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EXECUTIVE SUMMARYFor almost three decades radiochemical data have been collected from water samples that originate from saturated nuclear test cavities, chimney rubble zones, and satellite wells constructed adjacent to underground nuclear tests conducted on the Nevada Test Site (NTS)*_Although these data have been . collected regularly under the auspices of Defense Programs and Environmental Management funded programs, there has been less effort made at interpreting the data to define controls on the dissolved or mobile fraction of radionuclides in groundwater. The need for data analysis have been given more impetus by the contaminant transport in the near-field. These models rely on input parameters a d boundary conditions which can best be obtained through field observations of radionuclide migration adjacent to nuclear tests. The present report is an attempt to synthesize observations and analyze data returned from the near-field over the past ten years and draw general conclusions regarding the behavior of radionuclides in Nevada Test Site groundwaters.. recent effort to construct sophisticated models of hydrologic flow andThe complexity of the hot well problem deserves mention. Most nuclear tests were conducted between -500 and -1200 m below the surface. Besides the obvious residual radioactivity, the post-test environment is characterized by extreme heterogeneity due to the mechanical damage done to the rock units at the working (firing) point. For these reasons, the construction of wells and collection of water samples is a technically complex, expensive, and logistically complicated enterprise. While data exists, it must be interpreted with care and, for these reasons, data interpretations have been limited in scope. In the majority of cases, near-field or "hot" wells were completed opportunistically in post-shot re-entry borings into a nuclear test cavity or chimney, originally drilled to return solid samples diagnostic of device performance. Of the more than eight himdred nuclear tests conducted at the NTS, only fifteen near-field study sites regularly produce water. It must be emphasized that the near-field or "hot well" sampling program is not a monitoring program designed to detect the arrival or movement of radionuclides.. Rather the data from these wells can be used to establish bounds on those radionuclides in solution and to deduce controls on their solubility and mobility. Data returned from near-field wells are affected by well completion techniques and in particular by whether the water is pumped or bailed. Only pumped samples are truly representative of water in the sampling interval, and time-series analyses of pumped cavity water samples have proven particularly important in illuminating the nature of radionuclide migration.The justification for a program to collect near-field groundwater samples can be capsuled in five points. First, hot well sampling and analysis provides an the empirical measure of radionuclide solubility and resulting concentrations of importance to determining risk to ...
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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