Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.6
SOIL VAPOR SAMPLING STRATEGIES

7.6 SOIL VAPOR SAMPLING STRATEGIES

7.6.1 Determining When to Collect Soil Vapor Samples

Table 7-1 Decision Logic for Subsurface Vapor Hazards

Site Scenario

1Regularly Occupied Buildings within 100 ft of Source Area

Soil Vapor Data

Contaminants in Vadose Zone 2Soil and/or 3Groundwater Pose Potential Vapor Intrusion Hazards

Yes

Collect source area vapor data and data to evaluate potential vapor intrusion hazards.

No

Collect source area vapor data to evaluate potential future vapor intrusion hazards or, at a minimum, recommend soil vapor investigation prior to future subsurface work or construction of buildings.

Post-Remediation Confirmation of Previously Identified Vapor Intrusion Hazard

Yes or No

Collect soil vapor data to confirm and document absence of remaining, significant vapor intrusion hazards.

4No Potentially Significant Vapor Intrusion Hazards Identified

Yes or No

Collection of soil vapor samples not necessary; conclude in EHE that contamination does not pose significant vapor intrusion hazards.

Notes:

  1. For petroleum sources only - Source area within vertical thirty feet of building slab or crawl space.
  2. VOC concentrations above Tier 1 soil action levels for vapor intrusion, significant volume (e.g., >10m3) of VOC-contaminated soil is present, or potential for elevated vapors under a building slab otherwise suspected (e.g., PCE vapors under a dry cleaner).
  3. Free product on groundwater table or dissolved VOC concentrations above Tier 1 groundwater action levels for vapor intrusion.
  4. VOC concentrations below Tier 1 EALs for both soil or groundwater and significant volume (e.g., >10m3) of VOC-contaminated soil or other potential source of elevated vapors under a building slab not suspected.

An example, decision flow chart for the collection of soil vapor samples is presented in Table 7-1. Soil vapor samples are collected to help locate and characterize areas of contaminated soil and groundwater that pose vapor intrusion risks for existing or future buildings. Direct comparison of groundwater data to HDOH action levels intended to address potential vapor intrusion concerns in the absence of initial, soil vapor (or indoor air) data is generally acceptable (HDOH 2017a). The groundwater action levels are intended to be conservative, assuming that representative samples are collected. If action levels are exceeded then the additional collection of soil vapor samples is recommended. If a significant threat to indoor air is deemed likely, then the concurrent collection of indoor air samples is likewise recommended (Section 7.7).

Note, however, that groundwater action levels presented in the HDOH EHE guidance are not applicable for sites where the depth to groundwater is less than ten feet due to limitations in the models and data used to develop the levels. The direct collection of soil vapor samples is recommended in these scenarios.

Reliance on soil samples to adequately identify and characterize the presence of VOC-contaminated soil is, in contrast, significantly prone to errors. This is in part due to the small size of the soil aliquot typically tested by the laboratory for VOCs (five grams) and the heterogeneous nature of contaminants in soil (refer to Sections 3, 4 and 5 of the TGM). The chance that a small number of discrete, five-gram soil samples will be representative of the targeted area and volume of subsurface soils and capture a representative number of “hot spots” is minimal. The chemicals may also be present predominantly in vapor phase in very dry soil (e.g., beneath a dry cleaner building slab). This could be overlooked by the collection of only soil samples.

The collection of soil vapor samples is therefore recommended at all sites where a significant amount of VOC-contaminated soil could be present in the vadose-zone and/or the contaminant could be present primarily in the vapor phase. A soil volume of at least 10m3 is generally needed in order to pose significant, long-term vapor intrusion hazards, based on mass-balance models for assumed exposure duration and typical contaminant concentration in heavily-impacted soil; HDOH, 2007c, HDOH, 2016). This can be evaluated on a site-specific basis as needed, although short-term, acute or nuisance impacts must also be considered. Direct collection of soil vapor samples regardless of soil and/or groundwater data is also recommended for sites with a very high potential for the release of volatile chemicals. This includes gas stations and dry cleaners (see Section 7.6.2.2).

