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• S ECOR • -- Work Plan for Monitoring Well Installation July 31, 2007 Former ARCO Service Station No. 4932 Stockton, California Page 4 and BTEX. Of the 10 soil samples collected from the two borings, TPHg was detected in the 50-foot sample from the boring for well DMW-1 at 3.2 mg/kg, and in three samples collected between 27.5 feet and 47.5 feet bgs in the boring for well EW-1 at concentrations ranging from 7.2 mg/kg (EW-1, 40') to 780 mg/kg (EW-1, 275). Benzene was detected in the 50-foot and 69.5-foot samples from the boring for well DMW-1 and all four samples from the boring for well EW-1 at concentrations ranging from 0.0081 mg/kg (DMW-1, 695) to 5.1 mg/kg (EW-1, 275). Toluene was only detected in three samples collected between 27.5 feet and 47.5 feet bgs from the boring for well EW-1 at concentrations ranging from 0.25 mg/kg (EW-1, 475) to 73 mg/kg (EW-1, 275). Ethylbenzene and xylenes were detected in the 50-foot sample from the boring for well DMW-1 and all four samples collected from the boring for well EW-1, at concentrations of ethylbenzene ranging from 0.017 mg/kg (EW-1, 15') to 23 mg/kg (EW-1, 27.5'), and concentrations of xylenes ranging from 0.0098 mg/kg (EW-1, 15') to 120 mg/kg (EW-1, 275). Petroleum hydrocarbons were not detected above laboratory reporting limits in the deepest sample collected from the boring for well DMW-1 at 80 feet bgs. Of the fuel oxygenates analyzed by EPA Method 82608, MTBE was detected in eight of the ten samples collected from the two soil borings. MTBE was detected in four samples collected between 30 feet and 69.5 feet bgs from the boring for well DMW-1 and in all four samples collected between 15 feet and 47.5 feet bgs from the boring for well EW-1, at concentrations ranging from 0.24 mg/kg (EW-1, 475) to 290 mg/kg (EW-1, 275). One sample reported a concentration of TBA at 4.1 mg/kg (DMW-1, 15'). All other fuel oxygenates were not detected above laboratory detection limits.). On April 2 through 4, 2002, a step drawdown pumping test, constant rate drawdown pumping test, and aquifer test were completed. Using a submersible pump in well EW-1. Prior to the start of the test, water levels in pumping well EW-1 and the four observation wells (MW-1, MW- 4, MW-6, and MW-7) were monitored. Well EW-1 was subsequently pumped for a specified period at different pumping rates (steps) while monitoring changes in the water level in the pumping well. Results were as follows: • Capture zones of approximately 31 feet and 48 feet were calculated in the downgradient and cross-gradient directions, respectively. The capture zone was calculated using the Javandel and Tsang method. • A pumping rate of four and six gallons per minute was used for the constant rate drawdown pumping test. The well installation and aquifer pumping test is documented in SECOR's report titled, Site Assessment and Aquifer Testing Results, dated June 7, 2002. On June 10 through 13, 2002, two monitoring wells (MW-10 and MW-11), two deep monitoring wells (DMW-2 and DMW-3), and one extraction well (EW-2) were installed at the site by West Hazmat. Monitoring wells MW-10 and MW-11 were completed at 40 feet bgs and were installed with a 2-inch polyvinyl chloride (PVC) well casing screened from 20 to 40 feet bgs. These wells are located off-site and to the north - MW-10 is in an alley off EI Dorado Street and MW-11 is on the corner of EI Dorado Street and Sperry Avenue. Deep monitoring wells DMW-2 and DMW-3 were completed at 82 feet bgs and were installed with a 2-inch PVC well casing screened from 77 to 82 feet bgs. Extraction well EW-2 was completed at 43 feet bgs and was installed with a 6-inch PVC well casing with a stainless steel wire-wrap screen extending from 20 to 40 feet bgs. 4932 WP for MW Installation 2007.doc SECOR International Incorporated