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5.3 Considerations for Soil/Landfill Gas Sampling There are some additional sampling considerations for collecting grab samples (canister or Tedlar bag) from a soil boring, landfill boring, SVE system, or landfill gas(LFG) collection system. The general challenge with these samples arises from.the need to employ long lengths of tubing to direct the soil gas, landfill gas, or process air to the canister or Tedlar bag. Tubing introduces the potential for contamination and diluting the sample. • Use inert tubing. Teflon tubing is recommended. Tubing with an outer diameter of 1/4 in. works best with the fittings on the particulate filter. • Do not reuse tubing. $2 per foot for new tubing is a small price to pay for peace of mind. • Purge tubing adequately. A long length of tubing has significant volume of"dead air" inside. Without purging, this air will enter the canister and dilute the sample. Consider using a hand- held PIDIFID to confirm that you have purged the tubing and are drawing sample air through the tubing. • Avoid leaks in the sampling train. Leaks of ambient air through fittings between pieces of the sampling train (e.g., tubing to particulate filter) will dilute the sample. • Don't sample water. If moisture is visible in the sample tubing, the soil gas sample may be compromised. Soil gas probes should be at an appropriate depth to avoid reaching the water table. Additionally, subsurface vapor should not be collected immediately after measurable precipitation. • Purge the sample port. A sample port on a SVE system or LFG collection system can accumu- late solids or liquids depending upon the location of the port in the process and the orientation of the port. An influent sample port located upstream of a filter or moisture knock-out can be laden with particulates or saturated with water vapor. Heavy particulate matter can clog the particulate filter and foul the canister valve. It is important to prevent liquids from entering the canister. A sample port oriented downward may have liquid. standing in the valve. Purge the sample port adequately before connecting the sampling train. • Consider the effects of sampling a process under vacuum or pressure. When collecting a grab sample from a stationary source such as an SVE system or LFG collection system, some sample ports may be under vacuum or pressure relative to ambient conditions. When the sample port is under vacuum, such as the header pipe from the extraction well network, it may be difficult to fill the canister with the desired volume of sample. A vacuum pump can be used to collect a canister grab sample from a sample port under considerable vacuum. See the related discussion on sampling at altitude in Section 5.2. When the sample port is under pressure, such as the effluent 1C:r CpAIR TOXICS LTD. 23