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18 August 2023 AdvancedGeo Project No. 17-4073 An Employee-Owned Company Page 2 of 7 � 1.0. INTRODUCTION An unauthorized release of solvents was discovered at the site during the removal of eight underground storage tanks (USTs) in January 1991. Impacted soil that was excavated during the UST removals was subsequently utilized as backfill material. Between 1991 and 2003, numerous monitoring wells (MW-1 through MW-10 and MW-3R)were installed at the site, however, the total extent was never fully defined. In August 2006, two additional USTs were discovered but were not removed until May 2015. During the removal, San Joaquin County Environmental Health (SJCEHD) officials noted that two of the monitoring wells at the site (MW-2 and MW-7) were damaged and creating potential conduits for contaminant migration. MW-2 and MW-7 were destroyed in April 2021 and soil samples were collected from the site to determine the extent of the existing contamination. In December 2021, AGI prepared a Low-Threat Closure Evaluation Report to evaluate the site conditions against the criteria established in the State Water Resources Control Board (SWRCB) Resolution 2012-0016, Low-Threat Underground Storage Tank Case Closure Policy (LTCP). Based on the low-threat closure evaluation, AGI identified two impediments to closure and recommended a work plan be prepared to propose a scope of work to define petroleum hydrocarbon impact to the south and evaluate the potential for vapor intrusion. The CVRWQCB concurred with the recommendation in the January 2022 letter. Site background information was provided in the Low-Threat Closure Evaluation Report dated 15 December 2021. Between November 2022 and February 2023, two grab groundwater samples were collected upgradient of the site and four soil vapor wells (SV1 — SV4) were installed and sampled. Except for low concentrations of chloroform, total petroleum hydrocarbons as Stoddard Solvent (TPH-ss) and volatile organic compounds (VOCs) were not detected above laboratory reporting limits in the groundwater samples analyzed (Table 1). Chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE) were detected in SV2 and SV3 exceeding their respective cancer risk and non-cancer hazard Environmental Screening Levels (ESLs) for a commercial property. Benzene, ethylbenzene, and naphthalene concentrations detected in soil vapor were below the soil gas criteria established in the Low-Threat Closure Policy (Table 2). Based on the results of the investigation, the CVRWQCB directed preparation of a work plan to delineate VOCs in soil and soil-vapor and collect indoor air/crawlspace samples from the residence at 1426 North Van Buren Street, Stockton, California. 2.0. SCOPE OF WORK AND PROCEDURES To delineate the extent of VOC impact to soil and soil vapor and collect indoor air samples from the south adjacent residence, AGI proposes the following scope of work: