Assessing the Significance of Subsurface Contaminant Vapor Migration to Enclosed Spaces

Site-Specific Alternative to Generic Estimates

API Publication 4674
December 1998


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The move toward more structured risk-based corrective action (RBCA) approaches has led to an interest in better understanding vapor migration to enclosed spaces. The significance of this pathway is currently the subject of intense debate, with many believing that existing non-site-specific risk-based ("generic" or "Tier 1 ") screening levels are too conservative. As little data are available to justify generic approaches, this pathway must be addressed on a more site-specific basis. This document provides options for addressing the vapor migration pathway on a more site-specific basis.

Vapors in enclosed spaces pose two levels of concern. First, enclosed-space vapors may be found at concentrations near those that pose immediate flammability and/or health risks. These sites warrant immediate attention and response as required by most state and federal regulatory guidance. In the second class of sites, concentrations are lower and the concern is for longer term health risks. This report focuses exclusively on this second class of sites, where advection and diffusion occur through a soil layer and into an enclosed space and time is available to adequately address the problem on a site-specific basis.

The options considered in this document for refining generic vapor migration calculations and assessing the significance of this pathway on a more site-specific basis include:

• direct measurement through sampling of enclosed-space vapors,
  
• 
  
• use of near-foundation or near-surface soil gas sampling results,
  
  
• use of site-specific homogeneous and layered soil diffusion coefficient estimates in generic algorithms, and
  
  
• assessment of bio-attenuation potential.

Data requirements, data presentation, and data interpretation are discussed and illustrated for each option. As discussed in the document, it is envisioned that options (c) and (d) will be used much more often than options (a) and (b) for the assessment of longer term impacts, due to a variety of technical and practical considerations. Also, a vision for a simpler site-specific assessment approach is presented and accompanied by a discussion of the steps necessary to progress toward that goal. This improved approach considers bio-attenuation, but requires only soil moisture content measurements, or alternatively, in-situ diffusion coefficient measurements, to screen sites.