Numerical study of the building pressure cycling method for evaluating vapor intrusion from groundwater contamination.

Vapor intrusion (VI) risk assessments determine the cleanup level of groundwater in the absence of ingestion. In recent VI investigations, the building pressure cycling (BPC) method has been applied to help minimize ambiguity caused by temporal variability of indoor air samples that are important to risk assessments, and, consequently, determine groundwater cleanup level accurately. In this study, we use a three-dimensional numerical model to examine the dynamic migration of VOCs from groundwater after the application of BPC. First, we validated the numerical model with field measurements. Then, the verified model is used to investigate the effects of site-specific features in determining the performance of BPC operation. At last, we summarize past field applications of BPC to examine the simulated results. Our study indicates that the BPC-induced indoor depressurization can increase the building loading rate in the first 2-3 h, which would then drop to 2-3 times of that with natural conditions in most cases of groundwater contamination. In some cases involving a strong source, e.g., a vapor source above the capillary fringe or a groundwater source with sandy soil above the groundwater level, the normalized building loading rates can be maintained as high as 4-9 without decrease after the first 2-3 h. Significantly higher increase in building loading rate may indicate a potential presence of a preferential pathway between the groundwater contamination and concerned building.