Multiple-Lines-of-Evidence Approaches
to Address Complications to Vapour
Intrusion Pathway Assessments
Robert Ettinger
Geosyntec Consultants
October 28, 2014
Challenges to
Vapour Intrusion Assessments
 Sensitive subject for many
stakeholders
 The subject of new and
changing regulatory guidance
and litigation
 Closed sites reopened to
address vapour intrusion pathway
 Affecting property transactions
 Technically challenging pathway
 Data needs and interpretation for “MLE” assessments
 Evaluation of background contributions to indoor air
 Understanding uncertainties associated with vapour intrusion
modeling
2
Managing Uncertainties
 Compounding
conservative
assumptions can
lead to overly
conservative
conclusions
 Balance
uncertainties to
improve risk-based
decision making
process
Limited Site
Characterization
Greater understanding
of contaminant fate
(e.g., bioattenuation)
Reduced site
characterization
requirements
Less conservative
modeling
Detailed site
characterization
3
More conservative
risk mgmt. decisions
Direction of Regulatory Guidance
 Increased reliance on multiple lines of evidence “MLE”
assessments to address spatial and temporal variability
 Exclusion criteria for petroleum vapour intrusion
 More cautious screening evaluations
 Consideration of short-term action levels
 Less reliance on vapour intrusion modeling /
greater use of indoor air sampling
 Alternate lines of evidence to evaluate
indoor air background sources
 Increased emphasis on engineering controls
and pre-emptive mitigation
4
Multiple Lines of Evidence
Investigation Approach
Source
5
Vapour Intrusion To Building
• Soil Characteristics
• Building Characteristics
Source Characterization
• Groundwater, Soil, Soil
Vapour Concentration
Distribution
Top-Down
Bottom-Up
Indoor Air Evaluation
• Risk Management Decisions
• Background Contributions
• Mitigation Options
Importance of a
Conceptual Site Model
Develop
Initial CSM
Conduct
Investigation
 CSM should characterize
potential sources, fate and
transport pathways, and
receptors (e.g., buildings)
 Use CSM for investigation
planning and identify pros / cons
of different lines of evidence
Review/
Update CSM
 Not all lines of evidence have
equal weight in VI evaluation
Need
Add’l
Data?
Yes
No
Proceed with
Corrective Action
Planning / NFA
6
 Consider CSM when interpreting
investigation data
 Resolve differences between
data and CSM
Common Views of
Vapour Intrusion Models


There is a range of opinions about the use of
models for vapour intrusion pathway assessments
Most modeling questions arise from:





7
Different expectations for accuracy of model results
(i.e., typical vs RME estimates)
Deviation from conceptual model used for vapour
intrusion model development
Use of inappropriate input parameters
Uncertainty of significance of model input
parameters
Some combination of data collection and
modeling is usually appropriate
Vapour Intrusion Models
• There are many options for VI models available
• Model selection is dependent on what you know about the
site and the level of desired assessment
• Regulatory attenuation factors are simple VI models
Empirical


8
USEPA Database
USEPA VISL
Calculator
Analytical
Numerical

Johnson and Ettinger (1991)

VAPOURT (1989)

Little et al. (1991)

Sleep & Sykes (1989)

San Diego SAM

RUNSAT (1997)

VOLASOIL (1996)

Abreu & Johnson (2005)

Krylov and Ferguson (1998)

VIM (2007)

DLM - Johnson et al. (1999)

Brown University (2007)

DeVaull (2007)

