A Mountain-Scale Model for Characterizing Unsaturated Flow and Transport in Fractured Tuffs of Yucca Mountain

mountain-scale UZ flow model (Wu et al., 2003), which is in turn based on the analysis and results of the aboveWe present a large-scale modeling study characterizing fluid flow referenced work (as well as many other studies). Buildand tracer transport in the unsaturated zone (UZ) of Yucca Mountain, Nevada, the proposed underground repository site for storing highing on our previous investigations (Wu et al., 1999a, 1999b, level radioactive waste. The modeling study is conducted using a three2002a), we present more comprehensive three-dimendimensional numerical model, which incorporates a wide variety of sional model calibrations using field-measured pore water field data and takes into account the coupled processes of flow and Cl data, in addition to moisture and perched water transport in Yucca Mountain’s highly heterogeneous, unsaturated, fracdata. Moreover, the model we present here uses both tured porous rock. The modeling approach is based on a dual-contina much more refined grid and special connections beuum formulation. Using different conceptual models of unsaturated tween the gridblocks of rock matrix and fractures (at the flow, various scenarios of current and future climate conditions and interfaces between a few selected hydrogeologic units) their effects on the UZ are evaluated to aid in the assessment of the to represent the UZ system. Specifically, the current repository’s system performance. These models are calibrated against repository design, updated field data, and rock properfield-measured data. Model-predicted flow and transport processes under current and future climates are discussed. ties are incorporated into this model. The primary objective of this paper is to summarize the features of the current mountain-scale flow and transport model. First, we discuss how the model is caliI the past two decades, significant progress has been brated against saturation, water potential, perched water, made in characterizing flow and transport processes and Cl data. Second, we analyze flow behavior and in fractured rock of the UZ of the highly heterogeneous, patterns in the Yucca Mountain UZ system. As applicafractured tuff at Yucca Mountain, Nevada, the proposed tion examples, we use the model to simulate ambient permanent subsurface repository for storing high-level hydrological conditions for use in predicting the system radioactive waste. Since the 1980s, extensive field studresponse to future climate conditions. The emphasis is ies have been performed, and many types of data have on how to quantify moisture flow through the UZ under been collected to investigate flow and transport propresent-day and estimated future climate scenarios, and cesses. On the basis of these field data, a number of numerto estimate groundwater travel times from the proposed ical models have been developed for evaluating UZ repository level to the water table. hydrological conditions. Studies before the 1990s primarily used oneand two-dimensional models for understanding basic flow and transport behavior (Rulon MODEL DESCRIPTION et al., 1986; Tsang and Pruess, 1987). In the 1990s, Wittwer Geological Setting and Model Grid and coworkers (1992, 1995) began an effort to develop a three-dimensional model that incorporated many geoThe aerial domain of the mountain-scale UZ flow model logical and hydrological complexities. Ahlers et al. (1995) encompasses approximately 40 km2 of the Yucca Mounand Wu et al. (1997) continued the work of developing tain area (Fig. 1). As shown in Fig. 2 and 3, the UZ is the site-scale UZ model, but with increased spatial resobetween 500 and 700 m thick and overlies a relatively flat lution and incorporating more physical processes, such water table. Yucca Mountain is a structurally complex as gas and heat flow. Since then, more comprehensive geological system of Tertiary volcanic rocks, a heterogemodel calibrations and studies, using mountain-scale nuneous environment of layered, anisotropic, fractured merical models, have been made to investigate fluid flow tuffs (Scott and Bonk, 1984). The primary geological and radionuclide transport processes within the Yucca formations found at Yucca Mountain (from the land Mountain UZ (e.g., Viswanathan et al., 1998; Ahlers et surface down), as shown in Fig. 2 and 3, consist of the al., 1999; Bandurraga and Bodvarsson, 1999; Wu et al., Tiva Canyon, Yucca Mountain, Pah Canyon, and the 1999a, 1999b, 2002a, 2002b; Robinson et al., 2003). Topopah Spring tuffs of the Paintbrush Group. UnderThe modeling studies presented here represent our lying these units are the Calico Hills Formation and the continuing effort in development and application of the Prow Pass, Bullfrog, and Tram Tuffs of the Crater Flat Group (Buesch et al., 1995). These geological formations have been reorganized into hydrogeologic units Y.-S. Wu, G. Lu, K. Zhang, and G.S. Bodvarsson, Earth Sciences primarily on the basis of the degree of welding (MonDivision, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720. Received 26 Aug. 2003. Special Section: Research Advances tazer and Wilson, 1984): the Tiva Canyon welded (TCw) in Vadose Zone Hydrology through Simulations with the TOUGH Codes. *Corresponding author ([email protected]). Abbreviations: CFu, Crater Flat undifferentiated unit; CHn, the Calico Hills nonwelded unit; ESF, Exploratory Studies Facilities; PTn, Published in Vadose Zone Journal 3:796–805 (2004).  Soil Science Society of America Paintbrush nonwelded unit; TCw, Tiva Canyon welded hydrogeologic unit; TSw, Topopah Spring welded unit; UZ, unsaturated zone. 677 S. Segoe Rd., Madison, WI 53711 USA