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• 8260 to confirm this result, but the laboratory was contacted and asked to review the data for <br /> evidence of possible error After reviewing the data, the laboratory re-ran the sample on October 18 <br /> and the results were confirmed <br /> Two isocontour maps have been constructed from the October data Figure 4 shows the present <br /> interpretation of the extent of diesel in groundwater, and Figure 5 shows the interpreted extent of <br /> gasoline Both maps conform reasonably well with the interpreted groundwater gradient map, and <br /> suggest that the contaminant plume is elongate in a general east-west direction, roughly parallel to <br /> the Modesto Aquifer However, it appears that the shape of the plume has been modified by <br /> pumping of the Gillies domestic well, and perhaps also by the Bardot well Further modification of <br /> the plume is also likely to occur as the static water level continues to fall, because groundwater and <br /> contaminant flow directions will be different within the Riverbank Formation than they are in the <br /> overlying Modesto Formation When the static level begins to rise again during the late winter and <br /> spring,the shape of the plume will again change <br /> 4.0 RECOMMENDATIONS <br /> The direction of groundwater flow remains questionable at this site The problem apparently stems <br /> from a combination of a relatively flat gradient, seasonal changes in flow direction, pumping rates <br /> from nearby (and perhaps more distant) domestic wells, variations in water depth, and contrasts in <br /> the hydraulic conductivity of subsurface lithologic units Due to this complexity, it is rather unlikely <br /> . that a thorough understanding of the gradient could be reached without several additional wells and <br /> frequent and long-term groundwater monitoring The nature and magnitude of the contaminant <br /> plume does not appear to warrant such an expensive investigative effort Rather, we reaffirm our <br /> earlier conclusion that the most cost-effective approach would be to conduct a short-term pumping <br /> test of GT-10 to assess the response of the Modesto Aquifer to groundwater pump-and-treat <br /> technology Such a test could not be performed at this time, because of the low static water level It <br /> would be necessary to wait until the first quarter of 2000, when the water level is expected to rise <br /> sufficiently for pumping to be effective <br />