Tremendous Treatment

In the ongoing effort to address hazardous PFAS contamination in groundwater, a nature-based remediation approach is emerging as a promising and cost-effective solution.

PFAS (per- and polyfluoroalkyl substances) are persistent chemicals prevalent in industrial and consumer products, including firefighting foam, since the 1940s and known for their toxicity, mobility, and resistance to degradation (persistence). Traditional cleanup methods to protect PFAS groundwater contamination footprints from expanding — like pump-and-treat systems — are effective but costly, energy-intensive and difficult to sustain.

One alternative gaining attention is Phyto Enhanced In-situ Remediation Barrier (PEIR Barrier™), an engineered treatment strategy developed by WSP that incorporates plants to enhance proven in situ retention barriers that filter groundwater using sorbents like advanced carbon-based materials.

PEIR Barrier combines nature-based processes with engineered media to better manage the PFAS migration in groundwater through:

• Sorbent retention: Biochar, activated carbon or other sorbent media is trenched or injected into the subsurface to capture long-chain PFAS.
• Phytoremediation: Trees and plants are strategically planted to draw water through the treatment zone, actively managing groundwater flow, improving long chain PFAS sorption efficiency in the media and removing the more difficult to treat short-chain PFAS from the aquifer.

“PEIR Barrier combines tree-based phytoremediation with demonstrated carbon-based retention media to capture and retain PFAS over time,” said Len Mankowski, CPG, vice president of emerging contaminants research and development at WSP in the U.S. “This approach is engineered to be active, resilient and cost-effective, operating year-round — even during plant dormancy — thanks to the carbon media’s ongoing treatment capacity.”

The effectiveness of the PEIR Barrier is no longer theoretical. A recent field study provides real-world evidence as a viable remediation option.

As lead designer, developer and implementer of a first-of-its-kind pilot study at a former tannery site in Michigan, we demonstrated the efficacy of a phyto-based approach to enhance capture of PFAS-impacted groundwater in a permeable adsorbent barrier.

The site had significant and widespread PFAS contamination due to historical application of aqueous film-forming foam (AFFF) to address fires at the site. Addressing the broad area or soil impacted by AFFF was simply not feasible and would not address the PFAS impacts already in and migrating with groundwater.

We needed a solution that could compliment site redevelopment plans, prevent further migration of PFAS in groundwater and provide long-term protection to the nearby river and Great Lakes. We selected and installed the treatment components beginning in early 2022 and have monitored system performance and analyzed results for over three years.

“Engineered biochar was placed in the aquifer and high-water-use trees — black willow, hybrid poplars and white cedar — were planted in specially designed auger holes also augmented with the retention media,” Mankowski said. “This design enhances tree reliance on groundwater rather than precipitation and puts more of the groundwater-derived PFAS in contact with the retention media.”

The biochar acted as a powerful sorbent, retaining long chain PFAS in situ, while the trees served as active evapotranspiration “pumps,” drawing contaminated groundwater into the treatment zone and enhancing hydraulic control. The evapotranspiration process simultaneously uptakes short chain PFAS, which are poorly retained long-term on the sorbent media when in groundwater.

The testing performed during the study revealed a synergistic relationship: Bioavailable absorbent media (BAM – an engineered biochar) efficiently captured long-chain PFAS, while the trees primarily absorbed more mobile short-chain PFAS, helping eliminate transport of the short chain PFAS as well.

“Four years of performance monitoring has demonstrated remarkable results,” Mankowski said. “PFAS concentrations within the treatment zone decreased by more than 99 percent and previously impacted downgradient groundwater PFAS concentrations declined by as much as 82 percent. Impressive when we consider there is still residual PFAS adsorbed onto the aquifer matrix downgradient of our treatment area.”

BAM was placed in the rhizosphere to further limit long chain PFAS uptake into plant tissues and retain PFAS transferred from the aquifer. The testing of the biochar treatment effectiveness in the soil was tested by analyses of soil, groundwater and tree tissue and has revealed new insights into the fate of PFAS in the trees.

PFAS levels increased in tree leaves from spring to fall, following the water and nutrient uptake cycle. The PFAS levels in the tree leaves planted with extra sorption media are 68-to-99 percent lower than levels detected in untreated “wild” willow trees present at the site. More significantly, PFAS levels in leafdrop collected from our treat trees are nearly non-detectable.

So where does the PFAS that accumulated in the leaves all summer go? In the fall, the nutrient cycle reverses. Nutrients that provide the building blocks for chlorophyl in the next growing season, for example, are pulled back down to the roots.

The data we’ve collected suggests that the accumulated PFAS are also pulled back from the leaf tissue and down into the roots and soil. PFAS levels, including the usually hard-to-detect short-chain PFAS, are found to be increasing in the rhizosphere soil of the treated trees. The changes in PFAS levels detected in tree tissue over time, coupled with rhizosphere PFAS results, suggest PFAS cycle, with nutrients from the leaves ultimately stored in the roots and soil before leafdrop occurs.

The system’s capital and maintenance expenditure was a fraction of the cost of traditional remediation — about $0.03–$0.07 per 1,000 gallons treated versus $0.16–$0.29, which is typical for pump-and-treat with arguably better results. The system is achieving comparable PFAS concentration reductions but with a low carbon footprint.

“As opposed to most pump-and-treat applications, PEIR Barrier is powered by the sun and naturally derived gradients with nothing to ‘plug in.’” Mankowski said. “While pump-and-treat systems often generate a secondary waste stream to manage and maintain, to date, the effectiveness of the retention media design has not generated any PEIR Barrier secondary waste streams.”

The success of this demonstration highlights the potential for phyto-enhanced permeable adsorbent barriers to be a widely applicable, cost-effective alternative for control of PFAS concentrations and migration in groundwater. Ground has already been broken to expand the approach further at the site with planting to take place in spring 2026.

A Game-changing Advancement

“The project site study and extremely promising results are a critical step forward toward the use of nature-based approaches to address PFAS in soil or groundwater,” Mankowski said.

By harnessing the strengths of both nature and engineered use of established sorbent technologies, PEIR Barrier offers a rapid, reliable and sustainable pathway to PFAS reduction, one that will protect people and natural resources with a solution that is more achievable and affordable for communities and industry.

“This landmark field study represents a validated, nature-based breakthrough in PFAS groundwater remediation,” he added. “It confirms our belief that PEIR Barrier is emerging as a widely applicable and lower cost alternative to current PFAS groundwater technologies and in short order will bring rapid improvement to how we remediate and manage areas impacted by PFAS. It’s a game changer.”