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

Abstract

The key role of biodegradation in attenuating the migration of petroleum hydrocarbon vapors into the indoor environments has been deeply investigated in the last decades. Very recently, empirical screening levels for the separation distance from the source, above which the potential for vapor intrusion can be considered negligible, were defined. In this paper, an analytical solution that allows one to predict risk-based vertical screening distances for hydrocarbons compounds is presented. The proposed solution relies on a 1-D vapor intrusion model that incorporates a piecewise first-order aerobic biodegradation limited by oxygen availability and accounts also for the effect of the building footprint. The model predictions are shown to be consistent with the results obtained using a 3-D numerical model and with the empirical screening criteria defined by U.S.EPA and CRC care. However, the different simulations carried out show that in some specific cases (e.g., large building footprint, high methane concentration, and low attenuation in the capillary fringe), the respect of these empirical screening criteria could be insufficient to guarantee soil-gas concentrations below acceptable risk-based levels.

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

“…Based on the evidences provided by the above-mentioned empirical studies, U.S.EPA (2015a) suggested that additional investigation may be not necessary when the source to building vertical separation distance is greater than 1.8 m (6 ft) for dissolved contamination or 4.6 m (15 ft) for light non-aqueous phase liquid (LNAPL). Furthermore, results from numerical (Hers et al, 2000;Abreu and Johnson, 2006;Abreu et al, 2009;Knight and Davis, 2013;Hers et al, 2014) and analytical models (DeVaull, 2007;Yao et al, 2014;Verginelli and Baciocchi, 2014;Yao et al, 2016) were consistent with the empirical exclusion distance values reported above, showing that, in nearly all cases, a source to building vertical separation distance greater than 2 m or 5 m is sufficient to attenuate to acceptable risk-based levels petroleum hydrocarbon vapors from dissolved-phase or LNAPL sources, respectively.…”

“…The first-order steady-state reactive transport of petroleum vapor hydrocarbons can be described by a one-dimensional (1-D) diffusion differential equation with reaction terms (DeVaull, 2007;Verginelli and Baciocchi, 2014):…”

“…(11) and Eq. (13) a simple stoichiometric mass balance at the aerobic to anaerobic interface of oxygen migrating from the surface through the aerobic layer and the n degradable compounds diffusing from the source through the anaerobic layer can be written (Verginelli and Baciocchi, 2014):…”

“…λ N 0.05 h −1 ) (DeVaull, 2011). In Eq (21) C 0 * represents an acceptable reference concentration in the soil gas at the ground surface, which can be set equal to the threshold soil gas values reported by U.S. EPA (2013EPA ( , 2015a or estimated with the analytical solution extrapolated from the Johnson and Ettinger (1991) model (Verginelli and Baciocchi, 2014). Note that in Eq.…”

“…4b) for different source depths and three biodegradation rates representative of the values available in the literature (DeVaull, 2011). In the same figure the risk-based vertical exclusion distances predicted by the analytical model proposed by Verginelli and Baciocchi (2014) as a function of the source concentration are also reported for comparison. The input parameters used for the considered contamination scenarios are reported in Table 1.…”