Impact of bentonite quality on hydraulic conductivity of geosynthetic clay liners

The differences in hydraulic conductivity for two geosynthetic clay liners (GCLs) containing different qualities of bentonite are evaluated based on permeation with water and chemical solutions containing 5, 10, 20, 50, 100, and 500 mM CaCl2. The GCL with the higher quality bentonite (GCL-HQB) is characterized by a greater content of sodium montmorillonite (86 versus 77%), a higher plasticity index (548 versus 393%), and a higher cation exchange capacity (93 versus 64 meq/ 100 g) relative to the GCL with the lower quality bentonite (GCL-LQB). The tests using CaCl2 solutions as permeant liquids were continued until chemical equilibrium between the influent and effluent was established, resulting in test durations that ranged from less than 1 to more than 900 days. The hydraulic conductivity for GCL-HQB, k(HQB), is similar to3 x lower than the hydraulic conductivity for GCL-LQB, k(LQB), when specimens of both GCLs are permeated with water. However, the hydraulic conductivity for GCL-HQB is always higher than that for GCL-LQB when the specimens are permeated with the CaCl2 solutions. For example, k(HQB)/k(LQB) ranges from 2.0 to 2.6 for the tests performed using 5, 10, and 20 mM CaCl2 solutions as the permeant liquids, whereas the k(HQB)/k(LQB) values are similar to230, similar to100, and similar to40 for the tests performed using 50, 100, and 500 mM CaCl2 solutions as the permeant liquids, respectively. Thus, the GCL with the higher quality bentonite is more susceptible to chemical attack than the GCL with the lower quality bentonite.