Normalized soil gas concentration distribution for oxygen and hydrocarbon undergoing aerobic biodegradation with first-order rate λ = 0.18 h-1 and vapor source at concentrations of 20 mg
Introduction
This section provides general information about the subsurface-vapor-to-indoor-air exposure pathway
To define the following activities for collecting and interpreting soil gas samples:
Soil gas data is essential for evaluating the presence of petroleum hydrocarbon vapors in the subsurface, assessing the potential migration of these vapors into buildings, estimating indoor air concentrations, and identifying the natural processes that significantly reduce vapor transport.
Soil gas sampling has long been utilized to assess the distribution of chemicals in soil and groundwater, aiding in site characterization and monitoring remediation efforts Typically, the focus of soil gas data collection is on understanding the location and distribution of these chemicals rather than quantifying their concentrations in soil gas Consequently, traditional methodologies for site characterization may not effectively evaluate the exposure pathway from subsurface vapor to indoor air.
To evaluate the subsurface-vapor-to-indoor-air pathway, an initial screening identifies sites that require further investigation Due to the challenges associated with indoor air sampling, the assessment typically emphasizes soil gas collection and analysis, as outlined in API 1998 Soil gas samples are collected to facilitate this evaluation.
• Establish a snapshot of the concentrations of chemicals of concern in soil gas at a location along the exposure pathway between the source and the building location
• Analyze the potential for human receptors to be exposed in indoor environments
• Predict the expected indoor air concentration based on the soil gas concentrations using an estimated attenuation factor
Reproduced by IHS under license with API
• Account for the fate and transport processes between a sampling location and the indoor environment
Aerobic biodegradation of petroleum hydrocarbons can significantly reduce the concentrations of harmful chemicals in soil gas as vapors migrate towards buildings from contaminated soils or groundwater Consequently, soil gas data collected for assessing the subsurface-vapor-to-indoor-air exposure pathway should be purpose-driven and aligned with specific data-quality objectives.
The use of soil gas data to assess the subsurface-vapor-to-indoor-air exposure pathway is an emerging method with limited existing methodologies This document outlines various options for collecting, analyzing, and interpreting soil gas data It highlights the importance of conceptual models for vapor transport in developing sampling plans and data analysis, while also providing essential information for selecting appropriate sample locations and depths.
The collection and analysis of soil gas samples are enhanced by essential reminders and checklists included at the conclusion of each section Additionally, the data analysis section outlines a systematic approach to identify inconsistencies in the collected data and to determine scenarios that necessitate further investigation.
Specifically, five basic activities have been identified for the collection and interpretation of soil gas samples These activities include the following:
1 Collecting information to understand the characteristics of the site (Section 3.0, Appendix A)
2 Identifying the location or locations for soil gas sample collection (Section 4.0, Appendix B)
3 Determining the method or methods for collecting soil gas samples (Section 5.0, Appendix C)
4 Selecting the method or methods to analyze soil gas samples (Section 6.0, Appendix D)
5 Evaluating and interpreting the results (Section 7.0, Appendix E)
This article aims to provide a fundamental understanding of the processes that influence the subsurface-vapor-to-indoor-air exposure pathway, serving as a basis for soil gas sampling decisions It begins with an overview of soil gas transport, followed by an exploration of five key activities essential for the collection and interpretation of soil gas samples.
Soil Gas Transport and Soil Gas Profiles at Petroleum Hydrocarbon Impacted Sites
The conceptual model for soil gas migration and expectations for soil gas profiles at petroleum hydrocarbon sites are discussed here
To show how chemicals volatilize from impacted soil or groundwater and migrate to regions of lower chemical concentration (e.g., the atmosphere, conduits, basements)
Hydrocarbon vapor migration in the subsurface is mainly driven by diffusive transport processes Additionally, factors like advective soil gas flow, oxygen migration, and aerobic biodegradation play crucial roles in influencing the movement of soil gas towards buildings.
Soil gas migration occurs when chemicals volatilize from contaminated soil or groundwater and move towards areas of lower concentration, such as the atmosphere or basements While the illustration in Figure 2-1 primarily shows houses, vapor intrusion can also pose risks to commercial buildings and other structures This guidance is relevant for all types of building scenarios.
Figure 2-1.A typical conventional conceptual model of soil gas migration.
Soil Source (residual or LNAPL)
Chemical Vapor Migration crawl-space basement slab
Soil Source (residual or LNAPL)
Chemical Vapor Migration crawl-space basement slab
Reproduced by IHS under license with API
The migration of hydrocarbon vapors in the subsurface is primarily driven by diffusive transport processes, although atmospheric pressure fluctuations may also play a role in vapor dispersion The extent of this vapor spreading is influenced by the thickness and composition of the vadose zone In proximity to buildings or enclosed spaces, significant advective soil gas flow can occur due to pressure imbalances caused by indoor appliances, temperature variations, and interactions with wind.
Processes that facilitate the transport of hydrocarbon vapors can also introduce atmospheric oxygen into the subsurface In soils contaminated with petroleum hydrocarbons, the composition of soil gas and the overall migration of vapors may be influenced by both aerobic and anaerobic biodegradation.
Oxygen from the atmosphere infiltrates the subsurface while hydrocarbon vapors rise from the source, leading to partial or complete consumption of oxygen in areas of active aerobic biodegradation, which produces carbon dioxide (CO2) This process can diminish hydrocarbon vapor fluxes to the atmosphere and enclosed spaces, potentially creating oxygen-depleted zones in the subsurface near the source.
Anaerobic decomposition of residual LNAPL in soils and dissolved chemicals in groundwater can take place in oxygen-depleted source zones, leading to the generation of methane (CH4) This methane gas migrates upward towards enclosed spaces and the atmosphere, where it may undergo aerobic biodegradation in oxygen-rich subsurface areas, similar to the hydrocarbon vapors originating from the source.
Thus, a more complex conceptual model is needed to adequately describe subsurface petroleum hydrocarbon vapor migration One such conceptualization is shown in Figure 2-2
Vapor migration results from the following processes:
Diffusion is the process where molecules move randomly, leading to a net transfer of chemicals from regions of higher concentration to those of lower concentration The rate of this diffusive flux is directly proportional to the concentration gradient, which is the change in concentration divided by the distance Additionally, the effective diffusion coefficient of the porous medium plays a crucial role, with typical values ranging from 0.001 to 0.01 cm²/s for various chemicals under standard vadose zone conditions.
Advection refers to the transport of chemicals through the movement of soil gas, driven by pressure gradients created by variations in subsurface building pressure or changes in atmospheric pressure.
