Use of gamma emission computed tomography to study the effect of electrolyte concentration on regions of preferred flow and hydraulic conductivity in deep regolith materials

Abstract

Understanding fluid flow, displacement, and mixing processes in natural porous media is fundamentally dependent upon the accurate characterisation of complex 3-dimensional structures. This current study delineated the distribution of conducting regions within a suite of regolith materials as they interacted with electrolyte solutions of different concentrations. Previous studies on the effects of electrolyte concentration on clay swelling and dispersion and the concomitant changes in pore structure, and hence soil permeability, have mainly been carried out on repacked samples of disturbed surface soils. This study used unconsolidated materials recovered as undisturbed cores from a saline aquifer from the deeper regolith (8.0-55.8 m). Progressive dilution of the electrolyte concentration of the percolating fluid (while maintaining a constant sodium adsorption ratio) was used to alter the pore structure of these saturated regolith materials. The electrolyte concentration was reduced from an initial value of 383 m.e./L (the original electrolyte concentration of the saline aquifer) to below the threshold concentration, and finally the cores were rinsed with deionised water. The corresponding changes to the regions conducting fluid and therefore pore structure, and the major fluid pathways followed during the percolation process, were imaged using gamma emission computed tomography. Five experimental core samples from depths of 8, 28, 30 (x2), and 55 m were used in the experiments. The average hydraulic conductivity was measured and found to decrease as a function of electrolyte concentration. The regions containing the major fluid pathways were found to decrease in volume as a function of electrolyte concentration. Clay mineralogy, sodium adsorption ratio, and grain size characteristics were found to be positively correlated with reductions in the average hydraulic conductivity. This method has the potential to aid in our understanding of the fundamental processes that govern the dynamics of pore structure changes and hence fluid flow in porous regolith materials, particularly in relation to changes in the electrolyte concentration and sodium adsorption ratio of the pore fluid. Such data will add significantly to our understanding of factors that affect the hydraulic properties of regolith materials under saline/sodic conditions.