BioVapor (2010)
USEPA Empirical
Attenuation Factors
 Empirical data from over 900
buildings at over 40 sites from
across the country
 Data are predominantly from
residential buildings
 Majority of data from a few
sites
 Used paired data (indoor air and
sub-surface data) to calculate
empirical attenuation factors
 Filtered data to screen out poor
data quality and results impacted
by background sources
9
USEPA Empirical
Attenuation Factors
 US regulatory
agencies focus on
95%ile values
 USEPA database
results may be
biased by
background
impacts
 May not be
relevant to nonresidential
scenarios
95th percentile
J&E Model Prediction
10
 Be careful if simply
using empirical
factors
USEPA Empirical
Attenuation Factors
Empirical Attenuation Factors
Source
# of Data
Pairs
Median
95%ile
Model
Predict
Crawl Space
41
0.39
0.90
NC
Sub-Slab Soil
Gas
411
0.0027
0.026
0.0024
Soil Gas
106
0.0038
0.25
0.0013
Groundwater
743
0.000074
0.0012
0.00041
 Always keep limitations of empirical attenuation factors in
mind in risk-based decision making process
11
Vapor Intrusion Attenuation Factor
 USEPA VI database study is commonly referenced to estimate VI
attenuation factor, but limitations of study should be recognized
 Data predominantly from single family homes
 Difficult to completely address background effects
 Natural vadose-zone biodegradation effects not captured
 Use of 95%ile empirical factors will over-state potential risks
 Ability for site-specific modeling/assessment is important
12
Site-Specific Modeling
 Site-specific modeling can be useful tool to characterize
uncertainty in preliminary screening vapour intrusion
assessments
 Consider differences in site conceptual model from
default assumptions
 Focus site-specific inputs for “critical” parameters that
can be well characterized (see Johnson, 2003)
 Additional support may be needed if calculated results
are significantly different from expected range
13
Vapour Intrusion Critical
Processes for Modeling Evaluation
14
Process
Key
Considerations
Sensitivity
Measurements
Diffusive Transport
(Diffusive Flux)
Soil type,
moisture content,
presence of
groundwater
VI decreases when
higher moisture
content soils are
present
Continuous boring
logs, soil property
data, in-situ
diffusivity test, VOC
soil gas profile
Building Ventilation
Varies by building
use/design
Increasing ventilation
reduces indoor air
concentrations
Building ventilation
rate
Soil Gas
Convection
Default values
typically used
Key parameter for
sub-slab data or pos.
press.
Cross-slab pressure
Partitioning
Groundwater to
soil gas
relationship
Uncertainty reduced
by collection of soil
gas samples
Soil gas samples for
source
characterization
Evaluation Framework for
Petroleum Hydrocarbons
 PVI is different than VI for chlorinated compounds
 PVI rarely shown to be a complete
pathway due to natural biodegradation
in vadose-zone soils
 Investigation strategies for chlorinated
sources are not well-suited for many
petroleum release sites
PVI Conceptual Model
 PVI guidance focuses on identifying site conditions where PVI
is not of concern (exclusion criteria)
 USEPA OUST and ITRC developed guidance on parallel
tracks
From API, 2004
15
USEPA PVI Database
 Data from 74 sites
 Predominantly UST sites, but data from terminals,
refineries, and petrochemical sites included
 Data analysis focuses on paired soil vapour and
groundwater data to identify distance for vertical
attenuation in vadose zone
 Distance to attenuate to 50 – 100 µg/m3
 Different from analysis for CVOCs due to
background sources of petroleum compounds
16
USEPA PVI Database
Dissolved-Phase Source
17
From USEPA, 2013. Evaluation of Empirical Data to Support Soil Vapor Intrusion
Screening Criteria for Petroleum Hydrocarbon Compounds, EPA 510-R-13-001.
USEPA PVI Database
LNAPL Source
18
From USEPA, 2013. Evaluation of Empirical Data to Support Soil Vapor Intrusion
Screening Criteria for Petroleum Hydrocarbon Compounds, EPA 510-R-13-001.
Considerations for Sites That
Do Not Meet Exclusion Criteria
 Petroleum hydrocarbons degradation occurs for
wide range of petroleum contaminated sites
 Typically re-visit multiple-lines-of-evidence
approach
 Data collection to assess biodegradation (soil vapour
or sub-slab soil vapour probes)
 Modeling
 Indoor air sampling is challenging for petroleum
hydrocarbons
 Consider whether remediation and/or mitigation
is warranted
19
Indoor Air Sampling
 Indoor air concentration measurements are used to make
decisions about potential health risks, but there are
difficulties with sampling and interpretation.
 Challenges to indoor air sampling
 VOCs frequently detected
 Occupant disruption
 Temporal and spatial variability
 Interpretation for future
land development scenarios
 Background effects
20
WMS
Sampler
VOCs Commonly Detected
in Indoor Air
Total Percent Detections
0
10
20
30
40
50
60
70
80
90
Toluene (0.03 - 1.9)
m/p-Xylene (0.4 - 2.2)
Chemical (Reporting Limits in ug/m3)
Benzene (0.05 - 1.6)
o-Xylene (0.11 - 2.2)
Ethylbenzene (0.01 - 2.2)
Methylene chloride (0.12 - 3.5)
Carbon Tetrachloride (0.12 - 0.25)
Chloroform (0.02 - 2.4)
Tetrachloroethylene (0.03 - 3.4)
1,1,2-Trichloro-1,2,2-trifluoroethane (0.25)
Methyl tert-butyl ether (MTBE) (0.05 - 1.8)
1,1,1-Trichloroethane (0.12 - 2.7)
