Suitability of a hydraulic-conductivity model for predicting salt effects on swelling soils

Many empirical approaches have been developed to analyze changes in hydraulic conductivity due to concentration and composition of equilibrium solution. However, in swelling soils these approaches fail to perform satisfactorily, mainly due to the complex nature of clay minerals and soil-water interactions. The present study describes the changes in hydraulic conductivity of clay (Typic Haplustert) and clay-loam (Vertic Haplustept) soils with change in electrolyte concentration (TEC) and sodium-adsorption ratio (SAR) of equilibrium solution and assesses the suitability of a model developed by Russo and Bresler (1977) to describe the effects of mixed Na-Ca-Mg solutions on hydraulic conductivity. Four solutions encompassing two TEC levels viz., 5 and 50 mmolc L⁻¹ and two SAR levels viz., 2.5 and 30 mmol¹/² L⁻¹/² were synthesized to equilibrate the soil samples using pure chloride salts of Ca, Mg, and Na at Ca : Mg = 2:1. Diluting 50 mmolc L⁻¹ solution to 5 mmolc L⁻¹ reduced saturated hydraulic conductivity of both soils by 66%, and increasing SAR from 2.5 to 30 mmol¹/² L⁻¹/² decreased saturated hydraulic conductivity by 82% and 79% in clay and clay-loam soils, respectively. Near saturation, the magnitude of the change in unsaturated hydraulic conductivity due to the change in TEC and SAR was of 10³- and 10²-fold, and at volumetric water content of 0.20 cm³ cm⁻³, it was of 10¹⁴- and 10⁶-fold in clay and clay-loam soils, respectively. Differences between experimental and predicted values of saturated hydraulic conductivity ranged between 0.6% and 11% in clay and between 0.06% and 2.1% in clay-loam soils. Difference between experimental and predicted values of unsaturated hydraulic conductivity widened with drying in both soils. Predicted values were in good agreement with the experimental values of hydraulic conductivity in clay and clay-loam soils with R² values of 0.98 and 0.94, respectively. The model can be satisfactorily used to describe salt effects on hydraulic conductivity of swelling soils in arid and semiarid areas, where groundwater quality is poor.