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GeoSyntecGeoSyntec
Future DirectionsFuture Directionsfor Assessing Vapor Intrusionfor Assessing Vapor Intrusion
byTodd McAlary, GeoSyntec Consultants, Inc.
AEHS VI WorkshopOctober 19, 2004
GeoSyntecGeoSyntec
OutlineOutline
Improved Protocols – What’s ComingImproved Protocols – What’s Coming QsoilQsoil
Perimeter crack model vs 0.05>QPerimeter crack model vs 0.05>Qsoilsoil/Q/Qbldgbldg>0.0001>0.0001 Barometric PumpingBarometric Pumping Implications for Data VariabilityImplications for Data Variability
Alternative Sampling OptionsAlternative Sampling Options More discrete in time and spaceMore discrete in time and space More integratedMore integrated
GeoSyntecGeoSyntec
APIAPI - - Collecting and Interpreting Soil Gas Samples from the Vadose Zone: A Practical Strategy for Assessing the Subsurface Vapor-to-Indoor-Air Migration Pathway at Petroleum Hydrocarbon Sites – Final Draft, July 2004
Health Canada – Soil Vapour Intrusion Guidance for Health Canada Screening Level Risk Assessment (SLRA) – Final Draft, October 2004
EPRI - Best Practices Manual For Evaluating Subsurface Vapor Intrusion to Indoor Air – Preliminary Draft, Sept 2004
ITRC – Workshop next week to develop an outline Others?
Improved ProtocolsImproved Protocols
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Conceptual ModelsConceptual Models More tools in the toolboxMore tools in the toolbox StandardizationStandardization QA/QC checksQA/QC checks Interpretive ToolsInterpretive Tools
Areas for AdvancementAreas for Advancement
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QQsoilsoil: Perimeter Crack Model: Perimeter Crack Model
)/2ln(
2Qsoil
crackcrack
crack
RZ
Pxk
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
C CL L LS S SC SCL SI SIC SICL SIL SL
SCS Soil Type
Qso
il (
L/m
in)
0.8 L/min > Qsoil > 0.05 L/min0.8 L/min > Qsoil > 0.05 L/min
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QQsoilsoil: Current Recommendation: Current Recommendation
0.05 > Qsoil/Qbuilding > 0.0001 (Johnson, 2002)0.05 > Qsoil/Qbuilding > 0.0001 (Johnson, 2002)
(perhaps 0.01 to 0.0001 is more common)(perhaps 0.01 to 0.0001 is more common)
For a typical residence: V ~ 600m3, AER ~0.5 hr-1For a typical residence: V ~ 600m3, AER ~0.5 hr-1
Qbuilding ~ 300 m3/hr or 5,000 L/minQbuilding ~ 300 m3/hr or 5,000 L/min
250 L/min > Qsoil > 0.5 L/min 250 L/min > Qsoil > 0.5 L/min (perhaps 50 to 0.5)(perhaps 50 to 0.5)
Generally higher than perimeter crack modelGenerally higher than perimeter crack model
OSWER Guidance uses 5 L/min, strictly empiricalOSWER Guidance uses 5 L/min, strictly empirical
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Barometric PumpingBarometric Pumping
Increasing B.P.Increasing B.P. Decreasing B.P.Decreasing B.P.
Gas is compressible, so changes in B.P. either compress soil gas or allow it to expand. No net flow in the long-term, but short term...