Trichloroethylene (0.02 - 2.7)
1,2-Dichloroethane (0.02 - 0.25)
Vinyl chloride (0.01 - 0.25)
1,1-Dichloroethylene (0.01 - 2.0)
cis 1,2-Dichloroethylene (0.25 - 2.0)
1,1-Dichloroethane (0.08 - 2.0)
trans 1,2-Dichloroethylene (0.8 -2.0)
Dawson and McAlary, 2009, “Background Indoor Air,” Ground Water Monitoring & Remediation 29, No. 1
21
Detection of VOC in indoor air is not a useful single
line of evidence to assess vapour intrusion pathway
100
Indoor Petroleum Hydrocarbon Sources
20:50
Richard Wilson
Saatchi Gallery
Permanent Exhibit
22
Indoor Air Concentrations Are Greater
Than Outdoor Air Concentrations
23
From Sexton, et al., 2004. Comparison of Personal, Indoor, and Outdoor
Exposures to Hazardous Air Pollutants in Three Urban Communities
Approaches for Indoor Air
Background Assessment
 Various approaches available to evaluate background
contributions to indoor air quality measurements
 Traditional approach - chemical inventory
and comparison to literature values
 Real-time monitoring with portable GC/MS
 Compound ratio analysis / tracers
 Building pressure control
 Compound-specific stable isotope analysis (CSIA)
 Detailed data analysis
 Pros and cons to each of these methods
 Risk management considerations should be used to
assess need for detailed background evaluation
24
Indoor Air Sources
Source
Latex Paints
X
X
X
Alkyl Paints
X
Carpets
X
X
X
X
Glued Carpets
X
X
X
X
X
X
Wood Burning
X
X
X
X
Paint Removers
X
Spray Products
X
Adhesives/Tapes
X
X
X
X
Room Deodorants
X
Tobacco Smoke
X
X
Gasoline/driving
X
X
Dry Cleaning
X
X
Foam Board
Solvents
X
X
X
X
X
X
X
X
X
X
From Hers et al., 2001. The use of indoor air measurements to evaluate intrusion of
subsurface VOC vapors into buildings, J. Air & Waste Manage. Assoc. 51:1318-1331.
Expect detections of VOCs in any indoor air sample
25
Example Background
Indoor Air Concentrations
From Dawson and McAlary, 2009
26
Consider background range as well as typical values
Background Concentration of 1,2-DCA
CONCENTRATION
100%
1.0
90%
0.9
1,2-DCA Conc. (ug/m3)
1,2-DCA Detect. Freq. (%)
DETECTION FREQUENCY
80%
70%
60%
50%
40%
30%
20%
10%
0%
2004
2005
2006
2007
2008
Median 1,2-DCA Conc.
90%ile 1,2-DCA Conc.
0.8
0.7
0.6
0.5
0.4
USEPA INDOOR AIR LIMIT
0.3
0.2
0.1
0.0
<0.08
2004
<0.08
2005
<0.08
2006
1,2 DCA Background Source:
Detailed study by Hill AFB identified molded plastic
ornaments manufactured in China as source for 1,2 DCA.
Note: 1) 1,2-DCA = 1,2-dichloroethane
From McHugh et al., 2009. Also see Doucette et al., GWMR, 2010
27
2007
2008
Real Time Monitoring
with Portable GC/MS
 Building survey to sub-ppbv levels
 Use to identify preferential pathways or indoor sources
 HAPSITE GC/MS
 Analyze for target VOCs in SIM mode
 ~10 minute sample time
 Can also be run in scan mode
28
Building Pressure Control
 Negative pressure
= induced vapour
intrusion
 Positive pressure =
inhibited vapour
intrusion
29
Building Pressure Cycling
Concept
30
Short-Term EPA TCE
Response Action Levels (RAL)
 USEPA issued TCE toxicity reassessment Sept. 2011
 Strengthened confidence “that TCE is a human carcinogen”
 Identified non-cancer effects
 Decreased thymic weights
(immune system)
 Toxic nephropathy
(kidney)
 Conotruncal cardiac defects
(developmental)
 USEPA Region 9 recently
proposed short-term action levels
31
USEPA Region 9
TCE Indoor Air Screening Levels
Residential
6
Accelerated
RAL
(µg/m3)
2
Commercial
(8 hr/day)
Commercial
(10 hr/day)
24
8
3.0
21
7
2.4
Exposure Scenario
Urgent RAL
(µg/m3)
Chronic
RSL
(µg/m3)
0.48
• Accelerated RAL – Accelerated Response Action Level based on Hazard Quotient =1
Implement corrective action within a few weeks
• Urgent RAL – Urgent Response Action Level based on Hazard Quotient =3
Implement corrective action immediately
• Chronic RSL – Chronic Regional Screening Level based on 1x10-6 target risk level.
32
TCE Response Action Levels
 Technical questions have been raised regarding the
development of the response action level for TCE
 Laboratory test procedures
 Reproducibility of laboratory tests
 Calculation of acute reference concentration
 Expect further questions/
comments regarding the
TCE Response Action Level
33
From: Symposium on New Scientific Research Related to the
Health Effects of Trichloroethylene, Washington, DC.
February 26-27, 2004.
(http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=75934)
Considerations for Indoor Air Monitoring
Focus on short-term action levels and need for expedient
response may affect indoor air sampling strategies
 More difficult to address data with quality control issues
(e.g., false positives)
 Temporal variability in indoor air concentrations may
lead to requests for more frequent monitoring
 Consider longer duration sampling
(i.e., passive sampling)
 Allows for 2-3 week time-average samples
 Impractical to implement if sampling with HVAC off is required
 Expedited decisions require planning before sample collection
 Develop decision tree for contingent actions
 Consider whether expedited laboratory analysis provides value
34
Response Process
 Occupant relocation or indoor air
purification (if expedited action needed)
 Source removal and/or mitigation (e.g.,
excavation, soil vapour extraction)
 Local regulatory or building code
requirements
 Barriers to chemical entry




