Draft Petroleum Vapor Intrusion Information Paper Michael Lowry, RTI International Matthew Young, EPA OUST g,
Describe differences between petroleum vapor intrusion (PVI) and chlorinated solvent vapor intrusion (CVI) and the resulting influence on human exposure and risk Provide background and context for development of guidance specific to PVI Summarize current state of knowledge Thank you to the Petroleum Vapor Intrusion Workgroup for review and input!
Draft information paper May be downloaded d d at http://www.epa.gov/oust Contact for more information and comments: Matthew A. Young, P.G. Environmental Protection Specialist USEPA/OSWER/OUST Implementation Division Mail Code:5401P 1200 Pennsylvania Ave, NW Washington, DC 20460-0001 (703) 603-7143 Young.matthew@epa.gov Note: This paper discusses petroleum vapor intrusion and is intended as a reference and informational source for technical and programmatic environmental professionals and other interested parties. The information provided in this paper is not intended to replace or contradict federal or state regulations, nor is it a regulation itself - it does not impose legally binding requirements.
Petroleum Vapor Intrusion (PVI) Petroleum hydrocarbons such as gasoline, diesel, and jet fuel Typically gas station UST sites; also tank farms, terminals, etc. Chlorinated Vapor Intrusion (CVI) Most commonly chlorinated solvents such as dry cleaning chemical perchloroethylene (PCE) and the degreasing solvent trichoroethylene (TCE) and 111-trichloroethane (TCA) Typically dry cleaning and industrial sites
Biodegradation Most petroleum hydrocarbons biodegrade readily under aerobic conditions Chlorinated hydrocarbons typically biodegrade much more slowly and under anaerobic conditions Petroleum biodegradation in unsaturated zone can often provide an effective, naturally occurring contaminant removal mechanism, potentially preventing PVI impacts Free Product Density Light nonaqueous phase liquid (LNAPL petroleum) Light nonaqueous phase liquid (LNAPL, petroleum) Dense nonaqueous phase liquid (DNAPL, chlorinated solvents)
Petroleum hydrocarbons: Aerobic biodegradation zones around perimeter of plumes Oxygen transport (dashed arrows) LNAPL can collect at water table Biodegradation can prevent VI Chlorinated solvents: Slower anaerobic biodegradation in saturated zone Little to no biodegradation in unsaturated zone Generally larger plumes (vapor and dissolved) DNAPL can penetrate below water table
Surface Aerobic biodegradation common in unsaturated soils Typical vertical profile CO 2 O 2 VOCs (red) degrade Carbon dioxide (green) VOCs Aerobic produced Depth Biodegradation Oxygen (blue) is consumed. Zone Biodegradation zone extends over area of active biodegradation often along perimeter of highest concentration zones Maximum VOC concentrations and limited biodegradation in source zone. Concentration Typical unsaturated-zone vertical concentration profile Source Zone (anaerobic)
Direct contact between a contamination source (LNAPL or dissolved) d) and the building Insufficient separation distance between the source and the building Anaerobic conditions limited oxygen to support aerobic biodegradation Methane gas production potentially increased vapor flow from source and oxygen depletion Preferential transport pathways (e.g., fractures, utility corridors)
Direct Building Contact Shallow or perched water table brings source (LNAPL or dissolved) into direct contact with building foundation Foundation cracks or basement drainage systems (e.g., a sump) can bring LNAPL or dissolved source into building
Insufficient Separation Distance Insufficient clean, oxygenated soil between building foundation and a strong (high concentration) source Very shallow water tables, perched features, seasonal water level fluctuations. Anaerobic Conditions Large biological oxygen demand Contamination in shallow unsaturated zone Strong source concentrations Highly organic soils Surface/subsurface barriers to oxygen transport from atmosphere Large, impermeable building foundation Saturated, low-permeability zone Impermeable surfaces (e.g, pavement) Potential for methane production Additional oxygen demand from methane Potentially explosive concentrations
Preferential Transport Pathways. Geologic features (e.g., fractures, structured soils) Engineered features (e.g., utility lines) May connect sources with buildings, potentially enhanced transport? Potential route for source migration Biodegradation important Significant ifi attenuation from loss to atmosphere Not necessarily a problem Should be evaluated carefully
Is sufficient oxygen available and are soils thick enough for effective aerobic biodegradation? When present, clean, aerobic soil between building and source can act as a natural biofilter Can sufficient oxygen migrate through shallow soils and under buildings to replenish oxygen and support aerobic biodegradation? Do soil gas profiles (VOCs, carbon dioxide, and oxygen) show patterns characteristic of aerobic degradation?
Aerobic biodegradation prevents PVI under many conditions Most known, documented cases of PVI can be attributed to a limited number of recognizable site conditions PVI screening protocols should consider Inclusionary criteria -- Conditions where PVI is much more likely to occur e.g., direct building contact, insufficient separation distance, preferential transport pathways. Exclusionary criteria i Conditions where PVI is much less likely due to active biodegradation e.g., sufficient separation distance, available oxygen. Accounting for biodegradation in site screening will allow appropriate prioritization of limited resources to remediate contamination and protect public health.