1

Conceptual Model Scenarios for The Vapor Intrusion Pathway

Lilian Abreu, Ph.D. ARCADIS Inc.

San Francisco, CA

USEPA Vapor Intrusion WorkshopAEHS Conference, March 2010

San Diego, CA

-8

-6

-4

-2

0

Dep

th b

gs (m

)

Vapor Source

0.01

0.1

0.8

0.7

1E-3

0.9

0.9

0.90.20.1

0.010.10.20.3

2 © 2010 ARCADIS

Presentation Outline

• Objective• Vapor Intrusion (VI) Definition• Brief Description of A&J 3D VI Model • Vapor Fate & Transport Mechanisms• Effect of Site Conditions on VI• Remarks

3 © 2010 ARCADIS

Objective

• Provide an overview of key contaminant vapor fate and transport mechanisms for the vapor intrusion pathway

• Illustrate the effect of different site conditions on VOC concentration distribution in the subsurface and on the indoor air

4 © 2010 ARCADIS

Vapor Intrusion PathwayWhat is it?

1. Source (GW & Soil)2. Volatilization3. Migration through porous4. Infiltration through

building envelope5. Indoor sources can

confound

Contaminated Groundwater PlumeContaminated Groundwater Plume

Vadose Zone SoilsVadose Zone Soils

PAINT

Work BenchWork Bench

BasementBasement

Sewer line or other UtilitiesSewer line or other Utilities

Potentially exposed individualPotentially exposed individual

CracksCracks

CracksCracks

HVACHVACSystemSystem

11

22

33

44

55

Paint CanPaint Can

Courtesy of Chris Lutes

5 © 2010 ARCADIS

Foundation Entry Points

• Floor wall joints• Loose fitting pipes• Floor drains• Cracks in foundation

Abreu, 2005 (adapted from Loureiro, 1987)

6 © 2010 ARCADIS

Type of ContaminantsRecalcitrant

– Very persistent (No degradation or slow degradation under natural conditions)

– Chlorinated hydrocarbons (e.g., PCE, TCE)– Degradation products can be more toxic (e.g., Vinyl

chloride)

Aerobically Biodegradable (see presentation in the afternoon) – Readily degradable under natural conditions in the

presence of oxygen– Petroleum hydrocarbons (e.g., Benzene, Toluene)– Degradation products less toxic (e.g., H2O, CO2)– Methane byproduct under anaerobic conditions

7 © 2010 ARCADIS

• The three-dimensional transient model solves equations for:

• Soil gas flow

• Multi-species simultaneous vapor transport (e.g., O2, hydrocarbons)

• Reactions (e.g. degradation) in the vadose zone with user-defined kinetics

• Model simulates transient contaminant transport and heterogeneous subsurface conditions

• The model description and results have been published in peer-reviewed journals and presented at several conferences & workshops sponsored by EPA and API.

Abreu & Johnson 3D Model Overview

8 © 2010 ARCADIS

Configurations Handled by the Model

Can evaluate slab on grade or basement construction

Symmetrical scenario with source at the water table or within unsaturated zone

Nonsymmetrical scenario with source at the water table

Nonsymmetrical scenario with source within the unsaturated zone

9 © 2010 ARCADIS

Model Domain and Site Specific Inputs

0 10 20 30 40 50 60 70 80 90 100

x (m)

-8

-6

-4

-2

0

Dep

th b

gs (m

)

Building type, footprint, crack location and size

Source location, size and strength

Soil characteristics and properties

Building pressurizationand air exchange rate Ground surface

cover type

Vapor source idealized as uniform concentrationOnly vapor transport, No GW transport

10 © 2010 ARCADIS

Presentation of ResultsVertical cross-sections through center of building showing:

Concentration distribution normalized by source-zone vapor concentration VI attenuation factor,

α = Cindoor/ Csource

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Previously Existing Models1D models (e.g.; Johnson and Ettinger Model)

Simplistic geometry, one-dimensional transport, steady-state conditions, no biodegradation

A&J 3D Model Helped Advance Knowledge onsource-building lateral separation

site heterogeneous conditions,

Sub-foundation vs. near-foundation soil gas

oxygen limited aerobic biodegradation

State-of-Science Advancements

12 © 2010 ARCADIS

Key vapor fate and transport mechanisms

13 © 2010 ARCADIS

Transport due to concentration gradients

Dominant process close to the source

Diffusion

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

0.9

0.1

0.01

0.1

0.01

water table

x (m)

1E-4

1E-3

vaporsource

-8

-6

-4

-2

0

Dep

th b

gs (m

)

