SITE INFORMATION AND CORRESPONDENCE

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statrtec Interim Remedial Action Plan Former ARCO Service Station No. 443 July 10, 2008 surface (bgs) did not contain concentrations of gasoline range organics (GRO), benzene, methyl tert-butyl ether (MTBE), or 1,2-dichloroethane (1,2-DCA) above the laboratory method reporting limits (MRL). Grab groundwater samples collected from soil boring SB-18 at 62, 72, and 81 ft bgs contained concentrations of GRD ranging from 240 to 860 micrograms per liter (pg/L) and 1,2-DCA concentrations ranging from 25 to 470 pg/L. The maximum GRO and 1,2-DCA concentrations were reported the sample collected at 62 ft bgs. In November 2005, Cambria, on behalf of Shell, conducted an off-site assessment that included advancing two cone penetrometer test soil borings SB-26 and SB-27 on the site. SB-26 was located over the former eastern dispenser island and SB-27 was advanced approximately 50 ft north of the SB-26 and away from the former fueling facilities. Borings SB-26 and SB-27 were drilled to depths of approximately 91 and 150 ft bgs, respectively. A pore pressure dissipation test was performed in boring SB-27 to determine the approximate depth to groundwater. The test was performed at a depth of approximately 82 ft bgs. The test results indicated the estimated depth to groundwater was 53.5 ft bgs. The data agrees with monitoring well water level data at the Shell site. Soil samples collected from SB-26 at depths ranging from 20 to 70 ft bgs contained GRO with concentrations ranging from 1,600 to 19,000 milligrams per kilogram (mg/kg). Benzene was detected in four soil samples at concentrations ranging from 2.9 to 30 mg/kg. No fuel oxygenates or lead scavengers were detected in any of the soil samples collected from boring SB-26. In addition, one grab groundwater sample was collected from boring SB-26 at 83 ft bgs contained GRO, benzene, and 1,2-DCA at concentrations of 19,000, 1,300, and 57 pg/L, respectively. Soil samples collected from boring SB-27 at ten-foot intervals from ten to 70 ft bgs did not contain concentrations of GRO, benzene, or 1,2-DCA above the laboratory MRL. Grab groundwater samples were collected from SB-27 at depths of 83, 110, 130, and 150 ft bgs. The groundwater sample collected from SB-27 at 83 It bgs had detections of GRO and 1,2-DCA at concentrations of 13 and 430 pg/L, respectively. No benzene, toluene, ethylbenzene, and xylenes (BTEX), or fuel oxygenates were detected in the 83 ft bgs groundwater sample from boring SB-27. GRO, BTEX, fuel oxygenates, and lead scavengers were not detected in the groundwater samples collected from boring SB-27 at 110, 130, and 150 ft bgs. The purpose of the 2005 Cambria investigation was to delineate the northern extent of the Shell plume. Isoconcentration contour maps for GRO and 1,2-DCA created by Cambria indicate two separate plumes of GRO and 1,2-DCA dissolved in groundwater are present one beneath the Shell Station and one beneath the site. From June 12 through July 10, 2007, Stratus Environmental, Inc. of Cameron Park, California (Stratus), field staff observed Woodward Drilling Company of Rio Vista, California, (Woodward) drill and install four shallow on-site monitoring wells MW-1S through MW-4S to approximately 66.5 ft bgs and one deep on-site monitoring well MW-1D to approximately 90 ft bgs using B-61 and BK-81 drill rigs equipped with eight and ten-inch hollow-stem augers, respectively. GRD was detected in MW-1S at concentrations ranging from 6,600 mg/kg to 11,000 mg/kg at depths ranging from 21 to 66.5 ft. Benzene was detected in MW-1 S at concentrations ranging from 1.5 to 47 mg/kg at depths ranging from 41.5 to 66.5 ft. 2 443 IRAP.doc