Vapor Intrusion Screening at Petroleum UST Sites

Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening-level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil-gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16-year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil-gas screening levels of 30, 50, and 100 mu g/m3 and nonparametric Kaplan-Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are 30 mu g/m3 at any distance above a dissolved-phase hydrocarbon source. Dissolved-phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous-phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are 30 mu g/m3 for vertical screening distances of 13ft (4m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100ft (10 to 30m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.