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Mr. Roger Smith - 2 - 9 September 2021 The 2005 Revised Groundwater Cleanup and Contingency Plan and the 22 February 2006 Central Valley Water Board letter (collectively, the Cleanup Plan) established groundwater cleanup levels, threshold concentrations for the protection of human health for the vapor intrusion pathway, and performance criteria for the current remedy to monitor its effectiveness. Because regulatory vapor intrusion screening levels have been revised/updated over the years and VOCs are present in groundwater near and beneath a portion of the residential area at concentrations that exceed the current San Francisco Bay Regional Water Quality Control Board Environmental Screening Levels (ESLs) for vapor intrusion, Central Valley Water Board staff requested that the most up-to-date vapor intrusion screening levels be used to evaluate whether the current phytoremediation and MNA remedy is protective of human health. VOCs detected in groundwater beneath the Site include 1 ,2-dichloropropane (1,2-DCP), 1,2,3-trichloropropane (1,2,3-TCP), 1,2-dibromo-3-chloropropane (DBCP), and chloroform. During the most recent groundwater monitoring event (February 2021), 1,2-DCP and chloroform were detected at concentrations that exceeded their respective ESLs for vapor intrusion. REVIEW OF WORK PLAN Geosyntec proposes to collect soil vapor samples at two on-Site locations, near monitoring wells MW-5B and MW-713. The highest concentrations of 1,2-DCP and 1,2,3-TCP were detected at these wells during the most recent monitoring event. Soil vapor probes will be installed approximately 10 feet and 15 feet east of MW-513 and MW-713, respectively. The proposed soil vapor probes will be installed at depths of approximately 5 and 10 feet below ground surface at each location. Borings for the soil vapor probes will be advanced using direct push technology. Soil cores will be collected during drilling and will be visually logged and screened for the presence of VOCs using a hand-held photoionization detector. Soil samples will also be collected for analysis of physical properties. The soil vapor probes will be constructed using a 1/2-inch diameter, 6-inch-long stainless-steel screen attached to 1/4-inch diameter Nylaflow®tubing. The probes will be installed and sampled in accordance with the Department of Toxic Substances Control July 2015 Advisory - Active Soil Gas Investigations. Soil vapor sampling will be conducted a minimum of 48 hours following installation of the vapor probes. Prior to collecting soil vapor samples, shut-in testing, leak testing (using a shroud and helium as the tracer gas), and purging will be conducted. Soil gas samples will be collected using laboratory supplied 1-liter SUMMA canisters. Collected soil gas samples will be submitted to a State-of-California certified analytical laboratory under chain-of-custody procedures for analyses of VOCs using EPA Method TO-15 and helium by ASTM International D-1946.