Speciation studies of uranium contamination in soils at the US DOE Fernald Facility

The U.S. Dept. of Energy's former uranium production facility located at Fernald, OH (18 mi NW of Cincinnati) is the host site an Integrated Demonstration for remediation of uranium-contaminated soils. The soils in this area are principally glacial tills, but also contain silty clays, glacial lake clays, sand, and gravel. In addition, some of the soils are highly disturbed as a result of construction activities and/or limestone overburdens. A wide variety of source terms for uranium contamination have been identified, reflecting the diversity of operations at the facility. Most of the uranium contamination is contained in the top 1/2m of soil, but some has managed to migrate to perched waters. As a prelude to remediation, the authors are conducting uranium speciation studies using x-ray absorption and optical luminescence spectroscopies to determine the precise chemical forms of uranium in the soils, and attempting to correlate these data with source terms. Samples from five discrete sites have been analyzed. It has been found that at least 75% of the uranium exists as hexavalent UO22+ species even though many source terms consisted principally of tetravalent uranium species such as UO2. Much of the uranium exists as microcrystalline precipitates (secondary minerals) such as uranyl hydroxides and carbonates. These similarities in speciation at sites having different source terms suggest that soil and groundwater chemistry may be more important than source terms in defining the uranium speciation in these soils