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• with hydrochloric acid, and the samples were labeled, placed in a cooled ice chest, and transported to Sequoia Analytical Laboratories for hydrocarbon analysis Laboratory data from previous monitoring events are summarized in Table 2, and results of the April 24 samples are summarized in Table 3 The full laboratory report is included in Appendix B • None of the five gasoline oxygenates were detected in any of the wells, which is to be expected, since the USTs had been removed long before refiners began adding oxygenates to fuels As in past monitoring events conducted by Geological Audit and Clearwater, however, high concentrations of gasoline and volatile aromatic compounds (benzene, etc) were detected in MW-2 Gasoline and/or BTEX compounds were also detected in all other samples The surprisingly low concentrations detected in both vapor extraction wells may be due to the fact that water has been present in these wells for only a short time and may not have had enough time to dissolve large quantities of gasoline out of the soil 4.3 Dissolved Oxygen • In conjunction with quarterly monitoring on April 24, baseline measurements of the concentration of dissolved oxygen were made A portable dissolved oxygen sensor was used to measure the concentration in percent DO prior to purging the wells and at 2 or 3-gallon intervals during purging The dissolved oxygen data are shown in Appendix A, and final readings at the conclusion of purging are tabulated in Table 4 It is apparent from Appendix A that concentrations decrease during purging as oxygenated standing water is replaced by less oxygenated water from the formation Dissolved oxygen concentrations were re-measured two weeks after the initial monitoring and ORC infection to determine whether changes had occurred As before, measurements were made before, during, and after purging The results are shown in Table 4 and are plotted against time in Figure 10 Concentrations are low in most wells, but appear to be higher in MW-4 In most wells, concentrations measured on May 8 were lower than on April 24, suggesting that much of the slightly oxygenated water had been purged on April 24 and fresher, less altered groundwater was present in all wells on May 8 One well showed a significant increase in DO concentration after ORC infection Well SW-2 is located close to an infection boring, and may have been affected by the release of oxygen The initial DO concentration in MW-2 exceeded 40% on May 8, but decreased rapidly during purging, while that in SW-2 increased to 77% after 4 gallons had been purged Dissolved oxygen concentrations will be measured several more times over the next few months, and other trends may become apparent • 5