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i During the first quarter 2004, benzene concentrations have decreased in the groundwater samples collected from wells MW-4 and MW-5 since the previous monitoring event (December 10, 2003). A small increase in benzene concentration from 3,000 ttg/L (fourth quarter 2003) to 3,100 µg/L (first quarter 2004) was observed in the groundwater sample collected from well MW-8. Since the previous sampling event, MTBE concentrations were observed to have decreased in groundwater samples collected from wells MW-1, MW-3, MW-6, MW-9, and MW-10, while an increase in concentrations were observed in groundwater samples collected from wells MW-4, MW-5, and MW-8. The greatest decrease in MTBE concentration (from 46,000 pg/L to 37,000 pg/L) was observed in well MW-6, while the greatest increase (from 22,000 pg/L to 33,000 µg/L)was observed in well MW-4. During the first quarter 2004, GRO and benzene were not reported in wells MW-1 through MW-3, MW-7, MW-9, or MW-10. MTBE was not reported in wells MW-2 or MW-7. TAME (1.1 µg/L) was reported in well MW-5. 1,2-DCA was reported in groundwater samples collected from wells MW-7 (2.5 pg/L) and MW-9 (3.3 µg/L). TBA, DIPE, ETBE, EDB, or ethanol were not reported in any of the groundwater samples collected from the site. During the first quarter 2004, the average DO levels in observation wells (MW-3 through MW-10) varied in the range of 1.1 mg/L to 8.5 mg/L, while the average DO levels in background monitoring wells (MW-1 and MW-2) were in the range of 0.8 mg/L to 3.1 mg/L. During the first quarter 2004, higher DO levels in the range of 7.1 to 9.3 were observed in wells MW-7 and MW-10 (located approximately 10 feet from the injection point). COD concentrations have generally decreased in observation wells since the fourth quarter 2004. A consistent correlation between COD, TOC, and heterotrophic plate counts could not be identified for data collected during first quarter 2004 and/or in comparison to fourth quarter 2004 data. Based on the field and laboratory data collected to date, the ozone injection system appears to be supplying ozone and oxygen to the subsurface as designed. LIMITATIONS This report was prepared in general accordance with accepted standards of care that existed at the time this work was performed. No other warranty, expressed or implied is made. Conclusions and recommendations are based on field observations and data obtained from this work and previous investigations. It should be recognized that definition and evaluation of geologic conditions is a difficult and inexact art. Judgments leading to conclusions and recommendations are generally made with an incomplete knowledge of the subsurface conditions present. More extensive studies may be performed to reduce uncertainties. This report is solely for the use and information of our client unless otherwise noted. t P:1Bp-Arco for UR5154691Quarter1yM041Arco_5469_IQ04.doc SrRArUS 0 i r