Assessment of benzene and total petroleum hydrocarbons as risk drivers for petroleum vapor intrusion from LNAPL sources to indoor air

Petroleum vapor intrusion (PVI) is commonly evaluated by assuming benzene as the primary risk driver. This approach, however, does not reflect the complexity of LNAPL mixtures, whose constituents exhibit different physicochemical properties and biodegradation behavior during subsurface transport. To address this limitation, some studies have proposed TPH-to-benzene ratios to identify conditions under which non-benzene hydrocarbons may dominate PVI risk. These ratios, however, were derived assuming a constant mixture composition during subsurface transport. In this study, a two-dimensional numerical model was developed to assess the evolving composition of LNAPL-derived vapors in the subsurface and the relative contribution of benzene and other TPH to PVI risk. The model accounts for solubilization and volatilization from the LNAPL source and simulates diffusive transport and aerobic biodegradation in the vadose zone. The model was applied to gasoline and diesel contamination scenarios considering typical fuel compositions. Vapor intrusion risk was then quantified by introducing a compound-specific Risk Index that allows for a direct comparison between carcinogenic and non-carcinogenic effects. A sensitivity analysis was conducted to identify conditions under which non-benzene compounds become the dominant PVI risk drivers. This was achieved by varying the benzene mass fraction in the LNAPL source and identifying the conditions under which the cumulative Risk Index of TPH equals that of benzene. Results show that critical TPH-to-benzene ratios are strongly dependent on the environmental matrix, biodegradation and transport distance. Ratios may differ by several orders of magnitude among environmental matrices, while indoor-air thresholds remain comparatively stable across scenarios. Application of these ratios to the USEPA PVI database suggests that non-benzene petroleum hydrocarbons may represent the primary vapor intrusion risk drivers in approximately 10-60%% of observations, depending on the critical ratio applied. These results underscore the importance of accounting for the full petroleum hydrocarbon mixture to accurately characterize vapor intrusion risks.