As is the case for groundwater, volatile chemicals in subsurface soils tend to more evenly disperse over relatively large areas due to diffusion flow. A soil vapor sample is also representative of a significantly larger volume of soil (liters) than a discrete soil sample (five grams, around three milliliters). This emphasizes the usefulness of soil vapor samples to identify the presence or absence of significant VOC contamination in the subsurface. The use of multi-increment subsampling approaches can significantly increase the usefulness of VOC soil data from cores (see Section 5), but widely-spaced cores could still miss relatively small but still significant areas of VOC-contaminated soil that might pose leaching or vapor intrusion hazards. Even so, and as discussed elsewhere in this section and in Section 13.2, random, small-scale variability in VOC concentrations between closely located points can still be considerable and limits the reliability of data that represent very small volumes of vapor.

Although not explored in detail in this guidance document, soil vapor data can also be used to evaluate leaching hazards at sites contaminated with volatile chemicals. Traditional soil leaching models estimate the concentration of a contaminant in vadose-zone leachate based on input soil data (HDOH 2017a). This can be highly unreliable, due to complexities in soil composition, moisture content and other factors. In the case of VOCs, a more precise estimate of the dissolved-phase concentration of a contaminant in vadose-zone leachate can be made by simply dividing the concentration of the VOC in vapor samples by the Henry’s Constant (unitless) for that chemical. This approach is used to develop soil vapor screening levels for leaching and groundwater protection concerns in the Tropical Pacific edition of the HDOH Environmental Hazard Evaluation guidance (HDOH 2017b).

Additional guidance on the use of soil vapor samples to help evaluate potential leaching hazards at sites will be included in future editions of the TGM. In addition to the identification of subsurface VOC-contaminated soil, subsurface vapor samples are most commonly used to evaluate potential vapor intrusion hazards for existing or future buildings. The HEER Office recommends the following three-step approach for the initial evaluation of vapor intrusion hazards at sites where soil or groundwater is contaminated with volatile chemicals (HDOH, 2016):

  1. Compare groundwater and soil analytical data to appropriate HDOH environmental action levels (EALs) prescribed in Evaluation of Environmental Hazards at Sites with Contaminated Soil and Groundwater (HDOH, 2016) or site-specific action levels approved by HDOH. See Table C-1a for Groundwater Action Levels and Table C-1b for Soil Action Levels, located in Appendix 1 of the EHE document; or use the EAL surfer.
  2. Collect soil vapor samples immediately beneath building slab (preferred; LVP sampling methods recommended) or adjacent to buildings if groundwater EALs for vapor intrusion are approached or exceeded or if a potentially significant source of VOCs in vadose-zone soil is suspected, (see Section 7.6.2.2; see also HDOH, 2016, Table C-2 in Appendix 1). Collect soil vapor samples from within deeper, source areas if widespread, heavy contamination is known to be present (see Section 7.6.2.3). Collect soil vapor samples beneath the footprint of anticipated, future buildings if a building is not currently located in that area. Recommended sampling depths for uncovered (unpaved) locations proposed for future construction or uncovered locations adjacent to existing structures are discussed in the following section.
  3. Consider remedial actions at sites where Shallow Soil Gas Action Levels are approached or exceeded. This is necessarily site-specific, but could include sealing of floors and active treatment of source areas or the installation of vapor barriers under future buildings. Consider the collection of indoor air samples if the concentration of a VOC in vapors immediately beneath a building slab exceeds the soil gas action level and is greater than 1,000 times (sensitive land use, including residential) to 2,000 times (commercial/industrial) typical background indoor air (see Section 7.7.1). For crawl spaces, consider the collection of indoor air samples if the concentration of a targeted VOC is greater than ten times the anticipated indoor or outdoor background level. Compare results to Indoor Air Action Levels (HDOH, 2016, Table C-3 in Appendix 1) and known or anticipated background levels in indoor air.

Table 7-1 provides the decision logic for determining when soil vapor sampling is recommended (Step 2) based on the occurrence of VOCs in soil and/or groundwater and the distance between the building and the source area.