Diffusion is the primary process influencing gas movement away from a building and close to the source, while advection plays a more significant role near the building itself Although the impact of atmospheric pressure fluctuations on soil gas movement is not yet fully quantifiable, it is believed that these oscillations have a minimal effect on chemical migration in shallow soils and are comparable to diffusion in deeper soils.
Figure 2-2.Revised conceptual model of soil gas migration at petroleum hydrocarbon impacted sites.
For additional insights into soil gas migration processes and their connection to the vapor-to-indoor-air exposure pathway, refer to the works of API (1998), Johnson (2002), Johnson et al (1999), Little et al (1992), and Roggemans et al (2002).
Expectations for Soil Gas Profiles at Petroleum Hydrocarbon Impacted Sites
When choosing suitable locations and depths for soil-gas sampling, it is essential to anticipate the qualitative characteristics of soil gas profiles This understanding is vital for evaluating data quality post-sampling and analysis, as outlined in Section 7.0 This section will cover qualitative expectations for soil gas profiles and will include sample graphs in Section 2.2.
Based on the preceding discussion, the following observations are qualitatively expected at petroleum hydrocarbon impacted sites:
• The highest concentrations of chemicals of concern in soil gas will be found near the source
Chemical concentrations in soil gas diminish with increasing distance from the source zone, both vertically towards the ground surface and laterally However, hydrocarbon vapors can become trapped beneath fine-grained soils or moisture barriers, which may alter the distribution of these concentrations.
• The composition of the hydrocarbon vapors at the source will reflect the composition of the petroleum liquid and the chemical properties of those components Hydrocarbon
Soil Source (residual or LNAPL)
(this occurs frequently when the vapor source zone is anaerobic)
An a e ro b ic Zo n e Ae ro b ic Zo n e
Soil Source (residual or LNAPL)
(this occurs frequently when the vapor source zone is anaerobic)
An a e ro b ic Zo n e Ae ro b ic Zo n e
Reproduced by IHS under license with API
`,,```,,,,````-`-`,,`,,`,`,,` - vapors at the source also may contain a significant fraction of methane if the soil gas in the vapor source zone is oxygen-depleted (anaerobic) and methanogenesis is occurring
Oxygen levels in soil gas above contaminated soils and groundwater decline with increasing depth, often reaching zero directly above the source in soils affected by shallow hydrocarbon contamination.
Decreasing oxygen concentrations with depth can result from background oxygen demand, particularly in soils rich in natural organic matter This background oxygen utilization can be evaluated by monitoring soil gas in nearby unimpacted areas.
• In regions where oxygen concentrations decline with depth, increasing CO2 concentrations with depth are generally observed through the aerobic region of the subsurface
CO2 is produced during the aerobic biodegradation of petroleum hydrocarbons and the aerobic biodegradation of methane that might be produced under anaerobic conditions closer to some source zones
The decreases in oxygen with depth and the increases in CO2 with depth should be consistent with each other as discussed later in Section 7.0
• There will likely be some region of the subsurface in which aerobic biodegradation occurs
The effectiveness of this process in reducing harmful chemical concentrations in soil gas is largely influenced by surface conditions, which affect oxygen migration into the subsurface and hydrocarbon vapor escape, as well as subsurface conditions such as moisture content, lithology, and nutrient availability.
Estimating the Time Necessary To Achieve Near-Steady-State Conditions
API (1998) and Johnson et al (1999) emphasize the significance of assessing whether soil gas profiles have attained near-steady conditions They also present an equation to estimate the time required to reach these near-steady conditions, denoted as T ss.
The effective diffusion coefficient (\$D_{v \, \text{eff}}\$) typically ranges from 0.001 to 0.01 cm²/s It is influenced by the distance from the source to the ground surface (L, in cm), the vapor-filled void volume (\$θ_v\$), which usually falls between 0.1 and 0.3 cm³-voids/cm³-soil, and the vapor-phase retardation factor (\$R_v\$) While substances like propane, butane, and pentane have \$R_v\$ values close to unity, chemicals of greater health concern, such as monoaromatic hydrocarbons and MTBE, exhibit vapor-phase retardation factors around 10.
Estimates indicate that reaching near-steady conditions can take a few hours to days for shallow sites (less than 1 m depth to the vapor source), a few months to years for intermediate-depth sites (up to 3 m), and potentially a year to decades for deeper vapor sources (greater than 10 m depth) Significant aerobic biodegradation can expedite this process by shortening the path length over which concentrations decrease.
7 o Concentrations of chemicals of concern and the total hydrocarbon concentration in soil gas at the source o Thickness of the aerobic region o Rate of biodegradation reactions
Oxygen transport and aerobic biodegradation are crucial in establishing soil gas profiles, leading to significant differences between measurements taken near buildings and those beneath them This discrepancy is particularly evident when building foundations limit oxygen flux to the subsurface, resulting in minimal effects of aerobic biodegradation on the soil gas profile beneath the foundation.
Soil gas profiles are analyzed under the assumption that vapor sources have existed for a significant duration, allowing chemical concentrations in the soil gas to approach a near-steady state However, data suggests that these profiles can be influenced by seasonal variations, indicating that even near-steady conditions may still show some temporal fluctuations.
Measured Soil Gas Profiles at Petroleum Hydrocarbon Impacted Sites
The relationship between the conceptual model depicted in Figure 2-2 and the measured vertical soil gas profiles is illustrated through the sample profiles in Figures 2-3 and 2-4 While each profile aligns with the conceptual model, they exhibit distinct qualitative differences The plots display normalized soil gas concentrations, calculated by dividing actual values by the maximum concentration at each site, plotted against depth below the ground surface, represented as \( z/L \) (the actual depth to the soil gas sample divided by the depth to the source at that site).
Roggemans et al (2002) conducted an empirical assessment of soil gas profiles at petroleum hydrocarbon-impacted sites, categorizing the data into generalized hydrocarbon-oxygen soil gas profiles Figure 2-3 illustrates specific examples of these profiles, all sourced from sites affected by gasoline or other petroleum products It is important to note that the majority of the profiles presented were measured near buildings or under paved surfaces, with only a few taken beneath buildings.
Figures 2-3 and 2-4 illustrate the utilization of oxygen, indicated by decreasing concentrations with depth below the ground surface, alongside a variable reduction in hydrocarbon concentrations.
