Hydraulic conductivity, a property initially conceived as an intrinsic property of a homogenous porous media, is commonly used to describe the performance of Geosynthetic Clay Liners (GCLs). In engineering practice, it is also used as an index value to specify a minimum GCL performance threshold for project requirements. GCLs, however, are heterogeneous materials comprised of synthetic fibers and bentonite clay at various concentrations along a given cross section. A conceptual model of a GCL as layers of bentonite clay contaminated with synthetic fibers was conceived and tested. By considering heterogeneity, the model allows a more thorough Darcian analysis of GCL design performance to be incorporated into composite liner leakage calculations, transmissive flow over GCLs, and transmissive flow through GCL overlaps. A new permeameter configuration was used to measure transmissive flow through the non-woven geotextile component of a needle punched GCL.