Daniel Hunkeler

Current challenges in compound-specific stable isotope analysis of environmental organic contaminants

AbstractCompound-specific stable-isotope analysis (CSIA) has greatly facilitated assessment of sources and transformation processes of organic pollutants. Multielement isotope analysis is one of the most promising applications of CSIA because it even enables distinction of different transformation pathways. This review introduces the essential features of continuous-flow isotope-ratio mass spectrometry (IRMS) and highlights current challenges in environmental analysis as exemplified for the isotopes of nitrogen, hydrogen, chlorine, and oxygen. Strategies and recent advances to enable isotopic measurements of polar contaminants, for example pesticides or pharmaceuticals, are discussed with special emphasis on possible solutions for analysis of low concentrations of contaminants in environmental matrices. Finally, we discuss different levels of calibration and referencing and point out the urgent need for compound-specific isotope standards for gas chromatography–isotope-ratio mass spectrometry (GC–IRMS) of organic pollutants. FigureCompound-specific isotope analysis of environmental contaminants: chromatographic separation is followed by online conversion to a suitable measurement gas (M) and subsequent isotope ratio mass spectrometry. Current challenges in the field concern the analysis of multiple elements (C, H, N, O, Cl) in polar compounds, at low concentrations and in the presence of matrix interferences. An urgent need exists for contaminant-specific reference materials.

Intrinsic bioremediation of a petroleum hydrocarbon-contaminated aquifer and assessment of mineralization based on stable carbon isotopes

This study presents a stepwise concept to assess the in situ microbial mineralization of petroleum hydrocarbons (PHC) in aquifers. A new graphical method based on stable carbon isotope ratios (δ13C) was developed to verify the origin of dissolved inorganic carbon (DIC). The concept and the isotope method were applied to an aquifer in Studen, Switzerland, in which more than 34,000 liters of heating oil were accidentally released. Chemical analyses of ground water revealed that in this aquifer locally, anaerobic conditions prevailed, and that PHC mineralization was linked to the consumption of oxidants such as O2, NO2-, and SO42- and the production of reduced species such as Fe2+, Mn2+, HSS and CH4. However, alkalinity and DIC balances showed a quantitative disagreement in the link between oxidant consumption and DIC production, indicating that chemical data alone may not be a reliable assessment tool. δ13C ratios in DIC have been used before for bioremediation assessment, but results were reported to be negatively influenced by methanogenesis. Using the new graphical method to display δ13C data, it was possible to identify anomalies found in methanogenic monitoring wells. It could be shown that 88% of the DIC produced in the contaminated aquifer originated from microbial PHC mineralization. Thus, the new graphical method to display δ13C ratios appears to be a useful tool for the assessment of microbial hydrocarbon mineralization in a complex environment.

Environmental isotopes in biodegradation and bioremediation.

Isotope Fundamentals Fundamentals of Environmental Isotopes and Their Use in Biodegradation, P. Hohener and C.M. Aelion Analysis of Stable Isotopes, D. Hunkeler and S. Bernasconi Principles and Mechanisms of Isotope Fractionation, D. Hunkeler and M. Elsner Isotope Fractionation During Transformation Processes, D. Hunkeler and B. Morasch Isotopes and Microbial Processes Isotopes and Aerobic Degradation, C.M. Aelion and S.A. Mancini Isotopes and Methane Cycling, E.R.C. Hornibrook and R. Aravena Isotopes and Processes in the Nitrogen and Sulfur Cycles, R. Aravena and B. Mayer Isotopes in Field Applications Investigating the Origin and Fate of Organic Contaminants in Groundwater using Stable Isotope Analysis, D. Hunkeler and R. Aravena Stable Isotope Fractionation of Gases and Contaminant Vapors in the Unsaturated Zone, P. Hohener, D. Bouchard, and D. Hunkeler Isotope Emerging Areas Isotopic Labeling in Environmental and Biodegradation Studies, C.M. Aelion and R.S. Norman Combined Use of Radiocarbon and Stable Carbon Isotopes in Environmental and Degradation Studies, C.M. Aelion Nontraditional Stable Isotopes in Environmental Sciences, C.S. Romanek, B. Beard, A.D. Anbar, and C.F.T. Andrus Index

Compound-specific chlorine isotope analysis: a comparison of gas chromatography/isotope ratio mass spectrometry and gas chromatography/quadrupole mass spectrometry methods in an interlaboratory study.

