Viscous Microemulsions of Aqueous Film-Forming Foam (AFFF) and Jet Fuel A Inhibit Infiltration and Subsurface Transport

Abstract

Concentrations of per- and polyfluoroalkyl substance (PFAS) remain orders of magnitude greater than health advisory levels in groundwater in “source” zones where military firefighters repeatedly applied aqueous film-forming foams (AFFFs) to fuel-based fires. AFFFs and fuels, such as Jet A, have the potential to form separate, thermodynamically stable microemulsions that have physical properties very different from those of either of the original fluids. Phase behavior studies with AFFF and Jet A were conducted to determine the Winsor type of microemulsion that forms as a function of scale and mixing energy. At all salinities and water:oil ratios tested, five AFFFs and one fluorine-free AFFF formed viscous Winsor type II microemulsions, where surfactants migrated into the jet fuel. One-dimensional column experiments were also conducted to determine the impact of microemulsion formation on the flow properties of AFFF and jet fuel in porous media. We found evidence that viscous microemulsions formed in constant-flow and constant-pressure column experiments, indicating that infiltration into porous media provided adequate mixing energy for microemulsion formation. Formation of viscous Winsor type II microemulsions in which AFFF and Jet A comingled in the subsurface has likely contributed to long-term retention of PFAS in source zone soils.