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December 2008

Seeing the World in a Column of Sand

Study helps predict where uranium-tainted groundwater will go, when it will arrive

Results: By examining the behavior and characteristics of a small volume of sediment, scientists at Pacific Northwest National Laboratory developed a coupled experimental and computational approach that may better predict the behavior of uranium over relatively large areas in the field. Using experimentation and a computer model, the team accurately scaled the chemical effects they observed for a sediment component to describe the combined physical and chemical effects on uranium transport within the sediment as a whole. The scaling information was derived from experimentation with non-reactive tracers moving through the whole sediment.

At the Hanford Site, a former plutonium production complex in Washington State, groundwater contaminated with uranium is a concern. When and how much of this uranium reaches the nearby Columbia River is a question whose resolution could have large impacts on remediation.

Why It Matters: The Department of Energy, which manages the Site, must predict where along the riverbank the uranium will arrive and when. An accurate determination of river shore impacts will allow effective and efficient implementation of technologies to capture or stabilize the uranium before it enters the river. Chemical experimentation can determine the reactions responsible for controlling uranium movement, but such results are limited in their ability to predict field-scale movement in the natural environment. The laboratory results must be "scaled up" to include the physical and chemical components of the natural system. Scaling is complicated by the size and inherent variety of materials in the subsurface.

soil in column.
At Pacific Northwest National Laboratory, researchers developed a combined experimental and computational approach that may better predict the behavior of uranium in the field.

Methods: To understand how contaminants move through the subsurface, researchers collected sediment from the field. They separated the sediment into two fractions: one fraction of grains with a diameter greater than 2 millimeters, and the other of grains with a diameter smaller than 2 millimeters. Contaminants preferentially bonded to the finer grains, while large grains made up most of the soil by weight and volume.

The team studied the fine grains to define the chemical behavior of uranium. They used the results to establish a chemical model to describe the behavior of uranium in groundwater associated with the fine fraction of the sediment.

Then, the researchers packed about 60 pounds of whole sediment into a see-through plastic column with a diameter of 6 inches, and measured how fast a solution moved through the soil. The solution was designed to mimic uranium-contaminated groundwater, but the role of uranium was played by the nonradioactive chemical pentafluorobenzoic acid, or PFBA, which was chosen because it closely matched the physical behavior of uranium. For example, its rate of diffusion in an aqueous solution was close to the rate for uranium. For whole-sediment studies, the movement of uranium was thus isolated from the effects of chemical reaction. A non-reactive tracer, bromide, was used to determine the movement of the groundwater components other than uranium. 

The researchers found that the groundwater flow and migration of the uranium analogue were strongly influenced by the distribution of large grains (pebbles and cobbles).

The results from the large column experiment were used to build a physics-based model to describe uranium migration within the complex sand-pebble-cobble system. To predict the overall behavior of uranium movement in the field, the physics-based model was linked computationally with the chemical model of uranium reaction in the fine-grained portion of the sediment. The combined model was tested against experimentation with the same column using uranium-containing solutions: the measured results of uranium migration in the large column (responding to the chemical and physical components of the experimental system) validated the model.

The experimental and computational approaches developed in this research may be extended to predict uranium migration in field. The requirement for the field application is the direct field measurement of groundwater flow and nonradioactive chemical migration. The field information can then be linked with laboratory measurements of chemical reaction to predict uranium transport in field.

What's Next? The researchers will perform tests with undisturbed soils and conduct studies in the field at the Hanford Site.

Acknowledgments: The Office of Biological and Environmental Research at DOE funded this research through the Environmental Remediation Science Program. The DOE Office of Environmental Management also supported this work through the Hanford Remediation and Closure Science Project.

Chongxuan Liu, John Zachara, Nikolla Qafoku, and Zheming Wang at PNNL performed the research. They conducted the column tests in PNNL's 331 Building and performed the laser-induced fluorescence spectroscopy measurements at DOE's EMSL, a national scientific user facility at PNNL.

This work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.

Reference: Liu C, JM Zachara, N Qafoku, and Z Wang. 2008. "Scale-dependent Desorption of Uranium from Contaminated Subsurface Sediments." Water Resources Research 44(8):W08413, doi:10.1029/2007WR006478.

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