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S E C ® IIS Mr Harlin Knoll October 28, 2005 Page 3 never 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 in MW-3 from May 1996 to the most recent sampling event in July 2005 From Figure 1, it is evident that the concentrations of TPHg t and TPHd have remained fairly consistent at concentrations of 100/JgIL or less for the past 3 years in MW-3 Groundwater monitoring well MW-4 is located upgradient 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 trend in monitoring wells MW-4 and MW-3 after their observed "spikes" in November 1999 and January 2002 respectively (Figures 3 and 4) The last reported concentration of TPHg in MW-4 was 5,700 Ng/L This may indicate that some residual TPHg may still be present near the former tank excavation In monitoring wells MW4 and MW-3, a large increase in MtBE concentrations is observed between November 1999 and November 2001 (Figure 5) The concentrations of MtBE rapidly decrease after November 2001 and have shown a consistently decreasing trend throughout the remaining sampling events (Figure 6) In MW-3, 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 Ng/L or less over the past 3 years, 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 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 these constituents rapidly decrease after May 2002 Benzene has not been detected in either of these wells since October 2002 The most recent detections of MtBE to wells MW- 3 and MW-4, at concentrations of 100 Ng/L and 34 /rg/L, respectively, occurred during the July 28 2005 sampling event It is plausible these "spikes" represent when residual hydrocarbon material came into contact with groundwater Further evidence exists in the definite time lag between concentrations peaking in the upgradient well approximately two N 1Files\Protects\Chevron\Retadl#9-82641Per[odic Morntoring1200513Q2005 QMR doc