The migration of plutonium in groundwater (e.g. Mayak, RU, Nevada National Security Site (formerly Nevada Test Site), USA, and Hanford Site, USA) and surface water (e.g. Rocky Flats, USA, Little Forest Burial Ground, AU) has been documented at various locations in the world. In nearly all cases, colloid-facilitated Pu transport has been identified as the dominant mechanism leading to this migration. Importantly, Pu concentrations downgradient from contaminated sites have been consistently low. Both solubility constraints and colloid stability limits likely minimize the migration potential of Pu. Nevertheless, development of a credible conceptual model has been hampered by the lack of experimental data. The single most important process controlling Pu migration in groundwater is Pu stability and desorption from migrating colloids.<br/>To elucidate the mechanisms controlling Pu transport, we have investigated Pu desorption rates from montmorillonite and other mineral colloids under controlled laboratory conditions. These data suggest that Pu desorption rates are slow enough that colloid-facilitated transport of adsorbed Pu is possible at the field scale (km distances and decade timescales). Organic matter surface coatings on mineral surfaces could further decrease desorption rates and enhance colloid-facilitated transport. However, these data also suggest that Pu concentrations and transport distances will be inherently limited and very sensitive to the desorption rates and flow velocities expected in the field.<br/>One important alternative conceptual model that may be relevant at the NNSS and under hydrothermal repository conditions is Pu incorporation into secondary mineral colloids during rock alteration processes. This co-precipitation phenomenon could yield a truly irreversible association of Pu with colloids. This has been investigated through long-term hydrothermal alteration experiments. The results suggest that Pu co-precipitation with secondary phases could yield a “permanent” Pu association with migrating colloids in which colloid filtration rates, and not desorption rates, will limit transport distances at the field scale.<br/>This work was funded by U. S. DOE Office of Biological & Environmental Sciences, Subsurface Biogeochemistry Research Program, and performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Security, LLC under Contract DE-AC52-07NA27344.

Pu transport mechanisms in the environment: Field evidence, conceptual models, and experimental data

249th National Meeting (2015)

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