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I S E C O R IConceptual Site Model Chevron Service Station #9-6171 March 25, 2005 IPage 10 4.0 GROUNDWATER MONITORING AND SAMPLING I Groundwater monitoring and sampling has been ongoing since January 1987 During this time, the observed groundwater elevation fluctuations appear to be controlled by pumping of city and state owned public drinking water supply wells in the Immediate area (Table 6) Between 1990 and 1999, groundwater elevations increased approximately 24 feet This overall change in groundwater elevation was calculated using the groundwater elevations, with respect to mean sea level (MSL), for the highest recorded value in September 1990 and the lowest recorded value in May 1999 from MW-4 From 2001 to 2004, groundwater elevations in the shallow A-Zone aquifer, in general, decreased approximately 6 feet, causing several of the monitoring wells installed in 2001 to go "dry" This has prevented sampling of several of the monitoring wells for several quarters Although the groundwater flow direction has been shown to vary, the principal historical groundwater flow direction has been reported to the southeast (Figure 6) To determine the I effects, if any, of withdrawal of water from the system on the groundwater flow direction, two rose diagrams were created that match the time spans described above Plots of chemical concentration versus time are shown in Figures 7 through 17 Plots of chemical Iconcentration versus distance from the site are shown on Figures 18 and 19 Concentrations reported as non-detect are considered as zero values for graphing purposes in Figures 7-19 Breaks In the trend Imes indicate sampling events where a Iparticular constituent was not included in the laboratory analyses, that the well was inaccessible, or that the well contained insufficient water to sample Historical groundwater monitoring and sampling data are presented in Appendix A I 41 Chemical Concentrations versus Time IConcentrations of TPHg have typically declined in well MW-8 since monitoring and sampling began in 1991 Concentrations of benzene have typically declined in wells MW-2, MW-3, MW-4, MW-5, MW-6, and MW-7 since monitoring and sampling began in 1991 Concentrations of MtBE have typically declined in wells MW-2, MW-3, MW-4, MW-5, MW-6, MW-7, MW-8, and MW-11D since monitoring and sampling began in 1991 Concentrations r of TPHg have typically increased in wells MW-2, MW-3, MW-4, MW-5, MW- 6, MW-7, MW- 10, MW-11D, MW-13, and MW-14 since monitoring and sampling began In 1991 Benzene has never been detected in wells MW-8, MW-10, MW-11D, MW-13, and MW-14 MtBE has never been detected in wells MW-10, MW-13, and MW-14 I I IN(Files\Pro)ects\Chevron\Retadl#9-61711Site Conceptual Model\FINAL_CSM doc