A 300 d solubility study involving two carbonate-rich, uranium-contaminated soils from the Department of Energy's Fernald Environmental Management Project site was conducted to predict the behavior of uranium during on-site remediation of these soils. Geochemical modeling was performed on the aqueous species dissolved from these soils following the solubility measurements to predict the on-site uranium leaching and transport potential. Results showed that the soluble levels of the major components (total uranium, calcium, magnesium, and carbonate) increased continually for the first 4 weeks. After the first 4 weeks, these components either reached a steady-state or continued to increase linearly throughout the study. Soluble uranium levels of both soils and their correlation with alkalinity was strongly mediated by the source term of the contamination. Geochemical modeling predicted and anion exchange experiments confirmed that uranyl-carbonate complexes were the most stable and abundant complexes. Further modeling showed that uranium solubility in these soils and in onsite groundwater wells is controlled by UO2(H2PO4)2, but is also mediated by complexation with carbonate and the oxidation state of the uranium. For assessing the risk related to off-site transport of uranium, it should be recognized that the solubility of uranium-bearing minerals is the critical factor in controlling uranium solubility of these soils rather than sorption/desorption processes as measured by the uranium distribution coefficient (Kd) in these soils.