0.9

0.7

0.5

0.30.1

0.01

water table

1E-3

1E-4

vapor source

Dep

th b

gs (m

)

Normalized Vapor Concentration Profiles

14 © 2010 ARCADIS

Advection

Transport due to air pressure gradients

More significant close to the building

Normalized Gauge Pressure Field & Qs

0 2 4 6 8 10 12-12-11-10

-9-8-7-6-5-4-3-2-10

-3-2-10

Dep

th b

gs (m

)

0.01

0.050.10.2

0.050.10.20.3

x (m)0 2 4 6 8 10 12

0.0050.010.050.10.2

0.010.05

0.10.20.3

Qs = 3.1 L/min

Qs = 4.1 L/min

Qs = 1.1 L/min

Qs = 2.1 L/min

Crack on Perimeter of Slab Crack on Center of Slab

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0 10 20 30 40 50 60 70 80 90 100

x (m)

-8

-6

-4

-2

0

Dep

th b

gs (m

)

0 10 20 30 40 50 60 70 80 90 100

x (m)

-8

-6

-4

-2

0D

epth

bgs

(m)

a) Hydrocarbon

b) Oxygen

α bio = 1.1 x 10-13

0 10 20 30 40 50 60 70 80 90 100

x (m)

-8

-6

-4

-2

0

Dep

th b

gs (m

)

0 10 20 30 40 50 60 70 80 90 100

x (m)

-8

-6

-4

-2

0D

epth

bgs

(m)

a) Hydrocarbon

b) Oxygen

α bio = 1.1 x 10-13

(Abreu and Johnson, 2005 & 2006)

Aerobic Biodegradation

Decomposition of contaminants by microorganisms within vadose zoneMay significantly affect contaminant fate & transportOxygen distribution within vadose zone is key

(see afternoon section)

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Effect of different site conditions on the VI pathway

This presentation focus on recalcitrant compounds, the afternoon presentation focus on

aerobically biodegradable petroleum hydrocarbons

Steady-state conditions presented firstTransient conditions presented later

17 © 2010 ARCADIS

Source Vapor Concentration

Higher source concentration results in higher concentrations in subsurface gas and indoor air (if there is vapor intrusion and all other conditions are the same)

Vapor source Vapor source

Vapor source

(x1,000) (x100,000)

For recalcitrant compounds, normalized concentrations and α are independent of source concentration

18 © 2010 ARCADIS

Steady pressurization

If building is under-pressurized the air flows from soil into building and may lead to higher sub-slab and indoor air concentrations

If building is over-pressurized the air flows from building into soil and may lead to lower indoor air and sub-slab concentrations (if indoor air is clean)

Building Pressurization

Vapor source Vapor source

Vapor sourceVapor source

19 © 2010 ARCADIS

Increased source depth results in decrease in α(i.e., greater attenuation) • Non-linear relationship

Sub-slab concentrations and α for buildings with basements are slightly higher than slab-on-grade construction

Source Depth & Building Type

-3-2-10 0.10.2

0.70.9

0.10.20.5

0.7

0.9

0.8

0 2 4 6 8 10 12-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8 10 12

α = 1.7Ε−3 α = 1.2Ε−3

α = 5.7Ε−4 α = 2.8Ε−4

0.8

0.6

Dep

th b

gs (m

)

x (m)

0.9

0.8

0.7

0.10.2

0.3

0.4

0.5

0.6

0.9

0.8

0.7

0.1

0.20.3

0.4

0.5

0.6

Vapor source Vapor source

Vapor sourceVapor source

20 © 2010 ARCADIS

A 20 m (66 ft) lateral distance separation results in decrease in α by five orders of magnitude

Source Lateral DistanceShallow Groundwater

0 10 20 30 40 50 60 70 80 90 100-3-2-10

1E-41E-5

0.011E-3

0.30.2

0.5

0.10.4

0.9

α = 4.2Ε−8

0 10 20 30 40 50 60 70 80 90 100-3-2-10

1E-4 1E-50.01

1E-30.6

0.20.10.4

0.9

α = 1.9Ε−3

Dep

th b

gs (m

)

x (m)

Vapor source

Vapor source

21 © 2010 ARCADIS

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

1E-41E-5 0.011E-3

0.30.2

0.5

0.10.4

0.9

α = 1.3Ε−5

0.60.70.8

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

1E-4

0.011E-3

0.2

0.6

0.1

0.40.7

0.9

0.8

α = 1.2Ε−3

Dep

th b

gs (m

)D

epth

bgs

(m)

x (m)

