ABSTRACT
The spread of landfill leachates is prevented by the use of hydraulic barriers such as compacted clay liners (CCLs), geosynthetic clay liners (GCLs) and geomembranes. High quality bentonites, like sodium bentonite, are used in GCLs and mixed to soil in CCLs. Sodium bentonite clays are widely used because of their low hydraulic conductivity to water and favorable swelling properties (Mitchell, 1993; Shackelford et al., 2000; Egloffstein, 2001; Bouazza, 2001). Ca-bentonite is a lower quality bentonite but is more readily available and may be enhanced by sodium activation (i.e., saturating the exchange complex of the Ca-bentonite with sodium ions). However, exposure to high concentrations of organic or inorganic compounds present in landfill leachates can change the clay fabric of natural sodium or sodium-activated bentonites, increasing their hydraulic conductivity (Norrish, 1954; Mitchell, 1993). Another critical disadvantage of sodium activation is the consequent formation of calcium carbonate precipitates into the soil. This calcium carbonate may re-dissolve during contact with a dilute permeant, releasing calcium ions that exchange with the sodium in the clay (Guyonnet et al., 2005).This exchange leads to the development of a more permeable clay.
