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Volume 4: Investigative approach for petroleum VIAP

Attachment C - Data collection and evaluation for petroleum sites

If a facility or property is located within the lateral inclusion zone (LIZ) and is not able to be screened out using the vertical separation distances, if a site-specific approach is implemented, or if there are utilities that may directly transport vapors to a structure, then data collection is needed to evaluate risks. The data collection should be based on where the vapor source is in relation to the structure and is discussed in detail in the following sections:

  • Structure over a Vapor Source (Section C.1.0)
    • Vapor Source not in Contact with a Structure
    • Vapor Source in Contact with a Structure
  • Structure adjacent to a Vapor Source (Section C.2.0)
  • Utilities and Conduits (Section C.3.0)
  • Structures are Not Currently Present (Section C.4.0)

Types of Soil Gas Samples

At facilities that don’t screen out, the investigation approach after the vapor source has been adequately delineated will include soil gas sampling. Representative soil gas data allows for better risk-based decisions since soil gas data reflect the processes that are occurring in the vadose zone (e.g., partitioning, sorption, biodegradation) from the vapor source to the overlying receptor at the location being sampled. Three primary options are available for characterizing soil gas which differ by the sampling location relative to the structure under investigation (if present):

  • Subslab Soil Gas. These sampling points are located within the footprint of a building and are installed by drilling through the slab. They are not located outside of a building and require an actual structure to be present. Sampling depths are less than 1-foot below the bottom of the slab. They are the most representative and predictive of vapors located beneath the structure and their potential to cause an unacceptable health risk.
NOTE: Subslab soil gas samples are preferred over exterior samples for a PVI building evaluation unless the lateral migration of vapors is being evaluated. More information and methods for collecting soil gas samples and additional factors in sample placement are described in Volume 2 – Investigation Methods for the Volatilization to Indoor Air Pathway (VIAP).
  • Exterior Soil Gas. These subsurface sampling points are located at some distance (usually 10 linear feet or more) away from the building. Sample points are installed within the vadose zone and at least 5-feet below the ground surface. Factors considered for selecting sampling depth include (1) fluctuations in water table depth; (2) thickness of capillary fringe; (3) a minimum sampling depth; and (4) depth of the vapor source. See Volume 2 – Investigation Methods for the Volatilization to Indoor Air Pathway (VIAP) for other depth considerations.
  • Near-Slab Soil gas. These subsurface sampling points are located around the perimeter of the building (typically less than 5-feet from a building). In addition to the sampling depth considerations for external soil gas points, building features (such as depth of foundation) should be considered when selecting near-slab sampling depths. Subslab soil gas samples are preferred in evaluating petroleum over near-slab soil gas samples unless lateral migration is being evaluated. Near-slab samples are typically used only to evaluate the fill associated with a utility line or the lateral migration of vapors (see C.2.0)
NOTE: Pressure differential readings, as well as O2, CO2, and methane, should be collected prior to any samples being collected to aid in CSM development and to ensure that the pressure gradient is from the subsurface into the structure.

General advantages and disadvantages for each type of vapor sampling method for the investigation of a petroleum release are provided in Table C-1. More information is available in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP).

Table C-1: Advantages and Disadvantages of Various Investigative Strategies

Measurement Advantages Disadvantages Comments
Subslab soil gas
  • Gives concentrations immediately below building and receptors
  • Most reliable predictor of the potential exposure for the VIAP when the vapor source is not directly entering the structure
  • May have contaminants from interior sources
  • Highly intrusive; requires building access and drilling through slab/floor
  • Preferred approach for evaluating current structures.
Near-Slab Soil Gas (e.g., typically, less than 5-feet from a building)
  • Less chance of short-circuiting by atmospheric air
  • Temporal variations in concentration minimal at depth greater than 5-feet below ground surface.
  • Data can be collected outside the building
  • Can evaluate the lateral migration of a vapor source towards a structure.
  • May not reflect subslab concentrations
  • Requires an understanding of the building construction and size
  • Typically requires larger equipment to reach depths below a basement
  • Concentrations are likely to be different than those collected directly beneath the structure.
Soil Gas (>5-feet below grade)
  • Less chance of short-circuiting with atmospheric air
  • Temporal variations in concentrations are minimal the deeper the soil gas is collected
  • Does not account for aerobic biodegradation in soil layers less than 5 feet below ground
  • May not be representative of future uses where groundwater or shallow petroleum sources are present
  • Soil gas data can provide evidence of biodegradation as a function of vertical transport distance.
Shallow Soil Gas (<5-feet below grade)
  • At some sites, this is the only data able to be collected.
  • Will reliably provide information on oxygen and verify lateral migration.
  • The closer the borings get to the surface, the greater the chance for short circuiting.
  • Subject to temporal variations with atmospheric dilution
  • May not represent future uses nor vapor concentrations beneath a structure
  • Concentration of the vapor source and depths should be considered when evaluating shall soil gas samples – see Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP) for more details.


Vertical soil gas profiles can be acquired by installing a series of nested or clustered exterior or near-slab soil gas points at a range of depths. Such soil gas data may be useful for defining the zone of active biodegradation and demonstrating that the decrease in PHC concentrations with distance from the source is due to biodegradation. This may be useful in development of site-specific risk evaluations. Additional information on vertical soil gas profiles can be found in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP).

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