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Manifold Shut-In Check 1. Open valve on vapor sampling manifold and open 3-way valve#1 so the vacuum pump of the purging assembly can evacuate the vapor sampling manifold assembly(keep valves#2 and#3 closed to the Tedlar bag/vacuum chamber of the vapor screening assembly)(Figure 2). Start the vacuum pump.Do not open#I valve to the probe assembly,or the valve on the sample Summa canister.Allow manifold/tubing vacuum to stabilize at approximately 10"Hg. 2. Stop the vacuum pump,close 3-way valves#2 and#3 (to allow shut-in testing of vapor sampling manifold),and conduct a shut-in test by waiting at least 5 minutes(if using 150 inches of water gauge)or 10 minutes(if using 30 inches of mercury gauge).Monitor manifold vacuum gauge to test for leaks. If the vacuum decreases,rectify the leak before proceeding. Purze, Flow and Leak Check 1. Calculate purge volume and duration. Determine the desired total purge volume and purging duration for the equipment setup. A critical orifice flow restrictor is intended to limit the maximum purge and sampling flow rate(approximately 150 ml/min). If step testing is not required to better determine optimal purge volume,purge approximately 3 times the volume of the sampling assembly,tubing,vapor probe and void space or any probe/filter pack material below the concrete slab. 2. Leak-check enclosure. Place leak-check enclosure over vapor probe and seal to floor using hydrated bentonite or weather stripping. Introduce helium gas into the leak-check enclosure and monitor with the helium gas analyzer until it reads between 20%and 30% helium. 3. Conduct purging. Start vacuum pump and open 3-way valve#1 (and 3-way valves#2 and#3)so the vacuum pump can evacuate the probe. Do not over-purge. Closely monitor the flow on the rotameter and the vacuum on the vacuum gauge.For most samples flow should be limited to 150mL/min or less.If the vacuum remains below approximately 7"Hg,then sufficient flow is present to collect a representative sample (Cal/EPA 2012)and continue purging for the planned purge duration. 4. If the probe-side vacuum exceeds approximately 7"Hg,then insufficient flow may be present to collect a representative sample and this condition should be noted.Evaluate probe integrity or consider re-installation of probe,especially if probe installed in coarse- grain material. If no significant flow is attained,the sampling line may be plugged or the vapor probe may be positioned in a low permeability or saturated layer. If the probe cap is opened for probe inspection,record the inspection procedures and duration. If purging and sampling is resumed after opening the probe cap,this information will help determine the representativeness of the sample. To sample subslab gas under low flow conditions,follow this alternate sampling method derived from Appendix D,Cal/EPA 2012. Make a reasonable attempt to purge one purge volume.After purging,open sample Summa canister until sampling manifold vacuum threshold is achieved,then close Summa sample valve until probe vacuum dissipates. Repeat this sampling procedure as necessary to sufficiently fill the sample Summa canister. Alternatively,consider installing a subslab gas probe with a larger probe annulus space,or employing passive soil gas sampling methods. 5. When purge duration complete and ready to discontinue purging,close 3-way valve#1 so that the probe is connected to the sampling manifold,and then stop the vacuum pump. 3