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ATC Associates Inc 1117 Lone Palm Avenue, Suite B (: Modesto, Califorma 95351 V:VJ%Tc 209-579-2221 ASSOCIATES INC fax 209-579-2225 ANALY'T'ICAL RESULTS The groundwater sample collected from well MW4 contained detectable concentrations of benzene [2 3 micrograms per liter (ug/L)] The groundwater sample collected from well MW5 contained detectable concentrations of toluene (0 6 ug/L) Groundwater samples collected from_ wells MW 1 and MW6 through MW8 contained no detectable concentrations of TPHg or BTEX constituents Groundwater samples collected from wells MW1, MW4, MW5, MW6, MW7, and MW8 contained detectable concentrations of MTBE at, 2 4 ug/L, 520 ug/L, 770 ug/L, 1,500 ug/L, 5 6 ug/L and 4,200 ug/L, respectively The groundwater samples from wells MW5 and MW8 contained detectable concentrations of TAME at 15 ug/L and 38 ug/L, respectively Analytical results of groundwater samples are summarized in Table 2 Laboratory data sheets and chain-of-custody documentation are contained in Attachment 2 An isoconcentration map depicting the MTBE concentrations in on-site wells during the third quarter 2004 is shown on Figure 'j yrt GEOTRACKER DATA UPLOAD The depth to water data was submitted electronically to the State Water Resources Control Board (SWRCB) Geotracker database (confirmation number 4257099747) and the laboratory data was • also submitted electronically to the SWRCB Geotracker database (confirmation number 6327163755) Documentation of the data submittal is contained in Attachment 3 CONCLUSIONS Chenucal contaminant concentrations from wells MW through MW8 generally remain consistent with the results from the previous sampling event The direction of groundwater flow was --n reversed with respect to the previous monitoring event but consistent with the fourth quarter 2003 monitoring event The gradient Aron August 19, 2004 was 0 0026 ft/ft or approximately 14 ft/mile Groundwater elevations have decreased since March 2004 RECOMAIENDATIONS Based on the results of the third quarter 2004 monitoring episode, we recommend the following • Conduct the fourth quarter 2004 groundwater monitoring and sampling of wells MW 1 and MW4 through MW8 and analyze samples for TPHg and BTEX by EPA method 8021B and MTBE, ETBE, DIPE, TBA, TAME, 1,2-DCA, and EDB by EPA method 8260B • Advance geoprobe and CPT borings in accordance with ATC's Workplan for Additional Subsurface Investigation dated April 19, 2004 and approved July 12, 2004 :1\mwo> \25847\003Tre�\3qr-2004 dm 4