Comparisons of degradation kinetics of chloroethenes in groundwater between microcosms and field scale
Résumé
Groundwater pollution by chlorinated solvents is a major concern since several years. It has been demonstrated that in specific physicochemical conditions, microbial processes like direct reductive dechlorination allow contamination reduction at several sites. Therefore, determination of biodegradation kinetics of chloroethenes is crucial in applying Natural Attenuation protocols on contaminated sites and assessing the potential risks for human health and natural media Biodegradation of chlorinated solvents is effective in highly reduced conditions, which rarely concerns the whole contaminant plume. In this study, direct reductive dechlorination of chloroethenes was studied on two different scales, on microcosms in the laboratory and at the real scale that corresponds to the contaminated site Microcosms studies were conducted in three different ways. (1) sediments sampled from the site and mixed with groundwater modified or not by a synthetic electron donor (Na propionate, Na lactate, toluene), (2) composite sediments coming from several places of the site mixed with groundwater modified or not by a synthetic electron donor; (3) autoclaved sediments and groundwater modified or not by synthetic organic matter. Studies on the real scale were conducted by the achievement of a synthesis of historical data (hydrogeological, geological and physicochemical data) of a polluted site. The synthesis of physicochemical data and then modelling the real site revealed the presence of degradation products of chloroethenes in the plume : cis-1,2-DCE and VC The results of comparisons of degradation kinetics obtained on the laboratory and the field under the same physicochemical conditions showed significant differences. Indeed, biodegradation of chlorinated solvents were faster in lab studies than in the field at the global scale. The existence of chlorinated ethenes biotransformation in microcosms confirmed the presence of a bacterial population able to catalyse reductive dechlorination reaction until CV. It is also likely that the bacterial consortium permitted to degrade other species like electron acceptors; detection of sulphide ions and Fe(II) and the presence of a black precipitate of FeS are proofs of sulphate reducing, ferro reducing and dechlorinating activities. The clear difference that there is between kinetics of degradation on microcosms and field scale could be explained by differences in chemical conditions that are not optimal everywhere in the plume of pollutants. The differences of chemical conditions (electron acceptors, type of natural organic matter, pH, redox potential...) are investigated in details to explain the differences in kinetic constants
Domaines
Sciences de l'environnement
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