Effect of Municipal Solid Waste Leachate on Hydraulic Conductivity and Exchange Complex of Geosynthetic Clay Liners

Abstract

Tests were conducted to assess how permeation with synthetic and real municipal solid waste (MSW) leachate affects the hydraulic conductivity and exchange complex of geosynthetic clay liners (GCLs). GCLs were arranged in a composite barrier configuration for hydration on moist compacted subgrades for 30 or 90 days. After hydration, the geomembrane and subgrade soil were removed and the GCLs were permeated with typical and strong synthetic and real MSW leachates at different confining stresses for 342 to 1,281 days (4.9 to 148.9 pore volumes of flow). For most tests, hydraulic and chemical termination criteria were met (referred to as equilibrium), and in many cases the tests were conducted long after these criteria were achieved. Hydraulic conductivity of the GCLs increased no more than 5.6 times the hydraulic conductivity to deionized water (DIW), even though more than 80% of the sodium (Na) in the bentonite was replaced by other cations [predominantly calcium (Ca) and magnesium (Mg)]. Very long testing times can be required to reach equilibrium, especially at higher stresses. However, the combined effects of stress and cation exchange can be evaluated conservatively and more rapidly by first permeating the GCL to equilibrium at low stress, and then increasing the stress to simulate field conditions. Hydraulic conductivities obtained using this approach are no more than three times hydraulic conductivities obtained at equilibrium under elevated stress. Adding a requirement for equality of effluent and influent cation concentrations results in hydraulic conductivities to MSW leachate representative of long-term conditions.