PFAS model evaluation
PFAS model performance — and what it does not yet cover
What our SWAT+ and MODFLOW 6 PFAS models reproduce, how we score them, what failed, and the audit we have not earned the right to publish. Every figure here is from the Rogue River manuscript and the Wurtsmith study and every limit is stated in the same breath as the result it limits.
Figures are frozen artifacts from the Rogue River manuscript, read from source rather than retyped from a summary. That manuscript has not been accepted for publication. This is a case-study performance report; the portfolio-scale audit that exists for drainage area does not exist for PFAS.
- 846 wells → 73 grid cells
- A case series, not a population audit
A performance page that reports only good numbers gets read as marketing, and then the good numbers get discounted along with everything else. So this page is organised the other way round: each result appears next to the thing it cannot tell you, and the section that matters most is the one describing the audit we cannot yet run.
The short version. The flow model's head fit is not shown: it ran on one uniform recharge rate, not on recharge from the SWAT+ model. The PFAS comparison itself rests on far less independent evidence than its raw observation count suggests, and we report the denominator that makes that visible. The site study is not calibrated to PFOS magnitude, on purpose. And what we have is two studies, not a portfolio audit — which is a different and much weaker kind of claim than the one our drainage-area page makes.
FAQ
How well do the SWATGenX PFAS models reproduce measured PFAS?
The comparison is made in log space, cell by cell, and at that level the model shows no skill: re-pairing the simulated values at random does as well or better in about one in five of 10,000 permutations (19.7 %), and a constant at the observed mean scores lower error. Most of the agreement is the prescribed background, so a single headline accuracy figure would present the background as performance. The groundwater PFOS comparison rests on 846 wells, and the 477 with a detection aggregate to 73 model grid cells — 8 prescribed and 65 predicted — and 73 is the honest denominator for how much independent evidence the comparison carries. The simulated groundwater PFOS spans the observed range above the 10 ng/L background. The plume map is not validated. The paper's attribution comes from the river's PFAS composition, which shows a plume-type contribution in the lower river without a quantitative share, and does not use the transport model.
Have all SWATGenX PFAS models been audited?
No, and the distinction matters. Our drainage-area audit checks every generated model against an independent national reference at portfolio scale. There is no equivalent reference dataset for PFAS to audit against. There is one PFAS study on the Rogue River, a manuscript that has not been accepted for publication, and one site study at Wurtsmith, accepted for publication in the Journal of Contaminant Hydrology and not calibrated to PFOS magnitude. That is a case series, not a population audit, and this page reports it as one.
Why is the Wurtsmith study not calibrated to PFOS magnitude?
Because the study asks what a model with a prescribed source predicts, not how closely a fitted one can match. The source concentration is inverted from measured source soil (1,738 ng/g PFOS) through an assumed partition coefficient, and no parameter was adjusted to improve the agreement with measured PFOS, so the comparison with wells tests the plume's extent and depth, not its fit.
What can the PFAS transport model not do?
PFOS only. The two models run separately: no water and no PFAS pass between them, and MODFLOW 6 alone carries PFOS from a prescribed source, on the calibrated steady flow field, through the aquifer to the streambed. The paper's attribution of PFAS in the lower river comes from the river's PFAS composition and does not use the transport model. Transient transport runs on a steady flow field, and the source cap is a numerical bound, not a measured ceiling, and it sets most of the release. SWAT+ with MODFLOW builds can also fail on solver convergence where the SWAT+ build alone succeeds — we treat that as a failed delivery rather than shipping an unvalidated result.
Next steps
Last updated 2026-08-31. Figures are frozen artifacts from the Rogue River manuscript, which has not been accepted for publication.