Single photon emission computed tomography (SPECT) is an im-1990; Singh and Kanwar, 1991; White, 1985). Macropore aging technique that is widely used in medical diagnosis. This technique has never been applied to soils. The objective of this study flow results in rapid movement of water and solutes was to investigate the capabilities of SPECT scanning for visualizing through the soil profile with important implications for preferential flow in soil. This paper describes the principle of SPECT ground water quality. Short travel time may not allow scanning and its application to tracer breakthroughs in four large contaminants, such as pesticides, to be adsorbed on and undisturbed soil columns (800-mm length ϫ 77-mm diam.). This new into soil particles. Consequently, most solutes in the approach allows real-time analysis of flow patterns of radioactive water will be quickly delivered to the drains or ground tracers in 2-D using planar imaging, and in 3-D using the tomographic water aquifers without being degraded by chemical and capabilities of the SPECT scanner. Not only does SPECT scanning biological actions. However, quantification and predicprovide qualitative data, but it also allows for the quantification of a tion of preferential flow has been difficult because of tracer's spatial distribution. Our results characterized preferential flow the complexity of soil structure. very clearly in soil columns. SPECT scanning opens up new avenues for 2-D and 3-D tracer studies in porous media such as soils. Knowledge of soil structure, along with a suitable technique for measuring water and associated solute flow characteristics through soil, is essential to understanding the mechanisms of preferential flow. Unfortu-W ater and solute transport through soil macnately, progress in this area has been severely limited ropores can be quite significant and is now well by difficulties in obtaining direct and nondestructive documented (Perret et al., 1997; Perret et al., 2000; Bemeasurements of the preferential flow in a structured soil. Computer assisted tomography (CAT) scanning
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