The initial collection of soil vapor samples will generally focus on source area and immediately under overlying or nearby buildings. A lateral separation distance of 100 feet from a subsurface source area is considered adequate to prevent potentially significant vapor intrusion problems (ITRC 2007). The adequate vertical separation distance is highly site and contaminant specific. Vertical separation distances appropriate for attenuation of vapors associated with chlorinated solvents have not been adequately studied.

Layering of soil horizons due to weathering, past deposition of sediment, etc., can lead to the presence of clay-rich moist units with very low vapor permeability that significantly impede the upward diffusion of vapors (diffusion rates through water are typically four orders-of-magnitude slower than through soil; see Appendix 1 in HEER EHE guidance, HDOH, 2016). Thin lenses of perched groundwater can further reduce upward vapor flux. Aerobic biodegradation of non-chlorinated, vapor-phase, petroleum compounds can also result in a significant and often abrupt attenuation of vapors within a few feet of a source area (e.g., heavily contaminated soil or free product on groundwater).

A discussion of targeted chemicals of concern for petroleum releases is provided in Section 7.13.1.2 (see also Section 9 ). Recent studies have suggested that ten meters (thirty feet) of clean soil (i.e., TPH <100 mg/kg) is adequate to reduce vapor concentrations to below levels of concern for potential vapor intrusion hazards, regardless of the mass or concentration of petroleum in underlying soil or the presence of free product on groundwater (e.g., Abreu et. al 2009, McHugh 2010; USEPA 2013). For dissolved-phase contaminants a “vertical separation” distance of fifteen feet or less was observed to be adequate. These studies are ongoing, but appear to be consistent with observations in Hawai´i. With the exceptions noted below, these separation distances can be used to determine the need to collect actual soil vapor samples at a site. For example, if no contaminated soil is present in the upper thirty feet of the vadose zone then potentially significant vapor intrusion hazards can be ruled out without the collection of soil gas samples. If the water table is at a depth of greater than fifteen feet year round and no free product is present on groundwater and contaminated soil is not present in the vadose zone, then potential vapor intrusion hazards can again be ruled out without the collection of soil vapor samples.

Shorter vertical separation distances might be appropriate, but should be evaluated and supported on a site-specific basis before a concurrence to negate the need to collect additional soil vapor samples can be granted. This should include borings to characterize subsurface soil types and the collection of a small number of soil vapor samples (e.g., one to three) from an area considered to be representative of overall site conditions. In practice, significant long-term vapor intrusion hazards are unlikely to be posed by dissolved-phase petroleum contaminants in groundwater under any site scenario due to low source strength and rapid biodegradation of vapors in the vadose zone. The collection of soil vapor samples over dissolved-phase plumes can, however, help negate (or identify) the presence of previously unidentified petroleum contamination in the vadose zone. (For dissolved-phase solvent plumes, soil vapor samples are always strongly recommended if action levels for vapor intrusion are approached or exceeded, regardless of the depth of the plume.)

Shorter lateral separation distances (i.e., <100 ft) might also be appropriate at a site but again this should be evaluated on a site-by-site basis. Significant, lateral migration of petroleum vapors away from source areas is of particular concern at sites covered with pavement or buildings, where replenishment of oxygen in subsurface soils is hindered. Large volumes of shallow, contaminated soil or widespread free product on shallow groundwater (i.e., <30ft deep) could lead to the accumulation of vapors under caps and a progressive outward expansion of anaerobic conditions and migration of petroleum vapors over time.

Exceptions to the above guidelines are likely to be rare, but could include sites that directly overlie bedrock (e.g., fractured basalt) that could allow for significantly greater vertical and lateral migration of petroleum vapors prior to attenuation below target action levels. Other potential exceptions include substantial subsurface releases of petroleum in areas with a very deep water table (e.g., >50ft). This could lead to the presence of a thick, deep column of heavily contaminated soil. Anaerobic conditions could develop for a significant distance above and away from the plume, as the natural replenishment of oxygen is overwhelmed. Anaerobic conditions and less inhibited vapor migration could also develop under paved areas that overlie deep (i.e., >30ft) widespread, heavily contaminated soil or free product on groundwater. Such scenarios could be possible with large releases from fuel pipelines, fuel hydrant systems at airports, or large, aboveground tank facilities.

