Vapor Intrusion

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Vapor Intrusion

• Lisa K. Voyce
• Tetra Tech EMI
• 100 Enterprise Drive
• Suite 400
• Rockaway, NJ
• 973-659-9996 ext. 289
• Lisa.Voyce_at_ttemi.com

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Introduction

• Basic concepts and history
• Environmental and building factors
• Sampling issues
• Mitigation strategies

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Basic Concepts and History

• Soil vapor intrusion is the process by which
  volatile chemicals migrate form a subsurface
  source into the air of buildings through conduits
  and pressure gradients
• Part of IAQ evolution - from radon to RBCA to
  mold to soil vapor intrusion from VOCs and NAPL
• Largely ignored in the past - is a bath really as
  bad as a drink?

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Environmental Factors

• Soil conditions - dry coarse vs. wet fine
• Contaminants concentrations - VOCs and NAPL
• Geology - e.g., bedrock fractures
• Surface layer - impervious cover (or not)

• Underground conduits - path of least resistance
• Weather - wind, barometric pressure and
  temperature
• Biodegradation - a mixed blessing

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EPA Guidance Issues

• 100 foot distance - vertical and horizontal
• Groundwater migration
• Preferential pathways
• Potential vapor clouds
• Odors and symptoms, wet basements in contaminated
  areas
• Short-term safety issues (LEL, acute toxicity)

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Building Factors

• HVAC and anything else that vents - creates
  pressure gradient
• Building air exchange rates - drafty vs. tight
  buildings
• Foundation material - dirt, stone, concrete block

• Foundation type - slab, crawlspace, dirt,
  multiple
• Penetrating features - sump, sewer water lines
• Evaluate the building - know how the vapors get
  in and what you need to do to decrease risk and
  liability

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Sampling Issues

• Sampling related to vapor intrusion
• When to sample - screening vs. Site-specific
  sampling
• Sample types - soil sub-slab soil vapor, indoor
  outdoor air
• DQOs analytical considerations
• Conceptual site model

• Consider season, pavement, VOC sources, sensitive
  populations
• Evaluate in concert with other factors
• Compare to standards, guidance criteria - e.g.,
  0.00001 mcg/m3 for TCDD

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Mitigation Methods - by Foundation Type

• Vapor containment, ventilation, occupant
  relocation remedial action options
• Basement/slab
• Crawlspace
• Dirt floor
• Multiple foundation types

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Mitigation Methods for Basement/Slab on Grade

• Seal vapor entry points PLUS
• Sub-slab depressurization (SSD) system to prevent
  infiltration and discharge vapors to outdoor air

• Modify HVAC to maintain positive pressure
• Soil vapor extraction (SVE) system to collect and
  remove vapors

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Mitigation Methods for Crawlspace

• Seal vapor entry points PLUS
• Crawlspace ventilation
• Soil vapor retarder with sub-membrane
  depressurization (SMD) system

• Modify HVAC to maintain positive pressure
• Soil vapor extraction (SVE) system to collect and
  remove vapors

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Mitigation Methods for Dirt Floor/Multiple
Foundation Types

• Dirt floor - use either a SSD system with a newly
  poured slab or SMD system

• Multiple types - use appropriate methods,
  consider joining points and use of a common
  depressurization system and fan

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Why This May Seem Confusing

• Even EPA admits we have much less air pathway
  risk assessment experience
• The response options may differ mitigate vs.
  remediate?

• Vapor intrusion pathway uses indirect measures
  and modeling that adds uncertainty
• Indoor air sampling can be misleading - need to
  consider background other factors

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The Devil Is in the Details

• Problems with JE model
• TCE risk number
• Avoid creation of other hazards - backdrafts and
  fire code violations
• Sampling DQOs and getting the values right

• Re-opener concerns and the disconnect between BF
  approved remedies and vapor intrusion - natural
  attenuation vs. unacceptable risks