Geometry and position of light nonaqueous-phase liquid lenses in water-wetted porous media

Predicting the movement of LNAPLʹs (light nonaqueous-phase liquids) in the subsurface environment is critical for the design of effective remediatory action. The objective of this study was to develop a method for predicting the shape and extent of LNAPL lenses in the capillary fringe of the vadose zone. Two-dimensional experiments were performed in a glass chamber (50 cm × 60 cm × 0.95 cm) using four Miller-similar silica sands (12/20, 20/30, 30/40 and 40/50 sieve sizes) and two LNAPLʹs (Soltrol® 220 and Duoprime® 55 mineral oil). LNAPLʹs were released in water-wetted sands to simulate a point-source discharge above a water table. Observation of light transmission was used to delineate the changing LNAPL lens boundary during infiltration until equilibrium was established. At equilibrium, no zone above the capillary fringe remained at a NAPL saturation higher than the residual saturation. A previously published model for predicting vertical lens dimensions was tested and good agreement was found between measured and predicted lens thicknesses when the time-dependent nature of LNAPL—water interfacial tensions was considered. Less agreement between measured and predicted lens thicknesses was found when model equations were modified to a fully explicit, predictive form. Observed spatial variability in the emplacement of LNAPLʹs in the capillary fringe, compounded by the strong time dependence of a key variable, limits the use of the model as a predictive tool, but provides important insight into the low precision in prediction which is attainable even if a more complete model was developed. The results provide a means to better understand LNAPL behavior in the subsurface environment.