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ADVISORY-ACTIVE SOIL GAS INVESTIGATIONS • 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 Purge Volume Test The purpose of a purge volume test is to ensure that stagnant air is removed from the sampling system and to ensure that samples are representative of subsurface conditions. The purge volume test should be completed after the shut-in and leak test. The test well should be located near the contaminant source zone and in a lithologic unit where soil gas concentrations are anticipated to be elevated. The purge volume test is conducted by collecting and analyzing a sample for target compounds after removing one, three and 10 purge volumes. The purge volume test samples should be analyzed with the same analytical method as the constituents of concern. One purge volume includes the following volumes: • The internal volume of tubing; • The void space of the sand pack around the probe tip; and • The void space of the dry bentonite in the annular space. For permanent probes subject to frequent sampling, the purge volume can be reduced to one tubing volume if sufficient time, typically two weeks, has transpired between sampling events to allow the filter pack to come into equilibrium with the surrounding soil and the probe has remained sealed to ambient air. 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. Conduct the purge test at the same flow rate and applied vacuum as will be used to collect the soil gas samples. If the pump is battery-operated, the batteries should be checked before and during the operation to insure that a proper charge is maintained. As batteries lose charge the flow rate is lowered, effectively changing the purge rate. Select the appropriate purge volume based on the highest concentration of the compound(s) of concern detected during the purge volume test. To avoid extensive purging for soil gas samples collected at less than five feet bgs, a default of three purge volumes should be extracted prior to sampling. If VOCS are not detected in any of the step purge tests, a default of three purge volumes should be used. Include the purge test data in the report to support the purge volume selection. The data set should include the purge volume test as well as the flow rate, vacuum exerted on the formation, and duration of each purge step. Additionally, dependent on the objectives of the characterization activities, collecting pneumatic data during the purge April 2012 19