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IAPPENDIX A - HISTORICAL BACKGROUND • COMFORT AIR, INC. 1607 Turnpike Road, Stockton, California SITE BACKGROUND IThe Comfort Air facility is a heating and air conditioning sales and repair business located at 1607 Turnpike Road, Stockton, California,in an area of low topographic relief in southwestern Stockton UNDERGROUND STORAGE TANK REMOVAL ' One 550-gall0n 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) ' 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 (gg/l) and volatile aromatics (benzene, toluene, ethylbenzene and ' xylenes, BTEX) at concentrations as high 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 ug/1 and BTEX at concentrations as high as 1,400 gg/l 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 lig/l and 1 3 pg/l, respectively ' On 25 and 26 October 2000, three additional ground water monitoring wells were installed at the site 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 boring for MW-6 contained concentrations ofTPH-g as high as 4,100 mg/kg, benzene as high as 17 mg/kg, 1,2-dibromoethane (EDB) as high as 460 mg/kg and 1,2- ' dichloroethane (1,2-DCA) as high as 410 mg/kg Advanced GeoEnvironmental,Inc 1 1