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Figure 6A represents the Benzene groundwater plume in the A zone and shows a <br /> elongated plume extending form the former UST area west to GeoProbe hole P3 <br /> Figure B6 represents the Benzene groundwater plume in the B zone and shows a <br /> elongated plume extending from the eastern edge of the former pump island area to <br /> approximately 20 feet west of monitor well MW6B <br /> Figure 7A represents the MTBE groundwater plume in the A zone and indicates that a <br /> possibility of two plumes exist One plume originating from the UST area near cluster <br /> well RIA expands to the west past Cluster well MW6A and to the north past MW1 The <br /> second MTBE plume seems to originate east of MW5 and expands to MW5 <br /> Figure 7B represents the MTBE groundwater plume in the B zone and shows similar <br /> plumes as indicated in Figure 7A One plume originating from the UST area near cluster <br /> well RIB expands to the west past "Geoprobe" P9, to the south MW3 and to the north <br /> past MWI The second MTBE plume also seems to onginate east of MW5 and expands <br /> to MW5 2rJ I, <br /> v <br /> Figure 8 illustrates the subsurface lithology and the extent of the A and B[zones rar •-. ht <br /> groundwater plumes within the A and B zones Also illustrated on Figure 8 is the <br /> anticipated over-excavation depth- <br /> RECOMMENDATIONS <br /> epthRECOMMENDATIONS <br /> 1) Conduct pump test to determine feasibility of dewatering during overexcavation of <br /> former UST area and/or dewatering in conjunction with vapor extraction At the end <br /> of the dewatenng pump test, test the cluster wells for aquifer parameters and <br /> leakeage between the A and B zone <br /> 2) Over-excavate former UST - piping and dispenser areas to approximately 24 feet <br /> bgs, see Figure 8 Backfill excavation with piping and pea gravel for use in future <br /> vapor extraction/air sparging and groundwater recovery <br /> 3) Conduct airsparging/vapor extraction pilot test using air sparging in conjunction with <br /> the installation of horizontal vapor collection piping within a backfilled over- <br /> excavation area that traverses the former UST - dispenser areas <br /> TIME FRAME <br /> Prior to performing the next step, i e , Aquifer Pump Tests, obtain pre-approval for <br /> reimbursement from the Underground Storage Tank Cleanup Fund The pump test will <br /> be scheduled for two weeks after receiving written reimbursement preapproval <br /> 7 QM 4' 2001&I s' 2002 <br />