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a� <br /> connected to the upper end cap, to which '/4-inch outside diameter nylon (NylaFlowe) <br /> tubing is attached which extends to the ground surface for sample collection ('/s-inch <br /> outside diameter nylon tubing is also acceptable). Nylon tubing is recommended for all <br /> soil vapor sampling activities, as it has the lowest adsorptive capacity for VOCs. Each of <br /> the nylon tubing lines must be clearly and permanently marked at the land surface to <br /> denote its corresponding screened interval. <br /> Each 6 inch long screen is vertically centered in a 1 ft long interval containing standard <br /> sand pack,resulting in 3 inches of sand being above and below each screen. Be certain to <br /> correctly size the sand pack for the probe screen diameter. Each sand pack is covered <br /> with a 1 ft interval of dry granular bentonite, which is then covered with >_ 2 ft of <br /> hydrated granular bentonite to the bottom of the next sand pack (i.e., the next sampling <br /> interval). The dry granular bentonite is emplaced immediately above the sand pack to <br /> ensure that hydrated granular bentonite does not flow down to the probe screen and seal it <br /> off from the adjacent sediment. For multilevel probes the distance between sampling <br /> intervals is a function of the depth to groundwater, depth to known contaminant sources, <br /> and other site-specific factors. Generally, a spacing interval of> 5 ft is acceptable. In <br /> order to minimize the potential for short-circuiting of atmospheric air into soil vapor <br /> samples, the uppermost sampling interval should be > 5 ft from the ground surface. <br /> Following the emplacement of 1 ft of dry granular bentonite immediately above the <br /> uppermost sampling interval (sand pack), the remainder of the borehole should be filled <br /> with hydrated granular bentonite and, at the top, a 1 ft cement cap. A flush-mounted, <br /> locked utility vault of sufficient size to contain the tubing lines should be set in the <br /> cement cap. <br /> Each of the nylon tubing lines must be clearly and permanently marked at the land <br /> surface to denote its corresponding screened interval. Each tube must be fitted with a <br /> gas-tight cap (such as a Swagelok cap) at the ground surface to eliminate the potential <br /> for barometric pressure fluctuations to induce vapor transport between the subsurface and <br /> the atmosphere. <br /> ETC Soil Vapor Sampling Technical Toolkit 6 <br /> Version 1.4,September 7,2005 <br />