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Trimethylbenzoic Acids As Metabolite Signatures in the Biogeochemical Evolution of an Aquifer Contaminated with Jet Fuel Hydrocarbons

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Trimethylbenzoic Acids As Metabolite Signatures in the Biogeochemical Evolution of an Aquifer Contaminated with Jet Fuel Hydrocarbons Journal of Contaminant Hydrology 67 (2003) 177–194 www.elsevier.com/locate/jconhyd Trimethylbenzoic acids as metabolite signatures in the biogeochemical evolution of an aquifer contaminated with jet fuel hydrocarbons J.A. Namocatcata,1, J. Fanga,*, M.J. Barcelonaa,2, A.T.O. Quibuyenb, T.A. Abrajano Jr.c a National Center for Integrated Bioremediation Research and Development, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, MI 48109, USA b Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101, Philippines c Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Received 24 May 2002; accepted 21 March 2003 Abstract Evolution of trimethylbenzoic acids in the KC-135 aquifer at the former Wurtsmith Air Force Base (WAFB), Oscoda, MI was examined to determine the functionality of trimethylbenzoic acids as key metabolite signatures in the biogeochemical evolution of an aquifer contaminated with JP-4 fuel hydrocarbons. Changes in the composition of trimethylbenzoic acids and the distribution and concentration profiles exhibited by 2,4,6- and 2,3,5-trimethylbenzoic acids temporally and between multilevel wells reflect processes indicative of an actively evolving contaminant plume. The concentration levels of trimethylbenzoic acids were 3–10 orders higher than their tetramethylben- zene precursors, a condition attributed to slow metabolite turnover under sulfidogenic conditions. The observed degradation of tetramethylbenzenes into trimethylbenzoic acids obviates the use of these alkylbenzenes as non-labile tracers for other degradable aromatic hydrocarbons, but provides rare field evidence on the range of high molecular weight alkylbenzenes and isomeric assemblages amenable to anaerobic degradation in situ. The coupling of actual tetramethylbenzene loss with trimethylbenzoic acid production and the general decline in the concentrations of these compounds * Corresponding author. Present address: Department of Geological and Atmospheric Sciences, Iowa State University, 3010 Agronomy Building, Ames, IA 50011-3212, USA. Tel.: +1-515-294-6583; fax: +1-515-294- 6049. E-mail address: [email protected] (J. Fang). 1 Present address: Environmental Science Program, University of the Philippines, Diliman, Quezon City 1101, Philippines. 2 Present address: Department of Chemistry, Western Michigan University, 3442 Wood Hall, Kalamazoo, MI 49008, USA. 0169-7722/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-7722(03)00067-6 178 J.A. Namocatcat et al. / Journal of Contaminant Hydrology 67 (2003) 177–194 demonstrate the role of microbially mediated processes in the natural attenuation of hydrocarbons and may be a key indicator in the overall rate of hydrocarbon degradation and the biogeochemical evolution of the KC-135 aquifer. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Groundwater; Biodegradation; Intrinsic bioremediation; Jet fuel hydrocarbons; Biogeochemistry 1. Introduction Variability in hydrogeochemical conditions influences the type of microbial niches found in the subsurface (Haack and Bekins, 1988). Microbial processes, in turn, play a key role in subsurface reactions (McMahon and Chapelle, 1991) and are largely responsible for the observed biogeochemical changes occurring in aquifers contaminated with fuel hydrocarbons (Bennett et al., 1993; Baedecker et al., 1993; Eganhouse et al., 1993; Kelly et al., 1997). The surfeit organic carbon sources in fuel-contaminated aquifers typically deplete the dissolved oxygen supply (Lovley, 1997; Anderson et al., 1998) and poise the system for alternate anaerobic processes. The energetics of these processes suggests that utilization of alternate electron acceptors proceeds sequentially from nitrate reduction followed by iron reduction and so forth (Champ et al., 1979; Baedecker et al., 1993; Lovley and Chapelle, 1995). This simplified notion of spatial partitioning of redox processes occurs largely on a macro-scale, considering that concurrent redox reactions may also occur at the micro-scale as was shown for sulfate reduction occurring in the oxygenated surface regions of some microbial mats (Canfield and Des Marais, 1991; Fru¨nd and Cohen, 1991). Such concurrent redox processes are also observed in some subsurface investigations, albeit in strictly anoxic conditions as a function of electron donor availability in the case of competing sulfidogenic and methanogenic processes (Vroblesky et al., 1996), complexity of con- taminant plumes (Albrechtsen et al., 1999), or due to spatial variations within the hydrogeochemical framework (Ludvigsen