Applicability of Stable Isotope Fractionation Analysis for the Characterization of Benzene Biodegradation in a BTEX-contaminated Aquifer

Abstract

In recent years the analysis of stable isotope fractionation has increasingly been used for characterizing and quantifying biodegradation of contaminants in aquifers. The correlation of carbon and hydrogen isotope signatures of benzene in a BTEX-contaminated aquifer located in the area of a former hydrogenation plant gave indications that biodegradation mainly occurred under anoxic conditions. This finding was consistent with the investigation of hydrogeochemical conditions within the aquifer. Furthermore, the biodegradation of benzene was calculated by changes in carbon isotope signatures using the Rayleigh-equation-streamline approach. Since contaminant concentrations can be also affected by nonisotope-fractionating abiotic processes such as dilution, volatilization, or irreversible sorption to the aquifer matrix, the Rayleigh-equation-streamline approach was adjusted for scenarios assuming that biodegradation and abiotic processes occur either consecutively or simultaneously along a groundwater flow path between contaminant source and sampling well. The results of the scenarios differed significantly, indicating that an abiotic process (typically dilution) causes a decrease in benzene concentration within the investigated aquifer transect. This comparison of results derived from the different scenarios can help to identify whether biodegradation is the predominant process for decrease in contaminant concentration. However, for a proper quantification of biodegradation, the temporal sequence between biodegradation and dilution needs to be known. The uncertainty associated with the quantification of pollutant biodegradation by the Rayleigh-equation-streamline approach increases when nonisotope-fractionating abiotic processes cause a significant decrease in contaminant concentrations.

Field applicability of Compound-Specific Isotope Analysis (CSIA) for characterization and quantification of in situ contaminant degradation in aquifers

“…This may produce different Λ values even for apparently similar degradation mechanisms, as shown for organisms using benzylsuccinate synthase for the initial step of toluene and xylene degradation Herrmann et al 2009). Recent studies have proven the applicability and benefit of 2D hydrogen and carbon as well as chlorine and carbon CSIA for identification of dominant biodegradation pathways of groundwater contaminants such as BTEX (Fischer et al 2007Vogt et al 2008;Rakoczy et al 2011), MTBE (Kuder et al 2005;Zwank et al 2005;Elsner et al 2007;McKelvie et al 2009;Youngster et al 2010), chlorinated ethenes and ethanes (Abe et al 2009;Hofstetter and Berg 2011;Hunkeler et al 2011a), and quinoline ). However, the application of solely 2D CSIA may be of limited value for the distinction between aerobic and anaerobic MTBE degradation pathways in the field (Rosell et al 2007.…”

“…It is useful in cases where the degradation conditions are ambiguous or heterogeneous, or several different pathways potentially play a role under given conditions (Kuder et al 2005;Fischer et al 2007). The identification of pathways via 2D CSIA can support decision-making on implementation of enhanced NA measures such as biostimulation (Rakoczy et al 2011) and bioaugmentation.…”

“…In an open aquifer system, contaminant concentrations may decrease due to both isotope-fractionating biodegradation processes and non-isotope-fractionating dispersion/dilution down-gradient from the source (Abe and Hunkeler 2006;Mak et al 2006;Van Breukelen 2007b;Fischer et al 2007). Contaminants may leave the defined system (the flow path or aquifer transect considered as a closed system) without necessarily being degraded (Fischer et al 2007).…”

“…Contaminants may leave the defined system (the flow path or aquifer transect considered as a closed system) without necessarily being degraded (Fischer et al 2007). This impacts quantification of contaminant biodegradation using CSIA, as comprehensively illustrated by Thullner et al (2012).…”

“…Furthermore, the isotope effects associated with physical processes such as diffusion, volatilization, and sorption may in some cases not be negligible (LaBolle et al 2008;Kopinke et al 2005a;Kuder et al 2009;Bouchard et al 2008a, b). It may even be difficult to account for the impact of non-isotopefractionating processes on observed contaminant concentrations and isotope ratios in aquifers, such as dilution/dispersion/mixing and dissolution from non-aqueous phase liquid (NAPL) sources (Fischer et al 2007;Morrill et al 2009;Green et al 2010). For application of the Rayleigh equation in open field systems, the definition of a hypothetical "closed system" is of critical importance for quantification of biodegradation (Thullner et al 2012).…”