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Quarterly Monitoring Report July 2000 Canepa's Car Wash(Pacific Avenue) Page -5 ' 5.0 DISCUSSION OF RESULTS 5 1 DISCUSSION OF GROUNDWATER ELEVATIONS ' The average depth to groundwater measured during the July 2000 groundwater-monitoring event was approximately 32 8 feet below the monitor well measuring points, which is approximately 13 to 18 feet above the top of all monitoring well screens The groundwater elevation ranged from 19 32 to 20 33 feet I bmsl Since November 1993, the depth to groundwater at the site has ranged from approximately 54 5 feet to 31 8 feet bgs The groundwater elevation contour map (Figure 2, Appendix A) indicates groundwater mounding in the center of the site in the vicinity of MW-1 and MW-2 with a primary ' groundwater gradient to the west, but with a northeast groundwater gradient reversal on the east side of the mound The groundwater mounding effect identified during the July 2000 quarterly groundwater- monitoring event is generally consistent with previous monitoring results ' 5 2 DISCUSSION OF ANALYTICAL RESULTS The third quarter, 2000, laboratory analytical results (see Figures 3-5 and 7-9, Appendix A) are generally consistent with recent historical laboratory analytical results The laboratory analytical results indicate that none of the analytes of interest were detected at or above the laboratory reported detection limits in the groundwater samples collected from MW-5, MW-6, MW- 7, and MW-8 BTEX, TPH-G, and MTBE were detected in the groundwater samples collected from MW-1 and MW-2 BTEX and TPH-G were detected in the groundwater sample from SV-4, as well as an elevated concentration of MTBE There was a significant decrease in the concentrations of BTEX, TPH-G, and MTBE in the groundwater sample collected from SV-4 on July 11, 2000, compared to the groundwater sample collected from SV-4 on July 22, 1998 A low concentration of MTBE was detected in the groundwater sample collected from MW-3, and low concentrations of toluene and TPH-G were detected in the groundwater sample collected from MW-4 5 2.1 DISCUSSION OF BENZENE CONCENTRATIONS Concentrations of benzene have decreased over time in MW-1, but appear to be increasing in MW-2 (Figure 7, Appendix A) It appears that with an increase in groundwater elevation there is an increase to the concentiation of benzene in MW-2 The highest peaks in benzene concentration in Figure 7 occur at the sampling event following the peak in groundwater elevation Benzene concentrations in the soil vapor extraction wells (SV-1 through SV-9) have been decreasing as of late (Figure 8, Appendix A) I5 2.2 DISCUSSION OF MTBE CONCENTRATIONS MTBE concentrations have risen in both MW-i and MW-2 over time, with the greatest increase observed I in MW-1 (Figure 9, Appendix A) There is a recent decrease in concentration of MTBE in MW-1, but the general trend is increasing The highest concentration of MTBE was found in SV-4 (see Table 3, next page), which was the only well sampled that was not drowned i r� k `� CONDOR