Assessment of the petroleum vapour intrusion pathway: vertical screening distances for UK sites

Site screening at petroleum vapour intrusion (PVI) sites involves an initial assessment of vertical (and lateral) separation distances between a vapour source in soil or groundwater, and current or future building foundations. For sites with petroleum underground storage tanks, vertical screening distances of approximately 2 and 5 m have been recommended for use in the assessment of the PVI pathway based on empirical soil gas and groundwater data collected in the US and Australia. Where the screening distance is met or exceeded, the risk posed via the PVI pathway may be screened out as part of a site-specific risk assessment. No comparable empirical dataset existed in the UK, where the unsaturated zone at petroleum hydrocarbon sites almost always includes a greater depth of heterogenous Made Ground, due to a long history of site development. This study compiled a first database of coupled groundwater–soil-gas field data from UK sites impacted by petroleum hydrocarbons. Using similar assessment techniques to those applied in the US and Australian studies, the UK dataset was evaluated using the ‘Vertical Distance Method’. Two vertical screening distances suitable for screening BTEX (benzene, toluene, ethylbenzene, and xylenes) compounds and light-end aliphatic compounds were derived, which are consistent with the other international studies: • 2 m for sites where only dissolved phase hydrocarbon is present; and • 5 m for sites where LNAPL is present.