A new study published in the Journal of Hazardous Materials provides one of the most comprehensive empirical inventories to date of background PFAS across environmental media, reframing how we understand mass distribution, cross-media transfer, and site management strategy
Per- and polyfluoroalkyl substances (PFAS) are no longer viewed solely as contaminants emanating from discrete industrial sources. Increasingly, regulators and site managers are grappling with a more complex reality: PFAS are detectable at “background” locations where no direct release has occurred. Drawing on global and U.S. occurrence datasets spanning soil, groundwater, surface water (fresh and ocean), precipitation, air, biosolids, and wastewater, the authors establish a quantitative mass balance of background PFAS. Their central conclusion is both striking and consequential: surficial soil is the largest current reservoir of background PFAS globally and in the United States, surpassing groundwater, surface water, and even ocean water in estimated total mass.
Reframing “background” PFAS
The study adopts a pragmatic definition of background PFAS: concentrations observed in environmental media where no known direct release has occurred, even though the compounds themselves are anthropogenic. This distinction is critical. While PFAS at contaminated sites may originate from point sources such as AFFF use or manufacturing discharges, background PFAS reflect diffuse contributions, including:
- Long-range atmospheric transport and deposition
- Wastewater effluent and biosolids application
- Septic discharges
- Landfill leachate
- Consumer product degradation
The detection of PFAS in remote soils, ambient groundwater, and precipitation complicates site delineation, particularly when regulatory thresholds are in the low ng/L range. As the authors note, the ubiquity of low-level PFAS makes it difficult to distinguish plume margins from regional ambient concentrations.
An empirical mass inventory across media
Rather than relying solely on production or emission estimates, the study compiles occurrence data from peer-reviewed literature and regulatory datasets (2000–2025) to derive representative median concentrations for key compounds (including PFOS, PFOA, PFHxS, PFBS, PFBA, PFNA, PFHxA, 6:2 FTS, and HFPO-DA).
To estimate total mass, the authors:
- Selected central tendency (median-of-medians) concentrations by media.
- Estimated total media volumes.
- Applied corrective factors based on detection frequency and geographic relevance (e.g., limiting groundwater to “modern” water <50–75 years old).
This hybrid empirical–mass-balance approach yields a global PFOS mass distribution that is particularly illuminating.
PFOS mass by media (global background estimates)
- Soil: 1.3 × 10⁷ kg
- Groundwater: 1.1 × 10⁶ kg
- Ocean surface water: 7.5 × 10⁵ kg
- Fresh surface water: 2.7 × 10⁵ kg
- Precipitation and air: orders of magnitude lower
- Biosolids and wastewater (annual flux basis): relatively small but recurrent
In the U.S., the pattern is similar: soil again dominates, with approximately 2.8 × 10⁶ kg of PFOS estimated in surficial soils.
This finding challenges the long-standing assumption that oceans represent the primary environmental reservoir for legacy PFAS. Earlier studies often excluded soils due to limited data; this updated analysis demonstrates that soils are not merely transient storage compartments but dominant mass reservoirs.
Why soil dominates
Several factors explain the prominence of soil:
- Strong sorption of longer-chain PFAS (e.g., PFOS, PFNA) to organic matter and mineral surfaces.
- Accumulation from decades of atmospheric deposition.
- Retention in shallow intervals (upper 30 cm assumed conservatively).
- Slow migration rates relative to advective transport in surface water.
Notably, the estimated soil mass is consistent with prior emission-based inventories when depth assumptions are taken into account. A previous estimate of global soil PFOS mass (10 cm depth) was approximately one order of magnitude lower, aligning with the current study’s deeper (30 cm) assumption. This underscores soil as an intermediate to long-term PFAS reservoir, particularly for legacy perfluoroalkyl sulfonates and longer-chain carboxylates.
Precipitation: limited direct impact on groundwater
Despite widespread detection of PFAS in rainfall, the study’s modelling indicates that recent precipitation alone is unlikely to generate groundwater exceedances under most plausible conditions.
Using median U.S. precipitation concentrations (PFOS 0.29 ng/L; PFOA 0.33 ng/L; PFNA 0.10 ng/L), the authors simulated vadose-zone transport using the PFAS-LEACH model. Under baseline assumptions, predicted groundwater concentrations at receptor wells remained below drinking water standards for over 200 years.
