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Former Beacon Station No 419 July 23, 1999 Stockton California Groundwater Monitoring Report �i butyl alcohol (TBA), methanol, and ethanol, as well as the lead scavenger compounds 1,2- dichloroethane (ethylene dichlonde or EDC) and 1,2-dibromoethane (ethylene dibromide or EDB) by modified EPA Method 8260 Laboratory results are summarized in Tables 1 and la Copies of the laboratory data sheets and chain-of-custody reports are included as Attachment C h Conclusions Water-level data collected by Doulos on June 2, 1999 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the wells was 40 7 feet below surface grade (bsg) Water levels have risen an average of 0 6-foot since April 1999 The flow direction is generally toward the southwest (ranges from south to west) The groundwater gradient magnitude averages 0 002 foot/ foot beneath the site Data collected from this sampling event was used to construct a Benzene Isoconcentration Map (Figure 3) Detectable concentrations of benzene were reported in four of the eleven water samples collected No detectable concentrations of benzene were reported in wells MW-2, MW-4, MW-5, MW-6, and MW-8 Dissolved benzene concentrations have decreased in wells MW-7, MW-9, MW-10, and VW-1, while dissolved benzene concentrations have increased in wells MW-1 and MW-3 since the March 1999 sampling event No detectable concentrations of the oxygenated fuel additive compounds DIPS, ETBE, or TAME were reported for any of the groundwater samples analyzed this quarter Detectable concentrations of MTBE were reported in three of the eleven water samples collected No detectable concentrations of MTBE were reported in wells MW-1 through MW-6, MW-9, and MW-10 MTBE concentrations have increased in wells MW-7 and MW-8 since the March 1999 sampling event Detectable concentrations of TBA were reported in wells MW- 2, MW-7, MW-9, MW-10, and VW-1 Detectable concentrations of inethanol and ethanol were detected only in well VW-1 --- No detectable concentrations of the lead scavenger compound EDB were reported for the groundwater samples analyzed this quarter No detectable concentration of the lead scavenger compound EDC was reported for well MW-6 Detectable concentrations of EDC were reported in wells MW-I through MW-5, MW-7 through MW-10, and VW-1 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 also recommends the installation of a monitoring well with a depth-discrete screened interval to evaluate the HydroPunch® water sample collected in boring S13-6 and feasibility testing to evaluate the most effective method(s) of soil and groundwater remediation at the site 99gmr q2 Job No 1419 42 2 HORIZON ENVIRONMENTAL INC.