ASSESSING THE SIGNIFICANCE OF SUBSURFACE CONTAMINANT VAPOR MIGRATION TO ENCLOSED SPACES - SITE-SPECIFIC ALTERNATIVE TO GENERIC ESTIMATES
American Petroleum Institute
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. Data requirements, presentation and interpretations are discussed and illustrated for each option.
| Document Type | Standard |
| Status | Current |
| Publisher | American Petroleum Institute |
