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M CDIM Standard Operating Procedure 4.1 Soil Vapor Sampling "°'"""'"' Rev.1 6/30/2022 A shroud leak test is used to evaluate whether ambient air is introduced into the soil gas sample during the collection process which may result in an underestimation of actual site contaminant concentrations. The two types of leak check compounds available for use when soil gas sampling are liquid compounds and gaseous compounds. This SOP focuses on the employment of trace gas in conjunction with a shroud for leak detection. At the end of the purge, use the tracer gas detector to screen for tracer gas in the sampling chain. According to DTSC, an ambient air leak up to 5 percent is acceptable if quantitative tracer testing is performed by shrouding (DTSC, 2015). 3.3.6 Soil Vapor Sampling Once a vapor well sampling setup has successfully demonstrated a closed system, the system is ready to collect subsurface vapor samples. Procedures for sample collection are outlined in the list below. 1. Insert the trace gas detector into the shroud. 2. Apply tracer gas to counter any loss in concentration after purging (return to approximately 20%). 3. Open shroud enough to access the manifold. 4. Close the two-way valve used for purging so ambient air will not be included in the sample. 5. Open the sample canister valve completely to begin sampling. 6. Seal the shroud closed. The gas may be shut off at any time once the target concentration of around 20% tracer gas is reached. As needed, add gas to the system to maintain a steady concentration during sample collection. Efforts should be made to limit the elapsed time it takes to access the components inside the shroud to minimize the volume of tracer gas that escapes. 7. Monitor the Summa canister pressure gauge and down-well pressure throughout the sampling duration. 8. Once an appropriate sample volume has been collected, open the shroud and close the sample canister valve completely. 9. Confirm the canister is properly labeled before packaging. The sampling event should be documented as described in the project work plans, with appropriate labels, sample descriptions, and notes in the Soil Vapor Sampling Sheet (Attachment 4) as well as the field log. 3.3.7 Decontamination All reusable equipment shall be decontaminated prior to use and across sampling locations to prevent cross contamination. Tubing is not reusable and should not be decontaminated. Instead, new sampling tubing is to be used at each probe location. Equipment subject to decontamination between sample locations include but is not limited to hand auger and drill pipe used in the well construction. Equipment decontamination may consist of a three-stage decontamination process consisting of a wash with a non-phosphate detergent, a thorough scrub using a brush, and a final rinse with distilled water. Appropriate 7