The performance of geosynthetic clay liners (GCLs) above arsenic-rich gold mine tailings was investigated by the construction of a test cover comprised of three GCL products placed above the tailings at an abandoned gold mine in Nova Scotia, Canada. In parallel research, the performance of the three commercially available GCLs was assessed based on a series of laboratory column experiments. Of the three needle-punched GCLs tested, one has untreated Wyoming bentonite and a woven carrier geotextile; the second has a polymer-enhanced Wyoming bentonite and a scrim-reinforced nonwoven carrier geotextile, and the third has a polymer-enhanced Wyoming bentonite and woven carrier geotextile coated by a thin polypropylene geofilm. After 4 years of exposure in the field and laboratory, the bentonite structure of all exhumed GCLs appeared to be well hydrated with no visible cracks provided that the cover soil thickness was >0.7 m. The GCL with untreated bentonite exhibited the highest reduction in the exchangeable sodium percentage (ESP; decreased to 1-3% in field and 4-29% in laboratory) and swell index (decreased to 7-12 ml/ 2 g in both field and laboratory tests). Despite this reduction in the swelling capacity, the maximum measured hydraulic conductivity (k)for this GCL was 1 3 1010 m/s, which is still considered low. The GCLs with polymer-enhanced bentonite experienced less cation exchange (ESP 10-27% in the field and 17-53% in the laboratory) than the GCL with untreated bentonite. The k values of the GCL with polymer-enhanced bentonite and non-coated carrier geotextile (3.6 x 10(-11) to 7.9 x 10(-11) m/s in the field and 1.6 3 10 11 to 6.7 3 10 11 m/s in the laboratory) were lower than those measured for the GCL with untreated bentonite. The lowest measured k values (3.6 3 10(-12) to 9.4 3 1012 m/s) were for the GCL with polymer-enhanced bentonite and geofilm coating the carrier geotextile. All three GCLs prevented the migration of arsenic to the overlying cover soil, even when the GCL was resting directly above the tailings without a foundation layer.