This report summarizes the findings of a geochemical investigation of the Pahute Mesa-Oasis Valley groundwater flow system in southwestern Nevada, funded by the Underground Test Area project of the U.S. Department of Energy, National Nuclear Security Administration Nevada Operations Office. This work is intended to provide geochemical data and interpretations in support of flow and contaminant transport modeling for the Western and Central Pahute Mesa Corrective Action Units. The principal conclusions of this study are as follows: Geochemical and isotopic data in the Oasis Valley flow system are consistent with a direct groundwater flowpath between Pahute Mesa and Oasis Valley. This conclusion is in accord with hydraulic gradients and current hydrogeologic framework models of the area. The geochemical and isotopic data indicate that groundwater discharge in central Oasis Valley consists predominantly of deep regional groundwater flow beneath Pahute Mesa mixed with a smaller amount of local recharge. Geochemical signatures in the Oasis Valley discharge area are further affected by shallow groundwater recirculation or gas exchange through the unsaturated zone. Available data suggest that groundwater in northwestern Oasis Valley originates from north of Oasis Valley, and that groundwater from southeastern Pahute Mesa flows both into the Beatty Wash area of Oasis Valley and down Fortymile Canyon. The lack of tritium in groundwater indicates that travel times from Pahute Mesa to Oasis Valley are greater than 50 years. Corrected dissolved inorganic carbon and dissolved organic carbon 14 C groundwater ages indicate that average travel times from Pahute Mesa to Oasis Valley range from modern (greater than 50 years) to 8,500 years. Trace element data and U and Sr isotopic data are consistent with the proposed source areas, flowpaths, and mixing of groundwaters that discharge in Oasis Valley.

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vii • Pu sorption/desorption experiments on colloid minerals identified in NTS groundwater, and • Transport of Pu-doped colloids through fractured rock core. Chapter 1 is a background review of our current understanding of colloids and their role in facilitating contaminant transport. Chapters 2, and 3 are field studies that focused on characterizing natural colloids at different hydrologic environments at the NTS and address Ryan and Elimelech's (1996) first criteria regarding the existence and stability of colloids. Chapters 4, 5 and 6 are laboratory experimental studies that investigate the sorption/desorption behavior of Pu and other low-solubility radionuclides on colloid minerals observed in NTS groundwater. These studies evaluate Ryan and Elimelech's (1996) second criteria that the affinity of Pu for colloids must be high. Chapters 7, 8, 9, and 10 are laboratory studies that focus on whether colloids can be transported through fractures. These transport studies address Ryan and Elimelech's (1996) third criteria that colloids must be transported. Chapter 11 discusses the implications of the fracture flow experimental results. Chapter 12 provides recommendations for future work that would help reduce uncertainties associated with the prediction of colloid-facilitated radionuclide transport at the NTS.

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