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Beacon Station No 641 February 16,2000 Stockton,California Groundwater Monitoring Report (DIPE) , 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 Environmental Protection Agency (EPA) Method 8260B The laboratory results are summarized in Table I Copies of the laboratory data sheets and the chain-of-custody report are included as Attachment C Conclusions Water-level data collected on December 9, 1999 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the five groundwater monitoring wells was approximately 44 88 feet Water levels have nsen approximately 3 14 feet since August 1999 The flow direction is shown to be primarily toward the north and east at an average magnitude of 0 001 foot / foot The flow direction for the previous quarterly monitoring episode in June 1999 was primarily toward the south and east beneath the site Concentrations of benzene were not detected in the groundwater samples collected from monitoring wells MW-2, MW-3, MW-4, and sparge well SW-1 Detectable concentrations of benzene were reported in water samples collected from monitoring wells MW-1, MW-5, and vapor wells VW-1, VW-3, VW-5 and VW-7 A Benzene Concentration Map was constructed for the December 9, 1999 sampling (Figure 3) Dissolved benzene concentrations have decreased in wells MW-1, MW-2, MW-4, sparge well SW-1, and vapor . wells VW-1, while increasing in wells VW-3 since the August 1999 sampling event MTBE was detected in all of the wells sampled at the site Dissolved MTBE concentrations have decreased in wells MW-1, MW-2, MW-4, MW-5, sparge well SW-1, and vapor wells VW-1, and VW-3 while increasing in wells MW-3 since the August 1999 sampling event Additional oxygenate compounds were detected in some of the monitoring well analyses included with this report (see Table 1) Detectable concentrations of TBA were reported in the water samples collected from wells in all of the wells sampled at the site, except for wells MW-3, VW-5, and VW-7 Recommendations Horizon recommends continued quarterly monitoring and evaluation of recent additional assessment to evaluate if the site is adequately delineated Additional groundwater monitoring wells MW-6, MW-7, and MW-8 should be incorporated into the quarterly groundwater monitoring schedule Offsite groundwater monitoring well MW-9 will be installed after an offsite access agreement has been acquired from the property owner • A 16411421641 QMR99 Q4 doc Job No 164142 2 HORIZON ENVIRONMENTAL INC.