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APPENDIX A PROJECT BACKGROUND 2987 : Four ground water monitoring wells (MW-1, MW-2 , MW-3 , and MW-4) were installed to an approximate depth of 50 fbg in the immediate vicinity of four 5000-gallon underground storage tanks by Woodward-Clyde Consultants (WCC) . WCC also performed ground water monitoring and sampling of these wells. Ground water was encountered at 40 fbg. Analysis of the ground water samples detected total petroleum hydrocarbons as gasoline (TPH-G) ranging from 154 , 000 parts per billion (ppb) in MW--2 to 231, 000 ppb in MW-3 . These concentrations are indicative of emulsified free product. June 1988 : WCC subcontracted Pacific (NET) to perform a soil gas survey of the underground storage tank area. NET collected 26 soil gas samples and analyzed for TPH-G, and benzene, toluene, and xylenes (BTX) . Concentrations of TPH-G and/or BTX were detected in the majority of the samples collected at depths ranging from 7 to 33 fbg. The highest concentration of TPH-G (300 ppm) was detected in the area of the underground storage tanks at a sample depth of 7 fbg. . February 1989 : Four underground storage tanks, and the waste oil tank were removed by Placer Tractor Services. Eight soil samples were taken from the tank removal excavation and analyzed for total petroleum hydrocarbons as diesel (TPH-D) , TPH-G, BTX and ethylbenzene (BTEX) , organic lead, and ethylene dibromide. TPH-G was detected in the soil samples taken from the tank cavity with concentrations ranging from 0. 023 ppm in the west wall at 14 . 5 fbg to 3 . 7 ppm in the southeast corner at approximately 14 fbg. TPH-D was detected in two samples taken from the north end of the tank cavity at concentrations of 0. 1 ppm in the northwest corner at 15 fbg and 1 . 7 ppm in the northeast corner at 14 . 5 fbg. Organic lead and ethylene dibromide were not detected above reported detection limits in all soil samples. Under the direction of the California Regional Water Quality Control Board, Central Valley Region (RWQCB) , a site investigation was completed by WCC. The ob3ectives of the site investigation were to define the extent of subsurface contamination and identify remedial alternatives. March and April 1990: Monitoring Wells MW-1 through MW-4 were dry and five soil borings were drilled and sampled. TPH-G dnd BTEX were detected in soil samples collected at 31 and 55 fbg from MW--7 at concentrations of up to 20 ppm. The five soil borings were converted into ground water Monitoring Wells MW-5 through MW-9 . Water samples were collected . from MW-6 through MW-9 (MW-5 was dry) and analyzed for total Petroleum hydrocarbons and BTEX.