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Report: Groundwater-quality Monitoring—October 25,2002: 7500 West Eleventh Street, Tracy, CA. Page 13 subsurface via the casings of Wells 3A 313 and 12A that penetrate into them. As is discussed in Section 2.5 below, when the well-head cover of Monitoring Well MW-12 was opened on October 25, 2002, it was found to contain a significant volume of an unknown liquid having a chemical odor. As SJC has stated in an earlier report (The San Joaquin Company Inc. 2002b), we have been greatly concerned about the possibility that exactly such contamination could be introduced into deeper aquifers beneath the 7500 !. ! West Eleventh Street site via Monitoring Wells 3A, 313 or 12A because they penetrate through a clay aquitard that protects the deeper aquifer from surface spills and the affected groundwater that exists in the shallow aquifer that exists beneath the site. i However, for the identified compounds that appeared in each of those wells in the samples recovered on October 25, 2002 to have had a source related to surface spillage that passed downward through their casings, it would have been necessary for the spillage S to have occurred at two separate locations one around the well- p group MW-3, MW-3A and MW-3B and one around the well group MW-12 and MW-12A. This circumstance is ,.} unlikely, since, if the spillage had only affected deep aquifers penetrated by MW-12A, it F- would have taken much longer than a three-month period(e.g., from July 29, 2002, when none of the unidentified compounds were detected, to October 25, 2002 when they were), to migrate 200 feet down gradient to Monitoring Wells MW-3A and MW-3B. STL also considered the possibility that the compounds detected in the samples had a j source within the laboratory, but no source of materials similar to those detected in the s samples were found. Having eliminated a number of common sources of sample contamination and having determined that the probability of other plausible site-specific causes is very low, the apparent presence of the undetected compound seen in the samples recovered from } Monitoring Wells MW-3A, MW-3B and MW-12A is, at the present time, unexplained. Figures 2 through 5 provide an interpretation of the lateral and vertical extent to which the subsurface beneath the site is affected by components of fuel hydrocarbons that was developed from the results of the groundwater-quality sampling conducted on October 25, 2002. 2.5 Contaminated Water in Monitoring Well MW-12 Well-head Box When the well-head box that protects the top of the casing of Monitoring Well MW-12 was opened on October 25, 2002, that box was found to be filled with contaminated water. The depth of water in the well-head box was sufficient to have submerged the casing cap. Although clear, the water had a chemical odor reminiscent of some types of solvents and its surface was affected by a slight sheen. The bottom of the well-head box and the casing cap were covered in a thin sedimentation of a grey/blue material that could not be identified in the field. SJC bailed the contaminated water from the well-head box and cleaned the exposed casing and casing cap before carefully removing the cap to permit measurement of the depth to groundwater in the well. sic L �