Paul Johnson, Ph.D. Lilian Abreu Ph.D. Candidate Department of Civil and Environmental Engineering

The Ira A. Fulton School of Engineering Arizona State University

Paul Johnson, Ph.D. Lilian Abreu Ph.D. CandidateDepartment of Civil and Environmental EngineeringIra A. Fulton School of Engineering

Site-Specific Modeling in the Context of the OSWER Guidance?

OSWER Guidance


Tier 3: Site-Specific Pathway Assessment“Modeling is considered to be useful for determining which combination of complex factors (e.g., soil type, depth to groundwater, building characteristics, etc.) lead to the greatest impact and, consequently, aid in the selection of buildings to be sampled. It is recommended that sampling of sub-slab or crawlspace vapor concentrations and/or sampling of indoor air concentrations be conducted before a regulator makes a final decision…”

OSWER Guidance (11/29/02)


Tier 3: Site-Specific Pathway Assessment - Issues• Why are you limited to near-foundation (e.g., sub-slab) soil gas

data in Tier 3, when you can use soil gas data at any depth or groundwater data in Tier 2?

• Why is semi-site-specific J&E modeling used in Tier 2 to assess impacts, but site-specific J&E modeling is not allowed in Tier 3?

• If you allow site-specific modeling to decide on a subset of buildings does not need to be monitored - aren’t you using it to screen out sites?

• If you could do site-specific modeling in Tier 3 - who is qualified to perform it and who is qualified to review the output?

• Future use scenarios/no building currently present?

OSWER Guidance (11/29/02)


Site-Specific Modeling Options…[What might we do if we ignored the current language in the OSWER guidance?]

1. Site-specific -value determined from “tracer” input

2. Use of J&E model with site-specific inputs

3. Multi-dimensional numerical codes


Determination of, and use of, a site-specific -value

• measure soil gas and indoor concentration of tracer (a conservative chemical not expected to be confounded by ambient or indoor air background sources; radon, 1,1 DCE, etc.)

• Derive site-specific -value • Estimation indoor air

concentrations for chemicals of concern using that site-specific -value

Option #1


enclosed space

vapor migration

Diffusion [pseudo-

steady state]

Well-Mixed

Source[steady or transient]

Vapor Source

Use of J&E (1991) model for site-specific assessment

Option #2

Layered settings?

Measured Deff?

Perched water?

Fresh-water lens?

Site-specific =

Qsoil/Qb


A exp B

exp B A AC

exp B 1

Generalized Sensitivity Assessment of the J&E (1991) Model*

• The output only depends on three parameters (A, B, C)

• If you understand sensitivity to those three parameters, you can quickly assess the sensitivity to any specific input.

P.C. Johnson. 2002. Sensitivity Analysis and Identification of Critical and Non-Critical Parameters for the Johnson and Ettinger (1991) Vapor Intrusion Model. API Technical Bulletin.

A DT

eff

EB (VBAB

) LT

B (QsoilQB

) EB (VBAB

) Lcrack

Dcrackeff

C QsoilQB


Determine Reasonable Initial Estimates for Primary Inputs{(Qsoil/QB), (VB/AB), , Lcrack, LT, Deff

T, Deffcrack, EB}

Calculate Parameters* {A, B, C}

B<0.1 Other B>3

(AB/C)<0.1

Diffusion is thedominant

mechanism acrossfoundation

Advection is thedominant

mechanism acrossfoundation

Diffusionthrough soilis the over-

all rate-limitingprocess

(AB/C)>10(1+A)Other

Diffusionthrough

foundationis the over-


(A/C)<0.1

Diffusionthrough soilis the over-


(A/C)>10Other

Advectionthrough

foundation isthe over-allrate-limiting

process

Critical Non-Critical

(VB/AB) (Qsoil/QB)LT Lcrack

EB


(VB/AB) (Qsoil/QB)Lcrack LT

EB


(VB/AB) (Qsoil/QB)LT Lcrack

EB


(Qsoil/QB) (Qsoil/QB)Lcrack

LT

EB


(VB/AB) Lcrack

LT

EB

(Qsoil/QB)

