Thesis: Understanding the geochemical behavior of uranium (U) in the environment is crucial for the limitation of U dissemination in contaminated systems. The primary objective of this thesis was to determine the potential evolution of noncrystalline U phases over thousand years in naturally U-rich lacustrine sediments (up to more than 1000 µg/g) from Lake Nègre, in the Mercantour-Argentera Massif (South-East France). Using U isotopic ratios (δ238U and (234U/238U)) and U L3-edge X-ray absorption spectroscopy (XAS), we show that U is first deposited as organic-bound mononuclear species and is readily reduced to U(IV). In less than 700 years, these species transform into U(IV)-silica polymers with a local structure close to that of coffinite (USiO4·nH2O). This transformation highlights the role of ligand abundance in limiting the precipitation of crystalline U phases, but only slightly reduces the lability of uranium which potential mobility remains significant even after several thousand years. At the watershed scale, U originates from fractures in the granitic bedrock and is subsequently scavenged in the soils, especially in the wetland upstream of the lake (up to > 5000 µg/g), before being transported through erosion. Uranium scavenging in the soils occurs through complexation by organic matter including particular biological structures, followed by partial reduction to U(IV). U is thus mainly present in mononuclear species but also in polymeric U(VI) phases which may potentially result from U aging after thousand years of accumulation.