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Staff Accomplishments

Sediment characterization leads to answers about tank waste

August 2004
Cataclysmic floods thousands of years ago may affect how nuclear waste, leaked from underground tanks, behaves today. The floods and other events deposited thin layers of sand, silt, and clay in the Columbia Basin thousands of years ago. Today, 177 underground tanks containing radioactive waste rest in those layers. By understanding the deposition and chemical composition of the sediments, Pacific Northwest National Laboratory's researchers can determine how radioactive waste leaked from these tanks migrates through the sediment. As plans are finalized for removing this waste and preparing it for final storage, this research will also help plan for potential leaks during retrieval. Sediment Deposition
To carefully study the properties of the thin layers of sediment left by floods and other events, PNNL's researchers drill boreholes, about 4 inches wide and up to 250 feet in depth, into the contaminated vadose zone sediment near the tanks. By analyzing these samples and using their previous experience with Hanford's geology and geochemistry, the researchers provide detailed geological and stratigraphic data. Sediment samples are being taken at 6-inch intervals, where previous studies used much longer intervals, on the order of several feet. The small intervals give researchers a more detailed view of what is happening. As project manager Clark Lindenmeier notes, "Thin layers, or lenses, can completely change how waste migrates. Contaminants can use the thin lenses to move several kilometers from the tanks." How do the contaminants use the lenses to travel? Different contaminants in the waste react differently. Certain contaminants might attach to a layer of silt and stop. Other contaminants might not attach and migrate along the silt layer for several miles. Geochemistry Research
Using the soil samples from the tank farms and sophisticated instrumentation, PNNL's researchers are studying how the waste has reacted with the sediment. The highly saline and highly caustic nature of the waste changes the sediment's chemistry in ways not studied before. For example, previous research showed that technetium-99 and chromate do not significantly interact with the layers of fine sand, silt, or clay. The contaminants simply flow along with the water in the soil. However, PNNL's researchers showed that the high levels of mineral salts (salinity) and the basic pH (caustic) of the waste can dissolve certain sediment layers. This creates new conditions that change the properties of the technetium and chromate and partially sequester them in the sediments. The research began in 1997, said technical lead manager Jeff Serne. The $1.4 million per year project will finish in 2006 or perhaps continue on to revisit particularly complicated tank leak regions or to look at other in-soil waste disposal situations at Hanford, such as the cribs and trenches. Contacts: Clark Lindenmeier, Project Manager, (509) 376-8419, clark.lindenmeier@pnl.gov
Jeff Serne, Technical Lead, (509) 376-8429, jeff.serne@pnl.gov
Charlie Brandt, (509) 376-5345, charles.brandt@pnl.gov

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