Uranium and neptunium were used as reactive tracers in long-term laboratory desorption studies using saturated alluvium collectedfrom south of Yucca Mountain, Nevada. The objective of these long-term experiments is to make detailed observations of the desorption behavior of uranium and neptunium to provide Yucca Mountain with technical bases for a more realistic and potentially less conservative approach to predicting the transport of adsorbing radionuclides in the saturated alluvium. This

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... describes several long-term desorption experiments using a flow-through experimental method and groundwater and alluvium obtainedfrom boreholes along a potential groundwater flow path from the proposed repository site. In the long term desorption experiments, the percentages of uranium and neptunium sorbed as a firnction of time after different durations of sorption was determined. In addition, the desorbed activity as a function of time was Jit using a multi-site, multi-rate model to demonstrate that drfferent desorption rate constants ranging over several orders of magnitude exist for the desorption of uranium from Yucca Mountain saturated alluvium. This information will be used to support the development of a conceptual model that ultimately results,in effective Kd values much larger than those currently in use for predicting radionuclide transport at Yucca Mountain.