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Pombo Ranch Facility August 17,2004 Subsurface Investigation Report& Second Quarter Groundwater Monitoring from the high. A map of the groundwater elevations and interpreted flow directions is in Figure 3. Groundwater elevations in the well pair MW5 (shallow)/ ' MW 105 (deep discretely screened) were identical, indicating a lack of vertical groundwater movement. Cumulative groundwater elevations are shown in Figure 16. 3.2 Soils Contamination ' Laboratoryresults of analyses of soil samples y p es collected in this investigation are presented in the table "Laboratory Soil Sample Results (May 5, 2004)" and ' summarized in the Appendix A. Gasoline and volatile aromatic hydrocarbons were detected in the laboratory samples collected from boring EB at a depth 10 feet, in boring MW4 at a depth 10 feet, and in boring MW5 at a depth of 10 feet. Contamination was not reported in soil samples collected at depths greater than 10 feet. The borings are located to the northeast of the location of the former ' underground storage tanks. The shallow contaminated soil horizon probably represents a smear zone created during the initial release from the tanks. Cross sections of the benzene, petroleum hydrocarbons, and MTBE soil and groundwater j ' distributions are shown in Figures 10 through 15. MTBE was not detected in any of the soil samples. 3.3 Groundwater Contamination ' Laboratory results of analyses of groundwater samples collected in this investigation are presented in Appendix A and summarized in the table Laboratory Water Sample Results (May 14, 2004). Low concentrations of contaminants were detected in ' ground water samples from all wells but the up gradient well, MW3. Gasoline range petroleum hydrocarbons was detected at concentrations ranging from the detection limit, 0.05 to 0.41 mg/l. Benzene was detected at concentrations ranging from 0.8 to ' 3.8 below or slightly above the maximum contaminant level of 1.0. Toluene, Ethyl benzene and xylene were detected at concentrations between 0.5 to 16 ug/l., well below the maximum contaminant levels for the compounds. Maps of benzene, ' petroleum hydrocarbons, and MTBE distribution are shown in Figures 4, 5, and 6. Low concentrations of gasoline range petroleum hydrocarbons and volatile aromatic compounds were detected in the deep well, MW105, indicating migration of the contamination to a depth of 40 feet. MTBE was not detected in the deep well. MTBE was reported in one well (MW4) at a concentration of 20 ug/l, just above the ' established action level. Dissolved oxygen concentrations range from 0.89 to 0.30 percent with the oxygen low centered on well MW5. The low overall oxygen concentrations indicate that in situ biodegradation of the contaminats is in progress. A ' map of the dissolved oxygen concentrations is shown in Figures 7. i 10