Flow Focusing in Unsaturated Fracture Networks: A Numerical Investigation

and become focused. The focused unsaturated flow will increase relative permeability of its path, and cause flow A numerical modeling study is presented to investigate flow– further focusing. Focused flow paths exist in the unsatufocusing phenomena in a large-scale fracture network, constructed rated rock of the Yucca Mountain site. For example, using field data collected from the unsaturated zone of Yucca Mountain, Nevada, the proposed repository site for high-level nuclear waste. elevated levels of 36Cl originating from atmospheric nuThe two-dimensional fracture network for an area of 100 by 150 m clear tests conducted in the 1950s and 1960s were found contains more than 20 000 fractures. Steady state unsaturated flow in at several locations in an underground tunnel at the site the fracture network is investigated for different boundary conditions (Fabryka-Martin et al., 1996), an indication of fast flow and rock properties. Simulation results indicate that flow paths are behavior. Flow focusing within well-connected fracture generally vertical, and that horizontal fractures mainly provide pathnetworks play an important role in controlling distribuways between neighboring vertical paths. In addition to fracture proption of percolation fluxes through highly fractured tuffs erties, flow–focusing phenomena are also affected by rock–matrix (such as in the Topopah Spring welded tuff (TSw) unit, permeability, with lower matrix permeability leading to a high degree which will potentially house the repository drifts). Flow– of flow focusing. The simulation results further indicate that the averfocusing phenomena and discrete flow paths in the TSw age spacing between flow paths in a layered system tends to increase and flow tends to become more focused, with depth. unit are thus considered to be of significant importance to potential repository performance (Pruess, 1999). Because the fracture geometry and connectivity of sitespecific fractured rock is complex, flow behavior through I is widely recognized that fractures play an imporunsaturated fractures is difficult to characterize for a tant role in flow and transport process through unsatgiven site (Bodvarsson et al., 2003). Even with the signifurated geologic strata. Because fracture permeability is icant progress made in the last two decades, flow progenerally much greater than rock matrix permeability, cesses in unsaturated fractured rock are currently still fracture networks have the potential for being highly poorly understood. This poor understanding results effective pathways for conducting fluid. Recently, the mainly from technical difficulties in observing details of need to investigate flow and transport in the unsaturated flow processes and accurately describing such phenomzone (UZ) of Yucca Mountain, Nevada, the proposed perena within fractured rocks. However, flow processes ocmanent storage underground facility for geological discurring at this scale are important to many field-scale posal of high-level nuclear waste, has generated intenapplications. For the Yucca Mountain site, detailed sive research interest in modeling flow and transport knowledge of UZ flow processes is needed to predict processes in unsaturated fractured rocks. Different conthe degree of flow focusing, which is essential for assessceptual models have been proposed for handling water ing the performance of the proposed nuclear waste reflow and tracer transport in the UZ at Yucca Mountain. pository. Modeling approaches used in characterizing fracture In recent years, through field studies as well as matheflow in general include (i) the continuum model, such matical simulations, considerable progress has been made as the effective continuum model (ECM) and dual– in understanding flow–focusing processes within unsatupermeability models (e.g., Wu et al., 1999), (ii) discrete– rated fractured rocks. Glass et al. (1996) demonstrated fracture models (e.g., Rasmussen, 1987; Kwicklis and that the main flow mechanism for a vertical unsaturated Healey, 1993; Zimmerman and Bodvarsson, 1996; Liu fracture is fingering which results from gravitational et al., 2002), and (iii) other models with discrete geologic instability and aperture heterogeneities. Tokunaga and features to study discrete flow processes (e.g., Finsterle, Wan (1997) showed that film flow could be an important 2000). One of the main purposes of these modeling apmechanism at low fracture saturations. In their laboraproaches is to determine flow paths and to examine tory experiments, Su et al. (1999) demonstrated interflow focusing in complex fractured media. mittent flow behavior not considered by classical theory. Preferential flow pathways may exist in downward However, how to incorporate these small-scale mechadominated flow fields through unsaturated flow systems. nisms into field-scale models remains a challenge. The The phenomenon that flow is focused along these prefconventional approach for numerically describing flow erential paths or well-connected fracture networks is in partially saturated fractured media employs macroknown as flow focusing. Because of heterogeneity of fracscale continuum concepts (Peters and Klavetter, 1988). tures, flow will run through the path of least resistance Those approaches use large-scale volume averaging for homogenizing heterogeneous fracture and matrix perEarth Sciences Division, Lawrence Berkeley National Lab., MS 90meabilities. Some researchers have conceptualized het1116, 1 Cyclotron Road, Berkeley, CA 94720. Received 17 Apr. 2003. erogeneous fractured media with a stochastic spatial disOriginal Research Paper. *Corresponding author ([email protected]). tribution of fracture permeability (Gauthier et al., 1992; Published in Vadose Zone Journal 3:624–633 (2004).  Soil Science Society of America Abbreviations: ECM, effective continuum model; PTn, paintbrush nonwelded; TSw, Topopah Spring welded tuff; UZ, unsaturated zone. 677 S. Segoe Rd., Madison, WI 53711 USA