The performance of four geosynthetic clay liners (GCLs) used as a hydraulic barrier below concrete-lined sewage treatment lagoons was examined based on a series of laboratory tests aimed at measuring: (i) the lateral flow of synthetic wastewater through the interface between each GCL product and a 0.1 m thick cast-in-place concrete above the GCL over a 14-month period and, from this data, calculating the concrete/GCL interface transmissivity (theta); and (ii) the hydraulic conductivity (k) of the GCLs below poured concrete when exposed to (iia) synthetic wastewater under isothermal conditions, or (iib) a series of wet-dry and/or cool-heat cycles for up to 12 months. The four GCLs have either sodium or polymer-enhanced sodium bentonite, and either granular or powdered bentonite. When the wastewater head above the GCL was 1.0 m (stress on concrete, sigma = 10 kPa), L for the GCL with granular sodium bentonite was 4 X 10(-11) m(2)/s. For the same GCL, when the head increased to 2.5 m (sigma = 25 kPa), the value of L was reduced by about one order of magnitude to 2 X 10(-12) m(2)/s. For the GCL which has polymer-enhanced granular bentonite, the value of L was similar to (and possibly lower than) that for the GCL with untreated granular bentonite. For the GCL with powdered bentonite and cover geotextile impregnated with 1280 g/m(2) of bentonite, the values of L were 2 X 10(-12) and 4 X 10(-13) m(2)/s at 1.0 and 2.5 m head, respectively. With a reduction in the amount of the impregnated powdered bentonite in the cover geotextile to 840 g/m(2), theta was 1 to 9 X 10(-12) m(2)/s. The lowest k (3.2 X 10(-11) m/s) for a GCL below concrete and exposed to wastewater under isothermal conditions was measured for the GCL with polymer-enhanced granular bentonite, whereas the highest value of k (1.9 X 10(-10) m/s) was for the GCL with standard granular bentonite. Under 0.5 m head and 3 kPa stress, the k of the GCL below concrete and exposed to wet-dry cycles was 1.2-2.6 times the k of the GCL exposed to wastewater under isothermal conditions. Analytical calculations for the leakage through concrete/GCL liners for the four GCLs showed that the leakage was below the allowable limits specified by the Australian, British, and American standards.