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and TPHd concentrations are stable to decreasing, benzene concentrations are stable to <br /> decreasing, and MTBE is decreasing as TBA increases. Well MW-10 also contained di- <br /> isopropyl ether (DIPE),with stable concentrations trends. <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 (B-6 <br /> and B-7) indicate that TPHg, TPHd, benzene, and MTBE concentrations are decreasing <br /> and TBA concentrations are stable. Well MW-20 also contained DIPE and ethyl tertiary- <br /> butyl ether,with stable or decreasing concentrations trends. <br /> Both wells MW-34 and MW-36 are located off site and down gradient of well MW-20. <br /> These wells delineate the plume's down-gradient extent. <br /> 3.5.3 CROSS-GRADIENT WELLS <br /> Wells MW-18, MW-32, and MW-33 are used to verify concentration trends cross <br /> gradient of the site. Well MW-18 is cross gradient and east of the well MW-1 (truck <br /> loading rack) source area, and well MW-32 is located cross gradient and east-southeast <br /> of the well MW-10 (Tank 14) source area. As shown on the graph for well MW-18 (B-5), <br /> benzene is no longer detected, TPHg concentrations are stable, and TPHd concentrations <br /> are decreasing. MTBE is not a constituent of concern for this well. TBA concentrations <br /> are stable, ranging from 11 to 47 µg/L since 2001. Well MW-18 also contained DIPE, <br /> with stable concentrations trends. <br /> Well MW-32 is located off site and cross gradient east of wells MW-29 and MW-30. It <br /> delineates the plume's cross-gradient extent east of the site. <br /> Well MW-33 is located cross gradient and west of the well MW-1 source area. As shown <br /> on the graph for well MW-33 (B-8), the TPHg, TPHd, and benzene concentrations are <br /> decreasing. MTBE and TBA are not constituents of concern in this well. <br /> 241729128) 6 CONESTOGA-ROVERS&ASSOCIATES <br />