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I system has experienced some brief periods of down time due to failure o f the C-Sparger air compressor. 4 . 0 DISSOLVED OXYGEN READINGS { ASE no longer measures DO concentrations in the on-site wells. M 5 . 0 GROUNDWATER SAMPLING EVENTS Since the last 6-month remediation report in February 2003, the six groundwater monitoring wells have been sampled 2 times. The analytical results are tabulated in Table Two. Graphs showing the TPH-G, BTEX, MTBE, TAME, and TBA concentrations in each of the monitoring wells since April 2002 are in the Graphs Section of this report. April 2002 was chosen as the starting point because it shows two sampling events prior to the system being turned on. The site has experienced a downward trend in hydrocarbon and oxygenate concentrations since the remediation system was turned on. In monitoring well MW-2, the most severely contaminated well on site, the r _ concentrations of hydrocarbons and oxygenates have dropped an order of f magnitude and have either continued to drop or remain stable, see Table G Two and the graphs of the individual wells. i 6 . 0 PROPOSED REMEDIATION STRATEGY FOR THE NEXT SIX i MONTHS • ASE believes that the system should remain operational for another period of 6 months. • ASE has recently modified the ozone injection timing and duration of ozone-sparging well OS-1, which is nearest to monitoring well MW-2. We believe that by adding additional ozone to this sparge point, the concentrations of hydrocarbons and oxygenates should drop rather than remain stable as they have the last several sampling periods. • Groundwater sampling of the six monitoring wells should remain on a quarterly sampling frequency. A third evaluation report should be prepared during the month of February 2004 detailing the system's performance and provide recommendations for future remedial activities. ASE will continue to visit the site on a weekly basis to perform the necessary O&M activities. Singh's Unocal System Second Six Month's Operation Report — September 2003 -3-