Vapor Intrusion Screening at Petroleum UST Sites

Abstract

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 µg/m3 and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/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 µg/m3 for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) 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.

Numerical Model Investigation for Potential Methane Explosion and Benzene Vapor Intrusion Associated with High-Ethanol Blend Releases

“…40 This concentration falls within the Table 1. Simulated Scenarios for Table 2 and Figures 1−7 in the Main Text and Tables S2 and S3 and Figures S2−S8 in the SI CH 4 source concentration TPH source concentration benzene source concentration source gas pressure separation distance Table 2 75% 200 g/m 3 NA a 0 to 200 Pa 3 m Figure 2 0.015% to 75% 200 g/m 3 NA 0 Pa 1, 3, 6, and 13 m Figure 3 0.015% to 75% 200 g/m 3 NA 0 Pa 6 m Figure 4 75% 200 g/m 3 NA 0.1 to 200 Pa 1, 3, 6, and 13 m Figure 5 0.076% to 75% NA 0.1 g/m 3 0 Pa 1, 3, 6, and 13 m Figure 6 0.015% to 75% NA 0.1 g/m 3 0 Pa 6 m Figure 7 0.015% to 75% NA 0.1 g/m 3 0 Pa 6 m SI Table S2 75% 200 g/m 3 NA 0 to 200 Pa 1, 6, and 13 m SI Table S3 75% 200 g/m 3 NA 0 to 200 Pa 1, 6, and 13 m SI Figure S2 0.015% to 75% 200 g/m 3 NA 0 Pa 1 m SI Figure S3 0.015% to 75% 200 g/m 3 NA 0 Pa 3 m SI Figure S4 75% 200 g/m 3 NA 0 to 200 Pa 3 m SI Figure S5 0% to 75% NA 0.1 g/m 3 0 Pa 6 m SI Figure S6 0% to 75% NA 0.1 g/m 3 0 Pa 6 m SI Figure S7 0% to 75% NA 0.1 g/m 3 0 Pa 6 m SI Figure top 25% of benzene vapor concentrations measured near various NAPL sources and complied.…”

“…As CH 4 source concentrations increase from 0 to 75% (v/v), benzene indoor concentrations increase by 15-, 1.0 × 10 4 -, 6.2 × 10 7 -, and 6.3 × 10 16 -fold for separation distances of 1, 3, 6, and 13 m, respectively. Although previous studies indicate a low benzene vapor intrusion potential when the separation distance is larger than 10 m, 32,39,40 our simulations infer that if very high CH 4 concentrations (e.g., 75% v/v) are generated and subsequently consumed in the vadose zone (resulting in O 2 depletion), even a separation distance of 13 m may result in some benzene vapor intrusion and possibly exceed the EPA indoor air screening level of 3.1 × 10 −5 g/m 3 that corresponds to a 10 −4 lifetime risk 48 ( Figure 5). Note that US EPA provides target indoor air concentrations for three different lifetime risk levels: 10 −4 , 10 −5 , and 10 −6 .…”

Vapor Intrusion Screening Model for the Evaluation of Risk-Based Vertical Exclusion Distances at Petroleum Contaminated Sites

“…In Figure 3a the vertical exclusion distances observed in more than one hundred sites and reported in the U.S. EPA database 47 are also reported. Making reference to this comparison, it can be noticed that all the field data fall below the line corresponding to the exclusion distances calculated by the model assuming a low biodegradation; most of them are also Figure 3b, where the model predictions are compared with the recent screening criteria defined through a statistical analysis of a large set of field data collected at petroleum hydrocarbon sites, by U.S. EPA, 43 CRC care, 41 and Lahvis et al 2 The obtained results suggest that the trend predicted by the model is in line with these empirical screening criteria. For example, Figure 3b shows that, considering a groundwater benzene source concentration of 0.5 mg/L and a moderate biodegradation rate, the vertical exclusion distance predicted by the model is 1.9 m against the 1.6 m vertical screening distance for dissolved phase sources set by U.S. EPA 43 for UST sites, using the "clean soil method" (CS).…”

“…40 Namely, a statistical analysis of large empirical soil vapor data sets allowed to estimate separation screening distances from the source, beyond which the potential for vapor intrusion can be considered negligible. 2 For instance, as reported in a recent CRC care report, 41 Davis et al 42 have analyzed more than 200 benzene and total petroleum hydrocarbon vapor samples estimating that 1.5 m (5 ft) and 10 m (30 ft) thickness of clean soil is sufficient to attenuate to nondetectable levels, petroleum hydrocarbon vapors from dissolved-phase and LNAPL sources, respectively. McHugh et al 1 proposed a separation distance of 3 m (10 ft) for petroleum vapors resulting from dissolved phase groundwater sources and a separation distance of 10 m (30 ft) for LNAPL vapor sources.…”

Role of the source to building lateral separation distance in petroleum vapor intrusion

“…The significance of biodegradation in reducing vapor concentrations towards potential receptors has been deeply documented in the recent guidelines released by U.S.EPA (2015a); ITRC (2014) and CRC care (2013) where it was clearly highlighted that biodegradation by ubiquitous soil microbes can attenuate petroleum vapors of several orders of magnitude within few meters of clean soil, provided that sufficient oxygen is available to sustain the aerobic reaction. Namely, based on a statistical analysis of large empirical soil vapor data sets, different works (Davis, 2009;Peargin and Kolhatkar, 2011;Wright, 2011;Lahvis et al, 2013) have recently estimated the vertical separation screening distances (i.e. the thickness of clean biologically active soil between the source and the overlying receptor) beyond which the potential for petroleum vapor intrusion can be considered negligible.…”