PFAS fate & transport
How PFAS moves — and why it stays
A field guide to the chemistry that carries PFAS through soil, the vadose zone, groundwater, and streams — the mechanisms, the governing relationships, and real numbers from our coupled SWAT+/MODFLOW 6 models.
- Freundlich sorption
“Forever chemicals” is a good headline and a poor explanation. PFAS do eventually move — through the soil you stand on, the unsaturated ground beneath it, the aquifer below that, and out into streams and lakes — but they move slowly, selectively, and differently at every step. This page walks that journey compartment by compartment. Each section explains the controlling mechanism in plain terms, states the one relationship that governs it, and grounds it in numbers from the PFAS models we build and run — labeled honestly as simulated or reported, never invented.
What SWATGenX does
SWATGenX builds a SWAT+ watershed model for any USGS stream gauge in the contiguous United States from national data, and our research joins SWAT+ with MODFLOW 6 to carry PFAS through MODFLOW 6's groundwater and stream transport (GWT and SFT); daily two-way coupling is research work. Automatic builds deliver the SWAT+ model, and a SWAT+ with MODFLOW order adds a steady-state MODFLOW 6 groundwater model; site-specific PFAS fate-and-transport assessments are delivered as a quoted service; and a national inventory of public PFAS sampling records is mapped on the Watershed Explorer.
Read more: Services · SWAT+ and MODFLOW 6 coupling
What the engine does
| Component / link | Status | Mechanism |
|---|---|---|
| What an order delivers | Delivered | SWAT+ model. A SWAT+ with MODFLOW order (MAX and Department plans, states with well-log coverage) adds a steady-state MODFLOW 6 model: layers, conductivity, water table kriged from state well logs. Delivered side by side, not coupled. |
| Stream ↔ aquifer | Engine | Daily, both directions. SWAT+ channels ↔ MODFLOW 6 SFR reaches, in one process through BMI/XMI. Water-balance fix in progress. |
| Land → aquifer (recharge) | Engine | Daily. SWAT+ soil percolation → MODFLOW 6 cells, weighted by area overlap. |
| Lakes / wetlands ↔ aquifer | Not coupled | Lakes and wetlands are routed in SWAT+ only. |
| PFAS in soil | Engine | Every soil layer: Freundlich sorption, air–water interfacial sorption, solubility cap. Losses by runoff, lateral flow, percolation and eroded sediment. |
| PFAS in streams and reservoirs | Engine | Dissolved and sorbed phases: settling, resuspension, sediment–water diffusion, burial. Reservoirs are mixed tanks. No decay. |
| PFAS in the vadose zone | Engine | MODFLOW 6 UZF/UZT, modified by us for water-content-dependent solid and air–water sorption. One column per active cell with more than 0.3 m to the water table. |
| PFAS in groundwater | Engine | MODFLOW 6 GWT. Its source term is SWAT+ soil leaching, loaded directly into the GWT cells. |
| Vadose zone → aquifer | Engine | Inside MODFLOW 6: UZF → GWF (water), UZT → GWT (mass). |
| Land → vadose zone | Not coupled | Offline only: SWAT+ percolation is written as the UZF input file. |
| Aquifer → stream (PFAS) | Engine | Daily: groundwater discharge × groundwater concentration, added to each gaining reach. |
| Compounds | Engine | Soil and streams: any number, simulated together. Groundwater: one per run. |
| Mass balance | Engine | PFAS, soil phase: closure residual written on every run (Rogue: below 0.0001 %). Water in the two-way mode: fix in progress. |
| Time step | Engine | Flow daily. Groundwater transport every 30 days by default (configurable). |
| Water-quality codes | Engine | SWAT+ and MODFLOW 6 only. No WASP, no RT3D. |
1. What PFAS are, and why they don’t go away
PFAS — per- and polyfluoroalkyl substances — are built around a chain of carbon atoms whose hydrogens have been swapped for fluorine. The carbon–fluorine bond is the strongest single bond in organic chemistry. Nothing in ordinary soil, water, or biology has the energy to break it at scale: there is no microbe that eats it, no sunlight reaction in an aquifer, no natural weathering that takes it apart. That is the whole of “forever” — not that PFAS is immobile, but that it does not degrade. Whatever is released stays PFAS and simply redistributes.
