Effect of Permeant Water on the Hydraulic Conductivity of Exhumed GCLs

Hydraulic conductivity tests were performed on geosynthetic clay liners (GCLs) exhumed from composite barriers (i.e., geomembrane over GCL) in four landfill covers using three dilute permeant waters: type II deionized water (DW), 0.01M CaCl₂ (so called “standard water” (SW)), and a typical water having average characteristics of eluent from cover soils (“average water” (AW)). Depending on the exhumed state of the GCL, very different (up to four orders of magnitude) hydraulic conductivities were obtained with DW, AW, and SW. When macroscopic features were present in the GCL, similar hydraulic conductivities (1×10⁻⁹–2×10⁻⁷ m/s) were obtained with SW and AW, but lower hydraulic conductivities were obtained with DW (1×10⁻¹¹–3×10⁻¹⁰ m/s). For GCLs without macroscopic features, much higher hydraulic conductivities were obtained with SW (1×10⁻⁹–2×10⁻⁷ m/s) than AW or DW (<2×10⁻¹¹ m/s) if the exhumed GCL had lower water content (<46 %), whereas similar hydraulic conductivities (<5×10⁻¹¹ m/s) were obtained with all three waters if the GCL had higher water content (>53 %). For GCLs with lower water contents, permeation with AW or DW had minimal effect on the composition of bound cations. In contrast, permeation with SW reduced the mole fraction of monovalent bound cations. These findings demonstrate that the chemistry of the permeant water can have a significant effect on the hydraulic conductivity of exhumed GCLs even when the permeant water is dilute. To simulate typical conditions, a solution containing 1.3 mM NaCl and 0.8 mM CaCl₂ is recommended as the permeant water (73.8 mg of anhydrous NaCl and 87.0 mg of anhydrous CaCl₂/L DW). A conservative assessment of hydraulic conductivity can be obtained using 0.3 mM NaCl and 1.9 mM CaCl₂ (15.5 mg of anhydrous NaCl and 214.6 mg CaCl₂/L DW).