ASTM D7758-11(2016)

5.1 Passive soil gas samplers are a minimally invasive, easy-to-use technique in the field for identifying VOCs and SVOCs in the vadose zone. Similar to active soil gas and other field screening techniques, the simplicity and low cost of passive samplers enables them to be applied in large numbers, facilitating detailed mapping of contamination across a site, for the purpose of identifying source areas and release locations, focusing subsequent soil and groundwater sampling locations, focusing remediation plans, identifying vapor intrusion pathways, tracking groundwater plumes, and monitoring remediation progress. Data generated from passive soil gas sampling are semi-quantitative and are dependent on numerous factors both within and outside the control of the sampling personnel. Key variables are identified and briefly discussed in the following sections.

5.2 Application—The techniques described in this practice are suitable for sampling soil gas with sorbent samplers in a wide variety of geological settings for subsequent analysis for VOCs and SVOCs. The techniques also may prove useful for species other than VOCs and SVOCs, such as elemental mercury, with specialized sorbent media and analysis.

5.3 Limitations—Passive soil gas data are reported in mass of individual compounds or compound groups identified per sample location, with the reporting units generally in nanograms (ng) or micrograms (μg) per sampler and not a concentration (see 6.8). Ideally, the data produced using this method will be representative of time-weighted soil gas concentrations, present in the vicinity of the PSG sampler and sorbed on the sampler during the exposure period; however, non-uniformity of sampler design, starvation effects during sample collection, or an insufficient amount of sorbent that results in saturation of the sorbent surface area, or combinations thereof, will affect the relationship between sorbed mass and soil gas concentrations present. The degree to which these data are representative of any larger areas or different times depends on numerous site-specific factors. In general, information obtained from a passive soil gas sampling program alone is not sufficient to support a quantitative determination of soil gas concentrations.

5.4 Sampler Design—Passive soil gas is an effective investigatory/monitoring tool if the appropriate quality controls are included in the technology design, which includes uniformity in the construction of the sampler. At a minimum, controls should be in place to ensure that (1) the appropriate sorbents with hydrophobic properties are used to target the compounds of concern (see Practice D6196), (2) materials used to house the sorbents are chemically-inert, non-reactive or corrosive, and will not off-gas compounds or act as competing sorbents (see Guide D5314, paragraph 6.5.3), and (3) the sorbents are housed in suitable containers that protect the sorbents, allow diffusion of the soil gas to the sorbents, and facilitate installation of the sampler to the desired sampling depth.

5.5 Sampler Exposure Periods—Guidelines for PSG exposure periods for source identification, spatial variability assessment, and vapor intrusion evaluation should consider the project objectives, target compounds, required detection limits or anticipated soil gas concentrations or both, design of the passive sampler, matrix heterogeneity, soil types (total porosity), soil moisture level (water filled porosity), and depth to expected contaminants. Sites having coarse-grained dry soils, high concentrations, shallow groundwater or soil contamination or both, and volatile compounds typically require shorter exposure periods. Sites with finegrained, clays or moist soils or both, deep contaminant sources, low concentrations, or SVOCs, or combinations thereof, typically require longer exposure periods. Exposure periods typically range from days to weeks but can be as brief as one hour when high concentrations of target compounds are expected in the soil vapor.

5.6 Sampler Spacing—Grid designs can consist of regularly spaced sampler locations, random or irregular spaced, and as transects or varying spatial intervals (see Guide D6311). Biased spacing in which smaller sample spacing is used in areas with known or suspected targets (that is, source areas) and large spacing in areas not believed to be impacted are also used. For large area investigations, a staged or phased sampling program can be used. The investigation begins with a widely spaced regular grid design. The initial soil gas results are reviewed and subsequent sampling is conducted at locations where the target compounds were observed. The subsequent survey design consists of more closely spaced samples to resolve the feature of interest in greater detail. Multiple phases of soil gas sampling can be combined to provide one comprehensive image of the soil gas results. Staged or phased investigations require multiple deployments adding costs to the overall investigations. However, areas of the site that have nondetectable values in the soil gas may be removed from further investigation.

5.7 Sampling Depth—Consideration of project objectives should be taken into account when determining deployment depth. It is ideal, when possible, to deploy samplers at the same depth to ensure data consistency. PSG samplers are generally installed from a depth of 15-cm to 1.0-m BLS; however, holes may be advanced to greater depths when appropriate, and samplers can also be suspended beneath surface flux chambers or in permanent vapor ports.

5.8 Soil Types—In general, sandy soils tend to be more porous and permeable and, thus, require shorter exposure times. Conversely, soils with high clay contents tend to be less porous and permeable and typically have lower flux rates (see Practice D2487). Soil types vary in vapor permeability due to the differences in the number and interconnectivity of air-filled pores. The more air-filled, interconnected the pores are, the greater the potential flux of contaminants through the soil to the sampler. Starvation effects resulting in low bias are more likely to occur in low permeability soils where the flux through the soil matrix is limited.

5.9 Effects of Soil Moisture—Because diffusion of vapors from subsurface sources to passive samplers 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 mass of the contaminant available for adsorption by the sampling device. The use of hydrophobic sorbents minimizes the effect on sampler sensitivity, but does not change the impact of soil moisture on contaminant soil gas concentrations. 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 subsequent surveys performed at a site.

5.10 Effects of Target Compounds—In general, the larger the molecular weight of the compounds being targeted, the lower the vapor pressure and resulting concentrations in the soil gas, and therefore, the longer the required exposure time of the PSG samplers in the vadose zone.

5.11 Sealing (Plugging) the Top of the Hole—Once the PSG sampler is inserted in the ground, the top of the hole is plugged with a material that will effectively seal the hole, such as aluminum foil or cork, which can then be covered with soil. For concrete or asphalt surfacing, an approximately 5-mm-thick mortar or quick-setting concrete patch above the plug can be used as an option to maintain the integrity of the surface while the sampler is in the ground. The materials used to plug the hole should not contribute compounds of concern and the seal should be flush mounted to keep the sampler safe from harm, prevent ingress of ambient air or surface water, and not interrupt ongoing site activities during the exposure period.

5.12 Effects of Ambient Air While Installing/Retrieving Samplers—PSG samplers arrive at the site sealed to protect the sorbents from contaminants in ambient air during transport. Just prior to installation into the hole, and then again during retrieval, the sampler is exposed to ambient air for a brief period of time. The typical time of exposure to the ambient air is less than 15 s. In some instances, it may be necessary to collect a field blank using a PSG sampler to evaluate whether compounds in the ambient air potentially biased the results. To perform this quality control check, an identical PSG sampler is opened and exposed to the ambient air for approximately the same amount of time required to install and then later retrieve a PSG sampler at a designated location. The field blank is sealed at all other times and is transported to the laboratory along with the field samples. Care should be taken to minimize the sorbent exposure to ambient air during field activities. Obvious sources of contamination (e.g., gas-powered electrical generators or vehicle exhaust) should not be in close proximity when installing/retrieving a sampler.

1.1 Purpose—This practice covers standardized techniques for passively collecting soil gas samples from the vadose zone and is to be used in conjunction with Guide D5314.

1.2 Objectives—Objectives guiding the development of this practice are: (1) to synthesize and put in writing good commercial and customary practice for conducting passive soil gas sampling, (2) to ensure that the process for collecting and analyzing passive soil gas samples is practical and reasonable, and (3) to provide standard guidance for passive soil gas sampling performed in support of source identification, spatial variability/extent determinations, site assessment, site monitoring, and vapor intrusion investigations.

1.3 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.

1.4 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.5 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.

1.6 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 standard 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.