In profile A of Figure 2-3, oxygen penetrates approximately halfway to the vapor source before being rapidly consumed by aerobic biodegradation This process is evident in the hydrocarbon concentration profile, which reveals a significant decrease in hydrocarbon levels over a short distance near the anaerobic/anoxic transition zone Profile A was the most commonly observed by Roggemans et al (2002).
In profile B, oxygen is consistently found in the vadose zone, except at the interface of the vapor source zone This hydrocarbon profile indicates a decrease in hydrocarbon concentration due to aerobic biodegradation as the distance from the vapor source increases The impact of aerobic processes is evident in this reduction.
Reproduced by IHS under license with API
Biodegradation in Profile B is less pronounced than in Profile A, typically occurring at shallow sites with short transport distances This slower biodegradation process is relative to the oxygen diffusion time scale in the vadose zone, or it may occur in areas where vapor source concentrations are low compared to atmospheric oxygen levels, such as above dissolved hydrocarbon groundwater plumes.
Profile C, collected beneath a basement with a high concentration vapor source, is unique due to the absence of oxygen at monitoring points and reduced attenuation of hydrocarbon vapor concentration In contrast to Profile A, which is also linked to a high concentration vapor source beneath an uncovered surface, the data indicate that the building influences oxygen transport and the importance of aerobic biodegradation.
Profile D exhibits an oxygen profile akin to that of profile B; however, it shows a much more pronounced hydrocarbon attenuation with distance from the source, resulting in a four-order-of-magnitude decrease in concentration over a short depth This phenomenon occurs when the source is situated in an area with lower diffusion rates compared to the overlying soils, specifically in cases where vapors emanate from within or beneath the capillary fringe.
Figure 2-4 illustrates vapor concentration profiles from a site affected by heavier hydrocarbons, specifically those in the C12 to C24 range Notably, the figure highlights the generation of methane gas from hydrocarbon decomposition, which is quickly reduced by aerobic biodegradation processes over a short distance Qualitatively, this profile resembles profile A in Figure 2-3, with the key difference being that methane is the primary component of the hydrocarbon vapor concentration.
Figure 2-3 Soil gas profiles (Roggemans et al 2002)
C max (hydrocarbon)= 27 mg/L paved surface
C max (hydrocarbon)= 60 mg/L beneath basement
C max (hydrocarbon)= 73 mg/L uncovered surface
C max (hydrocarbon)= 27 mg/L paved surface
C max (hydrocarbon)= 60 mg/L beneath basement
C max (hydrocarbon)= 73 mg/L uncovered surface
C max (hydrocarbon)= 27 mg/L paved surface
C max (hydrocarbon)= 60 mg/L beneath basement
C max (hydrocarbon)= 73 mg/L uncovered surface
Reproduced by IHS under license with API
Figure 2-4 Soil gas profile at a site with methane production in the source zone (Johnson et al
2003) This figure shows the soil gas profiles for oxygen (circles) and methane (diamonds)
Roggemans et al (2002) found no clear correlations between soil gas profiles and site characteristics or surface cover conditions, indicating that profiles beneath paved surfaces do not consistently match those under buildings, and they vary significantly across different sites Consequently, predicting the reduction in hydrocarbon vapor flux due to biodegradation based on site properties remains challenging Therefore, soil-gas-profile data are essential for comprehending subsurface processes and their overall impact on hydrocarbon vapor migration into enclosed spaces.
Figures 2-5 and 2-6 from Abreu (2005) illustrate the significant impact of oxygen and aerobic biodegradation on vapor profiles at sites, particularly near buildings with basements The results from three-dimensional numerical simulations reveal that changes in source concentration, ranging from 20 mg/L to 200 mg/L, can greatly affect the chemical of concern, oxygen soil gas profiles, and the attenuation factor According to Roggemans et al (2002), source vapor concentrations exceeding 200 mg/L are indicative of gasoline source zones above the water table, while concentrations below 20 mg/L are typically found near dissolved plumes down-gradient Notably, the biodegradation effect is minimal at 200 mg/L, whereas at 2 mg/L, the attenuation differs by six orders of magnitude (α=5.6 x 10^{-11} versus 7.1 x 10^{-5}) A key distinction between the figures is the oxygen penetration depth beneath the building; at 20 mg/L, elevated oxygen levels facilitate aerobic biodegradation along most of the transport pathway These findings highlight the importance of concentration levels in understanding vapor transport dynamics.
11 on attenuation factors for aerobically biodegradable chemicals is expected to be more significant as the source depth is increased (Abreu 2005)
Figure 2-6 demonstrates how depth influences soil gas profiles and vapor attenuation coefficients for aerobically biodegradable chemicals, with simulations conducted for slab-on-grade foundations and high soil vapor source concentrations of 200 mg/L at depths between 1 m and 8 m below the ground surface (Abreu 2005) While the impact of biodegradation is minimal at shallower depths, it becomes significantly pronounced for sources located 8 m below the surface.
The simulations demonstrate that aerobic biodegradation's impact is closely associated with oxygen availability beneath a foundation Additionally, the degree of attenuation from aerobic biodegradation is expected to rise as the source depth increases and the source concentration decreases.
Reproduced by IHS under license with API
The distribution of normalized soil gas concentrations for oxygen and hydrocarbons during aerobic biodegradation is illustrated in Figure 2-5 This process follows a first-order rate of λ = 0.18 (h\(^{-1}\)), with vapor sources at concentrations of 20 mg/L, 100 mg/L, and 200 mg/L situated 8 meters below the ground surface beneath a basement foundation The contours for hydrocarbons and oxygen are normalized to the source and atmospheric concentrations, respectively (Abreu 2005).
The normalized distributions of soil gas concentrations for oxygen and hydrocarbons during aerobic biodegradation are illustrated in Figure 2-6 This process is characterized by a first-order rate constant of \$\lambda = 0.18 \, \text{h}^{-1}\$ and a vapor source concentration of 200 mg/L, situated beneath a slab-on-grade foundation at depths of 1 m, 3 m, 5 m, and 8 m below the ground surface The contours for hydrocarbons and oxygen are normalized to the source and atmospheric concentrations, respectively (Abreu 2005).
Reproduced by IHS under license with API
Conceptual Migration Model for Subsurface Vapor to Indoor Air
This section provides background information to assist in developing a conceptual migration model for a specific site The conceptual migration model is important for planning a soil-gas-sampling program
To define a conceptual migration model for describing the vapor source characteristics, the current and future building locations and features, and the geologic profile of the subsurface
The model conveys a working hypothesis of the movement of vapor-phase chemicals of concern within the subsurface to a current or future building
The conceptual migration model must clearly convey key aspects of the site's geology, hydrogeology, and the distribution and composition of petroleum hydrocarbons related to vapor migration Accurate characterization of the site is crucial for effective planning of soil gas sampling and laboratory analyses.
