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Watershed-to-site PFAS fate & transport modeling

Fifty years of an AFFF legacy at Wurtsmith AFB, reconstructed from a measured soil source term rather than a fitted flux — a HUC12 watershed model handing its boundary to a 30 m site model, explicit vadose-zone transport, and a plume you can orbit.

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Pick a point
anywhere in CONUS
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Watershed model
HUC12 · SWAT+/MODFLOW 6 · 250 m
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Site model
HUC14 · nested · 30 m
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Interactive 3D
plume, wells, layers

The plume, in 3D — live

This is the actual model output, interactive: orbit the aquifer, scrub five decades of the plume evolving, drop the concentration threshold to the regulatory limit, slice it top-to-bottom, and hover any well, source, layer, or plume cell for its numbers. Nothing is a screenshot.

Live · SGX3D interactive viewer— drag to orbit · press ▶ to animate · hover for values

Best on a desktop browser. Built with SGX3D — SWATGenX's interactive 3D viewer, which turns any built coupled model into a self-contained scene.

One pipeline, two scales

Plume-scale questions don't need the whole watershed, and watershed-scale hydrology can't resolve a plume. So we run both — the same automated pipeline, nested. The watershed model resolves regional flow and hands the site model its boundary conditions; the site model resolves the plume at 30 m. Every observation is validated at the scale that resolves it.

This is one nest, chosen for one site. Where to place the cut in general — at a gauge, a HUC12 outlet, or a single catchment — what can cross it, and the cases where nesting is physically indefensible are set out on nested watershed modeling; the site-scale build itself is described on site-scale (HUC14) modeling.

The pipeline is built on a two-way coupled SWAT+ / MODFLOW 6 simulation, and PFAS transport is native to every stage: land and soil in the SWAT+ engine, vadose zone via UZF/UZT, aquifer in MODFLOW 6 GWT, and streams in-engine. No WASP, RT3D, or third-party water-quality model is used anywhere in this workflow.

Watershed scale
HUC12 · 250 m
The full 732 km² Van Etten Lake–Pine River watershed: SWAT+ land-surface hydrology + a 3-layer MODFLOW 6 aquifer, kriged from 1,026 state well records. Resolves regional flow and boundary fluxes.
Site scale
HUC14 · 30 m
A 77 km² observation-anchored footprint — the catchments holding every monitoring well, surface-water station, and AFFF source — rebuilt at 30 m. The watershed model supplies its inflow and groundwater context.
Vadose → aquifer
explicit UZF/UZT
PFAS travels land → unsaturated zone → water table → plume → stream in one MODFLOW 6 simulation. The shallow ~3.5 m sandy vadose zone is where explicit transport, not a screening shortcut, changes the answer.

Three sites, three depths to water

How thick the unsaturated zone is decides whether land-applied PFAS reaches groundwater within a management horizon at all — so it is the variable the three modeled sites were chosen to span. Wurtsmith sits at the shallow end, 3.5 m of sand, where the plume arrives in years rather than decades. The same framework carries to the intermediate and deep cases below without changing the method.

Four-panel study-area figure: a locator map of three modeled AFFF sites, then each site's watershed — Wurtsmith AFB in Michigan, Gabreski ANGB in New York, and the Stegman Creek site in Michigan — with PFAS source zones marked.
Three modeled AFFF sites spanning the vadose spectrum: (a) where they are, then each site’s watershed and source zones — Wurtsmith AFB, MI (3.5 m to water), Gabreski ANGB, NY (10–12 m), and Stegman Creek (Rogue), MI (source cells 0–26 m to water).

The result — plume and vertical structure

Simulated 2017 PFOS plume at Wurtsmith AFB on aerial imagery
Simulated 2017 PFOS plume on aerial imagery: three AFFF source zones (stars), the 35-well network sized by observed PFOS, the model boundary (dashed). The simulated core sits on the former flightline; wells toward Lake Huron are low in both model and data. Imagery © Esri.
North–south cross-section through the plume core showing the vertical descent
A vertical slice through the plume core: PFOS enters at the water table (blue line) and the core plunges as a deep tail — the shallow-source / deep-plume signature that distinguishes a legacy AFFF site.

By the numbers

732 → 77 km²
watershed to site
the site footprint is 10% of the watershed drainage — yet holds all 35 plume wells and 3 source areas
1971–2024
five decades simulated
AFFF first use to today, from a measured soil source term, no fitted flux
κ = 0.64
plume agreement
detect / non-detect concordance against the 35-well network, uncalibrated
30 m
site resolution
fine-grid flow field where the plume needs it, nested inside the coarse watershed model

Uncalibrated, and presented as a research demonstration rather than a regulatory product; manuscript in preparation. The source term is measured fire-training-area soil PFAS, not a flux fitted to make the plume match — which is why the agreement above is worth quoting at all. Groundwater observations were extracted from public agency reports.

Model any watershed — or any site within it

Nothing here is bespoke. The watershed-to-site workflow is the one that builds every model on SWATGenX: pick a point, get a coupled SWAT+/MODFLOW 6 package, and refine to a 30 m site model where the plume needs it.