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r <br /> Mr Terry Tarditi <br /> April 17, 2001 <br /> • Page 3 <br /> Table 4. Comparison of Groundwater Analytical Results with Previous <br /> Sampling Event <br /> TPH-G Benzene MTBE 8260$ <br /> 4th tr,00 1st qtr,01 4th qtr,00 1st qtr,01 4th qtr,00 1st qtr,01 <br /> Highest concentration,(µg/L) 1,400 1,900 320 390 1,900 1,900 <br /> Wellwith highest concentration MW-3 MW-3 MW-3 MW-3 MW-1 MW-1 <br /> Number of wells with detection 1 2/4 2/4 3/4 3/4 3/4 2/4 <br /> Discussion. <br /> Based on this quarter's data, groundwater flows west under a hydraulic <br /> gradient of approximately 0 02 as shown in Figure 2 Benzene concentrations <br /> remained consistent with the previous quarter in wells MW-1, MW-2 and <br /> MW-3, and increased slightly in well MW-4 Toluene, ethylbenzene, and total <br /> xylenes concentrations remained consistent with historical results in all four <br /> wells TPH-G concentrations increased in MW-1 and MW-3 and were not <br /> detected in MW-2 and MW-4 MTBE concentrations increased in MW-4 <br /> MTBE concentrations remained consistent with previous quarter's results in <br /> MW-1 and MW-2 and was below the reporting limit in MW-3 <br /> • Based on historical data collected from December 1995 to January 2001, the <br /> general groundwater flow is to the southwest under an average hydraulic <br /> gradient of approximately 0 005 Figure 4 shows a rose diagram of groundwater <br /> flow directions and occurrence From this diagram it appears the predominant <br /> flow direction is to the southwest This diagram includes data from only those <br /> sampling events where the groundwater flow direction was not divided or <br /> mounding <br /> Benzene, TPH-G, and MTBE concentrations over time were evaluated to <br /> determine the status of constituents in each well Figure 5 shows benzene <br /> concentrations in each well over time, Figure 6 shows TPH-G concentrations in <br /> each well over time, and Figure 7 shows MTBE concentrations in each well <br /> over time Concentrations below the method reporting limit have been <br /> graphed using one-half the constituent method reporting limit <br /> Benzene concentration in MW-1 peaked during the third quarter 1998 and have <br /> decreased by two orders of magnitude since then Benzene concentrations in <br /> well MW-2 have been below method reporting limits since January 2000 Well <br /> MW-2 is up-gradient from the former UST Benzene concentrations in MW-3 <br /> which is located downgradient from the former UST, have remained <br /> • consistently elevated since August 1998 Benzene concentrations in MW-4 have <br /> decreased since a pear in April 1999 and remain from 1 to 3 µg/L <br /> Use or disclosure of data continued on this sheet is subject to the restriction specified at the beginning of this document <br /> 04/17/01P\16000\16271\Reports\ISTOIgtr do <br />