9- Iron And Manganese. Reactivity And Recycling (The Seine-Aval Scientific Programme

Le travail porte sur l'origine et le devenir du fer et du manganèse présents dans la Seine et son estuaire. Ces deux métaux, considérés comme non polluants, ont retenu l'attention en raison de leur capacité à jouer un rôle de marqueurs de processus impliquant des métaux à toxicité marquée. Leur comportement a été étudié dans la colonne d'eau, les particules en suspension et les sédiments. L'importance des réactions d'échange hétérogènes a été mise en évidence. Le sédiment ne se comporte pas uniquement comme un piège pour les métaux lourds mais peut constituer une source potentielle de contaminants métalliques liée aux variations physico-chimiques du milieu. Ces résultats devront être pris en compte lors des études d'impact d'aménagements (remaniements de sédiments, dragages...).

Introduction A little history. Chapter I - What are the sources of the iron and manganese in the Seine river? Iron and the manganese of natural origin Iron and manganese of anthropic origin Chapter II - What are the usual iron and manganese concentrations in waters and sediment particles of the Seine estuary Chapter III - What are the iron and manganese concentrations in other European estuaries? Chapter IV - How have iron and manganese concentrations varied with time in the Seine estuary In sediments In suspended matter In water Chapter V - What are the inputs of iron and manganese from the Seine river to the estuary Chapter VI - What are the anthropic inputs of iron and manganese into the Seine downstream of Poses? Chapter VII - How much iron and manganese in the sediments of the Seine can be attributed to human activities? Chapter VIII - Behaviour of iron and manganese in the water column during their transit in the estuary Conservativity and reactivity Case of iron Case of manganese What sedimentary fractions are iron and manganese associated to? What are the mechanisms involved in exchange reactions? Case of iron Case of manganes A chemical model to describe manganese behaviour in the Seine estuary Chapter IX - Anoxic sediments: a confined and reactive environment Confinement and diagenesis Degradation of the organic matter: sequence of reactions involved and consequences on the interstitial medium Oxygen is the first species consumed Nitrates take part in the oxidation of organic carbon Manganese and iron oxides and hydroxides are released into solution Sulphates are then reduced to sulphides What leads to the uptake at depth of dissolved iron and manganese produced beneath the sediment-water interface? Case of manganese Case of iron How rapidly do particulate iron sulphides appear in sediments? Sulphides: sink phases for trace metals Chapter X - Processes induced by the resuspension of anoxic sediments Increase of dissolved iron and manganese contents in the water column and erosion of the bottom sediments What is the influence of iron and manganese on the recycling of trace metals in the estuary and their export towards the ocean? How much iron and manganese can be remobilized by the various erosive sedimentary processes in the estuary? Erosive sedimentary processes Geochemical processes involved Conclusion and perspectives References Glossary