API 4674 : 1998

Executive Summary
1.0 Introduction
2.0 Current Approaches for the Development of
      Generic RBSLs
3.0 Key Technical Considerations
4.0 Site-Specific Assessment of the Significance
      of Vapor Migration to Enclosed Spaces
      4.1 Direct Measurement of Enclosed-Space
            Vapor Concentrations
      4.2 Use of Soil Gas Samples Collected Near-Surface
            or Near Foundation
      4.3 Use of Site-Specific Diffusion Coefficients in
            Generic RBSL Algorithms
      4.4 Use and Interpretation of Soil Gas Data with
            Depth
      4.5 Accounting for Attenuation Due to Biodegradation
      4.6 Other Refinements
5.0 An Opportunity for the Future
6.0 References
List of Figures
1 Schematic of vapor migration scenario and sampling
      options
2 Johnson and Ettinger (1991) site-specific vapor
      attenuation coefficient a=(C indoor/C source) estimate
      as a function of the overall effective vapor-phase
      porous media diffusion coefficient DT eff and distance
      between the source and foundation LT
3 Estimated time for non-retarded chemicals to reach
      near steady vapor concentrations (Tss/Rv) at the
      distance L from a source. For retarded compounds
      multiply the (Tss/Rv) value by the retardation factor
      Rv defined in Equation (4)
4 Sample presentation using data from a) BP (1997) and
      b) Fischer et al. (1996)
5 Vapor concentration data compared with predictions
      for one-dimensional transport through a layered system
      without degradation, using data from a) BP (1997) and
      b) Fischer et al. (1996)
6 Normalized hydrocarbon and oxygen soil gas
      concentrations in a shallow near-homogeneous setting:
      data from Ostendorf and Kampbell (1991). Lines show
      expected concentration profiles in homogeneous settings
      at near steady conditions for no degradation, and
      first-order degradation
7 Predicted vapor concentration profiles for a
      homogeneous system at steady-state with a first-order
      reaction using Equation (8)
8 Attenuation coefficient predicted by Equation (10) for
      the case of a homogeneous medium at steady-state with
      a first-order degradation reaction
9 Schematic of dominant layer model bio-attenuation
      scenario
10 Comparison of dominant layer model with data from
      Fischer et al. (1996)
11 Hypothetical plot showing conditions necessary for
      significant bio-attenuation
List of Tables
1 Refinement options and associated data collection and
      analysis needs
2 Sample use of field data (data from BP 1997) to
      determine site-specific effective vapor-phase diffusion
      coefficients
3 Inputs used in generating Figure 10 using the dominant
      layer model

This standard covers the options of refining generic vapor migration calculations and assessing the significance of this pathway on a more site-specific basis, being; assessment of bio-attenuation potential, 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, direct measurement through sampling of enclosed-space vapors.