At sites where soils or groundwater contain volatile and/or semi-volatile contaminants (semi-volatile: 0.04 < H < 4 · 10-6, where H = dimensionless Henry coefficient), there is the potential that contaminant vapors may migrate from the subsurface into overlaying buildings. Beside near-term safety hazards (e.g. explosions, acute health effects, aesthetic problems), which can be observed in the indoor air under worst-case conditions, most frequently contaminant concentrations are relatively low. However, an assessment is required to determine whether there is an unacceptable of health effects.
The subsurface to indoor air pathway represents a specific pathway of exposure in the human health risk assessment. The same definitions are valid as established in "Risk Assessment: Receptor Human Health".
The subsurface to indoor air pathway represents a specific pathway of exposure in the human health risk assessment. Receptor identification and characterization takes place according to the principles described in "Risk Assessment: Receptor Human Health".
An assessment of the subsurface to indoor air pathway considers a contaminant source located in some distance below floor of an enclosed building constructed with a basement or constructed slab-on-grade. The source is ether a soil-incorporated volatile/semi-volatile contaminant or a volatile/semi-volatile contaminant in groundwater. Driven by molecular diffusion the volatilized contaminant migrates from the top boundary of the source towards the soil surface, where it reaches the zone of influence of an overlaying building. There the contaminant vapor enters into the indoor air through cracks between the foundation and the basement slab floor or within the slab floor structure. This sweep effect is induced by convective air movement due to a pressure differential between the soil surface and the building. The negative pressure with respect to the soil surface within the structure is generated by stack effects because of heating of the interior air, and unbalanced mechanical ventilation.
Data required for the calculation of vapor intrusion into buildings can be obtained from the following sources:
American Petroleum Institute (1998):
Assessing the Significance of
Subsurface Contaminant Vapor Migration to Enclosed Spaces, Site-Specific
Alternatives to Generic Estimates. API Publication No. 4674. American
American Petroleum Institute. (2002):
Practical Guidance for Assessing the “Groundwater to Indoor Air” Vapor Migration Pathway at Petroleum Hydrocarbon Sites.
Ettinger, R. (2002):
Vapor Intrusion Modeling: Theory and Implications. Indoor Air Session, Technical Support Project Meeting.
Fischer, M. L., A. J. Bentley, K. A. Dunkin, A. T. Hodgson, W. W. Nazaroff, R. G. Sexto, and J. M. Daisey (1996):
Factors affecting indoor air concentrations of volatile organic compounds at a site of subsurface gasoline contamination. Environ. Sci. Technol., 30(10):2948-2957.
Johnson, P. C., R. A. Ettinger, J. Kurtz, R. Bryan , and J. E. Kester. (2002):
Migration of Soil Gas Vapors to Indoor Air: Determining
Vapor Attenuation Factors Using a Screening-Level Model and Field Data from the
Draft of the Guidance for Evaluating The
Vapor Intrusion to Indoor Air Pathway from Groundwater Soils (Subsurface Vapor
Intrusion Guidance). Office of Solid Waste and Emergency
User's Guide for Evaluating
Subsurface Vapor Intrusion into Buildings. Office of Emergency and Remedial