Technetium-99 (Tc) and plutonium (Pu) have been released to the subsurface worldwide, including the release of ~2.59 x 10¹³ Bq of Tc and ~1.85 x 10¹⁵ Bq of Pu at the Hanford Site (Washington State, U.S.A.). Tc and Pu contamination remain a major environmental problem at legacy nuclear reprocessing sites as subsurface mobility is influenced by complex interactions with sediments and co-contaminants within waste streams. Developing enhanced attenuation and efficient remediation strategies for released Tc and Pu requires a complete understanding of retardation processes and mass flux, including the different mechanisms by which Tc and Pu are immobilized in the subsurface and the effect of localized subsurface conditions. Here, sediments from Hanford waste disposal sites have been selected, based on historical information and sediment composition, for characterization of Tc and Pu sediment associations.
Characterization of Tc-containing phases in field-contaminated sediments using Tc K-edge X-ray absorption near edge structure spectroscopy revealed that Tc is present in vadose zone sediments as (i) TcO₄^- that remains in water films associated with fine-grained sediments; (ii) Tc co-precipitated with other lanthanides in phosphate mineral phases; and (iii) Tc present as TcO₂.nH₂O in localized reducing zones.
Previous investigations at Hanford have shown that Pu is associated with discrete particles as PuO₂, as secondary phases formed from interactions with waste or as adsorbed/incorporated species associated with sediments. In this work, evidence is presented for the existence PuO₂, PuO₂-Bi₂O₃ composites, and burnt Pu metal particles in near-surface sediments where acidic waste was disposed. Pu and americium (Am), the long-lived daughter product of Pu-241, moved deeper into the subsurface and exhibit different behavior with depth, suggesting different mobility controlling phases.
From these preliminary results, it is suggested that Pu and Am have limited mobility within the soil profile, and that stimulation of conditions that enhance Tc attenuation can be used to improve long-term Tc remediation strategies.

Sue Clark

Washington State Univ

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