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Attachment A Former Texaco Service Station Chevron Site ID 309075 3978 South Highway 99 Stockton, California In June 2008, ARCADIS supervised the installation of four groundwater monitoring wells (MW-2, MW-3, MW-4 and MW-5) and one soil boring (63) intended to evaluate the potential extent of petroleum hydrocarbon impacts to groundwater near the former UST complex. Only the soil samples collected from MW-3 and B3, both at 40 feet bgs, contained concentrations of petroleum constituents above their respective MDLs. Methanol and ethanol were detected in the soil sample collected from MW-3 at 40 feet bgs at concentrations of 0.25 and 0.76 mg/kg, respectively. Methanol was detected in the soil sample collected from B3 at 400 feet bgs at a concentration of 0.20 mg/kg. Groundwater samples were collected from the newly installed monitoring wells and the existing monitoring well, MW-1. Total petroleum hydrocarbons as diesel (TPH-DRO) were detected in the groundwater sample collected each monitoring well, at concentrations ranging from 280 to 520 pg/L. Given the distribution of similar concentrations across the site, TPH-DRO may be a regional issue in this area of Stockton (ARCADIS 2008). The groundwater sample collected MW-1 was the only sample to contain concentrations of other petroleum hydrocarbon constituents above their respective MDLs. In January 2009, ARCADIS supervised the advancement of one soil boring (SB-2) intended to evaluate the vertical extent of petroleum hydrocarbon impacts to soil and groundwater near groundwater monitoring well MW-1. The soil sample collected at 135 feet bgs contained a total petroleum hydrocarbon as diesel (TPH-DRO) concentration of 5.7 mg/kg and the soil sample collected from 100 feet bgs contained toluene and total xylenes concentrations of 0.001 and 0.002 mg/kg, respectively. Three soil samples, collected at 60, 70 and 80 feet bgs, contained 1,2-DCA concentrations of 0.005, 0.060 and 0.18 mg/kg, respectively. The soil sample collected at 60 feet bgs also contained ethylbenzene, total xylenes and methanol concentrations of 0.013, 0.074 and 0.40 mg/kg, respectively, and the soil sample collected at 70 feet bgs also contained a tertiary butyl alcohol (TBA) concentration of 0.028 mg/kg. The grab groundwater sample from SB-2 was collected from a screened interval of 115 to 120 feet bgs and contained TPH-DRO, total petroleum hydrocarbons as motor oil (TPH-MO), benzene, total xylenes and TBA concentrations of 2,700, 160, 4, 1 and 3 pg/L, respectively. In November 2009, ARCADIS supervised the advancement of two monitoring wells (MW-6 and MW-6D) intended to evaluate the down gradient extent of petroleum hydrocarbon impacts to soil and groundwater onsite. Twelve soil samples were collected from MW-61D at depths ranging from 10 to 120 feet bgs. Petroleum hydrocarbon concentrations were not detected above their ,r respective laboratory MDLs in any of the soil samples collected. A grab groundwater sample was collected from MW-6, at a screened interval of 55 to 70 feet bgs. The groundwater sample collected from MW-6 contained TPH-DRO and TPH-MO with silica gel clean-up concentrations of 62 and 86 pg/L, respectively. A grab groundwater sample was also collected from MW-61D, at a screened interval of 109 to 119. The groundwater sample collected from MW-61D contained TPH-DRO, TPH-DRO with silica gel clean-up, TPH-MO, TPH-MO with silica gel clean-up, ethylbenzene, toluene and total xylenes concentrations of 170, 80, 350, 230, 0.7, 2 and 2 pg/L, respectively. 3