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U Canepa's Car Wash 248 East Park Street,Stockton,California Second Quarter Groundwater Monitoring Report,April 2006 Page 3 of 5 •� Table 1 —Groundwater Elevation Data (April 28, 2006) Well Casing Monitoring Well TOC Elevation' Screen Interval Depth to Groundwater Groundwater (feet) Elevation(feet) (feet-b s) MW-1 1278 45 - 65 2341 -1063 MW-2 12 18 45 - 65 2438 1 -1220 MW-3 1357 45 - 65 2492 -1135 MW-4 1266 40 - 60 2295 -1029 MW-5 1306 40 -60 2467 -1161 MW-6 13 53 40 - 60 2485 -1132 MW-7 1286 40 -60 2383 -1097 ' TOC Elevation Elevations are measured from a mark on the top of the well casmg and are referenced to NGS benchmark HS0391 using NAVD 88 datum bgs=below ground surface All wells were surveyed on October 5,2001 All measurernents are in feet LABORATORY ANALYTICAL RESULTS The groundwater samples were analyzed by Argon Laboratories of Ceres, California, for total petroleum hydrocarbons quantified as gasoline (TPH-G) by EPA Method 8015M, and for benzene, toluene, ethyl benzene, and total xylenes (BTEX) and selected gasoline oxygenates/additives by EPA Method 8260B The selected gasoline oxygenates/additives included MTBE, tertiary-butanol (TBA), di-isopropyl ether (DIPE), ethyl tertiary-butyl ether (ETBE), tertiary-amyl methyl ether (TAME), 1,2-dichloroethane (1,2- DCA),and 1,2-dibromoethane (EDB) iMTBE was detected in the groundwater sample collected from monitor well MW-6 at a concentration of 14 micrograms per liter (µg/L) MTBE was not detected at or above the elevated detection limits of 10 µg/L and 50 }tg/L in the groundwater samples collected from monitor wells MW-2 and MW-5, respectively, nor at or above the detection limit of 0 5 gg/L in the groundwater samples collected from monitor wells MW-1, MW-3, MW-4, and MW-7 Low concentrations of MTBE have historically been detected sporadically in groundwater samples collected from monitoring wells MW-1, MW-3, MW-4, MW-6, and MW-7 MTBE has historically not been detected at or above raised laboratory reported detection limits in MW-2 and MW-5, with the exception of questionable laboratory analytical results in July 1997 for MW-2 and in April 1997 for MW-5 The detected concentration of benzene in monitoring well MW-2 has generally had an inverse relationship with groundwater elevation(Figure 6, Attachment A) In contrast, benzene concentrations in MW-5 have generally increased as the groundwater elevation has increased (Figure 7, Attachment A) Also during the current monitoring event, 1,2-DCA was detected in the groundwater sample collected from MW-5 at a concentration of 70 µg/L Historic groundwater analytical data are presented in Table A, Attachment B Analytical results for the current groundwater monitoring event are generally consistent with historic results, groundwater samples from MW-2 and MW-5 continue to exhibit a modest decline in analyte constituent concentrations comparable to pre-October 2001 levels A summary of the laboratory analytical results for the current monitoring event is presented in Table 2, on the next page Laboratory report sheets indicating analytical methods used, reporting limits, and the results of the analyses are included in Attachment D • �� CONDOR