While a majority of more recent modeling studies (e.g., Ho, 1995, Altman et al., 1996, Moyer et al., 1996, Wilson, 1996) have affirmed earlier conceptualizations of lateral flow in the PTn, others (e.g., Gauthier et al., 1992) have suggested that because of the ability of fractures to drain rapidly the underlying TSw, little lateral diversion occurs in the PTn.įollowing the detection of atmospheric radionuclides Fabryka-Martin et al., 1993, Fabryka-Martin et al., 1994 in surface-based boreholes, as well as the observation of a large number of fast flow paths following the construction of the Exploratory Studies Facility (ESF) in 1995, conceptual models were developed to include possible fast flow paths from the TCw into the TSw. Other reports released in the mid-1980s also supported the notion of lateral flow at the base of the PTn (e.g., Sinnock et al., 1984, Sinnock et al., 1987, Klavetter and Peters, 1986, Peters and Klavetter, 1988 as reported in Flint et al., 2001). This conceptualization was supported by one of the earlier two-dimensional numerical models investigating the unsaturated zone hydrology at Yucca Mountain (Rulon et al., 1986), which suggested that lateral flow occurred through the PTn, and that with increasing fluxes, the portion of diverted water was reduced. A subsequent model presented by Montazer and Wilson (1984) had most of the infiltrating water move laterally through the PTn. In an early conceptual model (Scott et al., 1983), water was thought to travel along vertical fractures in the overlying welded Tiva Canyon Tuff (TCw) before entering the PTn, where it continued to move vertically downwards as matrix flow. However, while the importance of the PTn to the hydrology of the site has been acknowledged, there does not seem to be a consensus on how water is conveyed through this layer of nonwelded tuff. The PTn is a major feature of conceptual models that have been developed to describe physical processes dominating unsaturated flow at Yucca Mountain. The nonwelded tuffs of the Paintbrush group (PTn) lie immediately above the welded tuff of the Topopah Spring Tuff (TSw), the host rock for the potential nuclear waste repository. Closest to the surface is the welded Tiva Canyon Tuffs, followed sequentially by the Yucca Mountain, Pah Canyon, and the Topopah Spring Tuffs of the Paintbrush Group. From this observation, we infer that a sequence of infiltration events separated by periods of up to a few months could convey water over increasing distances along the fault.Īlternating layers of welded and nonwelded ash flow and air fall tuffs comprise the subsurface formations at Yucca Mountain (Bodvarsson et al., 1999). As saturation increases in the matrix, less water imbibes along the fault and more water travels farther along the fault.
Once wetted, the matrix is able to retain the moisture over a period of months. Our field tests suggest that the dry porous PTn matrix is capable of attenuating episodic percolation fluxes in localized areas (such as around faults) where fast flow would be expected to dominate. During the experiments, changes in moisture content were monitored within the test bed, and a slot excavated below the test bed was visually inspected for seepage. To investigate the potential for fast flow through altered tuff of the nonwelded unit of the Paintbrush Group (PTn) at Yucca Mountain, Nevada, we carried out in situ field experiments using water released directly into the matrix and along a minor subvertical normal fault at Alcove 4 in the Exploratory Studies Facility (ESF).
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