1. Aerobic Biodegradation in the Vadose Zone
Workshop 1: Assessment and Evaluation of Vapor Intrusion at Petroleum Release Sites
Conceptual Model:
Aerobic Biodegradation in the Vadose Zone
The Association for Environmental Heath and Sciences Foundation
The 27th Annual International Conference on Soils, Sediments, Water, and Energy
Amherst, MA, October 17-20, 2011 1 1

2. Overview of Petroleum VI
n General VI Conceptual Model
n Vadose Zone Attenuation of Petroleum Vapors
n Oxygen Below Building Foundations

3. Groundwater-Bearing Unit
Conceptual Model for Vapor Intrusion: Regulatory Framework
BUILDING
Building Attenuation Due to Exchange with Ambient Air 3
Air Exchange
Advection and Diffusion Through Unsaturated Soil and Building Foundation
Unsaturated Soil 2
Affected Soil
Affected GW
Partitioning Between Source and Soil Vapor
Groundwater-Bearing Unit
Slide Topic: Conceptual model for vapor intrusion Diffusion from source and advection into building does not match industry experience.
The standard conceptual model of vapor intrusion has the following key elements:
Partitioning between the source and soil vapors. An infinite source of VOCs is assumed (i.e., no mass flux or mass balance considerations)
Diffusion and advection (but not biodegradation) through unsaturated soils and the building foundation
Attenuation in the building due to exchange with ambient air.
This conceptual model is based on the J&E model.
The U.S. EPA VI guidance was developed using this conceptual model of vapor intrusion.
Slide Presentation: Describe the conventional conceptual model for vapor intrusion.
Key Points:
This conceptual model does not distinguish between petroleum and chlorinated VOCs.
The model assumes an infinite source of VOCs
The model does not account for biodegradation in the vadose zone.
The model does not reflect industry experience with petroleum vapor intrusion impacts (see next slide). 1
KEY POINT:
Regulatory guidance focused on building impacts due to vapor migration.

4. Physical Barriers to Vapor Intrusion
Possible Barriers
No Barrier
Building Foundation:
(A) Low permeability foundation with no cracks or unsealed penetrations;
(B) Positive building pressure A B
Vadose Zone
Vadose Zone:
KEY POINT:
Vapors
(A) High moisture content or clay layer;
(B) Aerobic biodegradation
Presence of subsurface source does not always mean vapor intrusion is occurring. A B A B
Groundwater interface:
(A) Clean water lens
(B) Clay layer
Source Area
GW Aquifer

5. Overview of Petroleum VI
n General VI Conceptual Model
n Vadose Zone Attenuation of Petroleum Vapors
n Oxygen Below Building Foundations

6. Petroleum Biodegradation Conceptual Model
CHmin
Comax
Aerobic Biodegradation Possible
Co>Comin 
Oxygen L
No Aerobic Biodegradation
Co<Comin
Hydrocarbon
Comin
CHmax
Hydrocarbon Source
Vapor Concentration
KEY POINT:
Correlation between oxygen consumption and hydrocarbon attenuation.
From Roggemans et al., 2001, Vadose Zone Natural Attenuation of Hydrocarbon Vapors: An Empirical Assessment of Soil Gas Vertical Profile Data, API’s Soil and Groundwater Technical Task Force Bulletin No. 15.

7. VOC Concentration vs. Depth Biogenic Gases vs. Depth
Petroleum Biodegradation: Real Site Data
Diesel Release Site, North Dakota
VOC Concentration vs. Depth
Biogenic Gases vs. Depth

8. Correlation Between Groundwater Concentration and Indoor Air??IA ??
Petroleum Hydrocarbons
Chlorinated Solvents GW
Indoor Air Concentration ( ug/m3)
Indoor Air Concentration ( ug/m3)
CORRELATION ? YES (p <0.001)
CORRELATION ? NO (p = 0.11)
USEPA data base compiled by EPA, clumped together chlorinated and petroleum hydrocarbons.
GW Concentration (ug/L)
GW Concentration (ug/L)
Observable Relationship Cia vs. Cgw ?
n Petroleum Hydrocarbons: No
n Chlorinated Solvents: Yes
- Direct
Cgw = COC conc. In groundwater; Cia = COC conc. In indoor air; (p = 0.11) = Probability = 11% that slope of best-fit line = 0 (I.e., no trend).

9. Overview of Petroleum VI
n General VI Conceptual Model
n Vadose Zone Attenuation of Petroleum Vapors
n Oxygen Below Building Foundations

10. Oxygen Under Building Foundation
Key Question
n Is there enough oxygen below building foundations to support aerobic biodegradation?
aerobic zone Ct
anaerobic zone Cs
Vapor Source

11. Oxygen Under Foundation: Model Prediction
Numerical model predicts oxygen shadow below building, but…..
n Very strong vapor source (200,000,000 ug/m3)
n All flow into building is through perimeter crack
n No advective flow below building
Model does not account for key oxygen transport processes.
KEY POINT:
From Abreu and Johnson, ES&T, 2006, Vol. 40, pp 2304 to 2315.