Vapor source

Vapor source

A 20 m (66 ft) lateral distance separation results in decrease in α by two orders of magnitude

Source Lateral Distance Deep Groundwater

22 © 2010 ARCADIS

0 10 20 30 40 50 60 70 80 90 100

x (m)

Dep

th b

gs (m

)

1E-41E-5 0.011E-3

0.30.2

0.5

0.10.4

0.9

0.60.70.8

0 10 20 30 40 50 60 70 80 90 100

Dep

th b

gs (m

)

0.011E-3

0.2

0.60.1

0.1

0.7

0.9

0.8

1E-30.01

0.3

Changes in α with Source Position

α = 1.2 x 10-3 α = 8.4 x 10-4

α = 2.3 x 10-4 α = 1.3 x 10-5

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

-15 -10 -5 0 5 10 15 20 25

α

3 m bgs (basement)3 m bgs (slab-on-grade)8 m bgs (basement)8 m bgs (slab-on-grade)

source zone no longer beneath building

Source Edge - Building Center Separation (m)

10 m x 10 m building footprintsource size 30 m x 30 m

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

Dep

th b

gs (m

)

1E-4

0.011E-3

0.2

0.6

0.1

0.40.7

0.9

0.8

0 10 20 30 40 50 60 70 80 90 100

x (m)

-8

-6

-4

-2

0

Dep

th b

gs (m

)

1E-4

0.011E-3

0.2

0.60.1

0.4

0.7

0.2

0.8

0.1

23 © 2010 ARCADIS

α

Source Depth & Lateral DistanceUnder equal

conditionsEffect of lateral distance separation is more significant for shallow source scenariosSource depth has small effect when building is above source areaBldg. construction has small impact

Abreu and Johnson, EST 2005

Shallow GW

Deep GW

24 © 2010 ARCADIS

Other Considerations…

Site conditions may lead to opposite finding:

Source locationSurface sealBuilding ventilationMultiple sourcesSoil lithology

Under equal conditions, α for slab-on-grade building is smaller than for basement

-8

-6

-4

-2

0

Dep

th b

gs (m

)

α = 1.2Ε−3 α = 6Ε−4

0.010.20.10.01

0.8

0.20.1

0.9

0.60.3 0.30.7

-8

-6

-4

-2

0

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

x (m)

α = 2.2Ε−4α = 4.3Ε−5

Dep

th b

gs (m

)

α = 4.3Ε−5α = 1.7Ε−5

0.9

0.01

0.20.1

0.8

0.9

0.6

0.3

0.1

0.1

0.7

0.60.3

0.20.1

1E-31E-4

0.011E-31E-4

Dep

th b

gs (m

)

Impermeablecover

Vapor source

Vapor source

Vapor source

25 © 2010 ARCADIS

Building Conditions & Ventilation

Other building related factors also affect the indoor air concentrations (or αvalues)

Presence or absence of cracks Building ventilation (i.e., air exchange rate and building air flow rates)

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

α = 8.6Ε−4

No cracks or openingsIncomplete pathway

Building A

α = 0 α = 2.2Ε−4

Dep

th b

gs (m

)

Buildings B and C

α = 0 α = 6.4Ε−6 α = 4.3Ε−6

Dep

th b

gs (m

)

A B C

A B C

Full length perimeter cracks

Building B Cp (Pa) 1 7Qs (L/min) 1 7.3AER (1/h) 1 0.25Qbdg (m3/h) 300 75

Scenario 2

Vapor Source

0.90.80.7

0.5

0.6

0.90.80.7

0.1

0.20.5

0.6

0.01

1E-3

1E-4

1E-5

Building B Cp (Pa) 5 5Qs (L/min) 5.2 5.2AER (1/h) 0.25 1Qbdg (m3/h) 62.5 250

Scenario 1

Vapor Source

0.90.8

0.7

0.5

0.6

0.90.80.7

0.5

0.6

0.90.80.7

0.50.6

0.40.30.2

0.10.1 0.10.1

26 © 2010 ARCADIS

Dep

th b

gs (m

)

x (m)

y (m

)-8

-6

-4

-2

0

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

Vapor Source

6,000

Vapor Source

9,000

8,000

1,000

2,00

0

100

9,00

06,000

Vertical Cross-Section View

Plan View at 5 m bgs

1,000

100

1,00

0 2,0001,000

2,000100

Vapor source zone footprint

4,000

IA = 1.1 ug/m3

1,000

IA = 7.7 ug/m3

10,000

10,000

Multiple Sources

Identifying multiple sources and their locations is key in understanding contaminant distribution and identifying buildings with potentially higher VI risks

