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Volume 4: Investigative approach for petroleum VIAP

1. Introduction

PVI and the direct volatilization of a petroleum hydrocarbon into a structure is a subdivision of the VIAP and is the process by which PHCs volatilize into vapors and migrate into a structure with the potential to pose an unacceptable exposure risk to human health.

This document uses a scientifically based approach that has been supported by empirical data and is based on the ITRC Petroleum Vapor Intrusion – Fundamentals of Screening, Investigation, and Management (PVI -1) (ITRC 2014) so that decisions can be made to confidently screen out any area, place, parcel or parcels of a property, or portion of a parcel of a property and focus limited resources on the small fraction of petroleum-contaminated properties that warrant further evaluation, vapor control, or an additional response activity to prevent unacceptable exposure risks. This document is drafted to specifically address only PVI and is consistent with Part 201. However, the approaches found in this document are applicable and can be used under Part 213 though the terminology will be different due to the use of a Risk-Based Corrective Action program consistent with ASTM International E1739.

Note: The use of the information in this volume requires that the data collected be representative of the actual conditions and for the purpose that it is intended. It is the user’s responsibility to understand the strengths and weakness of the sampling methodology prior to utilizing and that the resulting data is what the decision is based on.

1.1. About this Volume

This volume is intended for petroleum releases that are sufficiently characterized to identify and evaluate vapor sources that rapidly biodegrade in the presence of O2 relative to receptor locations. It does not address chlorinated volatile organic compounds (CVOCs) or other aerobically recalcitrant non-petroleum hydrocarbons compounds above the VIAC. The screening method described in this document is applicable whether the release occurred from an underground storage tank (UST); aboveground storage tank (AST); manufactured gas plant (MGP); petroleum industrial terminal, refinery, pipeline; or other type of petroleum release. The screening and evaluation is applicable to petroleum releases in general and is not limited to the use of a property.

The VIAP assessment strategy is based upon the following stepwise approach shown on Figure 1-1.

1.2. Petroleum Vapor Intrusion and Vapor Intrusion

The defining feature of PVI that distinguishes it as a unique subdivision of VI is the rapid rate of attenuation of PHCs because of aerobic biodegradation. With PVI, vapor concentrations generally decrease with increasing distance from a subsurface vapor source due to aerobic biodegradation, and eventually at some distance, the concentrations become negligible (USEPA 2015a and 2015b). The extent and rate to which this natural biodegradation process occurs is strongly influenced by several factors cited by Lahvis and Baehr (1996), Suarez and Rifai (1999) and USEPA (2015a) and include: the concentration of the vapor source, the distance the vapors must travel to potential receptors, and the presence of oxygen (O2). Petroleum vapors are not expected to migrate more than 15 feet (ft) with most vapors being degraded within inches to a few feet.

Studies have documented the subsurface biodegradation of PHC vapors (McAlary et al. 2007; Ririe et al. 2002; Hers et al. 2000; Ostendorf et al. 2000). Recent evaluations of empirical soil gas data have demonstrated that biodegradation can limit the migration of PHC vapors from a subsurface vapor source (USEPA 2013; Lahvis et al. 2013; Davis 2009). These studies show that the potential for PVI is reduced because biodegradation minimizes the flux of PHC vapors in soil gas from a vapor source to overlying buildings.

General differences is discussed in Table 2-1 ITRC (2014) which is based on USEPA (2012a) and summarizes the differences between PVI and VI for PHCs and CVOCs. These differences form the basis for the PVI-specific facility screening approach discussed and detailed in this volume.

1.3. Biodegradation

Aerobic biodegradation is the most important fate and transport mechanisms for understanding PVI and is the basis for the screening strategy presented in Sections 4 and 5.

The processes of partitioning, diffusion, advection, and mixing are the same for PHCs and other compounds, including CVOCs. Further details on these processes, its uniqueness to PVI and the biogeochemical behavior of PHCs is discussed in Appendix C - Chemistry of Petroleum and Appendix M - Fate and Transport of Petroleum Vapors in ITRC (2014). A brief summary about the processes of the biodegradation is provided below.

