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4.0 REMEDIATION EFFECTIVENESS <br /> Charts 1 though 6 present hydrocarbon concentrations over time in <br /> monitoring wells MW-1 through MW-6, respectively They illustrate t h e <br /> period of time beginning on April 30, 2002, two sampling events prior to <br /> activation of the ozone-sparging system, and run through January 26, <br /> 2004, the current sampling event <br /> The charts most notably illustrate the hydrocarbon concentration <br /> decreases in monitoring wells MW-2 and MW-4 directly following <br /> activation of the ozone-sparging system TPH-G and MTBE concentrations <br /> in MW-2 decreased from 7,600 ppb and 3,700 ppb, respectively, on April <br /> 30, 2002 to < 50 ppb and 28 ppb, respectively, on January 26, 2004 TPH-G <br /> and MTBE concentrations in MW-4 decreased from 290 ppb and 540 ppb, <br /> respectively, on April 30, 2002 to < 50 ppb and 6 8 ppb, respectively, on <br /> January 26, 2004 The graphs show a decreasing or stable trend for all <br /> concentrations except TPH-G in monitoring well MW-1, which increased to <br /> 53 ppb on January 26, 2004, slightly above the laboratory method <br /> reporting limit of 50 ppb Note that the scale of the Y-axis varies between <br /> graphs to accommodate the different initial concentrations in the wells <br /> 5.0 CONCLUSIONS <br /> • The groundwater sample collected from monitoring well MW- 1 <br /> contained 12 ppb MTBE, 5 2 ppb tert-butonal (TBA), and 53 ppb TPH-G <br /> • The groundwater samples collected from monitoring well MW-2 <br /> contained 077 ppb benzene and 28 ppb MTBE <br /> • The groundwater samples collected from monitoring well MW-4 <br /> contained 68 ppb MTBE <br /> • The groundwater samples collected fro m monitoring well MW-5 <br /> contained 7 2 ppb MTBE and 9 7 ppb TBA <br /> • No other hydrocarbons were detected above the laboratory method <br /> reporting limits in any of the groundwater samples collected <br /> Although the MTBE concentration in the groundwater sample collected <br /> from monitoring well MW-2 increased slightly from 2 2 ppb to 28 ppb, <br /> when compared to the previous quarter, hydrocarbon concentrations <br /> remained substantially lower than those reported in the sample collected <br /> from the well six months ago Hydrocarbon concentrations in the other site <br /> wells remained relatively stable, at or near the laboratory method <br /> reporting limits <br /> Singh's Unocal Service Station Quarterly Report — January 2004 Sampling <br /> -3- <br />