Surfactant-Induced Flow Phenomena in the Vadose Zone: A Review of Data and Numerical Modeling

Surfactants may occur naturally in the subsurface or may be introduced anthropogenically. Because of their ability to reduce surface tension and modify the solid-liquid contact angle, surfactants affect capillarity in unsaturated porous media. We review the current state of knowledge regarding surfactant effects on unsaturated flow and transport in water-wetted porous media. Surfactant effects on moisture retention and unsaturated hydraulic conductivity are reviewed, as well as experimental evidence of surfactant effects on unsaturated flow. Surfactants can cause significant flow perturbations that do not occur in constant surface tension systems. Noteworthy effects include surfactant-induced unsaturated flow that arises from surfactant concentration-dependent surface tension gradients, as well as capillary fringe depression proportional to the surfactant-induced relative reduction in surface tension. Most of the available data is from laboratory experiments; consequently, questions still remain about the relative importance of surfactant-induced effects on field-scale flow and transport processes. Numerical models that account for surfactant effects on flow provide useful tools for assessing the importance of these effects and should prove useful for designing surfactant-based remedial schemes. We review simulations of unsaturated flow and transport in systems containing surfactants, as well as models that may be useful for conducting such simulations. Comparisons of simulated and experimental data indicate that hysteresis and dispersivity effects on simulation results can be important considerations. Future research directions should include the collection of additional field and laboratory-scale data and expanded modeling efforts.