Significance and Use
Soil-gas sampling results can be dependent on numerous factors both within and outside the control of the sampling personnel. Key variables are identified and briefly discussed below. Please see the documents listed in the Bibliography for more detailed information on the effect of various variables.
Application—The techniques described in this standard practice are suitable for collecting samples for subsequent analysis for VOCs by US EPA Method TO-15, US EPA Method TO-17, Test Method D5466, Practice D6196, or other VOC methods (for example, ISO 16017-1, US EPA Methods TO-3 and TO-12). In general, off-site analysis is employed when data are needed for input to a human health risk assessment and low- or sub-ppbv analytical sensitivity is required. On-site analysis typically has lesser analytical sensitivity and tends to be employed for screening level studies. The techniques also may prove useful for analytical categories other than VOCs, such as methane, ammonia, mercury, or hydrogen sulfide (See Test Method D5504).
Limitations:
This method only addresses collection of gas-phase species. Less volatile compounds, such as SVOCs, may be present in the environment both in the gas phase and sorbed onto particulate matter, as well as in liquid phase. In soil gas, the gas-phase fraction is the primary concern. In other potential sampling locations (for example, ambient or indoor air), however, sampling for the particulate phase fraction may also be of interest.
The data produced using this method should be representative of the soil gas concentrations in the geological materials in the immediate vicinity of the sample probe or well at the time of sample collection (that is, they represent a point-in-time and point-in-space measurement). The degree to which these data are representative of any larger areas or different times depends on numerous site-specific factors.
Effect of Purging of Dead Space—If a soil gas probe is to be sampled soon after installation, the gas within the probe and any sand pack will consist mostly of atmospheric air. This air must be purged before soil gas that is representative of the geologic materials can be obtained. If the probe has previously been sampled, it may be possible to collect a representative sample after a smaller volume of gas is purged, but the volume of gas in the probe tubing or pipe must be purged at a minimum. It is recommended that a minimum of three (3) dead volumes be purged from the sampling system immediately prior to sample collection. Larger purge volumes typically are not necessary to achieve stable readings and should be avoided for shallower probes or if the potential exists that the additional purging will affect the partitioning of the VOCs in the subsurface. Larger purge (and sample collection) volumes can result in migration of soil gas from locations some distance from the sampling probe. Preferential pathways within the soil may exist and so the uncertainty associated with the origin of the soil gas will tend to increase with increasing purge (and sample) volumes. The data, however, should still be representative of how VOCs will migrate in these subsurface conditions.
Effect of Sampling Rate—The faster the rate of sampling, the larger the pressure differential (that is, vacuum) that is induced at the point(s) where soil gas enters the sampling system. The relationship between the flow rate and the vacuum is primarily dependent on the gas-permeability of the subsurface materials. This pressure differential has the potential to affect the partitioning of the VOCs in the subsurface if the VOCs exist in two or more phases (for example, free phase, dissolved phase, gas phase, sorbed onto soil particles) at or near the sampling depth (for example, within 1 m of the sample probe ). Sampling at relatively high rates (for example, >200 mL/min) has the potential to introduce a positive bias to the results (that is, make the results more conservative). The magnitude of any such bias is believed to be at most a factor of two. If the sampling depth is not near the source of the vapors, faster sampling rates (or larger sampling volumes) are not expected to have a significant effect on data quality.
Effect of Induced Vacuum—If desired, the induced vacuum can be limited by some upper bound value (for example, 2500 Pa [10 in. of water column]). The induced vacuum, however, is dependent on variables such as soil moisture as well as length and internal diameter of sampling line that may not be under the control of the user. Most significantly, the use of an upper limit for induced vacuum may preclude the use of preset flow control devices that allow unattended sample collection into evacuated canisters.
Effect of System Volume and Length of Tubing—The system volume should be relatively small to minimize the volume of dead space that must be removed prior to sampling. In practice, this typically means that 0.32 or 0.64-cm (1/8or ¼ -in.) OD tubing is used for shallow probes. For deeper probes (for example, ≥10 m), larger diameter installations may be preferable to minimize potential for plugging over time. Larger diameter probes and tubing also may be needed for large volume sub-slab sampling. The length of any tubing used in the above-ground sample collection train also should be kept to a minimum. If the ambient air temperature is less than the bulk soil temperature, condensation may form in the above-ground sampling lines and remove polar compounds from the sample stream. The potential is greater if excess tubing is present, so the length of tubing extending from the probe or well to connect to the sampling device should be kept to a meter or less. When the ambient temperature is less than the soil gas temperature, collecting samples at or near the maximum obtainable flow rate for a given location will minimize the potential for condensation.
