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SITE CLOSURE REQUEST REPORT Former Exxon Service Station No.7-3330 1 . 3128 West Benjamin Holt Drive Stockton,California Delta Project No.D093-810 Page 10 4.0 DISTRIBUTION OF PETROLEUM HYDROCARBONS IN GROUND WATER 4.1 Total Petroleum Hydrocarbons as Gasoline The historical presence of TPH as gasoline in ground water beneath the site has been adequately assessed in each direction from the source area based on the existing ground water monitoring well network. Using the quarterly ground water monitoring results from March 26, 1992(bench mark), it was estimated that approximately 1.8 pounds (0.30 gallons) of dissolved TPH as gasoline existed in ground water. The calculation for the estimated dissolved TPH as gasoline in ground water (March 26, 1992) is included in Appendix D. Based on the laboratory's limits of detection for TPH as gasoline, it is estimated that less than 0.047 pounds (0.008 gallons) of dissolved gasoline could remain in ground water. This volume is based on the laboratory's limits of detection, however, it should be noted that concentrations of TPH as gasoline and benzene have been non-detect for four consecutive quarters and it is conceivable that no concentrations of hydrocarbons may remain in ground water. The calculations for the estimated TPH remaining in ground water is included in Appendix E. Ground water calculations indicate that since the • March 26, 1992, quarterly monitoring event, there has been a greater than 97.5 percent reduction in TPH quantified as gasoline in ground water. The initial estimated size of the hydrocarbon plume in ground water is shown in Figure 13 and the estimated volume of dissolved TPH as gasoline using the March 26, 1992, data is shown in Figure 14. Calculations for the reduction are included in Appendix F. The TPH as gasoline and BTEX concentrations have been below the laboratory's detection limits in all four ground water monitoring wells(MW-3 through MW-6) for four consecutive quarters. Ground water samples collected from monitoring well MW-5 indicate concentrations of TPH as gasoline to have been below the laboratory's detection limit for the last two years with the exception of when the ground water table has been seasonably high. This highest concentration was 110 micrograms per liter (pg/L) which was in April 1995. It is interpreted that only a localized area was impacted (in the vicinity of MW-5) since analytical results from the furthest downgradient well (MW-6)continues to show concentrations of petroleum hydrocarbons to be below the laboratory's detection limits. Since the highest recorded concentrations in April 1995, monitoring well MW-5 has been overpurged, and a soil vapor extraction test has been conducted on the RPT001.810