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and maintenance of the ozone injection system. The hour meter of the ozone injection system was observed as non-functional during the site visits conducted during the first quarter and was replaced on May 5, 2004. Field data sheets presenting field parameters collected between April 5, 2004 and June 28, 2004 are included in Appendix A, and certified analytical reports with chain-of-custody documentation are included in Appendix C. Analytical data were submitted to the State Water Resources Control Board as required by AB2886. The electronic delivery receipt is attached in Appendix C. Ozone Iniection Monitoring Plan Monitoring wells MW-3 through MW-10 are used as observation wells to monitor the performance of the ozone injection system. Monitoring wells MW-1 and MW-2 are used as background wells to evaluate and monitor for natural geo-chemical changes in the groundwater. During the second quarter 2004, field parameters were collected on a monthly basis and select laboratory parameters were collected on a quarterly basis. During the second quarter 2004 monitoring event, all samples were collected after purging three volumes of well casing. The modified ozone injection monitoring plan is listed below and a summary of frequency and significance of field and laboratory parameters is presented in Table 3. Field Parameters (Monthly): pH, Dissolved Oxygen (DO), and Specific Conductivity Laboratory Parameters (Quarterly): GRO, BTEX, five oxygenates, Chemical Oxygen Demand (COD), Ferrous Iron, Total Iron, and Heterotrophic Plate Count. Laboratory Parameters (Annually): Total Organic Carbon (TOC), Nitrates, Sulfates, Phosphates, and Total Dissolved Solids. Results Results of field and analytical parameters collected to date to evaluate and optimize the performance of the ozone injection system are presented in Tables 4 and 5. The GRO, benzene, and MTBE concentrations observed in all the groundwater monitoring wells during the second quarter 2004 are presented in Figure 4. Change in DO concentrations for select observation and background monitoring wells are presented in Figure 5. The historical GRO, benzene, and MTBE concentration variation over time at select monitoring wells (MW-4 through MW-8) are illustrated graphically in Figures 6 through 10. GRO concentrations have decreased in the groundwater samples collected from MW-4, MW-5, MW-6, and MW-8 since the previous sampling event (March 4, 2004). The greatest decrease of GRO (from 15,000 µg/L to 180 Ag/L) was reported in the sample collected from MW-8. The concentration of GRO reported for the groundwater sample collected from well MW-5 was due to a discrete peak at C5 indicating presence of MTBE only. The GRO concentrations continue to be lower than the historical high concentrations and appear to be decreasing. During the second quarter 2004, benzene concentrations followed a decreasing trend similar to that of GRO since the previous sampling event (March 4, 2004). Benzene was reported in samples collected from wells MW-4 and MW-8 at concentrations of 1,900 µglL and 990 µg/L, respectively. The highest decrease in benzene concentration from 3,100 µg/L (March 4, 2004) to 990 µglL was observed in well MW-8. Since the previous sampling event, MTBE concentrations appear to have decreased in groundwater samples collected from wells MW-1, MW-4, MW-5, and MW-6, while an increase in concentrations was observed in groundwater samples collected from wells MW-8, MW-9, and MW-10. The greatest decrease in MTBE concentration (from 37,000 pg/L to 15,000 pg/L) was observed in well MW-6, while the greatest increase (from 31,000 µglL to 32,000 pg/L) was observed in well MW-8. PAHp-Arco for URS154691Quarterly120041Arco_5469_2Q04,doe 5TI'ATU5