Inorganic transport through composite geosynthetics and compacted clay liners under geomembranes with multiple defects

Abbas El-Zein, Ingrid McCarroll and Mohsen Masoudian

Abstract

Geosynthetic clay liners (GCLs) and compacted clay liners (CCLs) are widely used in waste containment systems – usually in conjunction with a high-density polyethylene geomembrane – for the protection of groundwater from contamination. Defects in geomembranes have been shown to have detrimental impacts on their performances. These systems, including defects, have been studied mostly through the lens of hydraulic leakage. Previous contaminant migration studies of liner systems have assumed single rather than multiple defects or have simulated organic, rather than inorganic transport when multiple defects are present. Unlike organic chemicals, inorganic contaminants do not biologically decay and have extremely small coefficients of diffusion through the intact parts of the geomembrane. These differences create different transport regimes, with contaminant levels likely to take longer to build up and dissipate in the aquifer. This paper simulates the transport of inorganic contaminants in systems containing CCLs or GCLs, under a geomembrane with multiple defects. Specifically, we aim to a) assess the extent to which leakage rates are good predictors of concentrations of inorganic contaminants in the aquifer and b) quantify the relative effect of various design and field parameters on the degree to which defects in the geomembranes reduce the performance of these systems.

Two-dimensional models of liner systems with multiple defects are simulated with the finite-element based Soil Pollution Analysis System (SPAS) for the transport of chloride and cadmium in geosynthetic and compacted composite clay liners. The coupled, steady-state seepage equations and time-dependent reactive diffusion advection equations are solved in two-dimensional space in order to compute seepage velocities and chemical concentrations in the system, including the underlying aquifer. A finite mass boundary condition is applied at the top of the system, representing a finite intake of contaminants in the waste. Parametric analyses are conducted to characterise the relationship between, on the one hand, various design and field parameters (intake of contaminant, thickness of primary liner, frequency and size of defects, hydraulic conductivities of clay) and, on the other hand, leakage rates and maximum concentrations of contaminant in the aquifer.

We find that defects in the geomembrane lead to significant increases in maximum concentrations of inorganic contaminants in the aquifer. However, these maxima are predicted to occur a few hundred years after the closure of the landfill, i.e. beyond the usual regulatory limit of the design. Design/field parameters with the strongest effect on maximum contaminant levels in the aquifer are the hydraulic conductivities of the primary liner (CCL or GCL), the frequency of defects and, in the case of the CCL, the thickness of the primary liner. Finally, we find that leakage rates are sometimes poor indicators of the effects of design parameters on chemical concentrations in groundwater. The maximum specific discharge rate under defects can have an important effect on concentrations as well, though it is not usually taken directly into account, nor is it easily measurable.