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SITE CLOSURE REQUEST REPORT Farmer Exxon Service Station No.7-3330 3128 West Benjamin Holt Drive Stockton,California Delta Project No.D093-810 Page 11 well to address the locally impacted soil and ground water. The April 1996 quarterly monitoring event was conducted during the highest recorded ground water measurements for the subject site to date. Analytical results from ground water samples collected from each of the monitoring wells were below the laboratory's detection limits for all analytes. The April 1996 quarterly monitoring results, in combination with three additional consecutive quarters of laboratory analytical results below the laboratory's detection limit, indicate that concentrations of hydrocarbons in soil and ground water in the vicinity of monitoring well MW-5 have been reduced through overpurging, soil vapor extraction, and naturally occurring bioremediation to non-detectable levels. At the request of San Joaquin County, methyl tertiary butyl ether (MTBE) was analyzed in ground water samples collected from each well on January 26, and April 15, 1996. Analytical results reported MTBE was only present in MW-5 at a concentration of 12,ug/L during the January sampling event. All samples were below laboratory detection during the April 1996 sampling event. 4.2 Benzene Dissolved benzene in ground water has been adequately assessed in all directions from the source area based on the existing ground water monitoring well network data. Using the quarterly ground water monitoring results from March 26, 1992 (bench mark), it was estimated that approximately 0.50 pounds (0.07 gallons)of dissolved benzene existed in ground water. The calculations for the estimated dissolved benzene in ground water for March 26, 1992, is included in Appendix D. The estimated volume of dissolved benzene using the March 26, 1992, data is shown in Figure 15. Based on the laboratory's limits of detection for benzene, it is estimated that less than 0.0005 pounds (0.00007 gallons) of dissolved benzene could remain in ground water. Similarly, it should be noted that there have been four consecutive quarters of non-detectable results for benzene from all four wells and it is conceivable that no benzene may remain in ground water. The calculations for benzene potentially remaining in ground water is included in Appendix E. Ground water calculations indicate since the March 26, 1992, quarterly monitoring event, there has been a greater than 99.7 percent reduction in dissolved benzene concentrations in ground water. Percent reduction of dissolved benzene calculations are included in . Appendix F. RPT001.810