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Neither TPH-g nor BTE&X were detected above the detection limits in <br /> any of these samples. <br /> Based on these results, it appears that the plume of contamination <br /> within the vadose zone has been effectively assessed and remediated <br /> by overexcavation and drilling activities. Residual toluene and <br /> xylene concentrations up to 14 ppb have been detected in the <br /> vicinity of MW-1, but the average concentration between a depth of <br /> 20 feet and 50 feet is probably less than 1 ppb, and it is quite <br /> ' likely that the volume of contaminated soil is very limited. As <br /> discussed below, groundwater data support the interpretation that <br /> only a limited volume of soil remains contaminated. <br /> ' 5.2 Groundwater Contamination <br /> Table 4 shows the concentrations of all hydrocarbons detected in <br /> groundwater since monitoring began in June 1992 . It is evident that <br /> TPH and benzene concentrations in groundwater have decreased <br /> slightly since November 1992 but have been consistent during the <br /> ' last two monitoring events. If a significant volume of contaminated <br /> soil were present, the concentrations of hydrocarbons in water <br /> samples from MW-1 would probably fluctuate noticeably during <br /> oscillations of the water table. There is no evidence that <br /> groundwater has been further contaminated since the rise in <br /> groundwater levels during the winter of 1992-1993 . <br /> Additionally, groundwater monitoring has successfully established <br /> a consistent groundwater gradient for the past 15 months. During <br /> this time, there is no evidence that the contamination plume has <br /> ' migrated or that concentrations have varied significantly. In a <br /> letter to the Bank of Stockton, dated October 2, 1992 , San Joaquin <br /> PHS/EHD required that "the full extent of the contaminant plume <br /> ' (must) be characterized in order to design a proper remediation <br /> plan" . As shown in Figure 4 , the plume has been adequately <br /> delineated, and no additional assessment of its extent should be <br /> ' necessary. <br /> It is possible that the dissolved hydrocarbon plume will naturally <br /> degrade over time and that contaminant concentrations will decrease <br /> ' in future monitoring events. However, recent data suggest that <br /> concentrations have stabilized and that natural degradation may be <br /> slow. GeoAudit therefore recommends that a meeting be held with San <br /> ' Joaquin PHS/EHD and representatives of the Bank of Stockton to <br /> review the present status of the site, consider the need for <br /> developing a remediation plan, and discuss a schedule for site <br /> closure-. <br /> 12 <br />