Formation of Residual and Entrapped Dense Non-aqueous Phase Liquid.

The presence of residual non-aqueous liquid phases (NAPL) in the vadose zone has been demonstrated by many researchers, but the conditions under which residual NAPL is formed are poorly understood. Some researchers assumed that residual NAPL formed during drainage. In contrast, others assumed residual NAPL is formed by water entrapment during imbibition. We use the term residual saturation to describe different phenomenon. In a water-wet system, residual NAPL is NAPL not entrapped by water, but does not drain from the pore space when drainage is unrestricted.  The residual NAPL is assumed to occupy the smallest pores.  Any NAPL occluded by water in a water-wet system, is entrapped NAPL and both residual and entrapped NAPL are immobile. The surface tension of the NAPL and water also determines the arrangement of NAPL in the porous matrix. A spreading NAPL exists as a thin film and non-spreading NAPLs form a lens. Residual NAPL can be continuous or discontinuous in the pore space, but entrapped NAPL is always discontinuous. While models of three-phase constitutive properties (the relationships between the relative saturation of the three phases, water, NAPL and air with their respective pressures) have been proposed, the different assumptions about the mechanism of formation produce different mathematical formulations. One reason that this difference remains unresolved is that an experimental study of NAPL formation in a water-wet system where both water and NAPL heads are measured during imbibition and drainage has not been reported.  Herein, we report results of a column experiment where both NAPL and water heads were determined.