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Caractérisation et stabilité de la matière organique des sédiments des grandes retenues

Abstract : Hydroelectric dams induce sediment accumulation which represents an important stock of organic matter (OM) leading to management difficulties. Their elimination often carried out by flushing, temporarily depletes the biotope downstream of oxygen. This sediment is not stable, and can generate greenhouse gas emissions, especially under anaerobic conditions in situ. The OM mineralization from organo-mineral matrices is controlled by its biodegradability or bioaccessibility. For soil matrices, recent works established that OM mineralization mainly depends on its interaction with the organo-mineral matrix according to the “soil continuum model”. We therefore studied the mechanisms involved in the OM stability of dam sediments. We applied a sequential extraction method, which separates OM based on its strength interaction with the matrix. The biochemical nature and the potential for mineralization of each OM group showed that i) the carbon OM consists mainly of phenol, carbohydrate, and protein, in comparable proportions between the groups, ii) the mineralizable character of OM is related to its degree of attachment to the matrix. These results show the impact of organo-mineral interactions on the evolution of OM in sediments. Then, we tackled the OM evolution for two modes of sediment management. i) In anaerobic conditions, we looked for estimation tools of the methane potential of the sediment by comparing them to the production of biogas after 20 days of ex situ incubation. Two indices were retained, monitoring of the F420 cofactor (involved in methanogenesis pathway) at 7 days ex situ, and monitoring of the "density" parameter measured by an echosounder in situ. ii) In aerobic condition we developed an incubation method to separate the chemical and the biological oxygen demand. Chemical demand is strong and rapid, biological demand is slow and represents only 11 to 22% of the total oxygen demand of the sediment. These data could be of interest when cleaning the reservoirs.
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Sylvain Bascle. Caractérisation et stabilité de la matière organique des sédiments des grandes retenues. Sciences de la Terre. Université de Limoges, 2020. Français. ⟨NNT : 2020LIMO0083⟩. ⟨tel-03351890⟩

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