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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS When designing a field study, the tracer compound should be carefully selected. The following items should be considered when choosing a tracer compound. Excessive concentrations of the tracer can elevate analytical detection limits; Tracer compounds can cause interference with target analytes; Field detectors may produce biased results in the presence of water vapor or other compounds; The tracer compound may be naturally occurring; Field detectors may not be routinely calibrated; and Pressurized canisters of tracer gas may be dangerous to transport. 4.2.3 Purging The purpose of purging is to remove stagnant air from the sampling system so that representative samples can be collected from the subsurface. A default of three purge volumes should be used. Purge volume testing is no longer recommended. One purge volume includes the following: The internal volume of the tubing and probe tip. The void space of the sand pack around the probe tip. The void space of the dry bentonite in the annular space. No distinction should be made between new and old probes or whether probes are shallow or deep. All probes should be subject to similar procedures. Purging should commence after the shut-in test. Sample containers are not included in the purge volume calculation except when non-evacuated glass bulbs are used. In those instances, the volume of the non-evacuated glass bulbs should be added to the purge volume to account for mixing and dilution of gasses inside the glass bulb. Shallow soil gas and sub-slab probes, where screens and associated sand packs are less than five feet below surface grade, are subject to purging. For shallow soil gas probes, sample collection containers should be less than or equal to one liter to avoid excessive air removal, avoiding the possibility of ambient air entering the subsurface and into the sample. All permanent probes should have an air-tight seal or cap to prevent ambient air from entering the tubing or casing. Include the purg ing data in the report to verify an adequate volume of air was removed prior to sampling. The soil gas report data set should include the purged volume as well as the flow rate, and vacuum exerted on the formation. Additionally, dependent on the objectives of the characterization activities, collecting pneumatic data during purging may be warranted to determine the air permeability of the subsurface (see Appendix D for more information). July 2015 23