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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS 5.1.4 Sorbent Tubes Sorbent tubes are used with USEPA (1999) Compendium Method TO-17 (Method TO-17). Method TO-17 describes: Sorbent tube sampling procedures; Sorbent tube selection; Tube conditioning; Sampling apparatus; Sampling rates; Sample collection preparation; Flow rates; and Other sampling procedures. Method TO-17 is used for VOCs and SVOCs including naphthalene (See Appendix E for additional details on collecting and analyzing for naphthalene in soil gas). 5.1.5 Alternate Sample Containers Non-traditional sample containers are available for collecting soil gas samples. Mini- passivated stainless steel canisters (-400 milliliters), may be useful in many field applications. Evacuated glass bottles may also be used but their holding time should be limited to 48-hours. The use of non-traditional size or types of containers should be discussed in the workplan. 5.2 FIELD CONDITIONS Adjustments or modifications to the sampling program may be required by the regulating agency to accommodate changes mandated by evaluation of the data set or unforeseen site conditions. Field conditions, such as rainfall, irrigation, low permeability lithology or drilling conditions may affect the ability to collect soil gas samples. 5.2.1 Rainfall and Barometric Pressure (See Appendix G) Rainfall decreases the air-filled porosity of the shallow soil, thereby limiting diffusional transport of volatile contaminants. Also, soil gas contaminants may partition into the clean infiltrating rainwater, both of which may potentially bias soil gas sampling results. Hence, soil gas sampling should not occur during a significant rain event and should only occur after five days without a significant rain event. A significant rain event is defined as 1/2 inch or greater of rainfall during a 24-hour period. The waiting period is based upon soil drainage curves. Appendix G provides additional information. Irrigation or watering of soil should stop at least five days prior to the soil gas sampling event. Likewise, areas subject to soil gas sampling should be free of standing or ponded water for at least five days prior to sampling. Do not perform soil gas sampling in swales or depressions where water might have accumulated. However, soil gas sampling after rainfall can proceed where infiltration has not occurred, such as under buildings or beneath high-integrity pavement. July 2015 26