Before choosing sampling locations and depths, it is essential to create a site-specific conceptual migration model This model outlines the characteristics of the vapor source, such as its location, size, environmental media, and the concentrations of chemicals of concern, along with their potential changes over time Additionally, it should detail the anticipated distribution of soil gas concentrations and include a thorough discussion of these factors.
• whether or not this distribution has reached near-steady conditions under current site conditions (e.g., present locations of buildings and surface cover),
• whether or not future site uses might alter the soil gas distribution (e.g., future building locations and surface features), and
• how the soil gas profile is expected to be influenced by geologic features
The conceptual model should identify the current and reasonably potential future subsurface-vapor-to-indoor-air exposure pathways that may be present The exposure
Vapor Transport along Utility Conduits
Vapor transport from the source area to the building may happen through utility conduits, making it essential to consider vapor sampling within these conduits, as well as in manholes and sumps, alongside vadose-zone soil gas sampling.
This document does not provide specific guidance on utility sampling, but it emphasizes the importance of screening field instruments at utility access points to assess whether utilities are facilitating vapor migration Establishing the conceptual migration model is crucial to determine if vapor migration occurs along the utility backfill or if vapor is actually transported within the utility itself.
Any utility sampling program must include safety precautions to protect personnel (e.g., oxygen and combustible gas monitoring, confined-space entry requirements) and to avoid damage to utilities
Fifteen pathways must be established based on the vapor source, such as soil, groundwater, or light non-aqueous phase liquids (LNAPL), and the locations where the subsurface-vapor-to-indoor-air exposure pathway is a concern, including current or potential future buildings It is essential to identify specific buildings and future construction sites, along with a description of the existing or anticipated building types, such as slab on grade, basement, or multiple stories Additionally, the design aspects, including basement or floor-slab thickness, and the intended use of the buildings—whether residential, commercial, or industrial—should be clearly outlined.
In developing a conceptual migration model, it is crucial to assess existing information about the site to understand subsurface conditions and identify necessary data for evaluating vapor exposure pathways from subsurface to indoor air The model should clearly convey key aspects of the site's geology, hydrogeology, and the distribution and composition of petroleum hydrocarbons relevant to vapor migration Adequate site characterization is essential for effective planning of soil gas sampling and laboratory analyses.
• Types of petroleum hydrocarbons with volatile chemicals of concern (e.g., gasoline, jet fuel, diesel) that are currently or previously stored or handled at the site
Petroleum hydrocarbon chemicals, such as benzene, toluene, ethylbenzene, xylenes, MTBE, and naphthalene, are significant contaminants in soil and groundwater The specific chemicals of concern at a site, along with their concentrations, inform the selection of laboratory analytical methods, ensuring compliance with regulatory requirements and effective data utilization.
• Potential sources and source areas of vapors (e.g., soil, groundwater, LNAPL)
○ Presence of LNAPL, which may represent an expanding vapor source
○ Presence of residual LNAPL, which may represent a stable or reducing vapor source
Understanding the behavior of groundwater vapor sources is crucial, as it reveals whether the groundwater plume is expanding, stable, or shrinking This information indicates the temporal variability of concerning chemical concentrations in soil gas, helping predict whether future concentrations at a specific location will be higher or lower than current levels Additionally, evaluating fluctuations in the groundwater table is essential for accurately assessing the temporal variability of these groundwater sources.
The geology and hydrogeology of the site reveal distinct soil strata with notable qualitative differences, such as variations in grain size and moisture content Finer-grained soils typically exhibit higher moisture levels and are characterized by significant concentration gradients, while coarser-grained soils often serve as preferential zones for vapor migration.
Reproduced by IHS under license with API
When assessing vapor migration, it is crucial to evaluate the presence of thin, finer-grained soil layers, as they can significantly influence the movement of vapors but may be challenging to identify during drilling.
Identifying the approximate location of vapor sources in the subsurface, along with the lateral and vertical distances to the building site, is crucial for effective soil-gas sampling These distances play a significant role in determining the number and placement of soil-gas-sampling probes.
• Possible preferential vapor-migration conduits (e.g., utility conduits, sewers)
○ Of particular interest are the utilities that intersect a vapor source and also connect to a building
Significant vapor migration through preferential pathways can lead to soil gas measurements that do not accurately reflect the vapor concentrations entering indoor environments To better assess the subsurface-vapor-to-indoor-air exposure pathway, alternative investigative techniques such as utility vapor screening and indoor air measurements may yield more reliable data Users are advised to consult USEPA 2002a and relevant state regulatory guidance for additional methods to evaluate this pathway, including resources from PaDEP 2002, MaDEP 2002, WDHFS 2003, and NJDEP 2004.
• Construction features of existing or future buildings (e.g., size, age, presence of foundation cracks, entry points for utilities)
Appendix A provides a summary of the information that is useful for understanding the site
Development of a Strategy for Soil Gas Sampling
This section deals with articulating the questions to be answered by the soil gas sampling and developing a strategy for soil gas sampling
To develop a strategy for soil gas sampling based on:
• The practicality of collecting soil gas samples at a particular site
• A conceptual migration model for subsurface vapor to indoor air
• Questions to be answered by the sampling
To establish an effective soil-gas-sampling program for petroleum hydrocarbon impacted sites, it is essential to develop a conceptual migration model, discuss various sampling options, outline key considerations, and explore reasonable sampling scenarios.
Section 3.0 outlines the conceptual migration model for subsurface vapor to indoor air, focusing on soil gas sampling methods While specific guidance on sampling locations, depths, and frequencies is not provided due to site variability, the section offers a discussion of sampling options, key considerations, and reasonable scenarios for petroleum hydrocarbon-impacted sites It is important to note that local regulatory agencies may have different sampling guidelines Additionally, Appendix B includes worksheets for three typical scenarios to assist in planning sampling locations.
The options for soil gas sample collection discussed in this section are based on consideration of:
• The practicality of collecting soil gas samples at a particular site
• A conceptual migration model for subsurface vapor to indoor air
• Questions to be answered by the sampling
Information about site considerations for sampling is included in Section 5.0 and in Appendix C
When creating a soil-gas-sampling strategy, it is essential to clearly define the questions that the sampling aims to address before designing the plan At petroleum hydrocarbon impacted sites, these questions typically revolve around specific concerns related to contamination and its effects.
