Novel methods for measuring air–water interfacial area in unsaturated porous media

• Interfacial partitioning tracer tests (IPTT) are used to measure air–water interfacial area.
• Two novel alternative approaches for conducting IPTTs are presented.
• System monitoring during the tests revealed no measurable surfactant-induced drainage.
• The measured interfacial areas compared well to those obtained with the standard IPTT method.

Interfacial partitioning tracer tests (IPTT) are used to measure air–water interfacial area for unsaturated porous media. The standard IPTT method involves conducting tests wherein an aqueous surfactant solution is introduced into a packed column under unsaturated flow conditions. Surfactant-induced drainage has been observed to occur for this method in some cases, which can complicate data analysis and impart uncertainty to the measured values. Two novel alternative approaches for conducting IPTTs are presented herein that are designed in part to prevent surfactant-induced drainage. The two methods are termed the dual-surfactant IPTT (IPTT-DS) and the residual-air IPTT (IPTT-RA). The two methods were used to measure air–water interfacial areas for two natural porous media. System monitoring during the tests revealed no measurable surfactant-induced drainage. The measured interfacial areas compared well to those obtained with the standard IPTT method conducted in such a manner that surfactant-induced drainage was prevented.