Impact on groundwater of concurrent leakage and diffusion of dichloromethane through geomembranes in landfill liners

Abstract

Although intact geomembranes are excellent barriers to fluid flow, there are still two possible pathways for the transport of organic contaminants into the environment: leakage through defects in the geomembrane, and diffusion through the intact geomembrane. Analytical, semi-analytical and numerical techniques for calculating leakage rates have been developed. However, no previous attempt has been made to rigorously model the interaction between leakage through a hole in a wrinkle and diffusion. To address this shortcoming, finite-element analyses in two-dimensional space are performed to simulate the transport of dichloromethane (DCM) through a leaking HPDE geomembrane (GM) for eight cases involving composite liners, including both compacted clay liners and geosynthetic clay liners. Equations of steady-state seepage and time-dependent diffusion-advection in saturated soils are solved. Transport through the GMs is simulated by a novel, mass-conserving equivalent boundary condition that renders the problem computationally more tractable. Results are compared with those obtained from widely used one-dimensional methods, namely the Rowe equation for leakage through a wrinkle and the POLLUTEv7 program for mass transport. It is shown that predictions of leakage and transport by the one-dimensional method (POLLUTEv7) are within a maximum 30% of two-dimensional values under conditions of perfect mixing in the aquifer. When no horizontal mixing is present, concentrations of DCM are found to increase by up to 43% in two-dimensional analyses. It is concluded that one-dimensional analyses are adequate for most practical purposes.