Subsurface Vapour Intrusion Assessment

Verginelli, Iason. "Petroleum Vapor Intrusion." Advances in the Characterisation and Remediation of Sites Contaminated with Petroleum Hydrocarbons. Cham: Springer International Publishing, 2023. 139-169.

Petroleum vapor intrusion (PVI) refers to the migration of volatile petroleum hydrocarbons from contaminated groundwater or geological materials through the vadose zone into overlying buildings. PVI poses significant environmental and health risks, particularly when hydrocarbons enter indoor air. The natural attenuation of petroleum hydrocarbons in the source zone and during vapor transport is a key process that reduces or eliminates PVI potential. Aerobic biodegradation in the presence of oxygen plays a critical role in mitigating this risk.

The concept of vertical exclusion distance criteria has been introduced in some regions to establish safe distances between petroleum-contaminated sites and buildings. When this separation is insufficient, additional assessments are needed to evaluate PVI risks. Methods such as modeling, soil gas sampling, and indoor air measurements are utilized to gather data for a comprehensive risk assessment. A multi-method approach, combining these techniques, enhances the reliability of PVI evaluations and risk estimations.

While PVI is more easily mitigated in the presence of biodegradable compounds, the approach to PVI management differs significantly from that of less biodegradable substances, such as chlorinated solvents. This case study highlights the importance of monitoring and mitigating PVI through an understanding of natural attenuation, the use of vertical exclusion distance criteria, and a robust assessment framework.

Guo, Yuanming, et al. Environmental science & technology 49.22 (2015): 13472-13482.

Vapor intrusion (VI) is a critical environmental issue where vapors from contaminated soil or groundwater migrate into buildings, often through diffusion and underpressurization-induced soil gas flow. Traditional models have guided VI pathway assessments, typically focusing on upward diffusion through soil. However, recent studies have revealed that alternative pathways, such as neighborhood sewers, land drains, and other underground piping, can also contribute significantly to VI, extending beyond the footprint of soil and groundwater contamination.

This case study demonstrates how controlled-pressure-method testing (CPM), in conjunction with soil gas sampling and screening-level emissions calculations, can identify these alternative VI pathways. Data from a long-term study house revealed that under natural conditions, an alternative VI pathway was undetectable, but under CPM conditions, it was clearly identifiable. The measured emission rates were found to be two orders of magnitude greater than predictions from screening models, while subfoundation vertical soil gas profiles deviated from conventional VI expectations.

The findings highlight the utility of combining CPM testing with traditional methods to provide a more comprehensive understanding of VI pathways. This approach can uncover previously undetected pathways, leading to more accurate assessments and better-informed mitigation strategies for addressing petroleum vapor intrusion.