Wurtsmith AFB: from contaminated soil to aquifer to stream

The demonstration at a glance

All results are from an uncalibrated coupled model — concentrations agree with observations within a factor of ~4 without any parameter fitting. A manuscript describing the method is in preparation; until it completes peer review, treat this page as a research demonstration, not a peer-reviewed result.

The enigma the model had to reproduce

Monitoring at legacy AFFF sites often finds a puzzle: the shallow water table is nearly clean, while the aquifer below carries a mature contaminant plume. If contamination came from the surface, how did it get underneath clean water?

The answer is time. Decades of recharge slowly carried PFAS from fire-training-area soils down through the sandy vadose zone; the mass that reached groundwater kept sinking and migrating down-gradient while the source at the surface was excavated and controlled (~33 kg of PFOS removed at one fire-training area). By 2024, the plume core sits deep (~60 ft), the shallow water table has largely flushed — and a model that carries the full 1971–2024 history through soil, vadose zone, and aquifer reproduces exactly that vertical structure. A model initialized from today's surface alone cannot.

Why the vadose zone decides the physics

Wurtsmith's water table sits ~3.5 m below the surface in sand — PFAS reaches groundwater in years, so soil, aquifer, and stream are dynamically connected within the simulation window, and explicit unsaturated-zone transport (MODFLOW 6 UZF/UZT driven by daily SWAT+ percolation) is the right physics. Contrast a site with a 30 m vadose zone, where retardation-scaled travel times stretch to centuries and the soil signal is effectively decoupled from today's stream. The thickness of the unsaturated zone — not model sophistication — determines when the explicit pathway is required.

Built entirely from public data

Every input is public: the watershed model comes from the same national pipeline as any SWATGenX model; the subsurface comes from the national groundwater lithology inventory (1,026 wells in this single HUC12); the source term comes from measured soil concentrations of the kind collected in the national soil PFAS inventory; and the groundwater validation dataset was extracted from public EGLE investigation reports. The coupling method is described on the SWAT+MODFLOW-6 page.