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ADVISORY — ACTIVE SOIL GAS INVESTIGATIONS collecting a soil gas sample. The sampling volume should not exceed the dead space in the probe. This testing can be conducted in the field or by the stationary or mobile laboratory prior to using the tubing. If conducted by a stationary laboratory, the dead air in the assembled probe may be replaced by zero-grade air and this zero-grade air may be tested; c. Probe Tips. Probe tips can be blank tested with distilled water. The most common method is to pour distilled water over the tip and through the inside, collecting the water in a container appropriate to the analytical method. The collected water should be tested for all site contaminants. 3.2.3 Drive Point Method Post-run tubing (PRT) and drive point methods' used to create temporary soil gas wells may be used to rapidly acquire soil gas samples when carefully installed. Contractors should ensure that installation includes regularly checking and cleaning of the PRT tip threads and its seat and changing the 0-rings on a daily basis. Contractors should use stiff tubing to couple the PRT tip to the connective hose and use 1/4 inch outer diameter, thick-wall tubing to ensure sufficient torque is available to screw the tip tightly into the seat. If the 0-ring is not seated properly into the drill rod, ambient air from inside the rod could enter into the sampling system, introducing ambient air into the soil gas sample, biasing the sampling results. The integrity of the seal of the 0-ring cannot be readily evaluated with a leak check compound, and the inability to evaluate the integrity of the 0-ring seal should be evaluated with the DQO process. Representative soil gas samples may be difficult to obtain with PRT and drive point methods in certain lithologies. Drive point probes may be deflected by consolidated lithologies and strata containing cobbles or boulders, which can create gaps between the outer wall of the drive rod and the subsurface that are difficult to observe and equally difficult to seal. A hydrated bentonite plug at ground surface does not stop communication along the annular space. Samples collected under these circumstances will potentially draw soil gas primarily from the most permeable layer above the probe tip which may introduce a significant bias. Moreover, this condition is difficult to identify by a leak check compound applied at or near ground surface. Collecting representative soil gas samples in these conditions may require alternative sampling methods such as passive soil gas sampling or the installation of permanent sampling wells. 3.3 SOIL GAS WELL COMPLETION Soil gas wells should be secured, capped and completed to prevent infiltration of water or ambient air into the subsurface, and to prevent accidental damage or vandalism. Mark the tubing at the surface to identify the probe location and depth. For surface completions, the following components may be installed: 5 Drive point methods may be appropriate for certain site conditions or circumstances depending on DQ0s. The use of post-run tubing should be discussed with the regulating agency prior to inclusion in the workplan. July 2015 13