Impact of drainage pipe vapor intrusion pathways on sub-slab depressurization system mitigation performance

Sub-slab depressurization (SSD) systems are the most commonly employed technology for mitigating vapor intrusion (VI) impact to indoor air. However, as cases involving preferential VI pathways (e.g. sewers, land drains) are being reported, the efficacy of SSD systems to reduce VI impacts from such pathways is being questioned. In this study, field test and numerical modeling were performed to evaluate the effectiveness of an SSD system for the case of VI involving a drainage pipe preferential pathway. Field tests investigated the SSD performance at the study house under conditions with and without the impact of a drainage pipe pathway. A numerical model was developed to explore the influence of the preferential pathway on SSD mitigation under different scenarios. Both field and simulation results indicated that the sub-slab drainage system inhibits the performance of the SSD, consequently requiring a greater level of flow and depressurization for the system to protect the building. In addition, the SSD caused higher sub-slab vapor concentrations than under natural condition in both field testing and simulation results, which raised concerns for potential VI risk under standard SSD operation. Moreover, simulation results suggested the location of preferential pathways can affect the performance of an SSD system.