Contaminant-induced flow effects in variably saturated porous media

Dissolved organic contaminants that decrease the surface tension of water (surfactants) can have an effect on unsaturated flow through porous media due to the dependence of capillary pressure on surface tension. One and two-dimensional (1D, 2D) laboratory experiments and numerical simulations were conducted to study surfactant-induced unsaturated flow. The 1D experiments investigated differences in surfactant-induced flow as a function of contaminant mobility. The flow in a system contaminated with a high solubility, mobile surfactant, butanol, was much different than in a system contaminated with a sparingly soluble, relatively immobile surfactant, myristyl alcohol (MA). Because surface tension depression caused by MA was confined to the original source zone, the MA system was modeled using a standard unsaturated flow model (HYDRUS-1D) by assigning separate sets of hydraulic functions to the initially clean and source zones. To simulate the butanol system, HYDRUS-1D was modified to incorporate surfactant concentration-dependent changes to the moisture content-pressure head and unsaturated hydraulic conductivity functions. Following the 1D study, a two-dimensional flow cell (2.4 x 1.5 x 0.1 m) was used to investigate the infiltration of a surfactant contaminant plume from a point source on the soil surface, through the vadose zone, and toward a shallow aquifer. Above the top of the capillary fringe the advance of the surfactant solution caused a drainage front that radiated from the point source. Upon reaching the capillary fringe, the drainage front caused a localized depression of the capillary fringe and eventually a new capillary fringe height was established. Horizontal transport of surfactant in the depressed capillary fringe caused the propagation of a wedge-shaped drainage front in the downgradient direction. The numerical model HYDRUS-2D was modified to account for surfactant concentration-dependent effects on the unsaturated hydraulic functions and was successfully used to simulate the surfactant infiltration experiment. The extensive propagation of the drying front and the effect of vadose zone drainage on contaminant breakthrough time demonstrate the potential importance of considering surface tension effects on unsaturated flow and transport in systems containing surface-active organic contaminants or in systems where surfactants are used for remediation of the vadose zone or unconfined aquifers.