Natural enrichment of ²³⁴U with respect to ²³⁸U was investigated in perched water, vadose zone pore waters, and secondary minerals. The activity ratio (AR) for dissolved ²³⁴U increased from about 1.4 in runoff to >6 in perched water. The AR in the vadose zone increased sharply at a transition that correlated with increased magnetic susceptibility and decreasing hydraulic conductivity. This was evidently caused by auto—oxidative selective leaching controlled by matrix saturation. The direct recoil fractionation mechanism has been proposed for tuffs on the Nevada Test Site, and could imply enhanced retardation of U. Direct recoil was evaluated using a steady state isotopic mass balance formulated to represent matrix pore water, and including first order sorption and selective leaching. Matrix sorption parameters were estimated by selectively leaching intact core with hydroxylamine. Much U was recovered, fractionated similarly to pore water. Interpreting this as isotopic exchange limited by Fickian transport within a sorbent layer, the rate constant and distribution coefficient were estimated. Uranium—series analysis of fracture—lining MnO₂ indicated that isotopic exchange was operant. The isotopic mass balance showed that direct recoil is a minor contribution to fractionation, so the predominant mechanism is selective leaching. This result depends mainly on the sorption rate constant, and where direct recoil is likely such as in roll—front deposits, it implies that the rate constant is smaller than in typical oxidizing waters. Extrapolating matrix properties to formation scale transport, an upper bound on formation scale sorption was inferred from the isotopic mass balance. Formation scale sorption is greater where the perched water table lies in more porous, permeable tuff. The layer diffusion model predicts that the effective sorption rate constant decreases significantly for thicker sorbent layers. Thus although fracture lining MnO₂ minerals are common at Yucca Mountain, U retardation may be strongly rate limited. Elevated AR's (>5) generally signify conditions favorable to U retardation, based on hydraulic isolation from recharge. Elevated vadose zone AR's do not necessarily signify the former presence of perched water, but could be interpreted that way at Yucca Mountain if similar fractionation is not found where past perching is unlikely.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/191210 |
| Date | January 1996 |
| Creators | Hardin, Ernest Lauriston,1956- |
| Contributors | Bassett, Randy, Conklin, Martha H., Hiskey, J. Brent, LaBrecque, Douglas J., Zreda, Marek G. |
| Publisher | The University of Arizona. |
| Source Sets | University of Arizona |
| Language | English |
| Detected Language | English |
| Type | Dissertation-Reproduction (electronic), text |
| Rights | Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. |
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