Surfactant effects on unsaturated flow in porous media with hysteresis: horizontal column experiments and numerical modeling

Many organic compounds depress the surface tension of water relative to their aqueous concentration. These surface-active organic solutes have been shown to cause water flow in unsaturated porous media. Flow in these systems typically occurs from contaminated (high concentration) regions toward cleaner (lower concentration) regions. That flow is characterized by significant drainage and rewetting associated with the advance of the solute front. In the literature, modeling of unsaturated flow and transport in systems with solute concentration-dependent surface tensions has been limited to simulations without hysteresis in the hydraulic functions. However, experimental evidence is also presented which shows that hysteresis is a factor. We modified the hysteretic unsaturated flow and transport numerical model HYDRUS 5.0 to include concentration-dependent effects of a mobile organic solute. The moisture content-pressure head and unsaturated hydraulic conductivity functions were scaled for concentration-dependent surface tension and viscosity, respectively. The modified model successfully simulated data from surfactant-induced flow in 1-D horizontal column experiments. The effects of hysteresis were shown to be important to accurately simulate flow in these systems. For example, at final steady state, hysteretic simulations predicted uniform concentration and pressure profiles, but moisture contents that varied with distance. The final simulated moisture contents ranged from 0.12 to 0.25 along the column. In contrast, a non-hysteretic simulation would predict uniform distributions of concentration, pressure and moisture content. Dispersivity also had an effect on flow simulations. Lower dispersivity values caused sharper surfactant concentration gradients, which resulted in larger capillary pressure gradients and higher fluxes near the solute front. The modeling approach used here is expected to be applicable to many organic compounds of environmental interest that depress surface tension and thereby are capable of inducing unsaturated flow in porous media.