Pathway sealing
Sub-slab depressurization
Sub-slab venting
Aerated Flooring
 HVAC modifications to increase
ventilation or change building pressure
 Consider VI pathway in redevelopment
plan
35
Active Remediation vs
Engineering Controls
 Site-specific determinations are needed at balance shortterm and long-term vapor intrusion concerns
 Engineering controls can provide a short-term solution to
address vapor intrusion concerns
 Remediation may be better suited to address long-term
concerns
36
Sub-Slab Depressurization
37
37
Novel Mitigation Technique:
Aerated Flooring
A plastic form used to
create a continuous void
below concrete slabs
38
Concrete is poured over the forms
39
Vapour Intrusion Mitigation System
Considerations
 Design
 Implementability
(e.g., new vs existing structure)
 Effectiveness
 O&M Requirements
 Electrical costs
 Equipment upkeep
 Monitoring Requirements
 Requirements to demonstrate
effectiveness
 Cost Considerations
 Installation costs may be much less than monitoring costs
 Other Issues
 Impacts to building occupants
(i.e., aesthetics, costs)
40
Risk-Management Decision Matrix
Parameter 1
41
Increasing VI Risk
Parameter 2
Increasing VI Risk
Risk Management Assessment
Risk management in mitigation decision process
Increasing Sub-Slab
Concentration
Increasing Indoor Air Concentration
42
< RBSL and
< Background
> RBSL and
< Background
> RBSL and
> Background
<100 x RBSL
Pre-emptive
Mitigation /
Remediation
Pre-emptive
Mitigation /
Remediation
Mitigate /
Remediation
<10 x RBSL
Confirmation
Monitoring
Monitor/
Pre-emptive
Mitigation
Mitigate /
Remediation
<RBSL
No Further
Action
Confirmation
Monitoring
Monitor/
Background
Assessment
Summary
 Regulatory approaches for the vapour intrusion pathway
are continuing to change.
 Vapour intrusion evaluation methods continue to be
developed and improved, including methods for:
 Site investigation
 Site-specific modeling
 Identification of background sources
 Consider risk management, risk communication, and
long-term liabilities to address uncertainties associated
with vapour intrusion pathway assessments
43
Acknowledgements
 Special thanks to colleagues at Geosyntec who
contributed to this presentation:
 Nancy Bice
 Todd Creamer
 Helen Dawson
 David Folkes
 Todd McAlary
 Bill Wertz
44