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Beacon Station No 641 October 29, 1999 Stockton, California Groundwater MonitoringRepart total xylenes (BTEX), the five fuel oxygenates methyl tert-butyl ether (MTBE), dusopropyl ether (DIPE) , ethyl-t-butyl ether (ETBE), tert-amyl methyl ether (TAME), tert-butanol 91 (TBA), and the lead scavenger compounds 1,2-dichloroethane (ethylene dichloride or EDC) and 1,2-dtbromoethane (ethylene dtbromtde or EDB) utilizing Environmental Protection Agency (EPA) Method 8260B, and the two fuel oxygenates methanol and ethanol utilizing modified EPA Method 8260 The laboratory results are summarized to Table 1 Copies of the laboratory data sheets and the chain-of-custody report are included as Attachment C Conclusions Water-level data collected on September 30, 1999 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the five groundwater monitoring wells was approximately 48 02 feet Water levels have decreased approximately 5 56 feet since June 1999 The flow direction is shown to be primarily toward the south and east at an average magnitude of 0 003 foot/foot A concentration of benzene was not detected to the groundwater sample collected from monitoring well MW-3 Detectable concentrations of benzene were reported to water samples collected from monitoring wells MW-1, MW-2, MW-4, MW-5, SW-1, VW-1 and VW-3 A Benzene Concentration Map was constructed for the September 30, 1999 sampling (Figure 3) Dissolved benzene concentrations have decreased to well VW-3 since the June 1999 sampling An increase in benzene concentrations was reported in wells MW-1, MW-2, MW-4, MW-5, SW-1 and VW-1 MTBE was detected in all the wells sampled, except for well MW-3 Additional oxygenate compounds were detected to some of the monitoring well analyses included with this report (see Table 1) Recommendations Horizon recommends continued quarterly monitoring and additional assessment to evaluate the horizontal and vertical extent of gasoline-range hydrocarbons beneath the site area Proposed groundwater monitoring wells MW-6, MW-7 and MW-8 are scheduled to be installed at the site in October 1999 Offsite groundwater monitoring well MW-9 will be installed after an offsite access agreement has been acquired from the property owner 641QNIR99 q3 Job No 164142 2 HORIZON ENVIRONMENTAL INC.