Vapor Intrusion Attenuation Factors (AFs)

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Vapor Intrusion Attenuation Factors (AFs)
Measured vs. EPA DefaultsA Case Study

• Presented byManu Sharma and Jennifer DeAscentis
  
• Presented at EPA Vapor Intrusion Workshop
• Amherst, Massachusetts
• October 18, 2004

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Outline

• Site Setting
• Environmental Data
• Measured and Default AFs
• Role of Background
• Modifications to Default AF

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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

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Outline

• Site Setting
• Environmental Data
• Measured and Default AFs
• Role of Background
• Modifications to Default AF

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Sampling Locations
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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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Indoor Air and Groundwater Data
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Outline

• Site Setting
• Environmental Data
• Measured and Default AFs
• Role of Background
• Modifications to Default AF

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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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Measured vs. EPA Default AFs
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Measured vs. EPA Default AFs (Contd)

• 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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Outline

• Site Setting
• Environmental Data
• Measured and Default AFs
• Role of Background
• Modifications to Default AF

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Role of BackgroundRange of Measured Site Indoor
Air Concentrations versus Range of Literature
Background Concentrations
Background concentrations from McHugh et al.,
2004.
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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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Outline

• Site Setting
• Environmental Data
• Measured and Default AFs
• Role of Background
• Modifications to Default AF

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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