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Geosynthetic clay liners (GCLs) used in waste and chemical containment applications exhibit high swell and low hydraulic conductivity to water (e.g., k < 10−10 m/s), such that diffusion dominates contaminant transport occurring through the barrier. Thus, knowledge of expected diffusion coefficients for GCLs under relevant environmental conditions is required for performance-based design and accurate assessment of the barrier system. Unfortunately, diffusion testing for bentonites can be challenging and time consuming, limiting data availability for GCL diffusion coefficients. The dialysis leaching test (DLT) method has been utilized in recent studies for simple, time-efficient diffusion measurements for bulk sodium bentonites (NaB), enhanced bentonites, and NaB pastes. This study used a new, modified version of the DLT method for measuring diffusion in GCLs comprising NaB. Diffusion tests were performed using dilute (20 mM) and aggressive (100 mM) calcium chloride (CaCl2) solutions to measure apparent diffusion coefficients (Da) for chloride for the GCLs. Values of Da were in the range of 10−10 m2/s, consistent with expectations from the literature for longer-term traditional testing. Diffusion coefficients increased as CaCl2 concentration increased, as expected due to cation exchange and suppression of the diffuse double layer in the bentonite in the GCL. The results of the study demonstrate the potential use of a new, time-efficient test method for assessing diffusion properties of GCLs, further improving our ability to predict contaminant transport through barrier systems. 

Abstract Measuring the diffusion coefficient of clay-based liner materials is important in estimating and predicting long-term barrier performance in waste containment facilities. Various theoretical models, including the finite cylindrical model, have been commonly used to determine the diffusion properties of clay-based liner materials in leaching tests. However, the assumption of zero-concentration boundary conditions of the traditional finite cylindrical model contradicts the measured variation of concentration in real leaching tests, likely resulting in (1) underestimated and unconservative diffusion coefficient, or (2) requirement of a relatively large liquid-to-soil ratio and frequent leachate replacement in the experiment to maintain the zero-concentration boundary condition. In this study, a theoretical model was developed to evaluate the solute diffusion process within a soil specimen under arbitrary, time-dependent concentration boundary conditions. The proposed model, incorporating the time-dependent boundary conditions, provides efficient calculations of the concentration distribution and the cumulative fraction leached of solute across the soil specimen. The example application of the proposed model to experimental data demonstrates the capability of the proposed model to determine apparent diffusion coefficients of clay-based liner materials without introducing errors associated with the assumption of a zero concentration boundary condition. The proposed model provides a comprehensive method to investigate the dynamic transport behaviors of solutes through clay-based liner materials in future studies.