Ideal Gas Law: P1V1 = P2V2
Ground Surface
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Transient Effect of Barometric PumpingTransient Effect of Barometric Pumping
bpo
a
tP
PA
bpbpsoil
xQ
QQsoil-BPsoil-BP = transient soil gas flow rate induced by B.P. change = transient soil gas flow rate induced by B.P. change A = area of the footprint of the buildingA = area of the footprint of the building aa = air-filled porosity = air-filled porosity
xxbpbp = depth of barometric pressure propagation = depth of barometric pressure propagation
ΔΔP = barometric pressure change over time period “tP = barometric pressure change over time period “tbpbp””
PPoo = mean air pressure = mean air pressure
0.02 > 0.02 > ΔΔP/PP/Poottbpbp > 0.002 (Parker, 2002) > 0.002 (Parker, 2002)
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QQsoilsoil: Perimeter Crack Model vs Delta BP: Perimeter Crack Model vs Delta BP
Maximum Qsoil from Barometric Pumping
-5-4-3-2-1012345
0 2 4 6 8 10 12
Vadose Zone Thickness (m)
Qs
oil
(L/m
in)
Magnitude agrees well with empirical estimate of 5 L/minMagnitude agrees well with empirical estimate of 5 L/min
But in the short-term, Qsoil can be -5 to 5 L/min!But in the short-term, Qsoil can be -5 to 5 L/min!
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Barometric Pressure ChangesBarometric Pressure Changes
Arrows represent approximately 24 hours,
during which BP may change a lot (>200Pa in this example) , or hardly at all
Compare this to 4Pa stack effect!
(dP can be up to 5,000 Pa)
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Temporal VariabilityTemporal Variability
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Implications for Data VariabilityImplications for Data Variability
How much of this is due to short-term barometric pressure fluctuations?How much of this is due to short-term barometric pressure fluctuations?
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Alternative Sampling OptionsAlternative Sampling Options
Option 1: More Integrated Data Time-Integrated: sample over several barometric
cycles (e.g. 7 days) to smooth temporal variability Volume-Integrated: to smooth spatial variability
Option 2:More Discrete Data over time Real-time monitoring Vertical Profiling Monitor BP and delta P to establish correlation
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Time-Integrated SamplesTime-Integrated Samples
Summa CanistersSumma Canisters Smaller critical orificeSmaller critical orifice Solenoid valve “timer”Solenoid valve “timer”
ATD TubesATD Tubes Lower Flow Rate over Longer TimeLower Flow Rate over Longer Time
VOST Samplers – borrow stack testing tools?VOST Samplers – borrow stack testing tools? PUF: TO-13 designed for 300mPUF: TO-13 designed for 300m33 samples samples
10m by 10m house, 10 m vadose zone, 30% air-porosity…10m by 10m house, 10 m vadose zone, 30% air-porosity… Or Qsoil for 24 hours…Or Qsoil for 24 hours…
Radon analogy: passive electret samplersRadon analogy: passive electret samplers
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Volume Integrated SamplesVolume Integrated Samples
0
100
200
300
400
500
600
0 1 2 3 4 5 6Sampled Volume (L)
Co
nc
en
tra
tio
n (
pp
bv
)
1,1,1-TCATCE1,1-DCEc-1,2-DCE
(DiGiulio, 2004)
Representative Purge VolumeRepresentative Purge Volume
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High Purge Volume SamplingHigh Purge Volume Sampling
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High Purge Volume SamplingHigh Purge Volume Sampling
PID readings ~ 1,000 ppmv over 1,700,000 L (3 days)indicates concentrations are spatially uniform
Total mass removed ~ 10 kg (10-6 risk needs only 0.001 kg!)Combine with pneumatic testing, assess gas K
Q = 15 scfm
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Is Bigger Always Better?Is Bigger Always Better?
Huntington Beach Soil GasHUNTINGTON BEACH SITE - SOIL GAS
0
5
10
15
20
25
30
35
Time
Va
lue
(%
)
CH4
CO2
O2
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Summary & ConclusionsSummary & Conclusions
New and Improved Protocols are coming – data qualityNew and Improved Protocols are coming – data quality
Qsoil from BP changes is probably not negligibleQsoil from BP changes is probably not negligible
What is the contribution to data variability? What is the contribution to data variability?
$$ to find out?$$ to find out?
Risk assessment requires long-term average exposuresRisk assessment requires long-term average exposures
Time- or volume-integrated samples may helpTime- or volume-integrated samples may help
Vertical profiling needed for assessing biodegradationVertical profiling needed for assessing biodegradation
