The dirt on PFAS and biosolid land applications
As national debates intensify over bans on spreading biosolids on farmland, University of Arizona’s College of Agriculture, Life, and Environmental Science researchers are leading a national effort to assess the true risk of PFAS contamination.
Sedimentation tanks at a wastewater treatment facility, where gravity helps separate suspended solids. Sludge settles at the bottom while scum rises to the top, ensuring cleaner water before further treatment.
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In 2020, Pima County issued a moratorium on the land application of biosolids, one of the first of its kind in the United States. It was a decision made in response to growing concerns surrounding PFAS (per- and polyfluoroalkyl substances), a group of chemicals linked to health risks and environmental contamination. While the ban sought to protect the public from “forever chemicals,” the immediate consequence was steep.
“It raised the cost for utilization or disposal of biosolids by Pima County wastewater reclamation department from $1.3 million annually to $3.3 million annually,” explained Ian Pepper, Director of the University of Arizona’s Water & Energy Sustainable (WEST) Center and a regents professor in the Department of Environmental Science. “That’s a $2 million increase in just one year. For a local utility, that’s a huge hit.”
What are biosolids? Biosolids are nutrient-rich organic materials recovered from wastewater treatment. While the raw material is known as 'sewage sludge,' 'biosolids' is the term used to describe the final, treated product that has met established safety standards for recycling into our environment.
Ian Pepper in his office at the U of A's WEST Center
U of A CALES
To assess the real risk, Pepper’s team analyzed decades of Pima County land application plots, some dating back to 1985. Using recorded lifetime loading rates – the total biosolids applied over nearly 40 years – and measuring PFAS levels in soil, the team could see if the chemicals were moving toward groundwater.
“This is a valid approach, because these are, after all, “forever chemicals,” these do not change over time,” explained Pepper, who is BIO5 Institute member.
Their findings were reassuring: both the incidence and mobility of these PFAS were remarkably low. Armed with this data, the Pima County Board of Supervisors made a rare move in environmental policy: lifting the moratorium. It was a victory for evidence-based regulation, according to Pepper.
“A little light bulb went off in my head that we had had a local problem, which we solved with a local study,” he recalled. “What do you need for a national problem? You need a national study.”
It sparked what would become the National Collaborative PFAS Project, the largest study of its kind in the U.S., spanning 21 sites across 16 states and 25 species or compounds of PFAS.
The 4-million-ton question: Where does the waste go?
Biosolids in storage: treated sewage sludge processed to meet safety standards for environmental recycling.
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The U.S. Environmental Protection Agency estimates approximately 4 million dry metric tons of sewage sludge are produced each year. This staggering volume of byproducts is the inevitable result of modern wastewater treatment. According to Pepper, as we clean our water, the organic solids must go somewhere, but options for managing them are limited.
“You can put it in the air, the ocean, or soil. Those are your three options,” Pepper said. “Which one would you like?”
To break it down, each disposal method has significant environmental trade-offs, and these are the limited options we face:
Burn it – Incinerating reduces the volume but has the potential to release air pollutants.
Dump it in the ocean – While it may dilute the waste it can harm marine ecosystems, a risk that led to a national ban in 1988.
Landfills – While often used as a fallback, landfills are themselves significant sources of PFAS, essentially “doubling up” on the chemical load. Over time, chemicals like PFAS can leach out, concentrating in the surrounding environment and posing long-term contamination risks.
Soil – Currently 60% of all U.S. biosolids are land-applied, a practice that can recycle nutrients and sequester carbon, but there are real concerns surrounding the risk of contaminating soil and water, especially with persistent chemicals like PFAS.
To ban or not ban, that is the question
The disposal problem surrounding biosolids and the debate over the risks of these “forever chemicals” has shifted from local utility boardrooms to state legislatures. In 2022, Maine became the first state to enact a ban on biosolids land application, followed by Connecticut in 2024. Several other states are currently grappling with whether to follow suit.
This landscape is where Pepper’s big idea entered the fray. Arguably one of the most ambitious of his career – second only to his leadership in the application of wastewater epidemiology to support public health efforts during the COVID-19 pandemic – the effort rallied farmers, academic researchers, and public utilities across 16 states to build the largest and most comprehensive database of its kind in the United States.
Led by the principle that regulation must be science-based, the project was a massive undertaking that involved field studies across broad geographic regions of the U.S., with differing soils, climates, and depths to groundwater.
“The first year, people thought I was crazy, and the second year, I was a genius,” Pepper recalled.
The project relied on local utility and state agency personnel collecting more than 400 soil samples, which were sent to the U of A's Arizona Laboratory for Emerging Contaminants for PFAS analysis. To ensure the project’s integrity across so many different sites, Pepper’s team created an 18-minute instructional video to standardize sampling and prevent the accidental contamination of samples with “background PFAS.”
Weighing soil samples in the lab at the U of A's WEST Center, where researchers analyzed the samples for PFAS concentrations.
