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The laboratory analytical results indicate petroleum hydrocarbon constituents were detected in the groundwater samples collected from MW-1 through MW-5. Only the groundwater samples collected from wells MW-2 and MW-3 contained benzene at concentrations that exceeded the Maximum Contaminant Level (MCL) of 1.0 µg/1. All of the other constituents were reported well below their respective MCLS. MTBE was detected in the groundwater samples collected from wells MW-1 through MW-5. Only the groundwater sample collected from well MW-2 contained MTBE at a concentration that exceeded the Secondary MCL of 5.0 gg/l. Nitrate was detected in the groundwater samples collected from MW-1 through MW-8 at concentrations ranging from 7.2 milligrams per liter (mg/1) to 54 mg/l. Sulfate was detected in the groundwater samples collected from MW-1 through MW-8 at concentrations ranging from 90 mg/l to 280 mg/l. Alkalinity was reported in the groundwater samples collected from MW-1 through MW-8 at concentrations ranging from 460 mg/1 to 570 mg/l. Carbon dioxide was detected in the groundwater samples collected from MW-1 through MW-8 at concentrations ranging from 390 mg/I to 530 mg/l. Dissolved ferrous iron was not detected at or above the laboratory reported detection limit in the groundwater samples collected from MW-1 through MW-8. Groundwater analytical results are summarized in Table 2. The laboratory analytical reports are included in Attachment 2. Isoconcentration maps depicting benzene and MTBE concentrations detected in the groundwater samples are shown in Figures 4 and 5,respectively. BIOREMEDIATION OF PETROLEUM HYDROCARBONS Figures 4 and 5 show the distribution of benzene and MTBE in groundwater at the Fayette Manufacturing site. Although the benzene and MTBE groundwater plumes vary slightly, the plume locations and concentrations have been relatively consistent during recent historical groundwater monitoring events. Monitoring wells MW-2 and MW-3 are located in the center of the petroleum hydrocarbon plume. MW-1 is located on the upgradient side of the petroleum hydrocarbon plume and MW-4, MW7, and MW-8 are located on the downgradient side of the petroleum hydrocarbon plume. Monitoring wells MW-5 and MW-6 are perimeter wells that define the lateral extent of the petroleum hydrocarbon plume to the west. Recently published literature addresses the mechanism of intrinsic bioremediation and the geochemical evidence used to document its occurrence (Buscheck and others, 1993; Salanitro, 1993; McAllister and Chiang, 1994; Borden and others, 1995; Buscheck and Alcantar, 1995; Thierrin and others, 1995). In addition, both the U.S. Air Force and Chevron Corporation have developed sampling protocol for this purpose(Wiedemeier and others, 1995; Buscheck and O'Reilly, 1995). General Discussion of Intrinsic Bioremediation In biodegradation, microorganisms utilize hydrocarbons to obtain energy. This process is an oxidation- reduction reaction where the hydrocarbons are oxidized (i.e., donate electrons) and the electron acceptor (commonly oxygen) is reduced (i.e., accepts electrons). In the subsurface, oxygen (02) is the preferred electron acceptor because microorganisms gain more energy from aerobic reactions (Borden and others, 1995). However, because the transfer of oxygen from the atmosphere to groundwater is usually slow, biodegradation activity often leads to the depletion of oxygen in groundwater (Borden and Bedient, 1986). Under aerobic conditions,the biodegradation of benzene(C6116) is described by the following reaction: Oxygen: C6H6+ 7.5 02—> 6 CO2(g)+3 H2O s:\Environmenta1117861\reports\3gmr2005.doc 4 ATC Associates Inc.