The geometry of nonwetting liquids in the unsaturated zone

In hydrophilic aquifer material, gasoline, immiscible solvents, and similar nonaqueous phase liquids (NAPL) form a separate nonwetting phase. If such a nonwetting liquid is spilled onto soil, it will percolate into the soil and migrate downward and laterally under the influence of gravity and capillary forces. As the nonwetting liquid drains from a portion of soil, some of this liquid is retained by capillary forces. Although current models predict that the retained nonwetting liquid is immobile a growing body of evidence indicates that it may be displaced by percolating water. A theory is developed to describe the geometry of nonwetting or nonaqueous phase liquids in moist unsaturated media. This theory predicts that liquids with positive values of the classical two-dimensional spreading coefficient form pendular rings. Liquids with negative values form pendular rings in regions of high-water tension or in regions of relatively high concentration of the nonwetting fluid, but they form insular lenses in regions of low-water tension or regions of low concentration of the nonwetting fluid. A three-dimensional spreading coefficient was developed to predict this behavior. Infiltrating water can cause transitions by breaking the pendular rings of nonwetting liquids in the unsaturated zone, and forming larger blobs as these rings coalesce. These blobs may drain downward due to gravity. These conditions may contribute to the mobility of nonwetting liquids due to percolation of water through the unsaturated zone