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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS A sufficient number of sub-slab sampling events should be conducted to account for seasonal and spatial variability. At a minimum, two sub-slab sampling events are warranted before a final risk determination is made. Upon completion of all sampling, the sub-slab probes should be properly decommissioned. The probe tip, probe tubing, bentonite, and grout should be removed by over-drilling. The borehole should be filled with grout and concrete patch material. Surface restoration should include a follow-up visit for final sanding and finish work to restore the floor slab, and associated coverings, to their original condition. At least two sub-slab probes should be installed at each residential structure, with one probe installed in the center of the building's foundation. The probes should be installed in inconspicuous areas, such as utility closets or beneath stairs. Sub-slab probes should not be installed near the edges of the foundation due to the effects of wind on the representativeness of contaminant concentrations (Luo et al., 2009). Numerous sub-slab sampling devices, are commercially available. Cal/EPA encourages the use of innovative technologies as long as the devices are capable of meeting project DQ0s and established performance criteria. Accordingly, sampling procedures recommended in this Advisory should be followed when using innovative technologies. 3.6.1 High Purge Volume Sub-Slab Sampling Extracting a large volume of soil gas from beneath a foundation may provide representative average concentrations that minimize spatial variability in the data and can minimize the possibility of missing an area of elevated concentrations when using multiple discrete sampling points. Unlike conventional sub-slab sampling, high purge volume (HPV) testing provides information over a broader area by removing a larger volume of soil gas. The volume of gas beneath a typical residence in the engineered subgrade fill is a few thousand liters. In this context, the volumes of sub-slab soil gas samples currently collected for vapor intrusion assessments are very small, and larger samples may provide more spatially averaged concentrations. Examples of high purge volume methods can be found in Lewis and others (2004) and McAlary and others (2010). HPV sampling consists of extracting sub-slab soil gas at a slow, constant rate of five to ten liters per minute, or higher if warranted, and collecting a soil gas sample after about two hours. Hence, over five hundred liters of sub-slab gas can be removed prior to sample collection using the HPV technique. The use of HPV sampling should be based on the site conditions and the project DQ0s. All HPV sampling should follow the recommendations in this Advisory concerning leak checking, shut-in testing, and DQ0s. As HPV sampling becomes further developed, quantitatively accurate results for contaminant concentrations can be achieved and the HPV samples can be used in quantitative risk assessments. When utilizing HPV at a site, the protocols within McAlary and others (2010) should be followed. Some items to consider when using HPV are as follows: July 2015 17