Effect of Connections and Fittings—The number of connections and fittings also should be kept to a minimum, as these represent potential points for leaks to occur. If possible, all connections should be made above ground and visually inspected. For direct push approaches, this requires that slotted drive caps and pull caps be used, to allow the tubing connection to the PRT adapter or implant to be made above ground prior to probe installation. All fittings shall be leak checked prior to use (See 7.3.1).
Effect of Annular Seal—Soil gas probes installed in an augered or cored hole with a thick slurry of bentonite and water in the borehole annulus above the sand pack have the least risk of atmospheric air leakage down the borehole annulus or cross-communication of soil gas between different intervals during purging and sampling. This relative advantage compared with other techniques is most apparent for geologic materials with relatively low gas permeability.
Effect of Porosity—The effective porosity of a soil may be different than the total porosity. Large spaces (“macro pores”) such as fractures in fine-grained soils can impart a high permeability to materials that would otherwise have a low permeability. The emplacement of sampling probes in soil can cause compression or closure of macropores, resulting in a lower yield of soil gas than would otherwise occur through the uncompressed soil or formation.
Effect of Environmental Variables—In some cases, the soil gas concentrations may be affected by rainfall or changes in barometric pressure. The magnitude of any such effects is not well known, but is believed to be minimal at sampling depths ≥1.5 m. It is recommended that, at a minimum, hourly precipitation and barometric pressure data be obtained and reviewed for the 3-day period prior to sample collection as part of the data evaluation for any sampling of sub-slab probes or sampling depths <1.5 m.
Because diffusion of vapors from subsurface sources to the sampling probe relies on interconnected and air-filled pores within the soil column, soil moisture can have a significant effect on the flux of contaminants and, therefore, the concentration of the contaminant available at the sampling location. As a result, areas of high soil moisture may have significantly lower soil gas results than areas of low soil moisture, even though subsurface concentrations are similar in both areas. Therefore, some knowledge of the soil moisture conditions can help in interpreting soil gas results. This knowledge is also useful for comparing results from multiple rounds of sampling performed at a site.
Application of Results—The data generated using this method should be suitable for use in characterizing the nature and extent of gases and volatile chemicals in soil gas for developing a conceptual site model, as input to vapor intrusion pathway models, to estimate indoor air concentrations using attenuation factors, or for plume mapping. Data should be reviewed in conjunction with any drilling records, soil moisture data, groundwater and soil pollutant concentrations, and other relevant lines of evidence.
Note 1—The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D7663 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D7663 does not in itself assure reliable results. Reliable results depend on many factors; Practice D7663 provides a means of evaluating some of those factors.
Scope
1.1 Purpose—This practice covers standardized techniques for actively collecting soil gas samples from the vadose zone beneath or near dwellings and other buildings.
1.2 Objectives—Objectives guiding the development of this practice are: (1) to synthesize and put in writing good commercial and customary practice for active soil gas sampling, (2) to provide an industry standard for soil gas sampling performed in support of vapor intrusion evaluations that is practical and reasonable.
1.3 This practice allows a variety of techniques to be used for collecting soil gas samples because different techniques may offer certain advantages for specific applications. Three techniques are presented: sampling at discrete depths, sampling over a small screened interval, and sampling using permanent vapor monitoring wells.
1.4 Some of the recommendations require knowledge of pressure differential and tracer gas concentration measurements.
1.5 The values stated in SI units shall be regarded as standard. Other units are shown for information only.
1.6 This practice does not address requirements of any federal, state, or local regulations or guidance, or both, with respect to soil gas sampling. Users are cautioned that federal, state, and local guidance may impose specific requirements that differ from those of this practice.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
1.8 This practice offers a set of instructions for performing one or more specific operations. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this practice may be applicable in all circumstances. This ASTM practice is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word “Standard” in the title means only that the document has been approved through the ASTM consensus process.