Chlorine isotope analysis of chlorinated hydrocarbons like trichloroethylene (TCE) is of emerging demand because these species are important environmental pollutants. Continuous flow analysis of noncombusted TCE molecules, either by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) or by GC/quadrupole mass spectrometry (GC/qMS), was recently brought forward as innovative analytical solution. Despite early implementations, a benchmark for routine applications has been missing. This study systematically compared the performance of GC/qMS versus GC/IRMS in six laboratories involving eight different instruments (GC/IRMS, Isoprime and Thermo MAT-253; GC/qMS, Agilent 5973N, two Agilent 5975C, two Thermo DSQII, and one Thermo DSQI). Calibrations of (37)Cl/(35)Cl instrument data against the international SMOC scale (Standard Mean Ocean Chloride) deviated between instruments and over time. Therefore, at least two calibration standards are required to obtain true differences between samples. Amount dependency of δ(37)Cl was pronounced for some instruments, but could be eliminated by corrections, or by adjusting amplitudes of standards and samples. Precision decreased in the order GC/IRMS (1σ ≈ 0.1‰), to GC/qMS (1σ ≈ 0.2-0.5‰ for Agilent GC/qMS and 1σ ≈ 0.2-0.9‰ for Thermo GC/qMS). Nonetheless, δ(37)Cl values between laboratories showed good agreement when the same external standards were used. These results lend confidence to the methods and may serve as a benchmark for future applications.

Assessing chlorinated ethene degradation in a large scale contaminant plume by dual carbon–chlorine isotope analysis and quantitative PCR

The fate of chlorinated ethenes in a large contaminant plume originating from a tetrachloroethene (PCE) source in a sandy aquifer in Denmark was investigated using novel methods including compound-specific carbon and chlorine isotope analysis and quantitative real-time polymerase chain reaction (qPCR) methods targeting Dehaloccocoides sp. and vcrA genes. Redox conditions were characterized as well based on concentrations of dissolved redox sensitive compounds and sulfur isotopes in SO(4)(2-). In the first 400 m downgradient of the source, the plume was confined to the upper 20 m of the aquifer. Further downgradient it widened in vertical direction due to diverging groundwater flow reaching a depth of up to 50 m. As the plume dipped downward and moved away from the source, O(2) and NO(3)(-) decreased to below detection levels, while dissolved Fe(2+) and SO(4)(2-) increased above detectable concentrations, likely due to pyrite oxidation as confirmed by the depleted sulfur isotope signature of SO(4)(2-). In the same zone, PCE and trichloroethene (TCE) disappeared and cis-1,2-dichloroethene (cDCE) became the dominant chlorinated ethene. PCE and TCE were likely transformed by reductive dechlorination rather than abiotic reduction by pyrite as indicated by the formation of cDCE and stable carbon isotope data. TCE and cDCE showed carbon isotope trends typical for reductive dechlorination with an initial depletion of (13)C in the daughter products followed by an enrichment of (13)C as degradation proceeded. At 1000 m downgradient of the source, cDCE was the dominant chlorinated ethene and had reached the source δ(13)C value confirming that cDCE was not affected by abiotic or biotic degradation. Further downgradient (up to 1900 m), cDCE became enriched in (13)C by up to 8 ‰ demonstrating its further transformation while vinylchloride (VC) concentrations remained low (<1 μg/L) and ethene was not observed. The correlated shift of carbon and chlorine isotope ratios of cDCE by 8 and 3.9 ‰, respectively, the detection of Dehaloccocides sp genes, and strongly reducing conditions in this zone provide strong evidence for reductive dechlorination of cDCE. The significant enrichment of (13)C in VC indicates that VC was transformed further, although the mechanism could not be determined. The transformation of cDCE was the rate limiting step as no accumulation of VC occurred. In summary, the study demonstrates that carbon-chlorine isotope analysis and qPCR combined with traditional approaches can be used to gain detailed insight into the processes that control the fate of chlorinated ethenes in large scale plumes.

