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

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 C-labelled CB. Significant shift in the isotopic composition of CB over the flow path under prevailing iron reducing conditions and the detection of C-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 cisand 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. C values for transand cis-DCE substantially changed over the flow path and correlated