Porous Media Contamination: 3-Dimensional Visualization and Quantification Using X-Ray Computed Tomography
Abstract
Non-aqueous phase liquids (NAPLs), if spilled into the subsurface, will migrate downward, and a significant fraction will become trapped in the soil matrix. These trapped NAPL globules partition into the water and/or vapor phase, and serve as continuous sources of contamination (e.g. source zones). At present, the presence of NAPL in the subsurface is typically inferred from chemical analysis data. There are no accepted methodologies or protocols available for the direct characterization of NAPLs in the subsurface. Proven and cost-effective methodologies are needed to allow effective implementation of remediation technologies at NAPL contaminated sites. X-ray Computed Tomography (CT) has the potential to non-destructively quantify NAPL mass and distribution in soil cores due to this technology's ability to detect small atomic density differences of solid, liquid, gas, and NAPL phases present in a representative volume element. We have demonstrated that environmentally significant NAPLs, such as gasoline and other oil products, chlorinated solvents, and PCBs possess a characteristic and predictable X-ray attenuation coefficient that permits their quantification in porous media at incident beam energies, typical of medical and industrial X-ray CT scanners. As part of this study, methodologies were developed for generating and analyzing X-ray CT data for the study of NAPLs in natural porous media. Columns of NAPL-contaminated soils were scanned, flushed with solvents and water to remove entrapped NAPL, and re-scanned. X-ray CT data was analyzed to obtain numerical arrays of soil porosity, NAPL saturation, and NAPL volume at a spatial resolution of 1 mm. This methodology was validated using homogeneous and heterogeneous soil columns with known quantities of gasoline and tetrachloroethylene. NAPL volumes computed using X-ray CT data was compared with known volumes from volume balance calculations. Error analysis revealed that in a 5 cm long and 2.5 cm diameter soil column containing 0.5 ml NAPL (7,080 mg NAPL per Kg soil), the precision of calculated NAPL volumes was ±0.03 ml (6% error). Residual NAPL saturation in natural soil cores averaged 15% and varied spatially (inversely with porosity) from less than 1% to 70%. These results and others serve as proof-of-concept that a typical medical X-ray CT scanner has the potential to accurately quantify selected NAPLs in natural soils.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSMNS23A..10G
- Keywords:
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- 1831 Groundwater quality;
- 1894 Instruments and techniques;
- 9800 GENERAL OR MISCELLANEOUS