Quantification of surfactant -induced unsaturated flow in the vadose zone

Surfactant induced unsaturated flow refers to the movement of water in the unsaturated porous media, as a result of capillary pressure gradients caused by surface tension differences between surfactant-free and surfactant-contaminated regions. This thesis presents experimental and numerical studies for the quantification of surfactant-induced unsaturated flow in the vadose zone. Hysteretic soil moisture characteristics curves for a silica sand wetted with water and 7% butanol solution were measured in first section of the thesis. The applicability of using one branch hysteresis model in predicting the complete set of curves from a single measured curve for butanol was investigated.;The third section of the thesis presents the experimental results and numerical simulations for surfactant-induced unsaturated flow in a one-dimensional horizontal flow experiment. Compared to the previous research, the experimental setup used, employed continuous and precise measurement of moisture content and pressure head during the course of experiment. The continuous temporal and spatial moisture content and pressure head information provided a better understanding of the surfactant induced flow perturbation, including quantitative information regarding the capillary pressure gradients and their rate of change due to surface tension depression.;To describe the migration of surfactants in the vadose zone, it is essential to establish a description of the flow regime as well as transport model. Richards' equation is a mathematical model, which is often used to describe flow in the vadose zone. The fourth section of the thesis presents a conservative mixed formulation for the solution of the Richards equation of unsaturated flow by the finite element method. In this formulation the discharge velocity (volumetric flux) and pressure head are the primary field variables. Solution techniques for various boundary conditions including prescribed constant pressure head and constant flux are also presented in this section. The ability of the formulation to handle variably saturated domain and material heterogeneity is also established.;The fifth section of the thesis presents a numerical study of hysteretic surfactant induced unsaturated flow in two-dimensional systems. The hysteresis routine of HYDRUS 2D was modified to work with the small time steps and convergence criteria required to solve surfactant induced flow problems. The hysteretic model was used to simulate the two-dimensional surfactant infiltration and surfactant focused flow experiments reported in the literature. The comparison of hysteretic simulation results with the experimental data showed better agreement than the nonhysteretic simulation. Comparison of surfactant flow and transport with that of conservative tracer, under intermittent boundary conditions is also presented in this section. (Abstract shortened by UMI.).;Subsurface migration of organic contaminants can be strongly influenced by solution chemistry as a result of sensitivity of surface tension and contact angle to factors such as PH, concentration of surface-active solutes, and ionic strength. The second section of the thesis provides experimental evidence for surfactant induced unsaturated flow systems, where the surface tension is a function of soil water ionic strength. The experimental results indicate that additional consideration of surface tension as a function of soil water ionic strength is required in conceptual and numerical models of surfactant-induced unsaturated flow systems.