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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.