Aqueous phase permeability in the region of discontinuous NAPL

Abstract

In saturated soil, a nonaqueous phase liquid (NAPL) can become a discontinuous immobile phase when capillary forces are stronger than the opposing viscous and buoyancy forces. In regions where this occurs, the aqueous phase permeability is less than the water saturated permeability of the surrounding medium, which may cause treatment to bypass the targeted contaminated region. As an in situ treatment is applied, the saturation of residual NAPL (Srn) is reduced, while the aqueous phase saturation (Sa ) and the relative permeability of the aqueous phase (kra) increases. Hence, in situ treatment of NAPL must account for various levels of heterogeneity. In this study, column experiments measure Kra‐Sa during the application of a surfactant that enhances the dissolution of NAPL Into the aqueous phase. The results indicated that the surfactant initially decreases kra followed by a steady increase as Srn is reduced. In multiphase models, it is common practice to use empirical coefficients measured from pressure‐saturation (p‐S) data to predict kra‐Sa. This approach assumes that Kra‐Sa relationship in the region of discontinuous NAPL will follow the trend estimated in the continuous two‐phase flow region. The results document that this practice may not adequately predict Kra‐Sa at Sa> 1 ‐ Srn.