Result varies with changes in all primaryinputs { (VB/AB), Lcrack, LT, Deff

crack, DeffT,

. EB, (Qsoil/QB)}; however is constrainedto be less than A

A

1 A

DTeff

Dcrackeff

DTeff

Dcrackeff

DTeff

Dcrackeff

Dcrackeff

DTeff

Dcrackeff

DTeff

CB

C

A

1 AC

A

A DTeff

E B ( VBAB

) LT

, B (Qsoil

QB) EB ( VB

AB) L crack

Dcrackeff

, C QsoilQB

* ParameterEquations:

Generalized Sensitivity Assessment of the J&E (1991) Model


Generalized Sensitivity Assessment of the J&E (1991) Model

• If your analysis suggests “high” sensitivity to any inputs…you are probably using:

- an inconsistent set of input values, or

- an unreasonable set or unreasonable range of input values

Most of the time critical*, but pretty well-constrained:[(VB/AB), LT, DT

eff, EB]

Sometimes critical, but data hints at their reasonable values:[(Qs/QB)]

Rarely critical, and any reasonable value works:[, Lcrack, Dcrack

eff]


Needed Improvements…• Confusion stemming

from (improper) use of the EPA spreadsheets could be minimized with the following changes:

1) Reformat the calculations in terms of:[(VB/AB), LT, DT

eff, EB, , (Qs/QB), Lcrack, and Dcrack

eff]

2) Eliminate the Qs calculation and input (Qs/QB) values based on empirical analysis.

3) Input moisture saturations instead of individual moisture contents and total porosities

4) Integrate the spreadsheet with the graphical flowchart for identifying critical parameters

5) Constrain users to reasonable ranges and combinations of inputs…


small diameter tubing

In S

itu D

iffus

ion

Coe

ff. M

easu

rem

ents

syringe

tracer gas tracer gas

tracer gas

sample volume (≈ 9 cm radius)

1-L tedlar bag

Time = 0 Time = t1

Time = t2Time Soil Gas Withdrawn

% M

ass R

ecov

ered

Johnson et al. ES&T 1998


Effect of lateral separation between building and vapor

source?

Effect of building construction (slab vs.

basement)?

Sub-foundation vs. near-foundation soil

gas sampling?

Effect of aerobic biodegradation on

?

Near foundation soil characteristics

Variation in withconcentration,

depth and soil type?

Multi-dimensional multi-component numerical code

Option #3 Effect of changing atmospheric

conditions and occupant habits?

Future use scenarios?


0 10 20 30 40 50 60 70 80 90 100x (m)

-8

-6

-4

-2

0

Dept

h BG

S (m

)Sample details for simulations

Grid spacing is variable - finer detail near cracks, source boundaries, and domain boundaries

10 m x 10m footprint

30 m x 30 m constant source(200 mg/L-vapor)

Fine to medium sand

5 Pa constant building under-pressurization 1 mm wide full perimeter crack

12/d exchange

rate


A Sample Pressure Field…

Symmetrical Simulation - cross-section through plane of symmetry


0 10 20 30 40 50 60 70 80 90 100

x (m )

-8

-6

-4

-2

0

Dep

th B

GS

(m)

0.0001

0 10 20 30 40 50 60 70 80 90 100

x (m )

0

10

20

30

40

50

60

70

y (m

)

Changes in with Source Position and Depth…

No biodegradation

Alpha=1.2e-3 , Qs=4.1 L/min


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 )

0

10

20

30

40

50

60

70

y (m

)


No biodegradation

Alpha=9.3e-6 , Qs=4.1 L/min


No biodegradation


Source no longer under building


0 2 4 6 8 10 12 14 16 18 20 22 24

x (m )

