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TBA plume attenuates to 10 µg/L in perimeter wells. Well DW-2 contained 20 µg/L <br /> 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 <br /> water-bearing zone. To simplify this discussion and to be consistent with previous <br /> reports, indicator wells were selected to represent the dissolved-phase petroleum <br /> constituents beneath the site. Groundwater analytical data through the fourth quarter <br /> 2011 were used for this evaluation. The graphs are presented in Appendix C. <br /> 3.5.1 SOURCE AREA WELLS <br /> Wells MWA 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 (C-1 and C-2), the TPHd, TPHg, <br /> benzene, MTBE, and TBA concentration trends are decreasing. <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 (C-3 and C-4), <br /> TPHd, TPHg, and benzene concentrations are relatively stable, and MTBE is decreasing <br /> as TBA increases. The most recent TBA data point was not used in calculating the trend <br /> as it appears to be anomalous. Well MW-10 also contained 31.4 gg/L di-isopropyl <br /> ether (DIPE), which is within the range of historical detections. <br /> 3.5.2 DOWN-GRADIENT WELLS <br /> Wells MW-20, MW-34, and MW-36 are used to verify concentration trends down <br /> gradient of the site. Well MW-20 is located approximately 40 feet down gradient of <br /> source area well MW-1 at the southern property boundary. The graphs for this well (C-5 <br /> and C-6) indicate that TPHd, TPHg, benzene, MTBE and TBA concentrations are <br /> decreasing. Well MW-20 also contained DIPE and ethyl tertiary-butyl ether, with stable <br /> concentrations trends. <br /> 241729(32) 5 CONESTOGA-ROVERS&ASSOCIATES <br />