Surfactant-Induced Flow in Unsaturated Porous Media: Implications for Air-Water Interfacial Area Determination

Air-water interfacial area (A_I) in porous media is an important factor governing equilibrium contaminant retention, as well as the kinetics of interphase mass transfer. Interfacial-partitioning tracer (IPT) tests are a common technique for measuring A_I at a given moisture saturation (S_W), where A_I is calculated based on the ratio of arrival times of a surfactant and a non-reactive tracer. At surfactant concentrations often used, the aqueous surface tension of the interfacial tracer solution is ~30% lower than that of the resident porewater in the system, creating transient surface tension gradients during the IPT measurement. Because surface tension gradients create capillary pressure gradients, surfactant-induced unsaturated flow may occur during IPT tests, a process that would violate fundamental assumptions of constant S_W, of steady-state flow, and of nonreactive and surfactant tracers experiencing the same transport conditions. To examine the occurrence and magnitude of surfactant-induced flow, we conducted IPT tests for unsaturated systems at ~84% initial S_W using surfactant input concentrations that bracket concentrations commonly used. Despite constant boundary conditions (constant inlet flux and outlet pressure), the introduction of the surfactant solution induced considerable transience in column effluent flowrate and S_W. Real-time system mass measurements revealed drainage of 20-40% S_W, with the amount of drainage and the maximum rate of drainage proportional to the influent surfactant concentration, as would be expected. Because A_I is inversely related to S_W, the use of higher surfactant concentrations should yield larger A_I estimates. Measured A_I values, however, showed no clear relationship to surfactant concentration or the time-averaged S_W of the system. These findings cast doubt on the reliability of IPT for A_I determination.