1. Vapor Intrusion Evaluation Strategy and Modeling Developments
Robert Ettinger
Geosyntec Consultants
California Industrial Hygiene Council
16th Annual Conference
San Diego, CA
December 4 – 6, 2006

2. Timeline for the Vapor Intrusion Pathway
Radon Intrusion & Vapor Diffusion studies
OSWER Draft Guidance
CA, NY, NJ State Guidance Development
Air-Superfund
Guidance
Draft RCRA EI
Supplemental
Guidance
MA State
Guidance
J&E Model
Revised OSWER Guidance
ASTM RBCA
Standard
Response to Comments
RCRA EI Guidance
1980’s
1989
1991
1992
1993
1994
1999
2001
2002
2004
2005
2007
Hillside School
Hill AFB
CDOT
Redfields
MEW
Endicott

3. Vapor Migration to Indoor Air - General Conceptual Model

4. Vapor Intrusion Evaluation Strategy
Utilize Tiered Approach
Data collection and analysis increase in higher tiers
Target Indoor Air Levels
Risk-based levels, PELs, background
Media Sampled & Locations
Groundwater, soil gas, indoor air
Near, next to, or beneath buildings
Other Data
Geologic characterization, building characteristics
Modeling Options
Empirical, screening level, site-specific
Corrective Action Selection

5. Target Indoor Air Levels
Typically, indoor air target levels based on risk-based concentrations developed using EPA risk methodology
Need to consider
Target risk level
Occupational standards
Background concentrations
Example Target Indoor Air Levels
Basis
Benzene
PCE
10-6 Risk
0.25
0.32
10-5 Risk
2.5
3.2
10-4 Risk 25 32
Background
3 - 5
1 - 5
PEL (8-hr TWA)
3200
170,000

6. Site Characterization
What data are best to characterize vapor intrusion pathway?
Indoor Air
Groundwater
Soil
Soil Gas

7. Indoor Air Sampling
Indoor air sampling may seem to be a direct assessment approach, but is typically conducted during higher tier of investigation
Several challenges to indoor air sampling
Occupant disruption
Temporal and spatial variability
Background effects
May more practical to collect indoor air samples in occupational setting
Indoor air sampling guidance
Sample collection techniques
Analytical methods
Building survey examples

8. Source Characterization
Groundwater
Henry’s Law to evaluate partitioning
Mass transport limitations due to vertical concentration gradients in saturated zone
Soil
Gas-water and water-solid partitioning
Uncertainty in accuracy of partitioning equation
Soil Gas
Soil gas results can resolve uncertainty associated with groundwater or soil data
Typically provide better source characterization for vapor intrusion pathway.

9. Soil Gas Sample Location
Current regulatory focus on appropriate sampling locations
Near source
Exterior to building
Sub-slab
Soil gas profile may be affected by building
More significant for biodegradable compounds

10. Soil Gas Sampling
Soil gas sampling methods not as uniform as groundwater sampling methods, but approaches to meet investigation data quality objectives are available

11. Sub-Slab Soil Gas Sampling
Requires building access
Methods developed to limit intrusiveness
(DiGiulio, 2004)

12. Vapor Intrusion Modeling
Mixing in Breathing Zone
Convective Transport into Bldg
Diffusive Transport to Breathing Zone
Impacted Soil and/or Groundwater in Equilibrium with Soil Gas
Risk is proportional to (a) x (Csoil gas)

13. Empirical Attenuation Factor
(Dawson, 2004)

14. Johnson-Ettinger (1991) Attenuation Factor
Primary Parameters
Deff = Effective diffusion coefficient
LT = Depth to source
AB = Building area in contact with soil
QB = Building ventilation rate
Qsoil = Soil gas convection rate
Dcrack = Eff. diff. coeff. through cracks
Lcrack = Crack thickness
h = Building crack factor
Secondary Parameters
Deff = fn(H, Dwater, Dair, qT, qw) for each layer
LT = S(Li)
Qsoil = fn(k, DP, rcrack, zcrack, xcrack)
>30 inputs, but only a handful that are really sensitive
J & E Model has dozens of input parameters, how much data is required to use?

15. Common Screening Model Assumptions
One-dimensional vertical transport
Steady state conditions
No preferential pathways
Uniform mixing within building
Slab on grade or basement construction
No biodegradation
Homogeneous vadose-zone
Constant source concentration
No gas generation (e.g., municipal waste)
No barometric pumping
Prior to using model results, you need to ensure that model assumptions and site conditions are consistent

16. Constrained Model Use
Many problems with vapor intrusion modeling associated with improper inputs
Updated EPA spreadsheets will limit values allowed for inputs
Constraints based on Johnson, 2002

17. Biodegradation Modeling
Methods to model vadose zone biodegradation have been developed
Johnson et. al., 1999 – Dominant Layer Model
Abreu and Johnson, 2004 – 3D Numerical Model
Typically, additional site investigation data will be necessary to conduct biodegradation modeling
Soil gas concentration profile data
Analysis of biodegradation indicators (O2 , CO2)
Tracer compounds
Consider use of soil vapor profile data

18. Screening Level Biodegradation Model (Johnson, et al., 1999)
Mixing in Breathing Zone
Diffusive Transport
Partitioning
Convective Transport into Building
Biodegradation Zone
VOCs
Source
Requires additional data collection for bio indicators
Calibrate model with site soil gas data to determine biodegradation parameters
Reduce a by factor of 10 – 1000

19. Three-Dimensional Numerical Model (Abreu and Johnson, 2005)
Model Description
3-D vadose zone F&T model
Evaluate building type, source scenarios, and biodegradation kinetics
Model Results
Impact of biodegradation
Significance of lateral migration

20. Investigation Approach for Complex Sites
Soil surface
CO2
VOCs O2
Soil gas profile sampling points
Soil gas profile data recommended to assess biodegradation
Biodegradation significantly affects petroleum compound vapor migration
No common approach to use soil gas profile data to quantitatively evaluate vapor intrusion pathway

21. Soil Gas Profile Data
Soil gas profile underneath building may be different than that outside building footprint.
May need to assess potential exposure scenarios
Evaluate soil gas data to address uncertainty in sub-surface transport (diffusion and biodegradation)
Reassess vapor intrusion evaluation from subsurface source (include convection and ventilation effects)
Soil gas samples

22. Example Modeling Results

23. Institutional Controls Engineering Controls
Choice of Remedy
Active Remediation
Institutional Controls
Engineering Controls
“Radon System”
HVAC Modifications
Sealing
Filtration
Building Design (Brownfields)

24. Mitigation Options: Radon Sump
Cheap and reliable. $2K per system lasts a decade.

25. Summary
Selection of appropriate target levels is key factor in vapor intrusion assessment.
Site investigation methods require careful planning.
When modeling, assess whether site conditions are consistent with conceptual model assumptions and input parameters are reasonable.
Corrective action planning may reduce scope of vapor intrusion investigation.
Consider multiple lines of evidence to support conclusions. A balance of modeling and monitoring is typically appropriate.