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

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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OSWER Draft Guidance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

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Vapor Migration to Indoor Air -Vapor Migration to Indoor Air -General Conceptual ModelGeneral Conceptual Model

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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

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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

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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

Vapor Intrusion ModelingVapor Intrusion Modeling

• Convective Transport into Bldg

Risk is proportional to ( x (Csoil gas)

airC

gas soilC

gas soil

airC

C

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Empirical Attenuation Factor

(Dawson, 2004)

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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?

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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

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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

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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

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

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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)

Soil gas samples

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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

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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)

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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.