The Representativeness of Subslab Soil Gas Collection as Effected by Probe Construction and Sampling Methods

Subslab soil gas (SSSG) samples were collected as part of an investigation to evaluate vapor intrusion (VI) into a building. The June 2015 Office of Solid Waste and Emergency Response (OSWER) VI Guide (U.S. Environmental Protection Agency [U.S. EPA] 2015) does not provide specific, detailed recommendations regarding how to collect SSSG samples. The data collected in this study will be used to provide input into future OSWER VI Guidance documents on SSSG sample collection. To this end, three different types of subslab sampling ports were constructed with various sampling techniques within a hexagon‐shaped grid in near proximity to each other. Conventional‐, Vapor Pin‐, and California‐style ports were established in duplicate for continual analysis by onsite gas chromatography‐electron capture detection (GC‐ECD). Triplicate ports were established to evaluate active and passive long‐term sampling methods to determine short range temporal differences. Active sampling methods included evacuated stainless‐steel canisters fitted with capillary flow controllers (Modified U.S. EPA Method TO‐15 [U.S. EPA 1999a]) and sorbent tubes collected using a syringe (Modified EPA TO‐17 [U.S. EPA 1999b]). The Passive sampling method used was sorbent tube samples collected following the EPA TO‐17 sampling method (Modified). This study did not identify any systematic differences in sample results between conventional, Vapor Pin, and CA‐style probes for used in SSSG sampling. The decisions for site management would probably be the same for data from any subslab port style, active or passive sampling techniques over durations less than 2 weeks. This paper examines the influence of subslab soil gas (SSSG) probe construction and sampling method on the concentrations of volatile organic compounds (VOCs) in SSSG samples collected as part of vapor intrusion (VI) investigations. SSSG sampling is one of the primary lines of evidence used in VI assessments (U.S. EPA 2015). As part of the study, several techniques for constructing probes and collecting SSSG samples were compared using a dataset obtained in an industrial warehouse‐type building in coastal Virginia in which elevated VOC concentrations were previously identified.