A Model for Predicting Residual NAPL in the Vadose Zone

A constitutive model for relative permeability-saturation-pressure relationships is outlined for predicting free, trapped, and residual NAPL in the vadose zone. Residual NAPL is commonly defined as that NAPL which does not drain from the pore spaces under gravity. We conceptualize residual NAPL to be NAPL that becomes immobile after filling small pore wedges or spaces and after forming thin films or lenses on water and solid surfaces. As such, the residual NAPL can be continuous and/or discontinuous in the pore spaces. Because the residual NAPL is immobile, it does not contribute to free NAPL advection, regardless whether NAPL is draining from or imbibing into pore spaces. Trapped NAPL differs from residual NAPL because it is always discontinuous and occluded by water. The mechanisms for the formation of residual and trapped NAPL are different. We model residual NAPL using similar concepts as those used to model residual water. Using the hysteretic constitutive model, the water and NAPL (free, trapped, and residual) saturations are predicted from the capillary pressures in an air-NAPL-water fluid system and the water and total-liquid saturation-path histories. Water and free-NAPL relative permeabilities are predicted from integral expressions. When all of the NAPL will be in residual form, the free-NAPL relative permeability will be zero. The model can be easily incorporated in multifluid numerical flow codes. As a preliminary test of the model, the water and NAPL saturations and relative permeabilities are predicted from simulated air-NAPL and NAPL-water capillary pressures involving a scenario where a slug of NAPL infiltrates into a sandy porous medium and drains later to its residual value. The results are shown and discussed. The model can be used in numerical codes for more accurately predicting the behavior of NAPL in the subsurface. Modeling residual NAPL in the vadose zone is important because it is a long-term source for groundwater contamination.