Faculté des sciences

Assessment of in situ biodegradation of chlorinated solvents in aquifers and constructed wetlands using an integrative approach

Imfeld, Gwenaël ; Hunkeler, Daniel (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2008 ; Th.2069.

Knowledge about biogeochemical processes associated with natural attenuation of chlorinated solvents in the environment is currently limited. Though processes such as sorption, volatilization or dilution may contribute to contaminant natural attenuation, in situ biodegradation is the only process leading to destructive removal of contaminants. However, the distribution of hydrological and... Plus

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    Summary
    Knowledge about biogeochemical processes associated with natural attenuation of chlorinated solvents in the environment is currently limited. Though processes such as sorption, volatilization or dilution may contribute to contaminant natural attenuation, in situ biodegradation is the only process leading to destructive removal of contaminants. However, the distribution of hydrological and hydrochemical processes over both spatial and temporal scales influence degradation reactions, and thus should be taken into account when assessing in situ biodegradation in complex systems. This thesis aimed at gathering knowledge about in situ biodegradation of chlorinated solvents and associated biogeochemical processes within both contaminated wetlands and aquifers. The specific objectives of this thesis were i) to develop and apply an approach in order to demonstrate in situ degradation of toxic reductive dechlorination intermediates such as dichloroethenes (DCE), vinyl chloride (VC) and chlorobenzene (CB), ii) to investigate the biogeochemical development of model wetlands treating intermediate dechlorination compounds, iii) to identify the prevailing degradation pathways within the investigated systems, and iv) to investigate the relationships between the microbial community and ambient hydrochemical conditions. The different investigations carried out in the framework of this thesis were based on an integrative approach, which couples Compound Specific Isotope Analysis (CSIA) to hydrogeochemical and molecular techniques. The various resulting sets of data were explored and linked by means of multivariate statistics in order to gain additional insights into the development of biogeochemical processes. First, recent progresses made towards understanding how mechanisms attributed to various organic chemicals removal interact to form a functioning treatment wetland were reviewed (Chapter 1, section 2). In particular, possible complementary techniques and integrative approaches to follow up in situ degradation processes were discussed. Second, the biogeochemical processes and the in situ biodegradation activity were characterized in two different model constructed wetland systems for contaminated groundwater-surface water interfaces (Chapter 2). In a first study (Chapter 2, section 1), the biotransformation of CB was investigated in a constructed wetland supplied with contaminated groundwater by means of a detailed geochemical characterization, stable isotope composition analysis and in situ microcosm systems loaded with 13C-labelled CB. Significant shift in the isotopic composition of CB over the flow path under prevailing iron reducing conditions and the detection of 13C-labelled benzene indicated reductive dehalogenation of CB. In a second study (Chapter 2, section 2), the spatial and temporal biogeochemical development of a model system loaded with cis- and trans-1,2-dichloroethene contaminated groundwater was characterized over 430 days by means of hydrogeochemical and CSIA. The hydrogeochemistry dramatically changed over time from oxic to strongly reducing conditions, as emphasized by increasing concentrations of ferrous iron, sulphide and methane over time. In a complementary study (Chapter 2, section 3), the model wetland microbial community was characterized during the transition phase from a prevailing aerobic to an anaerobic regime. Non-metric dimensional scaling analysis of microbial fingerprints revealed that a dynamic community was associated with changes of redox-sensitive processes at the system scale. Microbial analyses emphasized the possible involvement of several microbial groups in the observed biogeochemical processes potentially influencing DCE transformation. The presence of a complex guild of putative dehalorespirers (Dehalobacter spp., Dehalococcoides spp. and Geobacter spp.) during the anoxic phase, where reductive dechlorination prevailed, was detected. 