12. Petroleum Hydrocarbons
Aerobic Biodegradation: Oxygen Mass Balance
bacteria
Hydrocarbon + Oxygen
Carbon dioxide + Water
1 kg CxHy + 3 kg O2
3.4 kg CO kg H2O
New
Cells
Electrons & Carbon +
Petroleum Hydrocarbons
Energy
Electrons
Electron
Acceptor
(e.g., O2)

13. Aerobic Biodegradation: Oxygen Mass Balance
n Atmospheric air (21% Oxygen) = 275 g/m3 oxygen > Provides capacity to degrade 92 g/m3 hydrocarbon vapors (= 92,000,000 ug/m3)
KEY POINT:
Even limited migration of oxygen into subsurface is sufficient to support aerobic biodegradation.

14. Bi-Directional Flow Across Foundation
Transport of Oxygen Under Foundation
Bi-Directional Flow Across Foundation
Wind Driven Advection
+/-
+/-
KEY POINT:
Advection drives oxygen below building foundation.

15. Transport of Oxygen Under Foundation
Conceptual Model
Field Data
0.0
Depth (m)
0.5
Wind-driving building ventilation
isoP
1.0
CH4
CO2
Wind Loading 02
1.5
Advection of subslab soil gas into bldg.
0.0
0.5
Biodegradation
1.0
Upwind-downwind advection in soil gas
Diffusion from deep sub-slab soil gas
1.5
0.0
Subslab VOC source
0.01
0.1 1 10
100
1000
Concentration (g m-3)
KEY POINT:
Conceptual model and field data indicate common presence of oxygen under building foundation.
From Fisher et al., 1996 Environmental Science and Technology, Vol. 30 No. 10, p

16. Transport of Oxygen Under Foundation
Soil Column Attenuation
Transport of Oxygen Under Foundation
Nitrogen Flooding Experiment:
Purge sub-foundation soils with nitrogen gas and observe oxygen recovery
Low Oxygen
Time = 0
Time > 0
3 m N
1.1
0.8
1.0
0.9
concrete
garage
Injection wells
% O2 (shallow)
% O2 After Flood
Oxygen Recovery Below Building
Slide Topic: Transport of Oxygen Under Foundation, Case Study
This slide and the next slide present the results of a field experiment conducted to measure the transport of oxygen below a building foundation. The test building was a residential structure in Santa Maria, CA built on fill material.
For the experiment, nitrogen was pumped under the building, displacing the air (which contained relatively high oxygen levels). Following the nitrogen flooding, monitoring points (separated from the N2 injection points) showed that the air had been displaced, resulting in very low oxygen levels below the building. Oxygen sensors were then used to monitor the recovery of oxygen under the slab.
The sensors (shown on the graph) indicate that oxygen levels recovered at many locations in less than two days and at all locations within two weeks. The sudden recovery in oxygen levels at just under time = 2 days corresponds to a high wind event, supporting the conceptual model that wind facilitates oxygen transport under the building foundation.
Slide Presentation: Describe the experiment and the results.
Key Points:
The results of this experiment indicate rapid oxygen transport under the building foundation and support the conceptual model of wind-driven oxygen transport.
Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”

17. Transport of Oxygen Under Foundation
Soil Column Attenuation
Transport of Oxygen Under Foundation
Nitrogen Flooding Experiment:
Purge sub-foundation soils with nitrogen gas and observe oxygen recovery
Low Oxygen
Time = 0
High Oxygen
Time = 2 weeks
KEY POINT:
1.1
0.8
1.0
0.9
concrete
3 m N
garage
16.6
18.4
19.4
15.4
14.0
15.2
12.2
14.5
13.7
15.9
3 m N
garage
concrete
Rapid recovery of oxygen below building foundation supports petroleum biodegradation.
% O2 After Flood
Injection wells
% O2 (shallow)
Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”

18. Advective Transport Processes
Lower building pressure
Residence in winter (chimney effect); bathroom, kitchen vents
EXAMPLES
Gas flow from subsurface into building
Higher building pressure
Building HVAC designed to maintain positive pressure
Gas flow from building into subsurface
Variable building pressure
Barometric pumping; variable wind effects
Bi-directional flow between building and subsurface
Flow in
Flow out
Reversible flow
Low Pressure
High Pressure
UPWARD TRANSPORT
High Pressure
Low Pressure
DOWNWARD TRANSPORT