The Process of Biodegradation

PHC-degrading bacteria are found in biologically active soil in most environments (USEPA 2015a) in Michigan. PHC-degrading bacteria can consume hydrocarbons rapidly in the presence of O2. This process limits the transport of PHC vapors. Although PHCs can be biodegraded in the absence of O2, the most rapid rates of biodegradation typically occur under aerobic conditions. The vadose zone above an area contaminated by a petroleum NAPL is normally an aerobic environment in which O2 can be readily replenished from the atmosphere. USEPA (2013 and 2015b) identified a total petroleum hydrocarbon (TPH) concentration of 250,000 micrograms per kilogram (µg/kg) as a metric for clean, biologically active soil absent of NAPL. This aligns with EGLE’s lines of evidence for indicators of when NAPL is present (see EGLE’s Non-Aqueous Phase Liquid – Petroleum Releases Characterization, Remediation, and Management Guidance dated June 2023) and identifies where the aerobic environment will be outside of the NAPL body. The rates of petroleum vapor biodegradation typically exceed the rates of petroleum vapor transport via diffusion; therefore, petroleum vapors are often fully attenuated by aerobic biodegradation processes in the vadose zone. This process is fundamental in understanding why unsaturated soil that does not contain NAPL does not pose a risk to the VIAP and use of the screening distances for the maximum distances the petroleum vapors are likely to migrate before degradation.

Environmental Effects on Biodegradation

While there is the general reliability of aerobic biodegradation in reducing PVI, there are some environmental factors can hinder this process, such as arid climates that lack of soil moisture (USEPA 2015a), which are not common in Michigan. The most significant factor in biodegradation is the availability of O2, which is a necessary electron acceptor and enzyme reactant in the aerobic biodegradation of PHCs. Roggemans et al. (2001) showed O2 concentrations of 2% by volume to be supportive of aerobic biodegradation. Factors that can limit the biodegradation are described in ITRC (2014). In Michigan, these factors can often be identified by the lack of O2 present in the subsurface.

1.4. Vapor Intrusion for Petroleum Conceptual Site Model

A CSM provides an iterative representation of the site data and information collected from the property or properties and guides the decision-making process. The CSM should be refined throughout the life of the project as new information is acquired. Because of the importance of biodegradation to PVI, the VIAP portion of the CSM for any petroleum release should incorporate biodegradation. Information to construct the CSM is acquired from historical research, facility characterization (e.g., sample collection), and an understanding of contaminant behavior, among other sources.

Vapor Source for a Petroleum Release

CSMs assist in defining and depicting the nature and extent of the vapor source and identifying where a potentially unacceptable risk for the VIAP may occur to guide further evaluation or response actions. The vapor source is key to understanding and identifying where potentially unacceptable risks for the VIAP may occur.

For the VIAP, the vapor source is represented by concentrations in exceedance of applicable unrestricted residential soil and groundwater VIAC. As discussed above, aerobic biodegradation is an important factor to PVI because it rapidly degrades PHC vapors over relatively short distances (ITRC 2014). Site data shows that when petroleum is most likely to pose a risk to the VIAP are limited to NAPL being close to structures (e.g., less than 15 feet); dissolved phase PHCs or NAPL in direct contact with or entry into building foundations (e.g., basements, elevator pits, etc.); and dissolved phase PHCs or NAPL entering into subsurface utilities (McHugh et al. 2010). Therefore, petroleum-contaminated soil in exceedance of appliable unrestricted residential soil VIAC that has been determined through multiple lines of evidence to be without NAPL will not be utilized in PVI risk evaluations.

Specifically, for PVI, the NAPL and contaminated groundwater in exceedance of applicable unrestricted residential VIAC are the primary areas of the vapor source where risk evaluation and characterization is warranted. In these areas, it is important to identify utilities that may allow the direct entry of contaminated groundwater or mobile NAPL to a current (or future) building or enter into a conduit that can transmit vapors to a current or future building. Volume 6 – Volatilization to the Indoor Air Criteria provides a detailed description of the equations, inputs, and assumptions used in the development of applicable risk based VIAC with EGLE’s VIAP-SL’s calculator.

NOTE: If an evaluation is made in accordance with Rule 299.14(5) and Rule 299.24(5) using more representative data such as soil gas, it may be possible to show that the NAPL or petroleum-contaminated groundwater above applicable VIAC does not represent a VIAP risk for the current structure. The need for land or resource use restrictions will be highly dependent on-site conditions, how or where the soil gas samples were collected in relation to the vapor source, and if future land uses can be evaluated with representative soil gas samples. See Section 6 for more details.
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