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Work Plan jor Refined Plume Definition and Management of Floating Product-7500 W IN th St., Tracy, CA, Page 78 aquitard be thoroughly sealed immediately to prevent cross-contamination. Following that finding, the design of the groundwater-quality monitoring wells installed at the site at that { time was revised so that neither the well casings nor well borings penetrated through the aquifer. However, despite SJC's vigorous expression of concern, the SJCEHD insisted that additional wells penetrating through the aquitard at the base of the shallow aquifer be installed (San Joaquin County Environmental Health Department 2001a, 2001b). Consequently, when the Extended Site Characterization program was implemented in April 2002, Monitoring Wells MW-3A, MW-3B and MW-12A were installed. Wells MW-3A and MW-12A penetrate through the aquitard at the base of the shallow aquifer and are screened in an aquifer that is present beneath that silty clay formation. As noted previously, Monitoring Well MW-3B is of even greater depth and penetrates through two aquitards and into a third uncontaminated aquifer at depth beneath the site. As was evident from the hydrostratigraphy of the site that was developed early in the site characterization work (see Table 1), groundwater phreatic surface elevation data shows that .. the hydraulic heads in each of the three aquifers that are currently monitored by wells MW-3 and MW-12, MW-3A and MW-12A and MW-3B have persistently exhibited differences that indicate that they are, at least to a significant degree, hydrogeologically isolated one from the �.. other. The hydrostratigraphic and hydraulic evidence for the isolation of the aquifers discussed +- above - at least over the area of concern with respect to the contamination of groundwater at the Navarra Property - is also strongly supported by an extensive geochemical database. Since samples were first recovered from Monitoring Wells MW-3A, MW-3B and MW-12A in April 2002, no components of fuel hydrocarbons have been detected in them. On occasion, the apparent presence of very low concentrations of organic compounds having molecular length in the range C9 to C24, which coincides with that for components of diesel, have appeared on the chromatograms produced by the sample analysis procedure. However, as would be clear to an experienced engineer or analytical chemist, the chromatogram produced from the analysis of the groundwater sample from MW-12A recovered on January 20, 2003 that was included in the monitoring report for that date (The San Joaquin Company Inc. -� 2003c), the chromatographic signature of the organic compound that was detected in that sample was of insignificant magnitude and bore no relationship to the complex signature of any fuel hydrocarbons or components. In addition, by application of the well-established methodology of silica gel pretreatment and ` sample filtering of duplicate samples (Army and D. A. Wright 1997, Foote, et al 1997, Zemo 1997b, Zemo and Synkowiec 1995, Girard and Edleman 1994) and inclusion and exclusion of pretreatment in the analyses of samples from successive rounds of groundwater-quality . monitoring, the occasional apparent presence of an organic compound in samples from the deeper aquifers have been demonstrated to be false positive results for the presence of diesel or any petroleum fuel hydrocarbon (The San Joaquin Company Inc. 2003c). Note: - Use of the silica gel cleanup procedure was approved in 1995 by the technical v committee that was convened by the State of California in 1995 to advise on { sic