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Groundwater samples collected from monitoring well MW-15 contained <br /> 1 1 ppb MTBE These results show a continued decreasing trend in <br /> concentrations since the remediation system startup <br /> Groundwater samples collected from monitoring well MW-19 contained <br /> 590 ppb TPH-G, 0 70 ppb benzene, 77 ppb MTBE and 12 ppb TBA These <br /> results show a very slight increase from the December 2000 sampling <br /> However, there still appears to be a significant decreasing trend i n <br /> concentrations since the beginning of remediation <br /> 11 . 0 MASS FLUX CALCULATIONS <br /> Calculations of mass flux at this site present multiple problems A <br /> transect at the northern edge of the site is difficult to use because (a) <br /> free-floating hydrocarbons are present in monitoring well MW-5, and (b) <br /> the full extent of contamination to the west is still undefined By the time <br /> the plume reaches the first CPT transect off-site (illustrated in cross- <br /> section B-B'), there is (a) very limited horizontal data within the width of <br /> the plume (boring CPT-18 providing the only significant data), and (b) <br /> unknown groundwater flow velocities in the deeper zones Given these <br /> limitations, the most reasonable transect for mass flux calculations would <br /> be that along the north shoulder of West Yosemite Avenue At this point, <br /> the thickness of this zone is known to be approximately 5-feet (the <br /> distance from the water table to the underlying aquitard) No deeper <br /> zones contain detectable concentrations of any compound at this point <br /> However, this location is still not suitable for mass flux calculations since <br /> the extent of contamination to the west of MW-19 is still undefined For <br /> this reason, a calculation of mass flux across this transect was n o t <br /> completed because it would be highly inaccurate and misleading This <br /> transect would, however, be the most suitable location to conduct these <br /> calculations if an additional well was placed west of MW-19 a n d <br /> completely defined the horizontal extent of contamination <br /> Please note, however, that CPT-26, north and downgradient of the line of <br /> wells along the north shoulder of West Yosemite Avenue, shows MTBE <br /> concentrations in deeper zones even higher than the MTBE <br /> concentrations in the water-bearing zone above 20-feet bgs For this <br /> reason, the calculations of mass flux across the north shoulder of West <br /> Yosemite Avenue would be of very limited use <br /> • <br /> Franks One Stop Report -- March 2001 <br /> -18- <br />