Acknowledgements

1. Microtomographic Imaging of the Air-Water Interface in Unsaturated Porous MediaMolly S. Costanza-Robinson, Middlebury College, VT The air-water interface (AWI) in porous media retains solutes and colloids and also governs the dynamics of interphase mass- and energy transfer in the subsurface. Advances in synchrotron X-ray microtomography (µCT) enable direct visualization and quantification of air-water interfacial areas (AI). µCT AI measurements serve as a benchmark by which more conventional & cost-effective means of estimating AI can be evaluated. Conventional AI measurements involve the use of surfactant tracers. However, surfactants can significantly alter the very property being measured by creating a surface-tension gradient which induces water drainage (left). Drainage leads to a consequent low-bias in surfactant-estimated AI. The magnitude of this effect was evaluated by comparison of surfactant-based AI estimations to those made by µCT. Acknowledgements Microtomographic imaging was performed at GeoSoilEnviroCARS Beamline, Advanced Photon Source, Argonne National Laboratory, which is supported by NSF, the State of Illinois, and DOE. Surfactant-based AI estimates are consistently lower than µCT values, an effect that is more pronounced under wetter conditions (right).