-8

-6

-4

-2

0

Dep

th B

GS

(m)

0 . 1

0 . 9

0 2 4 6 8 10 12 14 16 18 20 22 24

x (m )

-8

-6

-4

-2

0

Dep

th B

GS

(m)

0 . 1

alpha=1.2e-3; Qs=4.0 L/min

alpha=6.1e-4 , Qs=5.1 L/min

Changes in with Building Cons-truction

No biodegradation


0 2 4 6 8 10 12 14 16 18 20 22 24

x (m )

-8

-6

-4

-2

0

Dep

th B

GS

(m)

0 2 4 6 8 10 12 14 16 18 20 22 24

x (m )

-8

-6

-4

-2

0

Dep

th B

GS

(m)Near-

Foundation vs. Through-the-Foundation Sampling?

alpha=1.4e-4 (w/biodegradation)alpha=1.2e-3 (no bio)


0 2 4 6 8 10 12 14 16 18 20 22 24

x (m)

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

Dep

th B

GS

(m)

0 2 4 6 8 10 12 14 16 18 20 22 24

x (m)

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

Dep

th B

GS

(m)0.001

1E-05

Changes in with Depth with Bio-decay

alpha=1.3e-18 (w/biodegradation)alpha=5.7e-4 (no bio)


In Progress…1. Manuscript #1 – Model development and application to

study of lateral distance and depths vs. impacts.2. Manuscript #2 – Effects of aerobic biodegradation on

impacts (source strength, depth, distance)3. Study of role of sub-slab characteristics, pressure

fluctuations, wind effects, etc.4. Use of model to develop nomograph identifying sites where

impacts may not be significant, based on• Building footprint• Depth to vapor source• Vapor source strength


Final Thoughts…1. Draft OSWER Guidance is inconsistent with respect

to the role of modeling for site-specific pathway assessment (and the role of modeling in general..)

2. If the role of site-specific modeling is expanded, then we need to be prepared to address:

• What options are allowed?

• What data is required?

• How to ensure that the use of site-specific modeling is technically credible?


Discussion


Groundwater Data Interpretation IssuesWith samples collected across conventional well screen intervals, there are multiple realizations that would correspond to the same depth-averaged groundwater concentration (in other words, the measured concentrations do not correspond to a unique vapor transport scenario)

C1 > C2 C2 > C3


Input Thoughts Reasonable*H, Dair Tabulated Chemical Properties Actual ValueE 10 - 20 d-1 (energy efficiency studies) 12 d-1

(VB/AB) 2 - 3 m (= ceiling height) 2.5 m

(Qsoil /QB) <0.01 (radon studies and Colorado field data) 0.0001 - 0.01

LT 0.5 - 50 m actual value

(m/T) 10% - 50% (vadose zone + crack) 0.10

Lcapillary 1 - 100 cm 5 cm

(m/T) 90% - 98% (capillary fringe) 0.95

T 0.25 - 0.50 0.35

Lcrack 15 - 60 cm (6 - 24 inches) 15

0.0001 - 0.01 (1=bare dirt floor) 0.001* reasonable conservative value

Model Inputs - what do we know?


1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 0.2 0.4 0.6 0.8 1M /T

T = 0.50

T = 0.35Vadose zone soils

DTeff / Dair

Dw

Dair H i

T 1.33

T 1.33

capi

llary

zone

Sands/ Gravels

Silts/Clays

Sensitivity of DTeff to Moisture Content..

• DTeff not very sensitive to

reasonable variation in moisture content for a given soil type.

• DTeff more sensitive to

variation across gross changes in soil types (i.e. sands -> clay about 5X change).

• The most significant change occurs between vadose zone and capillary fringe soils **however** the magnitude depends on H (beware at small H!)

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Paul Johnson, Ph.D. Lilian Abreu Ph.D. Candidate Department of Civil and Environmental Engineering