Influence des transformations minéralogiques sur la mobilité de l'arsenic dans les milieux anoxiques - Application au cas des eaux souterraines du delta du Bengale

摘要

Natural wide-scale poisoning is today affecting the populations that rely on drinking water from aquifers of the Bengal delta. The aim of this thesis is therefore to obtain a better understanding of arsenic mobility within these aquifers by describing, based on the study site at Chakdaha in West Bengal (India), interactions between the mineralogical transformations and the microbial processes. The interactions between iron-reducing or sulfate-reducing bacteria and As-doped ferrihydrite were studied in batch and column experiments under anoxic conditions. The coupling of biotic and abiotic processes is taken into account in various numerical models. A first energetically-driven model was designed and tested on batch experiments and on published data with Bengali sediment incubations. The model provides an explanation of the observed decoupling between the release of Fe(II) and As(III). The column experiments then enabled the reproduction of a possible redox transition with dissolution of iron oxides and precipitation of iron sulfides. Besides, observations at Chakdaha revealed the bioreduction of arsenic-bearing ferric phases and the possible role of carbonates in zones of high arsenic concentrations. All the experiments and field data then made it possible to develop a 1D reactive transfer model of arsenic in these deltaic aquifers. Different scenarios on the sequence between the various electron acceptors were tested and compared with the field data, in particular the Fe(II)/Fe solid ratios. The reactive transport models coupled with microbial processes will play a significant role in the future for predicting the transfer of pollutants in the environment.