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Volume 4: Investigative approach for petroleum VIAP

C.1.1 Vapor source not in contact with a structure

A vapor source is not in contact with a structure when, after considering seasonal variation to the depth to groundwater and the capillary fringe, a measurable distance of vadose zone between the vapor source and the structure is maintained. The structure is measured from the bottom of the slab; or the depth of footings if they can transport vapor into the structure (poured footings cannot); or the subsurface utilities (including the sump) if they can transport vapor or the vapor source.

When the vapor source directly underlies a building, subslab soil gas samples should be collected based on the number of sampling locations and frequency described in Tables C-2 and C-3. Exterior soil gas samples in lieu of subslab soil gas samples may be appropriate for structures when all of the following apply:

  • The vapor source is the same concentration on all sides of the structure,
  • There is no direct conduit or pathway between the vapor source and the structure that must be evaluated,
  • The soil gas sampling point can be installed using proper techniques at a depth >5-feet below the ground surface and greater than the depth of the structure considering capillary zone, depth of footings, and subsurface utilities (including the sump),
  • The structure is less than 1,000 feet2 or is a single-family structure, and
  • A soil gas sample is collected from at least two opposing sides of the structure.

Other soil gas sampling approaches may be considered by EGLE if it can be demonstrated that the data provided will account for the spatial and/or temporal variability associated with the facility (e.g., vapor source partially under a structure). If there are exceedances of the VIAC, then a party should proceed with continued evaluation, or an appropriate response activity as further detailed in Section 6.

Number of Sampling Locations

The actual number of subslab soil gas sample points should be justified and based on the overall aerial extent of the vapor source beneath the structure, number of slabs or multiple levels in contact with the soil (e.g., multiple slabs-on-grade in a large warehouse), and foundation type (e.g., combined basement and slab-on-grade in a residence). A standard number of sampling points based on the size of the structure and assuming that the vapor source is under the entire structure is detailed in Table C-2.

To obtain the most representative results, collect vapor samples at least 5-feet inside foundation edges and towards the center of the structure or over where the vapor source is located. In addition, subslab soil gas samples should be collected during periods where vapor intrusion is turned on – there is a higher pressure below the structure than in the structure. Additional samples should be collected near utility trenches (i.e., vapor transport) that intersect plumes of contamination. It is understood that a sample location may need to be adjusted to accommodate the actual structure and building layout. More information can be found in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP), including information that may allow for a reduction in sampling points.

Table C-2: Number of Sample Locations

Building Size Sample Density Minimum Number of Sampling Locations
Less than 1,000-ft2 Not Applicable 2
1,000-feet - 10,000-ft2 3 + one additional sample per 1,500 ft2 of building over 1,000 ft2 3
Greater than 10,000-ft2 9 + one additional sample per 2,500 ft2 of building over 10,000 ft2 9

When evaluating PVI potential beneath single-family residences, at least two samples should be collected, regardless of the building size. In general, one of these samples should be collected from beneath the center of the structure and the second from between the center of the structure and the wall nearest the source of contamination.

Number of Sampling Events

The number of soil gas sampling events for petroleum should be justified and based on the presence and the distance of NAPL and/or the dissolved source petroleum-contaminated groundwater above the VIAC. The general number of soil gas sampling events required for a petroleum release is identified in Table C-3 below.

Table C-3: Number of Sampling Events for Petroleum Sites

Potential Vapor Source and Distance to Receptor Soil Gas Sample Results Minimum Number of Rounds
NAPL ≤5-feet < VIAC 4
NAPL 5 - 10-feet < VIAC 3
NAPL 10 - 15-feet < VIAC 2
Dissolved Groundwater Source < VIAC 1

Prior to use in any decision-making process, the data should be evaluated for whether the data is of sufficient quality to complete the evaluation. If the data does not meet established data quality requirements, additional data collection may be warranted. Table C-3 provides minimum rounds of samples, and data trends should be evaluated to determine the actual number of rounds of samples. For example, if the data trends suggest the concentrations, either in the vapor source or soil gas, are increasing, additional sampling may be warranted.

Additional information can be found in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP) about the data evaluation process. If soil gas data exceeds the VIAC, continued evaluation and/or appropriate response actions will be necessary.  For more information see Section 6.0 Response Activity, Attachment D, Section 7 - Alternate Evaluation Approaches, and Attachment E for more information.

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