Effect of Low Electrolyte Concentration on Hydraulic Conductivity of Sodium/Calcium‐Montmorillonite‐Sand System

The effect of electrolyte concentration in the percolating solution on hydraulic conductivity (HC) of Na/Ca‐montmorillonite‐sand mixtures at exchangeable sodium percentage (ESP) 5, 10, and 20 was studied. The HC was decreased following a decrease in electrolyte concentration. The HC of the system at sodium adsorption ratio (SAR) 10 dropped to a very low value (4.2 × 10 −10 m s −1 ) and no clay was observed in the leachate, when the 10 mol c m −3 solution was replaced with solutions of lower concentration prior to introducing distilled water. On the contrary, when the 10 mol c m −3 solution was replaced by distilled water the HC dropped sharply followed by a sharp increase and clay leaving the column. The time it took to obtain the constant HC depends upon ESP. The higher the ESP the longer the time. The degree of swelling of clay before replacing the solution with electrolyte concentration below the flocculation value (FV) determines whether clay will leave the system. A subsequent introduction of a solution with electrolyte concentration below FV, causes a further decrease in pore radii and part of the dispersed tactoids at SAR 10 and 20 are trapped in the narrow pores. Thus, because of the sieving effect, the HC further decreased but no clay appeared in the leachate. At SAR 5, an increase in HC was obtained when 1 mol c m −3 solution was replaced with distilled water. This increase was due to clay dispersion and movement out of the system because swelling is limited at SAR 5.