27 © 2010 ARCADIS

Layered Soils

Moist, fine-grained soils may act as diffusive barrier

Higher concentration belowLower concentration above

-8

-6

-4

-2

0

0 2 4 6 8 10 12-8

-6

-4

-2

0

-8

-6

-4

-2

0

Dep

th b

gs (m

)

Qs = 3.2 L/min

Qs = 1.9 L/min

x (m)

Qs= 4 L/min

Sand

Sand

Silt

Sand

Silt

Vapor source

Vapor source

Vapor source

-8

-6

-4

-2

0

-8

-6

-4

-2

0

0.4

0.1

0.5

0.20.3

0.40.60.7

0.8

0.9

0.50.60.9

0.10.20.3

0 2 4 6 8 10 12-8

-6

-4

-2

0 α = 9.7Ε−4

α = 3.0Ε−4

0.6

0.7

0.9

0.8

x (m)

α = 1.2Ε−3

CMSCMS ResultsResults

28 © 2010 ARCADIS

Layered Soils & Lateral Source

High moisture content layer on ground surface may result in slight increase of lateral vapor migration

ResultsCMS

High moisture content layer above the vapor source may significantly decrease vapor migration

-8

-6

-4

-2

0

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

-8

-6

-4

-2

0

Dep

th b

gs (m

)

x (m)

Qs = 4 L/min

Qs = 3.8 L/min

Qs = 2.3 L/min

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0

-8

-6

-4

-2

0

1E-41E-5 0.011E-3

0.30.2

0.5

0.10.4

0.9

α = 1.3Ε−5

0.60.70.8

α = 4.3Ε−7

1E-51E-4

0.011E-3 0.1

0.9

1E-7 1E-6

-8

-6

-4

-2

0

0.20.5

0.1 0.4

0.9

0.6

0.7

0.8

0.011E-3

α = 7.6Ε−5

0.2

x (m)

Vapor source

Vapor source

Vapor source

Sand

Silt

Sand

Sand

Silt

α = 4.3E-7

α = 7.6E-5

α = 1.3E-5

Qs= 3.8 L/min

Qs= 2.3 L/min

Qs= 4 L/min

29 © 2010 ARCADIS

-8

-6

-4

-2

0

-8

-6

-4

-2

0

Dep

th b

gs (m

)

A

B

Qs = 4 L/min Qs = 4 L/min

Qs = 3.9 L/minQs = 3.3 L/min

Qsoil = 3.5 L/min Qsoil = 4.0 L/min

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0 C

x (m)

Heterogeneous SubsurfaceCMS

x (m)

-8

-6

-4

-2

0

-8

-6

-4

-2

0α = 1.2Ε−3 α = 1.2Ε−3

0.01

0.8

0.2 0.1

0.9

0.01

0.8

0.20.1

0.9

0.70.70.3 0.3

α = 1.1Ε−3α = 3.0Ε−4

0.01

0.8

0.2

0.1

0.9

0.7

0.1

0.1

0.9

0.1 0.3

0.01

0.1

0.1

0.3

0 10 20 30 40 50 60 70 80 90 100-8

-6

-4

-2

0α = 7.0Ε−5 α = 1.0Ε−3

0.01

0.1

0.8

0.7

1E-3

0.9

0.9

0.90.20.1

0.010.10.20.3

Silt

Sand

Sand

Results

Sand

Clay

Results in spatial variability of soil gas concentration distribution

Vapor source

Vapor source

Vapor source

Qs= 4 L/min

Qs= 3.5 L/min Qs= 4 L/min

Qs= 3.9 L/min

Qs= 4 L/min

Qs= 3.3 L/min

α = 1.2E-3 α = 1.2E-3

α = 1.1E-3α = 3.0E-4

α = 7.0E-5 α = 1.0E-3

30 © 2010 ARCADIS

Steady Wind LoadCan create non-uniform distribution of sub-slab concentrations

20 25 30 35 4020

25

30

35

40

y (m

)

101

0.1

20 25 30 35 40

0.11

110

150

x (m)

Hydrocarbon subslab concentration

20 25 30 35 4020

25

30

35

40

y (m

)

0

0.1

1

10

30

50

70

90

110

130

150

mg/L

NE wind (11 miles/h) North wind (11 miles/h)Kg = 1E-10 m2

Building footprintSlab on grade

31 © 2010 ARCADIS

Transient Transport by DiffusionProfiles & Time Since Source Release

Moisture content retards the migration of contaminants

0 2 4 6 8 10 12

-8

-6

-4

-2

0

-8

-6

-4

-2

0

0 2 4 6 8 10 12-8

-6

-4

-2

0

1 year

3 years

1 month .

