BioVapor A 1-D Vapor Intrusion Model with Oxygen- limited
Aerobic Biodegradation Application of BioVapor to Petroleum Vapor
Intrusion Sites
Slide 2
Course Outline Overview of Petroleum Vapor Intrusion (60 min)
Introduction to BioVapor Model (45 min) Break (15 min) Case Study
1: GW Screening Values (30 min) Case Study 1: Dissolved Hydrocarbon
Plume (30 min) Case Study 2: Gasoline Vapor Source (30 min)
Questions (30 min) Overview of Petroleum Vapor Intrusion (60 min)
Introduction to BioVapor Model (45 min) Break (15 min) Case Study
1: GW Screening Values (30 min) Case Study 1: Dissolved Hydrocarbon
Plume (30 min) Case Study 2: Gasoline Vapor Source (30 min)
Questions (30 min)
Slide 3
Gettin the Goods How Download at: www.api.org/pvi (Registration
information used so we can notify users of updates. No spam.) Roger
Claff [email protected] (202) 682-8399 Bruce Bauman [email protected]
(202) 682-8345 Who BioVapor Model
Slide 4
Meet the Trainers Introduction Thomas McHugh GSI Environmental
Developer of BioVapor Interface George DeVaull Shell Global
Solutions Developer of BioVapor Model Jim Weaver US EPA, Office of
Research and Development Petroleum Vapor Intrusion Research and
Policy
Slide 5
Overview of Petroleum VI General VI Conceptual Model Vadose
Zone Attenuation of Petroleum Vapors Oxygen Below Building
Foundations Framework for Evaluation of Petroleum VI General VI
Conceptual Model Vadose Zone Attenuation of Petroleum Vapors Oxygen
Below Building Foundations Framework for Evaluation of Petroleum
VI
Slide 6
Conceptual Model for Vapor Intrusion: Regulatory Framework KEY
POINT: Regulatory guidance focused on building impacts due to vapor
migration. Building Attenuation Due to Exchange with Ambient Air
Advection and Diffusion Through Unsaturated Soil and Building
Foundation Partitioning Between Source and Soil Vapor Groundwater-
Bearing Unit Air Exchange BUILDING Unsaturated Soil 3 2 1 Affected
GW Affected Soil
Slide 7
A B A B A B Physical Barriers to Vapor Intrusion
Slide 8
Overview of Petroleum VI General VI Conceptual Model Vadose
Zone Attenuation of Petroleum Vapors Oxygen Below Building
Foundations Framework for Evaluation of Petroleum VI General VI
Conceptual Model Vadose Zone Attenuation of Petroleum Vapors Oxygen
Below Building Foundations Framework for Evaluation of Petroleum
VI
Slide 9
Oxygen Aerobic Biodegradation Possible C o >C o min No
Aerobic Biodegradation C o
Aerobic Biodegradation: Oxygen Mass Balance Atmospheric air
(21% Oxygen) = 275 g/m 3 oxygen > Provides capacity to degrade
92 g/m 3 hydrocarbon vapors (= 92,000,000 ug/m 3 ) Even limited
migration of oxygen into subsurface is sufficient to support
aerobic biodegradation. KEY POINT:
Slide 24
Transport of Oxygen Under Foundation Wind Driven Advection
Bi-Directional Flow Across Foundation KEY POINT: Advection drives
oxygen below building foundation. +/-
Slide 25
Conceptual Model Field Data 0202 CO 2 CH 4 isoP Concentration
(g m -3 ) 0.010.11101001000 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0
Depth (m) 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.
2948. Wind Loading Wind-driving building ventilation Advection of
subslab soil gas into bldg. Biodegradation Diffusion from deep
sub-slab soil gas Upwind-downwind advection in soil gas Subslab VOC
source Transport of Oxygen Under Foundation
Slide 26
Soil Column Attenuation Transport of Oxygen Under Foundation
Nitrogen Flooding Experiment: Data from Lundegard, Johnson, and
Dahlen. Sub-slab Nitrogen Flood-Oxygen Re-entry Test. 3 m N 1.1 0.8
1.0 0.8 0.9 concrete garage Injection wells % O 2 (shallow) % O 2
After Flood Oxygen Recovery Below Building Low Oxygen Time = 0 Time
> 0 Purge sub-foundation soils with nitrogen gas and observe
oxygen recovery
Slide 27
1.1 0.8 1.0 0.8 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 High Oxygen
Time = 2 weeks 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
KEY POINT: Rapid recovery of oxygen below building foundation
supports petroleum biodegradation. Injection wells % O 2 (shallow)
% O 2 After Flood Data from Lundegard, Johnson, and Dahlen.
Sub-slab Nitrogen Flood-Oxygen Re-entry Test.
