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z • Figure l,shows proposed locations for four wells in the 3`d Riverbank aquifer (GT-12 through GT-15) These wells will accomplish four goals , ■ Determine the hydraulic gradient and groundwater flow direction in this aquifer ■ Provide thickness and lithologic data for mapping the geometry of the aquifer ■ -Provide soil samples for determining the impact to the saturated zone below 100 feet ■ Provide groundwater samples for mapping the extent of the groundwater impact to this aquifer Depending on the results of these samples, the proposed wells may either be sufficient to delineate the impact or indicate where additional sample points are needed The wells will be constructed of 2-inch diameter PVC casing They will be drilled through the sand body into the next underlying aquitard so that an isopach map of the sand body can be constructed,but only the upper 10 feet of the sand body will be screened This will insure that the well will sample both the top few feet of fine-grained sand and a few feet of coarser-grained sand below, which may have somewhat higher permeability The wells will be drilled with 10-inch diameter hollow-stem augers so that wells screened in upper Riverbank sand bodies can be nested in the same borehole (see below) Figure 2 shows the proposed well construction 3 0 SECOND RIVERBANK AQUIFER • The 2ad Riverbank sand body lies in the depth interval 70-100 feet and has been investigated in ten borings to date (GT-1, 2, 3, 11 and CPT-1 through 6) An isopach map showing the geometry of this sand body has not been included in previous reports, but a tentative outline of its trend can be gained from Figure 3 This sand body is thickest in GT-3 and GT-11, and thins eastward toward GT-1 and CPT-I Farther east, it is absent in CPT-3 The sand body also thins westward from GT-3 to CPT-4 and GT-2, and eastward to CPT-5 These data suggest that the axis of the sand body lies west of the UST excavation and trends generally north-south in the vicinity of GT-3 Its trend north of the excavation is uncertain, the axis could lie between CPT-2 and CPT-6 or it could curve eastward between CPT-2 and GT-8, as shown in Figure 3 Forty-nine well samples and five grab (CPT) samples have been collected from this aquifer (not including samples from G-1, which may also be sampling this aquifer) Diesel-range hydrocarbons were detected in the wells at moderate concentrations through 1999,but since then concentrations have been less than 100 ppb A concentration of 140 ppb was detected in CPT-1 in 2003, but concentrations were below the 50 ppb detection limit in the other four CPT samples Hence, we believe that diesel fuel is relatively limited in extent in this aquifer,and the six wells in Figure 3 should be adequate to fully delineate the plume Three of the proposed wells are situated on possible sand body axes Three wells (GT-12, GT-14, and GT-15) will be nested with wells screened in the 3`d Riverbank aquifer Two others(GT-16 and—17) will be nested with wells in the 1$` Riverbank aquifer GT-18 will be screened only in the 2nd Riverbank aquifer