Estimation of contaminant subslab concentration in petroleum vapor intrusion

Abstract

In this study, the development and partial validation are presented for an analytical approximation method for prediction of subslab contaminant concentrations in PVI. The method involves combining an analytic approximation to soil vapor transport with a piecewise first-order biodegradation model (together called the analytic approximation method, including biodegradation, AAMB), the result of which calculation provides an estimate of contaminant subslab concentrations, independent of building operation conditions. Comparisons with three-dimensional (3-D) simulations and another PVI screening tool, BioVapor, show that the AAMB is suitable for application in a scenario involving a building with an impermeable foundation surrounded by open ground surface, where the atmosphere is regarded as the primary oxygen source. Predictions from the AAMB can be used to determine the required vertical source-building separation, given a subslab screening concentration, allowing identification of buildings at risk for PVI. This equation shows that the “vertical screening distance” suggested by U.S. EPA is sufficient in most cases, as long as the total petroleum hydrocarbon (TPH) soil gas concentration at the vapor source does not exceed 50–100 mg/L. When the TPH soil gas concentration of the vapor source approaches a typical limit, i.e. 400 mg/L, the “vertical screening distance” required would be much greater.

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.…”

A Petroleum Vapor Intrusion Model Involving Upward Advective Soil Gas Flow Due to Methane Generation

“…According to the development of the model, the increase of the moisture content can decrease the effective diffusivities of soil gas and the upward advection, but increasing the biodegradation rate. In sum, a higher moisture content would lead to lower source-to-indoor air concentration attenuation factors with current conditions, similar to models based on diffusion dominated transport (20–22, 42). Finally, it should be noted the influence of indoor air exchange rate is not significant here, differently from the sensitivity analysis results of the Johnson-Ettinger model in Johnston et al (40–41), where different variations of indoor air exchange rate were employed.…”

“…As employed in previous models (9, 20–22, 24), a piecewise aerobic biodegradation is assumed here, shown as equation (T3) in Table 1, which means that a first-order reaction occurs if the oxygen concentration is larger than 1% v/v, otherwise, there is no reaction. The boundary conditions for contaminants and oxygen are shown in equation (T4) in Table 1.…”

An Excel®‐Based Visualization Tool of Two‐Dimensional Soil Gas Concentration Profiles in Petroleum Vapor Intrusion

“…In recent decades, there have been proposed numerous one‐dimensional (1D) analytical models (e.g. DeVaull ; Davis et al ; Verginelli and Baciocchi ; Yao et al ) and multidimensional numerical models (e.g. Abreu and Johnson ; Abreu et al ; Hers et al ) for estimating the migration of subsurface petroleum vapors into potentially impacted buildings.…”