Additional guidance on the investigation and evaluation of petroleum releases is provided in the HEER guidance Long-Term Management of Petroleum-Contaminated Soil and Groundwater HDOH, 2007c).

7.6.2 Soil Vapor Sampling Design

7.6.2.1 Overview

The design of a soil vapor sampling plan should reflect the objectives of the investigation. Investigations are typically carried out to identify large-scale vapor plume patterns vs vapor intrusion assessment. Factors considered in the design of a soil vapor investigation include the objectives of the investigation, soil type, depth to groundwater, the number and size of existing buildings, and current site use or future development plans. Additional considerations for the sampling strategy include access to building interiors or through concrete slabs, the conceptual migration model, and regulatory requirements.

Small-volume (e.g., one-liter) soil vapor samples are typically used for in situ characterization of subsurface vapor plumes (see Section 7.2). Large-scale patterns implied by the data can be used to help identify the presence of VOC sources, locate points for collection of LVP samples and design remedial options. Small-scale patterns depicted by single sample points are less reliable, due to potential random variability of VOC concentrations within a vapor plume at the scale of a few liters or less. The use of Large Volume Purge (LVP) methods for collection of vapor samples immediately beneath building slabs and more direct assessment of vapor intrusion risk are discussed separately in Section 7.5 and Section 7.8.5. The collection of one or more LVP samples to initially assess vapor intrusion risk is appropriate for general due diligence purposes, especially in absence of known or suspect, vadose-zone source area. The collection of multiple, small-volume vapor samples for in situ characterization of large-scale, vapor plume patterns is recommended if localized source areas are known or suspected beneath a building slab. The data can then be used to designate LVP sample collection points for more direct evaluation of vapor intrusion risk.

As discussed below, soil vapor sampling locations are selected based on areas the CSM identifies as having the potential for complete exposure pathways from the subsurface to the building interior. The sample locations can be selected to investigate a single point or based on lateral and vertical delineation considerations. Following the selection of sample locations, soil vapor samples can be collected using temporary driven probes or by installing permanent soil vapor sampling probes (see Section 7.9). When assessing the source of subsurface vapors, samples are typically collected within the suspected or known source area, and upgradient, downgradient, and cross-gradient of the source area because soil vapor can migrate in a different direction than groundwater flow. When assessing upward, vertical migration, vapor samples from multiple depths may be useful or even required to evaluate upward attenuation of vapors or highlight the need to identify preferential pathways through otherwise low-permeability soils that might connect deeper sources to overlying buildings.

As also discussed in more detail below, the frequency of soil vapor sampling is dependent upon the purpose of the soil vapor investigation. Characterization and delineation can require one or two surveys, while remediation assessment or long term monitoring can require repeated surveys on a pre-determined schedule (e.g., weekly for remediation assessment and semi-annually or annually for long term monitoring). Remediation assessment and long term monitoring of contaminants of concern are typically refined during the characterization and delineation phases of the project. Remediation assessment or long term monitoring generally should be conducted using permanent probes to ensure data comparability.

As noted, this guidance does not address safety or hazard mitigation efforts required in the event of explosive vapor accumulation (i.e., methane); however, methane concentrations should be monitored to determine whether these hazards exist. Methane is a non-toxic, lighter than air gas, which is an explosive hazard when present at concentrations in excess of five percent (%) by volume in air (approximately 50,000 parts per million by volume, which is referred to as the Lower Explosive Limit [LEL] for methane). At contaminated sites, additional soil vapor sampling events and possible interim corrective measures should be considered if methane exceeds 1/10 of the LEL (see Section 9.4).

7.6.2.2 Soil Vapor Sampling Point Locations

Figure-7-4

Figure 7-4: Schematic of Soil Vapor Concentration Profile. VOCs volatilize out of a groundwater plume and diffuse vertically toward the surface. Vapor phase concentrations are highest at the groundwater-vadose zone interface and decrease with decreasing depth. Vapors can accumulate under buildings or paved areas as the ability to diffuse outward and be emitted to the atmosphere becomes limited or as anaerobic conditions develop due to insufficient replenishment of oxygen.

