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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 21 <br /> 6.0 COMPONENTS OF FUEL HYDROCARBONS IN GROUNDWATER <br /> Following our evaluations of the changes in the distribution of components of fuel <br /> hydrocarbons in the groundwater at the Navarra Site, the results of the analyses of the <br /> samples recovered on March 4-9, 2005 are compared with those recovered in the <br /> previous ground-water-quality monitoring round conducted in October 2004. <br /> 6.1 Groundwater Quality in the Near-Surface Unconfined Aquifer <br /> As is shown on Figure 3, there are two plumes of affected ground water emanating from <br /> the 7500 West Eleventh Street site. The primary plume originates beneath the former <br /> pump island area and there is a small secondary plume originating from a location near <br /> the southern boundary of the property, at the rear of the Casa Mendoza restaurant. <br /> -:� 6.1.1 Analytes of Concern in Groundwater AlongPrincipal Plume Axis <br /> - <br /> Monitoring Wells MW-3, MW-13, MW-7, MW-14, MW-16, MW-18, MW-11 and MW- <br /> 19 are, as is shown on Figure 3, arrayed along the approximate long axis of the primary <br /> y plume of affected groundwater. As can be seen by inspection of Table 2, the <br /> concentrations of analytes of concern that were detected in the groundwater samples <br /> 'i recovered from those wells in March 2005 were within the range of fluctuation of <br /> concentrations previously observed. They do not exhibit any unusual variations from the <br /> concentrations measured during the October 2004 monitoring round. For example, the <br /> := concentrations of TPHg in the groundwater in Monitoring Well MW-3 rose from 1,200 <br /> gg/L to 2,300 gg/L while TPHd concentrations in the same well fell from 74 gg/L to 65 <br /> gg/L. The concentrations of TPHg in the sample recovered from Monitoring Well MW-7 <br /> ""rj in March 2005 rose from 14,000 gg/L to 15,000 gg/L over the same period and the TPHd <br /> in the same well rose from 11,000 gg/L to 18,000 gg/L. However, the concentration of <br /> TPHg of Monitoring Well MW-13 had fallen by March 2005 to 4,000 gg/L from 11,000 <br /> gg/L in October, and there was a commensurate fall in the concentration of TPHd from <br /> 2,500 gg/L in October 2004 to 360 gg/L in March 2005. <br /> The results of the March 2005 round of groundwater quality monitoring show that there <br /> I were only trace concentrations of TPHg and MTBE in the sample recovered from <br /> Monitoring Well MW-18, only a trace, at a concentration of 14 gg/L of MTBE was <br /> detected in the groundwater in Monitoring Well MW-11, and the groundwater in <br /> Monitoring Well MW-19 continues to be free of any detectable concentrations of <br /> analytes of concern. These findings confirm the continued stability of the plume of <br /> affected groundwater with respect to down-gradient migration in a northerly direction. <br /> 6.1.2 Analytes of Concern. in Groundwater around Fringes of Principal Plume <br /> The results of the groundwater-duality monitoring round conducted in March 2005 <br /> indicate that the plume perimeter on the eastern side has remained relatively stable since <br /> f` the monitoring round conducted in October 2004. However, due to the development of <br /> the groundwater mound described in Section 2.1 of this report, there has been some <br /> 1 <br /> sic <br /> .J <br />