There are two (2) scenarios and CSM that must be considered when a building directly overlies a vapor source. Section C.1.1 describes the data collection and evaluation that should be conducted for a facility where the vapor source is not in contact with a building and Section C.1.2 describes where it is in contact with a building.
C.3 1.1 - Vapor Source Not in Contact with a Building
A vapor source is not in contact with a building when a two (2) foot measurable distance between the vapor source and the building is maintained. For a chronic exposure scenario, the average depth to groundwater can be utilized and for an acute exposure scenario, the minimum measured distance should be used. The building is measured from the bottom of the slab or the subsurface utilities penetration (including the foundation drain into the sump) if they can transport vapor or the vapor source. If the sump is isolated from the subsurface, it will not transmit vapors.
When the vapor source is directly under a building, sub-slab soil gas samples should be collected based on the number of sampling locations and frequency described below. If the vapor source is directly under a building, exterior soil gas samples in lieu of sub-slab soil gas samples are appropriate for buildings when all the following apply:
- The vapor source concentration is within one-order of magnitude on all sides of the building (it is relatively similar),
- There is no conduit or preferential pathway that connects the vapor source to the building that must be evaluated,
- The soil gas sampling point can be installed using proper techniques at a depth > three (3) feet below the ground surface and greater than the depth of the building considering depth of foundation, and subsurface utilities (including the sump),
- The building is less than 1,000 feet2 or is a single-family building, and
- A soil gas sample is collected from at least two opposing sides of the building.
When the vapor source is not underneath an occupied building or partially under a building, other soil gas sampling approaches that utilize exterior soil gas samples in lieu of sub-slab soil gas samples may be appropriate if it can be demonstrated that the data provided will account for the vapor migration to the building, spatial, and/or temporal variability associated with the facility. An example scenario of site conditions that may warrant exterior soil gas include, situations where the building is less than 2,000 feet2 and there is at least five (5) feet of vertical separation between the vapor source and building foundation.
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.
Sampling density
The number or density of soil gas sampling points depends on building size, proximity to sources, the scale of soil and groundwater impacts, heterogeneity in subsurface conditions, and the purpose of the data collection. As a rule, the greater the heterogeneity in a particular exposure unit and/or the greater the potential for unacceptable risk, the more samples are required for accurate characterization. Additional samples also may be necessary to reduce uncertainty and can be iterative to increase confidence in vapor plume characterization and site evaluation. See Table C.3 – 2 below for a brief discussion of these factors and their influence on a sampling program.
Table C.3 – 2: Influences on Sampling Density
|
Factor |
Influence on Sampling Program |
Rationale |
|---|---|---|
|
Near Primary Spill/Release Area |
Increased Sample Density |
Soil contamination can produce heterogeneous contaminant distribution; high concentrations can result in a disproportionately large influence on indoor air quality |
|
Large Scale Site |
Reduced Sample Density |
Groundwater as the primary vapor source tends to be more homogeneous than soil sources; contaminant concentrations within larger plumes are more spatially uniform |
|
Reconnaissance Sampling Mode |
Reduced Sample Density |
Lower precision required. The primary objective is to define geographic area of concern, not assess risk/compliance |
|
Geologic Heterogeneity |
Increased Sample Density |
VI migration rates are sensitive to soil properties, and additional samples are needed to define subsurface variability |
|
Increasing Building Size |
Reduced Sample Density |
Conditions tend to be more homogenous in larger commonly ventilated spaces. |
Recommended Number of Sampling Locations
Investigations of sub-slab soil gas and/or indoor air contamination should proceed outward in all directions from known or suspected vapor sources, as appropriate, until the extent of subsurface vapor has been characterized. In addition, all current and future human exposures have been identified, evaluated, and addressed if necessary. In cases where widespread soil gas contamination is present with similarly constructed buildings, statistically valid sampling within a representative number of buildings within the study area (rather than all buildings) may be acceptable. It is recommended that any sampling plan that deviates from this guidance be discussed with EGLE prior to implementation.
The actual number of sub-slab soil gas sample points should be justified and based on the overall aerial extent of the vapor source beneath the building, 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 building and assuming that the vapor source is under the entire building is detailed in Table C.3 – 3. At least one of these samples should be collected from beneath the center of the building and a second from between the center of the building and the wall nearest the source of contamination. It is understood that the sample locations may need to be adjusted to accommodate the actual site conditions and building layout.
Note: For larger buildings that have a vapor source that extends entirely beneath it, or a large area that would require a large number of sampling locations, it is generally recommended to start with a selecting a few key sampling locations based on the highest potential for vapors to exist to assess if a response action is warranted or whether additional sampling locations would allow for a demonstration that the vapor source does not pose an unacceptable risk.
The site CSM, including existing environmental data (e.g., soil gas, groundwater, and soil data) and building construction details (e.g., basement, slab-on-grade, or multiple types of foundations, HVAC systems, etc.) must be considered when selecting locations within buildings for sub-slab soil gas sampling points. It is important for the investigator to take into consideration the potential for sub-slab soil gas sampling results to vary both spatially and temporally when planning for and conducting sub-slab soil gas sampling.
