EMPLACEMENT OF NONAQUEOUS LIQUIDS IN THE VADOSE ZONE

The results of experimental and analytical investigations of the movement of lighter than water organic liquids in the vadose zone are presented. The experiments consisted of physical model tests simulating nonaqueous phase liquid (NAPL) spills in unsaturated, two-dimensional domain above the water table. The evolution of the plume was observed through the transparent side of a tank containing sand, and the contaminant front was traced at appropriate intervals. Visual observations were supplemented with pressure and saturation measurements suitable for quantitative analysis of the process. The experiments show that during infiltration the front advances at constant speed (area/time). After spreading comes to a halt, the bulk of the contaminant is contained within a pancake-shaped lens situated on top of the capillary fringe. Fluctuations of the phreatic surface result in trapping the NAPL below the water table and in spreading the contaminant over a larger area. In layered samples, the behavior is largely geometry dependent, although heterogeneities invariably result in extensive horizontal spreading. Simplified analytical relationships were developed to estimate the spreading rate of the contaminant during infiltration and the thickness of the final immobilized oil lens observed during the experiments. These relationships are based on capillary pressure considerations, which have to be included in order to model the immobilization of the plume at saturations higher than the residual values, which typically occur in vadose zone, when the NAPL encounters the capillary fringe in its downward migration.