1 Is the subsurface-vapor-to-indoor-air exposure pathway currently complete for individual chemicals of concern?
Reproduced by IHS under license with API
2 Are concentrations of chemicals of concern in soil gas currently above applicable regulatory action levels or other levels of concern?
3 Is biodegradation contributing to the attenuation of hydrocarbon vapors between the source and building at this site?
4 How might the answers to questions (1) to (3) change when considering plausible future activity and land-use scenarios?
The chosen sampling approach for a site is influenced by its specific conditions and the questions that need to be addressed When selecting sampling locations and depths, it is essential to consider the conceptual migration model, which includes factors such as the location of vapor sources, relevant buildings, expected vapor distribution, and subsurface characteristics Some sites may require a phased program of soil gas sampling, while others may necessitate specific sampling types and locations The fundamental sampling approaches encompass various methodologies tailored to the site's unique requirements.
• Point samples at specific depths in one or more lateral locations These may be collected using temporary driven probes or by installing permanent soil-gas-sampling probes
Vertical profiles of soil gas samples can be obtained at multiple depths and lateral locations, known as transects These samples may be collected using temporary driven probes or by installing permanent soil-gas-sampling probes.
General Approach
Below are three basic steps in the selection of soil-gas-sampling locations
1 Develop a conceptual migration model describing the current or potential future subsurface-vapor-to-indoor-air exposure pathways The model may require modification as soil gas data are collected
2 Identify the questions to be answered by the soil gas sampling and any regulatory requirements
3 Select the sampling approach, the locations, and depths to provide information sufficient to assess the pathway and to answer the questions posed for the site.
Point Sampling
Point sampling is the collection of an individual soil gas sample from a specific depth at a single sample location Often, soil gas sampling begins with point sampling Consideration should be
Varied Sample Locations and Depths
Designing a sampling program to collect soil gas samples at specific locations or depths where concentrations are anticipated to exceed laboratory detection limits is essential for verifying both field and laboratory methods If analytical results fall below these detection limits, it becomes challenging to confirm the proper implementation of field methods, such as ensuring sample collection without dilution.
Increased confidence in data is achieved when samples are collected at two or more depths, as this ensures that the results are internally consistent and align with the conceptual migration model.
To accurately represent the concentrations of chemicals of concern in soil gas, it is essential to consider both spatial and temporal factors Collecting multiple point soil gas samples may be necessary to gain a comprehensive understanding of the distribution of these concentrations For further details on sampling frequency, please refer to Section 4.6.
Point sampling is crucial for assessing the subsurface-vapor-to-indoor-air exposure pathway, involving the collection of soil gas samples at specific depths to evaluate chemical concentrations against target levels These concentrations are also compared to ambient air samples, with sampling typically occurring near building foundations or vapor sources, assuming the latter's location is accurately identified If soil gas concentrations exceed target levels, further site-specific analysis or remedial actions are necessary; conversely, concentrations below target levels suggest a minimal exposure pathway For detailed data analysis, refer to Section 7.0.
Point sampling can be effectively used to assess conservative models of vapor transport in soil and groundwater When modeling suggests potential chemical concentrations exceeding target levels near a building, it is advisable to gather multiple point soil gas samples This approach allows for a comparison with model results and aids in determining whether the vapor transport pathway is indeed complete.
Point samples can be obtained using either temporary driven probes or permanent probes, as outlined in Section 5.1 To ensure the soil gas sample is suitable for comparison, careful consideration must be given to the location and depth of the point sample Factors such as the vapor source (including soil, groundwater, or LNAPL), the mobility of the source (like groundwater), and the effects of soil heterogeneity should all be taken into account.
Transects and Vertical Profiles
Selection of Lateral Positions for Soil Gas Transects
Soil gas transects are essential for assessing vapor sources, such as soil, LNAPL, or groundwater, that are not directly beneath a building site These transects involve multiple sampling points positioned between the source and the building, allowing for the demonstration and quantification of the reduction in concentrations of harmful chemicals in soil gas as they migrate laterally towards the structure.
A basic sampling program might involve three sampling locations For example:
• One sampling location at the edge of the source zone closest to the building of concern
• One sampling location mid-way between the source and building
• One sampling location at the building
Based on the authors' experience, while three sampling locations are generally adequate, users should consider adding extra intermediate points if the distance between sampling probes exceeds 50 feet.
Transect samples can be obtained using either temporary driven probes or permanent probes, as outlined in Section 5.1 When determining the location and depth for these samples, it is essential to take into account the vapor source, such as soil or LNAPL groundwater, the mobility of the source medium like groundwater, and the effects of soil heterogeneity.
Recent discussions highlight the necessity for a threshold lateral distance criterion in environmental assessments The USEPA (2002a) suggests that if a known source is more than 100 feet away from a building, the pathway is considered incomplete, and sampling may not be required This criterion is primarily based on the authors' professional judgment and practical considerations However, theoretical insights indicate that this distance should also take into account factors such as the depth to the vapor source, the strength of the vapor source, indoor air target levels, and local geological conditions (Abreu and Johnson 2004).
The USEPA's 100-foot distance may not universally ensure protection at all sites, as noted by Lowell and Eklund (2004) However, for sites impacted by petroleum hydrocarbons, this distance is generally adequate, assuming the vapor source edge is clearly defined, vertical oxygen migration is not significantly obstructed by surface or subsurface features, and there are no conditions that could encourage lateral migration, such as landfill gas production or highly layered soils.
Reproduced by IHS under license with API
Vertical Profiles
A soil-gas vertical profile involves collecting multiple samples from a single location, spanning from the top of the source to the ground surface or building foundation This process aims to quantify the reduction of chemical concentrations in soil gas as it moves vertically In cases where the vapor source is located directly beneath a building, these profiles help assess the potential for vapor migration into the structure When selecting vertical sampling depths, several factors must be taken into account.
• To estimate the maximum concentration of chemicals of concern in soil gas, a sample is needed immediately above the vapor source
• To estimate the concentration of chemicals of concern in soil gas near the foundation, samples should be collected immediately adjacent to, or beneath, the building foundation
• To establish the occurrence and significance of aerobic biodegradation, two or more soil gas samples should be collected between the vapor source and the building foundation
The vertical distance between adjacent samples is typically no less than 2 feet, which can limit the number of samples collected at certain sites due to practical constraints.
For deeper sources located more than 40 feet from the building foundation, it is essential to use profiles with five or more sampling depths to accurately resolve the soil gas profile.
