Surfactant-induced unsaturated flow: Instrumented horizontal flow experiment and hysteretic modeling
Organic compounds with surface-active properties reduce the surface tension of water and can change soil water contact angles depending on their aqueous concentration. The presence of such compounds in the unsaturated zone affects soil moisture characteristics and unsaturated hydraulic conductivity relations, and can cause flow as a result of induced soil water pressure gradients. Some studies have used horizontal column experiments to measure the surfactant-driven movement of water in unsaturated media; however, they have all used destructive sampling methods to determine water contents and did not measure soil water pressures. We attempted to gain better insight into this unsaturated soil water flow process through continual monitoring of water contents with time domain reflectometry and soil water pressures with pressure transducer equipped tensiometers. One half of the horizontal one-dimensional flow cell was prewetted (but unsaturated) with water while the other half contained the same fluid content of 7% butanol solution. The temporal and spatial water content and soil water pressure information improved our understanding of the surfactant-induced flow, including perturbations associated with solute gradients. Backflow in the flow cell was observed at later times as the water-content-induced component of the hydraulic gradients became more important. Hysteretic numerical simulations of the one-dimensional horizontal flow cell using a modified version of HYDRUS 5.0 including coupled flow and transport through concentration-dependent surface tensions were performed for the case in which butanol is the surface-active compound. The numerical simulations, which used independently measured flow and transport parameters, provided a good fit to the experimental data and provided further insight into the induced flow behaviors.