Transcript No Slide Title

Site-Specific
Modeling in the
Context of the
OSWER
Guidance?
OSWER
Guidance
Paul Johnson, Ph.D.
Lilian Abreu Ph.D. Candidate
Department of Civil and
Environmental Engineering
Ira A. Fulton School of Engineering
The Ira A. Fulton School of Engineering
Arizona State University
OSWER Guidance (11/29/02)
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…”
The Ira A. Fulton School of Engineering
Arizona State University
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?
The Ira A. Fulton School of Engineering
Arizona State University
Site-Specific Modeling Options…
[What might we do if we ignored the current language in the OSWER guidance?]
1. Site-specific a-value
determined from “tracer”
input
2. Use of J&E model with sitespecific inputs
3. Multi-dimensional
numerical codes
The Ira A. Fulton School of Engineering
Arizona State University
Option #1
Determination of, and use of, a sitespecific a-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 a-value
• Estimation indoor air
concentrations for chemicals of
concern using that site-specific
a-value
The Ira A. Fulton School of Engineering
Arizona State University
Option #2
Use of J&E (1991)
model for site-specific
assessment
Site-specific
a = Qsoil/Qb
Well-Mixed
Layered
settings?
enclosed space
Measured
Deff?
Diffusion
[pseudosteady state]
vapor
migration
Perched
water?
Source
Vapor
Source
Freshwater
lens?
[steady or transient]
The Ira A. Fulton School of Engineering
Arizona State University
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. 
a
A exp B
A
exp B  A   exp B  1
C 


 Qsoil

VB
(
)
E
(
)
L



B A
crack 
Deff
QB
T
B
 B  

A  
V
eff
E ( B ) L 


Dcrack 
B
T




AB





Q

C   soil 
 Q B 
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.
The Ira A. Fulton School of Engineering
Arizona State University
Det ermine Reasonable Init ial Est imat es for P rimary Input s
{(Qsoil/QB), (VB/AB), , Lcrack , LT, Deff T, Deff crack , EB}
Calculat e P aramet ers*
{A, B, C}
Generalized
Sensitivity
Assessment
of the J&E
(1991)
Model
Diffusion is the
dominant
mechanism across
foundation
(AB/C)<0.1
B<0.1
Ot her
Other
a

Critical

Diffusion
through
foundation
is the overall ratelimiting
process
A
1 A
a
Non-
 Critical
Critical
(V B/AB)
(A/C)<0.1
(AB/C)>10(1+A)
Diffusion
through soil
is the overall ratelimiting
process
B>3
(V B/A B)
Other
(A/C)>10
Diffusion
through soil
is the overall ratelimiting
process
C
B
Advection
through
foundation is
the over-all
rate-limiting
process
aA

Non-
Critical
a C

Critical
Non-
Critical
(Qso il/QB)
Advection is the
dominant
mechanism across
foundation
Critical
(Qso il/QB)
(V B/AB)
NonCritical
(Qso il/QB)
(Qso il/QB) (Qso il/QB)
LT
Lcr ack
Lcr ack
LT
LT
Lcr ack
Lcr ack
D eff
T
EB
D eff
crack

D eff
crack
EB

Deff
T
D eff
T
EB
D eff
crack

D eff
crack







Result varies with changes in all primary
inputs { (VB/AB), Lcr ack, LT, Def fcr ack, Def fT,
. EB, (Qso il/QB)}; however a is constrained
to be less than A
a



 D eff

T
,
A  
V
E ( B ) L 
B
T


AB


 Q soil

VB
( Q ) E B ( A ) L crack
B
,
B   B
D eff


crack 




(V B/A B)
LT
D eff
T
Q 
C   soil 
 Q B 

LT
EB
A
A
C
1
Critical
* Parameter
Equations:
D eff
T

NonCritical
Lcr ack
D eff
crack

EB
(Q
so il/QB)

