Uranium Desorption From Contaminated Sediments at the USDOE IFC Research Site in Rifle, CO: From Batch to Field

Uranium contamination in the subsurface is a part of the legacy of nuclear weapons and energy production, resulting from both mining activities and nuclear waste disposal. The Rifle IFC project focuses on gaining a better mechanistic understanding of U mobility in the subsurface and the use of bioremediation to achieve groundwater U concentrations below the MCL at a former U mill tailings site, integrating biological, geochemical, and hydrological studies. As a part of this project we have performed a series of experiments to better understand the U(VI) sorption-desorption and transport behavior under oxidizing conditions at this site. A series of U(VI) desorption experiments on aquifer sediment was conducted at the batch, column, and field scales. As one proceeds from the batch to the field scale, there is an increase in complexity and heterogeneity in both the geochemical and hydrological conditions. A surface-complexation model for U(VI) developed using batch adsorption and desorption experiments on homogenized sediments was applied to describe U(VI) desorption and transport behavior under high alkalinity conditions in a column experiment with the same sediments. An array of multi-level samplers was installed at the Rifle field site in order to investigate U behavior in both 3-dimensional spatial and temporal scales. A high degree of geochemical and hydrological heterogeneity was observed through the investigation of sediment core samples, nonreactive tracer tests, and geochemical groundwater sampling. A U(VI) desorption tracer test was performed in the field under high alkalinity conditions to compare with the batch and column work. The results from these multiple scales investigations are being integrated to assess the impact of the observed field-scale heterogeneities on U reactive transport in contaminated aquifers.