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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
California Industrial Hygiene CouncilCalifornia Industrial Hygiene Council1616thth Annual Conference Annual Conference
San Diego, CA San Diego, CA December 4 – 6, 2006December 4 – 6, 2006
2
Timeline for the Vapor Timeline for the Vapor Intrusion PathwayIntrusion Pathway
1989
Hillside SchoolHill AFB
J&E Model
1991
Air-SuperfundGuidance
1992
ASTM RBCA
Standard
19942001
Draft RCRA EISupplemental Guidance
OSWER Draft Guidance
20021999
RCRA EI Guidance
2004
Response to Comments
Radon Intrusion & Vapor Diffusion studies
1980’s2005
2007
CA, NY, NJ State Guidance Development
Revised OSWER Guidance
CDOTRedfields
EndicottMEW
MA StateGuidance
1993
3
Vapor Migration to Indoor Air -Vapor Migration to Indoor Air -General Conceptual ModelGeneral Conceptual Model
4
Vapor Intrusion Evaluation Vapor Intrusion Evaluation StrategyStrategy
Utilize Tiered ApproachUtilize Tiered Approach Data collection and analysis increase in higher Data collection and analysis increase in higher
tierstiers Target Indoor Air LevelsTarget Indoor Air Levels
Risk-based levels, PELs, backgroundRisk-based levels, PELs, background Media Sampled & LocationsMedia Sampled & Locations
Groundwater, soil gas, indoor airGroundwater, soil gas, indoor air Near, next to, or beneath buildingsNear, next to, or beneath buildings
Other DataOther Data Geologic characterization, building Geologic characterization, building
characteristicscharacteristics Modeling OptionsModeling Options
Empirical, screening level, site-specificEmpirical, screening level, site-specific Corrective Action SelectionCorrective Action Selection
5
Target Indoor Air LevelsTarget Indoor Air Levels
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
6
Site CharacterizationSite Characterization
Indoor Air
Groundwater
Soil
Soil Gas
What data are best to characterize vapor intrusion pathway?
7
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
8
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.
9
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
10
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
11
Sub-Slab Soil Gas Sampling
Requires building accessRequires building access Methods developed to limit intrusivenessMethods developed to limit intrusiveness
(DiGiulio, 2004)
12
• Mixing in Breathing Zone
• Diffusive Transport toBreathing Zone
• Impacted Soil and/or Groundwaterin Equilibrium with Soil Gas
Vapor Intrusion ModelingVapor Intrusion Modeling
• Convective Transport into Bldg
Risk is proportional to ( x (Csoil gas)
airC
gas soilC
gas soil
airC
C
13
Empirical Attenuation Factor
(Dawson, 2004)
14
Johnson-Ettinger (1991) Attenuation Factor
1expexp
exp
Beffcrack
cracksoil
Tsoil
BeffT
TB
BeffT
Beffcrack
cracksoil
Beffcrack
cracksoil
TB
BeffT
AD
LQ
LQAD
LQAD
AD
LQ
AD
LQ
LQAD
Primary Parameters
• Deff = Effective diffusion coefficient
• LT = Depth to source
• AB = Building area in contact with soil
• QB = Building ventilation rate
• Qsoil = Soil gas convection rate
• Dcrack = Eff. diff. coeff. through cracks
• Lcrack = Crack thickness
• = Building crack factor
Secondary Parameters• Deff = fn(H, Dwater, Dair, T, w)
for each layer• LT = (Li)• Qsoil = fn(k, DP, rcrack, zcrack,
xcrack)
J & E Model has dozens of input parameters, how much data is required to use?
15
Common Screening Model Common Screening Model AssumptionsAssumptions
One-dimensional One-dimensional vertical transportvertical transport
Steady state conditionsSteady state conditions No preferential No preferential
pathwayspathways Uniform mixing within Uniform mixing within
buildingbuilding Slab on grade or Slab on grade or
basement constructionbasement construction
No biodegradationNo biodegradation Homogeneous vadose-Homogeneous vadose-
zonezone Constant source Constant source
concentrationconcentration No gas generation No gas generation
(e.g., municipal waste)(e.g., municipal waste) No barometric No barometric
pumpingpumping
Prior to using model results, you need to ensure that model assumptions and site conditions are consistent
16
Constrained Model UseConstrained Model Use
Many problems Many problems with vapor with vapor intrusion modeling intrusion modeling associated with associated with improper inputsimproper inputs
Updated EPA Updated EPA spreadsheets will spreadsheets will limit values limit values allowed for inputsallowed for inputs
Constraints based Constraints based on Johnson, 2002on Johnson, 2002
US EPA VAPOR INTRUSION ASSESSMENT MODEL (VIA_MODEL.xls)
Site Name:Note: Cells with borders indicate parameters that may be changed by the user.
