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1 r' r - 1 <br /> MTBE was detected for the first time, in the deep vapor extraction well at a concentration of 2 7 <br /> ppb, well below the action level of 35 ppb currently in use by the State Water Quality Control <br /> ' Board Also for the first time, water samples were analyzed for gasoline oxygenates other than <br /> MTBE Tertiary butanol was detected in three wells, including MW2, where no contaminants had <br /> previously been detected The concentration ranged from 100-130 ppb, slightly above the detection <br /> ' limit of 100 ppb <br /> 5.0 CONCLUSIONS <br /> During the quarter, the total volume of extracted vapors declined from previous quarters but the <br /> concentration of hydrocarbons in the vapor stream increased The decreased flow was due to system <br /> ' breakdowns,which limited the operational time of the extraction unit The increased concentrations <br /> were probably due to seasonl lowering of the water table, exposing hydrocarbons that had <br /> previously been trapped in the saturated zone <br /> Groundwater remains slightly contaminated, but gasoline and BTEX concentrations declined <br /> ` considerably from their levels in the second quarter This may be due to the seasonal lowering of <br /> ' the water table, leaving previously dissolved hydrocarbons in the capillary fringe Two gasoline <br /> oxygenates, including MTBE, were also detected, but at levels that are unlikely to pose a significant <br /> health threat The source of the oxygenates may be the former UST, but it is not known for certain <br /> ' whether the UST was stili in service at the time tertiary butanol and methyl-tertiary-butyl-ether <br /> were first added to gasoline The UST was reportedly taken out of service in the 1980's <br /> 1 <br /> 1 <br /> r <br />� 6 <br />