Petroleum hydrocarbon mineralization in anaerobic laboratory aquifer columns

The anaerobic biodegradation of hydrocarbons at mineral oil contaminated sites has gathered increasing interest as a naturally occurring remediation process. The aim of this study was to investigate biodegradation of hydrocarbons in laboratory aquifer columns in the absence of O2 and NO3−, and to calculate a mass balance of the anaerobic biodegradation processes. The laboratory columns contained aquifer material from a diesel fuel contaminated aquifer. They were operated at 25°C for 65 days with artificial groundwater that contained only SO42− and CO2 as externally supplied oxidants. After 31 days of column operation, stable concentration profiles were found for most of the measured dissolved species. Within 14 h residence time, about 0.24 mM SO42− were consumed and dissolved Fe(II) (up to 0.012 mM), Mn(II) (up to 0.06 mM), and CH4 (up to 0.38 mM) were produced. The alkalinity and the dissolved inorganic carbon (DIC) concentration increased and the DIC became enriched in 13C. In the column, n-alkanes were selectively removed while branched alkanes persisted, suggesting a biological degradation. Furthermore, based on changes of concentrations of aromatic compounds with similar physical–chemical properties in the effluent, it was concluded that toluene, p-xylene and naphthalene were degraded. A carbon mass balance revealed that 65% of the hydrocarbons removed from the column were recovered as DIC, 20% were recovered as CH4, and 15% were eluted from the column. The calculations indicated that hydrocarbon mineralization coupled to SO42− reduction and methanogenesis contributed in equal proportions to the hydrocarbon removal. Hydrocarbon mineralization coupled to Fe(III) and Mn(IV) reduction was of minor importance. DIC, alkalinity, and stable carbon isotope balances were shown to be a useful tool to verify hydrocarbon mineralization.

Evidence of Substantial Carbon Isotope Fractionation among Substrate, Inorganic Carbon, and Biomass during Aerobic Mineralization of 1,2-Dichloroethane by Xanthobacter autotrophicus

ABSTRACT Carbon isotope fractionation during aerobic mineralization of 1,2-dichloroethane (1,2-DCA) by Xanthobacter autotrophicusGJ10 was investigated. A strong enrichment of 13C in residual 1,2-DCA was observed, with a mean fractionation factor α ± standard deviation of 0.968 ± 0.0013 to 0.973 ± 0.0015. In addition, a large carbon isotope fractionation between biomass and inorganic carbon occurred. A mechanistic model that links the fractionation factor α to the rate constants of the first catabolic enzyme was developed. Based on the model, it was concluded that the strong enrichment of 13C in 1,2-DCA arises because the first irreversible step of the initial enzymatic transformation of 1,2-DCA consists of an SN2 nucleophilic substitution. SN2 reactions are accompanied by a large kinetic isotope effect. The substantial carbon isotope fractionation between biomass and inorganic carbon could be explained by the kinetic isotope effect associated with the initial 1,2-DCA transformation and by the metabolic pathway of 1,2-DCA degradation. Carbon isotope fractionation during 1,2-DCA mineralization leads to 1,2-DCA, inorganic carbon, and biomass with characteristic carbon isotope compositions, which may be used to trace the process in contaminated environments.

Review: From multi-scale conceptualization to a classification system for inland groundwater-dependent ecosystems

