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y. <br /> 4WD <br /> during this quarter indicate that the groundwater table decreased approximately 3 feet from the <br /> last monitoring event Table 1 presents the groundwater elevation data <br /> Water table elevations and a groundwater contour map for September 17, 1996, are depicted on <br /> Figure 1 The data indicate that groundwater flows generally to the northwest The average <br /> hydraulic gradient is approximately 0 003 feet per foot The flow direction and gradient are <br /> similar to the second quarter's monitoring event <br /> BRE personnel collected groundwater samples from 8 monitoring wells during this field event <br /> Groundwater samples were transported to a California certified laboratory, Centrum Analytical <br /> Laboratories, Inc , in Redlands California, and analyzed for Total Petroleum Hydrocarbons - <br /> Gasoline range organics (TPHg) by EPA Method 8015 (modified) and total volatile organic <br /> aromatics (TVOA), including methyl tertiary butyl ether (MTBE), by EPA Method 8020 <br /> Laboratory analysis of the groundwater samples indicated the presence of dissolved phase <br /> petroleum hydrocarbons above laboratory method detection limits (MDLs) in 5 of the monitoring <br /> wells Concentrations of TPHg ranged from below detection limit (MW-4, MW-6, MW-7, and <br /> MW-8) to 5,000 pg/L (MW-5), and concentrations of TVOA ranged from below detection limit <br /> (MW-3, MW-6, MW-7, and MW-8) to 1,263 pg/L. (MW-5) Concentrations of MTBE ranged <br /> from below detection limit (MW-3, MW-4, and MW-7) to 8 µg/L (MW-5) The contaminate <br /> concentrations in MW-1, MW-2 and MW-5 have decreased since the last monitoring event MW- <br /> 4 shows an increase in concentration for benzene The concentration of MTBE was higher for <br /> MW-8, while MW-3 and MW-7 concentrations remained. the same Table 1 presents the' <br /> analytical testing data The analytical data is depicted on a site plan as Figure 2 The complete <br /> analytical laboratory report, including the chain of custody, is included as Appendix C <br /> Quality assurance and quality control (QA/QC) samples were collected during this quarterly <br /> sampling event The QA/QC samples consisted of one duplicate sample (obtained from well MW- <br /> 1), one trip blank, and one equipment blank No hydrocarbon constituents were detected in either <br /> the trip blank or the equipment blank <br /> 1 , <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> Perimeter monitoring wells MW-6, MW-7, and MW-8 did not contain hydrocarbon constituents <br /> during this quarterly sampling event as in the past several,quarterly'sampling events, indicating ' <br /> that the plume is defined to the north, west and south of the Site BRE recommends reducing'the <br /> frequency of monitoring for these wells to semi-annual events To complete the delineation of the <br /> hydrocarbon plume, BRE recommended installing an additional well to the east This well was <br /> proposed in a Workplan for Continued Assessment, submitted on August 28, 1996 The location <br /> of this proposed well is shown on Figure 1 of the Workplan The Workplan also .1 <br /> 1 I I I {I <br /> I li II <br /> V1', <br /> �� <br /> IE I f t 1411 <br /> I i I <br /> I <br />