UNDERSTANDING GEOCHEMICAL PROCESSES CONTROLLING COLLOID-FACILITATED TRANSPORT OF PLUTONIUM: FROM 10-INCH CORE EXPERIMENTS TO KILOMETER SCALE FIELD OBSERVATIONS

A major scientific challenge in environmental sciences is to reliably predict and control the cycling and mobility of highly toxic and long-lived radionuclides (e.g. plutonium (Pu)). Over 2000 metric tons of Pu have been deposited in the geosphere, representing a significant long-term environmental risk. It is generally thought that due to its low solubility and high sorption affinity, Pu migration occurs when facilitated by transport on particulate matter (i.e colloids, <1 micron particles of either inorganic, organic or microbial material). The ability of colloids to facilitate the long-term transport of Pu is a function of its redox chemistry, initial sorption to colloid surfaces, but also its desorption rate from surfaces. Despite the importance of colloid-facilitated transport of Pu, the mechanisms controlling coupled Pu-colloid interfacial processes are not well understood. Significant breakthroughs in our conceptual model of colloid-facilitated transport can occur through advances in our understanding of coupled processes, from colloid-water surfaces to the field scale.

To better understand the mechanisms controlling colloid-facilitated transport of Pu, a series of fracture -flow laboratory experiments were conducted with soluble Pu and Pu pre-sorbed to mineral colloids. The radionuclide cocktail was injected in polished fractured volcanic tuff cores. The effluent was analyzed and autoradiography and secondary ion mass spectrometry was used to image the colloid-Pu-rock interactions. Experimental conditions employed mimic conditions expected for the kilometer scale transport of Pu observed downgradient from underground nuclear tests at the Nevada Nuclear Security Site (NNSS, formerly called NTS). In our experiments, ~30% more Pu was eluted through the fractures with the colloids compared to soluble Pu. The majority of Pu retained in the fractured volcanic tuff was associated with the various minerals in the altered volcanic tuff cores, but significantly more Pu was associated with the minor Mn and Fe oxides. Our efforts combining fracture transport experiments, flow cell sorption/desorption experiments, and field scale monitoring at NNSS are helping to elucidate the dominant processes that control colloid-facilitated transport.

Prepared by LLNL under Contract DE-AC52-07NA27344.