Date of this Version
Pacific Northwest National Laboratory, October 2007
Uranium (U) is an important risk-driving contaminant at the Hanford Site. Over 200,000 kg have been released to the vadose zone over the course of site operations, and a number of vadose zone and groundwater plumes containing the uranyl cation [UO22+, U(VI)] have been identified. U is recognized to be of moderate-to-high mobility, conditions dependent. The site is currently making decisions on several of these plumes with long-lasting implications, and others are soon to come.
Uranium is one of nature’s most intriguing and chemically complex elements. The fate and transport of U(VI) has been studied over the long lifetime of the Hanford Site by various contractors, along with the Pacific Northwest National Laboratory (PNNL) and its collaborators. Significant research has more recently been contributed by the national scientific community with support from the U.S. Department of Energy’s (DOE) Office of Science through its Environmental Remediation Sciences Division (ERSD). This report represents a first attempt to integrate these findings into a cohesive view of the subsurface geochemistry of U at the Hanford Site. The objective is to inform all interested Hanford parties about the in-ground inventory of U and its geochemical behavior. This report also comments on the prospects for the development of a robust generic model to more accurately forecast future U(VI) migration at different Hanford waste sites, along with further research necessary to reach this goal.
To accomplish the report objectives, the environmental geochemistry of U at the Hanford Site is discussed in terms of both the vadose and saturated zone, to the extent that it is known. Hexavalent uranium [U(VI)] is the dominant valence form of U under the predominantly oxidizing subsurface conditions at the Hanford Site, and the researchers’ analyses consequently emphasize this species. The nature and concentration of background U in Hanford subsurface sediments is identified to place contaminant U(VI) concentrations and behavior in perspective to the natural system. In-ground U-waste inventories are quantified and characterized with regard to source term, to the extent possible, and the most important sites from an inventory perspective are identified. The U-isotopic content of various waste streams are discussed from the perspective of waste-source tracking. The geochemical attenuation processes responsible for slowing the rate of subsurface U migration, relative to the transporting water front, are illustrated through careful consideration of both field characterization studies of existing U vadose-zone and groundwater plumes, and laboratory studies of derived contaminated and uncontaminated sediments. Both empirical and more mechanistic models of these attenuation processes are considered as well as the parameters that define attenuation magnitude. Attention is given to the behavior of contaminant U(VI) that has been in contact with Hanford sediments for extended periods (circa 10–50 years), as long contact imparts unique character to U(VI) geochemical behavior by allowing 1) slow reactions to reach completion, and 2) diffusional access to intra-grain fractures and pores.