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previous maps show a relatively uniform southward primary gradient Such a uniform t7adient is not consistent with the new data from GT-10, however At this time, none of the interpretations in Figures 13 and 14 can be eliminated All of them are consistent with the fact that the highest concentrations of contaminants were detected n GT-10, which is located in a groundwater depression on all maps Maps 13C and D and 14C and J offer an explanation for the lack of contaminants in GT-9, in these maps, this well is located upE radient of the UST site and would therefore not be expected to be contaminated In contrast, GT-9 is located downgradient of the UST site in maps 13A and B and 14A and B, and might tht refore be contaminated (note that a diesel concentration of 150 ppb was detected in this well in October 1996, Table 3) Perhaps the true gradient will become clearer with further monitont Lg If not, additional wells will be needed in order to draw firm conclusions about the gradient and flow direction 4.0 NEARBY WELL SURVEY A survey to identify domestic and other wells within 2000 feet of the site that could bt potential receptors of contaminated groundwater has been conducted, as requested by PHS/EHD The survey included a review of the water-well files at the California Department of Water Re,ources in Sacramento and a drive-by inspection of the area The DWR files were very incomplete and most of the more than 30 wells that were located were found during the drive-by inspection (T ible 5 and Figure 15) However, not all properties could be viewed or accessed, so additional wells may be present The two closest wells to the Gillies contaminant plume are the Gillies and Barbo domestic wells, which were previously known, a third nearby well (#52 in Figure 15) is iocat:,d on the property just to the north of the Gillies facility Not all of the wells in the DWR files were identified with a specific address; for example, well #36 in Table 5 and Figure 15 is describes as being located "0 5 miles south of Wilson Way on Newton Road", but it is uncertain on which ,ide 8 i street this well is located We have shown it as the domestic well on the Barbot prop,rty Newton Road), but this is speculative, partly because we do not know when the Barboi well was drilled It is our understanding that the Gillies well was drilled in the 1970's or earlier, so it appears that this well was not listed in the DWR files Most wells appear to be small-volume producers used for domestic water supply, but oihers have larger holding tanks and are used for imgation or industrial purposes. A few of the wells are perforated in the 150-200 foot depth range, which is probably within the lower lb-,erbank or Turlock Lake formations, and these wells are unlikely to affect or be affected by the con}amination at the Gillies facility The depth of the perforated interval is unknown for the wells that were located during the dnve-by inspection and for most of the wells in the DWR file,,, and no information about the typical pumping rate is available for any of the wells Therefore, th,, potential of these wells to affect migration of the diesel plume or become impacted by it cannot be determined 9