Sampling locations can be chosen based on the site's lithology, particularly in areas with more permeable soils, whether natural or artificial, or where concentration changes are anticipated.
When evaluating sites with affected groundwater, LNAPL, or a smear zone as the vapor source, it is advisable to increase the density of sampling near the groundwater table or capillary fringe This involves taking multiple samples spaced 2 feet apart, as concentration gradients tend to be most pronounced at this depth.
A vertical profile can be established with just three sample depths, but site-specific factors such as the depth to the source area and lithology may necessitate four or more depths In certain cases, the use of vertically-nested soil gas probes or vertical profiling may not be feasible Generally, if groundwater is located within a few feet of a foundation, it is advisable to limit soil-gas sampling to a single depth Ultimately, the number of vertical samples required will depend on the site's characteristics and physical limitations.
Vertical profile samples can be obtained through temporary or permanent probes, as outlined in Section 5.1 When determining the number of samples, it is essential to take into account the vapor source, such as soil, groundwater, or LNAPL, as well as the depth of the source and the effects of soil heterogeneity Additionally, factors like the lithologic profile and surface topography should be considered.
When installing vertical profile probes at various lateral positions, it is crucial to consider the depth to the vapor source, particularly at a depth of 23 In relatively flat sites with consistent stratigraphy, using uniform sampling depths facilitates easier comparison of results However, in areas with significant spatial variability, it is advisable to collect the deepest samples at the vapor source elevation and the shallowest samples at the foundation or basement level For intermediate sampling points, depths should be chosen based on expected significant changes in concentration, such as above and below fine-grained soils or near the capillary fringe of groundwater sources.
Figure 4-1 Considerations for vertical profiles at relatively flat sites (e.g., consistent distance between the ground surface and the vapor source depth) with consistent stratigraphy
Reproduced by IHS under license with API
Figure 4-2 Considerations for vertical profiles at sites with significant spatial variability in the distance between ground surface and the vapor source depth.
Summary of Sampling Depth and Location Selection Considerations
Tables 4-1 to 4-5 summarize the use of data from various sample depth locations, providing key comments and cautions for each Users should evaluate whether the samples accurately represent the concentrations of chemicals of concern at each location The effectiveness of the locations and depths will depend on the conceptual migration model and the specific questions being addressed.
Table 4-1 Considerations for Samples Collected Immediately above the Vapor Source
Soil gas samples collected immediately above the vapor source (e.g., highest concentrations of chemicals of concern in soil or groundwater)
Use of Data Comments and Cautions
These samples should represent the highest concentrations of chemicals of concern present in soil gas
In assessing if the pathway is complete or significant, these samples can be used to generate a conservative exposure concentration estimate for a present or future building scenario
These concentrations are generally greater than concentrations of chemicals of concern in soil gas at the building
Understanding (through modeling or empirical data analysis) of the estimated soil-gas-vapor attenuation factor is needed to estimate concentrations of chemicals of concern in the building
The models employed for these concentrations are primarily conservative screening models that overlook biodegradation Consequently, the estimated indoor air concentrations tend to be biased, projecting higher values than what is realistically expected at the site.
As the distance between the sample location and the building increases, the uncertainty in estimating concentrations of chemicals of concern in indoor air will likely increase
These concentrations reach near-steady conditions quickly and tend to be stable seasonally and are relatively unaffected by near- surface changes (e.g., surface cover, weather changes)
If the concentrations in these samples are below target levels, then the vapor-to-indoor-air pathway is not likely to be significant (see Section 7.0) groundwater source groundwater source
Reproduced by IHS under license with API
Table 4-2 Considerations for Samples Collected Laterally Mid-Way between the Vapor Source and the Building Location
Soil gas samples collected from a location laterally mid-way between the vapor source and the building location
Use of Data Comments and Cautions
Used in conjunction with source vapor sampling, this sample may indicate site- specific attenuation along the subsurface-vapor-migration pathway
To accurately estimate the concentrations of chemicals of concern in buildings, it is essential to understand the soil-gas-vapor attenuation factor through modeling or empirical data analysis Typically, conservative screening models are employed, which do not account for biodegradation Consequently, this approach may lead to biased estimates, suggesting higher indoor air concentrations than what is realistically expected at the site.
Interpreting these concentrations is challenging without the corresponding levels of chemicals of concern in soil gas at the source zone and a precise conceptual migration model for groundwater sources.
Table 4-3 Considerations for Samples Collected Adjacent to the Base of an Existing Building Foundation or Basement
Soil gas samples collected adjacent to the base of an existing building foundation or basement
Use of Data Comments and Cautions
Because these samples are collected close to the exposure location, they may be a useful predictor of concentrations of chemicals of concern in indoor air
At this location, the concentrations of chemicals of concern are likely less attenuated along the subsurface-vapor-to-indoor-air exposure pathway compared to more distant sample locations, as indicated in Tables 4-1 and 4-2.
The entry point of subsurface vapors into a building, influenced by the building's foundation and the surrounding oxygen distribution, can affect the representativeness of soil vapor samples Consequently, these samples may not accurately reflect the actual concentrations of concerning chemicals infiltrating the building.
Depending on the distance from the source to the building, these concentrations may not reach near-steady conditions for some time after the release (see Section 2.1)
These samples are more likely to be affected by changes in near- surface conditions (e.g., temperature, precipitation, barometric pressure fluctuations) groundwater source groundwater source
Reproduced by IHS under license with API
Table 4-4 Considerations for Samples Collected Immediately below the Building Foundation or Basement
Soil gas samples collected immediately below the building foundation or basement
Use of Data Comments and Cautions
Because these samples are collected close to the exposure location, they may be a useful predictor of concentrations of chemicals of concern in indoor air
Logistical issues are associated with sample collection (e.g., building access, placing sampling probes, maintenance of sampling probes)
These samples are more likely to be variable with time as they are affected by changes in near-surface conditions (e.g., temperature, precipitation, barometric pressure fluctuations, HVAC systems)
The concentrations of chemicals of concern in soil gas under a building foundation may also be spatially variable More than one sampling location may be required to develop a representative concentration
Empirical relationships are utilized to estimate the concentrations of chemicals of concern in indoor air based on concentration data These relationships account for the effects of biodegradation between the vapor source and the building, ensuring that the estimated indoor air concentrations are unbiased.
The entry points of subsurface vapors into a building, influenced by the building's foundation and the surrounding oxygen distribution, can affect whether a sample location accurately reflects the actual concentrations of concerning chemicals in the soil gas infiltrating the structure.