Recharge further limits direct loading: approximately 21% of rainfall in the U.S. becomes groundwater recharge. Even when scaled nationally, the annual PFOS mass contribution from precipitation to groundwater represents only 5–14% of the estimated background groundwater mass. This finding has regulatory implications: precipitation is not currently the dominant driver of groundwater PFAS exceedances at background levels.
Soil-to-groundwater: a more consequential pathway
In contrast, when representative background soil concentrations (PFOS 700 ng/kg; PFOA 581 ng/kg; PFNA 244 ng/kg) were modelled, the results were more nuanced.
Baseline simulations incorporating dilution–attenuation factors predicted groundwater concentrations below standards at downgradient receptors. However, sensitivity analyses that reduced retention parameters and increased infiltration rates (the “Upper” scenario) produced predicted exceedances of PFOS and PFOA.
This demonstrates:
- Background soil concentrations alone can produce groundwater levels in the 0.1–10 ng/L range.
- Under conservative assumptions (reduced attenuation, high recharge), exceedances are plausible.
- Soil retention processes strongly control peak timing and magnitude.
For regulators developing soil-to-groundwater screening levels, these findings reinforce the importance of incorporating retention mechanisms rather than relying solely on default dilution factors.
Surface water: the role of wastewater and groundwater discharge
Median PFAS concentrations in freshwater bodies were generally higher than in precipitation and were similar to or slightly below groundwater concentrations.
Simple mixing calculations suggest:
- Precipitation-to-surface-water dilution factors: 1.4–2.5.
- Groundwater discharge dilution factors: 1.6–3.3.
These values imply modest contributions from precipitation and groundwater alone. In contrast, wastewater effluent typically exhibits higher PFAS concentrations (even in the absence of industrial inputs), making it a more plausible contributor to ambient surface water PFAS levels.
Thus, wastewater return flows likely represent a significant driver of background PFAS in surface water, consistent with national-scale drinking water intake analyses.
Strategic implications for site management
The findings carry several high-level implications for environmental professionals:
1. Soil background must be quantified during site delineation
Surficial soils represent the largest reservoir of PFAS mass. Failure to establish ambient soil baselines risks overestimating site-related impact.
2. Atmospheric deposition likely explains widespread soil burdens
The geographic breadth of soil PFAS suggests long-range atmospheric transport rather than localised direct releases.
3. Background soil can drive groundwater risk
Even in the absence of site releases, background soil PFAS may yield groundwater concentrations near regulatory thresholds under certain hydrogeologic conditions.
4. Surface water background reflects integrated watershed inputs
Wastewater discharges, groundwater baseflow, and runoff collectively shape surface-water PFAS signatures.
5. Oceans remain major sinks—but not necessarily dominant reservoirs
While oceans are key endpoints, the updated empirical data highlight soils as a currently larger reservoir of legacy PFAS mass.
Research frontiers
The study also identifies knowledge gaps relevant to academic research:
- Historical PFAS concentrations in precipitation versus current values.
- Depth distribution of PFAS in soils beyond 30 cm.
- Sediment inventories in freshwater and marine systems.
- Influence of colloid-facilitated transport and extreme rainfall.
- Role of precursors and transformation products in mass accounting.
Future work integrating sediment cores, deep ocean sampling, and long-term vadose zone monitoring will refine these inventories.
A paradigm shift: managing the legacy reservoir
The key insight from this comprehensive inventory is not simply that PFAS are ubiquitous—it is that the environmental legacy is disproportionately stored in soils. This reservoir is diffuse, geographically widespread, and largely independent of identifiable point sources.
As PFAS manufacturing declines and regulatory controls intensify, soil may remain a dominant upstream source for decades due to slow leaching and gradual redistribution. Managing PFAS risk, therefore, is no longer only about intercepting plumes; it is about understanding and contextualising background mass.
For environmental professionals navigating site closure, risk assessment, and remediation cost optimisation, this study offers an essential recalibration. Background PFAS are not marginal—they are system-defining. And soils are at the centre of that system.
