Skip to main content
Attachment C.3 - Data collection and evaluation

Volume 3: Investigative approach for VIAP
Attachment C.3 - Data collection and evaluation

Download the PDF

If a building or property is located within the lateral inclusion zone (LIZ), if a site-specific approach is implemented, or if there are conduits that a vapor source can enter and will directly transport vapors to a building, then data collection is needed to evaluate VIAP risks. The data collection should be based on where the vapor source is in relation to the building and the transport mechanism for vapors to enter the building. The following sections discuss the data collection in detail:

  • Building over Vapor Source (Section C.3 1.0)
    • Vapor Source not in Contact with a Building
    • Vapor Source in Contact with a Building
  • Building adjacent to a Vapor Source (Section C.3 2.0)
  • Utilities (Section C.3 3.0)
    • Vapor Source within Utility Backfill Material
    • Vapor Source in an Underground Conduit
  • Buildings are Not Currently Present (Section C.3 4.0)
  • Redefining the LIZ (Section C.3 5.0)

Types of Soil Gas Samples

The investigation approach after the vapor source has been adequately delineated will include soil gas sampling if appropriate. Representative soil gas data allows for better risk-based decisions since soil gas data reflect the processes that are occurring at the water table and in the vadose zone (e.g., partitioning, sorption). Three primary options are available for evaluating and characterizing soil gas which differ by the sampling location relative to the building under investigation (if present):

  • Sub-slab 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 building to be present. Sampling depths are less than one (1) foot below the bottom of the slab. They are the most representative and predictive of vapors located beneath the building and their potential to cause an unacceptable health risk. 
Note: Sub-slab soil gas samples are preferred over exterior samples for a 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.
  • Exterior Soil Gas. These subsurface sampling points are located at some distance (usually ten (10) linear feet or more) away from the building. Sample points are installed within the vadose zone and ideally at least five (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) minimum sampling depth; and (4) depth of the vapor source. See Volume 2 – Investigation Methods for the Volatilization to Indoor Air Pathway for other depth considerations. 
  • Near-Slab Soil gas. These subsurface sampling points are located around the perimeter of the building (typically less than five (5) feet horizontally 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. Sub-slab soil gas samples are preferred over near-slab soil gas samples unless lateral migration is evaluated. Near-slab samples are typically used to evaluate the fill associated with a utility line or the lateral migration of vapors and the vapor source is close to the building (see Section C.3 2.0)
Note: Pressure differential readings, as well as O2, CO2, and methane, should be collected prior to any soil gas samples being collected to aid in CSM development and to ensure that the pressure gradient is from the subsurface into the building. 

General advantages and disadvantages for each type of vapor sampling method for the investigation of a release is provided in Table C.3 – 1. More information is available in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway.

Table C.3 – 1: Advantages and Disadvantages of Various Investigative Strategies

Measurement

Advantages

Disadvantages

Comments

Sub-slab soil gas
  • Gives concentration immediately below building and receptors
  • Most reliable predictor of the potential exposure for the VIAP when the vapor source is not directly entering the building.
  • May have contaminants from interior sources
  • Highly intrusive; requires building access and drilling through slab/floor
  • Preferred approach for evaluating current buildings.

Near-Slab Soil Gas

(e.g., typically,
less than 5-feet horizontally from a building)
  • Less chance of short-circuiting by atmospheric air
  • Data can be collected outside thebuilding
  • Can evaluate the lateral migration of a vapor source towards a building.
  • May not reflect sub-slab concentrations
  • Requires an understanding of the building construction and size
  • May requires larger equipment to reach depths below a basement
  • Concentrations may be different than those collected directly beneath the building.
  • Collection should occur below the elevation of the lowest floor slab
  • May require different depths to assess the lateral migration of vapors

Exterior 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
  • May not be representative of future uses where groundwater in contact or shallow vapor sources are present
  • Soil gas data can provide evidence of lateral migration.
  • Soil gas can show that soil and groundwater concentrations above screening level or criteria are not a vapor source and risks are acceptable.

Exterior Shallow Soil Gas (3-5-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.
  • Potentially subject to increased temporal variations with atmospheric dilution which will result in increased number of sampling events to ensure data is representative.
  • Soil gas data can provide evidence of lateral migration.
  • Soil gas can show that soil and groundwater concentrations above screening level or criteria are not a vapor source or do not pose an unacceptable risk. Consideration of the sampling depth should be made and could lead to increased data collection.

Exterior Shallow Soil Gas (<3-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 sampling location is to the surface, the greater the chance for short circuiting and sampling atmospheric air
  • May not represent future uses nor vapor concentrations beneath a building built in the future
  • Data will not likely support a vapor source does not pose an unacceptable risk from vertical migration into a building
  • Soil gas data can provide evidence of lateral migration.
  • Concentration of the vapor source, and depths should be considered when evaluating shallow soil gas samples – see Volume 2 – Investigation Methods for the Volatilization to Indoor Air Pathway 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. This may be useful in the 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 Indoor Air Pathway.

Previous PageNext Page