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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS Soil gas samples collected immediately above the source of contamination are more likely to be representative of what may be in contact with the building's foundation (Hers et al., 2006; DiGiulio and Cody, 2006; and USEPA, 2012). Likewise, the numerical modeling conducted by Abreu and Johnson (2005), Abreu and others (2006), and Bozkurt and others (2009) also suggests this relationship. Hence, risk estimates may be biased low if quantified with shallow soil gas measurements when using the Johnson and Ettinger (1991) model. Maximum subsurface concentrations, or soil vapor concentrations representative of source conditions, should be used for preliminary vapor intrusion screening evaluations. A five foot deep sample is not appropriate if there is a deeper source. Accordingly, collecting soil gas samples near contaminant sources is recommended for vapor intrusion modeling. Vertical soil gas sampling should be conducted to determine the source of subsurface contamination. Ideally, numerous vertical profiles of soil gas should be developed at the site to accurately locate subsurface sources. Once located, soil gas collection can be targeted at these depths site-wide. Typically, contaminant sources are adjacent to the areas of highest subsurface concentration. 3.2 INSTALLATION PROCEDURES Soil gas well installation procedures are described below. Soil gas well construction should ensure a good seal between the formation and sampling assembly, and minimize ambient air breakthrough. Additional standards may be required by local oversight agencies. 3.2.1 Installation Methods and Design Soil gas wells may be installed using a variety of drilling methods such as direct push, hollow stem auger or hand auger. Certain drilling methods that significantly disrupt soil gas equilibrium, such as air rotary and rotosonic, may be employed if longer equilibration times are used prior to sampling. The mud rotary drilling method is not acceptable for soil gas probe emplacement under any circumstances. Following is a step-by-step guide to soil gas well (probe) installation after the borehole has been drilled: Sand Pack. Install a sand pack to minimize disruption of airflow to the sampling tip. A tremie pipe should be used for soil gas wells deeper than 15 feet to avoid bridging or segregation during placement of the sand pack and annular seal. Place the probe tip midway in the sand pack, as shown on Figure 1 with at least six inches of sand above and below the probe tip. Transition Seal. Emplace at least six inches of dry granular bentonite on top of each sand pack, as shown on Figure 1. Granular bentonite is preferred over bentonite in powdered or pellet form for the sand pack / annular seal transition. Following the dry bentonite, fill the borehole to the surface with an appropriate annular sealing material (see below). The purpose of the dry granular bentonite between the sand pack and the annular seal is to prevent water and clay from the annular seal infiltrating into the sand pack. A tremie pipe should be used for the emplacement of the transition seal in soil gas wells that are deeper than 15 feet, otherwise the material can be gravity fed (freefall) from the surface. July 2015 9