Gaseous Tracer Technique for Estimating Air–Water Interfacial Areas and Interface Mobility

A series of gaseous miscible displacement experiments were conducted to estimate specific air–water interfacial areas ( a i ) and water contents in an unsaturated sand column. A straight‐chain hydrocarbon ( n ‐decane) was used as the gaseous interfacial tracer and methylene chloride and chloroform were used as the water‐partitioning gaseous tracers. A gas chromatographic technique was employed for the tracer experiments conducted at room temperature using nitrogen as the mobile phase and water as the immobile liquid. Tracer experiments covered a water saturation ( S w ) range of 1.5 to 56%. The largest a i value (∼1500 cm 2 cm −3 ), measured at the lowest S w (1.5%), was somewhat smaller than the solid surface area (∼2000 cm 2 cm −3 ) determined using the nitrogen‐sorption technique. As S w increased, a i values decreased exponentially to ∼80 cm 2 cm −3 at S w of 56%. Within a limited S w range (0.29 < S w < 0.55), where both aqueous and gaseous interfacial tracer data were measured, the a i values measured using a gaseous tracer ( n ‐decane) were 2 to 3 times larger than those measured in a previous study using an aqueous interfacial tracer (sodium dodecylbenzene sulfonate [SDBS]). The velocity of the air–water interface was estimated to be between 23 and 36% of the bulk pore‐water velocity. The water contents measured using water‐partitioning tracers were within ±5% of those based on gravimetric measurements.