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• LETTER WORKPLAN FOR SOIL VAPOR PROBE INSTALLATION Former PG&E Tracy Manufactured Gas Plant 502 E. Grant Line Road,Tracy, CA As approved by the California Department of Toxic Substances Control, Parsons has excavated 6 Areas of Concern (AOIs) to remediate identified soil impacts associated with historical operation of a manufactured gas plant (MGP). As part of post-remediation monitoring, Parsons has proposed the installation of one soil gas probe within one of the remediated areas to determine the presence of soil vapor concentrations in excess of Site cleanup goals. Post-remediation soil gas probe installation/sampling was approved by the California Department of Toxic Substances Control (DTSC) as described in Parsons' Health Risk Assessment/Removal Action Workplan (HRA/RAW) dated June 16, 2011. The proposed location of the soil gas probe is shown on the attached figure. The probe will be constructed by hand augering a 3-inch diameter soil boring to a depth of 5.5 feet below ground surface (bgs) (i.e., approximately 2 feet into native soil). Approximately 6 inches of#2/12 filter sand will be emplaced in the bottom of the boring, and a 6-inch long, 3/8-inch diameter stainless steel mesh probe implant will then be installed from 4.5 to 5 feet bgs. The soil gas probe will then be attached to 1/4-inch outside diameter polytetrafluoroethylene (PTFE [Teflon'"]) tubing that will extend to the surface where it will be capped with a 3-way valve to be used for sampling. The annular space surrounding the probe implant will consist of#2/12 sand to approximately 6 inches above the implant (i.e., to 4 feet bgs). A seal consisting of granular bentonite will then be emplaced and hydrated in 6-inch lifts from 1 to 4 feet bgs. The remaining annular space will consist of a 6-inch diameter, traffic-rated, flush mount well box set in concrete. The probe tubing and valve will be coiled within the well box for protection between sampling events. This soil gas probe construction is similar to all soil gas probes previously installed at the Site under the direction of the County of San Joaquin.