Investigating the Influence of Surfactants on Vadose Zone Flow and Transport through Dynamic Capillary Rise Experiments

With climate change, greywater containing large amounts of surfactants is increasingly being used in the semi-arid and arid regions. However, the research on flow, transport and fate of surfactants in the vadose zone is limited. Surfactants affect soil capillarity through surface tension and contact angle changes and thus alter flow and transport characteristics of the vadose zone. Capillary rise is a phenomenon that occurs spontaneously where liquids rise into relatively dry hydrophilic porous media against friction and gravity forces. Capillarity at the pore scale depends on four factors: surface tension and the density of the liquid, the contact angle at the solid-liquid interface and the pore radius. Surfactants' concentration dependent effects on capillarity were investigated by conducting dynamic capillary rise experiments on a pre-wetted well-characterized silica sand. An analytical method, which utilizes Brooks-Corey and modified Green-Ampt model, was used to analyze the experimental results and fit a pressure potential, volumetric water content and unsaturated hydraulic conductivity at the wetting front, as well as providing an indirect estimate of the advancing contact angle. Additionally, using a drop shape analyzer, independent advancing contact angle measurements were performed for the imbibing solutions. Direct measurements and the indirect estimates of the contact angles were found to be in accordance with each other. Moreover, an independent Young-Laplace pressure potential scaling check was applied to the fitted pressure potentials by the analytical method. Fitted pressure potentials and scaled pressure potentials were also found to be in line. Results of this study provide a holistic overview of the effects of surfactants in the vadose zone.