14. Transport of Immiscible Fluids

The previous Chapters described the transport of miscible compounds in aquifers. Here, mechanisms of immiscible compound (also called non-aqueous phase liquid, NAPL) transport and propagation is presented. To do so, relevant properties of a multi-phase system (i.e., wettability, interfacial tension and capillary pressure, effective and relative permeability, drainage and imbibition) are illustrated. Migration of NAPLs in the subsurface is then qualitatively described, separately discussing the behavior of light- and dense-NAPLs. Light-NAPLs spilled at the surface that reach the saturated zone tend to accumulate and float on the water table and spread horizontally, progressively releasing their soluble fraction into the groundwater. Dense-NAPLs are denser than water, so if they reach the saturated zone they tend to displace groundwater from the pores, penetrating vertically in the aquifer. Depending on the volume of the release, Dense-NAPL contamination may affect the entire saturated thickness, and the compound may move along base of the aquifer. As for light-NAPL, also dense NAPLs present as a pure phase act as a continuous source of contaminant by releasing their soluble fraction into the groundwater. Finally, a quantitative approach to immiscible compound transport is provided, allowing for the estimation of mass distribution in the different phases in the saturated and unsaturated zones.