Impact of Bentonite Quality on Hydraulic Conductivity of Geosynthetic Clay Liners

TLDR

The high‑quality bentonite GCL contains more sodium montmorillonite, a higher plasticity index, and a greater cation‑exchange capacity than the low‑quality GCL. Hydraulic conductivity was compared for the two GCLs by permeating specimens with water and CaCl₂ solutions (5–500 mM) until equilibrium, with test durations ranging from less than 1 to over 900 days. While the high‑quality GCL’s conductivity is about three times lower than the low‑quality GCL’s in water, it is consistently higher in CaCl₂ solutions, with k_HQB/k_LQB ratios of 2–2.6 at 5–20 mM and 40–230 at 50–500 mM, indicating greater chemical susceptibility.

Abstract

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 500mMCaCl2. 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 64meq∕100g) 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 900days. The hydraulic conductivity for GCL-HQB, kHQB, is ∼3× lower than the hydraulic conductivity for GCL-LQB, kLQB, 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, kHQB∕kLQB ranges from 2.0 to 2.6 for the tests performed using 5, 10, and 20mMCaCl2 solutions as the permeant liquids, whereas the kHQB∕kLQB values are ∼230, ∼100, and ∼40 for the tests performed using 50, 100, and 500mMCaCl2 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.