Numerical Experiments on Flow and Transport of Various Surfactant Solutions in the Vadose Zone

ABSTRACT: Surfactants exist in the subsurface either naturally or as a result of anthropogenic activities related to agriculture or soil/groundwater remediation. With climate change, greywater containing large amounts of surfactants is increasingly being used in the semi-arid and arid regions. Surfactants affect soil capillarity through surface tension and contact angle changes and thus alter flow and transport characteristics of the vadose zone. The research on how flow is affected by the surfactants is very limited. Most of the conceptual and numerical models assume that flow is independent of the solute concentration, which does not provide accurate representation of the flow and transport of surfactant solutions in the unsaturated zone. This research uses a modified version of unsaturated flow and transport code Hydrus 2D, which couples flow and transport by implementing concentration dependent capillarity effects. Infiltration characteristics and flow and transport of two different surfactant solutions in the vadose zone is simulated in this research. The flow and transport of surfactants is compared with flow of water and associated transport of a conservative solute. In both cases, surfactant solutions led to notable differences in comparison to pure water and conservative solute. The differences were result of surfactant induced pressure gradients and hysteretic behavior under monotonic flow conditions. Additionally, the results also indicate that the flow and transport of surfactants is not only dependent on concentration dependent capillarity effects but also on different sorption characteristics of the surfactants. The current research will help in enhancing our understanding of the flow, transport and fate of the surfactants in the vadose zone and their possible link to future environmental problems.