Aquifers provide water, nutrients and energy with various patterns for many aquatic and terrestrial ecosystems. Groundwater-dependent ecosystems (GDEs) are increasingly recognized for their ecological and socio-economic values. The current knowledge of the processes governing the ecohydrological functioning of inland GDEs is reviewed, in order to assess the key drivers constraining their viability. These processes occur both at the watershed and emergence scale. Recharge patterns, geomorphology, internal geometry and geochemistry of aquifers control water availability and nutritive status of groundwater. The interface structure between the groundwater system and the biocenoses may modify the groundwater features by physicochemical or biological processes, for which biocenoses need to adapt. Four major types of aquifer-GDE interface have been described: springs, surface waters, peatlands and terrestrial ecosystems. The ecological roles of groundwater are conditioned by morphological characteristics for spring GDEs, by the hyporheic zone structure for surface waters, by the organic soil structure and volume for peatland GDEs, and by water-table fluctuation and surface floods in terrestrial GDEs. Based on these considerations, an ecohydrological classification system for GDEs is proposed and applied to Central and Western-Central Europe, as a basis for modeling approaches for GDEs and as a tool for groundwater and landscape management.RésuméLes aquifères fournissent eau, nutriments et énergie, selon des mécanismes variés, à beaucoup d’écosystèmes aquatiques et terrestres. Les écosystèmes dépendant des eaux souterraines (EDES) sont de plus en plus reconnus pour leur valeur écologique et socio-économique. Les connaissances actuelles sur les processus contrôlant le fonctionnement éco-hydrologique des EDES continentaux sont passées en revue, de façon à identifier les facteurs-clés qui conditionnent leur viabilité. Ces processus ont lieu à la fois à l’échelle du bassin versant et de l’émergence. Les types de recharge, la géomorphologie, la structure et la géochimie des aquifères contrôlent la disponibilité en eau nutritive des nappes. La structure de l’interface système souterrain-biocénoses peut modifier les caractéristiques des nappes du fait des processus physico-chimiques et biologiques, auxquels les biocénoses doivent s’adapter. Quatre types majeurs d’interface aquifère-EDES sont décrits : sources, eaux de surface, tourbières et écosystèmes terrestres. Le rôle écologique des eaux souterraines est conditionné par les caractéristiques morphologiques pour l’interface sources-EDES, par la structure de la zone hyporhéïque pour les eaux de surface, par la structure et le volume du sol organique pour l’interface tourbières-EDES et par la fluctuation de la surface libre et par les écoulements de surface dans les EDES terrestres. Sur la base de ces considérations, un système de classification éco-hydrologique pour les EDES est proposé et appliqué à l’Europe du Centre et du Centre-Ouest, comme fondement pour des modélisation des EDES et comme outil de gestion des eaux souterraines et des paysages.ResumenLos acuíferos proporcionan agua, nutrientes y energía con varios esquemas para muchos ecosistemas acuáticos y terrestres. Los ecosistemas dependientes del agua subterránea (GDEs) son cada vez más reconocidos por sus valores ecológicos y socioeconómicos. Se realiza una revisión del conocimiento actual de los procesos que gobiernan el funcionamiento ecohidrológico de los GDEs interiores, para evaluar los principales factores que limitan su viabilidad. Estos procesos ocurren tanto en escala de cuenca como de la emergencia. Los esquemas de la recarga, la geomorfología, la geometría interna y la geoquímica de los acuíferos controlan la disponibilidad del agua y el estado de los nutrientes del agua subterránea. La estructura de interfaz entre el sistema de agua subterránea y la biocenosis pueden modificar las características del agua subterránea por procesos fisicoquímicos y biológicos, para los cuales la biocenosis necesita adaptarse. Cuatro tipos principales de la interfaz de acuíferos GDEF han sido descriptos: manantiales, aguas superficiales, turberas y ecosistemas terrestres. Los roles ecológicos del agua subterránea están condicionados por las características morfológicas para los manantiales GDEs, por la estructura de la zona hiporreica para las aguas superficiales, por la estructura orgánica del suelo y por el volumen de los GDEs con turberas, y por la fluctuación de los niveles freáticos y las inundaciones de superficie en los GDEs terrestres. Basado en estas consideraciones se propone un sistema de clasificación ecohidrológico para los GDEs y es aplicado para el centro – oeste y el centro de Europa., como una base para modelar enfoques para los GDEs y como una herramienta para el manejo del agua subterránea y el paisaje.摘要含水层能够为许多水生和陆地生态系统提供水以及各种类型的营养和能量。依赖地下水的生态系统(GDEs)的生态及社会经济价值越来越被认可。本文综述了控制内陆GDEs生态水文功能的过程研究现状,从而评价限制其发展的主要驱动力。这些过程发生在流域及更大尺度上。含水层的补给类型、地形地貌、内部几何形状以及地球化学控制着水资源可利用量和地下水的营养状况。地下水系统及生物群落之间的界面结构可通过物理化学和生物过程改变地下水特征,并需要生物群落的适应。本文对四个主要类型的含水层—GDE界面进行描述;泉、地表水、沼泽、陆地生态系统。地下水的生态作用由形态学特征约束,对泉水GDEs而言,由伏流区结构约束,对地表水,由有机土壤结构和体积约束,对沼泽GDEs,由地下水位波动约束,对陆地GDEs由地表洪水约束。基于这些因素,本文提出一个GDEs生态水文分类系统,并应用到欧洲中部和中西部,从而为建立GDEs模型打下基础,为地下水及景观管理提供工具。ResumoOs aquíferos fornecem água, nutrientes e energia com vários padrões para muitos ecossistemas aquáticos e terrestres. Os ecossistemas dependentes das águas subterrâneas (EDAS) são crescentemente reconhecidos pelos seus valores ecológicos e socioeconómicos. Revê-se o conhecimento actual dos processos que determinam o funcionamento eco-hidrológico dos EDAS interiores de forma a avaliar os factores principais que condicionam a sua viabilidade. Estes processos ocorrem tanto à escala da bacia hidrográfica como à escala da emergência. Os padrões de recarga, geomorfologia, geometria interna e geoquímica dos aquíferos controlam a disponibilidade de água e o estado nutritivo das águas subterrâneas. A estrutura da interface entre o sistema de águas subterrâneas e as biocenoses pode modificar as características das águas subterrâneas por processos físico-químicos ou biológicos para os quais as biocenoses precisam de se adaptar. Descreveram-se quatro tipos principais de interface aquífero-EDAS: nascentes, águas superficiais, turfeiras e ecossistemas terrestres. O papel ecológico das águas subterrâneas está condicionado pelas características morfológicas no caso dos EDAS associados a nascentes, pela estrutura da zona hiporreica para as águas superficiais, pela estrutura e volume do solo orgânico no caso dos EDAS associados a turfeiras e pela flutuação do nível freático e das cheias de águas superficiais no caso dos EDAS terrestres. Com base nestas considerações, propõe-se um sistema de classificação ecohidrológica para os EDAS e faz-se a sua aplicação à Europa Central e Centro-ocidental como uma base para as abordagens de modelação de EDAS e como uma ferramenta para a gestão das águas subterrâneas e da paisagem.

