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' SITE BACKGROUND ' The Comfort Air facility is a heating and air conditioning sates and repair business located at 1607 Turnpike Road, Stockton, California, in an area of low topographic relief in southwestern Stockton I UNDERGROUND STORAGE TANK REMOVAL ' One 550-gallon gasoline (UST) was removed from the site in January 1989 Following tank removal, soil samples were collected beneath the UST and submitted to a laboratory for analysis ' for petroleum hydrocarbons Total petroleum hydrocarbons as gasoline (TPH-g) was reported in soil samples at concentrations as high as 5,800 milligrams per kilogram (mg/kg) 1 li PREVIOUS SITE ASSESSMENT ' In February 1992, three groundwater monitoring wells (MW-1, MW-2 and MW-3) were installed near the former UST area by RESNA Industries of Escalon, California Evidence of petroleum ' hydrocarbon-impacted soil was detected in all borings at depths between 20 and 35 feet below surface grade (bsg) Initial ground water samples contained TPH-g at concentrations as high as 300,000 micrograms per liter (µg/1) and volatile aromatics (benzene, toluene, ethylbenzene and ' xylenes, BTEX) at concentrations as lvgh as 17,000 µg/1 A fourth groundwater monitoring well was installed northeast of the former UST area in.October ' 1993 Evidence of petroleum hydrocarbon soil contamination was detected in soil samples collected at 40 and 45 feet bsg Initial ground water samples detected TPH-g at concentrations as high as 6,800 µg/1 and BTEX at concentrations as high as 1,400 µg/1 ' In May 1994, a fifth ground water monitoring well (MW-5) was advanced near the northeast corner of the site Petroleum hydrocarbons were not reported in soil samples submitted for ' analysis TPH-g and BTEX were reported in initial ground water samples collected from MW-5 at concentrations up to 93 µg/1 and 13 µg/1, respectively ' Three additional ground water momtonng wells were installed at the site on 25 and 26 October 2000 Two of the wells (MW-7 and MW-8) were installed in first encountered ground water and ' one well (MW-6)was advanced to a depth of 65 feet bsg to monitor deeper ground water Soil samples collected from the pilot bonng for MW-6 contained concentrations of TPH-g as high as 4,100 mg/kg, benzene as high as 17 mg/kg, ethyldibromide (EDB) as high as 460 mg/kg and 1,2- ' dichloroethane (1,2-DCA) as lugh as 410 mg/kg Contamination was detected in the deepest recovered sample from MW-6 at a depth of 65 feet bsg One soil sample collected from the pilot boring for MW-8 contained 1,2-DCA at a concentration of 15 mg/kg Soil samples from the pilot ' boring for MW-7 did not contain detectable concentrations of analytes of concern TPH-g,BTEX and 1,2-DCA were detected in the initial ground water samples collected from MW-6, MW-7 and MW-8 Details of the procedures and results of the monitoring well installation were