Dep

th b

gs (m

)

0.01

1E-3

0.01

1E-70.20.1

0.01

0.30.40.5

0.6

0.3

0.2

0.30.40.50.6

0.2

0.7

0.1

0.20.1

0.01

0.3

0.20.1

0.3

0.1

0.1

Sandα = 1.4Ε−6 α = 8.0Ε−21

α = 4.3Ε−7α = 3.3Ε−5

α = 3.8Ε−5 α =9.0Ε−6

0.9

0.9

0.90.90.7

0.7

SiltEffect of Soil Moisture Content

32 © 2010 ARCADIS

Transient Transport by DiffusionProfiles & Time Since Source Release

Chemical sorption to soil particles retards the migration of the contaminant

Effect of foc & koc

0 2 4 6 8 10 12

-8

-6

-4

-2

0

-8

-6

-4

-2

0

0 2 4 6 8 10 12-8

-6

-4

-2

0

1 year

3 years

1 month

Dep

th b

gs (m

)

x (m)

α = 3.3Ε−5

α = 3.8Ε−5

α = 7.7Ε−8

α =5.7Ε−6

1E-4

0.01

1E-3

0.01

1E-70.20.1

0.01

0.30.40.5

0.6

0.3

0.2

0.30.40.50.6

0.2

0.7

0.1

0.20.1

0.01

0.3

0.20.1

0.3

0.1

0.1

foc = 0.001, koc = 60 cm /g3 foc = 0.005, koc = 600 cm /g3

α = 1.4Ε−6 α = 1Ε−21

foc = fraction of organic carbon in soil

koc = sorption coefficient of chemical to organic carbon

33 © 2010 ARCADIS

Time to Reach Steady-State by DiffusionTime is affected by

Source depth & lateral distanceSoil moisture & focContaminant properties (koc)

Non-retarded Time to Reach Near SS

Johnson et al. (1999)

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛ ⋅⋅++=

nHg

bocfnocK

nHg

wvR

φ

ρ

φ

φ ,1

Multiply τss/Rv byChemical retardation factor

clay

sand

34 © 2010 ARCADIS

Transient Pressure Fluctuations

Fluctuations in building pressurization can affect the transport of soil gas into buildings as the air flow direction is reversed

Atmospheric Pressure Fluctuations

101,225

101,250

101,275

101,300

101,325

101,350

101,375

0 5 10 15 20

time (h)

Abs

olut

e pr

essu

re (P

a)

Δ=100 Pa

Pressure difference indoor-sub-slab air

-10.0

-5.0

0.0

5.0

10.0

0 5 10 15 20

time (h)

Pind

oor-P

sub-

slab

(Pa)

kg = 1E-11 m2 kg = 1E-12 m2

(+) Building over-pressurized

Air flow through slab crack

-5

-2.5

0

2.5

5

0 5 10 15 20

time (h)

Air

flow

(L/m

in)

kg = 1E-11 m2 kg = 1E-12 m2

(+) Flow from subsurface to building

35 © 2010 ARCADIS

Fluctuations in Water Table Elevation

Groundwater sourceGroundwater source

Groundwater source

Contaminated soilClean soil

Rising Falling Original

Clean groundwater

Contaminated soil

Original Rising Falling

Contaminated groundwater

Contaminated groundwater

Residual soil source

Schematic illustration (Not model simulated)

Fluctuations in the water table elevation may affect the distribution of contaminants in the subsurface and the source-foundation separation

Soil Source

Groundwater Source

36 © 2010 ARCADIS

RemarksVOC concentration distribution in the subsurface may not be uniform and can exhibit spatial and temporal variability (including the sub-slab concentration distribution).Factors that influence subsurface contaminant distribution and indoor air concentration:

Source concentration, depth and lateral distanceSoil physical propertiesSite geology (i.e., subsurface heterogeneities)Building conditions (construction, pressure, AER)Surface seal (impermeable cover)Time since contaminant release occurredAerobic biodegradation (see afternoon presentation)

37 © 2010 ARCADIS

RemarksThese vapor intrusion conceptual model scenarios can help practitioners develop a site-specific conceptual model, plan VI investigations, and interpret the results.The presented simulations are based on idealized and simplified assumptions about conceptual model scenarios to illustrate VI processes. These scenarios may not be representative of site-specific conditions, but may be used to guide site-specific VI evaluations.