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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS Barometric pressure fluctuations associated with the passage of frontal systems can introduce atmospheric air into the shallow vadose zone. Therefore, soil gas sampling should be delayed until frontal systems have passed the area. 5.2.2 Wet Conditions Moisture may have the effect of biasing the soil gas samples. If moisture is observed in the sample tubing or container, the vapor well should be abandoned and a new well installed. Moisture entrainment can be avoided by sampling a sufficient distance from the water table or by avoiding highly saturated soil. Also, experiences with soil gas sampling indicate that larger diameter tubing is less susceptible to plugging by moisture and fine-grained materials. All protocols for soil gas sampling in this Advisory apply to larger tubing. The degree of potential sample bias is a function of subsurface equilibrium. When soil moisture and soil gas are in equilibrium, the VOCs in the pore water entrained in the sampling system may be released into the air stream during the agitation of the water, biasing the sampling results high. Conversely, when soil moisture and soil gas are not in equilibrium, soil gas contamination may partition into newly introduced pore water, biasing the sampling results low. Hence, vapor samples could be biased either low or high depending on the subsurface equilibrium. In either scenario, moisture should be avoided to alleviate potential bias. Areas of high soil moisture or areas where irrigation is present, should be allowed to drain, if possible, prior to soil gas sampling. If no flow or low flow conditions are encountered where water is drawn into the sampling system due to wet soils caused by rain or irrigation, cease soil gas sampling and wait five days for the soils to drain. 5.2.3 Soil Gas Sampling in Low-Permeability Soil Soil gas sampling in silt and clay-rich soils is feasible by following the sampling protocols described in Appendix D. Low flow or no flow conditions correspond to conditions where the minimum flow rate of 100 mL/min cannot be sustained at the maximum applied vacuum of 100 inches of water. High quality data can be produced by implementing the following field practices: Good annular seals; Careful monitoring of flow rate and vacuum during purging; and Use of tracer gas for leak-testing. If the soil gas permeability is too low to allow sustainable purging at appreciable flow rates without applying excessive vacuum, follow the protocols described in Appendix D by using an alternative sample collection method or re-drilling and constructing a soil gas well in a non-traditional manner. If low flow or no flow conditions are encountered, a new soil gas well in a coarser lithology at a different depth or lateral location may be installed. The following should be considered if low-flow conditions persist: July 2015 27