X-ray Computed Tomography (CT) provides three-dimensional images of the interior features of objects based on density differences. X-ray CT technology has been used traditionally in the medical sciences, but is being increasingly used in the areas of geology, environmental science and petroleum engineering for measurements of porous media properties such as bulk density, porosity, permeability, heterogeneity, microstructure imaging, preferential flow pathways, and fluid phase saturations. An X-ray CT scanning technique for accurate, three-dimensional, and non-destructive characterization of the volume and distribution of non-aqueous phase liquids (NAPLs) in contaminated soil cores has been developed in our laboratory. The knowledge of the mobility, distribution and morphology of NAPLs such as petroleum liquids, chlorinated solvents and coal tars in the subsurface may facilitate our ability to effectively assess the long-term impacts of the NAPL discharge on groundwater quality and, to remediate impacted sites. X-ray CT data were analyzed by an image subtraction technique to obtain three-dimensional arrays of soil porosity, NAPL saturation, and NAPL volume at a spatial resolution of approximately 0.7 mm in the scan plane. The computed NAPL saturation was graphically represented as 3-D images of NAPLs in porous medium. X-ray CT derived NAPL volume is subject to errors ranging from 3 - 16%, depending on the density of the NAPL. The experimental technique was used to determine how freezing and freeze-and-thaw conditions, commonly encountered in cold climates, influences the morphology, displacement and migration of petroleum oils in subsurface soils. Significant mobility and changes in the morphology of NAPLs were observed due to freeze-thaw cycles in soil.

An overview of applications of X-ray CT scanning in environmental science and engineering, and experimental techniques for non-destructive visualization and quantification of NAPL volumes and saturation levels in natural soil columns using a medical X-ray CT scanner, will be presented.

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