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Field notes indicate D.O.readings of less than 0.5 mg/L which is considered the minimum 7 reproducible detection limit. In the degradation of BTEX,nitrate is converted to aqueous nitrogen by a process known as denitrification where nitrate is the electron acceptor. The nitrate concentration around the zone of significant contamination (MW-DIS-2 and 3)is below the detection limit. Background nitrate concentration appears to be on the order of 5 to 10 mg/L(MW-DIS-4 and 9). Insoluble Fe+3 accepts an electron and becomes soluble Fe+2 in the degradation of BTEX. Figure 3 shows elevated levels of Fe+z in the zone of significant contamination with a concentration of between 2.3 and 0.9 mg/L,for MW-DIS-1 and 2 respectively. Sulfate is converted to aqueous hydrogen sulfide in the degradation of BTEX. Figure 3 shows depressed levels (<5.0 mg/L) of sulfate in the zone of significant contamination in contrast to 10 to 40 mg/L outside the zone (MW-DIS-4 and 9). The anaerobic fermentation of BTEX converts carbon dioxide into dissolved methane. Fermentation is most pronounced in the heart of the source area near MW-DIS-1,and it falls off rapidly near the fringe of the plume. The dissolved methane concentration in MW-DIS-1 of 0.04 mg/L is an indication of the capacity of the formation for fe e tion. The measured change in concentration of dissolved oxygen,nitrate,ferrous(FEZ*)iron,and sulfate between background samples and the samples collected from within the BTEX plume,provides a means for estimating the rate of BTEX removal from groundwater. { Table 5 presents the concentration of these natural attenuation indicators. TABLE 6 Geochemical Indicators of Natural Attenuation De!Monte Disco Property Indicator Background* In the Plume* Change Dissolved Oxygen 4.6 mg/L <0.5 mg/L 4.1 mg/L { Nitrate 7.5 mg/L ¢0.05 mg/L 7.45 mg/L Ferrous Iron <0.1 mg/L 1.6 mg/L 1.5 mg/L Sulfate 25 mg/L" 3 mg/L 22 mg/L Dissolved Methane 0.0 mg/L 0.04 mg/L 0.04 mg/L f ' Based on average values from quarterly data. * Based on the average of average for MW-DIS-4 and 9. ' Based on a mass ratio between each indicator and a benzene molecule (C6H6),the mass removal of BTEX can be determined for each indicator as described in Table 6. For each liter of background water that enters the BTEX plume, 7.6 mg of BTEX is potentially removed. 1 I SFOIG:IPROJECTSIDELMONTEIPLANT 3310RPT2-9AQTR-REP.DOC 15