Estimation of the Number of Subslab Soil Gas Samples to Collect to Characterize Vapor Intrusion Under a Large Building

Upward migration of vapors from subsurface contamination into overlying buildings is known as vapor intrusion (VI) and can result in exposure of the building’s inhabitants to contaminants that can cause detrimental health effects. Multiple lines of evidence (MLEs), such as groundwater, soil, soil gas, and indoor air volatile organic compound (VOC) concentrations, are used to evaluate a building for VI and potential risk of occupant exposure. Background sources of contaminants contained within a building can result in a false positive determination of VI and installation of mitigation systems that are not needed. To avoid a false positive determination, some VI guidance documents recommend the prioritization of subslab soil gas (SSSG) concentrations over indoor air concentrations for determination of a VI issue. If the SSSG VOC concentrations are above a determined concentration, then VI is assumed to be possible; depending upon the concentration, immediate mitigation may be required. The major challenge to characterizing VI potential is the number of samples needed to confidently assess VI exposures due to the extreme variability in vapor concentrations across both time and space, and this study explores variability in SSSG and how many SSSG samples are needed. To address this issue, SSSG samples were collected between December 2020 and April 2022 from six commercial buildings in Fairbanks, Alaska, and between May 2019 and June 2021 from a large, compartmentalized warehouse at a coastal site in Virginia. Types of samples collected included indoor air; outdoor air; SSSG; soil gas; radon; differential pressure; indoor and outdoor temperature; heating, ventilation, and air conditioning (HVAC) parameters; and other environmental factors. To illustrate how these results can inform estimates of expected SSSG variability and thus the number of samples required to characterize variability, the temporal and spatial variabilities of the results observed at the test sites were used as a “similar population” to estimate necessary sample sizes for characterization of VI levels and to explore how temporal and spatial factors may influence estimates. The estimated SSSG sample requirement ranged from 1 to 80 samples and thus showed the substantial sensitivity of the systematic project planning equation to cases in which the action level and the average concentration are similar. We recommend that the estimated number of samples generated from the collected data for the buildings should only be used as a starting point for planning purposes. The number of SSSG samples to initially collect at a large building to characterize VI can be calculated, but the actual number should include adjustments for features of a building (e.g., past usage, separate foundations, and footers) and conditions at a site (e.g., proximity to source and depth to groundwater) that may alter the required number of SSSG samples.