At the Nevada National Security Site (formerly Nevada Test Site), colloid-facilitated Pu transport has been identified as the dominant mechanism leading to downgradient migration of Pu.  In 1999, the transport of Pu from the Benham underground nuclear test to a well 1.3 km downgradient (ER-20-5) was detected.  Since that time, several new wells have been drilled nearby.  Groundwater from one of these wells, ER-20-7, located 300 meters downgradient from ER-20-5, was found to contain Pu.  Isotopic ratio measurements have identified the source of Pu as the Benham test.  These results, combined with tritium data, indicate that trace Pu is migrating at this location.  To elucidate the mechanisms controlling Pu transport at the NNSS (and other field sites), we have investigated the Pu desorption rates from montmorillonite 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).  However, the transport distances are very sensitive to the desorption rates and flow velocities expected in the field. An alternative scenario at the NNSS is Pu incorporation into secondary mineral colloids during nuclear melt glass alteration processes which can lead to irreversible association of Pu with colloids.  This mechanism is being investigated through longterm hydrothermal alteration experiments and Pu desorption/extraction experiments.  We will summarize the field and laboratory investigations of Pu colloid-facilitated transport and discuss the underlying mechanisms controlling Pu transport at the NNSS and other field sites.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.

Colloid-facilitated Pu transport mechanisms at the Nevada National Security Site: Linking field evidence, laboratory desorption kinetics, and Pu desorption from glass alteration products

248th National Meeting (2014)

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