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0 1117 Lone Palm Avenue,Suite 201 <br /> Modesto,California 95351 <br /> ATC 209-579-2221 <br /> Shaping the Future Fax:209-579-2225 <br /> �1 <br /> In addition, considering the site is located in an urban area served by a public water system, it <br /> seems unlikely that additional amounts of groundwater will be utilized for drinking water <br /> —' purposes beyond current levels. Since the site vicinity is an older established neighborhood, the <br /> groundwater utilization rates do not appear likely to change in the vicinity of the site. <br /> /Criterion 6: The plume size is stable and sufficiently limited in lateral and vertical extent and <br /> V contaminant concentrations detected in groundwater show a decreasing trend with <br /> time. One hydrologic cycle (four quarters) of monitoring after remediation <br /> measures have ceased is usually considered to be the minimum necessary to <br /> determine site groundwater and plume conditions. <br /> Historically, wells MWI through MW7 have been the most impacted wells. Figures 16 through <br /> 29 show that the concentrations of petroleum hydrocarbons are declining over time in <br /> groundwater samples collected from wells MWI, MW2, MW3, MW4, MWS, MW6, and MW7. <br /> During the most recent sampling event, groundwater collected from well MW9 contained a low <br /> concentration of TPHg and well MW10 contained a low concentration 1,2-DCA. Wells MW8 <br /> and MW 11 did not contain detectable concentrations of any constituent. <br /> MTBE has been detected in samples collected during the latest sampling event from MWI, <br /> MW2, MW3, MW4 and MW7. Figures 15, 20, 23, and 26 show that MTBE is decreasing in <br /> samples associated with these wells and the most recently analyzed samples show that <br /> concentrations of MTBE are below the MCL of 13 ug/L in all but two of the wells and the <br /> „r projected trend is for concentrations to further decrease. <br /> TBA has been detected in samples collected during the latest sampling event from MWI, MW2, <br /> MW5 and MW6(Figures 17 and 30). TBA concentrations in samples have been at or below the <br /> California Notification Level of 12 ug/L in all wells except well MW2 and the projected trend is <br /> for concentrations to further decrease. <br /> During the latest sampling event wells MWI through MW7 and MW10 contained 1,2-DCA <br /> concentrations above the MCL of 0.5 ug/L. Figures 27 through 29 show that concentrations of <br /> 1,2-DCA are decreasing in samples collected from each of the monitoring wells except wells <br /> MW4 and MW7, however the source of the release has been removed and there are no receptors <br /> that would be adversely impacted or threatened by petroleum hydrocarbons from the site. <br /> Concentrations of TPHg have been recently detected in samples collected from wells MWI, <br /> MW2, MW5, MW6 and MW9. Concentrations of TPHg detected in samples collected from <br /> MWI, MW2,MW5 and MW6 were greater than the notification level of 100 ug/L. However,the <br /> trends depicted in Figures 16, 1.8, 22, 24, and 25 show that concentrations of TPHg are <br /> decreasing in samples collected from each of the monitoring wells. It is estimated that <br /> concentrations of TPHg will decrease to below 100 ug/L within approximately 12 years. <br /> S:Enviromental\22493\Reports\Eggiman Closure Report.doe l I <br /> r <br /> r <br />