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SITE CLOSURE REQUEST REPORT Former Exxon Service Station No.7-3330 3128 West Benjamin Holt Drive ` Stockton,California Delta Project No. D093-810 Page 12 5.0 MONITORING WELL MW-5 EVALUATION 5.1 Overpurging Monitoring Well MW-5 Monitoring well MW-5 was overpurged(approximately 200 gallons)prior to performing quarterly ground water sampling on June 1, 1995. A decrease in benzene and TPH as gasoline concentrations was observed compared to the previous quarterly sampling (Table 3). A ground water pumping test was performed on monitoring well MW-5 on June 29 and 30, 1995, to evaluate the effectiveness of pumping to reduce the concentrations of petroleum hydrocarbons in ground water in the vicinity of MW-5. 5.2 Pumping-Test On June 29 and 30, 1995, ground water was pumped from MW-5 at a rate of four gallons per minute. Approximately 5,800 gallons of ground water were removed in a 24-hour period. Ground water samples were collected approximately 30 minutes, 12.5 hours, and 24 hours into the test. Ground water sample analytical results reported benzene concentrations increased from G 0.5 Mcg/L to 14µg/L over the duration • of the test, and TPH as gasoline remained below the laboratory's detection limit (Table 3). The results of the pumping test are presented in a letter report prepared by Delta dated July 26, 1995. Monitoring well MW-5 was again overpurged(250 gallons) on July 27, 1995, to further evaluate ground water pumping as a remediation technique. Benzene concentrations increased slightly following purging (0.62 lgIL prior to purging, 1.2 µg/L following purging) but remained below 2.0 /cg/L. 5.3 Soil Vap2r Extraction SVE On Monitoring Well MW-5 On November 14, 1995, Delta conducted a soil vapor extraction pilot test on monitoring well MW-5. The purpose of this test was to assess the viability of vapor extraction on petroleum hydrocarbons in soil and ground water in the area of MW-5. The SVE pilot test was performed on monitoring well MW-5 for approximately four hours. The SVE test consisted of inducing a vacuum on monitoring well MW-5 using a portable vacuum blower. The blower exhaust was routed through two 200-pound vapor phase granular activated carbon columns in series prior to atmospheric discharge. An initial soil vapor extraction sample was collected during the RPTo01.810