Tracer and radionuclide sorption to vitric tuffs of Busted Butte, Nevada

Abstract

Field-scale unsaturated-zone tracer tests have been performed at Busted Butte, Nevada, near the potential high-level radioactive waste repository at Yucca Mountain. These tests are intended to improve our understanding of unsaturated-zone transport processes, and to test our ability to predict field-scale behavior using laboratory-scale measurements. The field tests use a mixture of nonreactive and reactive tracers. Nonreactive tracers include Br −, I −, 5 different fluorobenzoic acids, Na fluorescein, and a pyridone derivative. Reactive tracers include the metals Li, Mn, Co, and Ni, and the organic dye rhodamine WT. Rock samples from 3 different stratigraphic units at the Busted Butte test facility have been extensively characterized lithologically and mineralogically, and analyzed for Fe and Mn oxide content. Sorption of Np, Pu, Am and the field tracers onto these 3 rocks has been measured using batch techniques. Results confirm that the nonreactive tracers are indeed nonreactive, and show that sorption of radionuclides and sorbing tracers increases with increasing degree of rock alteration, as evidenced by increasing levels of smectite and Fe and Mn oxides. Among the radionuclides, Am and Pu sorb much more strongly than Np; the tracers' degree of sorption is rhodamine WTLi⪡Mn<Ni<Co. Pu, Co, Mn, Ni and rhodamine WT exhibits strongly nonlinear sorption; Mn and Ni behavior may reflect competition with Co for sorption sites.