Long-term hydraulic performance of geosynthetic clay liners subjected to inorganic salt solutions

Abstract

Specimens of two geosynthetic clay liners (GCLs) containing sodium bentonite, commonly used as liners or liner components in waste containment, are permeated with water and inorganic salt solutions containing from 5 mM to 500 mM calcium chloride (CaCl2) for the purpose of evaluating the long-term hydraulic performance of the GCLs. The results are evaluated in terms of criteria commonly used to terminate hydraulic conductivity tests involving non-standard permeant liquids (i.e., liquids other than water), the potential effects of quality of bentonite and prehydration on the hydraulic conductivities, and the potential correlation between index properties of the bentonite and hydraulic conductivity of the GCLs. All hydraulic conductivity tests involving CaCl2 solutions were conducted until equilibrium between the effluent and influent chemistry was achieved, resulting in test durations ranging from less than 1 day to more than 900 days, with longer test durations associated with lower CaCl2 concentrations. In addition, a new index property, referred to as the solution retention capacity (SRC) is developed and evaluated as an alternative to the use of the swell index approach for providing a qualitative indication of the relative impact of CaCl2 solutions on the hydraulic performance of the GCLs. Finally, the potential effect of Ca+ diffusion on the semipermeable membrane behavior is evaluated on the basis of a combined chemico-osmotic/diffusion test conducted on one of the GCLs. In terms of termination criteria, only equilibrium between influent and effluent Ca2+ results in equilibrium in hydraulic conductivity, regardless of prehydration or quality of bentonite, particularly for the influent concentrations ≤ 20 mM CaCl2. Also, the GCL containing the higher quality bentonite (GCL-HQB) is found to be more susceptible to incompatibility in hydraulic conductivity than the GCL with the lower quality bentonite (GCL-LQB) when permeated with CaCl2 solutions. In fact, measured hydraulic conductivity values range from 2.4 x 10-9 cm/s to 1.9 x 10-6 cm/s for GCL-LQB and from 7.0 X 10-10 cm/s to 6.2 x 10-5 cm/s for GCL-HQB, respectively, when permeated with solutions containing from 0 (water) to 500 mM CaCl2. The results also indicate that prehydration has little, if any, effect on hydraulic conductivity of GCL-LQB when permeated with solutions containing ≤ 50 mM CaCl2, i.e., as long as the tests are conducted until equilibrium between the effluent and influent Ca2+ is achieved. Also, a strong correlation between the SRC and swell index is shown over the full range of CaCl2 concentrations used in the study, suggesting that the SRC approach may be used as an alternative to the swell index approach. In general, correlations between the magnitude of the change in index property (e.g., liquid limit, sedimentation volume) when subjected to a CaCl2 solution relative to that based on water and a change in hydraulic conductivity when permeated with the same CaCl2 solution relative to permeation with water are shown to be a function of the magnitude of the change in hydraulic conductivity being considered (e.g., 10X or 100X), the index property, and the bentonite quality. Finally, time-dependent membrane efficiency is observed for a specimen of GCL-LQB, and the time required for effective destruction of semipermeable membrane property of the GCL correlates well with the time required to achieve steady-state Ca2+ diffusion.