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Appendix A - Historical Background PACIFIC MINI-MART 4511 Pacific Avenue, Stockton, California was calculated at 205 ftz/day(equivalent to 1,536 gallons per day/ft) and a storativity value was calculated at 0 054 • Based on a transmissivity value of 205 W/day and the extraction well screen thickness equal to 25 ft (screen interval at EW-1), a hydraulic conductivity value was calculated at 61 44 ft/day, or 0 022 cm/s, a hydraulic conductivity of 0 022 cm/s is consistent with a screened interval located within a well sorted sand to clean sand • Laboratory analysis of extracted water during the pumping test detected elevated Iconcentrations of TPH-g, BTEX and MTBE On 05 and 06 June 2001 four ground water monitoring wells were installed off-site as part of the investigation Borings MW13 through MW16 were completed as ground water monitoring wells MW-13 through MW-16 MTBE was detected in the soil samples collected from thirty feet bsg from borings MW 15 and MW 16, these concentrations maybe more reflective of impacted groundwater in the pore spaces of the soil than of adsorbed MTBE The highest concentration of dissolved hydrocarbons quantified as TPH-g,TPH-d and overall BTEX compounds occurs in the areas east and northeast of the dispenser islands, on-site The highest concentration of MTBE detected in the ground water occurs in monitoring well MW-11, which is ' situated in the median of Pacific Avenue Lower or non-detectable concentrations in the down- gradient and cross gradient wells define the dissolved MTBE plume toward the north and west This suggests that the dissolved MTBE plume is detaching from the TPH/BTEX hydrocarbon plume In September and October 2001, a low-flow, ground water extraction and treatment system was installed at the site The system utilizes the newly installed ground water extraction well on site and ' a small volume treatment system As of 14 December 2004, over two-million gallons of hydrocarbon-impacted ground water have been extracted and treated for disposal ' On 04 and 05 February 2003,two additional ground water monitoring wells were installed off-site as part of the investigation Boring MW 17 was completed as groundwater monitoring well MW-17 using two-inch diameter PVC casing with 0 020-inch slotted screen installed from approximately 25 feet to 45 feet bsg and boring MWI8 was completed as ground water monitoring well MW-18 using two-inch diameter PVC casing with 0 020-inch slotted screen installed from approximately 60 feet to 65 feet bsg and with blank casing extended to the surface On 15 and 16 March 2004, one additional ground water extraction well and three soil vapor extraction wells(EW-2, VW-2 through VW-4)were installed at the site The extraction well EW-2 was completed using 6-inch diameter PVC casing, with 0 03-inch slotted screen installed from 35 to 55 feet bsg, while well VW-2 through VW-4 were completed as vapor extraction wells using 2-inch diameter PVC casing, with 0 02-inch slotted screen installed from 20 to 35 feet bsg Advanced GeoEnvironmental,Inc