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Appendix A - Historical. Background PACIFIC MINI-MART 4511 Pacific Avenue, Stockton, California • Based on a transmissivity value of 205 ft2/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 concentrations 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 MW15 and MW16, these concentrations maybe more reflective of impacted ground water 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 I 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 1 situated in the median ofPacific Avenue High concentrations of MTBE exist in well MW-11,lower 1 i or non-detectable concentrations in the down-gradient and cross gradient wells defined the dissolved F MTBE plume toward the north and west This may Indicate 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 16 December 2002, approximately 1,315,454 gallons of hydrocarbon-impacted ground have been extracted and treated for disposal IOn 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 MW 18 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 At that time,the dissolved MTBE plume has been defined east of well MW-16 bywells MW-17118 • Advanced GeoEnvironmental Inc J