Indoor Air as a Source of VOC Contamination in Shallow Soils Below Buildings

Abstract

US EPA and many state regulatory agency guidance documents recommend belowfoundation vapor sampling as a key element of site investigations to determine if vapor migration from underlying soil into buildings is a completed exposure pathway (USEPA, 2002; WIDHFS, 2003; San Diego County, 2004; PADEP, 2004). If volatile organic compounds (VOCs) are detected below the building foundation, then VOC migration from the subsurface is assumed to be occurring, and further investigation is needed to determine the extent of the VOC impact. These guidance documents are predicated on the assumption that VOCs detected in below-foundation samples have originated from deeper within the subsurface. However, detection of VOCs in below-foundation vapor samples alone is not sufficient to conclude that the VOCs are migrating from the subsurface upward towards a building. VOCs detected in below-foundation vapor samples can originate from indoor sources, migrating down through the slab by diffusion or advection. Commonly referenced conceptual models for vapor intrusion address VOC migration from the subsurface into buildings but do not consider the potential for VOC migration from buildings into the subsurface (USEPA, 2002;Johnson and Ettinger, 1991;Parker, 2003). The advective and diffusive forces that lead to the migration of VOCs from the subsurface into buildings are equally likely to result in the migration of VOCs from buildings into the subsurface when pressure or concentration gradients support such migration. In this paper we present: i) pressure gradient measurements indicating bidirectional advective flow across building foundations, ii) simple modeling indicating that indoor sources of VOCs may cause subsurface impacts through advection across the building foundation, and iii) field data from a site where indoor sources rather than subsurface contamination were the source of VOCs detected in below-foundation vapor samples.

Evaluation of Vapor Attenuation at Petroleum Hydrocarbon Sites: Considerations for Site Screening and Investigation

“…In addition to wind-driven oxygen transport, in buildings with continuous or transient positive pressure conditions, air will flow from the building to the shallow soils through any foundation cracks or other penetrations, providing an additional source of oxygen below the foundation (McHugh et al, 2006). Many commercial buildings are maintained at a positive pressure relative to the atmosphere by the HVAC system, and passively ventilated buildings (e.g., typical single-family residences) typically fluctuate between positive and negative pressure due to wind effects and other transient conditions.…”

Development and Application of a Three-Dimensional Finite Element Vapor Intrusion Model

“…The other portion of the diffusing contaminant enters the atmosphere from the open surface of the ground surrounding the structure. It should be noted that for the scenarios presented herein, a negative pressure differential was assumed to control advection into the building; however, positive pressure differentials can also occur and result in indoor air entering the subsurface 28. The model presented herein is appropriate for both positive and negative disturbance pressures.…”

Exposure Assessment Modeling for Volatiles—Towards an Australian Indoor Vapor Intrusion Model

“…Ririe et al (2002) observed similar outcomes in an examination of field data against the Orange County Health Care Agency (OCHCA) vapor diffusion model. Evaluation of related models consistently demonstrates variability and uncertainty (Johnson, 2002;Hers et al, 2002) and identifies the importance of processes such as biodegradation (Johnson et al, 1999;Hers at al., 2000;Lahvis, 2005) particularly aerobic degradation and soil oxygen levels (Abreu & Johnson 2006), advection , pressure differentials (Hers et al, 2000;Folkes & Kurtz, 2002;McHugh et al, 2006), and building factors (Fugler & Adomait, 1997;McHugh et al, 2004b). Validation undertaken to assess JEM constructs has been limited to partial field validation exercises or assessment of empirical data on soil gas and indoor air levels of volatile hydrocarbons.…”

“…Increasing attention to the development of strategies for subsurface measurement of soil gas was also undertaken to facilitate assessment of indoor air quality (Wong et al, 2003), while indoor air measurement has been highlighted as problematic due to complex site-specific behaviors exhibiting spatial and temporal variations complicating interpretation (Hers et al, 2001). Recent research also highlighted the potential for VOC migration from the indoor environment to subsurface due to changes in pressure gradients resulting in confounding of source apportionment of measured subsurface VOCs (McHugh et al, 2006).…”