Parameter Units Symbol Value Default Flag Comment
Source Characteristics:Source medium Source Groundwater
Groundwater concentration (ug/L) Cmedium 100
Depth below grade to water table (m) Ls 3.00
Average groundwater temperature (oC) Ts 15 15
Calc: Source vapor concentration (ug/m3) Cs 44484
Chemical:Chemical Name Chem Tetrachloroethylene
CAS No. CAS 127184
Toxicity Factors
Unit risk factor (ug/m3)-1 URF 5.90E-06 5.90E-06
Reference concentration (ug/m3) RfC 6.00E+02 6.00E+02
Building Characteristics:Building setting Bldg_Setting Residential Residential
Foundation type Found_Type Basement w/ slab Basement w/ slab
Depth below grade to base of foundation (m) Lb 2.00 2.00
Foundation thickness (m) Lf 0.10 0.10
Fraction of foundation area with cracks (-) eta 1.00E-03 1.00E-03
Enclosed space floor area (m2) Ab 150 150
Enclosed space mixing height (m) Hb 3.66 3.66
Indoor air exchange rate (1/hr) ach 0.50 0.50
Qsoil/Qbuilding (-) Qsoil_Qb 0.020 0.020
Calc: Building ventilation rate (m3/hr) Qb 274.50 274.50
Calc: Average vapor flow rate into building (m3/hr) Qsoil 5.49 5.49
Vadose zone characteristics:Stratum A (Top of soil profile):
Stratum A SCS soil type SCS_A Sand
Stratum A thickness (from surface) (m) hSA 3.00
Stratum A total porosity (-) nSA 0.375 0.375
Stratum A water-filled porosity (-) nwSA 0.054 0.054
Stratum A bulk density (g/cm3) rhoSA 1.660 1.660
Stratum B (Soil layer below Stratum A):
Stratum B SCS soil type SCS_B Not Present
Stratum B thickness (m) hSB
Stratum B total porosity (-) nSB
Stratum B water-filled porosity (-) nwSB
Stratum B bulk density (g/cm3) rhoSB
Statum C (Soil layer below Stratum B):
Stratum C SCS soil type SCS_C Not Present
Stratum C thickness (m) hSC
Stratum C total porosity (-) nSC
Stratum C water-filled porosity (-) nwSC
Stratum C bulk density (g/cm3) rhoSC
Stratum directly above the water table
Stratum A, B, or C src_soil Stratum A
Height of capillary fringe (m) hcz 0.170 0.170
Capillary zone total porosity (-) ncz 0.375 0.375
Capillary zone water filled porosity (-) nwcz 0.253 0.253
Exposure Parameters:Target risk for carcinogens (-) Target_CR 1.00E-06 1.00E-06
Target hazard quotient for non-carcinogens (-) Target_HQ 1 1
Exposure Scenario Scenario Residential Residential
Averaging time for carcinogens (yrs) ATc 70 70
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
17
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
18
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
19
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
20
Investigation Approach for Investigation Approach for Complex SitesComplex Sites
Soil gas profile data Soil gas profile data recommended to recommended to assess biodegradationassess biodegradation
Biodegradation Biodegradation significantly affects significantly affects petroleum compound petroleum compound vapor migrationvapor migration
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
21
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)
Soil gas samples
22
Example Modeling ResultsExample Modeling Results
Cluster 2
0
10
20
30
40
50
60
1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00
Dimensionless Concentration (C/Csource)
Dep
th (
ft)
Benzene Detects Benzene ND DLM PCE Detects PCE NDs JEM
t1/2 = 2.8 dlambda = 0.25 day-1DLM = 1-10 ft bgs
PCE Cgw = 0.79 ppbBenzene Cgw = 52500 ppb
Lithology
23
Choice of RemedyChoice of Remedy
Active RemediationActive Remediation Institutional ControlsInstitutional Controls Engineering ControlsEngineering Controls
o “Radon System”o HVAC Modificationso Sealingo Filtrationo Building Design (Brownfields)
24
Mitigation Options: Radon Sump
http://www.bre.co.uk/radon/reduce.html
25
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.