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Mr Kenneth Earnest Ultramar Inc }n May 10, 1994 t Page 3 The inferred water table contours illustrated in Figure 6 indicate that ground water was flowing toward the south with a hydraulic gradient of approximately 0 003 foot per foot ' Discussion of Mindirip Laboratory analyses indicate that one or more soil samples from each of the borings contained ' detectable concentrations of petroleum hydrocarbons The highest concentration of TPH as gasoline (10,000 milligrams per kilogram (mg/kg)) was present in the soil sample collected at a depth of 40 feet from the boring for monitoring well MW-28 The highest concentration of TPH as diesel ' (26,000 mg/kg) was present in the soil sample collected at a depth of 35 feet from the boring for vapor extraction well VEW-2 The approximate lateral extent of vadose soil containing TPH as gasoline and TPH as diesel in concentrations greater than 100 mg/kg is shown in Figures 7 and 8, respectively The approximate vertical extent of vadose soil containing TPH as gasoline along geologic cross-sections A-A', B-B', and GC' is shown in Figures 9 and 10, and the approximate vertical extent of vadose soil containing TPH as diesel along geologic cross-sections A-A', B-B', and C-C' is shown in Figures 11 and 12 As indicated in Figures 9 through 12, the highest concentrations of petroleum hydrocarbons were typically detected in samples collected at depths of 35 to 45 feet This may be due to the presence of a clay layer at that depth, and because the ground water table was reportedly present at a depth of approximately 45 feet when the first wells were installed at the site in 1985 These conditions might have slowed downward migration of the hydrocarbon plume and caused the plume to spread laterally Future Work It is anticipated that quarterly ground water monitoring, and start-up of the soil vapor extraction system will be performed in April 1994 ' Remarks and Si natures ' The interpretations contained in this report represent our professional opinions, and are based in part on information supplied by the client These opinions are based on currently available information and are arrived at in accordance with currently accepted hydrogeologic and engineering practices at this time and location Other than this, no warranty is implied or intended - We recommend that copies of this report be forwarded to Ms Linda Turkette Mr James B Maughan Environmental Health Division California Regional Water Quality Public Health Services Control Board, Central Valley Region San Joaquin County 3443 Routier Road Post Office Box 2009 Sacramento, California 95827 Stockton, California 95201 i