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I Q , 1 crease iron reduction capacity. MICROBIAL BREAKDOWN Methane distribution is shown in Figure 4. COMPONENTS A direct relationship occurred between ele- A ground water sample was collected at vated methane and total BTEX concentra- Point A (Figure 1) for analysis of phenols tions, which indicated that methanogenesis and aliphatic/aromatic acids. The technique was occurring. Background methane was involved liquid-liquid extraction, derivatiza- near 1 mg/L while the highest plume meth- tion, and gas chromatography/mass spec- i ane concentration was 14.6 mg/L. Assimi- trometry analysis. Major components de- kI lation capacity during methanogenesis tected were branched heptanoic and octa- ! based on stoichiometry would be at least noic acids, trimethylbenzonic acids, di- fif 17,400 pg/L of total BTEX (Table 5). Actual methylbenzonic acids, and some lower methanogenic assimilative capacity could molecular weight acids such as propinoic be much higher because the amount of and butyric. The presence of these fatty available carbon dioxide was not included. acid components in the BTEX plume is Methane-corrected BTEX concentrations at further evidence that viable microbial bio- points A, B and C also provided evidence degradation processes are functional at the that intrinsic bioremediation was occurring site. (Table 3). RATE CONSTANT ASSIMILATIVE CAPACITY A first order biological decay rate was cal- The expressed BTEX assimilative capacity culated using methane as a surrogate of the site ground water was 18,690 µg/L tracer. We assumed that once methane (Table 5) based on stoichiometry and site was produced from the fuel biodegradation geochemical data. Since the highest dis- it was stable and, therefore, could be used solved BTEX concentration at the site was as a tracer. Table 2 lists methane-corrected 7300 µg/L, the ground water had sufficient BTEX and the amount lost between the A, capacity to degrade dissolved BTEX that B and C points. The biodegradation line t had partitioned oily residual from the soil slope between points A and C approxi- into ground water before the plume moved mates a first order process. The average beyond 250 meters downgradient from the decay rate using retarded solute transport source. The remaining assimilative capacity velocity for total BTEX was 0.014 week-'. could be consumed by other gasoline com- This was within the range reported by Wil- ponents such as aliphatic hydrocarbons. son et al. [2]. i CONCLUSION Three lines of evidence to identify intrinsic bioremediation at the site were loss of contaminants at field scale, geochemical ---- .- data, and the presence of intermediate mi- Aerobic respiration Jsite _ crobial BTEX breakdown products. Con- Ferric Hydioxide Reductiontaminant loss showed that natural attenua- tion was occurring. Ground water chemistry f Methanogenesis determined the relative importance of each I Total operating natural attenuation mechanism. Highest total BTEX The presence of volatile organic acids showed that microbial biodegradation proc- Tabie 5. Assimilattve:_capactty of esses were viable. Aerobic respiration and Proceedings of the 10th Annual Conference on Hazardous Waste Research. 7 i