Vertical screening distances for total petroleum hydrocarbon for vapour intrusion risk assessment at petroleum underground storage tank sites

摘要

The US Environmental Protection Agency Office of Underground Storage Tanks (US EPA OUST) and Interstate Technology and Regulatory Council (ITRC) have recently published guidance indicating that buildings separated from light non-aqueous phase liquid (LNAPL) sources by more than 15 ft (4.6 m) are not a concern for vapour intrusion at petroleum underground storage tank (UST) sites. At this distance, there is a greater than 95% probability that soil gas concentrations for all key constituents of potential concern are below risk-based screening levels (RBSLs) between the LNAPL source and building foundation. Vertical screening distances for bulk Total Petroleum Hydrocarbon (TPH), common TPH fractions (C-5-C-8 aliphatics, C-9-C-12 aliphatics, and C-9-C-10 aromatics), and indicator compounds (n-hexane and naphthalene) are less certain, however, because of a general lack of empirical data and highly variable soil-gas RBSLs. This study addresses these gaps through a statistical analysis of hundreds of soil-gas samples collected above LNAPL sources at more than 30 petroleum UST release sites. Resulting vertical screening distances for the various TPH fractions range between 0 and 7 ft (2.1 m) depending primarily on soil-gas RBSLs, which vary by over 5 orders of magnitude. Vertical screening distances for naphthalene are generally 3 ft (0.91 m). Vertical screening distances for n-hexane range from 12 (3.7) to 16 ft (4.9 m) and vary geographically. Vertical screening distances for total (bulk) TPH generally exceed 15 ft (4.6 m). Petroleum vapour intrusion risk assessments involving total (bulk) TPH are not recommended, however, because of uncertainties in quantifying soil-gas and indoor-air concentrations and defining RBSLs for fuel mixtures with variable source composition, toxicity, and fate and transport properties. Overall, the findings of this study validate the 15-ft screening distance recommended by ITRC and US EPA for application at petroleum UST sites with LNAPL sources.