16S rRNA gene libraries revealed substantial changes of the bacterial composition between the supplied groundwater and the model wetland pore water. Proteobacteria accounted for > 50 % of 16S rRNA gene clone library of the wetland, and about 17 % of the sequences could be related to sulphate reducers.Overall, the investigation of the model system treating DCE contaminated groundwater i) demonstrated the linkage between the hydrogeochemical variability and the ongoing degradation processes, ii) highlighted the potential of CSIA to trace the temporal and spatial changes of the dominant degradation mechanism of DCE in natural or engineered systems, and iii) underscores progressive changes of both wetland microbial community structures and degrading guilds during the transition from mostly oxic to anoxic conditions that also coincided with changing degradation mechanism. The use of an integrated approach enabled monitoring in situ biodegradation of DCE and CB in model wetland systems, and to gather parallel information on the associated biogeochemical processes and prevailing contaminant transformation pathways.Third, in situ biodegradation of chlorinated solvents was studied in two different groundwater systems (Chapter 3). In a first study (Chapter 3, section 1), biodegradation of mostly recalcitrant chlorobenzenes was assessed at an anoxic aquifer by combining hydrogeochemical and stable isotope analyses. In situ microcosm analysis evidenced microbial assimilation of CB derived carbon, whereas laboratory investigations asserted mineralization of CB. An isotope balance was applied to overcome the limit of interpretation posed by the simultaneous enrichment and depletion in 13C of reductive dechlorination intermediates during sequential in situ degradation. The enrichment obtained by cumulating the concentration and isotopic composition values of single chlorobenzene species indicated CB biodegradation at various zones of the aquifer. Additionally, the relationship between hydrogeochemical conditions and degradation activity was investigated by principal component analysis. This analysis underlines variable hydrogeochemical conditions associated with degradation activity at the plume scale. In a second study (Chapter 3, section 2), the ongoing in situ biodegradation of chlorinated ethenes within a hydrogeologically complex groundwater system (Bitterfeld, Germany) was investigated at the plume scale. The assessment of hydrogeochemical species and chlorinated ethenes concentrations distribution by principal component analysis (PCA), in combination with carbon stable isotope composition analysis, revealed that chlorinated ethenes were subjected to substantial biodegradation. Changes in isotopic composition values up to 20.4, 13.9, 20.1 and 31.4 ‰ were observed between geological units for tetrachloroethene (PCE), trichloroethene (TCE), cis-dichloroethene (cDCE) and vinyl chloride (VC), respectively. The use of specific biomarkers (16S rRNA gene) indicated the presence of Dehalococcoides sp. DNA in 20 of the 33 evaluated samples. Analysis of bacterial community structures variation in the aquifers using canonical correspondence analysis (CCA) indicated a predominant influence of the contaminant concentrations. To gain additional knowledge on biogeochemical variability within this groundwater system, a third study assessed the bacterial community structures, hydrogeochemical indicators and the carbon stable isotope composition of chlorinated ethenes at discrete-depth intervals along a hydrogeologically heterogeneous vertical profile (Chapter 3, section 3). Substantial variation of the hydrogeochemistry, bacterial community structures and the distribution of putative dehalorespirers (Dehalobacter spp., Desulfitobacterium spp., Dehalococcoides spp. or Geobacter spp.) were observed with depth, according to the contaminant concentration. The compound specific isotope analysis of the single chlorinated ethenes species was interpreted globally based on an isotope balance. Hydrogeochemical and microbiological indicators as well as the isotope balance, revealing isotopic enrichment above 8 ‰ between the sources and the plume fringe area, clearly demonstrated the occurrence of reductive dechlorination of chlorinated ethenes at the investigated depths. Isotope balance values revealed enrichment from -14.4 to -4.4 ‰ over the vertical profile, which suggested that chlorinated ethenes biodegradation reaction varied spatially. Overall, a relationship could be established between bacterial community structures, hydrogeochemical variables and hydrogeological conditions. In summary, the different investigations presented in this thesis underscored the relevance of conducting integrative studies based on several, complementary techniques to improve the assessment of relevant chlorinated solvents in situ biogeochemical processes in complex systems. In order to facilitate the exploration and interpretation of the relationships between subsurface biocenoses and biogeochemical contaminant removal processes, complex microbial and hydrochemical sets of data can be efficiently treated by multivariate analysis. Several concepts related to the isotopic analysis of chlorinated solvents developed and applied in the framework of this thesis (i.e. the elucidation of DCE degradation pathways, the isotope balance and the use of in situ microcosms experiments) may be adapted and employed for further investigation of the environmental fate of chlorinated solvents as well as for other organic chemicals. Furthermore, the investigation demonstrated that concepts and approaches currently being applied in groundwater systems for assessing in situ biodegradation and elucidating degradation pathways of chlorinated solvents can be efficiently transposed to monitor and study organic chemicals in hydrologically and hydrochemically heterogeneous wetland systems.