To clarify the mechanisms of uranium distribution in river sediment and its speciation in river water near a phosphate mining region, the sediment and river water in the Mianyuan River in China were investigated. Using linear correlation analysis, it was determined that uranium distribution in upstream sediment is mainly controlled by the adsorption of organic compounds ( R TOC-U 2 = 0.848 ), whereas distribution is controlled by physical deposition of albite ( R Na-U 2 = 0.773 ) in downstream sediment. In addition, it was determined that Se and As had strong and positive linear correlations with uranium in sediment, regardless of whether it was upstream or downstream sediment. This type of relationship was also found in phosphate ores, which suggests that the ore is the common source of U, Se and As in the sediment. The results of speciation-solubility modelling suggest that no precipitation of uranium minerals occurred in river water and the dominant uranium species in river water included: UO2(HPO4)22−, Ca2UO2(CO3)3 and UO2(CO3)34−. In addition, our modelling indicated the relative species distribution was mainly affected by the pH and the total phosphate concentration.
