Assessing Variability in Petroleum Vapor Intrusion with...
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Assessing Variability in
Petroleum Vapor Intrusion with
PVIScreenJim Weaver
(US EPA, retired)
26TH National Tanks Conference
Louisville, KY
Sept 10, 2018
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Outline
• Characteristics of Environmental Models
• Vapor Intrusion and Petroleum Vapor Intrusion
• PVIScreen model
• Excerpts from examples
– PVI indicated versus not indicated
• Secrets of PVIScreen
• Summary
• Availability
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Why vapor intrusion and models?
• Technical Challenges
– ambient air contamination, internal sources/sinks, temporal changes
• Social
– RP or homeowner reluctance to sample
• In some cases—redeveloping a site—no building exists for
testing, so models are relied upon
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Vapor Intrusion and Models
• Series of articles in the Denver Post in 2000
– The vapor intrusion model (Johnson-Ettinger) over-predicted
indoor air concentrations sometimes and under-predicted
indoor air concentrations sometimes
– Model used with defaults and very few site specific values
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Example: Darcy’s Law
• Darcy flux q = -K dh/dl
– Relationship from
Darcy’s sand tank
experiments
– Empirical coefficient,
the hydraulic
conductivity (K), from
experiment:
measuring the flow (q)
Material Value (m/d)
Gravel 102 to 104
Sand 10-1 to 103
Clay 10-8 to 10-3
Sandstone 10-5 to 10
Basalt 10-6 to 10-2
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Limits to Predictability
• Note the work of N. Oreskes on ideal
applications for models:
– Weather forecasting
• Forecast given and received with uncertainties
• Oreskes, Naomi, 2003, The role of quantitative models in science,
in Models in Ecosystem Science, C.D. Canham and W.K.
Lauenroth, eds. Princeton University Press, 13-31
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Chlorinated Solvent (left) petroleum (right) are distinguished by
prospects for biodegradation
U.S. EPA, 2012, Petroleum Hydrocarbons And
Chlorinated Hydrocarbons Differ In Their Potential For
Vapor Intrusion, U.S. Environmental Protection Agency,
Washington, DC., March.
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Petroleum Vapor Intrusion and biodegradation:
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PVIScreen rests on a foundation of field data:
Robin V. Davis, 2009, Update on Recent Studies and Proposed
Screening Criteria for the Vapor-Intrusion Pathway, LUSTLine Bulletin
61, pp 11-14.9
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Site assessment flow
chart from OUST guide
on PVI
Model Use:
•NOT without mitigating
immediate threats
•NOT without site
characterization
•As a line of evidence
for related to
determination of
vertical separation
distance*
*
*
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Consider PVIScreen usage in marginal cases as a second line of evidence
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http://www.epa.gov/land-research/pviscreen
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PVIScreen
• PVIScreen includes:– BioVapor equations, recoded in Java for speed
– Automated Monte Carlo uncertainty analysis• the native way the code is used
– Soil gas or ground water source
– Comparison to screening levels
– Flexible and customizable unit choices
– Automated Report
• Primary focus:– To add line of evidence for site assessment and
closure decisions
– To make uncertainty analysis practical by giving a prediction and estimate of its uncertainty
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PVIScreen Sources: Soil Gas Data
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PVIScreen generates schematic
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PVIScreen Sources: Ground Water Data
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0.001 0.01 0.1 1
Relative TCE Concentration
1
0.8
0.6
0.4
0.2
0
Dep
th (
m)
LegendDrainage
Imbibition
Water Table
Top of Tension Saturated Zone Imbibition
Top of Tension Saturated Zone Drainage
0 0.1 0.2 0.3 0.4
Moisture Content
1
0.8
0.6
0.4
0.2
0
10 x Reduction
100 x Reduction
Concentration
relationships in the
capillary fringe: from
one data set*•Concentration reduction by
~1/10 through the capillary
fringe
*McCarthy, K.A. and Johnson,
R.L., 1993, Transport of volatile
organic compounds across the
capillary fringe, Water Resources
Research, 29(6) 1675-1683.
