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Hydraulic conductivity tests were conducted on a geosynthetic clay liner ͑GCL͒ for more than 2.5 years and as many as 686 pore volumes of flow ͑PVF͒ using single-species salt solutions ͑NaCl, KCl, or CaCl 2 ͒ to ͑1͒ evaluate how the long-term hydraulic conductivity ͑K L ͒ is affected by cation concentration and valence and ͑2͒ demonstrate the relevance and importance of termination criteria when measuring hydraulic conductivity of GCLs to salt solutions. Permeation with CaCl 2 solutions resulted in an increase in the hydraulic conductivity of 1 order of magnitude or more. The rate at which these changes occurred depended on concentration, with slower changes ͑years and hundreds of PVF͒ occurring for weaker solutions. In contrast, permeation with 100 mM NaCl or KCl solutions or de-ionized ͑DI͒ water resulted in no appreciable change in hydraulic conductivity, regardless of the duration of permeation or number of pore volumes of flow. Hydraulic conductivities determined in accordance with ASTM D 5084 and D 6766 ͑K 5084 and K 6766 ͒ equaled K L when the permeant solution contained NaCl, KCl, or was a strong ͑ജ50 mM͒ CaCl 2 solution. In contrast, when the permeant liquid was a weak ͑ഛ20 mM͒ CaCl 2 solution, K 6766 and K 5084 were 2-13 times lower than K L. Closer agreement between K 6766 and K L ͑3 ϫ ͒ was obtained for weak CaCl 2 solutions when the electrical conductivity ratio criterion was tightened to ±5%. Hydraulic conductivities obtained after comparable influent and effluent concentrations of the permeant salt ͑±10%͒ were approximately 2ϫ lower than K L for weak CaCl 2 solutions. Hydraulic conductivities equal to K L were obtained from the tests permeated with weak CaCl 2 solutions only when Na was no longer eluted at detectable levels.

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