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real-time leak detection activities, using helium as the tracer compound, be conducted as part of the Phase 2 sampling activities. For the RI portion of this project (Phase 1), the compound used as the tracer was isobutane in the form of Barbasol® shaving cream. During the SSI (Phase 2), helium was used as the real time leak detection compound. Details of the procedures for leak detection determination used at the Site during the RI and SSI are provided in subsequent sections: Phase I Leak Detection Compound—Isobutane To determine if there were leaks in the soil gas sampling system, Barbasol® shaving cream containing isobutane was applied to the top of each boring prior to sampling as a leak-check compound (DTSC, 2003). At the point where the soil gas sample tubing daylights at the surface in each boring location, the shaving cream was sprayed in the well box until the soil gas tubing and the ground, where it daylighted, was completely covered. Additionally, the shaving cream was placed at each connection point in the sampling system. The shaving cream was sprayed into a clean, dry rag that was secured around the connection points in the system such that any leaks in the system would be detected. The samples were then analyzed for isobutane to determine if there is a leak in the system (i.e., if isobutane is detected above the Advisory guidance of 10 micrograms per liter(ug/L), the system has a leak). Phase 2 Real-Time Leak Detection Compound—Helium Real-time leak testing procedures using helium as the tracer compound were accomplished as follows: 1. The tracer gas compound was introduced around the sample probe and canister by filling a shroud positioned over the sample location. 2. The shroud was suitably sealed to the ground surface. 3. The tracer gas was introduced into the shroud. Helium was injected into the shroud at a maximum of 5 pounds per square inch. The shroud had a valve fitting at the top of the chamber to introduce the tracer gas into the shroud and a valve fitting at the bottom to let the ambient air out while introducing tracer gas. The helium concentration within the shroud was monitored with a field helium detector and maintained at>30%. 4. The shroud had a gas-tight fitting or sealable penetration to allow the soil vapor sample probe tubing to pass through and exit the chamber. 5. The sample probe tubing exiting the shroud was attached to a pump that was pre- calibrated to extract soil vapor at a rate of 200 ml/min. 6. Prior to collecting the sample in the canister, leak detections samples were collected in a 1-liter Tedlar bag. The Tedlar bag sample was analyzed with the handheld helium detector for the tracer gas and the detected concentrations were recorded on a field summary sheet. If the concentration of the tracer was >10% of the concentration measured from the shroud, the probe was re-sealed. The tracer test was performed again, and sample collection proceeded when the tracer concentration was <10% of the concentration in the shroud. 7. Following collection of the sample in the Summa canister, an additional soil gas sample was collected in a Tedlar bag and monitored for helium. If both the pre- and post- sampling Tedlar® bag samples contain helium at <10% of the shroud concentration, then the sample passed the leak test. If not, additional corrective measures were undertaken in PARSONS 23 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010