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Report:Groundwater-quality Monitoring—July 27,2004: 7500 West Eleventh.street, Tracy,CA. Page 11 <br /> Ethyl benzene EPA Method 8260B <br /> Total Xylene Isomers EPA Method 8260B <br /> tertiary-Butyl alcohol EPA Method 8260B <br /> Methyl-tertiary butyl ether EPA Method 8260B <br /> Di-isopropyl ether EPA Method 8260B <br /> Ethyl tertiary-butyl ether EPA Method 8260B <br /> -' Tertiary-amyl methyl ether EPA Method 8260B <br /> - STL is certified to perform the specified analyses by the California Department of Health <br /> Services(DHS). <br /> The results of the analyses of samples of groundwater are presented in Table 2, which <br /> also includes the results of analyses of groundwater samples recovered from the previous <br /> sampling rounds. <br /> 2.5 Groundwater Quality in the Near-Surface Unconfined Aquifer <br /> The results of analyses of samples of groundwater recovered on July 26 and 27, 2004 are <br /> evaluated below. <br /> 2.5.1 Analytes of Concern in Groundwater Along Principal Plume Axis <br /> -1 <br /> Monitoring Wells MW-3, MW-13, MW-7, MW-14, MW-16 and MW-18, which, as is <br /> shown on Figure 3, are arrayed along the approximate long axis of the primary plume of <br /> affected groundwater. As can be seen by inspection of Table 2, compared to the <br /> concentrations of analytes of concern that were found in the groundwater samples <br /> recovered from those wells in April 2004, the concentrations in the samples recovered in <br /> r> July 2004 were generally higher. For example, the concentration of TPHg in the <br /> groundwater in Monitoring Well MW-3 rose from 990 to 180 µg/L and TPHg <br /> concentrations in Monitoring Well MW-13 rose from 5,200 to 2,100 µg/l, and, with <br /> minor exceptions, there were parallel increases in the concentrations of BTEX in those <br /> wells. Further down-gradient along the axis of the plume, at the locations of Monitoring <br /> Wells MW-14, MW-16, and MW-18, which are located to the north of West Eleventh <br /> Street, the relative increase in the concentrations of analytes of concern in the period <br /> l between the two sampling rounds was minor, or as in the case of the groundwater in <br /> Monitoring Well MW-18, there was a slight decrease in the traces of analytes of concern. <br /> As is detailed in Section 3.0 below, between April and July 2004, the concentration of <br /> ' TPHg in Monitoring Well MW-7 increased from 10,000 to 37,000 µg/L. This large <br /> increase can be attributed to the water table rising and submerging the clay strata in the <br /> sic <br /> Ll <br />