Vapor Intrusion Vapor Intrusion Evaluation Strategy and Evaluation Strategy and Modeling Developments Modeling Developments Robert Ettinger Robert Ettinger Geosyntec Consultants Geosyntec Consultants California Industrial Hygiene California Industrial Hygiene Council Council 16 16 th th Annual Conference Annual Conference San Diego, CA San Diego, CA December 4 – 6, 2006 December 4 – 6, 2006
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Vapor Intrusion Evaluation Strategy and Modeling Developments
Vapor Intrusion Evaluation Strategy and Modeling Developments. Robert Ettinger Geosyntec Consultants California Industrial Hygiene Council 16 th Annual Conference San Diego, CA December 4 – 6, 2006. Timeline for the Vapor Intrusion Pathway. Radon Intrusion & Vapor Diffusion studies. - PowerPoint PPT Presentation
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Vapor IntrusionVapor IntrusionEvaluation Strategy and Evaluation Strategy and Modeling DevelopmentsModeling Developments
Robert EttingerRobert EttingerGeosyntec ConsultantsGeosyntec Consultants
Typically, indoor air target levels based on risk-Typically, indoor air target levels based on risk-based concentrations developed using EPA risk based concentrations developed using EPA risk methodologymethodology
Need to considerNeed to consider Target risk levelTarget risk level Occupational standardsOccupational standards Background concentrationsBackground concentrations
Basis Benzene PCE
10-6 Risk 0.25 0.32
10-5 Risk 2.5 3.2
10-4 Risk 25 32
Background 3 - 5 1 - 5
PEL (8-hr TWA) 3200 170,000
Example Target Indoor Air Levels
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Site CharacterizationSite Characterization
Indoor Air
Groundwater
Soil
Soil Gas
What data are best to characterize vapor intrusion pathway?
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Indoor Air SamplingIndoor Air Sampling
Indoor air sampling may seem to be a Indoor air sampling may seem to be a direct assessment approach, but is direct assessment approach, but is typically conducted during higher tier of typically conducted during higher tier of investigationinvestigation
Several challenges to indoor air samplingSeveral challenges to indoor air sampling Occupant disruptionOccupant disruption Temporal and spatial variabilityTemporal and spatial variability Background effectsBackground effects
May more practical to collect indoor May more practical to collect indoor air samples in occupational settingair samples in occupational setting
Indoor air sampling guidanceIndoor air sampling guidance Sample collection techniquesSample collection techniques Analytical methodsAnalytical methods Building survey examples Building survey examples
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Source Source CharacterizationCharacterization
GroundwaterGroundwater Henry’s Law to evaluate partitioningHenry’s Law to evaluate partitioning Mass transport limitations due to vertical Mass transport limitations due to vertical
concentration gradients in saturated zoneconcentration gradients in saturated zone SoilSoil
Gas-water and water-solid partitioningGas-water and water-solid partitioning Uncertainty in accuracy of partitioning equationUncertainty in accuracy of partitioning equation
Soil GasSoil Gas Soil gas results can resolve uncertainty Soil gas results can resolve uncertainty
associated with groundwater or soil dataassociated with groundwater or soil data Typically provide better source characterization Typically provide better source characterization
for vapor intrusion pathway. for vapor intrusion pathway.
