Identification of Alternative Vapor Intrusion Pathways Using Controlled Pressure Testing, Soil Gas Monitoring, and...

Area of Science:

• Environmental Science
• Geology
• Public Health

Background:

• The conventional vapor intrusion (VI) conceptual model assumes upward diffusion through soil.
• Alternative VI pathways, including sewers and drains, can contribute significantly, even beyond contamination zones.
• Identifying these alternative pathways is critical for accurate risk assessment.

Purpose of the Study:

• To demonstrate the utility of controlled-pressure-method testing (CPM) for identifying alternative VI pathways.
• To illustrate how CPM, soil gas sampling, and emissions calculations can detect previously unrecognized VI contributors.
• To show that alternative pathways may be significant but undetectable under natural conditions.

Main Methods:

• Controlled-pressure-method testing (CPM) to manipulate soil gas flow.
• Soil gas sampling to analyze vapor composition and movement.
• Screening-level emissions calculations to estimate vapor transfer rates.
• Comparison of data under natural and CPM conditions.

Main Results:

• A significant alternative VI pathway was identified at a study house using CPM.
• This pathway was undetectable under natural conditions but clearly identified during CPM.
• Measured emission rates during CPM were two orders of magnitude higher than screening estimates.
• Subfoundation soil gas profiles shifted, deviating from the conventional VI model under CPM.

Conclusions:

• CPM is effective in identifying significant alternative vapor intrusion pathways.
• Alternative pathways can be active and impactful even when not apparent under natural conditions.
• Conventional sampling may fail to detect these alternative VI routes, necessitating advanced testing methods.