The second fact that governs everything downstream is shape. A PFAS molecule is a surfactant — a soap. One end is a fluorinated tail that repels water; the other is a charged head (a sulfonate for PFOS, a carboxylate for PFOA) that dissolves happily in it. A molecule that is half water-hating and half water-loving cannot be comfortable anywhere, so it collects at surfaces: the boundary between water and air, water and a soil grain, water and organic matter. Every retention mechanism in the rest of this page is a consequence of that split personality. It is also why PFAS makes foam.
Most of the contamination we model is AFFF — aqueous film-forming foam, the firefighting agent used from the 1960s onward at airports and military bases to smother fuel fires. Decades of fire-training exercises poured concentrated PFAS onto bare ground at the same spots, year after year. Those training areas are the point sources behind most of the groundwater plumes in the United States, including the site this page uses as its worked example: the former Wurtsmith Air Force Base in Oscoda, Michigan.
References
- Rafiei, V. et al. (2023). Watershed-scale PFAS fate and transport model for source identification and management implications. Water Research 240, 120073. doi.org/10.1016/j.watres.2023.120073
- Raschke, A. et al. (2022). Opportunities and challenges of integrated large-scale PFAS modeling: a case study for PFAS modeling at a watershed scale. Journal of Environmental Engineering 148(9), 05022005. doi.org/10.1061/(ASCE)EE.1943-7870.0002034
- Rafiei, V. (2026, under review). Vadose-zone control of PFOS delivery to groundwater: explicit unsaturated-zone transport with solid-phase and air–water interfacial sorption (Wurtsmith, Gabreski, Stegman Creek).
- Li, F. et al. (2019). Adsorption of perfluorinated acids onto soils: Kinetics, isotherms, and influencing factors. Science of the Total Environment 649, 504–514. doi.org/10.1016/j.scitotenv.2018.08.209
- Higgins, C.P. & Luthy, R.G. (2006). Sorption of perfluorinated surfactants on sediments. Environmental Science & Technology 40(23), 7251–7256. doi.org/10.1021/es061000n
- Brusseau, M.L. (2018). Assessing the potential contributions of additional retention processes to PFAS retardation in the subsurface. Science of the Total Environment 613–614, 176–185. doi.org/10.1016/j.scitotenv.2017.09.065
- Guo, B., Zeng, J. & Brusseau, M.L. (2020). A mathematical model for the release, transport, and retention of PFAS in the vadose zone. Water Resources Research 56, e2019WR026667. doi.org/10.1029/2019WR026667
- Adamson, D.T. et al. (2020). Mass-based, field-scale demonstration of PFAS retention within AFFF-associated source areas. Environmental Science & Technology 54(24), 15768–15777. doi.org/10.1021/acs.est.0c04472
- U.S. EPA (2024). PFAS National Primary Drinking Water Regulation — maximum contaminant level (MCL) 4 ng/L for PFOA and PFOS. 89 FR 32532. www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas
- Michigan EGLE / MPART. Wurtsmith (Oscoda) PFAS response: surface-water and foam monitoring, Van Etten Lake and Clark’s Marsh. Public agency reports. www.michigan.gov/pfasresponse
All model parameters and simulated site numbers on this page come from the SWATGenX coupled SWAT+/MODFLOW 6 PFAS models and their manuscripts; agency and literature values are labeled reported and cited above. The soil-sorption approach follows Rafiei et al. (2023), Water Research, a watershed-scale SWAT model of PFAS fate and transport in surface water (see also Raschke et al., 2022); the SWAT+ implementation and its coupling with MODFLOW 6 are research work.