Abstract

Typic Chromoxeret-sand and Lithic Ruptic Xerochrept-sand mixtures, of high sodicity, and containing 8 and ~0% CaCO₃, respectively, were packed in columns and leached with electrolyte solutions. When a reference solution of 500 mmol/L was replaced successively with solutions containing 100, 50, 10, 5, and 1 mmol/L and then with deionised water (gradual leaching), the saturated hydraulic conductivity (K_s) decreased gradually to steady-state values, and no clay was observed in the leachate. Under deionised leaching, the relative K_s (K_s/K_s0) (the ratio between the observed K_s and that under leaching with the reference solution) in the Chromoxeret mixture at exchangeable sodium percentages (ESPs) of 30, 38, and 63 was 0.83, 0.47, and 0.41, respectively, and that in the Xerochrept mixture at ESPs of 23, 28, and 36 were 0.82, 0.71, and 0.39 respectively. When a solution of 50 mmol/L was replaced directly with deionised water (abrupt leaching), the K_s/K_s0 of the Chromoxeret mixture at the above ESPs dropped sharply to minimum values of 0.25, 0.18, and 0.11, respectively, and that of the Xerochrept mixture to 0.42, 0.04, and 0.13, respectively, and dispersed clay was observed in the leachate. It is hypothesised that the abrupt leaching with deionised water generated a steep concentration gradient between the solutions within and around the aggregates, leading to clay dispersion and rapid K_s/K_s0 reduction; under gradual leaching, no steep concentration gradient developed, there was no clay dispersion, and clay swelling caused gradual K_s/K_s0 reduction. In the Chromoxeret, the CaCO₃ decreased the clay swelling and dispersion, and probably acted as a cementing agent that stabilised the soil aggregates against slaking during the initial wetting of the mixture.