Thesis: Uranium scavenging in soils and sediments located downstream from former U mines is expected to naturally limit uranium dispersion in downstream waterways. However, uranium mobility in such contaminated sites may depend on the identity of U traps as well as the geochemical conditions. The aim of this thesis was to improve our knowledge on the geochemical behavior and the mobility of uranium in U contaminated lacustrine sediments and wetland soils, whose reducing conditions is expected to mitigate uranium mobility because U(IV) species are less soluble than U(VI) ones. X-ray absorption spectroscopy and scanning electron microscopy analyzes combined with geochemical analyzes were carried out. In U contaminated lake sediments, we show that indirect reduction of U(VI) by Fe(II) associated to clay minerals may be a major diagenetic process responsible for the scavenging of uranium. For organic-rich weltand soils, we show a sharp uranium redox boundary mainly controlled by the water-table. For both sites, U(IV) mononuclear species and U(IV)-phosphate minerals were identified as the major species controlling uranium solubility, while uraninite is virtually absent. For the highly U-contaminated wetland soil, we suggest a major uranium redistribution via the oxidative dissolution of U(IV)-minerals followed by U(VI) organic matter complexation. Soil incubation experiments have confirmed these redistribution mechanisms and suggest different geochemical behaviors for lermontovite (U(PO4)(OH)•H2O) and ningyoite (CaU(PO4)2•2H2O). These experiments also highlight the role of organic matter in the control of uranium mobility, favoring the remobilization of U(IV) organic complexes under reducing conditions. Altogether, our results call for the need to consider both non-uraninite U(IV) minerals and mononuclear U(IV) complexes in such anoxic environments as major species controlling uranium solubility.