Plutonium Transport in Vadose Zone Sediments Under Acidic Solution Conditions at the Hanford Site, United States
Teresa Baumer1*, Annie B. Kersting1, Mavrik Zavarin1 (firstname.lastname@example.org), Carolyn Pearce2, Hilary P. Emerson2
1Lawrence Livermore National Laboratory, Livermore, CA; 2Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA
Between 1955 to 1962 over 4 million liters of acidic processing waste, containing an estimated 40–150 kg of plutonium (Pu), were released into the sediments of the 216-Z-9 (Z-9) trench at the Hanford Site in southeastern WA. The waste was characterized as having a high ionic strength (~5 M nitrate), low pH (~pH 2.5), and contained organic processing solvents including tributyl phosphate (TBP). The majority of the Pu precipitated in the first several centimeters beneath the unlined trench, but a small fraction was detected in the vadose zone at depths up to 37 m (Cantrell and Riley 2008). Previous research has shown that the Pu and TBP are detected concurrently in the subsurface in the sediments with the lowest pH 1; however, the mechanisms controlling past and future Pu mobility beneath the trench are unknown.
In an effort to better understand Pu migration below the Z-9 trench, scientists undertook a series of bench-scale saturated column experiments using uncontaminated Hanford sediments and Pu in a range of high nitrate, acidic solution compositions with and without TBP in dodecane. Two types of experiments were designed to investigate (1) Pu mobility in low pH aqueous fluids and (2) Pu mobility with mixed aqueous organic fluids. The effluent was analyzed for Pu, changes in pH, and solution chemistry, and the data was compared and modeled. Results show that Pu does not become mobile until the pH of the sediments is reduced below pH 4. In low pH aqueous fluids, significant Pu mobility (14% total Pu breakthrough) is not observed until pH < 2. Pu in organic TBP-containing solvents is highly mobile in sediments that have been treated with high nitrate low pH acidic fluids. Pu can travel with these organic solvents virtually uninhibited with 94.8% and 86.8% total Pu breakthrough, observed at pH 1 and pH 3 respectively. The results of this study show that 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 at pH < 4. Pu migration in the subsurface will be limited by the natural buffering capacity of the sediments as well as the dispersal of the nitrate plume.
Cantrell, K. J., and R. G. Riley. 2008. A Review of Subsurface Behavior of Plutonium and Americium at the 200-PW-1/3/6 Operable Units, PNNL-SA-58953. Pacific Northwest National Laboratory. https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-SA-58953.pdf