Transcript Document

Workshop 1:
Assessment and Evaluation of Vapor
Intrusion at Petroleum Release Sites
Conceptual Model:
Aerobic Biodegradation in the Vadose Zone
The Association for Environmental Heath and Sciences Foundation
The 27th Annual International Conference on
Soils, Sediments, Water, and Energy
Amherst, MA, October 17-20, 2011
Overview of Petroleum VI
n
General VI Conceptual Model
n
Vadose Zone Attenuation of Petroleum Vapors
n
Oxygen Below Building Foundations
Conceptual Model for Vapor Intrusion:
Regulatory Framework
BUILDING
3
Air
Exchange
Unsaturated
Soil
Affected Soil
Affected GW
GroundwaterBearing Unit
KEY
POINT:
2
Building
Attenuation Due
to Exchange with
Ambient Air
Advection and
Diffusion
Through
Unsaturated Soil
and Building
Foundation
1
Regulatory guidance focused on building
impacts due to vapor migration.
Partitioning
Between Source
and Soil Vapor
Physical
Barriers to
Vapor
Intrusion
No Barrier
Possible
Barriers
A
KEY POINT:
Presence of
subsurface
source does
not always
mean vapor
intrusion is
occurring.
Vadose
Zone
B
Building Foundation:
(A) Low permeability
foundation with no
cracks or unsealed
penetrations;
(B) Positive building
pressure
Vadose Zone:
Vapors
A
B
A
B
(A) High moisture content
or clay layer;
(B) Aerobic biodegradation
Groundwater interface:
Source
Area
GW Aquifer
(A) Clean water lens
(B) Clay layer
Overview of Petroleum VI
n
General VI Conceptual Model
n
Vadose Zone Attenuation of Petroleum Vapors
n
Oxygen Below Building Foundations
Petroleum Biodegradation
Conceptual Model
Comax
CHmin
Aerobic
Biodegradation
Possible
Co>Comin
No Aerobic
Biodegradation
Co<Comin

Oxygen
L
Hydrocarbon
Comin
Hydrocarbon Source
CHmax
Vapor
Concentration
KEY
POINT:
Correlation between oxygen consumption and
hydrocarbon attenuation.
From Roggemans et al., 2001, Vadose Zone Natural Attenuation of Hydrocarbon Vapors:
An Empirical Assessment of Soil Gas Vertical Profile Data, API’s Soil and Groundwater Technical Task Force Bulletin No. 15.
Petroleum Biodegradation: Real Site Data
Diesel Release Site, North Dakota
VOC
Concentration
vs. Depth
Biogenic
Gases vs.
Depth
Petroleum Hydrocarbons
1000
100
10
1
CORRELATION ? NO (p = 0.11)
0.1
10
100
1000
10000
GW Concentration (ug/L)
Observable Relationship
Cia vs. Cgw ?
Cgw = COC conc. In groundwater;
Indoor Air Concentration ( ug/m3)
Indoor Air Concentration ( ug/m3)
Correlation Between Groundwater
Concentration and Indoor Air??
1000
IA
??
Chlorinated Solvents
GW
100
10
1
0.1
CORRELATION ?
YES (p <0.001)
0.01
0.001
0.1
1
10
100
1000
10000
GW Concentration (ug/L)
n Petroleum Hydrocarbons:
No
n Chlorinated Solvents:
Yes
Cia = COC conc. In indoor air;
- Direct
(p = 0.11) = Probability = 11% that slope of best-fit line = 0 (I.e., no trend).
Overview of Petroleum VI
n
General VI Conceptual Model
n
Vadose Zone Attenuation of Petroleum Vapors
n
Oxygen Below Building Foundations
Oxygen Under Building Foundation
Key Question
n Is there enough
oxygen below
building
foundations to
support aerobic
biodegradation?
aerobic zone
Ct
anaerobic zone
Cs
Vapor Source
Oxygen Under Foundation: Model Prediction
Numerical model predicts oxygen
shadow below building, but…..
n Very strong vapor source
(200,000,000 ug/m3)
n All flow into building is through
perimeter crack
n No advective flow below building
Model does not account for
KEY
POINT: key oxygen transport
processes.
From Abreu and Johnson, ES&T, 2006, Vol. 40, pp 2304 to 2315.
Aerobic Biodegradation: Oxygen Mass Balance
Hydrocarbon + Oxygen
bacteria
1 kg CxHy + 3 kg O2
Carbon dioxide + Water
3.4 kg CO2 + 0.7 kg H2O
New
Cells
Petroleum
Hydrocarbons
+
Energy
Electron
Acceptor
(e.g., O2)
Aerobic Biodegradation: Oxygen Mass Balance
n Atmospheric air
(21% Oxygen)
= 275 g/m3 oxygen
> Provides capacity to
degrade 92 g/m3
hydrocarbon vapors
(= 92,000,000 ug/m3)
KEY
Even limited migration of
POINT: oxygen into subsurface
is sufficient to support
aerobic biodegradation.
Transport of Oxygen Under Foundation
Bi-Directional Flow
Across Foundation
Wind Driven Advection
+/+/ KEY
POINT:
Advection drives oxygen below building foundation.
Transport of Oxygen Under Foundation
Conceptual Model
Field Data
Depth (m)
0.0
Wind-driving
building
ventilation
Wind
Loading
Advection of
subslab soil gas
into bldg.
Upwind-downwind
advection in
soil gas
Diffusion from deep sub-slab soil gas
Subslab VOC source
KEY
POINT:
isoP
CH4
CO2
02
1.0
1.5
0.0
Depth (m)
Biodegradation
0.5
0.5
1.0
1.5
0.0
0.01
0.1
1
10
Concentration (g
100
m-3)
Conceptual model and field data indicate common
presence of oxygen under building foundation.
