Videomicroscopy Study of Vertical Flow of Organic Liquids in Wet and Dry Heterogeneous Porous Media

Videomicroscopy was used with a novel bead-pack micromodel to observe directly movement and distribution of toluene in the unsaturated zone during its initial entry, subsequent water infiltration and gravity drainage, and externally imposed fluctuations in the water table. In a partially water-saturated medium gravity drainage of toluene to the water table occurred as thin layers which flowed along continuous air-water interfaces, leaving a very low residual saturation. Experiments with presaturated toluene and water and with n-butanol and water showed that such drainage was faster and more complete for liquids that spread spontaneously on water, i.e., that had zero contact angle. However, whatever the spreading properties, drainage in such layers did not occur at all unless the gravitational driving force for downward flow exceeded the vertical capillary pressure gradient opposing flow. Toluene trapped in zones of low permeability or hydraulic conductivity during the first infiltration of water following toluene injection was not displaced during subsequent water infiltration and gravity drainage cycles. Fluctuations in the water table redistributed the toluene, in some cases trapping mobile toluene while in others mobilizing trapped toluene. The implications of these results for contamination and cleanup of ground water aquifers are discussed.