Beginning with the Manhattan Project in 1943, 1.85 × 1015 Bq (200 kg) of plutonium (Pu) were released into over 80 unlined cribs, trenches, and field tiles at the Hanford Site, Washington, USA. Specifically, the Z-9 trench received over 4 million liters of Pu-laden reprocessing waste, containing an estimated 50-140 kg Pu, between 1955 and 1962. Approximately 90% of the waste deposited in the trench consisted of high ionic strength (~5 M nitrate, ~ 0.6 M Al), acidic (pH ~ 2.5) aqueous solutions, while the rest of the waste consisted of organic solvents including tributyl phosphate (TBP) and its degradation products, carbon tetrachloride (CCl4), kerosene, and lard oil. The majority of the Pu was observed to precipitate in the first several centimeters beneath the trench, but a small fraction of the Pu was detected in the vadose zone at depths of 37 meters. The exact mechanisms controlling Pu mobility beneath the trench are unknown, but Pu migration is likely driven by weak sorption of aqueous Pu under low pH conditions as well as the formation of Pu-TBP-nitrate complexes in the organic phase. In this study, the reactive transport of Pu in uncontaminated Hanford Site sediments was investigated using column experiments. The sediments were first thoroughly characterized then loaded into glass columns. Pu-bearing aqueous solutions ([Pu] = 1 x 10-8 M, [NaNO3] = 5 M, pH 1 – 2.5) were pumped through the sediments at a rate of 0.022 mL/min using a piston pump. The pH of the effluent was measured and analyzed by liquid scintillation counting and ion chromatography. Pu-bearing 15% TBP in dodecane ([Pu] = 1 x 10-8 M) was also pumped through select sediment columns. Pu transport in the aqueous and the organic sediment columns was compared and modeled.

Plutonium transport in sediments from the Hanford site, USA

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