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SOP No.021,REV.1 TO E RO E K SOIL GAS SAMPLING METHODS „ AssoCI ns INC. PAGE 4 allow for subsurface conditions to equilibrate,the following equilibration times are recommended: • 30 minutes for probes installed by hand auger method. • 20 minutes for probes installed by direct-push method where the drive rod remains in the ground. • 30 minutes for probes installed by direct-push method where the drive rod does not remain in the ground. • 48 hours for probes installed by hollow-stem auger drilling method. Once fitted with the sampling tube,the ambient air within the sampling system is purged. Usually, three to five system volumes are purged prior to sample collection. If the sampling system purge volume cannot be measured,then a standard purge time of 3 to 5 minutes should be used. A flame ionization detector(FID)or photoionization detector(PID)may be used to purge ambient air while also providing a qualitative means to indicate when purging is complete,i.e., when volatile organic compounds (VOC) are detected. After the system is purged of ambient air but before the pump is turned off, approximately 2 inches of the sampling line closest to the entrance port of the pump should be folded over itself, and the tubing should be clamped to keep ambient air from reentering the system. After the purged system is sealed to ambient air, the pump should be turned off. Sample collection can now proceed using a Tedlar®bag, a syringe, a Summa canister, or a sorbent tube. 2.3 SAMPLE COLLECTION USING TEDLARO BAGS Soil gas can be collected for chemical analysis in a 500-cc Tedlar® sampling bag. This is accomplished by using a pump to induce a vacuum on the exterior of the bag. This will cause the Tedlar®bag to be inflated with soil gas. The following procedures should be used: 1. Connect the free end of the Tygon® or equivalent tubing (previously inserted through the top of the vacuum chamber) to the Tedlar® sampling bag. Open the valve on the sampling bag and place the bag into the body of the vacuum chamber. 2. Place the top on the vacuum chamber. 3. Connect the free end of the evacuation tube to the pump. 4. Turn on the pump. This will create a vacuum in the chamber, and the Tedlar® bag should fill at a rate of approximately 2 liters per minute. The rate at which the Tedlar° sampling bag fills will depend on the porosity and permeability of the soil. 5. The minimum amount of soil gas needed for analysis is approximately 0.25 liters.