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does not extend eastward as far as GT-21 or southward as far as GT-17 and CPT-5, not does it extend westward beyond GT-13 For the fust time, the highest concentrations of diesel in groundwater were detected in the 2"d Riverbank Aquifer Concentrations in the 350 ppb range were reported in both GT-3 and GT-14A west of the forniei UST cavity The northern and southern margins of the plume are fairly well delineated by GT-12A, GT-1, and GT-17A, but the eastern and western margins extend beyond the existing NNells screened in this aquifer Thiee of the six water samples from the 3 Riverbank Aquifet were slightly contaminated with diesel As in November 2004, diesel was detected in GT-14B However, both GT-13B and GT-1513 were below the detection limit Diesel was detected in both of the newly installed wells, however Concentiations of 61 and 110 parts per billion were detected in GT-1$B and GT-12B, respectively Oveiall, the shape of the plume in the 3rd Riverbank Aquifer is very similar to that in the 2"d Riveibank, but the concentrations are lower Because the water elevation data suggest that these aquifeis are in communication, it is not surprising that the extent of groundwater contamination is similai in both 5 0 RECOMMENDATIONS In the Fourth Quarter 2004 Report, we recommended moving the location of GT-19 northwai d approximately 100 feet to place it west of GT-13 This was due to the detection of diesel fuel in G"1'-13 However, based on the results of the March samples from GT-13, it does not appear necessary to install a well to the west Therefore, we recommend deferring the dialling of GT-19 until additional water samples have been collected in futwe quarters 6 0 WORK PLAN FOR CONFIRMATION BORINGS 61 Rationale Figure 16 is a diesel isoconcentration map for the Modesto Formation at a depth of 50 feet The map compiles all soil data for this depth from all phases of drilling completed to date The map illustrates that the area of contaminated soil in the formation is long and narrow at this depth, due to confinement within the narrow axial region of the Modesto Channel Until GT-21 was drilled during the most recent phase of fieldwork, it appeated that the eastern non-detect line for soil was located near GT-6 However, the detection of diesel in GT-21 confirms that contamination extends eastward along the talweg fatlher than previously thought Additionally, as explained in the Fourth Quarter= 2004 Report, there is some question as to how comparable the TPH-d concentrations in samples collected during the 1990's ate to those collected today If the concentrations have declined (or increased, for that matter, due to leaching of groundwater smearing) during the 15 years since the investigation began, then using old Sai7lple results to calculate the mass of m-place hydrocarbons in 2005 would not yield accurate results 10