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12 . 0 DISCUSSION AND CONCLUSIONS <br /> Isoconcentration contours for MTBE in groundwater samples collected <br /> shallower than 20-feet bgs are shown on Figure 11 Isoconcentration <br /> contours for MTBE detected in groundwater samples collected at depths <br /> between 30-feet bgs and 70-feet bgs are shown on Figure 12 <br /> Isoconcentration contours for MTBE detected in groundwater samples <br /> collected from depths deeper than 70-feet bgs are shown on Figure 13 <br /> Please note that concentrations on these figures represent the most <br /> recent results in the case of monitoring wells, the highest results ever <br /> detected in the case of domestic wells, and the highest results detected in <br /> the given depth range for the CPT borings <br /> There appears to be minimal groundwater contamination in zones below <br /> 19-feet bgs on-site, as well as on the north shoulder of West Yosemite <br /> Avenue across the street from the site However, in the distance between <br /> the north shoulder of West Yosemite Avenue and CPT-26 on the north <br /> side of the vacant property across the street, significant MTBE <br /> concentrations reach deeper zones, including up to 2,600 ppb MTBE in <br /> the groundwater sample collected from 41-feet bgs Groundwater <br /> samples collected from even deeper zones at 91-feet bgs and 103-feet <br /> bgs still contained 58 ppb and 3 ppb MTBE, respectively The <br /> concentrations of the deeper groundwater samples are consistent with <br /> the concentrations in the nearby domestic wells which intake water at <br /> similar depths Since MTBE does not appear to have significantly <br /> impacted the deeper zones on-site or directly across West Yosemite <br /> Avenue, it appears likely that there is a conduit for this deeper migration <br /> on the vacant property across West Yosemite Avenue This conduit may <br /> possibly be an improperly abandoned water well on that property <br /> Benzene, TAME and TBA were also detected in groundwater samples <br /> collected from 15-feet bgs in boring CPT-26, however, these compounds <br /> were not detected in any of the deeper groundwater samples collected <br /> from this or other borings An isoconcentration map for benzene in <br /> groundwater samples collected shallower than 20-feet bgs is presented as <br /> Figure 14 <br /> Groundwater samples collected from boring CPT-18, further <br /> downgradient of CPT-26, also contained MTBE at concentrations as high <br /> as 92 ppb Groundwater samples collected from the deeper zones in this <br /> boring also contained MTBE, including 21 ppb at a depth of 97-feet bgs <br /> • No MTBE, or any of the other compound analyzed, was detected in any of <br /> Franks One Stop Report — March 2001 <br /> -19- <br />