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4 <br /> • _ (a) the apparent reduction in the hydrocarbon mass present in the primary source area, noted by the reduction in dissolved <br /> concentrations in downgradient well MW-11, operational data of the <br /> VES, and confirmation soil borings. <br /> (b) a stable to shrinking dissolved phase plume with a <br /> defined, finite migrational distance. The stable to shrinking <br /> nature of the hydrocarbon plume is based on the historical <br /> nondetect to minimal hydrocarbon presence in downgradient wells MW- <br /> 10, -12, -13 and -14, and the reduction to nondetectable dissolved <br /> BTEX concentrations in well MW-11. Monitor well MW-11 has exhibited <br /> nondetectable concentrations of BTEX compounds during the four most <br /> recent quarterly sampling events. <br /> (c) the absence of sensitive receptors to possibly impacted <br /> groundwater within the boundary of the nondetect downgradient <br /> wells. <br /> (d) the presence of adequate dissolved concentrations of <br /> electron acceptor compounds at the site are sufficient to inhibit <br /> the off-site migration of dissolved hydrocarbons. <br /> 6) A natural attenuation/monitoring only strategy of addressing a <br /> stable or shrinking dissolved phase plume is consistent with the <br /> recommendations in the following guidance documents: <br /> • (a) Recommendations to Improve the Cleanup Process for <br /> California 's Leaking Underground Fuel Tanks (LLNL, October 16, <br /> 1995) . <br /> (b) Regional Board Supplemental Instructions to State Water <br /> Board December 8, 1995 Interim Guidance on Required Cleanup at Low- <br /> Risk fuel Sites (CRWQCB - SF Bay Region, January 5, 1996) . <br /> (c) U.S. EPA Position on LLNL's Report and Recommendations on <br /> California 's leaking Underground Storage Tank Program (U.S. EPA <br /> Region 9, February 6, 1996) . <br /> Please call me at CTN 242-1383 if you would like to discuss this <br /> site further. <br /> CY 4Z- <br /> �!I 0 4 <br /> cc <br /> Cil f <br /> f N ~f <br /> Sheldon N. Nelson Curt <br /> 5 <br />