The Ira A. Fulton School of Engineering
Arizona State University
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
The Ira A. Fulton School of Engineering
Most of the time critical*, but
pretty well-constrained:
[(VB/AB), LT, DTeff, EB]
Sometimes critical, but data
hints at their reasonable values:
[(Qs/QB)]
Rarely critical, and any
reasonable value works:
[, Lcrack, Dcrackeff]
Arizona State University
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, DTeff, EB, ,
(Qs/QB), Lcrack, and Dcrackeff]
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…
The Ira A. Fulton School of Engineering
Arizona State University
syringe
small
diameter
tubing
tracer gas
tracer gas
Time = 0
Time = t1
1-L tedlar bag
sample volume
(≈ 9 cm radius)
tracer gas
Time = t2
The Ira A. Fulton School of Engineering
% Mass Recovered
In Situ Diffusion Coeff. Measurements
Johnson et al. ES&T 1998
Time Soil Gas Withdrawn
Arizona State University
Effect of changing
atmospheric
conditions and
occupant habits?
Option #3
Multi-dimensional
multi-component
numerical code
Future use
scenarios?
Effect of building
construction (slab vs.
basement)?
Near foundation
soil characteristics
Effect of aerobic
biodegradation on
a?
Effect of lateral
separation between
building and vapor
source?
Variation in a
withconcentration,
depth and soil
type?
Sub-foundation vs.
near-foundation soil
gas sampling?
The Ira A. Fulton School of Engineering
Arizona State University
Sample details for simulations
10 m x 10m footprint
5 Pa constant building
under-pressurization
1 mm wide full perimeter crack
Depth BGS (m)
0
-2
12/d
exchange
rate
-4
Fine to medium
sand
-6
-8
0
10
20
30
40
Grid spacing is variable - finer
detail near cracks, source
boundaries, and domain
boundaries
The Ira A. Fulton School of Engineering
50
x (m)
60
70
80
90
100
30 m x 30 m constant source
(200 mg/L-vapor)
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A Sample Pressure Field…
Symmetrical Simulation - cross-section through plane of symmetry
The Ira A. Fulton School of Engineering
Arizona State University
Alpha=1.2e-3 , Qs=4.1 L/min
0. 00
-2
01
-4
-6
-8
0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
x (m)
70
60
50
y (m)
Changes
in a
with
Source
Position
and
Depth…
Depth BGS (m)
0
40
30
20
10
0
No biodegradation
0
The Ira A. Fulton School of Engineering
10
20
30
40
50
x (m)
Arizona State University
Alpha=9.3e-6 , Qs=4.1 L/min
-2
-4
-6
-8
0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
x (m)
70
60
50
y (m)
Changes
in a
with
Source
Position
and
Depth…
Depth BGS (m)
0
40
30
20
10
0
0
No biodegradation
The Ira A. Fulton School of Engineering
10
20
30
40
50
x (m)
Arizona State University
Changes
in a
with
Source
Position
and
Depth…
Source no longer
under building
No biodegradation
The Ira A. Fulton School of Engineering
Arizona State University
alpha=1.2e-3; Qs=4.0 L/min
0
-2
-4
-6
0.9
-8
0
2
4
6
8
10
12
14
16
18
20
22
24
x (m)
alpha=6.1e-4 , Qs=5.1 L/min
0
0.1
Depth BGS (m)
Changes
in a
with
Building
Construction
Depth BGS (m)
0.1
-2
-4
-6
-8
0
No biodegradation
The Ira A. Fulton School of Engineering
2
4
6
8
10
12
14
16
18
20
22
24
x (m)
Arizona State University
alpha=1.4e-4 (w/biodegradation)
alpha=1.2e-3 (no bio)
Depth BGS (m)
-2
-4
-6
-8
0
2
4
6
8
10
12
14
16
18
20
22
24
14
16
18
20
22
24
x (m)
0
Depth BGS (m)
NearFoundation
vs.
ThroughtheFoundation
Sampling?
0
-2
-4
-6
-8
0
2
4
6
8
10
12
x (m)
The Ira A. Fulton School of Engineering
Arizona State University
0
0
-2
-2
-4
-4
1E-05
-6
0.001
-8
-10
-6
Depth BGS (m)
Depth BGS (m)
Changes
in a with
Depth
with Biodecay
alpha=1.3e-18 (w/biodegradation)
alpha=5.7e-4 (no bio)
-8
-10
-12
-12
-14
-14
-16
-16
-18
-18
0
2
4
6
8 10 12 14 16 18 20 22 24
x (m)
The Ira A. Fulton School of Engineering
0
2
4
6
8 10 12 14 16 18 20 22 24
x (m)
Arizona State University
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
The Ira A. Fulton School of Engineering
Arizona State University
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?
The Ira A. Fulton School of Engineering
Arizona State University
Discussion
The Ira A. Fulton School of Engineering
Arizona State University
Groundwater Data
Interpretation Issues
With 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
The Ira A. Fulton School of Engineering
C2 > C3
Arizona State University
Model Inputs - what do we know?
Input
Thoughts
Reasonable*
H, Dair
Tabulated Chemical Properties
Actual Value
E
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
(qm/qT)
10% - 50% (vadose zone + crack)
0.10
Lcapillary
1 - 100 cm
5 cm
(qm/qT)
90% - 98% (capillary fringe)
0.95
qT
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
The Ira A. Fulton School of Engineering
Arizona State University
Sensitivity of DTeff to Moisture Content..
eff
air
• D not very sensitive to
D
/
D
T


reasonable variation in
eff
T
1.E+00
• 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!)
The Ira A. Fulton School of Engineering
1.E-01
qT = 0.50
1.E-02
Vadose zone soils
1.E-03
Sands/
Gravels
1.E-04
0
0.2
Silts/
Clays
capillary zone
q T 1.33
moisture content for a
given soil type.
qT = 0.35
  Dw  

 q T 1.33
D
 air H  

i 
0.4
0.6
qM / qT 
0.8
1
Arizona State University