New research published in the Journal of Hazardous Materials provides one of the most comprehensive empirical inventories to date of PFAS in the context across environmental media, reshaping how we understand bulk distribution, cross-media transfer, and site management strategies.
Per- and polyfluoroalkyl substances (PFAS) are no longer considered solely as pollutants originating from discrete industrial sources. Regulators and field managers are increasingly faced with complex realities. This means that PFAS can be detected even in “background” locations where direct releases are not occurring. The authors utilize global and U.S. occurrence datasets spanning soil, groundwater, surface water (freshwater and marine), precipitation, air, biosolids, and wastewater to establish a quantitative mass balance of background PFAS. Their central conclusion is shocking and consequential. In other words, surface soils are currently the largest reservoir of background PFAS worldwide and in the United States, exceeding groundwater, surface water, and even seawater by estimated total mass.
Reconstitution of “background” PFAS
This study adopts a working definition of background PFAS. That is, concentrations observed in environmental media with no known direct release, even though the compound itself is of anthropogenic origin. This distinction is important. While PFAS at a contaminated site may originate from point sources such as AFFF use or manufacturing emissions, background PFAS reflects diffuse contributions such as:
Long-distance atmospheric transport and deposition Wastewater discharge and biosolids utilization Septic tank discharges Landfill leachate Consumer product deterioration
Detection of PFAS in remote soils, surrounding groundwater, and precipitation complicates site delimitation, especially when regulatory thresholds are in the low ng/L range. As the authors note, the ubiquity of low levels of PFAS makes it difficult to distinguish between plume margins and regional ambient concentrations.
Empirical mass inventory across media
Rather than relying solely on production and emissions estimates, this 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 the total mass, the authors did the following:
Selected central tendency (median of medians) concentrations by medium. Estimated total amount of media. Apply correction factors based on detection frequency and geographic relevance (e.g. limit groundwater to “modern” water) 50-75 years old).
This hybrid experimental and mass balance approach provides a particularly useful global PFOS mass distribution.
PFOS mass by medium (global background estimate)
Soil: 1.3 × 10⁷ kg Groundwater: 1.1 × 10⁶ kg Marine surface water: 7.5 × 10⁵ kg Freshwater surface water: 2.7 × 10⁵ kg Precipitation and atmosphere: Order of magnitude lower Biosolids and wastewater (on an annual flux basis): Relatively low but recurring
The pattern is similar in the United States, again dominated by soil, with approximately 2.8 × 10⁶ kg of PFOS estimated in surface soil.
This discovery challenges the long-held assumption that the ocean is the primary environmental reservoir for traditional PFAS. Previous studies often excluded soil due to limited data. This latest analysis shows that soil is a major material reservoir, not just a temporary storage compartment.
Why is soil dominant?
There are several factors that explain soil heaviness.
Strong adsorption of long-chain PFAS (PFOS, PFNA, etc.) onto organic and mineral surfaces. Accumulation due to decades of atmospheric deposition. Keep them shallowly spaced (assume conservatively the top 30 cm). The movement speed is slow compared to advective transport of surface water.
In particular, the estimated soil mass is consistent with previous emissions-based inventories when depth assumptions are taken into account. Previous estimates of global soil PFOS mass (10 cm depth) are about an order of magnitude lower, consistent with the deeper (30 cm) assumption of the current study. This highlights that soil is a medium- to long-term PFAS reservoir, especially for traditional perfluoroalkyl sulfonates and long-chain carboxylates.
Precipitation: Limited direct impact on groundwater
Despite the widespread detection of PFAS in rainfall, the study’s models indicate that recent precipitation alone is unlikely to cause groundwater exceedances under the most likely conditions.
The authors simulated vadose zone transport using the PFAS-LEACH model using US median sediment concentrations (PFOS 0.29 ng/L, PFOA 0.33 ng/L, PFNA 0.10 ng/L). Under baseline assumptions, predicted groundwater concentrations in receiving wells remained below drinking water standards for more than 200 years.
Recharge further limits direct loads. Approximately 21% of rainfall in the United States goes into groundwater recharge. Even when scaled nationally, the annual PFOS mass contribution to groundwater from precipitation is only 5-14% of the estimated background groundwater mass. This finding has regulatory implications. Precipitation is currently not the primary driver of PFAS exceedances in groundwater at background levels.
From soil to groundwater: the more important pathway
In contrast, when representative background soil concentrations (PFOS 700 ng/kg, PFOA 581 ng/kg, PFNA 244 ng/kg) were modeled, the results were more nuanced.
Baseline simulations incorporating dilution attenuation factors predicted groundwater concentrations in downgradient receptors to be below the standard. However, a sensitivity analysis of decreasing retention parameters and increasing infiltration rates (the “upper” scenario) resulted in predicted exceedances of PFOS and PFOA.
This shows that:
Background soil concentrations alone can cause groundwater levels to range from 0.1 to 10 ng/L. Under conservative assumptions (decreased damping, increased recharge), excesses can occur. Soil retention processes strongly control the timing and magnitude of peaks.
For regulatory agencies 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 areas and similar to or slightly lower than groundwater concentrations.
A simple mixture calculation reveals the following:
Precipitation versus surface water dilution factor: 1.4 to 2.5. Groundwater discharge dilution factor: 1.6 to 3.3.
These values suggest a modest contribution from precipitation and groundwater only. In contrast, wastewater runoff typically exhibits higher PFAS concentrations (even in the absence of industrial inputs) and is therefore more likely to contribute to PFAS levels in surrounding surface waters.
Therefore, consistent with national drinking water intake analyses, wastewater return flow is likely to be an important driver of background PFAS in surface waters.
Strategic implications for site management
This finding has several high-level implications for environmental professionals.
1. Soil background must be quantified during site demarcation
Surface soils represent the largest reservoir of PFAS mass. Without establishing a surrounding soil baseline, you risk overestimating site-related impacts.
2. Atmospheric deposition likely accounts for widespread soil loading
The geographic spread of soil PFAS suggests long-range atmospheric transport rather than local direct emissions.
3. Background soils can pose groundwater risks
Even in the absence of site releases, PFAS in background soils can result in groundwater concentrations near regulatory thresholds under certain hydrogeological conditions.
4. Surface water background reflects integrated watershed inputs
Wastewater discharges, groundwater baseflow, and runoff collectively form the PFAS signature of surface waters.
5. Oceans remain major sinks, but not necessarily major reservoirs
Although the ocean is an important endpoint, the latest empirical data now highlights soil as a larger reservoir of legacy PFAS mass.
frontiers of research
The study also identifies knowledge gaps related to academic research.
Historical and current concentrations of PFAS in precipitation. Depth distribution of PFAS in soil greater than 30 cm. Sediment inventory of freshwater and marine systems. Colloid-facilitated transport and the effects of extreme rainfall. The role of precursors and transformation products in mass accounting.
Future studies integrating sediment cores, deep-sea sampling, and long-term vadose zone monitoring will further refine these inventories.
Paradigm Shift: Legacy Reservoir Management
The key insight from this comprehensive inventory is not just that PFAS are ubiquitous, but that their environmental legacy is disproportionately accumulated in soils. This reservoir is diffuse, geographically widespread, and largely independent of any identifiable point source.
As PFAS production declines and regulatory controls tighten, soils are likely to remain a major upstream source for decades due to slow leaching and gradual redistribution. Therefore, managing PFAS risks is no longer just about shutting down the plume. It’s about understanding and contextualizing the background mass.
This research provides an important recalibration for environmental professionals working to optimize site closures, risk assessments, and remediation costs. Background PFAS are system-defining, not limiting. And soil is at the center of that system.
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