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
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
Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
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
Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
Gradient CORPORATION
Sampling Locations
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
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
Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
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
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
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
Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
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
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
Gradient CORPORATION
Outline
Site Setting
Environmental Data
Measured and Default AFs
Role of Background
Modifications to Default AF
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