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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS Vacuum during purging should not exceed 100 inches of water. The anticipated spatial influence of the HPV should not extend beyond the building's footprint, otherwise, ambient air may enter into the sample from the edges of the foundation. This situation can be alleviated by installing probes subject to HPV in the interior portions of the building. A leak check compound should be used during the purging and sampling. The amount of purge volume should be measured with a calibrated extraction pump. During the purging, site contaminants should be measured with a field instrument, such as a photoionization detector capable of reading in the parts per billion range, at the start of purging and then every 30 minutes thereafter, at a minimum. Fixed gases, such as oxygen and carbon dioxide, should be monitored every 10 minutes during purging to evaluate potential ambient air breakthrough into the sample system. Typically, oxygen and carbon dioxide should remain constant during the purging of sub-slab soil gas. Changes of fixed gas concentrations may be indicative of unacceptable leakage through the foundation slab. To evaluate the radial influence of the HPV sampling, vacuum measurements should be continuously monitored at a nearby sub-slab probe locations. These data, along with all other field data, should be reported to the oversight agency. As an additional line of evidence for the integrity of the final laboratory sample, the leakage of air through the foundation slab should be quantified by the methods within McAlary and others (2010). If ambient air breakthrough from the foundation slab occurs, the contaminant data collected upon completion of the purging should not be used for risk assessment purposes. Data indicating breakthrough include the detection of the leak check compound, significant increases of oxygen and/or significant decreases in carbon dioxide while purging. Likewise, significant decreases in VOC concentrations, as collected with a field instrument during purging, may be indicative of breakthrough. July 2015 18