PROGRESS IN CHARACTERIZING AND SIMULATING CONTAMINANT MIGRATION AT FORMER URANIUM MILL SITES IN THE WESTERN UNITED STATES

The Department of Energy (DOE) Office of Legacy Management manages 26 sites regulated under the Uranium Mill Tailings Radiation Control Act of 1978. Uranium milling occurred at these sites more than 50 years ago and the environment continues to be actively monitored to ensure contamination does not pose a threat to public health. The extraction of uranium from ore-bearing rocks left a legacy of mill tailings and concentrated solutions that have impacted water resources. To improve site conditions, DOE stabilized or relocated mill tailings in the 1980-90’s, but these efforts did not address groundwater plumes. Predictions made using conventional advection-dispersion models assuming linear, equilibrium sorption indicated contaminants would naturally flush from sites within 100 years. However, this approach proved to be too simplistic, particularly because it failed to account for multiple physicochemical phenomena, such as secondary contaminant sources, dual-domain transport or multi-rate mass transfer, geochemical rock-water interactions, and transient flow effects from river flooding. Monitoring data from the former mill sites indicate contaminant flushing is much slower and more complex than previously assumed.

Progress in characterizing groundwater flow and transport at former uranium mill sites is being made using improved field, laboratory, and modeling techniques. Numerous investigators from DOE national laboratories, the U.S. Geological Survey, universities, and private industry are contributing to a better informed understanding of contaminant transport. An iterative, rather than linear, project management approach is being taken to continuously improve conceptual models, assess boundary conditions, obtain lab and field data, and develop models that fit the necessary level of complexity. For example, a variety of reactive-transport models are available for simulating rock-water interaction, and surface complexation codes can replace traditional distribution-coefficient (K_d) methods for simulating species retardation due to sorption. With an enhanced understanding of the mechanisms affecting contaminant fate, the role of natural and engineered remedies can be better evaluated and optimized, leading to improved management of legacy groundwater contamination.