Examining the role of density-driven transport on chlorinated vapor intrusion

This study investigates the influence of density-driven transport on chlorinated vapor intrusion through modeling and experiments. Density-driven transport involves downward vapor advection, potentially reducing vapor intrusion into buildings. A 1-D steady-state numerical model was developed using COMSOL Multiphysics, considering upward diffusion and downward density-driven advection in the subsoil and in the granular fill layer beneath building's foundations. Source vapor concentration and granular fill layer permeability emerged as the key factors affecting density-driven transport. Regardless of building characteristics, for permeabilities in the granular fill layer exceeding 10−7 m2, density-driven transport is expected to become relevant at vapor concentrations of 1 mg m−3, while for lower soil permeabilities (10−8-10−10 m2), density-driven transport impact is expected for vapor concentrations exceeding 1 g m−3. The results of laboratory column trichloroethylene (TCE) diffusion tests through sand and gravel supported these findings, showing a vertical stratification of TCE vapor concentrations consistent with the model. The trends expected by modeling also align with the findings of different field studies, where the source to building attenuation factors (AF) were found to decrease with increasing source vapor concentration. These outcomes highlight that the common approach adopted for vapor intrusion screening from soil gas data based on default AF values independent of source vapor concentration, may potentially lead to an overestimation of indoor concentrations in the presence of high vapor concentrations and highly permeable soils. Given the use of permeable granular fill layers in building construction, this study underscores the importance of accounting for density-driven transport to improve the accuracy of vapor intrusion risk assessment.