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� v 0 ' ASSOCIATES I NC iFigure 6 TPHg isoconcentration contours estimated for the second quarter 2004 quarterly groundwater monitoring event conducted on June 4, 2004 are shown on Figure 5 ' • Well MW2 is located approximately 24 feet to the north of the former tank excavation pit, in a hydraulically downgradient direction TPHg and BTEX compounds have been detected in the groundwater concentrations collected from well MW2 at very low concentrations since monitoring began TPHg concentrations have ranged from 540 ug/l to nondetectable concentrations Benzene concentrations have ranged from 5 36 ug/l to nondetectable concentrations since February 1999 During the June 2004 groundwater monitoring event, TPHg and benzene were detected in the groundwater samples collected from well MW2 at concentrations of 240 ug/l and 4.2 ug/1, respectively. No oxygenates ' (including MTBE) have ever been detected in well MW2 A hydrograph depicting TPHg and benzene concentrations associated with MW 1 is shown on Figure 7 i • Well MW3 is located approximately 40 feet to the southwest of the former tank excavation pit, in a hydraulically upgradient to crossgradient direction Low levels of TPHg and ' BTEX constituents have consistently been detected in the groundwater samples collected from well MW3 Benzene was not detected in groundwater samples collected from well MW3 from December 2002 to March 2004 During the second quarter 2004 quarterly groundwater monitoring event conducted on June 4, 2004, TPHg and benzene were detected at concentrations of 1,600 ug/l and 2 4 ug/l, respectively MTBE has never been detected in MW3 A hydrograph depicting TPHg concentrations associated with MW3 is ' shown on Figure 8 4 2 3 Mass Balance Calculations - Groundwater Usinggroundwater analytical results resented in Table 2 the most recent data available ATC � yt P , estimated the lateral extent of dissolved TPHg concentrations beneath the site as depicted on ' Figure 5 The calculated mass of TPHg remaining in the groundwater is estimated to be approximately 0 36 kilograms or 0 8 pounds The calculated mass of benzene remaining in the ' groundwater is estimated to be approximately 0 003 kilograms or 0 006 pounds Mass balance calculations are included in Appendix C ' 5.0 GROUNDWATER RECEPTOR SURVEY ATC reviewed San Joaquin County groundwater well construction logs to identify any ' groundwater wells located within a 2,000-foot radius of the site The search revealed twelve potential wells located within approximately 2,000 feet of the site A figure showing the locations of wells indentified within 2,000 feet of the site is included in Appendix D The closest well is approximately 300 feet north of the site on North Walnut Avenue, located • in the downgradient direction This well is identified as public water supply Well #4 (labeled Well No 4 on the map) and is in operation by the City of Ripon The well is 257 feet deep w\14981\repor \clos=doe 6