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Former Beacon Station No 4I9 October 7, 1999 Stockton, California Groundwater Monitoring Resort Environmental Protection Agency (EPA) Method 8260B Laboratory results are summarized in Tables I and la Copies of the laboratory data sheets and chain-of-custody reports are included as Attachment C Conclusions Water-level data collected by Doulos on August 31, 1999 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the wells was 45 5 feet below surface grade (bsg) Water levels have decreased an average of 4 8 feet since June 1999 The flow direction was generally toward the west at an average magnitude of 0 002 foot/ foot beneath the site Concentrations of benzene were not detected in the groundwater samples from eight of the ten monitoring wells Detectable concentrations of benzene were reported in water samples from wells MW-3 and MW-9 Concentrations of benzene have decreased in monitoring well MW-1, while increasing in wells MW-3 and MW-9 since June 1999 Data collected from this sampling event was used to construct a Benzene Isoconcentration Map (Figure 3) Concentrations of the oxygenated fuel additive compounds DIPS, ETBE, or TAME were not detected for any of the groundwater samples analyzed this quarter Detectable concentrations of MTBE were reported in water samples collected from wells MW-7, MW-8 and MW-9 at levels of 12, 1 6 and 0 55 parts per billion(ppb), respectively Concentrations of MTBE have decreased in monitoring well MW-8, while increasing in wells MW-7 and MW-9 since June 1999 Detectable concentrations of TBA were reported in wells MW-1, MW-2, MW-3, MW-7, MW-9, and MW-10 Concentrations of TBA have decreased in monitoring wells MW-2, MW-7, MW-9 and MW-10, while increasing in wells MW-1 and MW-3 since June 1999 Concentrations of methanol and ethanol were not detected for any of the groundwater samples analyzed this quarter Concentrations of the lead scavenger compound EDB were not detected for the groundwater samples analyzed this quarter Concentrations of the lead scavenger compound EDC were not detected for well MW-6 Detectable concentrations of EDC were reported in wells MW- 1 through MW-5 and MW-7 through MW-10 Concentrations of EDC have decreased in monitoring wells MW-2, MW-4, MW-5, MW-7, MW-8, MW-9 and MW-10, while increasing in wells MW-1 and MW-3 since June 1999 Recommendations Horizon recommends continued quarterly monitoring be performed at this site to evaluate trends in groundwater levels and concentrations of dissolved hydrocarbons over time Based on the data collected during the Additional Subsurface Invests ation (Horizon February 17, 1999), Horizon has recommended the installation of a monitoring well with a depth-discrete screened interval to evaluate the HydroPunch® water sample collected in boring SB-6, and feasibility testing to evaluate the most effective method(s) of soil and groundwater remediation at the site Honzon is currently awaiting a response from the San Joaquin 99gmr q3 .lob No 1419 42 2 HORIZON ENVIRONMENTAL INC.