Slide 28
Advective Transport Processes High Pressure Low Pressure
DOWNWARD TRANSPORT Low Pressure High Pressure UPWARD TRANSPORT
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 EXAMPLES Gas flow from building into
subsurface Variable building pressure Barometric pumping; variable
wind effects EXAMPLES Bi-directional flow between building and
subsurface Flow in Flow out Reversible flow
Slide 29
Pressure Gradient Measurements: School Building, Houston, Texas
Differential Pressure (Pascals) Time (July 14-15, 2005) Neg.
Pressure (Flow into Bldg) Pos. Pressure (Flow out of Bldg) Pressure
gradient frequently switches between positive and negative within a
single day. Continuous inward flow does not occur. KEY POINTS:
Slide 30
Advection Through Building Foundation: Field Evidence S S
INDOOR AIR BELOW SLAB VOCs from indoor air typically detected in
sub-slab samples: - alpha pinene - limonene - p-dichlorobenzene
Oxygen transported below foundation by same mechanism KEY POINT:
Reversing pressure gradient drives air (and VOCs and oxygen)
through building foundation.
Groundwater- Bearing Unit BUILDING Unsaturated Soil >3 to 10
m LNAPL Proposed Action: Evaluate presence of preferential flow
pathways or other site-specific risk factors. Testing for
hydrocarbons in shallow soil gas below or directly adjacent to
building foundation may be appropriate. 1) Adapted from McHugh,
Davis, DeVaull, Hopkins, Menatti, and Peargin, 2010. Evaluation of
Vapor Attenuation at Petroleum Hydrocarbon Sites: Considerations
for Site Screening and Investigation, Soil and Sediment
Contamination: An International Journal, November/December 2010,
Vol. 19, No. 5. (>10 to 30 ft) Framework for Evaluation of
Petroleum Vapor Intrusion Sites: Based on Modeling and Empirical
Data (McHugh et al. 1 ) 3 LNAPL present >10 to 30 ft below
building foundation: LOWER RISK
Slide 40
BUILDING Unsaturated Soil >1.5 to 3 m Affected GW 1) Adapted
from McHugh, Davis, DeVaull, Hopkins, Menatti, and Peargin, 2010.
Evaluation of Vapor Attenuation at Petroleum Hydrocarbon Sites:
Considerations for Site Screening and Investigation, Soil and
Sediment Contamination: An International Journal, November/December
2010, Vol. 19, No. 5. (>5 to 10 ft) Framework for Evaluation of
Petroleum Vapor Intrusion Sites: Based on Modeling and Empirical
Data (McHugh et al. 1 ) 4 Dissolved hydrocarbon plume 5 to 10 ft
below building: LOWER RISK Proposed Action: Evaluate presence of
preferential flow pathways or other site-specific risk
factors.
Slide 41
Course Outline Overview of Petroleum Vapor Intrusion (60 min)
Introduction to BioVapor Model (45 min) Break (15 min) Case Study
1: GW Screening Values (30 min) Case Study 2: Dissolved Hydrocarbon
Plume (30 min) Case Study 3: Gasoline Vapor Source (30 min)
Questions (30 min) Overview of Petroleum Vapor Intrusion (60 min)
Introduction to BioVapor Model (45 min) Break (15 min) Case Study
1: GW Screening Values (30 min) Case Study 2: Dissolved Hydrocarbon
Plume (30 min) Case Study 3: Gasoline Vapor Source (30 min)
Questions (30 min)
Slide 42
Types of Vapor Intrusion Models Wide range of approaches to
vapor intrusion modeling, varying in complexity and specificity.
KEY POINT: Empirical (Tier 1) Analytical (Tier 2) Predictions based
on observations from other sites (e.g., attenuation factors).
