PFAS, Part 2: How Do PFAS Interact With Soil?
Before we look at PFAS contamination in WNY, let’s back up and explore whether PFAS contamination in water and soil is a concern for veggie and herb gardeners.
As discussed in PFAS article 1, PFAS are widespread in the environment and bioaccumulate in animal and human bodies. In their 2020 research review, Costello and Lee note that PFAS are so ubiquitous that “finding pristine soils is rare.” A review of soils in North America with no known sources of PFAS contamination found background levels of PFOS and PFOA in every sample. A worldwide soil survey of 32 PFAS in 62 locations with “limited to no anthropogenic impact” also turned up PFAS in every sample (Costello & Lee). Even those of us who live far from industrial activity and conventional agriculture (which may apply PFAS-containing pesticides and biosolids to fields) have PFAS in our local soils.
In this article and the next, I ask the following questions:
How do PFAS interact with soil?
To what extent are PFAS taken up by plants?
Buckle up for a somewhat technical sciencey ride. My exploration of the questions above outgrew one article, so I’ve split it into two. (If you think this article is boring, know that I edited it down from 14 pages!) If this is an area of knowledge/training for you and you spot a misunderstanding, please reach out and share the correction. The resources cited below were a lot to digest, and I’ve got much more to learn.
Thanks to Christina Schilling Costello and Linda S. Lee for their 2020 review and compilation of research on plant uptake of PFAS, which I reference frequently. Their article expands on a 2018 review by Rossella Ghisi et al.
Image description: My partner Patrick applying horse manure to four new garden beds. Our porch and weeping willow are in the background.
A few concepts and definitions:
Sorption occurs when a chemical is pulled out of watery solution by a soil solid, due to their attraction to one another. The chemical may remain at the surface of the solid, or be pulled within. This serves to retain or bind that chemical in the soil.
Sorpted chemicals won't easily leach out of the soil into the groundwater or be readily taken up by a plant beyond the root surface. The polarity or ionic charge of a chemical, plus the charge of the soil components, is one factor that affects sorption.
Cations are positively charged ions. Anions are negatively charged ions. Ions like to bind with oppositely charged ions.
Long-chain PFAS are PFAS with 6 or more carbon atoms in their carbon chain. For example, “C8” PFAS like PFOS possess 8 carbon atoms.
Short-chain PFAS may have 7 or fewer carbons.
The overlap of short- and long-chain PFAS at 6-7 carbons is due to the variation in the rest of the molecule’s composition, beyond the carbon atoms (Waste360).
Short-chain PFAS include newer GenX chemicals that are replacing long-chain PFAS like PFOA and PFOS.
Precursors are PFAS compounds that can degrade to shorter-chain PFAS metabolites, such as PFAAs. A precursor may also release long-chain PFOS as it degrades. PFOS is considered a terminal metabolite, since it doesn’t easily break down further.
PFAS and Soil
Here’s some info I’ve gleaned:
In general, PFAS stay in the soil long enough to be taken up by plants, and plants are able to take them up. And—“PFAS” is a huge group of chemicals that includes very different types of molecules, which interact with soil and plants very differently.
In general, a long-chain, non-polar, hydrophobic molecule like PFOS or PFOA is more likely to be bound up (“sorbed”) in the soil than shorter-chain, water-soluble PFAS.
Soil sorption makes long-chain PFAS less bioavailable to plants—but it also means that a reservoir of long-chain PFAS and precursors may accumulate in the soil over time (Costello & Lee).
The fact that shorter-chain, water-soluble PFAS molecules and metabolites are less likely to be bound in the soil makes them more mobile—both to exit to the water table, or to migrate into plants. Short-chain PFAS even have the potential to move to the gas phase and migrate via air (Waste360).
Many other variables may affect PFAS plant uptake. These include:
soil carbon (which increases with more organic matter—like compost)
the growing medium: field soil vs. container vs. a hydroponic system (where plants grow in fertilized water with no soil)
soil pH
charge of soil particles
and many more.
In general, the higher the soil carbon and organic matter in the soil, the greater the sorption of many contaminants (Costello & Lee). The more sorption to soil organic matter, the less the contaminant is taken up by plant roots.
Soil carbon is generally higher in organic gardens, pastures, and “virgin” (non-agricultural) soils than in conventional farmland (Magdoff).
Carbon is low in sandy desert soils.
Clay soils—like the soil in my backyard—are able to retain more carbon (Magdoff). (Perhaps I have a reason to feel good about all that sticky clay!) One can imagine nutrients and contaminants being easily washed away in sandy soils, where there is less soil structure to hold them.
Interestingly, a review by Pereira et al. found no consistent pattern between soil carbon concentration and PFAS sorption. Costello & Lee note that different organic matter sources may alter PFAS retention and plant uptake potential (2020). Sounds like an area for more study.
