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# AM B R I A Mr Jeffrey Wong July 27, 2005 influence to be approximately 19 to 20 feet (Table 3) (Figure 2 depicts the coverage of SVE based on this estimate ) Therefore, the older bonngs with elevated concentrations and much of the area below the USTs are within the influence of the SVE wells In applying SVE at any site, it is understood that soil vapors drawn from areas with less impacted soil will dilute soil vapors drawn from areas with higher impacted soil As previously discussed, SVE wells were placed within 7 5 feet and 6 feet of the older soil bonngs with the highest MTBE concentrations in soil, and within approximately 5 feet of the UST complex SVE testing was conducted from each SVE well at an applied vacuum of approximately 14 inches of water column-gauge and flow of approximately 60 standard cubic feet per minute At these operating conditions and within such close proximity to the source area, SVE was sufficient to yield the highest available soil vapor concentrations Initial MTBE vapor concentrations were less than 10 parts per million by volume (ppmv) After 24 hours of SVE from each SVE well, the maximum MTBE vapor concentration was 26 ppmv, which is considered low Although an increasing MTBE vapor concentration trend is evident, because the concentrations remained relatively low, the data do not suggest that extended SVE would result in a significant increase of MTBE concentrations Due to the well's proximity to the presumed source area, we would expect much higher vapor concentrations within 24 hours of SVE if a significant source remained in soil Similarly, the radius of influence and soil vapor analytical data do not suggest that significant MTBE vapor concentrations are being diluted to these low level MTBE vapor concentrations Again, we would expect much higher concentrations in a presumed source area, even with dilution occurring CONCLUSIONS AND RECOMMENDATIONS As discussed above, the most recent soil matrix data indicate that MTBE is no longer present at significant concentrations near the USTs MTBE vapor concentration data from SVE testing also support this assertion Furthermore, MTBE concentrations in groundwater have dramatically decreased This decreasing trend would not have occurred or been as prevalent if significant source area impact remained Therefore, based on these three separate data sets, Cambria does not believe that results from the 1996 and 1999 soil bonngs represent residual concentrations in soil at the present time Given the conflict of the older boring data with these three more recent data sets, an estimate of MTBE mass remaining in the vadose zone soil would have to include assumptions resulting in an unreasonable amount of error Cambria does not recommend that decision-making be based on such an estimate 6