NUMERICAL MODELING TO ASSESS NON-STEADY FLOW BEHAVIOR DURING LABORATORY MEASUREMENT OF AIR-WATER INTERFACIAL AREA

Interfacial-partitioning tracer (IPT) tests are used in the laboratory to determine air-water interfacial area (A_I), an important parameter that influences the transport of solutes which partition to the interface in unsaturated porous media. In IPT tests, Ai is determined based on the relative travel times of interfacial and non-reactive tracers during column experiments. The interfacial tracers are typically surfactants. Though surfactants have been shown to lower the surface tension of water, resulting in capillary pressure gradients which can induce flow in unsaturated media, little attention has been paid to this phenomenon with respect to IPT. Our recent experimental work has shown that surfactant-induced flow can occur during conventional IPT tests, resulting in unsteady flow and changes in moisture content---violations of the assumptions underlying IPT. In the present work, we conducted numerical modeling of IPT to examine the surfactant-induced flow behavior in more detail. We looked at the effect of initial moisture content and partitioning tracer concentration. In general, more water is displaced in systems initially at higher moisture content. All else being equal, systems with higher surfactant concentrations displace more water because they result in greater surface tension differences and higher capillary pressure gradients. Because surfactant-induced flow has been shown to cause flow perturbations in conventional IPT tests, we also used numerical modeling to evaluate whether modifications to the experimental system could reduce the magnitude of the induced flow. It appears that there are modifications that could reduce, but not eliminate the impact of the induced flow on the IPT measurement technique.