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Former Thrifty Co.Station No.172 Workplan for Additional Soil and Groundwater Assessment May 8,2017 WORKPLAN FOR A SOIL VAPOR INTRUSION STUDY 1.0 SITE BACKGROUND On behalf of Phillips 66, ATC Group Services LLC (ATC) has prepared this workplan for Former Thrifty Oil Co. Station No. 172, located at 7647 Pacific Avenue in Stockton, California (Figures 1 and 2). In correspondence dated March 9, 2017, the Central Valley Regional Water Quality Control Board (CVRWQCB) directed additional site activities including submitting a workplan to evaluate groundwater quality in the shallow and deep aquifer in the vicinity of monitoring well MW-23 and soil in the vicinity of the former waste oil tank (Appendix A). 2.0 SITE LOCATION The site is a former retail gasoline station and convenience store located on the southwest corner of Pacific Avenue and Rivara Road in Stockton, California. The site has been redeveloped into a 7-Eleven-branded convenience store. A site plan based on historical figures provided by previous consultants, recent well surveys, and redevelopment information provided by Torres Architects, Inc., is presented in Figure 2. It should be noted that some discrepancies regarding un-surveyed boring locations on historical figures used to create the site plan remain unresolved. 2.1 Previous Investigations From May 23 through June 6, 1991, SSB Environmental Consultants, Inc. (SSB) installed four groundwater monitoring wells (MW-1 through MW-4). Two monitoring wells were located adjacent to the underground storage tanks (USTs), and two monitoring wells were installed near the pump islands. The monitoring wells were installed to depths of 76 to 80 feet (ft), and groundwater was encountered at 58 ft below ground surface (bgs). Laboratory analysis of a soil sample collected from well MW-2 at a depth of 20 ft indicated the highest concentrations of total petroleum hydrocarbons (TPH, also reported as total purgeable petroleum hydrocarbons TPPH), and benzene at 380 milligrams per kilogram (mg/kg) and 33 mg/kg, respectively. Laboratory analysis of groundwater samples collected ranged from 36.4 micrograms per liter (pg/L) to 5,270 pg/L benzene in wells MW-4 and MW-2, respectively. On April 22, 1994, Thrifty installed three vadose zone wells (VWA, VWB, and VWC) to depths of 35 ft, 48 ft, and 45 ft bgs, respectively. Thrifty also installed two groundwater monitoring wells (MW-7 and MW-8) to 56 ft bgs and 58 ft bgs, respectively. Groundwater was encountered at approximately 50 ft bgs. Laboratory analysis of soil samples detected TPH concentrations up to 21,000 mg/kg, and benzene concentrations up to 36 mg/kg. Laboratory analysis of groundwater samples detected 5,500 pg/L TPH and 830 pg/L benzene in MW-7, and 130,000 pg/L TPH and 7,200 pg/L benzene in MW-8. Thrifty investigated the OPW vault area near MW-8. The vault was approximately 10 inches in diameter and covered what appeared to be a sewer line clean- 1