The concentrations of contaminants may take time to achieve near-steady conditions after their release, depending on the distance from the source to the building.
Table 4-5 Considerations for Samples Collected within the Footprint of a Future Building Location
Soil gas samples collected within the footprint of a future building location
Use of Data Comments and Cautions
The concentrations of chemicals of concern in soil gas, when analyzed through modeling or empirical methods, can help estimate the potential impacts of subsurface vapor exposure on indoor air quality in the future.
The samples collected at the source depth will likely be representative of future conditions (unless groundwater is very shallow)
Intermediate and shallow samples may not accurately reflect future conditions due to surface cover influences, as building foundations and HVAC systems can impact oxygen transport, thereby affecting the importance of aerobic biodegradation.
Some Comments on Sample Collection Adjacent to and Beneath Buildings
Sampling directly through foundation slabs of existing buildings (i.e., through-slab sampling) presents significant logistical and practical issues, including:
• Disturbance of residents or building occupants
• Representativeness of the samples (depending on the actual vapor entry points to the building, these sample locations may or may not represent vapors that are entering the building)
• Ability to install permanent sampling installations that can be used for multiple sampling events
• Maintenance of permanent sampling locations
• Limitations on the types of sampling installations and depths that can be used
Sampling and the installation of permanent soil-gas-sampling probes near buildings are often considered essential practices However, there has yet to be a widely recognized study focusing on groundwater sources in relation to future building developments.
Reproduced by IHS under license with API
When assessing the representativeness of samples collected near or beneath a building for chemical concentrations, there is no consensus on the most reliable sampling location However, some regulatory agencies advocate for through-slab sampling as a recommended practice.
The following should be considered for sampling locations near buildings:
When a building is situated above a vapor source, the samples taken from the source zone depth remain largely unaffected by surface conditions Consequently, samples collected at this depth, both adjacent to and beneath the building, should show similar concentration levels Therefore, if the pathway assessment utilizes deeper soil gas samples, those collected next to the building will be adequate for analysis.
The foundation of a building, including its size, age—especially concerning any cracks—and the entry points for utilities, significantly influence the likelihood of vapor migration from the surrounding soil into the indoor environment These characteristics must be taken into account when developing a conceptual migration model and determining appropriate sampling locations.
Sampling beneath a building is crucial for demonstrating the significance of attenuation through aerobic biodegradation This is due to the limited transport of oxygen to areas beneath the building footprint compared to outside it Consequently, oxygen depletion may occur, diminishing the effectiveness of aerobic biodegradation in reducing contaminants.
The differences in samples collected near and beneath buildings are likely to be most pronounced for shallow vapor sources, particularly when there is less than a 10-foot vertical separation between the basement or building foundation and the vapor source In contrast, for deeper vapor sources, where the separation exceeds 40 feet, the differences in samples are expected to be minimal.
Before conducting through-slab sampling, it is essential to create a site-specific sampling and data analysis plan This plan must outline the number and locations of samples to be collected, specify the analytical methods to be employed, establish the necessary detection limits, and, when relevant, detail the selection and use of tracers.
The plan should include a discussion of the rationale for each of the elements (e.g., sample locations) and specifications (e.g., assessment of variability) in the plan
Collecting tracer gas samples, such as radon, from within a building and beneath its foundation can provide valuable insights into the site-specific sub-slab attenuation factor (\(\alpha\)), which is essential for evaluating the indoor impacts of petroleum hydrocarbon chemicals For effective assessment, the measured indoor air concentrations of tracer gases should exceed ten times the expected background levels or analytical detection limits However, radon may not be suitable as a tracer in certain locations due to low soil gas concentrations or indoor sources, such as building materials (Hartman, 2004a) The attenuation factor is defined as:
=C α C and is represented in Figure 4-3
Figure 4-3 Sub-slab-to-indoor-air attenuation.
Sampling Frequency
Soil gas profiles generally remain stable over time, unless there are significant changes in the vapor source, vadose zone properties, or ground surface conditions For instance, rising groundwater levels can restrict vapor migration from residual LNAPL in the soil when the water level exceeds the top of the LNAPL Additionally, infiltration events, such as irrigation, can impact vapor profiles by altering air-filled porosity, submerging the source, or temporarily diluting the concentrations of chemicals in shallow groundwater.
To ensure effective data collection, sampling frequencies must align with seasonal variations at the site It is essential to gather samples during both the "wet" and "dry" seasons, as well as during periods of "high" and "low" groundwater levels.
One sampling event may be adequate for several sites, particularly those where the concentrations of chemicals of concern in soil gas within the source area are below target levels.
• other data are consistent with the measured soil gas concentration (e.g., groundwater and soil data consistency as assessed through equilibrium partitioning calculations, see Section 4.7 and Appendix E), and
• all other lines of evidence gathered for the site support the conclusion that soil gas concentrations would not increase to concentrations of concern in the future groundwater
Reproduced by IHS under license with API
At sites with elevated concentrations of chemicals in soil gas, particularly those exceeding 1,000 times the indoor air target level, multiple sampling events may be essential This is also true when there is a need to demonstrate the stability of vapor concentrations, especially if the time since the release is insufficient to ensure that near-steady vapor profiles have formed.
Additional Considerations to Increase Confidence in Data Sets and the Interpretation of Soil-Gas-
Point sampling can effectively demonstrate pathway completeness at certain sites, as noted in Section 4.2 However, for other locations, additional methods may be necessary to enhance confidence in the pathway assessment.
• Collection of soil gas vertical profiles and transects generally provides an added level of confidence in the data set
Confidence in the data set improves when it encompasses both low or non-detect concentrations of chemicals of concern at the ground surface and elevated concentrations near the source.
Confidence in the dataset and its interpretation is enhanced when the soil gas profiles and transects align with the conceptual migration model, as well as with the hydrocarbon and oxygen profiles observed at other locations.
Confidence in the data set is enhanced when the near-source concentrations of chemicals of concern align closely with expected values For instance, in the case of groundwater sources, collecting a groundwater sample and measuring groundwater elevation simultaneously with soil gas samples can provide valuable insights These groundwater samples can be obtained from the same location as the soil gas samples to ensure consistency and accuracy in the data.
Calculation of a soil gas concentration based on a groundwater concentration provides an estimate of the expected soil gas concentration
Several factors can cause the actual soil gas concentration to differ from the estimated values, including submerged LNAPL sources, LNAPL sources in the vadose zone, long groundwater well screens, rapid biodegradation in the vadose zone, and shallow soil gas sampling intervals relative to the groundwater table Additionally, vapor samples cannot be collected directly above the water table due to high water saturation in the capillary fringe There is a decreasing concentration gradient from the capillary fringe to the ground surface, leading to lower vapor concentrations in samples taken above the capillary fringe compared to those predicted at the water table Comparing measured soil gas concentrations with calculated expected concentrations serves as a relative measure of data confidence and indicates the applicability of the conceptual migration model.
Groundwater samples are essential for estimating soil gas concentrations, allowing for a comparison between expected and measured values to validate collected data For further details and a calculation worksheet, refer to Appendix E Additionally, groundwater elevation measurements can help assess the likelihood of submerged sources influencing soil gas profiles due to fluctuations in groundwater levels.
Collecting samples from multiple sampling events enhances the reliability of the data set, as it allows for the evaluation of temporal changes influenced by a fluctuating water table and seasonal variations.
In addition, the following supplemental data, which may have already been generated during the site investigations, are beneficial in developing a better understanding of vapor migration at a given site:
This article presents a photo log of a soil core along with laboratory analyses of key physical properties of the vadose zone, as outlined by Ririe et al (2002) The significant soil layers are examined for various attributes, including soil moisture, bulk density, air-filled porosity, water-filled porosity, total organic carbon, hydraulic conductivity, and air permeability.
• Recent precipitation record for the area (easily obtained from weather-monitoring data)
• Surface cover (based on visual inspection)
• Groundwater elevation history (from groundwater elevation measurements at the site or from nearby sites)
These and other data needs are included in Appendix C
Reproduced by IHS under license with API
Scope-of-Work Action Items:
To create an effective conceptual migration model for assessing subsurface vapor to indoor air exposure pathways, it is essential to consider key site characteristics These include the nature of the source, the depth to groundwater, and the distance from the source to the exposure location Additionally, the model should account for whether there is an existing building or plans for future construction, the locations of utilities and process piping, as well as the types of soil present It is also important to differentiate between a stratified vadose zone and a homogeneous vadose zone in the analysis.
• Define questions to be answered by the soil gas sampling
• Determine if there are any regulatory requirements for the sampling locations or frequencies
• Determine the applicable target levels and whether ambient air sampling will be conducted
To create an effective sampling plan, utilize the cross-sections in Appendix B or other visual aids to identify optimal sampling locations and depths Assess whether a phased sampling approach is suitable for your project, and carefully choose the specific locations, depths, and frequencies for sampling to ensure comprehensive data collection.
Soil Gas Sample Collection
Collection methods for soil gas samples are described in this section
To discuss common challenges associated with soil gas sampling and to present possible alternatives
When selecting a soil-gas monitoring method, it is essential to evaluate the specific conditions of the site Factors such as the lithology of the vadose zone, site configuration, depth to the vapor source, and anticipated sampling frequency play a crucial role in determining the most suitable approach Each method comes with its own set of advantages and limitations that must be carefully considered.
This section outlines the fundamental methods for collecting soil gas samples and the equipment utilized in the process For further details, please refer to Appendix C, which includes additional references and resources.
Soil-gas-monitoring installations may not be feasible at all sites due to challenges such as fine-grained soils and high moisture levels, which hinder the collection of representative soil gas samples Additionally, using permanent probes for shallow vapor sources can be problematic because of the increased risk of surface air leakage Site access restrictions and physical limitations may further restrict the placement and maintenance of soil gas probes Therefore, it is essential to evaluate the practicality of each soil-gas-sampling method at a specific site before proceeding with the outlined tasks.
Basic Monitoring Installation Options
Permanent Probes
Permanent probes, used for groundwater monitoring, are typically installed through augered soil borings or direct push techniques In the augering process, a borehole is created to the depth of the lowest monitoring interval, where a sampling probe is placed in sand-pack material This probe consists of small-diameter tubing, often made of materials like copper or stainless steel, extending from the surface to the sampling depth The tubing's end may feature a fine screen or connect to a perforated section of pipe or a metal mesh tube, and it is often supported by a more rigid structure, such as a PVC pipe.
When installing three or more soil gas probes in a single boring, the probes are typically positioned just above the vapor source, at various intermediate depths, and at a shallow depth The boring is sealed with bentonite above the sand-pack interval up to the next deepest sampling point, where an additional probe is placed in the sand-pack material This process is repeated, sealing the boring with bentonite above each sand-pack interval until reaching the ground surface for the shallowest probe.
Typically, the surface seal (the seal above the last sampling interval) will be approximately 3 feet in thickness
Direct-push techniques involve the installation of a single sampling probe by pushing it to the required depth These probes are typically constructed from rigid tubing equipped with disposable drive points, and in certain instances, the rods are driven down and left in position.
Probes can be installed using hollow, removable drive rods, where the rods are driven down, and the probe assembly is lowered into the rod before it is removed Typically, installers depend on the natural collapse of the surrounding formation to secure the probes; however, in some cases, a sand pack and seal are added during the removal of the drive rod.
To ensure accurate soil gas sampling, it is crucial to seal the sampling probe to prevent atmospheric air exchange and to guarantee that the retrieved sample reflects the soil gas at the specified depth Leak testing poses challenges since air sampling does not allow for visual detection of leaks Therefore, the integrity of the soil-gas-sampling probes is typically assessed through sampling volumes, pressures, and laboratory analysis results Adhering to field procedures, such as sealing above each sampling interval for permanent probes, and carefully installing the probes is essential Additionally, sealing the sampling tubing at the ground surface between sampling events is vital, as fluctuations in barometric pressure can lead to the unintended inhalation of ambient air and exhalation of soil gas, jeopardizing the sample's representativeness.
The article discusses the use of 37 small-diameter stainless steel mesh tubes, typically ranging from 1/8 to 1/2 inch in diameter and 6 inches in length These tubes are connected to the ground surface via flexible tubing, which is also small in diameter, usually between 1/8 and 1/4 inch The sampling interval for these probes is generally short, typically between 6 to 12 inches.
Temporary Driven Probes
Hollow metal rods, such as those made from stainless steel, can be driven into the soil manually or with direct-push vehicles, equipped with either disposable or retrievable drive points Once the probes reach the desired depth, sampling occurs either at that depth or slightly above it, utilizing a sampling tip This process involves tubing connected to the sampling tip or extracting vapors through the drive rods After sampling, the rods are either removed or pushed deeper for additional samples However, in certain conditions, such as with finer-grained soils or at greater depths, removing the drive rods can pose challenges.