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Soil Gas Sample LocationSoil Gas Sample Location
Current regulatory Current regulatory focus on appropriate focus on appropriate sampling locationssampling locations Near sourceNear source Exterior to buildingExterior to building Sub-slabSub-slab
Soil gas profile may Soil gas profile may be affected by be affected by buildingbuilding More significant for More significant for
biodegradable biodegradable compoundscompounds
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Soil Gas SamplingSoil Gas Sampling
Soil gas sampling methods not as uniform Soil gas sampling methods not as uniform as groundwater sampling methods, but as groundwater sampling methods, but approaches to meet investigation data approaches to meet investigation data quality objectives are availablequality objectives are available
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Sub-Slab Soil Gas Sampling
Requires building accessRequires building access Methods developed to limit intrusivenessMethods developed to limit intrusiveness
(DiGiulio, 2004)
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• Mixing in Breathing Zone
• Diffusive Transport toBreathing Zone
• Impacted Soil and/or Groundwaterin Equilibrium with Soil Gas
Averaging time for non-carcinogens (yrs) ATnc 30 30
Exposure duration (yrs) ED 30 30
Exposure frequency (days/yr) EF 350 350
Exposure time (hrs/24 hrs) ET 24 24
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Biodegradation ModelingBiodegradation Modeling
Methods to model vadose zone Methods to model vadose zone biodegradation have been developed biodegradation have been developed Johnson et. al., 1999 – Dominant Layer Model Johnson et. al., 1999 – Dominant Layer Model Abreu and Johnson, 2004 – 3D Numerical Abreu and Johnson, 2004 – 3D Numerical
ModelModel Typically, additional site investigation data Typically, additional site investigation data
will be necessary to conduct biodegradation will be necessary to conduct biodegradation modelingmodeling Soil gas concentration profile dataSoil gas concentration profile data Analysis of biodegradation indicators (OAnalysis of biodegradation indicators (O22 , CO , CO22)) Tracer compoundsTracer compounds
Consider use of soil vapor profile dataConsider use of soil vapor profile data
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Screening Level Biodegradation Model
(Johnson, et al., 1999)
Requires additional data collection for bio Requires additional data collection for bio indicators indicators
Calibrate model with site soil gas data to determine Calibrate model with site soil gas data to determine biodegradation parametersbiodegradation parameters
Reduce Reduce by factor of 10 – 1000 by factor of 10 – 1000
Biodegradation Zone
Mixing in Breathing Zone
Diffusive Transport
Partitioning
Convective Transport into Building
Source
VOCs
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Three-Dimensional Numerical Model
(Abreu and Johnson, 2005)
0 10 20 30 40 50 60 70 80 90 100
x (m )
-8
-6
-4
-2
0
De
pth
bg
s (m
)
Model DescriptionModel Description 3-D vadose zone F&T model 3-D vadose zone F&T model Evaluate building type, source Evaluate building type, source
scenarios, scenarios, and biodegradation kineticsand biodegradation kinetics
Model ResultsModel Results Impact of biodegradation Impact of biodegradation Significance of lateral migrationSignificance of lateral migration
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Investigation Approach for Investigation Approach for Complex SitesComplex Sites
Soil gas profile data Soil gas profile data recommended to recommended to assess biodegradationassess biodegradation
No common approach No common approach to use soil gas profile to use soil gas profile data to quantitatively data to quantitatively evaluate vapor evaluate vapor intrusion pathwayintrusion pathway
Soil surface
CO2
VOCs
O2
Soil gas profile sampling points
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Soil Gas Profile DataSoil Gas Profile Data
Soil gas profile underneath building may be different than that outside building footprint.
May need to assess potential exposure scenarios
Evaluate soil gas data to address uncertainty in sub-surface transport (diffusion and biodegradation)
Reassess vapor intrusion Reassess vapor intrusion evaluation from evaluation from subsurface source subsurface source (include convection and (include convection and ventilation effects)ventilation effects)
Active RemediationActive Remediation Institutional ControlsInstitutional Controls Engineering ControlsEngineering Controls
o “Radon System”o HVAC Modificationso Sealingo Filtrationo Building Design (Brownfields)
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Mitigation Options: Radon Sump
http://www.bre.co.uk/radon/reduce.html
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SummarySummary
Selection of appropriate target levels is key Selection of appropriate target levels is key factor in vapor intrusion assessment.factor in vapor intrusion assessment.
Site investigation methods require careful Site investigation methods require careful planning.planning.
When modeling, assess whether site conditions When modeling, assess whether site conditions are consistent with conceptual model are consistent with conceptual model assumptions and input parameters are assumptions and input parameters are reasonable.reasonable.
Corrective action planning may reduce scope of Corrective action planning may reduce scope of vapor intrusion investigation.vapor intrusion investigation.
Consider multiple lines of evidence to support Consider multiple lines of evidence to support conclusions. A balance of modeling and conclusions. A balance of modeling and monitoring is typically appropriate.monitoring is typically appropriate.