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

Subsurface soil vapor samples are collected to help locate and characterize areas of contaminated soil and groundwater that might require remediation or long-term management. The direct collection of groundwater samples is generally adequate to identify contamination with volatile chemicals. This is because contamination tends to become dispersed over relatively large areas due to diffusion and groundwater flow. The additional collection of soil vapor samples to assist in the identification of areas of contaminated groundwater is typically not necessary or required.

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 and to a lesser extent advective flow. A soil vapor sample is also representative of a significantly larger volume of soil (several 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 significant areas of VOC-contaminated soil that might pose leaching or vapor intrusion hazards.

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):

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.
  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 or within crawl spaces (preferred), or adjacent to buildings and compare results to Shallow Soil Gas Action Levels for areas over or near a plume where Groundwater and/or Soil EALs for vapor intrusion are approached or exceeded or sites where a potentially significant source area is otherwise suspected, (see Section 7.6.2.2; see also HDOH 2016, Table C-2 in Appendix 1). Collection of soil vapor samples from source areas is recommended if widespread, heavy contamination is known to be present (see Section 7.6.2.3). In the case of anticipated future construction, collection of soil vapor samples from the future building footprint is strongly recommended. Recommended sampling depths for uncovered (unpaved) locations proposed for future construction or uncovered locations adjacent to existing structures are discussed in the following section. Note that soil vapor samples are recommended at sites where significant amounts of VOC-contaminated soil may be present, regardless of soil data (see following discussion).
  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

A soil vapor sampling strategy depends on site-specific conditions, including soil types and groundwater levels, 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

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.

Point sampling is currently the primary method used to characterize subsurface soil vapors (see Section 7.2). The HEER Office is investigating the use of Decision Unit (DUs) and Multi-Increment Sample (MIS) approaches to obtain more representative data and better establish site investigation objectives (refer to Sections 2 and Section 3). Decision Units designated for vapor intrusion would necessarily be tied to the footprint of existing or future buildings but may not cover the entire footprint. The current HEER Office EHE guidance recommends that soil DUs for potential vapor intrusion hazards be conservatively limited to a default building footprint of 100m2 (HDOH 2016). This is the default area referenced in the models used to develop soil action levels for vapor intrusion. A similar DU area is most likely appropriate for soil vapor, but the number and/or volume of individual soil vapor samples or “increment” locations necessary to adequately represent the DU mean has not been well studied. Sample timing and frequency is discussed in Section 7.10.1.

Point samples are used during the initial phase of investigation to determine the extent and magnitude of subsurface vapor plumes and to assess exposure pathway completeness and guide selection of future sampling locations. A relatively small number of soil vapor samples (e.g., three to five) are typically used to identify or negate the presence of VOCs above soil gas action levels within or directly above primary source areas. The collection of soil gas 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 greater than 5 feet (see Section 7.6.2.3). If concentrations exceed Shallow Soil Gas Action Levels (HDOH 2016, Table C-2 in Appendix 1), additional investigation or remedial action is warranted. If concentrations are below action levels and are deemed to be representative of long-term, site conditions then no further action is necessary with respect to potential vapor intrusion hazards (see also Section 7.10.1).

The lateral boundaries of a vapor plume can be challenging to define once a plume with significant concentration of VOCs has been identified in a source area. The shape of a vapor plume might, or more likely 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. It is not uncommon for vapor plumes to become detached from the primary source area and be isolated some distance away, making them even more difficult to detect or making identification of the actual source area location difficult.

Locations for soil vapor sampling should be selected based on the objectives of the investigation (see also USEPA 2012d). If the objective is to identify and map a soil vapor plume, 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. Lateral spacing between sample locations should take into consideration existing utilities and buildings or planned future site use.

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 gas samples should also be considered (see Section 7.12). The collection of soil vapor samples from immediately beneath building foundations (i.e., below the concrete slab or within crawl spaces) is recommended to most directly assess the current potential for vapor intrusion into buildings. Drier soil under slabs can serve to enhance vapor concentrations in these soils, in comparison to soils with a higher moisture content (USEPA 2012d). Samples from utility trenches may also 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, there is severely restricted access to the building interior, or subsurface utilities are present. 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 due to low-oxygen conditions or advective flow. A soil vapor sampling strategy that incorporates samples from both immediately beneath the building foundation and adjacent to building foundations can also be considered depending on site conditions.

Concentrations of VOCs beneath the slab of a home or building are likely to be heterogeneous (USEPA 2012d; Brewer et al. 2014, in prep). A minimum of three vapor points is currently recommended to assess potential vapor intrusion hazards. As a worst-case scenario, one or more vapor points should be placed in the center of the slab, where the concentration of VOCs is predicted to be the highest in the absence of specific “hot spots” of contamination beneath the 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). Examples include areas where utilities penetrate the building slab, or areas where cracks in the floor could serve as preferential vapor pathways. Sampling points should also be placed in areas of the building suspected to overlie the highest concentration of a known or suspected vapor plume.

The number of probes that can be installed will in part be limited by cost and logistical considerations, such as availability of suitable locations and the need to minimize disturbance to building occupants. The HEER Office intends to prepare more detailed guidance on the collection of soil vapor samples beneath buildings and for site investigation purposes in general in the future. As discussed in Section 7.9.3, recent cheaper and faster options for the collection of subslab soil vapor samples (e.g., pin-type vapor points for slabs) hold promise for the collection of a larger number of samples and reduced uncertainties regarding potential vapor intrusion hazards.

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 out to HDOH Tier 1 soil gas action levels which take into consideration the potential intrusion of vapors into residential structures (HDOH 2016). Less conservative soil gas action levels may be appropriate for final decision making purposes at a commercial/industrial site. 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 soil vapor samples in 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 gas 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). 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 gas action levels even though the measured concentrations of volatile chemicals in actual soil gas 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).

In open areas where future buildings are to be constructed or at locations adjacent to buildings, the recommended sampling depth for evaluation of potential vapor intrusion hazards 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

Soil vapor samples are traditionally collected though a temporary or permanent six-inch screen or “implant.” The optimal depth interval over which a soil vapor sample should be collected has not been well studied, however, nor has the optimal number and volume of soil vapor samples needed to be representative of site conditions. 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). For now the current guidance is assumed to be conservative and adequate. These issues continue to be studied, however.

For subsurface slab samples in particular, the total depth interval covered by the sample point, including sand pack above and below the sample screen, should be limited to a thickness of one foot. Note that the area of influence associated with a typical, one-liter soil vapor sample is not expected to extend more than a few feet from the well screen (e.g., approximately three-times the volume of a one-liter canister or two feet, assuming radial flow and a thirty-percent soil porosity). Additional guidance on the collection of representative soil vapor samples with respect to long-term, vapor intrusion concerns is currently being developed by both the HDOH and other environmental agencies and private entities in the US. Updates will be incorporated into the TGM as available.