Petroleum Vapor Intrusion

Petroleum Vapor Intrusion (PVI) refers to the process by which petroleum vapors may originate from petroleum-contaminated groundwater, soil, or LNAPL and diffuse through vadose zone soil or preferential pathways into areas of concern, such as utility corridors or structures. This OPS assessment guidance provides a process for screening, investigating and evaluating PVI to be protective of human health and the environment.

This assessment guidance should not be used in emergency situations; responses to emergency situations should be addressed by the Emergency Response section of this guidance.

Recent advances in PVI science have lead to an improved understanding of petroleum vapor migration, biodegradation, assessment and mitigation. Most relevant to these advancements are multi-stepped processes developed by ITRC and EPA.

By utilizing this process, the vapor pathway can often be safely eliminated early in the assessment phase, allowing necessary resources to be utilized on the small number of releases that require petroleum vapor mitigation. The image below (courtesy of ITRC) depicts the relative ratio of sites where the vapor pathway is eliminated by site screening versus site investigation using the guidance developed by ITRC and adopted by OPS.

Relative ratio of sites requiring Site Investigation and Mitigation based on Site Screening process (ITRC, 2014).

PVI 101

Biodegradation is a naturally-occurring process where chemicals are broken down by microorganisms. Fortunately, petroleum vapors are attenuated relatively quickly in vadose zone soils by aerobic biodegradation. The generalized concept is depicted in the image below (courtesy of EPA).

Schematic depiction of the biodegradation process for petroleum hydrocarbons (EPA, 2015).

Previous PVI assessment did not account for biodegradation in the vadose zone. An understanding of the role biodegradation plays in petroleum vapor attenuation and migration lays the foundation for the remainder of our PVI guidance.

PVI Site Screening

The first step of the PVI assessment strategy developed by ITRC is to screen the site for certain conditions. The three-step site screening process includes:

  1. Developing a preliminary conceptual site model (CSM).
  2. Evaluating nearby structures for precluding factors and lateral inclusion.
  3. Conducting screening of nearby structures based on vertical separation distance.

The figure below depicts the PVI site screening process.

Flow chart of the PVI Site Screening Process (ITRC, 2014).

 

Colorado Division of Oil and Public Safety

633 17th Street, Suite 500, Denver, CO 80202

Phone: 303-318-8525

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