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Ms Margaret Lagorio, SJCPHS September 28,2004 Dual Phase Extraction Test Report Joe's Truck Plaza,Lathrop,California Page 3 concentrations of 120 µg/L, 440 µg/L, 3 1 µg/L, and 1,800 µg/L, respectively The site was subsequently placed on a quarterly groundwater-moni ton ng program In January 2001, Avalon advanced four soil borings (GP-1 through GP-4) along the northern border of the site, and six soil bonngs (GP-5 through GP-10) on the adjacent property to the north for further lateral characterization of the dissolved plume to the northeast (predominant groundwater flow direction) MTBE was reported in the grab groundwater samples collected from every boring advanced during this round of investigation activity, with the maximum concentration of MTBE reported at 2,600 µg/L In February 2002, Avalon installed four more monitoring wells to further evaluate the vertical and lateral extent of petroleum hydrocarbon impact to groundwater Three of these wells (MWO-1 through MWO-3) were installed in the adjacent property to the north and northeast of the study site The fourth well (MWO-1X) was installed in the vicinity of monitoring well MW-2, but screened in a deeper horizon In September 2003, Stratus prepared and submitted a Site Conceptual Model (SCM) Report to the San Joaquin County Environmental Health Department (SJCEHD) The SCM concluded that additional assessment was required to adequately characterize the petroleum hydrocarbon impact to soil and groundwater During May 2004, Stratus oversaw installation of eight cone penetration test (CPT) bonngs and one on-site groundwater monitoring well (MW-7) to further assess the lateral and vertical extent of petroleum hydrocarbon impact to soil and groundwater, and to facilitate completion of DPE feasibility testing using well MW-7 SITE GEOLOGY AND HYDROGEOLOGY SUMMARY The site geology and hydrogeology information presented below, and the summary of extent of petroleum hydrocarbon impact discussed in subsequent sections, are summarized from Stratus' SCM (dated September 4, 2003) and Subsurface Assessment Report (dated August 11, 2004) • Soil beneath the study area consists primarily of fine-grained material (predominantly silty clay, clayey silt, and clay, with minor amounts of silt, sandy silt, and material classified as "stiff fine grained") to the total depth explored of 100 feet bgs • Three laterally extensive sand strata (identified as Sand A, Sand B, and Sand C) were encountered within this fine-grained unit, along with several thin, laterally restricted sand bodies P Voe s Truck Plaza\Reports\DPE Test'Joes Truck DPE Test Report doc 5 7-RA rug