Many communities are forced to confront PFAS contamination in their groundwater. A key hurdle in addressing this harmful group of chemicals lies in unravelling how they move through a region of the environment called the unsaturated zone. A new study by the University of Wisconsin–Madison researchers offers a simplified new way of understanding PFAS movement through this zone, with improved PFAS models.
PFAS is an abbreviation for perfluoroalkyl and polyfluoroalkyl substances. Synthetic chemicals have been used for decades in products ranging from nonstick cookware to firefighting foams. Some PFAS chemicals are associated with health risks and can persist in the environment indefinitely. Modelling their flow through the unsaturated or vadose zone is essential. The chemicals can linger there for years or decades, slowly leaching into aquifers many communities use to provide drinking water.
Unfortunately for those tasked with this job, the complexity of the unsaturated zone and the molecular structure of the PFAS chemicals make this crucial work a considerable challenge.
“The unsaturated zone is really complex because you have air, you have grains, and you have water. They are all moving dynamically all the time,” said Will Gnesda, a graduate student in the UW–Madison Department of Geoscience.
“It’s always been a big issue for all types of contaminants, understanding how the unsaturated zone works,” Gnesda says. “But PFAS add another layer of complexity.”
That’s largely because PFAS molecules are attracted to the boundary between air and water.
“The unsaturated zone is full of those boundaries,” says Gnesda.
For these reasons, modelling the movement of PFAS through the unsaturated zone has traditionally required a lot of guesswork and immense computational power. Gnesda, who works in the lab of geoscience professor Christopher Zahasky, is striving to improve and simplify this modelling work.
PFAS models need work, simplification and improvement essential
Gnesda and his colleagues have produced a simplified framework that promises to reduce the computing power and time required to model PFAS movement through the ground. The framework can be applied to specific sites. It is an essential factor for utilities and environmental consultants attempting to predict how PFAS contamination may affect local reservoirs in geologically unique settings.
The work was recently published in the journal Environmental Science & Technology.
The researchers applied their modelling framework to a real-world site near Rhinelander. It is a city of about 8,000 in Wisconsin’s Northwoods, where two municipal wells were contaminated with PFAS in 2019. The site’s geology has been extensively studied, providing the team with valuable data for testing the modelling framework.
They found that several factors significantly influence where and how long harmful PFAS chemicals stay locked in the ground before flowing below the water table. These factors include the amount and location of organic carbon held in a site’s rocks, the amount of gravelly sand and the porosity of soils and rocks.
The research points toward a more accessible approach for modelling PFAS flow in the ground. More analyses is needed to refine and validate the framework. That is the focus of a new collaborative project led by Zahask. This project is underway as Gnesda and his colleagues attempt to track PFAS molecules flowing through a simulated unsaturated zone and aquifer in a lab back on the UW–Madison campus.
“We’re going to see how well our theory connects to the lab,” said Gnesda. He expects the experiments to refine the modelling framework further so they can ultimately be applied to more real-world scenarios.
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