Microtomographic Measurements of Total Air-Water Interfacial Areas for Soils

Synchrotron X-ray microtomography (XMT) was used to measure total air-water interfacial areas (A_aw) as a function of water saturation (S_w) for several soils that comprise a range of physical and geochemical properties. Measurements were also conducted for glass beads and quartz sands for comparison. Apparent near-linear A_aw-S_w relationships are observed for the three sands and the three sandy soils. In contrast, the measured interfacial areas for two soils that contain greater proportions of silt and clay are strongly nonlinear functions of water saturation. The greater degree of nonlinearity observed for these two soils is due to their much greater particle-size distributions (i.e., uniformity coefficients) and their concomitant greater range in pore sizes. Interfacial areas determined with the thermodynamic method were used to benchmark the XMT measurements. XMT-measured interfacial areas compare well to the thermodynamic-determined values for the sands and sandy soils. In contrast, the XMT-measured interfacial areas for the two soils with larger particle-size distributions are not fully congruent with the thermodynamic-determined values. Both of these soils have large fractions of pore space comprising nominal pore diameters smaller than the resolution of the XMT imaging. These results suggest that air-water interfacial area may not always be fully characterized by standard XMT for soils with large particle-size distributions.