A vapor intrusion model incorporating soil stratification and source depletion dynamics

Vapor intrusion (VI) represents a critical exposure pathway for volatile organic compounds (VOCs) at groundwater-contaminated sites and has become a central concern in site risk assessment and remediation planning as emphasized in U.S. EPA vapor intrusion guidance. Existing screening models are widely used for evaluation. However, they typically assume a constant source concentration and homogeneous soil properties. While these simplifications facilitate analytical tractability, they neglect two key processes: the time-dependent depletion of contaminant sources resulting from natural attenuation or remediation activities, and the intrinsic heterogeneity of layered soils. The omission of these factors may introduce systematic bias into vapor intrusion estimation and overconservative remediation decisions. To address these limitations, this study develops an enhanced one-dimensional transient analytical model that explicitly couples VOC migration in layered soils with source depletion dynamics, serving as an incremental enhancement to traditional screening tools. The model performance is systematically validated against experimental data and numerical simulations to ensure predictive reliability. Furthermore, a comprehensive sensitivity analysis is conducted to quantify the relative influences of source depletion parameters, soil stratification heterogeneity, and building characteristics on subsurface vapor transport and indoor contaminant concentrations. The results indicate that neglecting source depletion can overestimate the peak indoor contaminant concentration by a maximum of 70%. Moreover, soil stratified heterogeneity exerts a critical control on vapor migration by inducing layer-specific retardation and storage effects. The concentration differences predicted by the layered model and the traditional equivalent homogeneous soil model can reach up to 90% within the soil, and up to 20% in indoor environments. By integrating transient source behavior with layered soil structure, this model can serve as a Tier 2 analytical evaluation tool that improves predictive accuracy within the scope of its simplifying assumptions, providing a reference basis for vapor intrusion assessment and remediation analysis.