API PUBL 4775:2009

Subsurface migration of volatile compounds and vapor intrusion to indoor air is a potential exposure pathway for human occupants of buildings over or near contaminated soils and groundwater. In the past decade, there has been a significant increase in attention to vapor intrusion issues and several new regulatory guidance documents have been developed by State and Federal agencies for assessment and management of vapor intrusion risks. These guidance documents generally provide a framework for screening sites to assess whether vapor intrusion poses no significant risk or may require further evaluation, including assessment, remediation, or exposure controls. Most regulatory guidance documents use conservative assumptions to account for uncertainties in the screening process. This results in decisions to further evaluate sites more frequently than may actually be necessary. It is expected that screening procedures will become less conservative as we learn more about the processes affecting vapor intrusion. To date, most vapor intrusion screening procedures either assume that biodegradation does not occur, or allow for an arbitrary 10-fold reduction in the predicted indoor air concentration for petroleum hydrocarbons. Many petroleum hydrocarbons are metabolized by ubiquitous, naturally occurring soil microbes provided that sufficient oxygen is present in the subsurface. Several modeling studies and empirical data reviews have shown that aerobic biodegradation in the unsaturated zone can significantly attenuate vapor intrusion of petroleum hydrocarbons in some settings (i.e. DeVaull, 2007; Abreu and Johnson, 2006, Roggemans, et al. 2001). For example, the Abreu and Johnson (2006) study showed significant reduction in vapor intrusion for deeper and weaker sources and little to no reduction for shallower and stronger sources. This work builds on the Abreu and Johnson (2006) study by focusing specifically on low-concentration petroleum hydrocarbon vapor sources, as this may be a common case for buildings down-gradient of petroleum source zones and overlying dissolved petroleum hydrocarbon plumes. Simulations were performed using the three-dimensional mathematical model developed by Abreu and Johnson (2006, 2005) for a range of scenarios to develop relationships between the site-specific conditions and the vapor intrusion attenuation factor a, which is defined as the ratio of the indoor air concentration of a chemical divided by its subsurface vapor source concentration. This mathematical modeling study is intended to provide insight to the significance of bio-attenuation for a wide range of scenarios. It is anticipated that the results may be used to revise regulatory guidance for assessing vapor intrusion to buildings at dissolved petroleum hydrocarbonimpacted sites.