Small-volume point samples are used during the initial phase of investigation to identify large-scale vapor plume patterns and initially estimate potential vapor intrusion risks to overlying, existing, or future buildings. A relatively small number of soil vapor samples (e.g., three to ten) are typically used to initially identify the presence or absence of potential subsurface VOC source areas. Samples are typically collected from within suspect soil source areas or immediately above suspect groundwater sources. The additional collection of soil vapor samples from the fill material immediately under the building slab is recommended for initial site characterization at sites where the distance to the source area is greater than 5 feet (see Section 7.6.2.3). This will provide information on the upward attenuation of VOCs away from a source area. (Note that reliable correlation between vapor points will be limited by uncertainty regarding the magnitude of random, small-scale variability within the vapor plume). Confirmation of the plume boundaries based on multiple points is necessary to avoid false negatives and under estimation of the overall plume size. The location and shape of a vapor plume might not mimic the shape of the primary source area (i.e., contaminated soil or groundwater). This is because the outward, lateral migration of vapors away from the source area is strongly influenced by small-scale heterogeneities in the soil and associated preferential pathways that may not be obvious in the field. In the experience of the HEER Office, high concentration areas of vapor plumes can be located some distance from the primary source area, complicating identification of the latter based on soil vapor data alone.

Locations for soil vapor sampling should be selected based on the objectives of the investigation. If the objective is to identify and map large-scale vapor plume patterns, then strategically located sampling points over and around the suspected source area are appropriate, with samples collected at similar depths or targeted to suspected preferential pathways. Samples collected directly within a suspect vadose-zone source area or immediately above a groundwater source can be useful for evaluating the strength of the source. Lateral spacing between sample locations should take into consideration subsurface utilities, building foundations, or planned future use of the site. Care should be taken to avoid utilities when collecting vapor samples within or nearby utility corridors.

If the objective of the investigation is to assess potential vapor intrusion impacts at an existing building, then targeted sampling locations at the building, at the vapor source, and possibly in-between may be appropriate. Grids of passive soil vapor samples should also be considered (see Section 7.8.3 and see Section 7.12). The collection of small-volume soil vapor samples from immediately beneath building foundations (i.e., below the concrete slab or within crawl spaces) can also assist in subsequent designation of LVP subslab vapor points for more direct assessment of potential vapor intrusion risk (Section 7.8.5). For example, LVP samples could be collected directly within high-concentration areas of a plume in order to assess worst-case, vapor intrusion conditions. In contrast, LVP samples might be collected from localized, low-concentration areas within a plume suspected to indicate active vapor intrusion, as less-impacted air is advectively drawn into this area of the plume.

Note that dry soil under slabs can serve to enhance vapor concentrations in comparison to soils with a higher moisture content, even though the total concentration/mass of VOCs in both scenarios is similar (USEPA 2012d). Small- and/or large-volume samples from utility corridors may be warranted, since coarse fill in the trenches can serve as a conduit for vapors to the slab as well as to utility penetrations and other potential preferential pathways through the floor and into the building (see also USEPA 2012d). Sample collection adjacent to buildings can be considered if the source of contamination is not below the building or the collection of vapor samples directly beneath the building is limited due access issues or the presence of subsurface utilities. If this is the case then samples should be conservatively collected from a depth of five to ten feet below ground surface (or no more than two to three feet above groundwater for shallow water tables) in order to take into consideration the potential buildup of vapors under existing or future building slabs.

Small-volume soil vapor sample data for in situ characterization of a subslab vapor plume can be collected above suspect sources areas beneath the building slab, in the vicinity of utility corridors that could serve as preferential pathways for vapor migration, beneath high-risk areas of the building based on use or penetrations in the slab or, in the absence of other information, from the center of the building slab (USEPA 2012d; CalEPA 2011; see Section 7.7.2). Vapor points also should be placed in the vicinity of the building where vapor intrusion is considered to be most likely, as well as between the center of the building and adjacent sources that do not directly underlie the building (see Section 7.7.2).The number of probes that can be installed for in situ characterization of a vapor plume will in part be limited by cost and logistical considerations, including accessibility of locations for sample collection and the presence of subslab utilities.