U of A WEST Center
Because PFAS are found in countless everyday products, the sample teams’ own clothing or equipment could easily compromise a sample. To prevent this, the 18-minute video provided a strict list of don’ts for sampling personnel, including:
• No water-resistant or non-stick clothing or gear
• Avoid showering on scheduled sample collection days
• Regulated cooling packs to prevent PFAS contamination
• Restrictions on personal care products
• Even small items like sticky notes were banned in the field
Sifting fact from fear: The power of soil
By combining these modern sampling efforts with decades of historical land-use data, the findings provided a reality check: the real risk to groundwater is much lower than the “chaos” surrounding proposed blanket bans might suggest, Pepper explained.
“The take-home message from the National Collaborative Study is that the overall concentrations of PFAS in soil are very low, far lower than one might have expected,” he said. “Especially compared to industrially contaminated sites where levels can be 80 to 500 times higher.”
The data confirmed that soil acts as a surprisingly effective natural filter, Pepper explained. On average across the 20+ field sites, the study found a 69% attenuation rate just six feet below the surface, and some compounds like PFOS reached 92% attenuation.
Attenuation measures how much soil reduces PFAS concentrations as they move downward. The deeper you go, the fewer chemicals remain. That’s because PFAS molecules don’t just flow through soil, they stick to it. This process, called sorption, binds the chemicals to organic matter and clay particles, in most cases reducing the potential for groundwater contamination.
“By six feet down, we saw the greatest amount of non-detects,” Pepper said. “The soil is very good at attenuating PFAS.”
Pepper's research team collecting samples with soil auger in southern Arizona.
U of A WEST Center
The study examined soils from sandy to clay-heavy, although a simple national correlation between soil texture and PFAS levels didn’t emerge. Pepper notes that too many confounding soil factors, as well as varying biosolid loads and local climates, were at play.
Still, it’s clear from site-specific findings that not all soils and not all species of PFAS are created equal in this equation.
“The more clay you have in the soil, the less leaching you're going to have,” Pepper said. “And this protects against groundwater contamination.”
Clay provides a low-permeability barrier, and higher organic matter increases sorption, keeping PFAS from moving downward.
“It's resting, sorbed to organic matter or clay colloids,” Pepper said. “Very similar, actually, to the same issue many years ago with heavy metals.”
This sorption helps explain why not all PFAS are equal, according to Pepper. A compound’s “chain length” or the number of fluorinated carbons it has strongly influences mobility. Longer-chain PFAS tend to bind more readily to soil, showing higher attenuation, while shorter or intermediate-chain species travel more easily, he explained.
But even soil composition isn’t the whole story.
Remember, in southern Arizona the moratorium was ultimately lifted. Groundwater in parts of Pima County sits roughly 180 feet below the surface. Now compare that to states like Maine, where groundwater can lie just 18 inches below the ground. The difference is stark. The same land-application practice can carry very different risks depending on geography and the depth of the aquifer, a reality Pepper said complicates blanket, one-size-fits-all bans.
Industry vs. municipal sources and the ubiquity of PFAS
One of the clearest findings of the national project was the sharp divide between typical municipal biosolids and materials influenced by industrial waste streams. In fact, even sites with high cumulative biosolid applications generally showed low median PFAS concentrations. That wasn’t the case with sites impacted by industrial sources – such as utilities receiving landfill leachate or the contaminated liquid that drains from decomposing waste – which recorded significantly higher levels of PFAS.
“If it’s just municipal biosolids, rarely are you going to see groundwater contamination based on our results,” Pepper said, pointing to one outlier in the study: a facility with what he described as a “direct line from a landfill into the headworks of the utility.” That industrial input was one of only few substantially elevated concentrations observed.
The distinction, he argues, is critical for policymakers weighing statewide bans.
“Utilities should identify industrial inputs of PFAS and preclude them from their wastewaters,” Pepper said. “That's exactly what we did with metals. And point-source control of metals completely took metals off the table as an issue.”
The study also discovered PFAS are so widespread that they appear even in areas where biosolids were never used. Control plots in the study, those which never received biosolid applications, still showed levels of PFAS, likely from wind-blown deposition, irrigation, or precipitation, he explained.
The bottom line
PFAS in biosolids aren’t a simple yes-or-no problem. Risks depend on groundwater depth, soil type, chemical species, and whether industrial waste sneaks into the equation. Pepper stresses nuance over blanket bans: control industrial inputs, respect the natural “soil shield,” and recognize that even ubiquitous compounds behave differently depending on context.
“I started January the 15th, 1977, Gerald Ford was still president. This is the start of my 50th year at the U of A. Fall will be 100 semesters,” he said. “In the last five years, I’ve worked on wastewater-based epidemiology, and now PFAS. Those are two of the most important projects of my career.”
What’s next for Pepper? He’s staying in the fight with tough science. The team is now tackling two major environmental questions: how PFAS move into crops, with implications for food safety, and how microplastics interact with soils, affecting contaminant transport and soil health.
Solid-phase extraction of plant tissue samples at the Arizona Laboratory for Emerging Contaminants. The research will be used to help understand how crops interact with PFAS.
U of A WEST Center
Ian L. Pepper, Sarah M. Prasek, Jon D. Chorover, Frank J. Prevatt, Danielle M. Barrientes, Greg Kester, Mark L. Brusseau (2026). National collaborative study on the incidence and mobility of PFAS following land application of biosolids. Science of The Total Environment, Volume 1012, 2026. 181197, ISSN 0048-9697. https://doi.org/10.1016/j.scitotenv.2025.181197.