et al., 1998). In general, redox processes in contaminant plumes affect biodegradation rates of aromatic hydrocarbons (Vroblesky and Chapelle, 1994). The shift from aerobic to anaerobic conditions is characterized by the disappearance of À À 2 oxidants such as dissolved O2,NO3 , and SO4 and the subsequent evolution of reduced species not previously found that is coupled with the degradation of hydrocarbons (Bennett et al., 1993; Baedecker et al., 1993; Borden et al., 1994; Prommer et al., 1999). These species represent varied redox regimes predominant at a particular time and space during the geochemical evolution of the aquifer. These redox regimes are also associated with changes in microbial community structure. For example, shift to ferridogenic conditions is accompanied by increase in population of iron bacteria responsible for the solubilization of ferric hydroxides that is coupled with the degradation of organic compounds (Lovley et al., 1989; Lovley, 1991; Anderson et al., 1998). Similarly, shift to sulfidogenic conditions stimulated the activity of sulfate-reducing bacteria that degrade benzene (Anderson and Lovley, 2000), substantiated by increase in cyclopropyl fatty acids (Fang et al., 1997; Fang and Barcelona, 1998), biomarkers of J.A. Namocatcat et al. / Journal of Contaminant Hydrology 67 (2003) 177–194 179 sulfate-reducing bacteria (Dowling et al., 1986; Vestal and White, 1989; Findlay and Dobbs, 1993). The extent of microbial role in the degradation of organic contaminants in situ is difficult to quantify (Reinhard et al., 1997), but most studies suggest that intrinsic bioremediation is an important process in the natural attenuation of aromatic hydrocarbons in the subsurface (Baedecker et al., 1993; Eganhouse et al., 1993; Eganhouse et al., 1996; Curtis and Lammey, 1998; Gieg et al., 1999). No single factor is considered sufficient to demonstrate the occurrence of intrinsic bioremediation in the field (NRC, 1993). Instead, multiple strategies are required to show that microbial attenuation occurs. However, these strategies are constrained by the inherently complex hydrogeochemical settings of the aquifer and the compositional heterogeneity of the dissolved contaminant mixtures. While typical groundwater geo- chemical signatures demonstrate the occurrence of intrinsic bioremediation processes (Borden et al., 1994; McNab, 1999), production of metabolic intermediates in the contaminant plume concurrent with hydrocarbon losses provides unequivocal evidence that hydrocarbons are being actively degraded in situ (Fang et al., 1997; Cozzarelli et al., 1990). Organic acid metabolites such as aromatic acids typically observed in the anoxic zone of the contaminant plume are considered biogeochemical indicators in the degradation of aromatic hydrocarbons in situ (Fang et al., 1997; Cozzarelli et al., 1990; Beller, 2000). These compounds were neither observed in pristine areas of the aquifer nor were they components of the original hydrocarbon mass (Cozzarelli et al., 1990, 1994, 1995). Because aromatic acids persist in the anoxic plume longer than alicylic or aliphatic acids, aromatic acids are ideal metabolite signatures to document the microbially mediated, contaminant breakdown reactions, processes which characterize the biogeochemical evolution of the aquifer. Reports on trimethylbenzoic acids as products of anaerobic metabolism of tetramethyl- benzenes are exceedingly rare (Beller, 2000). This paper investigates the role of trime- thylbenzoic acids as potential metabolite signatures in the biogeochemical evolution of KC- 135 aquifer at the former Wurtsmith Air Force Base (WAFB), Oscoda, MI with a history of JP-4 fuel contamination. Trimethylbenzoic acid production is traced to the degradation of tetramethylbenzene precursors (Cozzarelli et al., 1990). All three tetramethylbenzene isomers—prenitene (1,2,3,4-), izodurene (1,2,3,5-), and durene (1,2,4,5-) found active as genotoxic compounds (Janik-Spiechowicz and Wyszynska, 1999) were detected in this aquifer. Although toluene, xylenes, ethylbenzene, and trimethylbenzenes are the major hydrocarbon components of the contaminant plume in KC-135 with tetramethylbenzenes comprising only a minor fraction, trimethylbenzoic acids are the dominant metabolic intermediates. This paper also reports the detection of additional trimethylbenzoic acids with unknown methylation, which were not documented in other contaminant plumes. 2. Field and laboratory methods This study was part of the multidisciplinary investigation at the KC-135 Crash Site in the decommissioned Wurtsmith Air Force Base (WAFB) in Oscoda, MI. The aquifer was contaminated with JP-4 fuel resulting from the tragic crash of a KC-135 fuel tanker in October 1988, spilling at least 3000 gal of jet fuel. Unknown quantity of the fuel burned as a result of the crash, while the remainder
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