Researchers step toward understanding how toxic PFAS chemicals spread from release sites

Researchers step toward understanding how toxic PFAS chemicals spread from release sites

New lab studies are helping researchers to better understand how so called “forever chemicals” behave in soil and water, which can help in understanding how these contaminants spread.

A study led by Brown University researchers sheds new light on how pollutants found in firefighting foams are distributed in water and surface soil at release sites. The findings could help researchers to better predict how pollutants in these foams spread from the spill or release sites — fire training areas or airplane crash sites, for example — into drinking water supplies.

Firefighting foams, also known as aqueous film forming foams (AFFF), are often used to combat fires involving highly flammable liquids like jet fuel. The foams contain a wide range of  per- and polyfluoroalkyl substances (PFAS) including PFOA, PFOS and FOSA. Many of these compounds have been linked to cancer, developmental problems and other conditions in adults and children. PFAS are sometimes referred to as “forever chemicals” because they are difficult to  break down in the environment and can lead to long-term contamination of soil and water supplies.

“We’re interested in what’s referred to as the fate and transport of these chemicals,” said Kurt Pennell, a professor in Brown’s School of Engineering and co-author of the research. “When these foams get into the soil, we want to be able to predict how long it’s going to take to reach a water body or a drinking water well, and how long the water will need to be treated to remove the contaminants.”

It had been shown previously that PFAS compounds tend to accumulate at interfaces between water and other substances. Near the surface, for example, PFAS tend to collect at the air-water interface — the moist but unsaturated soil at the top of an aquifer. However, prior experiments showing this interface activity were conducted only with individual PFAS compounds, not with complex mixtures of compounds like firefighting foams.

“You can’t assume that PFOS or PFOA alone are going to act the same way as a mixture with other compounds,” said Pennell, who is also a fellow at the Institute at Brown for Environment and Society. “So this was an effort to try to tease out the differences between the individual compounds, and to see how they behave in these more complex mixtures like firefighting foams.”

Using a series of laboratory experiments described in the journal Environmental Science and Technology, Pennell and his colleagues showed that the firefighting foam mixture does indeed behave much differently than individual compounds. The research showed that the foams had a far greater affinity for the air-water interface than individual compounds. The foams had more than twice the interface activity of PFOS alone, for example.

Pennell says that insights like these can help researchers to model how PFAS compounds migrate from contaminated sites.

“We want to come up with the basic equations that describe the behavior of these compounds in the lab, then incorporate those equations into models that can be applied in field,” Pennell said. “This work is the beginning of that process, and we’ll scale it up from here.”

Ultimately, the hope is that a better understanding of the fate and transport of these compounds could help to  identify wells and waterways at risk for contamination, and aid in cleaning those sites up.

Co-authors on the paper Jed Costanza and Linda Abriola. Funding for the research was provided by the Strategic Environmental Research and Development Program (W912HQ-18-C-0014).


Study Abstract

Perfluoroalkyl acids spontaneously concentrate at air–water and non-aqueous phase liquid (NAPL)–water interfaces, which can influence their retention during subsurface transport. This work presents measurements of air– and NAPL–water interfacial tension for synthetic groundwater containing perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorooctanesulfonamide (FOSA), or aqueous film-forming foam (AFFF) formulations at concentrations ranging from 0.1 to greater than 1000 mg/L. The NAPLs tested included dodecane, tetrachloroethylene, and jet fuel. AFFF formulations were less efficient at lowering interfacial tension than PFOA, FPOS, or FOSA substances below 100 mg/L, while above 100 mg/L, these formulations were more effective, achieving tensions of less than 3 mN/m. Infiltration of solutions with such low tension could lead to mobilization of residual NAPL. Equations based on interfacial tension measurements show that concentrations of PFOA, PFOS, and FOSA at the air–water interface were from 2 to 16 times greater than at the NAPL–water interface below 100 mg/L and were 10–50 times greater for AFFF below 20 mg/L. Calculations for unsaturated soil estimate that up to 87% of PFOS mass was at the air–water interface and less than 4% at the dodecane–water interface for bulk-water concentrations below 1 mg/L.