Thermodynamic Interpretations of Chemical Analyses of Unsaturated Zone Water From Yucca Mountain, Nevada
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ABSTRACT Analytical pore water compositions from Yucca Mountain were evaluated for internal thermodynamic consistency. Significant ionic charge imbalances, unequilibrated aqueous speciation relationships, and erratic variability with depth in some species concentrations were found. Thermodynamic consistency was restored by introducing measured CO 2 gas pressure as a constraint, imposing equilibrium aqueous speciation, and adjusting pH to achieve charge balance. Reinterpreted water chemistry data were used to evaluate and interpret vertical and lateral variations in water chemistry, differences between unsaturated zone pore and perched water compositions, and water-rock equilibria. INTRODUCTION Understanding unsaturated zone (UZ) groundwater chemistry is necessary in predicting the longterm performance of Yucca Mountain as a possible high level nuclear waste disposal facility. The composition and evolution of UZ waters may affect corrosion of engineered barriers, waste form alteration, radionuclide release, retardation of radionuclide transport, dissolution/precipitation of minerals, and changes in porosity and permeability. Predictions of waste package failure times and radionuclide release rates are particularly sensitive to the evolution of carbonate ion concentration and pH [1]. Consequently, the quantity and chemistry of groundwater contacting the engineered barriers constitute a key element of subsystem abstraction for performance assessment. Ironically, it is difficult to obtain reliable groundwater compositions from unsaturated zone environments that may otherwise be desirable for isolation of nuclear waste. Yang et al. [2,3] measured chemical compositions of pore water and perched water from Yucca Mountain. Perched waters were sampled from boreholes using plastic bailers and pore waters were extracted from borehole core samples using high-pressure uniaxial compression techniques. Although the accuracy of the resultant pore water chemistry data was unavoidably compromised by air drilling of core samples, pore water evaporation, and compression techniques [2,3], they provide a valuable characterization of groundwater chemistry at Yucca Mountain. This paper describes equilibrium speciation calculations performed to characterize uncertainties in analytical pore water data [2,3]. We restore thermodynamic consistency to the data given a specific set of assumptions. Revised data were used to evaluate and interpret vertical and lateral variations in UZ groundwater chemistry in terms of water-rock equilibria and to compare unsaturated zone, saturated zone, and perched water compositions. BACKGROUND: UNSATURATED ZONE WATER CHEMISTRY ANALYSES Yang et al. [2,3] extracted pore waters from core samples from boreholes USW UZ-14 and UE-25 UZ#16, SD-7, SD-9, and SD-12. Stratigraphic units penetrated by the boreholes are (in descending order): the Paintbrush Group (comprising the Tiva Canyon Tuff, Yucca Mountain Tuff, Pah Canyon Tuff, and Topopah Spring Tuft), the Calico Hills Formation, and the Prow Pass Tuff. Perched water
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