Hydraulic Conductivity of Geosynthetic Clay Liners Exhumed from Landfill Final Covers

Abstract

Samples of geosynthetic clay liners (GCLs) from four landfill covers were tested for water content, swell index, hydraulic conductivity, and exchangeable cations. Exchange of Ca and Mg for Na occurred in all of the exhumed GCLs, and the bentonite had a swell index similar to that for Ca or Mg bentonite. Hydraulic conductivities of the GCLs varied over 5 orders of magnitude regardless of cover soil thickness or presence of a geomembrane. Hydraulic conductivity was strongly related to the water content at the time of sampling. Controlled desiccation and rehydration of exhumed GCLs that had low hydraulic conductivity $({10}^{-9}\text{to}{10}^{-7}\mathrm{cm}∕\mathrm{s})$ resulted in increases in hydraulic conductivity of 1.5–4 orders of magnitude, even with overburden pressure simulating a $1\text{-}\mathrm{m}$ -thick cover. Comparison of these data with other data from the United States and Europe indicates that exchange of Ca and/or Mg for Na is likely to occur in the field unless the overlying cover soil is sodic (sodium rich). The comparison also shows that hydraulic conductivities on the order of ${10}^{-6}\text{to}{10}^{-4}\mathrm{cm}∕\mathrm{s}$ should be expected if exchange occurs coincidently with dehydration, and the effects of dehydration are permanent once the water content of the GCL drops below approximately 100%. Evaluation of the field data also shows that covering a GCL with a soil layer $750–\mathrm{1,000}\mathrm{mm}$ thick or with a geomembrane overlain by soil does not ensure protection against ion exchange or large increases in hydraulic conductivity.