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Pre-Design Investigation Workplan Former Manufactured Gas Plant— 1.()di, California Following a successful vacuum shut-in test, the chemical tracer leak test will be conducted during and after soil gas probe purging and during sampling at each soil gas sample probe (5- and 15-foot sample depths) using helium gas to check the ground surface seal and probe interface for ambient air leaks into the subsurface. A solid shroud, consisting of a plastic cap or box, will be placed to encompass only the soil gas probe at the surface. For at least 10 percent of the samples, a larger shroud, consisting of a larger plastic box, will be utilized to enclose the soil gas probe at the surface as well as the aboveground sampling train and the 1-liter SummaTM canister. Once the shroud is in place, helium will be introduced into the shroud prior to purging the soil gas probe. The helium concentration in the shroud will be maintained at a minimum concentration of 30 percent by volume or at least two orders of magnitude higher than the reporting limit of the field helium detection meter used. During purging, the helium concentration within the shroud will be measured with a handheld helium detector to maintain the required minimum concentration of 30 percent by volume throughout the purge period. Once the required volume is purged from the soil gas probe, but before SummaTM canister sample collection, a soil gas sample will be collected from the soil gas probe in a 1-liter Tedlar bag (using a "lung box"). The helium concentration in soil gas collected in the Tedlar bag will be measured using the helium detector. If the helium concentration within the Tedlar bag is less than or equal to 5 percent of the minimum concentration maintained in the shroud during probe purging, then the soil gas sample will be considered to not have a significant leak, and the SummaTM canister sample will be collected. All leak check test helium concentrations measured in the Tedlar bag will be recorded on soil gas sampling forms for each soil gas sample. However, if the measured helium concentration within the Tedlar bag is more than 5 percent of the minimum helium concentration maintained in the shroud, then the chemical leak check test will be considered to have failed, and the shroud and sample train connections will be checked, readjusted, and replaced as needed. The vacuum shut-in test will be repeated followed by purging a minimum of 1 purge volume through the probe and sampling train to clear ambient air that may have entered the tubing during trouble-shooting. The chemical leak check test will then be repeated as described above. If the probe continues to fail the chemical leak check test, the probe will be properly abandoned and a replacement probe will be installed, as appropriate. Replacement soil gasr probes will be installed at least 5 feet from the location of the original probe,depending on Site conditions. The helium gas will continue to be applied to the shroud throughout the collection of the soil gas sample in the SummaTM canister. As indicated in section 5.0 of this Workplan, helium will be analyzed in 10 percent of the Summar" canister samples submitted to the off-site analytical laboratory. 4.8.3 Purge Volume Test According to DTSC ASGI guidance, a purge volume test is not required when soil gas samples are collected using SummaTM canisters (DTSC, 2010). Rather, as specified in DTSC ASGI guidance, three purge volumes will be extracted from each probe prior to collecting the helium leak test Tedlar bag sample and SummaTM canister soil gas sample. The purge volume (also referred to as the "dead space volume' for each probe will be estimated by adding the following volumes: (1) the internal volume of tubing used over the length of probe from the tee-valve at the surface down-hole to the stainless steel probe, (2) the estimated void space of the sand pack around the probe tip, and (3) the TERRA PACIFIC GROUP Page 16 April 4. 2012