Influence of aggregate size on solute transport as measured using computed tomography

Abstract

Application of computed tomography (CT) using selected tracers for in-situ monitoring of solute transport in soil cores is useful for identifying the influence of soil structure on solute movement. The objective of this study was to apply X-ray CT techniques for measurement of solute breakthrough curves for selected soil aggregates and to determine the two-dimensional velocity distributions within soil cores. Soil material from the A horizon of a Mexico silt loam (Udollic Ochraqualf) was brought to the laboratory, passed through selected sieves, and packed in soil cores (76 mm × 76 mm). Solute breakthrough experiments were conducted using a 1 % KI solution with the cores held in an apparatus used to control the bulk fluid velocity. Shapes of average breakthrough curves measured using CT generally compared well with curves measured from effluent samples. Aggregate sizes of 0.50–1.00 and 1.00–2.00 mm gave dispersion coefficients of 0.37 and 1.13 mm 2/s, respectively, using a pore water velocity of 0.11 mm/s. Aggregate sizes of 0.25–0.50 and 1.00–2.00 mm gave dispersion coefficients of 0.042 and 0.123 mm 2/s, respectively, using a pore water velocity of 0.05 mm/s. Techniques were developed for calculating the pore water velocity distribution as a function of location within the soil core. Velocities can be calculated at approximately 17,000 locations within a 76-mm diameter soil core, thus achieving a level of detail not previously obtained.