MLE and the number of lines of evidence used is customizable to site characteristics and project objectives. The use of all of these tools may not be required in every PFAS site characterization. However, we find that it is important to keep in mind all of the available tools in our toolbox, so they can be used when needed.

We find that this MLE approach to PFAS site characterization has helped us understand the sources, transport and fate of this class of compounds, which are a class that is coming under increasing regulatory scrutiny and public concern. Furthermore, the multiple lines of evidence approach provides robust data that can be relied upon to design remediation strategies.

Stefano Marconetto, MSc, P.Eng., is with Golder. This article appears in ES&E Magazine’s April 2019 issue.

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