Incorporating lateral bypass in the design of horizontal permeable reactive barriers (HPRBs) for chlorinated vapor intrusion mitigation

Horizontal Permeable Reactive Barriers (HPRBs) installed in the unsaturated zone beneath buildings recently emerged as promising passive strategies to mitigate chlorinated vapor intrusion at contaminated sites. To date, experimental investigations have been conducted only at the laboratory scale. To progress toward full-scale implementation, modeling tools are essential for barrier design and predicting field performance. Existing modeling approaches assume infinitely wide barriers failing to account for the potential lateral bypass of vapors around the barrier. This assumption hinders the accurate assessment of field performance, as vapors may still reach buildings through longer diffusion pathways. In this work a 2-D numerical model that incorporates both vertical and horizontal dimensions of the reactive barrier was developed in Comsol Multiphysics. The results highlight that vapor lateral bypass is a critical factor. Barriers twice the width of the building footprint may achieve attenuation factors in the order of 10-3, providing adequate protection only for low contamination levels (slightly above MCLs). For higher contamination scenarios, additional vertical barriers are required to intercept lateral vapors. This study explores several configurations using purely horizontal barriers or integrating them with reactive vertical layers or HDPE liners. The latter solutions are capable of achieving attenuation factors in the order of 10-10, ensuring acceptable indoor risk for various chlorinated solvents, even under saturated groundwater conditions. To support the selection and design of barrier configurations, this study provides a set of nomographs as practical tools tailored to the target subsurface vapor attenuation levels, facilitating the field-scale implementation of this mitigation approach.