As discussed above, the type of chemicals present in the soil vapor should also be considered in selecting soil vapor sampling locations. Biodegradation can play an important role in the subsurface migration of petroleum-related contaminants and can significantly reduce the concentration of VOCs in vapors over short distances. At sites where the chemicals of concern are chlorinated compounds (e.g., dry cleaner sites), however, biodegradation is unlikely to be an important process, and elevated concentrations of VOCs can persist for significant distances. Elevated concentrations of VOCs in soil vapors can also persist for long periods of time in the vadose zone following active, in situ remediation of contaminated groundwater (“residual vapor plume,” see Table 7-1). The San Diego County Site Assessment and Mitigation (SAM) Manual, among other references, provides a useful source of soil vapor sampling strategies for a variety of site scenarios (SDC 2011).

7.6.2.3 Soil Vapor Sample Depths and Depth Intervals

The depth of soil vapor points depends on the objectives of the investigation (Figure 7-4). Characterization of known or suspected source areas should consider such factors as the nature and magnitude of the release, the subsurface geology and the depth to groundwater. The investigation of potential vapor intrusion hazards will require the placement of sample points within shallow, vapor flow pathways, including utility trenches and fill material immediately beneath slabs (e.g., first 6 to 12 inches of soil beneath building slab).

Ideally, the lateral and vertical extent of vapor plumes should be delineated to HDOH Tier 1 soil vapor action levels applicable to residential land use (HDOH, 2016). Small-volume sample data are currently most appropriate to accomplish this task, due to limitations on the collection of LVP samples from deeper soil or from open, uncapped areas and the potential for breakthrough to outdoor air. Less conservative soil vapor action levels may be appropriate for assessment of vapor intrusion risk at commercial/industrial sites. Failure to compare site data to residential action levels may impose the need for a land use restriction on the site, however.

The collection of small-volume vapor samples and/or LVP samples from the fill material immediately beneath a building slab (e.g., first 6 to 12 inches of soil) is an important part of a vapor intrusion investigation. Relatively permeable, sandy silts are typically used as fill material under building slabs to provide structural stability. This fill material is often more permeable to vapors than the native, clayey soils in Hawai‘i and can serve as a preferential pathway for subsurface vapors via connecting utility trenches or other conduits.

Soil vapor samples should therefore always be collected in the fill material immediately beneath the slab for evaluation of current vapor intrusion hazards, even if deeper samples are also collected. A focus on deeper soil vapor sample data can be misleading, since the samples do not take into account upward attenuation from the source area (especially important for petroleum). Deeper data could also miss contamination that is restricted to the fill material immediately beneath the building slab associated with indoor spills of solvents and other VOCs and downward migration through the floor or through broken drain pipes. Underlying soils might be relatively un-impacted, even though the concentrations of VOCs in vapors within the fill material are extremely high. This is a common scenario for dry cleaners, where high levels of PCE and related VOCs may be detected in subslab soil gas but not in deeper soil samples (or even soil samples collected from under the slab)

The presence of a building slab or other paving also significantly slows, or prevents, soil vapor from diffusing upwards and escaping to the atmosphere. This can result in elevated soil vapor VOC concentrations beneath the slab/paving in comparison to adjacent, uncovered areas. Note, however, that diffusive VOC transport can never lead to higher concentrations under the slab than at the source.

