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Former Thrifty Co.Station No.172 Workplan for Additional Soil and Groundwater Assessment May 8,2017 out. A 24-inch diameter hole was excavated adjacent to the OPW vault and a cast iron pipe that appeared to be a sewer line was encountered at approximately 3 feet bgs. The pipe was connected to the OPW pipe opening, but was not conclusively identified as a sewer line. In July 1995, Thrifty Oil Company installed three off-site groundwater monitoring wells (MW-5, MW-6, and MW-9) to approximately 60 feet bgs. Petroleum hydrocarbons were not detected in the soil samples collected from MW-5, MW-6, and MW-9. Dissolved-phase hydrocarbons were detected in groundwater at maximum concentrations of 1,000 pg/L TPH (MW-9), 3.4 pg/L benzene (MW-9), 34 pg/L ethylbenzene (MW-9), and 2.0 pg/L xylenes (MW-9). In July 1995, JDR Environmental (JDR) properly destroyed three vadose wells (VWA, VWB, and VWC) and three groundwater monitoring wells (MW-1, MW-2, and MW-8) as required by the SJCEHD prior to station remodeling activities. In July 1995, five single-walled, steel USTs were removed from the site and disposed of by Erickson of Richmond, California. Approximately 1,500 cubic yards of soil were removed from the UST excavation and transported to Forward Incorporated, of Stockton, California, for disposal. Ten soil samples (Tank Pit-1 through Tank Pit-9 and Tank-E-9') were collected from beneath the five USTs after excavation, and seven soil samples (PL-1 through PL-7) were collected at depths of 2 to 4 ft from beneath the product lines. TPH and benzene were detected in soil samples collected from beneath the USTs at concentrations up to 12,000 mg/kg TPH and 33 mg/kg benzene. Soil was excavated to approximately 15 feet bgs. In May 1996, AquaGeo Environmental Consulting (AquaGeo) installed six wells (VW-2, VW-4, VW-5, VW-6, MW-10/VW-1 and MW11/VW-3) to depths ranging from 16.5 to 58.5 ft bgs. Monitoring wells MW-10NW-1 and MW-11NW-3 were installed as nested wells. Well VW-2 was completed as a dual-completion vadose zone monitoring well. Wells VW-4, VW-5, and VW-6 were completed as triple-completion (A,B,C) vadose zone monitoring wells. Groundwater was encountered during drilling activities at depths of approximately 44 to 50 feet bgs. TPH, benzene, and methyl tertiary butyl ether (MTBE) were detected at maximum concentrations of 6,600 mg/kg (VW-6-25), 69 mg/kg (MW-11-40), and 670 mg/kg (VW-6-40), respectively. Groundwater samples collected from wells MW-10 and MW-11 during the third and fourth quarters of 1996 indicated that TPH, benzene, and MTBE were detected at maximum concentrations of 370,000 pg/L (MW-11), 33,000 pg/L (MW-11), and 880 pg/L (MW-11), respectively. In March 1999, The Park Corporation (Park) installed one groundwater monitoring well (MW-12) to a depth of 73 feet bgs. Groundwater was encountered at a depth of approximately 33 ft bgs. Laboratory analysis of soil samples detected TPH concentrations from 1.4 mg/kg to 6,400 mg/kg, benzene concentrations from 0.26 mg/kg to 100 mg/kg, and MTBE concentrations up to 35 mg/kg (EPA Method 8020). The presence of MTBE in the soil samples was confirmed at concentrations ranging from ND to 1.1 mg/kg (EPA Method 8260). The highest concentrations of aromatic volatiles in soil samples were detected in the 50 foot bgs sample. Laboratory analysis of groundwater samples detected 21,000 pg/L TPH, 3,200 pg/L benzene, and 150 pg/L MTBE (EPA Method 8020). The presence of MTBE was confirmed at a concentration of 120 pg/L (EPA Method 8260). In the surrounding groundwater monitoring wells, TPH was detected at concentrations up to 760 pg/L, benzene was detected at concentrations up to 2.1 pg/L, and MTBE was detected at concentrations up to 540 pg/L (EPA Method 8020). The presence of MTBE was confirmed at concentrations up to 490 pg/L (EPA Method 8260). Monitoring well MW-7 was redeveloped during assessment activities to enhance the well's productivity. A 12- 2