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r_ { ' Beacon Station No 641 August 13, 1999 Stockton California Groundwater Monitoring Report total xylenes (BTEX) utilizing Environmental Protection Agency (EPA) Method 8260B, the seven fuel oxygenates methyl tert-butyl ether (MtBE), diisopropyl ether (D1PE) , ethyl-t- butyl ether (ETBE), tert-amyl methyl ether (TAME), tert-butanol (TBA), methanol and ethanol, and the lead scavenger compounds 1,2-dichloroethane (ethylene dichloride or EDC) and 1,2-dibromoethane (ethylene dibromide or EDB) utilizing modified EPA Method 8260 The laboratory results are summarized in Table 1 Copies of the laboratory data sheets and the cham-of-custody report are included as Attachment C Conclusions Water-level data collected on June 2, 1999 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the five groundwater monitoring wells was approximately 42 46 feet Water levels have decreased approximately 1 06 feet since March 1998 The flow direction is shown to be toward the east at an average magnitude of 0 002 foot / foot Benzene was not detected in the groundwater samples collected from monitoring wells MW- 3 and MW-4 Detectable concentrations of benzene were reported in water samples collected from monitoring wells MW-2, MW-5, VW-1, VW-3, and VW-7 A Benzene Concentration Map was constructed for the June 2, 1999 sampling (Figure 3) Dissolved benzene concentrations have decreased in wells MW-1 and MW-2 since the March 1999 sampling An increase in benzene concentrations was reported in well MW-5 MtBE was detected in all the wells sampled, except for well MW-3 Additional oxygenate compounds were detected in 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 i 641 QMR99 q2 Job No 1641 42 2 HORIZON ENVIRONMENTAL INC