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

Abstract

Microbial processes govern the fate of organic contaminants in aquifers to a major extent. Therefore, the evaluation of in situ biodegradation is essential for the implementation of Natural Attenuation (NA) concepts in groundwater management. Laboratory degradation experiments and biogeochemical approaches are often biased and provide only indirect evidence of in situ degradation potential. Compound-Specific Isotope Analysis (CSIA) is at present among the most promising tools for assessment of the in situ contaminant degradation within aquifers. One- and two-dimensional (2D) CSIA provides qualitative and quantitative information on in situ contaminant transformation; it is applicable for proving in situ degradation and characterizing degradation conditions and reaction mechanisms. However, field application of CSIA is challenging due to a number of influencing factors, namely those affecting the observed isotope fractionation during biodegradation (e.g., non-isotope-fractionating rate-limiting steps, limited bioavailability), potential isotope effects caused by processes other than biodegradation (e.g., sorption, volatilization, diffusion), as well as non-isotope-fractionating physical processes such as dispersion and dilution. This mini-review aims at guiding practical users towards the sound interpretation of CSIA field data for the characterization of in situ contaminant degradation. It focuses on the relevance of various constraints and influencing factors in CSIA field applications and provides advice on when and how to account for these constraints. We first evaluate factors that can influence isotope fractionation during biodegradation, as well as potential isotope-fractionating and non-isotope-fractionating physical processes governing observed isotope fractionation in the field. Finally, the potentials of the CSIA approach for site characterization and the proper ways to account for various constraints are illustrated by means of a comprehensive CSIA field study at the benzene, toluene, ethylbenzene, and xylene (BTEX)-contaminated site Zeitz.

Carbon and Chlorine Isotope Analysis to Identify Abiotic Degradation Pathways of 1,1,1-Trichloroethane

“…Carbon isotope ratios (i.e., 13 C/ 12 C) of 1,1,1-TCA and daughter products (1,1-DCE, 1,1-DCA, ethene and ethane) were determined by GC− IRMS (see details in the Supporting Information). Chlorine isotope ratios (i.e., 37 Cl, n is the number of Cl atoms, k is the number of 37 Cl isotopes, 37 Cl (k) 35 Cl (n−k) and 37 Cl (k−1) 35 Cl (n−k+1) represent the isotopologues containing k and (k−1) heavy isotopes, respectively, and I indicates the ion peak intensities. Isotope ratios of individual compounds were reported using the delta notation (eq 2)…”

“…30−34 For organic contaminants, transformation-induced isotope fractionation is generally larger than the one related to phase transfer processes such as sorption or volatilization. 35 Although isotope fractionation of one element alone (e.g., ε bulk C ) could provide pathway distinction in laboratory experiments, 36 it is not possible under field conditions. Here, contaminant concentration changes related to processes other than its transformation (such as sorption or dilution) cannot be excluded, preventing accurate calculation of ε bulk values.…”

Documentation of time-scales for onset of natural attenuation in an aquifer treated by a crude-oil recovery system

“…Both compounds undergo significant isotopic enrichment along flow paths in the source zone. Recent studies showed that physical processes such as sorption and diffusion could lead to isotope fractionation and to an overestimation of biodegradation, although the importance of these processes in the field has not been shown yet (for a review, see Braeckevelt et al, 2012). Using a model indoor aquifer and a transient toluene pulse, Qiu et al (2013) were able to determine a specific isotope enrichment factor attributable to sorption only and accounting for about 15% of the isotope enrichment factor attributable to biodegradation only.…”

Evolution of carbon isotope signatures during reactive transport of hydrocarbons in heterogeneous aquifers

“…Numerous non-destructive isotope-fractionating processes (e.g., dissolution, sorption, volatilization, diffusion) potentially impact biodegradation-induced shifts in isotope signals to a different degree (Braeckevelt et al, 2012). Processes previously believed to be non-isotope-fractionating were recently found to also cause fractionation such as dispersion and mixing (Eckert et al, 2012;Rolle et al, 2010;Van Breukelen and Rolle, 2012).…”

“…Consequently the biodegradation rates of dissolved contaminants can be highly variable during aquifer passage between the contaminant source zone and downgradient sampling locations (e.g., Abe and Hunkeler, 2006;Stehmeier et al, 1999). Thus, a reliable quantification of the accumulated extent of in situ contaminant degradation becomes challenging (e.g., Braeckevelt et al, 2012;Fischer et al, 2007;Thullner et al, 2012;Van Breukelen and Prommer, 2008). For instance, in complex field situations CSIAbased in situ biodegradation estimates for BTEX compounds can be accompanied by relatively high uncertainties for a variety of reasons and accordingly restrict its applicability to being only a qualitative indicator (e.g., Van Keer et al, 2012).…”