Rationale for Study
PFAS are fluorinated compounds that comprise a family of anthropogenic chemicals used for decades to make products resistant to heat, oil stains, grease, and water. Of these, perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) have been the most prominently used in the U.S. PFAS are now regarded by EPA to be “emerging contaminants”.
PFAS are known as “forever chemicals” because most are resistant to microbial degradation, so that once introduced into the environment, they persist. They are ubiquitous and found in most soils, sediments, and water. All people in the U.S. are thought to have PFAS in their blood, with longer chain compounds like PFOA and PFOS remain in the body for many years.
Potential household exposure to PFAS include: textiles, carpets, cleaning agents, food wrappers, food products, drinking water, and household dust. Because of potential adverse health effects, PFOS and PFOA were voluntarily phased out of production in the U.S. beginning in the early 2000s and completed by 2015. The potential health effects have not been consistently demonstrated in humans, but laboratory studies indicate changes in liver and thyroid activity and reproductive problems. More recently, PFAS have been suspected of being potential carcinogenic agents. Based on this the EPA has established lifetime health advisory limits for drinking water of 70 parts per trillion (ppt) for PFOA and PFOS.
Biosolids are the solid endpoint of wastewater treatment and over 60% of biosolids produced nationally in the U.S. are land applied. In many parts of the country, land application has long been, and remains, the most efficient use for biosolids. In Pima County, 100% of locally produced biosolids have been land applied until recently. Biosolids are a proven effective organic fertilizer with multiple benefits for plant growth, crop production, and soil health. However, they also contain minute amounts of PFAS in the low parts per billion (ppb) range. Because of this, the Pima County Board of Supervisors took a conservative approach of enacting a temporary moratorium on land application of biosolids until the risks from PFAS in biosolids could be fully evaluated. In particular, concern was expressed over the possibility of groundwater contamination. This moratorium was initiated January 1, 2020 resulting in a requirement for all biosolids generated in Pima County to be landfilled. This action has doubled management costs for biosolids and removed availability of beneficial product widely employed by local farmers for agricultural production. This action has also been the impetus for this current study, the goal of which was to fully evaluate the potential impact of land application on groundwater contamination by PFAS.
The study, one of the largest of its kind ever undertaken, was conducted at long term biosolids land application sites in Pima County. Through collaboration between Pima County Regional Wastewater Reclamation and local farmers, agricultural sites were identified where Class B Biosolids had been land applied since 1984, with known recorded application rates. Samples of soil, well water, and biosolids were collected and analyzed for a suite of PFAS compounds. The analyses were conducted by an international certified analytical laboratory, Eurofins TestAmerica, specializing in PFAS determination in soils. During sample collection, strict adherence to recommended precautions for eliminating sample contamination were followed. In all, 109 samples were collected including 72 soil samples at depths of 1, 3 or 6 feet from the surface, 9 groundwater samples from irrigation wells, 4 biosolids samples, and a surface soil sample. Soil and groundwater samples were collected from 5 field types: undisturbed desert soil (no agriculture); irrigated agricultural soil with no biosolids; and irrigated agricultural soils with biosolids at three different cumulative loading rates: ≤20 tons; 21-30 tons; and > 30 tons.
Results and Discussion
Analysis of soil samples from undisturbed sites with no history of land application of biosolids or irrigation were all nondetectable at all soil depths tested. In contrast, samples from irrigated agricultural sites with no history of biosolids application demonstrates low concentrations of PFAS, less than 3 ppb. Analysis of some irrigation waters were also positive for PFAS, suggesting that irrigation alone can be a source of PFAS in soils. PFAS in irrigation water did not appear to correlate to biosolids application. Agricultural soils that were irrigated and received biosolids contained PFAS, but again at very low concentrations. PFAS soil concentrations detected were low and minimally increased with increased biosolids loading, from less than 2 ppb at the lowest biosolids application rate to 4 ppb at the highest loading rate. These concentrations are well 1Environmental Toxic Substance Assessment; Per- and Polyfluoroalkyl Substances (PFAS) in Pima County Water, December 2019. PFAS in Biosolids: A Southern Arizona Case Study 2 below health-based screening levels developed for residential soil established by EPA.
Another significant result was the limited migration of PFAS within soils. Even in the irrigated agricultural soils, over 90% of the PFAS was attenuated within the top 6 feet of soil, with the vast majority being trapped in the surface foot of soil. Because depth to groundwater in Pima County regional agricultural areas is typically 150 – 200 feet beneath the surface, contamination of groundwater from biosolids application is extremely unlikely and is supported by the groundwater results.
This study took a broad look at PFAS contamination, retention, and migration in farm soils where biosolids were historically land applied. The data presented demonstrates very low concentrations of PFAS compounds in soils receiving biosolids with migration attenuated in the first few feet of soil. The concentrations are lower than published health-based screening criteria published by the USEPA. The low concentrations of PFAS in biosolids coupled with depth to groundwater of 150’ – 400’ and Arizona’s low rainfall alleviates help minimize the impact on public health and groundwater.
Full report available here: https://casaweb.org/wp-content/uploads/2020/11/PFAS-in-Biosolids_A-Southern-Arizona-Case-Study.pdf