Characterizing air-water interfacial area in variably saturated sandy porous media

Abstract

Air-water interface plays an important role in the transport of many contaminants in the vadose zone. It is also a limiting factor for many processes involve mass or energy transfer between air and water phases in vadose zone. In this research, the gas-phase partitioning tracer method was used to measure air-water interfacial area for eight porous media. The experimental results were used to investigate the influencing factors of the magnitude of air-water interfacial area and the relationship between the air-water interfacial area and water saturation, and capillary pressure. The porous media comprised a series of sands with narrow particle-size ranges, a sand with a wider particle-size distribution, a sandy soil, and a loamy sandy soil. The measurement range was extended to very low water contents in an attempt to determine upper limits for air-water interfacial areas. The measured values were compared to the normalized surface areas of the porous media. The results of the experiments showed that the magnitude of the air-water interfacial areas increased with decreasing water saturation, and approached that of the normalized surface areas. Generally, air-water interfacial areas were larger for media with larger specific surface areas. The change in air-water interfacial area with changing water saturation was less near saturated water contents and greater at smaller values. In addition, the change was greater for the poorly-sorted media than the well-sorted media. An empirical model was developed to describe the observed relationship between air-water interfacial area and water saturation. The coefficients of the model were found to correlate to the porous-medium uniformity coefficient. With this model and associated correlations, only bulk density, specific surface area, and uniformity coefficient are needed to estimate air-water interfacial area for a given water saturation. The model was shown to provide a reasonable description of a literature data set. Potential relationships between air-water interfacial area and capillary pressure under higher water-content conditions are investigated for unsaturated sandy porous media. A conceptual relationship between air-water interfacial area and capillary pressure is hypothesized, and is tested using air-water interfacial area data obtained from gas-phase tracer tests and saturation-pressure data obtained from water-drainage experiments. The results show that the magnitude of the air-water interfacial area increases with increasing capillary pressure, which corresponds to decreasing water content. (Abstract shortened by UMI.)