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The distribution of DO, nitrate, manganese (Mn) and sulfate in May 2006 samples from five site wells are shown in Figures 4, 5, 6 and 7 The data used in these figures were collected from site monitoring wells in May 2006 The wells that sampled are as follows • Well MW-3R This well is hydraulically upgradient and screened across the water ' table Water within it is considered representative of ambient shallow groundwater which enters the site • Wells MW-1R, MW-4RS and well VW-2D are screened across the water table and considered to be within the zone of hydrocarbon impact They are expected to show depletion of electron acceptors Wells MW-4RS and VW-2D are expected to show ' greater depletion of election acceptors than well MW-1R because they are further downgradient and closer to the former UST location ' • Well MW-4RD represents deeper groundwater within or beneath the plume core It is screened beneath the water table tThe distribution of DO, nitrate, Mn and sulfate indicate the following • DO is relatively depleted throughout the site including in the upgradient direction This indicates that bacteria within site groundwater are oxygen-deprived and probably are utilizing less desirable electron acceptors such as nitrate • Nitrate values show relative depletion within the hydrocarbon plume (i c , within wells VW-2D, MW-4RS and MW-4RD) compared to upgradient (MW-3R and MW-IR) ' o Mn values are elevated by a factor of 2 or 3 within the shallow plume-core (i e , wells VW-21) and MW-4RS) compared to upgradient wells or deeper well MW-4RD This indicates the use of Mn OZ as an election acceptor ' G Sulfate is relatively depleted within all three plume-core wells compared to upgradient wells, by a factor of about 700 or greater This indicates that nearly all electron t receptors have been depleted by biodegradation processes within the fuel plume and that methanogenesis may be occurring 3.5 Conceptual Model of Hydrogeology and Contamination 1 Data collected during this investigation indicate that the vadose zone contains little hydrocarbon content and that the saturated zone contains a significant laterally dispersed hydrocarbon mass ' from about 24 to 54 feet bgs It is inferred that in the mid-201h century gasoline leaked and migrated downward to a water table that was as deep as approximately 54 feet bgs Separate-phase product (SPH) that accumulated was partially displaced when groundwater rose over it, eventually reaching the depths of 18 to 25 feet bgs observed since the mid-late 1990s Both annual oscillation and longer term variation in depth of the water table have resulted in a vertically attenuated source that continues to include discrete zones of submerged SPH and thicker widespread zones of absorbed and dissolved-phase hydrocarbons ' ,110P.N 1TWhPu61121211 Don Rode 1',IsORISprmg 200G SIR{Aug,,,O d. 3_4 82306