Hydraulic Conductivity of Geosynthetic Clay Liners to Recirculated Municipal Solid Waste Leachates

Tests were conducted to assess how permeation with actual recirculated leachate from municipal solid waste (MSW) landfills affects the hydraulic conductivity and exchange complex of geosynthetic clay liners (GCLs). Long-term hydraulic conductivity tests were conducted (∼1 to 5 years) on GCL specimens with conventional sodium bentonite using seven recirculated MSW leachates and one conventional (nonrecirculated) MSW leachate as permeant liquids. Effects of stress and temperature were also assessed. Hydraulic termination criteria were achieved for all tests, and chemical termination criteria were achieved for nearly all. The long-term hydraulic conductivity of the GCL specimens to recirculated leachates (1.0–2.0×1011 m/s) was slightly less than the long-term hydraulic conductivity to the conventional leachate (2.0×10−11 m/s). Furthermore, the ratio of the hydraulic conductivity to recirculated MSW leachate to the hydraulic conductivity to DI water for GCL specimens (1.4–2.9) fell within the range reported in other studies where GCLs were permeated with conventional MSW leachate (0.05–2.6). The long-term hydraulic conductivities to recirculated and conventional MSW leachate at 70 kPa were low (<2.0×10−11 m/s) because of the relatively low ionic strength of the leachates (<249 mM) and the preponderance of monovalent cations in the leachate. Geosynthetic clay liner specimens tested at higher stress (70 versus 20 kPa) had lower hydraulic conductivity (2.3–3.0 times lower), which slowed mass delivery of exchangeable cations and thereby delayed exchange processes and corresponding changes in hydraulic conductivity. Gas production from biological activity can be minimized during leachate compatibility testing by reducing the temperature to 4°C without compromising cation exchange processes and related effects on hydraulic conductivity. Leakage data from two landfills showed that leakage rates from composite liners with a GCL can be very low (<0.34 L/m2-year) when the landfill is operated with leachate recirculation.