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Schematic showing ground water source
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Perils of Ground Water as the Source for
PVIScreen
• Does the ground water concentration
represent the capillary fringe?
• Borehole dilution and screen length?
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How does it work?
• PVIScreen is written in Java
• The interface: JavaFX
• All inputs and outputs saved in text files:
– File extensions managed by User Interface:• Input: ProblemName.pvi
• Output: ProblemName-DateTime.PVIScreen.Result.csv
• Input and output files are ASCII text files in comma-separated value
format—direct editing not advised.
• Runs from Windows Directory (double click)
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PVIScreen Interface:
The options run from right to left, and buttons become
active when the option is available
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Approach to uncertainty: allow parameters to be
treated as uncertain, but incorporate all known
parameter values
•GUI allows
•Constant
•min to max range
•Command line also allows empirical and parametric
distributions
•(not included in GUI or today’s presentation)
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Example inputs: constants or ranges
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Inputs of multiple constituents•all oxygen should NOT go to degrade only benzene,
•Include TPH or petroleum fractions
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Input of Screening Levels:
State-specific or EPA RSLhttps://www.epa.gov/risk/regional-screening-levels-rsls-generic-
tables
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Factors controlling biodegradation are uncertain, variable
•Hydrocarbon degradation rates vary by factor of 100
•How does this impact PVI?
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“Suggested” Values
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0.0
1.0
0.0
1.0
0.0
1.0
0.0
1.0
0.0
1.0
0.0
1.0
frequency
parameter value
frequency
frequency
parameter value
parameter value
A constant
Uniform
distribution:
min, max
Data-driven
empirical distribution
0.0
1.0
Results: indoor air
concentration
frequencyMultiple
Model
Runs
InputsResults
All other inputs,
Other distributions can
be used:
Triangular, truncated
normal
Log normal
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Soil gas input data example from a site in Utah:
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Impacts to Off-Site Restaurant? PVIScreen
‘driven’ by soil gas data at 3’ below the surface
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PVIScreen generates schematic
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Results: PVIScreen model runs indicate no impact
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PVIScreen Result
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No graph because all
Results below screening level
(and plotting limits)
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Statistics
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If you used the average of all parameters in one simulation
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Old style output:
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Example with impact indicated:
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Impact indicated
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Simulation with equilavent soil gas inputs
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Automated Report:
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Model Output – all parameter values saved with results
Method to display parameter values which exceed risk levels is being developed
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Secrets of PVIScreen…• Java must be enabled.
• Always start PVIScreen by opening an existing file (examples or
templates).
– Template files regenerate every time the model is run—save under
different name.
– Create project directories for each project.
– All files are saved with date/time stamp (can pile up).
– If results are not displayed, exit and restart PVIScreen.
• If you see an orange line across the screen..
– Restart PVIScreen
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Secrets of PVIScreen…• Use the correct template to begin.
• Concentrations needed to drive model.
• Biodegradation is always treated as being uncertain.
• When an impact is shown…
– Because of randomness, % will very with each simulation
– If result has marginal exceedances (say <5%) consider refining
ranges of parameters.
• DON’T only simulate benzene or BTEX.– you will probably never see an impact
– because TPH needs to be included.
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Results fit
within PVI
guidance
framework
--one line of
evidence
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Summary
• Immediate threats must be handled first.
• Site characterization and development of a
Conceptual Site Model next.
• Model use (including PVIScreen) should be
embedded with site assessment.
• PVIScreen incorporates parameter.
uncertainty into PVI modeling.
• Results can add a line of evidence to an
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Available at http://www.epa.gov/land-
research/pviscreen
(look for copy dated Sept 2018)
• EPA Contact: [email protected]
• The views expressed in this presentation are
those of the author and do not necessarily
represent the views or policies of the U.S.
Environmental Protection Agency
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