Assessment of Consolidation-Induced VOC Transport for a GML/GCL Composite Liner System

Abstract

This paper presents a numerical investigation of the effect of consolidation on the transport of a volatile organic compound (VOC), trichloroethylene (TCE), through a composite liner system comprising a geomembrane liner (GML) overlying and intimate contact with a geosynthetic clay liner (GCL). The numerical simulations were conducted using the model CST2, and considered coupled consolidation and contaminant transport with representative geometry, material properties, and applied stress conditions. The simulation results indicate that, depending on conditions, GCL consolidation can have significant effects on TCE mass flux, cumulative mass outflow, and distribution of contaminant concentration within the GCL, not only during the course of consolidation but also long after consolidation has ceased. Because of the small thickness of the GCL, consolidation-induced advection is insignificant. However, consolidation can significantly impact TCE transport through the GCL via changes in GCL material properties, including decreases in thickness, porosity, and effective diffusion coefficient. In general, the effects of GCL consolidation increase with increasing magnitude of applied stress, decreasing loading period (i.e., increasing loading rate), and increasing variation in the effective diffusion coefficient. The traditional performance assessment approach neglects GCL consolidation and fails to consider associated changes in material properties and, thus, can lead to significantly different results. Simulation results indicate that, in general, when the applied stress is lower than 125 kPa or the exponent for porosity-dependent effective diffusion coefficient is lower than 4, the effect of consolidation on TCE transport through the GML/GCL composite liner is insignificant, with differences in performance of the GML/GCL composite liner resulting from consolidation being less than 15%.