Geochemistry of Natural Components in the Near-Field Environment, Yucca Mountain, Nevada
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Geochemistry of Natural Components in the Near-Field Environment, Yucca Mountain, Nevada Zell E. Peterman1 and Thomas A. Oliver2 1 Yucca Mountain Project Branch, U.S. Geological Survey, MS 963 Box 25046 Denver Federal Center, 6th and Kipling Sts., Denver, CO, 80225 2 c/o U.S. Geological Survey, S.M. Stoller Corporation, MS 421 Box 25046 Denver Federal Center, Denver, CO, 80225 ABSTRACT The natural near-field environment in and around the emplacement drifts of the proposed nuclear waste repository at Yucca Mountain, Nevada, includes the host rock, dust, seepage, and pore water. The chemical compositions of these components have been determined for assessing possible chemical and mineralogical reactions that may occur after nuclear waste is emplaced. The rock hosting the proposed repository is relatively uniform as shown by a mean coefficient of variation (CV) of 9 percent for major elements. In contrast, compositional variations of dust (bulk and water-soluble fractions), pore water, and seepage are large with mean CVs ranging from 28 to 64 percent. INTRODUCTION This paper summarizes the geochemistry of the natural components of the near-field environment, which is the area within the proposed repository at Yucca Mountain, Nevada, that will be affected by emplacement of nuclear waste. The natural components of the near-field environment are the host rock and contained pore water, seepage into the drifts, and dust. The Enhanced Characterization of the Repository Block (ECRB) Cross Drift and the Exploratory Studies Facility (ESF) (Figure 1) have accessed the rock mass for a variety of studies, including collection of water, rock, and dust samples. Because of the elevated temperatures associated with the proposed repository, chemical reactions of these materials may occur. After the drifts are sealed, temperatures will increase to above boiling (greater than 96 degrees Celsius at the elevation of Yucca Mountain), producing a dry-out zone in the adjacent rock mass. As the temperatures decrease with cooling of the waste, humidity will increase to nearly 100 percent over 104 to 106 years [1] to a condition similar to that in a sealed part of the ECRB Cross Drift [2]. The geochemistry of the natural components of the near-field environment is presented in the context of the hydrogeologic framework of Yucca Mountain. The mountain is composed of stratified welded and nonwelded felsic volcanic rocks [3]. The rock units of interest, which compose the Miocene Paintbrush Group [4], are, in descending stratigraphic order, (1) the Tiva Canyon Tuff (Tpc), which crops out over much of Yucca Mountain; (2) a sequence of named and unnamed nonwelded tuffs including the Yucca Mountain and the Pah Canyon Tuffs; and (3) the Topopah Spring Tuff (Tpt), which will host the proposed repository. The Tpc and Tpt are compositionally zoned, mostly densely welded units with the lower two-thirds to three-quarters composed of phenocryst-poor (crystal-poor) rhyolite (Tpcp and Tptp) and the upper parts transitioning from rhyolite to phenoc
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