Two‐dimensional modeling of flow and transport in the vadose zone with surfactant‐induced flow

Surface‐active solutes (surfactants) can induce capillary pressure gradients and water fluxes within the vadose zone. Common numerical models do not account for the effects of surfactant‐induced capillary pressure gradients on unsaturated flow and transport. We modified the variably saturated flow and transport model HYDRUS‐2D [Simunek et al., 1999] to incorporate the effects of surfactant concentration‐dependent changes to surface tension and viscosity on moisture retention and hydraulic conductivity, respectively. The model was tested by simulating the two‐dimensional surfactant infiltration experiment of Henry and Smith [2002]. The model successfully captured the major processes associated with the advance of a surface‐active contaminant plume through the vadose zone, including drainage of the vadose zone and depression of the capillary fringe associated with the solute front. The simulations also provided insight into unusual features of surfactant‐induced flow such as the upward flow of surfactant solution that was associated with capillary fringe drainage.