19. Pressure Gradient Measurements: School Building, Houston, Texas
KEY POINTS:
Pos. Pressure
(Flow out of Bldg)
• Pressure gradient frequently switches between positive and negative within a single day.
• Continuous inward flow does not occur.
Differential Pressure (Pascals)
Neg. Pressure
(Flow into Bldg)
Time (July 14-15, 2005)

20. Advection Through Building Foundation: Field Evidence
n VOCs from indoor air typically detected in sub-slab samples: - alpha pinene - limonene - p-dichlorobenzene
n Oxygen transported below foundation by same mechanism
INDOOR AIR S
BELOW SLAB
KEY POINT:
Reversing pressure gradient drives air (and VOCs and oxygen) through building foundation. S

21. Fill, “crust,” sandy silt Fill, dredged river sand
Chatterton Research Site, British Columbia, Canada
(Hers, et al 2000)
Building
Feet Below Grade
SG-BC, 10/1/97
SG-BR 5/14/97
<1000
Fill, “crust,” sandy silt
11%
10 %
<1000
80,000
10%
8 %
1.0%
50,000,000
Oxygen, %
Benzene, ug/m3
Sub-Slab vapor sample point
Sub-Surface vapor sample point
KEY
SG-BC
Vapor sample point identifier
55,000 3% 6%
25,000,000
50,000,000
Fill, dredged river sand
1.0%
50,000,000 5
60,000,000
Small building lies ~8 feet above benzene-rich LNAPL.
Vapors fully attenuated with ~7 feet clean soil.
Strong source puts strong demand on oxygen regardless of building presence.
Building does not occlude oxygen.
1.0% 20
Feet, horizontal 10
LNAPL, benzene-rich
Slide from Robin Davis, UDEQ

22. Hal’s, Green River, Utah (Utah DEQ, 8/26/06)
Feet Below Grade
Motel Office
Breezeway
Café/Bar
VW-7
Asphalt
8.4
850
Basement
Basement
14%
Clayey Silt
7.0
380
VW-4
VW-5
20% 51 22 10
2800
1600
7.7
9.5%
18%
Silt
250
12%
570 87
70,000
12,000
260,000
4.1%
11%
33,000,000
2.5%
Benzene in GW 1,000-5,000 ug/L 20
Small- and Medium-sized buildings overlie very strong source of soil and groundwater gasoline contamination.
Aerobic biodegradation of vapors with ~5 feet of clean overlying soil regardless of building presence.
Buildings do not occlude oxygen or impede aerobic biodegradation if sufficient thickness of clean soil overlies the source.
Vapor intrusion not likely.
Exterior soil vapor sample points are adequately representative of sub-building conditions.
KEY
260,000
2.5%
Benzene, ug/m3
Oxygen, %
VW-7
Multi-depth vapor monitoring well
Sub-Surface vapor sample point
TPH-gro, ug/m3
LNAPL, gasoline
Feet, horizontal 20
33,000,000
Slide from Robin Davis, UDEQ

23. Perth, Australia (B. Patterson & G. Davis, 2009)
Very Large Building
410
Feet Below Grade
Uncovered open ground
<2
19,000,000
<50,000
<50,000
<50,000
<0.5%
10.7%
19.9%
18.8%
Lateral Extent of Oxygen & Biodegradation
Sand
35,000,000
<50,000
<50,000
<50,000 5
<0.5%
8.2%
14.5%
15.9%
35,000,000
1,200,000
<50,000
<50,000
<0.5%
8.2%
4.5%
4.6%
Very large building overlies very strong source of kerosene LNAPL.
For ~30 feet laterally from center of building to building edge, ~5 feet of clean overlying attenuates vapors associated with very strong source as evidenced by low soil vapor and high Oxygen concentrations.
Building >30 feet wide/deep occludes oxygen (“casts a shadow”) and impedes aerobic biodegradation.
Interior and Exterior soil vapor sample points may be necessary for sites with very large buildings.
NOTE: Indoor air TPH concentration does not exceed health-based criteria for kerosene VOCs.
Conclusion: Very strong source in close vertical proximity to a very large building produces very high vapor concentrations that cannot degrade perhaps due to the size of the building and occlusion of oxygen. 10
KEY
LNAPL Kerosene
(very low BTEX)
Outdoor air sample
Indoor air sample
Sub-slab vapor sample
Sub-surface vapor sample 20
Feet, horizontal
1,200,000
Total Petroleum Hydrocarbons, ug/m3
8.2%
Oxygen, %
Slide from Robin Davis, UDEQ

24. Oxygen Under Building Foundation
Summary
n Wind and building pressure drive atmospheric air below building foundation
n Even modest oxygen transport sufficient aerobic biodegradation