Watershed-to-site PFAS fate & transport modeling

From a HUC12 watershed down to a 30 m site model of the plume itself — one automated coupled SWAT+/MODFLOW 6 pipeline, explicit vadose-zone transport, and an interactive 3D result. Demonstrated on the Wurtsmith AFB AFFF legacy.

Pick a point

anywhere in CONUS

→

Watershed model

HUC12 · SWAT+/MODFLOW 6 · 250 m

→

Site model

HUC14 · nested · 30 m

→

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.

The pipeline is built on the first direct coupling of SWAT+ with MODFLOW 6 — SWATGenX is the only platform that ships it, 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.

Where — three end-members of the vadose spectrum

The unsaturated-zone thickness sets whether land-applied PFAS reaches groundwater within a management horizon at all. Wurtsmith (3.5 m sandy) is the shallow end-member where the plume arrives in years; the same framework spans to intermediate and deep sites.

The three modeled AFFF sites across the vadose-zone spectrum — Three modeled AFFF sites spanning the vadose spectrum — Wurtsmith (MI, shallow sand), Gabreski (NY, intermediate), Rogue (MI, deep). Basemap © Esri.

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 coupled model presented as a research demonstration; manuscript in preparation. Source term is measured fire-training-area soil PFAS, not a fitted flux. Groundwater observations extracted from public agency reports.

Model any watershed — or any site within it

The watershed-to-site workflow is the same one that builds every model on SWATGenX: pick a point, get a coupled SWAT+/MODFLOW 6 package, refine to a 30 m site model where the plume needs it.

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