Relative Abundance of Monovalent and Divalent Cations and the Impact of Desiccation on Geosynthetic Clay Liners

Laboratory experiments were conducted on a geosynthetic clay liner (GCL) containing Na-bentonite to determine how the swell index and hydraulic conductivity of GCLs are affected by wet-dry cycling with solutions having different relative abundance of monovalent and multivalent cations. Relative abundance of monovalent and multivalent cations was characterized by the RMD of the test solution, which is defined as the ratio of the total molarity of monovalent cations to the square root of the total molarity of multivalent cations at a given ionic strength. RMD was found to control the final swell index, relative abundance of monovalent and divalent cations in the final exchange complex, and the final hydraulic conductivity of bentonite exposed to wet-dry cycling. Ionic strength affects the number of wet-dry cycles required for a change in hydraulic conductivity to occur and the rate of change in swell index. Large increases in hydraulic conductivity and loss of swelling capacity occurred for solutions having RMD <= 0.07 M-1/2. Modest or small changes in hydraulic conductivity and swell index were obtained when the RMD was >= 0.14 M-1/2. These findings suggest that chemical analysis of the pore water in cover soils may prove useful in evaluating the compatibility of GCLs and cover soils used in applications where wet-dry cycling may occur.