To obtain the most representative results, collect vapor samples at least five (5) feet inside foundation edges and towards the center of the building or over where the vapor source is located. It is understood that a sample location may need to be adjusted to accommodate the actual building and building layout.
More information can be found in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway, including information that may allow for a reduction in sampling points.
Table C.3 – 3: Number of Sample Locations within a Building based on Subslab soil gas
|
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 |
Number of Sampling Events
Table C.3 – 4 provides a general guide on the minimum rounds of samples that should be considered during the evaluation process to evaluate trends and determine if an unacceptable risk is present. If the data trends suggest the concentrations, either in the vapor source or soil gas, are increasing, additional sampling may be warranted. Table C.3 – 4 also assumes that the vapor source has been identified, is stable, and there is a need to demonstrate that the vapor source does not pose an unacceptable risk from a vapor source or there is only a need to assess the lateral migration of vapors from a vapor source. Additional sampling events may be necessary if the vapor source is not stable, or site-specific conditions, such as the construction of a new building, change. However, in most cases, the exceedance of the appropriate criterion, especially in a sub slab vapor point, may be sufficient to determine if a response action is warranted.
Table C.3 – 4: Soil Gas Sampling Events for Determining that an Unacceptable Risk is Not Present
|
Event |
Sub-slab Soil Gas |
Soil Gas > 5’ bgs |
Shallow Soil Gas 3’-5’ bgs |
Shallow Soil Gas < 3’ bgs |
|---|---|---|---|---|
|
Initial characterization or after source removal (conditions have restabilized) and initial sampling events (two) have analytical results 10x less than the SSVIAC or other applicable screening levels. |
2 |
2 |
4 |
2* |
|
After a remedial action has taken place, a vapor source will remain |
3 |
3 |
5 |
3* |
|
With a known vapor source to remain and no remedial actions.
Determination of soil and groundwater above SSVIAC is a vapor source and analytical results not 10x less than SSVIAC or screening levels. |
4 |
4 |
6 |
4* |
*Shallow soil gas samples exterior of a building used to evaluate the lateral migration of vapors only.
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, which includes when collecting sub-slab soil gas samples that they were collected during periods where vapor intrusion is most probable. This is during the potential for advective transport of gas into the building and is determined by measuring the pressure differential between the indoor air and the sub-slab. When there is a higher pressure below the building than in the building the advective transport of gas is into the building. Additional samples may need to be collected near utility trenches (i.e., vapor source transport) that intersect plumes of contamination.
If the data does not meet established data quality requirements, additional data collection may be warranted. Information can be found in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway about the data evaluation process. If soil vapor data exceeds the VIAC, continued or additional evaluation and/or appropriate response actions will be necessary.
Recommended Timing for Sampling Events
Soil gas concentrations are subject to seasonal variation driven by changes in soil temperature, moisture levels in the soil column, groundwater levels, and barometric pressure. Sampling across multiple seasons may be necessary to capture representative conditions, especially when considering closure or a no further action under Part 201. Timing and evaluating the potential for seasonal variation is critical to ensure that the data is representative. Sampling events across different seasons is typically necessary to capture the range of variability in vapor concentrations. Interpretation of soil gas data should consider environmental factors that influence gas movement, including soil saturation, freeze/thaw cycles, and barometric fluctuations.
Note: Observed differences in soil gas concentrations between sampling events may reflect seasonal variations rather than changes in contaminant sources or releases.
When a criterion is exceeded, a party should proceed to an appropriate additional evaluation or site management strategy. The evaluation may include a new site-specific criterion that will be developed and proposed, see Volume 6 for more information on the development of site-specific criteria.
C.3 1.2 - Vapor Source in Contact with a Building
A vapor source is in contact with a building when there is not a measurable distance between the vapor source and the building. This can be for any current building or a reasonably anticipated future building. For sites where there is groundwater that may come into contact with the building, the evaluation should consider seasonal variation to the depth to groundwater. The use of average groundwater depth may be applicable for the evaluation of a chronic risk; however, in most cases the use of an average is not applicable for the evaluation of chemicals with identified acute toxicity. The building is measured from the bottom of the slab; the subsurface utilities (including a foundation drain) if they can transport the vapor source and result in direct volatilization to the building, or the bottom of any elevator shaft or pit, whichever is lower. If there is some measurable distance, but less than two (2) feet between the vapor source or if only an elevator shaft or pit will be in contact with the vapor source, a site-specific evaluation will be needed. See Volume 6 for information on site specific criteria.
When a vapor source is in contact with a building, the exposure scenario is direct volatilization and will initially be evaluated with groundwater data. Soil gas data does not represent the potential risks from direct volatilization so cannot be used for the evaluation unless they are compared to acceptable indoor air concentrations. For current occupied buildings, the evaluation of direct volatilization will be the sampling of the groundwater that is entering the building (e.g., in a sump), the sampling of the headspace (e.g., headspace of the sump or foundation drain), and sampling of the indoor air. If the indoor air exceeds the screening levels an appropriate response activity should be implemented to prevent direct volatilization into the building (see Section 6). To account for the potential variations in concentrations, EGLE recommends at least three sampling events.