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S E C ® lR f . Mr Harlin Knoll January 18, 2006 Page 3 Groundwater monitoring well MW-3 is located downgradient, and to the east, of MW-1 and the former UST pit BTEX has not been detected at or above its MDL in groundwater samples collected from MW-3 since January 2002 Benzene was detected at a concentration of 46 pg/L during the May 2000 sampling event Benzene has not been detected, either before or after this sampling event, at or above its MDL TPHg and TPHd have been reported regularly in groundwater samples collected from MW-3 Figure 1 presents TPHg and TPHd detections from May 1996 to the present sampling event From Figure 1, it is evident that the concentrations of TPHg and TPHd have remained fairly consistent at concentrations of 100 pg/L or less for the past thirteen consecutive monitoring events Groundwater monitoring well MW-4 is located upgradient of MW-1 and adjacent to the former UST pit It shows a similar pattern, or "spike", in TPHd and TPHg concentrations as was observed in the downgradient well MW-3 (Figures 1 and 2) The spike in concentrations of TPHd and TPHg occurs approximately two years earlier in well MW-4 than the spike in these constituent concentrations observed in downgradient well MW-3 The concentrations of TPHd and TPHg show decreasing trends in monitoring wells MW-4 and MW-3 after their observed "spikes" in November 1999 and January 2002, respectively • The last reported concentration of TPHg in MW-4 was 4,100 pg/L This may indicate that some residual TPHg may still be present near the former tank excavation In monitoring wells MW-3 and MW-4, a large increase in MtBE concentrations is observed between November 1999 and November 2001 (Figure 3) The concentrations of MtBE rapidly decrease after November 2001 and have shown a consistently decreasing trend throughout the remaining sampling events In well MW-4, there is a noticeable spike in the concentrations of BTEX and MtBE that occurs from May 2001 until May 2002 This spike may represent residual petroleum hydrocarbons that were released during the removal of the former USTs Concentrations of BTEX and MtBE quickly returned to previous levels and have remained at 100 pg/L or less over the past eight consecutive monitoring events, showing a similar trend to TPHd and TPHg CONCLUSIONS Only random, low concentrations of TPHg, TPHd, benzene, and MtBE have been reported in groundwater monitoring wells MW-1 and MW-2 Benzene and MtBE have never been detected in these wells at or above their primary MCL TPHd detections may represent weathered gasoline, but additional chemical analyses would have to be performed to verify this Although TPHg, TPHd, benzene, and MtBE have been detected in wells MW-3 and MW-4 • over the past 10 years of quarterly sampling, a distinct "spike" in the concentrations of these constituents occurred between November 1999 and May 2002 The concentrations of all N 1Fdes\Projects\Chevron\Retadl#9-82641Penodic 1V1onitonng1200514Q1V1R0514Q2005 QMR doc i