Finite difference solution for contaminant transport through a GCL composite cutoff wall-aquifer system considering semipermeable membrane effect

Abstract

Semipermeable membrane effect is an important factor affecting the contaminant transport behavior through the bentonite-based barriers for waste containment. However, this effect has been ignored in majority of the existing analytical and numerical studies on contaminant transport, although it has been widely demonstrated in laboratory and field tests. Taking into account the semipermeable membrane effect, this study presents a numerical model for one-dimensional contaminant transport through a geosynthetic clay liner (GCL) composite cutoff wall-aquifer system consisting of a bentonite-based wall, a GCL and an aquifer. The finite difference method is used to obtain the solution. The numerical solution is validated against the laboratory experimental data, the existing analytical solution and COMSOL software simulations. A series of parametric analyses are conducted using the proposed numerical solution. The results indicate that the semipermeable membrane effect has a significant impact on the contaminant transport behavior and ignoring this effect can greatly underestimate the contaminant breakthrough time and overestimate contaminant outflow. For the conditions examined in this paper, the inclusion of semipermeable membrane effect results in a 7.1%∼78.6% increase in breakthrough time and a 5.3%∼40.7% decrease in mass flux, and the semipermeable membrane effect in bentonite-based wall, relative to that in the GCL, has greater impact on contaminant transport. The concentration-dependent semipermeable membrane efficiency coefficient ω more significantly slows down the contaminant transport relative to the constant ω, particularly under the conditions of low hydraulic conductivity, low source concentration and low hydraulic gradient. Moreover, it’s important to avoid simultaneous deterioration of both the bentonite-based wall and the GCL, otherwise the breakthrough time of the GCL composite cutoff wall can decrease significantly.