Cation exchange and long-term hydraulic conductivity of geosynthetic clay liners (GCLs) permeated with inorganic salt solutions

Hydraulic conductivity tests were conducted on a geosynthetic clay liners (GCLs) and bentonite using inorganic salt solutions to evaluate how the long-term (>2.5 yr) hydraulic conductivity is affected by cation concentration and valence and relationships between the gradual change in hydraulic conductivity and properties of bentonite. A model was developed for simulating cation transport through bentonite in GCLs that incorporates diffusion-limited cation exchange to evaluate factors affecting the time required to establish chemical equilibrium in GCLs. Small changes (i.e., ≤2X) in hydraulic conductivity occurred during the test duration when the permeant solution was deionized (DI) water or solutions having monovalent cations. For weak CaCl₂ solutions (≤20 mM), the hydraulic conductivities gradually increased by a factor of 2 to 13 over a period of nearly 2 yr. In contrast, the GCL permeated with strong CaCl ₂ solutions (≥50 mM) reached equilibrium nearly immediately, with a hydraulic conductivity approximately 2 orders of magnitude higher than the hydraulic conductivity to DI water. In series of replicate hydraulic conductivity tests, for the nonprehydrated bentonite permeated with the CaCl₂ solutions (20 mM and 40 mM), the hydraulic conductivity increased and the free swell, water content, and void ratio decreased as exchange of Ca 2+ for Na+ occurred. Even though exchange of Ca 2+ for Na+ was essentially complete at the end of testing, the hydraulic conductivity obtained with the 20 mM CaCl₂ solution was lower and the water content and void ratio were higher than those obtained with the 40 mM CaCl₂. Similarly, even when exchange was complete, the prehydrated bentonite had lower hydraulic conductivity, higher water content, and larger void ratio than the non-prehydrated bentonite at the end of testing. The predictions made with the model indicate that the time required to establish chemical equilibrium is controlled by the rate of mass delivery to the pore space, the rate of mass transfer between the liquid phases, and the number of sites available for sorption.