Effect of geomembrane defects on the degradable contaminant transport through composite geomembrane cutoff walls: A novel transient model

Composite geomembrane cutoff walls (CGCW) are among the most effective techniques for managing contaminated sites. Accurately predicting contaminant transport through CGCW is essential to the efficient remediation and management of contaminated sites. Previous prediction models have predominantly focused on contaminant diffusion in intact CGCW, with insufficient attention given to CGCW with geomembrane (GMB) defects. This study introduces fully transient models for contaminant transport in a CGCW-aquifer system, comprising an upstream slurry wall, a GMB with defects, a downstream slurry wall, and the adjacent aquifers. The model allows for three types of GMB defects, i.e., joint defects, bottom gaps, and GMB holes, and incorporates the contaminant advection-diffusion-adsorption-degradation process in the CGCW-aquifer system. Results indicate that existing CGCW models overlook the critical impact of GMB positioning on the containment performance of CGCW with GMB defects. The semi-infinite boundary assumption and the non-degradation assumption commonly adopted in previous models lead to, respectively, an underestimation and overestimation of contaminant plume concentrations in the downstream aquifer. Additionally, increasing the widths of joint defects, bottom gaps, and GMB holes significantly reduces the containment efficiency of CGCW, especially under conditions of high hydraulic head and low slurry wall permeability. To address this, an equivalent design curve for CGCW with GMB defects is proposed, offering engineers a practical reference for determining the required wall thickness.