The effect of vertical non-aqueous phase liquid film flow on soil vapor extractio

Surface properties of some non-aqueous phase liquids (NAPLs) suggest that they should spread spontaneously on the air-water interface. NAPLs from a pool on the water table spreading upward over water films present in the vadose zone will enhance the efficiency of soil vapor extraction by increasing the size of the zone of contact between extracted soil gas and liquid NAPL. The fluid mechanics of a uniformly thick NAPL film being drawn upwards over a uniformly thick water film on a flat solid substrate were evaluated and extended to porous media. NAPL film heights predicted for spreading NAPLs in water-wet, uniform spherical grains increased with increasing spreading coefficient, decreasing NAPL density, decreasing NAPL vapor pressure, decreasing grain radius, and increasing water film thickness, with predicted soil vapor extraction efficiency increasing by as much as a factor greater than 10 for small grain radii and thick water films.;Five experiments were carried out to investigate the behavior of NAPL films on water-wet surfaces and porous media. Spreading NAPLs were introduced (1) into porous media with varying moisture content, (2) onto water-wet inclined and horizontal flat plates, (3) into a glass box containing water-wet unsaturated sand over a small water-saturated zone, (4) onto the top of the water capillary fringe in meter-scale columns containing uniform glass beads or sand, and (5) into capillary tubes under varying conditions. NAPL film spreading was dependent on moisture content with spreading occurring on very dry and very wet glass beads but not at intermediate moisture contents. NAPLs spread on inclined and horizontal air-water interfaces, however, vigorous NAPL spreading caused the underlying water film to rupture, hindering further spreading. In the sand box experiment, the nonvolatile dye from a dyed NAPL on the saturated zone was left a centimeter or so above the initial NAPL location indicating vertical NAPL film flow. When a spreading NAPL was introduced into meter-scale columns of water-wet glass beads or sand, films large enough to significantly impact soil vapor extraction were not observed. The method of NAPL introduction may have caused water film rupture. In the capillary tubes, higher films were observed when hexane was placed on the air-water interface and water drained from the tube than when hexane was placed on a static air water interface.;Films large enough to impact soil vapor extraction were not observed in these experiments. The moisture content and spreading coefficient strongly impact NAPL film spreading in porous media. Too little moisture or too vigorous initial spreading may inhibit film spreading.