Amending the soil with phosphate has been correlated with decreased sorption of PFOS. In one study, this was most noticeable in soils with low organic matter and high ferric oxide (iron) levels (Qian et al). Ferric oxide is used as a soil amendment to increase iron (Chivine).
“Given the importance of phosphorus as a nutrient in agricultural systems, further studies on the effect of phosphorus…on sorption of additional PFAS are warranted” (Costello & Lee).
Rain events that saturate the soil may mobilize sorbed PFAS temporarily (Costello & Lee)
I’m interested in asking a soil scientist if this points to practical implications. For example, is it better to harvest veggies before or after a heavy rain?
Image description: A black garden cart sits in front of our veggie garden, which is bordered by white electric tape, wildflowers, and quaking aspen with bright yellow leaves.
Nature is complicated: Soil and environmental variables affect different PFAS compounds differently.
For example, in this study:
The effects of cations on sorption were noted only with shorter-chain PFAS.
For PFCAs with carbon chains of 5-8, the “charge” of soil organic matter is strongly correlated with sorption. However, for longer-chain PFAS, pH is a better predictor of sorption (Pereira).
And, different variables impact one another.
For example:
“Cation effects were more varied and complicated since their addition can invoke pH changes as well” (Costello & Lee). PFAAs in soil organic matter become more anionic with a higher soil pH. A lower sorption rate can be expected for anionic PFAAs in more alkaline soil (Pereira et al).
Variables can’t be isolated and tested on their own; that isn’t how nature operates.
More research is needed.
Just like research on the human health impacts of PFAS, much of PFAS research in agriculture has focused on just two compounds (PFOS and PFOA) even though thousands of PFAS are in use (EPA). Research on and testing for precursors and PFAS-containing metabolites has only picked up in the last few years.
For example, EtFOSA is the active ingredient in a pesticide that’s popular in South America. It’s a known precursor to PFOS, and was recently shown to degrade into PFOS in a wheat uptake study. PFOS was found in both earthworms and the soil itself (Zhao S et al). Costello and Lee note that “A small subset of these now-known precursors is starting to be routinely quantified; however, currently, there are still many known precursors that are often not quantified…There are likely still many unknown and difficult to quantify precursors that may be a significant fraction of the total PFAS present and could serve as a continual source of PFAAs [in the soil] depending on degradation potential and degradation rates” (Costello & Lee).
Final Thoughts
The fact that short-chain PFAS don’t bioaccumulate to the extent of long-chain compounds has justified the replacement of long-chain PFAS with short-chain GenX chemicals in manufacturing. However, the fact that short-chain PFAS are more readily taken up by plants and more mobile in the environment is cause for concern. More studies on the health effects of short-chain PFAS and their behavior in soil are certainly called for.
Stay tuned for the next article, PFAS, article 3: How do PFAS interact with Plants?
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Image description: Sunlight filters through trees in a deciduous forest behind a WNY creek at sunset.
Sources
Chivine. “Ferric Oxide.” Viewed Oct 28, 2022. webpage.
Costello, M. Christina Schilling and Linda S. Lee. “Sources, Fate, and Plant Uptake in Agricultural Systems of Per- and Polyfluoroalkyl Substances.” Current Pollution Report (2020). webpage.
Environmental Protection Agency (EPA). “PFAS Explained.” Updated April 28, 2022. webpage.
Ghisi, Rossella et al. “Accumulation of perfluorinated alkyl substances (PFAS) in agricultural plants: A review.” Environmental Research. Vol 169, pp. 326-341. Feb 2019. webpage.
Magdoff, Fred and Harold van Es. Building Soils for Better Crops, Ch 3: “Amount of Organic Matter in Soils.” Sustainable Agriculture Research and Education (SARE). 2021. webpage.
Mei, Weiping et al. “Per- and polyfluoroalkyl substances (PFASs) in the soil–plant system: Sorption, root uptake, and translocation.” Environment International. Volume 156. November 2021. webpage.
Minnesota Department of Health. “PFAS and Homegrown Garden Produce.” Jan 27, 2022. webpage.
Pereira, Hugo Campos. “Sorption of perfluoroalkyl substances (PFASs) to an organic soil horizon – Effect of cation composition and pH.” Chemosphere. Volume 207, Pages 183-191. September 2018. webpage.
Qian J. et al. “Adsorption of perfluorooctane sulfonate on soils: effects of soil characteristics and phosphate competition.” Chemosphere. 168: pp. 1383–8. 2017. webpage.
Waste260. “PFAS Carbon Chain Length — It is Just a Number or is it?” The History of PFAS. Aug 16, 2021. webpage.
Zhao S, Zhou T, Wang B, Zhu L, Chen M, Li D, et al. “Different biotransformation behaviors of perfluorooctane sulfonamide in wheat (Triticum aestivum L.) from earthworms (Eisenia fetida).” Journal of Hazardous Material. 346: pp. 191–8. 2018. webpage.
