vapor intrusion attenuation factors (afs) – measured vs . epa defaults a case study
DESCRIPTION
Vapor Intrusion Attenuation Factors (AFs) – Measured vs . EPA Defaults A Case Study. Presented by Manu Sharma and Jennifer DeAscentis Presented at EPA Vapor Intrusion Workshop Amherst, Massachusetts October 18, 2004. Outline. Site Setting Environmental Data Measured and Default AFs - PowerPoint PPT PresentationTRANSCRIPT
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Gradient CORPORATION
Vapor Intrusion Attenuation Factors (AFs) – Measured vs. EPA Defaults
A Case Study
Presented byManu Sharma and Jennifer DeAscentis
Presented at EPA Vapor Intrusion Workshop
Amherst, Massachusetts
October 18, 2004
![Page 2: Vapor Intrusion Attenuation Factors (AFs) – Measured vs . EPA Defaults A Case Study](https://reader035.vdocument.in/reader035/viewer/2022062520/56815acd550346895dc89ee3/html5/thumbnails/2.jpg)
Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
![Page 3: Vapor Intrusion Attenuation Factors (AFs) – Measured vs . EPA Defaults A Case Study](https://reader035.vdocument.in/reader035/viewer/2022062520/56815acd550346895dc89ee3/html5/thumbnails/3.jpg)
Gradient CORPORATION
Site Setting
Active equipment testing facility located in the northeast US
Urban setting – highway; other industrial/ commercial operations nearby
Long history of industrial activities – low volumes of chlorinated organics and hydrocarbon usage
Glacial outwash deposits – relatively high hydraulic conductivity
Depth to groundwater ~ 6 meters
![Page 4: Vapor Intrusion Attenuation Factors (AFs) – Measured vs . EPA Defaults A Case Study](https://reader035.vdocument.in/reader035/viewer/2022062520/56815acd550346895dc89ee3/html5/thumbnails/4.jpg)
Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
![Page 5: Vapor Intrusion Attenuation Factors (AFs) – Measured vs . EPA Defaults A Case Study](https://reader035.vdocument.in/reader035/viewer/2022062520/56815acd550346895dc89ee3/html5/thumbnails/5.jpg)
Gradient CORPORATION
Sampling Locations
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Gradient CORPORATION
Environmental Data
Sources and groundwater plume reasonably defined
Low VOC concentrations present in groundwater – order of 1 to 100 ug/l
Indoor air sampling locations
• Known sources of VOCs (e.g., historical solvent usage areas)
• Small and large buildings with different ventilation characteristics
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Gradient CORPORATION
Indoor Air and Groundwater Data
Indoor Air Background
(ug/m3) aIndoor Air Concentrations
(ug/m3) b
Indoor Air Sample #1
Trichloroethene 0.15-9.01 5.8 20 - 120
Indoor Air Sample #2
Tetrachloroethene 0.29-27.8 0.39-0.41Trichloroethene 0.15-9.01 0.63
Indoor Air Sample #3
Benzene 2.14-16.8 1.2-1.9
Trichloroethane, 1,1,1- NA g 0.49
Indoor Air Sample #4
Benzene 2.14-16.8 0.59-4.5
Dichlorodifluoromethane NA g 3.6-7.9 49 - 60Ethylbenzene 5.08-14.3 0.3-3.1Tetrachloroethene 0.29-27.8 0.51-4.2 1.2 - 1.6Toluene 26.9-80.0 1.1-14 1 - 49
Trichloroethane, 1,1,1- NA g 2.9-150Xylenes 20.0-24.2 0.72-11 2.1 - 220
1.3
5.8
96
25
26
22
Groundwater
Concentrations (ug/L) b
5
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Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
![Page 9: Vapor Intrusion Attenuation Factors (AFs) – Measured vs . EPA Defaults A Case Study](https://reader035.vdocument.in/reader035/viewer/2022062520/56815acd550346895dc89ee3/html5/thumbnails/9.jpg)
Gradient CORPORATION
AF Calculation
Measured AFs
• Range: average air concentration and range of measured groundwater concentrations at associated wells
• Recommended Estimate: average air concentration and average groundwater concentration (temporal and spatial) at associated wells
Default AF from Figure 3b of EPA guidance
• Depth to contamination = 4 m
• Sandy soils
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Gradient CORPORATION
Measured vs. EPA Default AFsFigure 3: Measured Site Specific AFs by Constituent versus USEPA-recommended Semi-Site Specific AF
2.7E-04
1.9E-04
2.7E-05
1.6E-05
5.3E-05
1.4E-03
2.8E-05
1.9E-04
6.9E-04
4.5E-043.6E-04 3.9E-04
1.1E-04
2.3E-05
5.5E-03
6.5E-02
2.2E-02
2.0E-03
2.6E-05
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
Mea
sure
d S
ite
Sp
ecif
ic A
tten
tuat
ion
Fac
tor
(Min
imu
m, M
axim
um
)
USEPA Semi-Site Specific Attenuation Factor (1.0E-03)
Benzene Dichlorodifluoro-methane
Ethyl-benzene
Tetrachloro-ethene
Toluene Trichloro-ethane, 1,1,1-
Trichloro-ethene
Xylenes
n=2
n=3
n=1
n=3
n=2
n=2
n=4
n=3
(AS-3)
(AS-4)
(AS-4)
(AS-4)
(AS-4) (AS-4)
(AS-4)
(AS-2)
(AS-1)
(AS-1)
(AS-3)
Note:AF = Attenuation FactorHighlighted attenuation factor indicates low levels of constituent detected in groundwater (i.e., equal to or less than 5 µg/L, the typical Method Detection Limit (MDL) for VOCs)
– - recommended AF by constituent and air sampling location
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Gradient CORPORATION
Measured vs. EPA Default AFs (Cont’d)
Measured AF about an order of magnitude less than default AF (10-4 vs. 10-3)
True AF may be even lower because:
• Air sampling locations near sources – air concentrations biased high
• Monitoring wells located downgradient of sources – groundwater concentrations biased low
No trend in measured AFs vs. building size or ventilation noted
Background may be masking indoor air concentrations attributable to subsurface contamination
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Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
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Gradient CORPORATION
Role of BackgroundRange of Measured Site Indoor Air Concentrations versus Range of Literature Background Concentrations
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14Benzene Ethyl-benzene
Tetrachloro-ethene
Toluene Trichloro-ethene
Xylenes
80
Background concentrations from McHugh et al., 2004.
Site indoor air concentration range
Background indoor air concentration range
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Gradient CORPORATION
Role of Background
Measured indoor air concentrations at the Site consistent with literature background
Contributions from subsurface contamination vs. background at the Site cannot be conclusively apportioned
• Site-specific background data needed
Background indoor and outdoor air concentrations should be characterized to place risks in proper perspective
Challenges in background definition
• Variability in source and characteristics
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Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
![Page 16: Vapor Intrusion Attenuation Factors (AFs) – Measured vs . EPA Defaults A Case Study](https://reader035.vdocument.in/reader035/viewer/2022062520/56815acd550346895dc89ee3/html5/thumbnails/16.jpg)
Gradient CORPORATION
Modifications to Default AF
Building Area
• Current value (100 m2) only applicable for small residential buildings
• Need to develop AF graphs for other building sizes (medium, large) to expand applicability
Air Exchange Rate (AER)
• Current value (0.25 hr-1) also applicable at small residential buildings
• Users should have flexibility to use higher AERs – commercial buildings, warmer setting