Use of Compound-Specific Stable Carbon Isotope Analyses To Demonstrate Anaerobic Biodegradation of MTBE in Groundwater at a Gasoline Release Site

Abstract

Currently it is unclear if natural attenuation is an appropriate remedial approach for groundwater impacted by methyl tertiary butyl ether (MTBE). Site-characterization data at most gasoline release sites are adequate to evaluate attenuation in MTBE concentrations over time or distance. But, demonstrating natural biodegradation of MTBE requires laboratory microcosm studies, which could be expensive and time-consuming. Recently, compound-specific carbon isotope ratio analyses (13C/12C expressed in delta13C notation) have been used to demonstrate aerobic biodegradation of MTBE in laboratory incubations. This study explored the potential of this approach to distinguish MTBE biodegradation from other abiotic processes in an anaerobic groundwater plume that showed extensive decrease in MTBE concentrations. To our knowledge, this is the first study to use delta13C of MTBE data in groundwater and laboratory microcosms to demonstrate anaerobic biodegradation of MTBE. The delta13C of MTBE in monitoring wells increased by up to 31 per thousand (-25.5 per thousand to +5.5 per thousand) along with a 40-fold decrease in MTBE concentrations. Anaerobic incubations in laboratory microcosms indicated up to 20-fold reduction in MTBE concentrations with a corresponding increase in delta13C of MTBE of up to 33.4 per thousand (-28.7 per thousand to +4.7 per thousand) in live microcosms. Little enrichment was observed in autoclaved controls. These results demonstrate that anaerobic biodegradation was the dominant natural attenuation mechanism for MTBE at this site. The estimated isotopic enrichment factors (epsilon(field) = -8.10 per thousand and epsilon(lab) = -9.16 per thousand) were considerably larger than the range (-1.4 per thousand to -2.4 per thousand) previously reported for aerobic biodegradation of MTBE in laboratory incubations. These observations strongly suggest that delta13C of MTBE could be potentially useful as an "indicator" of in-situ MTBE biodegradation.

Does the Rayleigh Equation Apply to Evaluate Field Isotope Data in Contaminant Hydrogeology?

“…For the previously mentioned field studies (1,6,9), the systematic effect and random uncertainty of kRayleigh were quantified for the most downgradient wells (Table 2). In addition, the kRayleigh values were calculated using eq 13 where R and R0 were calculated from the measured isotope ratios.…”

“…Isotope fractionation is usually quantified by the Rayleigh equation which relates the normalized isotope ratio and the normalized residual concentration by an isotope fractionation factor. For a number of field studies, the Rayleigh equation has been used to evaluate whether field data follow a Rayleigh trend (1,8,9), to calculate the extent of contaminant biodegradation (1-6, 9, 13, 14), or to estimate first-order rate constants by combining the Rayleigh equation with the first-order rate law (14). In the first case, the goal is usually to substantiate the importance of reactive processes and not to derive fieldbased isotope enrichment factors for further use.…”

“…Although the Rayleigh equation was originally developed for homogeneous batch systems (18), it is increasingly applied for flow-through systems such as column and field studies (1)(2)(3)(4)(5)(6)(7)(8)(9)(11)(12)(13)(14)(15). In a flow through system, physical and chemical heterogeneity influences the interpretation of isotope data and is expected to lead to an underestimation of the amount of biodegradation when the Rayleigh equation is applied, as recently shown by Kopinke et al 2005 (17).…”

Carbon isotope fractionation during aerobic biodegradation of n-alkanes and aromatic compounds in unsaturated sand

“…Usually intrinsic biodegradation by indigenous microbial populations is the key process for contaminant elimination and methods are required to demonstrate its efficacy. In groundwater studies, compound-specific isotope analysis (CSIA) has been used increasingly to demonstrate in situ biodegradation of various types of organic contaminants (Kelley et al, 1997;Sturchio et al, 1998;Hunkeler et al, 1999;Sherwood Lollar et al, 2001;Kolhatkar et al, 2002;Mancini et al, 2002;Meckenstock et al, 2002;Song et al, 2002;Kirtland et al, 2003;Richnow et al, 2003a,b;Chu et al, 2004;Griebler et al, 2004;Steinbach et al, 2004;Hunkeler et al, 2005;Morrill et al, 2005). In contrast, only a few studies have investigated the use of CSIA in the unsaturated zone (Stehmeier et al, 1999;Kirtland et al, 2005).…”

Carbon isotopic fractionation during vaporization of low molecular weight hydrocarbons (C6–C12)

“…Various parameters based on the chemical and isotopic compositions of these LMWHs have been widely utilized to make oil/source correlations (Bjorøy et al 1994;Ten Haven 1996;Odden et al 1998;Whiticar and Snowdon 1999;Obermajer et al 2000;Wever 2000), assess the thermal maturity of oils and condensates (Thompson 1983;Mango 2000), determine the source allocation of mixed oils (Chung et al 1998;Rooney et al 1998), and identify various secondary alterations of crude oils (George et al 2002;Pasadakis et al 2004;Zhang et al 2005). The characterization of these light compounds is also a powerful tool for tracing the source of petroleum-related contaminants and understanding the environmental processes that control the transport and fate of these contaminants (Dempster et al 1997;Kelley et al 1997;Gray et al 2002;Kolhatkar et al 2002;Mancini et al 2002Mancini et al , 2008Smallwood et al 2002;Zwank et al 2003).…”