Evaluating the fate of chlorinated ethenes in streambed sediments by combining stable isotope, geochemical and microbial methods

The occurrence of chlorinated ethene transformation in a streambed was investigated using concentration and carbon isotope data from water samples taken at different locations and depths within a 15 x 25 m study area across which a tetrachloroethene (PCE) plume discharges. Furthermore, it was evaluated how the degree of transformation is related to groundwater discharge rates, redox conditions, solid organic matter content (SOM) and microbial factors. Groundwater discharge rates were quantified based on streambed temperatures, and redox conditions using concentrations of dissolved redox-sensitive species. The degree of chlorinated ethene transformation was highly variable in space from no transformation to transformation beyond ethene. Complete reductive dechlorination to ethane and ethene occurred at locations with at least sulfate-reducing conditions and with a residence time in the samples streambed zone (80 cm depth) of at least 10 days. Among these locations, Dehalococcoides was detected using a PCR method where SOM contents were >2% w/w and where transformation proceeded beyond ethene. However, it was not detected at locations with low SOM, which may cause an insufficient H(2) supply to sustain a detectably dense Dehalococcoides population. Additionally, it is possible that other organisms are responsible for the biodegradation. A microcosm study with streambed sediments demonstrated the potential of VC oxidation throughout the site even at locations without a pre-exposure to VC, consistent with the detection of the epoxyalkane:coenzyme M transferase (EaCoMT) gene involved in the degradation of chlorinated ethenes via epoxidation. In contrast, no aerobic transformation of cDCE in microcosms over a period of 1.5 years was observed. In summary, the study demonstrated that carbon isotope analysis is a sensitive tool to identify the degree of chlorinated ethene transformation even in hydrologically and geochemically complex streambed systems. In addition, it was observed that the degree of transformation is related to redox conditions, which in turn depend on groundwater discharge rates.

Engineered in situ bioremediation of a petroleum hydrocarbon-contaminated aquifer: assessment of mineralization based on alkalinity, inorganic carbon and stable carbon isotope balances

Abstract A concept is proposed to assess in situ petroleum hydrocarbon mineralization by combining data on oxidant consumption, production of reduced species, CH 4 , alkalinity and dissolved inorganic carbon (DIC) with measurements of stable isotope ratios. The concept was applied to a diesel fuel contaminated aquifer in Menziken, Switzerland, which was treated by engineered in situ bioremediation. In the contaminated aquifer, added oxidants (O 2 and NO 3 − ) were consumed, elevated concentrations of Fe(II), Mn(II), CH 4 , alkalinity and DIC were detected and the DIC was generally depleted in 13 C compared to the background. The DIC production was larger than expected based on the consumption of dissolved oxidants and the production of reduced species. Stable carbon isotope balances revealed that the DIC production in the aquifer originated mainly from microbial petroleum hydrocarbon mineralization, and that geochemical reactions such as carbonate dissolution produced little DIC. This suggests that petroleum hydrocarbon mineralization can be underestimated if it is determined based on concentrations of dissolved oxidants and reduced species.