Experimental study of LNAPL migration in the vicinity of a steep groundwater table

Migration of Light Non-Aqueous Phase Liquids (LNAPLs) in porous soils was studied experimentally, using one-dimensional and two-dimensional laboratory tests. The movement of crude oil under constant pressure was observed in vertical and horizontal transparent columns filled with sand. It was found that in both cases the average oil saturation does not change during the oil propagation. Its value depends on the initial water saturation in sand and the mobility of air in front of the oil front. The hydraulic conductivity to oil was found to be constant along the vertical column, while it decreased exponentially when oil was spreading along the horizontal column. Two-dimensional experiments were carried out in a sand tank with simultaneous water flow and the oil migration. Oil leaked into the sand from a thin feeder above the water table with constant oil level. The experiments were conducted with three types of LNAPL: crude oil, vegetable oil, and engine oil. Evolution of the oil mound was recorded through the transparent front wall of the sand tank, while the water table was monitored using a series of piezometers. Presence of the water table had a major effect on the oil mound shape and size. Downwards movement of the oil front slowed down around the top of the capillary fringe and completely stopped near the water table. The depth of penetration into the capillary fringe depended on the oil viscosity and the pressure at the oil source. Lateral spreading was more pronounced in the direction of the water movement.