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3.4.4 MTBE ISOCONCENTRATION MAP <br /> As shown on Figure 7, maximum MTBE concentrations occur in well <br /> MW-1 (21,000 µg/L), near the truck loading rack. The MTBE plume attenuates to <br /> 1.0 µg/L in perimeter wells with the exceptions of wells MW-2 (15 µg/L), <br /> MW-3 (2.7µg/L), MW-4 (20 µg/L), MW-9 (1.2 µg/L), MW-23 (3.4 µg/L), and <br /> MW-25 (1.3 µg/L). Historical groundwater data from well DW-2 define the vertical <br /> extent of the MTBE plume. <br /> 3.4.5 TBA ISOCONCENTRATION MAP <br /> As shown on Figure 8, maximum TBA concentrations occur in well MW-1 (14,000 µg/L) <br /> and well MW-10 (10,000 µg/L). Except for wells MW-18 (36 µg/L) and <br /> MW-39 (13 µg/L), the TBA plume attenuates to 10 µg/l, in perimeter wells. Well DW-2 <br /> contained 21 µg/L TBA. <br /> 3.5 PETROLEUM CONSTITUENT <br /> CONCENTRATION TRENDS AND PLUME STABILITY <br /> This section presents a discussion of well concentration trends and plume stability for <br /> wells located in source areas, around the site's perimeter and in the deeper water- <br /> bearing zone. To simplify this discussion and to be consistent with previous reports, <br /> indicator wells were selected to represent the dissolved-phase petroleum constituents <br /> beneath the site. Groundwater analytical data through the fourth quarter 2010 were <br /> used for this evaluation. Well concentration trend graphs were prepared for wells with <br /> sufficient data. The graphs are presented in Appendix B. <br /> 3.5.1 SOURCE AREA WELLS <br /> Wells MW-1 and MW-10 are the source area wells. Well MW-1 is located down gradient <br /> of the truck loading rack and is an active pumping well. No SPHs have been detected in <br /> MW-1 since May 2006. As shown on the graphs (B-1 and B-2), the TPHg, TPHd, <br /> benzene, MTBE, and TBA concentration trends are decreasing. Well MW-1 also <br /> contained tertiary-amyl methyl ether,with stable concentrations trends. <br /> Well MW-10 is located within the tank farm north of Tank 14. No SPHs have been <br /> detected in this well since November 1998. As shown on the graphs (B-3 and B-4),TPHg <br /> 241729(28) 5 CONESTOGA-ROVERS&AssociATES <br />