Effect of sodium adsorption ratio and electrolyte concentrations on the saturated hydraulic conductivity of clay-sand mixtures

We did flow experiments under saturated conditions on homoionic (Na) kaolin-sand and illite-sand systems, containing 5, 10, and 15% clay, to validate a drainage model, and evaluate the effect of the different chemical composition of the percolating solutions on the hydraulic properties of the systems. Column drainage experiments, under a constant hydraulic head, were carried out using solutions with two sodium adsorption ratios (SAR 0 and infinity) and three electrolyte concentrations (10(-2), 10(-3), and 10(-4) M). We calculated the saturated hydraulic conductivity, K-sat, of the systems using Darcy's law when these showed linear relationships between effluent volume and time. The drainage model was applied to characterize the flow of non-steady-state drainage of solutions through the porous systems. This model describes and characterizes quantitatively the drainage of solutions from soil columns that vary in intrinsic permeability, k, because of structural modifications that occur within the solid matrix of the systems. For both the systems investigated we always observed a decrease in the flow rate as electrolyte concentration decreased, or clay percentage increased. Under the same conditions the flow was faster for the kaolin system than the illite system, even though kaolin dispersed more than illite. Non-linear relations were also observed at the smallest electrolyte concentration (10(-4) M). In all cases, the equations proposed correlated well with the experimental data, confirming the soundness of the model. The flow rates observed in the experiments at SAR infinity were unexpectedly greater than those observed at SAR 0, for the two systems, when leaching with solutions at 10(-3) and 10(-4) M. The values of pH and electrical conductivity of the eluates support the idea that the clay hydrolysis occurred during the saturation and, to a lesser extent, during the leaching phase of the flow.