2.1 What Is the VIAP?
The VIAP is the process by which contaminants in the subsurface volatilize and migrate into a structure with the potential to pose an unacceptable exposure risk to human health. VIAP scenarios are highly variable on a site-by-site basis. To choose the proper investigation, remediation, and/or mitigation techniques you must first determine if vapor intrusion or direct volatilization is occurring at the site.
2.2 Vapor Intrusion
The presence of volatile compounds in soil, groundwater, NAPL, and/or buried waste material results in the potential for volatile hazardous substances to enter into a vapor phase (i.e., soil gas). The soil gas can then migrate through the vadose zone and into the breathing space within a structure. The movement of vapors from the subsurface to the indoor air of a structure is called indirect volatilization or more commonly vapor intrusion. Soil gas vapors can migrate through the vadose zone via diffusion. Once the vapors are adjacent to a structure the migration into the structure will also be influenced by advection. Note that the vapor source does not need to be directly beneath a structure to present a VIAP concern as soil gas can migrate both vertically and laterally in the vadose zone.
Key terms:
- Vadose Zone - Unsaturated area of subsurface soils extending from the ground surface to the top of the water table.
- Diffusion – Movement due to a concentration gradient, migration from an area of high concentration to an area of low concentration.
- Advection – Movement due to a pressure gradient, migration from an area of higher pressure to an area of lower pressure.
2.3 Direct Volatilization
Direct volatilization is the process by which substances directly volatilize into a structure’s breathing space without migrating through the vadose zone. This can result from contaminated groundwater or NAPL directly entering a structure or when contaminants migrate into a structure through a preferential pathway or conduit.
Examples:
- Contaminated groundwater in a sump and/or foundation drain
- Contaminated groundwater seeping through a basement floor and/or walls
- Contaminated groundwater and/or NAPL in a conduit that can transmit vapors to a structure (e.g., sanitary sewer pipe)
Not all utilities are conduits for direct volatilization – the vapor source must be able to enter the conduit, and the conduit must connect to the structure’s indoor air.
2.4 Petroleum VIAP
Petroleum hydrocarbons are a subset of the list of volatile hazardous substances. They are distinguished from other volatile chemicals by their rapid rate of attenuation due to aerobic biodegradation in the vadose zone. This makes the VIAP evaluation of petroleum releases fundamentally different. Petroleum is addressed separately in Volume 4 – Investigative Approach for Petroleum Volatilization to Indoor Air Pathway.
2.5 Methane
EGLE recognizes that methane sites are also fundamentally different from other vapor intrusion sites and require separate guidance. Please see the guidance document Development on or Near Methane Generating Sources focused on identifying and mitigating risks associated with methane as it relates to the VIAP and fire and explosion hazards.
2.6 Conceptual Site Models
A thorough understanding of site conditions is crucial for evaluating the VIAP and determining potential risks. This is accomplished by developing a Conceptual Site Model (CSM) and collecting data to support or modify it. A CSM should include a current understanding of the following:
- Relevant site features
- Surface and subsurface conditions
- Chemical sources
- Extent of the contamination in environmental media
- Transport mechanisms
- Potential risks posed by the contamination
Please refer to ITRC’s Conceptual Site Models and Investigative Strategies document or to ASTM’s Standard Guide for Developing Conceptual Site Models for Contaminated Sites. If petroleum hydrocarbons are the only contaminants at the site, refer to Volume 4 – Investigative Approach for Petroleum VIAP. Methane sites should refer to the Development on or Near Methane Generating Sources guidance document.
2.7 Site Specific
Criteria EGLE is allowed under Part 201 of the Natural Resources and Environmental Protection Act to approve numeric or nonnumeric site-specific criteria if it better reflects the best available toxicity or exposure risk information compared to generic criteria. Refer to Volume 5 – Response Activity for the Volatilization to Indoor Air Pathway.