The interaction of dissolved rare earth elements with basin-fill materials from the Yucca Flat area of the Nevada Test Site, with inferences to actinide mass transfer potential

The geochemical processes influencing potentially infiltrating surface water quality have been investigated at Yucca Flat, Nevada. This research has focused on the aqueous behavior of the rare earth elements (REE) or lanthanide series elements (lanthanum through lutetium on the periodic chart, atomic numbers 57 to 71) in hydrogeologic environments as both geochemical tracers and indicators for the potential mobility of fission products. This research is presented in an introduction followed by three manuscripts describing the geochemical constraints on dissolved REE behavior, and the behavior of dissolved REE in the presence of basin-fill sediments from Yucca Flat. These studies indicate that differences in basin-fill sediments influences the major ion chemistry of potentially infiltrating waters as well as the attenuation of dissolved REE.;The first manuscript describes the physical, chemical, and mineralogical properties of the Yucca Flat sediments. Variations in the basin-fill sediments includes saturation paste extract composition and many parameters influential to contaminant attenuation. Shale normalized bulk REE analyses exhibit europium depletion and some suggestion for REE fractionation by preferential removal of heavy REE during weathering. The variation between basin-fill material types results in a greater than two order of magnitude difference in partition coefficients for dissolved REE.;The second manuscript describes REE stable isotope and radioisotope sorption experiments conducted using the Yucca Flat basin-fill materials and REE dissolved in solutions equivalent to the saturation paste extracts. Experimental results indicate that aqueous REE behavior is strongly influenced by the presence and distribution of carbonate phases. In the presence of high dissolved carbonate species concentrations, REE partitioning is inhibited. In the presence of high solid phase carbonate concentrations, sorption is enhanced.;The third manuscript evaluates the results from the REE sorption experiments in light of an REE solution speciation model. In the saturation paste extracts inferred to represent potentially infiltrating waters, dissolved REE are present predominantly as dicarbonato complexes. The degree of REE-dicarbonato complexation inhibits the partitioning onto the solid phase and appears to retard the rate of uptake. REE fractionation is also enhanced in the presence of higher dissolved free carbonate concentrations.;This research indicates that variations in basin-fill properties create differences in REE sorption that make scale an important parameter in potential radionuclide migration studies at Yucca Flat. The similarities between the REE and the trivalent transuranic actinide elements suggest that variations in dissolved actinide behavior within the basin are also to be expected.