The collection of soil vapor samples from both the fill material immediately under the building slab and the suspected or known source area is recommended at sites where the distance to the source area is 5 feet or greater, but no closer than 2-3 ft to the water table to avoid pulling water into the sample collection device (see Figure 7-4; see also Sections 7.9.3 and 7.10.1). Small-volume sample data can be used to assess large-scale, vapor plume patterns. The collection of LVP sample data is recommended for more direct assessment of vapor intrusion risk (Section 7.8.5). This will help assess the need to seal cracks and utility gaps in the building floor as an added measure of precaution, in the event that nearby portions of the vapor plume exceed subslab soil vapor action levels, even though the measured concentrations of volatile chemicals in actual soil vapor do not, and potential preferential pathways into the building were overlooked (se Section 7.14.1). As discussed below, in cases where the extent and magnitude of contamination is relatively small, the site could still receive case closure without further monitoring or action (see also HDOH 2007c). In other cases additional monitoring to verify that adverse, vapor intrusion impacts are unlikely to occur will be needed (see Section 7.10.1). This will typically require the periodic collection of LVP vapor samples beneath targeted areas of the slab, similar to the collection of periodic samples from groundwater monitoring wells. Reliance on small-volume samples might, however, be required for monitoring of vapors beneath building slabs that cannot be sufficiently sealed for LVP sample collection. The collection of LVP data likewise might not be feasible for sites with low-permeability soil immediately beneath the building slab, although this would likewise reduce the risk of advective flow of vapors into the building.

Collection depths for small-volume sample data to be used to assess vapor intrusion risk in open areas where LVP sample data are not practical depends in part on the VOCs present. At sites with recalcitrant compounds (e.g. chlorinated solvents) soil vapor samples should be collected from no less than five feet below ground surface. Soil vapor samples collected from depths of less than five feet can underestimate the concentrations of recalcitrant compounds that could accumulate if a building were present. Soil vapor samples should be collected from a minimum depth of ten feet for petroleum-contaminated sites or no more than two to three feet above groundwater for sites with a shallow water table. This is necessary in order to take into consideration the potential buildup of vapors under existing or future building slabs due to low-oxygen conditions and a reduced potential for biodegradation.

Additional sample depths will depend on site-specific conditions and the investigation focus. In some cases it may also be desirable to assess the vertical distribution of vapor-phase contaminants between the source media and the ground surface or the foundation of a building. This will require the collection of samples from a minimum of two depths, typically one within or immediately above the source and one at the target receptor point. Three or more sample depths may be beneficial at sites with deep sources or water tables.

The site geology should also be considered when identifying sampling depths. In general, installation of vapor sampling probes in relatively high permeability horizons is preferred; however, the overall CSM should be taken into account as well. Permanent soil vapor probes should be installed above the maximum-anticipated, seasonally- or tidally-influenced elevation of the water table.

7.6.2.4 Soil Vapor Sample Screen Intervals

Screens used for subslab samples should match the thickness of permeable, fill material immediately beneath the slab, typically four to six inches. Both small-volume and LVP vapor samples are typically collected though a temporary or permanent six-inch (15cm) screen or “implant.” The length and placement of the screen depends on the investigation objectives. Longer screening might be warranted for more reliable characterization of large-scale, vapor plume patterns. As discussed above and in Section 7.8.5, data for small volumes of vapor can be useful for identification of large-scale plume patterns but are not necessarily pertinent to assessment of vapor intrusion. This is similar to issues related to the use of discrete sample data for very general screening purposes versus the use of “large-mass,” Multi Increment soil sample data to more directly evaluate risk.

Six- to twelve-inch vapor point screens are generally desirable for characterization of subslab vapors, since HDOH soil vapor action levels are intended to apply to vapors within the assumed narrow, advective zone in the immediate vicinity of a vapor entry point. Much longer screens might be desirable for in situ, larger-scale characterization of deeper portions of a vapor plume. For example, a five-foot (1.5m) length of a two inch-diameter (15 cm) well screen contains approximately 30 liters of air. Allowing the air inside of the well screen to equilibrate with vapors in the surrounding soil would allow a sample collected from the well screen to represent a much larger volume of vapor than the vapor actually captured within a canister. The resulting data would provide a more reliable and reproducible characterization of the plume at that specific location in terms of vapor intrusion risk. Replicate samples could be collected over time to assess data precision and temporal variability within the vapor plume.

Note that the same is true with respect to the representativeness of a groundwater sample collected from a five-foot, monitoring well screen or from a much smaller interval using grab samples or passive diffusion bags (see Section 6). Additional guidance on this topic, including the concept of “Decision Units” for the collection or groundwater sample data will be incorporated into Section 6 of this guidance document in the future.