FIELD AND LABORATORY TRANSPORT STUDIES FOR THE YMP AT LOS ALAMOS NATIONAL LABORATORY

Geochemists at Los Alamos National Laboratory have conducted several field and laboratory transport experiments in the past few years to support site characterization and performance assessment of the proposed high-level nuclear waste repository at Yucca Mountain. Three single-well injection-withdrawal tracer tests were conducted in saturated alluvium at the Alluvial Testing Complex (ATC) from late November 2000 through April 2001. The tests involved the simultaneous injection of nonsorbing solute tracers with different diffusion coefficients (a halide and a fluorinated benzoate) and the use of three different tracer rest periods in the aquifer before pumping. The tests indicated that the saturated alluvium behaved as a single-porosity system, and they also provided estimates of groundwater flow rates at the site.

Laboratory radionuclide batch sorption and column transport experiments have been conducted using representative samples of alluvium along the projected flow pathway(s) from the proposed repository. Unlike previous sorption experiments, which focused primarily on determining partition coefficients (Kd values), these experiments placed greater emphasis on determining the desorption behavior of the radionuclides. Multi-step desorption experiments (repeatedly introducing fresh water to try to induce desorption) and long-duration column experiments were designed to determine distributions of desorption rate constants and to estimate fractions of irreversibly sorbed radionuclides.

Finally, column transport experiments have been conducted using Pu (initially Pu(V) in solution) sorbed onto natural colloids collected from the saturated alluvium at the ATC. Results show that colloids smaller than about 200-nm may play a major role in enhancing Pu transport. To our knowledge, these experiments represent the first colloid-facilitated Pu transport experiments conducted using natural colloids, natural water, and natural porous media.