How we model PFAS from soil to stream

PFAS released at the surface has a long way to go before it reaches a stream: through the soil, the unsaturated zone below it, and the aquifer. We model each one. In the soil, SWAT+ splits PFAS three ways, in every land unit and every layer: dissolved in the water, stuck to the soil grains, and held at the surfaces where air meets water. It leaves the soil in runoff, on eroded sediment, and in the water that drains down. Below the roots, MODFLOW 6 carries that water down through the unsaturated zone. Our transport code holds the PFAS back on both kinds of surface, recomputed at every step from how wet each cell is. A drier column holds more. At the water table, MODFLOW 6 moves it through the aquifer: carried by the flow, spreading, and sorbing as it goes. Where groundwater feeds a stream, the PFAS enters SWAT+'s channels, splits between water and sediment, settles, and is buried or stirred up again. We studied the unsaturated zone at two firefighting-foam sites and one tannery-waste site. At the shallow foam site, sorption in the unsaturated zone left far less PFOS in the deep aquifer, decades after release, than delivering the same mass straight to the water table. It is delayed, not gone: the leading edge arrives much sooner, while most of the mass is held for decades. That study is accepted for publication in the Journal of Contaminant Hydrology. The unsaturated-zone model runs on the engine's output rather than inside it, and the stream leg is not yet validated. Soil, unsaturated zone, aquifer, stream: we model PFAS in every one.