GCLs used in landfill cover applications typically contain natural sodium bentonite. To examine the effect of ion exchange on the performance of GCLs, tests were performed in 60 cm diameter cells with 1 m of calcium rich soil below the GCL and a confining stress of 15 kN/m². The uptake of moisture and calcium from the underlying soil caused the moisture content of GCL to increase to 86% in the first 279 days of the test and then decrease to 68% after 625 days. The hydraulic conductivity of the GCL changed with cation exchange of Ca⁺⁺ for Na⁺ and varied from 1-3 × 10⁻¹¹ at the start of the test to 2 × 10⁻¹⁰ m/s after 625 days.