IMPACTS OF REDUCING CONDITIONS IN THE SATURATED ZONE AT YUCCA MOUNTAIN

Abstract

Variations in groundwater redox chemistry in the saturated zone near Yucca Mountain could have significant repository to the accessible environment. This study examines geochemical data relevant to the distribution of redox impacts on processes associated with the potential transport of redox-sensitive radionuclides from the proposed conditions in the saturated zone, the relationships between redox state and solubility and sorption coefficients for technetium and neptunium, and sensitivity in transport model simulations. Results indicate evidence for a zone of reducing conditions in the volcanic rocks of the saturated zone located to the east and south of the repository and along the inferred flow paths from the repository. A working hypothesis is that these reducing conditions are related to the presence of minor pyrite in the matrix of some volcanic units. Chemical equilibrium modeling of technetium solubility using EQ3/6 software codes is used to estimate the value of solubility limits as a function of Eh. Surface complexation modeling with the EQ3 code is used to estimate neptunium sorption coefficient values as a function of Eh. A general analytical approach, one-dimensional reactive transport modeling, and the three-dimensional saturated zone site-scale transport model using the FEHM software code are used to evaluate the impacts ofmore » solubility limits and enhanced sorption in reducing zones on the simulated transport of technetium and neptunium in the saturated zone. Results show that if precipitation occurs in response to flow through a reducing zone, then the peak concentration released to the accessible environment will be restricted to the solubility limit. Simulations also show that enhanced sorption within a reducing zone of modest width leads to significantly greater retardation of radionuclides in the saturated zone.« less