From Fisher et al., 1996 Environmental Science and Technology, Vol. 30 No. 10, p. 2948.
1000
Soil Column Attenuation
Transport of Oxygen Under Foundation
Nitrogen
Flooding
Experiment:
Purge sub-foundation soils with nitrogen
gas and observe oxygen recovery
Time > 0
Time = 0
Low Oxygen
Oxygen Recovery Below Building
0.9
0.8
1.1
0.8
0.9
0.8
0.8
0.9
0.8
0.8
0.8
1.0
garage
3m
% O2 After Flood
Injection wells
% O2 (shallow)
concrete
Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”
Soil Column Attenuation
Transport of Oxygen Under Foundation
Nitrogen
Flooding
Experiment:
Purge sub-foundation soils with nitrogen
gas and observe oxygen recovery
Time = 0
Time = 2 weeks
0.9
1.1
0.8
0.8
0.8
0.8
15.9
16.6
0.9
0.8
0.9
0.8
KEY POINT:
High Oxygen
Low Oxygen
0.8
16.6
18.4
16.6
15.4
14.5
14.0
15.2
13.7
12.2
% O2 After Flood
19.4
1.0
garage
Rapid recovery
of oxygen below
building foundation
supports petroleum
biodegradation.
garage
3m
3m
concrete
concrete
Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”
Injection wells
% O2 (shallow)
Advective Transport Processes
Gas flow from subsurface into
Lower
building
pressure
Gas flow from
Higher
building
pressure
EXAMPLES
Residence in winter
(chimney effect);
bathroom, kitchen
vents
Flow in
building
into subsurface
EXAMPLES
Flow out
Bi-directional flow between
Variable
building
pressure
building
Building HVAC
designed to maintain
positive pressure
building
High Pressure
UPWARD
TRANSPORT
High
Pressure
and subsurface
EXAMPLES
Reversible
flow
Low
Pressure
Low Pressure
Barometric pumping;
variable wind effects
DOWNWARD
TRANSPORT
Pressure Gradient Measurements:
School Building, Houston, Texas
KEY POINTS:
40
Differential
Pressure (Pascals)
Pos. Pressure
30
(Flow out of Bldg)
20
10
0
-10
-20
-30
Neg. Pressure
(Flow into Bldg)
-40
6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00
Time (July 14-15, 2005)
• Pressure
gradient
frequently
switches
between
positive and
negative within
a single day.
• Continuous
inward flow
does not occur.
Advection Through Building Foundation:
Field Evidence
n VOCs from indoor
air typically detected
in sub-slab samples:
- alpha pinene
- limonene
- p-dichlorobenzene
INDOOR AIR
S
n Oxygen transported
below foundation by
same mechanism
BELOW SLAB
KEY
POINT:
Reversing pressure gradient drives
air (and VOCs and oxygen) through
building foundation.
S
Chatterton Research Site,
British Columbia, Canada
(Hers, et al 2000)
Building
Feet
Below
Grade
0
SG-BC,
10/1/97
Fill, “crust,” sandy silt
Fill, dredged
river sand
5
<1000
11%
<1000
10%
55,000
6%
25,000,000
1.0%
50,000,000
1.0%
SG-BR
5/14/97
10 %
80,000
8%
3%
KEY
SG-BC
50,000,000
Sub-Slab vapor
sample point
60,000,000
Sub-Surface vapor
sample point
50,000,000
1.0%
10
Slide from Robin Davis, UDEQ
0
LNAPL, benzene-rich
Vapor sample
point identifier
Benzene, ug/m3
Oxygen, %
20
Feet, horizontal
Hal’s, Green River, Utah
Feet
Below
Grade
0
Motel Office Breezeway
10
51
2800
9.5%
Café/Bar
VW-7
Clayey
Silt
Silt
7.0
380
20%
VW-5
22
1600
18%
570
70,000
87
12,000
4.1%
11%
Asphalt
8.4
850
14%
Basement
Basement
VW-4
(Utah DEQ, 8/26/06)
7.7
250
12%
260,000
33,000,000
2.5%
20
KEY
Multi-depth vapor
monitoring well
Sub-Surface vapor
sample point
260,000 Benzene, ug/m3
33,000,000 TPH-gro, ug/m3
2.5% Oxygen, %
VW-7
Slide from Robin Davis, UDEQ
Benzene in GW
1,000-5,000 ug/L
LNAPL,
gasoline
0
20
Feet, horizontal
Perth, Australia
Feet
Below
Grade
0
Uncovered
open
ground
Very Large
Building
(B. Patterson & G. Davis, 2009)
410
19,000,000
<0.5%
<2
<50,000
10.7%
<50,000
19.9%
<50,000
18.8%
Lateral Extent of Oxygen & Biodegradation
5
35,000,000
<0.5%
Sand
35,000,000
<0.5%
<50,000
8.2%
<50,000
14.5%
<50,000
15.9%
1,200,000
8.2%
<50,000
4.5%
<50,000
4.6%
10
LNAPL
Kerosene
(very low
BTEX)
Slide from Robin Davis, UDEQ
KEY
Outdoor air sample
Indoor air sample
Sub-slab vapor sample
Sub-surface vapor sample
1,200,000 Total Petroleum Hydrocarbons, ug/m3
8.2% Oxygen, %
0
20
Feet, horizontal
Oxygen Under Building Foundation
Summary
n Wind and building
pressure drive
atmospheric air
below building
foundation
n Even modest oxygen
transport sufficient
aerobic
biodegradation