A multilayer analytical model for reactive contaminant transport through cutoff wall-aquifer systems: Addressing pre-installation contamination and source removal scenarios

Abstract

Cutoff walls are a type of engineered barrier widely used to control contaminant transport in the subsurface environment, particularly at legacy contaminated sites. However, most existing models assume initially uncontaminated strata and a persistent source of release, which limits their applicability under realistic site conditions. This study aims to develop a multilayer analytical model that incorporates spatially variable initial contaminant concentrations and accommodates both source-release and source-removal scenarios. The model explicitly couples contaminant advection, dispersion, adsorption, and degradation across a three-domain system (inside aquifer-cutoff wall-outside aquifer), allowing for more accurate performance prediction of vertical barriers. A series of scenario comparisons were conducted using the proposed model, examining the effects of initial pollution status, source dynamics, hydraulic gradients, and barrier permeability. Results show that ignoring pre-existing contamination may lead to an overestimation of breakthrough time by more than 200 %, while source removal can reduce outlet concentrations by up to 50 % depending on wall placement. Sensitivity analysis indicates that reducing barrier permeability (from 10⁻⁸ to 10⁻⁹ m/s) or reversing hydraulic gradients can delay contaminant breakthrough time by an order of magnitude. Practical design curves are introduced to optimize barrier thickness, installation timing, and position, demonstrating that early installation, expanded control zones, and high-adsorption/low-permeability backfilling significantly reduce required barrier dimensions. This work advances predictive tools for cutoff wall performance at legacy contaminated sites, providing actionable insights for engineers to balance remediation costs and environmental safety.