1. General Principles for Hydrocarbon Vapor Intrusion
G. Todd Ririe BP
La Palma, CA
AEHS Fall PVI Workshop,
October 2011

2. EPA OUST Workgroup
First “white paper” or deliverable from the workgroup:
OUST recently finalized: “Petroleum Hydrocarbons And Chlorinated Hydrocarbons Differ In Their Potential For Vapor Intrusion”; which is now available on EPA's website at
OUST is also developing a compendium of information about petroleum vapor intrusion (PVI) which will be available in the next several weeks via OUST's home page at If you want more information about the petroleum hydrocarbons and chlorinated hydrocarbons document, the PVI compendium, or PVI in general, please contact Hal White of my staff at or
Carolyn Hoskinson, Director
EPA's Office of Underground Storage Tanks

4. With aerobic biodegradation in unsaturated soils, VOCs (red) degrade, carbon dioxide (green) is produced, and oxygen (blue) is consumed. The aerobic biodegradation zone extends over the area of active biodegradation. The source zone, which is anaerobic, is characterized by the maximum VOC concentrations and little biodegradation.

5. Clean Soil Model for HC Vapors
Reaction Zone
HC + O2 CO2 + H2O
Conceptual model for clean soils shows that above the hydrocarbon reaction zone there is no more hydrocarbon vapor migration if there is oxygen present. The key to identifying if sites are clean or dirty is to determine if there is sufficient oxygen present in the vadose zone over an interval of several feet.

6. Clean Soils vs Dirty Soils
Have oxygen concentrations above 3%
Have no residual petroleum hydrocarbons
PID values will be below 100 ppm
Usually have low methane and carbon dioxide values
Non-detect or very low petroleum hydrocarbon vapors
DIRTY SOILS
Have low oxygen concentrations (at or near 0%)
Can have residual petroleum hydrocarbons
PID reading at or above 100 ppm
Usually have high methane and carbon dioxide values
Elevated petroleum hydrocarbon vapors

7. Importance of Biodegradation of Petroleum Hydrocarbons
Recent publication covering effect of oxygen on biodegradation of petroleum hydrocarbons based on field work in Australia. 7

8. Oxygen versus Petroleum Hydrocarbon Plots
Notice that the when oxygen is present most of the petroleum hydrocarbon vapors are limited; however there were some outliers in the syringe data. However, once dedicated sample probes were installed (in situ data) the outliers disappeared. This is attributed to some leakage of oxygen into the samples collected in the field into syringes. Take away message is that once oxygen is present in measurable amounts the hydrocarbon vapors are no longer present.
Manual collected O2 data:
Leakage of O2 into syringe
In situ O2 data
From Davis et al., 2009 8

9. Is There Enough Oxygen? Aerobic Biodegradation
Hydrocarbon to Oxygen use ratio: 1 : 3 (kg/kg)
Atmospheric air (21% Oxygen; 275 g/m3 oxygen) provides the capacity to degrade 92 g/m3 hydrocarbon vapors (92,000,000 ug/m3)
Oxygen below a Foundation: can it get there?
Through the foundation
Cracks; concrete does have permeability to air
Around the foundation edges (bonus)
Oxygen has been found in sufficiently high quantities under most buildings
Large buildings or buildings built over dirty soils can be areas of low oxygen concentration
George DeVaull

10. Dirty Soil Model for HC Vapors
Conceptual model for clean soils shows that above the hydrocarbon reaction zone there is no more hydrocarbon vapor migration if there is oxygen present. The key to identifying if sites are clean or dirty is to determine if there is sufficient oxygen present in the vadose zone over an interval of several feet.

11. A very shallow or perched water table can bring contaminants (LNAPL or contaminated groundwater) into direct contact with a building foundation. Foundation cracks or basement drainage systems (e.g., a sump) can bring source materials into the interior space. Volatilization from these sources likely results in PVI.

12. PHC vapors migrate preferentially within the permeable backfill of a utility trench that intersects contamination. Vapors may migrate preferentially through the more permeable backfill (arrows); however, oxygen may also migrate more readily through these materials, allowing aerobic biodegradation to counter the preferential vapor migration.

13. Petroleum Vapor Intrusion: Petroleum Industry Experience
BUILDING 2
NAPL directly impacts building wall or floor. 3
Preferential pathway allows vapors to enter building.
Unsaturated Soil
NAPL
NAPL
Sump draws NAPL or dissolved hydrocarbons into building.
Affected GW 1 4
Vapors from NAPL diffuse through vadose zone.
NAPL
Based on industry experience, petroleum vapor intrusion impacts are generally associated with:
Direct NAPL impacts on a building foundation
NAPL or dissolved hydrocarbon impact on building sump
NAPL impact on preferential flow pathway or
Diffusion of vapors from subsurface NAPL source
Key Points:
Current USEPA VI guidance provides GW screening concentrations for benzene and other petroleum hydrocarbons in the low ug/L range (i.e., 5 ug/L for benzene). These low screening concentrations are not consistent with industry experience that vapor intrusion impacts are not associated with low concentrations of petroleum hydrocarbons dissolved in groundwater.
We believe that the science available today is sufficient to support the development of separate attenuation factors and screening criteria for petroleum and chlorinated VOCs
Groundwater-Bearing Unit
KEY POINT:
For petroleum sites, vapor intrusion is generally associated with i) direct impacts or ii) NAPL sources, but not diffusion of vapors from dissolved plumes.
Delineation of vapor sources is important for screening 13