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.

Interfacial areas between nonwetting‐wetting (NW‐W) liquids in natural porous media were measured using a modified version of the interfacial partitioning tracer test (IPTT) method that employed simultaneous two‐phase flow conditions, which allowed measurement at NW saturations higher than trapped residual saturation. Measurements were conducted over a range of saturations for a well‐sorted quartz sand under three wetting scenarios of primary drainage (PD), secondary imbibition (SI), and secondary drainage (SD). Limited sets of experiments were also conducted for a model glass‐bead medium and for a soil. The measured interfacial areas were compared to interfacial areas measured using the standard IPTT method for liquid‐liquid systems, which employs residual NW saturations. In addition, the theoretical maximum interfacial areas estimated from the measured data are compared to specific solid surface areas measured with the N2/BET method and estimated based on geometrical calculations for smooth spheres. Interfacial areas increase linearly with decreasing W‐phase (water) saturation over the range of saturations employed. The maximum interfacial areas determined for the glass beads, which have no surface roughness, are 32 ± 4 and 36 ± 5 cm−1 for PD and SI cycles, respectively. The values are similar to the geometric specific solid surface area (31 ± 2 cm−1) and the N2/BET solid surface area (28 ± 2 cm−1). The maximum interfacial areas are 274 ± 38, 235 ± 27, and 581 ± 160 cm−1 for the sand for PD, SI, and SD cycles, respectively, and ∼7625 cm−1 for the soil for PD and SI. The maximum interfacial areas for the sand and soil are significantly larger than the estimated smooth‐sphere specific solid surface areas (107 ± 8 cm−1 and 152 ± 8 cm−1, respectively), but much smaller than the N2/BET solid surface area (1387 ± 92 cm−1 and 55224 cm−1, respectively). The NW‐W interfacial areas measured with the two‐phase flow method compare well to values measured using the standard IPTT method.

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The gas‐absorption/chemical‐reaction (GACR) method used in chemical engineering to quantify gas‐liquid interfacial area in reactor systems is adapted for the first time to measure the effective air‐water interfacial area of natural porous media. Experiments were conducted with the GACR method, and two standard methods (X‐ray microtomographic imaging and interfacial partitioning tracer tests) for comparison, using model glass beads and a natural sand. The results of a series of experiments conducted under identical conditions demonstrated that the GACR method exhibited excellent repeatability for measurement of interfacial area (Aia). Coefficients of variation for Aia were 3.5% for the glass beads and 11% for the sand. Extrapolated maximum interfacial areas (Am) obtained with the GACR method were statistically identical to independent measures of the specific solid surface areas of the media. For example, the Am for the glass beads is 29 (±1) cm−1, compared to 32 (±3), 30 (±2), and 31 (±2) cm−1 determined from geometric calculation, N2/BET measurement, and microtomographic measurement, respectively. This indicates that the method produced accurate measures of interfacial area. Interfacial areas determined with the GACR method were similar to those obtained with the standard methods. For example, Aias of 47 and 44 cm−1 were measured with the GACR and XMT methods, respectively, for the sand at a water saturation of 0.57. The results of the study indicate that the GACR method is a viable alternative for measuring air‐water interfacial areas. The method is relatively quick, inexpensive, and requires no specialized instrumentation compared to the standard methods.