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• WORK PLAN FOR OFF-SITE ASSESSMENT and FEASIBILITY TESTING Valley Motors 800 East Main Street Stockton, CA 1.0 INTRODUCTION Valley Motors is a wholesale used-automobile dealership located at 800 East Main Street in Stockton, California (Figure 1) Prior to 1987, the property was owned by a taxi company , which operated two underground gasoline tanks to fuel its taxi fleet Valley Motors removed the tanks in 1990, and soil contamination was discovered Assessment of the contamination began in 1994 and groundwater contamination was discovered in 1996 The extent of soil and groundwater contamination beneath the Valley Motors property was delineated by mid-1998, and Upgradient Environmental submitted in early 1999 a plan to excavate the contaminated soil Howe ger, partly because the assessment indicated that groundwater contamination extends an unknown di,tante off- site to the west and north, this plan was not approved by the lead regulatory agency for the investigation Instead, in written communication dated March 9, 1999, San Joaquin County Public Health Services, Environmental Health Division directed Valley Motors to submit a work plan to complete the delineation of groundwater contamination Subsequently, in the First Quarter 1999 Report, Upgradient recommended that the work plan also address the completion of the remaining tasks necessary for the completion of a Problem Assessment Report These include an aquifer test, • various feasbility tests, and a sensitive receptor survey 2.0 BACKGROUND William Hunter & Associates drilled 5 borings and collected 40 soil samples for analys s in 1994 (Figure 2) Neither gasoline nor diesel was detected in any of these, but Stoddard Solvent was reported at concentrations ranging from 10 to 183 mg/kg and ethylbenzene, toluene, and`or xylene were detected at concentrations ranging from 0 04 to 120 mg/kg between the depths of 14 and 25 feet in S13-1, SB-2, and S13-3 Upgradient Environmental subsequently drilled VM-1 in 1995 and found that gasoline, diesel, and xylene were present at a depth of 25 feet and found that gasoline, diesel, and all of the volatile aromatic compounds (BTEX) were present in the ground water The chlorinated hydrocarbons 1,2-dichloroethane, trans-l,2-dichloroethene, cis-l,2-dichloroethene, trichloroethene, and tetrachloroethene were also present at concentrations well above the Maximum Contaminant Levels allowed in California These results were confirmed and extended vilien VM- 2, VM-3, and HP-2 were drilled in 1996 and HP-4 and VM-4 were drilled in 1998 Ga'oline, but not diesel, was detected from 15 to 20 feet in VM-2 (387 to 440 mg/kg) and at 32 feet in HP-4 (1 8 mg/kg) The BTEX compounds were detected in both of these borings as well in the dep h interval between 15 and 35 feet Gasoline and BTEX, and several chlorinated hydrocarbons, have been detected in several groundwater samples from VM-1 and VM-2, and MTBE has been r.-ported at concentrations ranging from 1 1 to 43 0 pg/1 in VM-1 and VM-4 (higher concentrations gave been reported using EPA method 8020 in VM-1 and VM-2, but these concentrations were not ,,-onfirmed with method 8240)