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Appendix A - Historical Background PACIFIC MINI-MART 4511 Pacific Avenue, Stockton, California ' installation of the additional ground water monitoring wells A vapor extraction test was conducted in December 1993 Since that time, ground water has risen from approximately 55 feet bsg to as high as 30 feet bsg,reducing the vertical extent of the vadose zone open to the screened interval of the vapor extraction well The rise of ground water elevation has also eliminated most of the coarse-grained soil from the vadose zone, leaving predominantly fine-grained soil in contact with the reduced open screened interval of the vapor extraction well These changes can potentially drastically reduce the zone of influence of the vapor extraction well(s) ' and reduce the concentrations of hydrocarbon vapor recovered by the extraction process On 29 and 30 April and 01 May 1998, seven probe borings (B 13 through B 19) were advanced off- site of the subject property to collect soil and grab ground water samples as part of on-going assessment of the release of fuel hydrocarbons from the former UST facility TPH-g, TPH-d and BTEX were not detected in any of the soil samples analyzed,however, MTBE was detected at 180 micrograms per kilogram (pg/kg) in the soil sample from boring B16 at a depth of 40 feet bsg TPH-g was not detected in any of the grab water samples collected,but TPH-d was detected in seven of the eight grab water samples at concentrations as high as 1,200 µg/l Various BTEX compounds iwere detected in the grab water samples from borings B13, B14 and B17 at concentrations as high as 1 4 micrograms per liter (µg/l), benzene was detected only in the sample collected from boring B 13 at a depth of 60 feet bsg, at a concentration of 0 61 µg/l In addition, MTBE was detected in seven of the eight grab ground water samples collected at a maximum concentration of 1,400 µg/l in sample B 16-H20 TBA was detected in sample B 13-H2OB at a concentration of 16 dig/l On 20 March 2001, ground water extraction well EW-1 was installed at the site for performance of a ground water pumping test At the regulatory directive of the EHD a twelve-hour aquifer pump test was performed on 27 March 2001 to assess the general ground water aquifer characteristics and to assess aquifer response to ground water extraction at a constant pumping rate utilizing well EW-1 for ground water extraction and wells MW-1, MW-2, MW-4, MW-5 and MW-6 and vapor well VW-I as observation wells Three ground water samples were collected from the extracted ground water and purged ground water was containerized in an on-site 6,500-gallon poly tank The implications from the March 2001 aquifer ground water pumping test are • Ground water draw-down of at least 11 15 feet was observed in extraction well EW-1 following 700 minutes of continuous pumping (5 GPM), ground water draw-down was observed in well MW-5 (located 15 feet to the east ) at I 1 feet,well MW-4 (located 60 feet toward the southeast) at 0 1 feet, well MW-6 (located 50 feet toward the north-northwest) at 0 12 feet and in well vapor well VW-1 (located 60 feet toward the west) at 0 06 feet • Utilizing a semi-logarithmic plot of ground water draw-down vs observation well distance • to pumping well, a best-fit line can be utilized to project a radius of influence of approximately 65 feet at 5 GPM Utilizing the Distance-Drawdown Method,a transmissivity Advanced GeoEnvironmental,Inc