Mathematical equation based on simplification of site conditions
(e.g., Johnson and Ettinger). Numerical models: - Abreu and
Johnson, Bozkurt et al. Mass flux model, foundation transport
model, etc. Others (Tier 3) SIMPLE MATH Vapor Intrusion Models
Slide 43
Johnson and Ettinger Model (Tier 2) Building Attenuation Due to
Exchange with Ambient Air Advection and Diffusion Through
Unsaturated Soil and Building Foundation Equilibrium Partitioning
Between GW and Soil Vapor C sv = C gw x H KEY POINT: Site-specific
predictions based on soil type, depth to groundwater, and building
characteristics. source area Groundwater- Bearing Unit Air Exchange
RESIDENTIAL BUILDING Unsaturated Soil H = Henrys Law Constant 1 2 3
Vapor Intrusion Models
Slide 44
J&E Model: Key Assumptions KEY POINT: J&E model is
generally conservative, but model error can be very large
(orders-of-magnitude). soil vapor Affected GW Plume 1-D Steady-
State Model Infinite Source Does not account for heterogeneities,
preferential pathways, or temporal variation. No mass balance; mass
flux into building can exceed available source mass. Does not
account for biotransformation in the vadose zone No Bio-
degradation Vapor Intrusion Models
Slide 45
Conceptual Model Model Inputs Model Outputs Example Model
Validation Conceptual Model Model Inputs Model Outputs Example
Model Validation BioVapor: 1-D VI Model w/ Bio
Slide 46
What is BioVapor? Easy-to-use vapor intrusion model that
accounts for oxygen-limited aerobic vapor intrusion. Free download
at: www.api.org/vi KEY POINT: 1-D Analytical Model Oxygen Mass
Balance Version of Johnson & Ettinger vapor intrusion model
modified to include aerobic biodegradation (DeVaull, 2007). Uses
iterative calculation method to account for limited availability of
oxygen in vadose zone. Simple interface intended to facilitate use
by wide range of environmental professionals. User- Friendly O2O2
HC SIMPLE MATH Conceptual Model
Slide 47
BioVapor: Conceptual Model Conceptual Model Vapor Source CsCs
CsCs CtCt CtCt aerobic zone anaerobic zone 3Advection, diffusion,
and dilution through building foundation 2Diffusion & 1 st
order biodegradation in aerobic zone 1Diffusion only in anaerobic
zone
Slide 48
BioVapor: Oxygen Mass Balance Conceptual Model Calculate oxygen
demand: - depth of aerobic zone - HC vapor concentration - 1st
order biodegradation Iterative Calculation Method Vapor Source
anaerobic interface ?? Final Model Solution Yes No Increase or
decrease depth of aerobic zone Calculations are cheap & quick
KEY POINT: O 2 demand = supply?
Slide 49
BioVapor: Intended Application Conceptual Model Obtain improved
understanding of petroleum vapor intrusion. Calculate oxygen
concentration/flux required to support aerobic biodegradation.
Identify important model input parameters and evaluate model
sensitivity. 1-D Model: Does not account for spatial variability
Steady State: Does not account for temporal variability Single
Source: Does not account for indoor sources and other background
sources of petroleum VOCs Simplifying Assumptions Yes
Slide 50
Conceptual Model Model Inputs Model Outputs Example Model
Validation Conceptual Model Model Inputs Model Outputs Example
Model Validation BioVapor: 1-D VI Model w/ Bio
Slide 51
Model Inputs Data Requirements
Slide 52
Human Health Risk Chemical Toxicity Exposed Dose COC Fate &
Transport x = x Baseline Risk Calculation Risk-Based Cleanup Level
Calculation RISK = ? SSTL = ? START W / COC CONC COC = Chemical of
Concern; SSTL = Site-Specific Target Level START W / RISK LIMIT
Forward and Backward Calculations Model Inputs
Slide 53
Human Health Risk Chemical Toxicity Exposed Dose COC Fate &
Transport x =x Backward Calculations: Conc. Vs. Risk Model Inputs
OPTION 1: Calculation based on target indoor air concentration
(from BioVapor database) OPTION 2: Calculation based on target
indoor air risk limits (enter by user)
Slide 54
Model Inputs Environmental Factors
Slide 55
Model Inputs Environmental Factors Model inputs similar to
J&E, plus a few new inputs related to oxygen-limited
biodegradation: - New inputs can be measured or estimated. KEY
POINT:
Slide 56
Oxygen Boundary Condition Open Soil: (Constant O 2 Conc.) Solid
Foundation: (Constant Air Flow) Constant oxygen concentration at
top of vadose zone: - 21% oxygen in dirt crawl space - Measured
oxygen concentration below solid foundation Constant oxygen flux
across top of vadose zone: - Air flow from atmosphere to below
building foundation User-specified depth of aerobic zone: - Based
on measured vertical profile in vadose zone - No O 2 mass balance
Fixed Aerobic Depth Model Inputs Dirt Crawl Space 21% O 2 Solid
Foundation Aerobic Anaerobic
Slide 57
Baseline Soil Respiration Rate Conceptual Model No Hydrocarbon
Source Oxygen concentration WHAT: Rate of oxygen consumption in
absence of hydrocarbon vapors (due to existing soil bacteria)
OPTION 1: Enter directly OPTION 2: Estimate from soil organic
carbon Base,O 2 (equation from, DeVaull, 2007 based on data from
several studies) = 1.69 x f o c f oc >0.02 - baseline
respiration can be very high. f oc
Fresh Gasoline Moderately Weathered Gasoline Weathered Crude
Oil * = Value based on MCL, risk-based number would be lower.
Benzene T, E, X Other Aromatic HCs Aliphatic HCs* 0.25 - 1%1 - 2 %
1 - 4%5 - 15%
