Modeling capillary barriers in unsaturated fractured rock

Abstract

[1] This work presents a series of numerical modeling studies that investigate the hydrogeologic conditions required to form capillary barriers and the effect that capillary barriers have on fluid flow and tracer transport processes in the unsaturated fractured rock of Yucca Mountain, Nevada, a potential site for storing high-level radioactive waste. The modeling approach is based on a dual-continuum formulation of coupled multiphase fluid and tracer transport through fractured porous rock. The numerical modeling results showed that effective capillary barriers can develop where both matrix and fracture capillary gradients tend to move water upward. Under the current hydrogeologic conceptualization of Yucca Mountain, strong capillary barrier effects exist for diverting a significant amount of moisture flow through the relatively shallow Paintbrush nonwelded unit, with major faults observed at the site serving as major downward pathways for laterally diverted percolation fluxes. In addition, we used observed field liquid saturation and goechemical isotopic data to check model results and found consistent agreement.

Temporal damping effect of the Yucca Mountain fractured unsaturated rock on transient infiltration pulses

“…Other modeling studies indicate that the damping effect may be caused by lateral flow within the PTn unit (Wu et al, 2002b;Liu et al, 2003b). However, 1-D and 2-D models generally have difficulty describing the 3-D unsaturated flow system--for example, the lateral flow paths and flow focusing phenomena through heterogeneous 3-D layers of the Yucca Mountain UZ.…”

“…One of these geological layers identified during site characteriation is the layered, nonwelded PTn unit tuffs that exist between the ground surface and the repository horizon. Infiltrating water descending from the land surface may be effectively damped spatially and temporally by these layers, and thus percolation could be approximated as a steady-state condition once passing them (Montazer and Wilson, 1984;Wu et al, 2000;2002a;2002b).…”

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

“…These studies consistently indicate that simulation results using the base-case flow fields with the present-day, mean infiltration provide an overall better match with the observed chloride data than the alternative model results. This is because the base-case flow fields in general predict relatively large lateral diversion (Wu et al, 2002b). Note that, as discussed above (in the indicates that large lateral diversion may occur in percolation fluxes throughout the PTn unit, which has a direct impact on chloride transport and distributions.…”

“…Ahlers et al (1995) and Wu et al (1997) continued the work of developing the site-scale UZ model, but with increased spatial resolution and incorporating more physical processes, such as gas and heat flow. Since then, more comprehensive model calibrations and studies, using mountain-scale numerical models, have been made to investigate fluid flow and radionuclide transport processes within the Yucca Mountain UZ (e.g., Viswanathan et al, 1998;Ahlers et al, 1999;Bandurraga and Bodvarsson, 1999;Wu et al, 1999a;1999b;2002a;2002b;Robinson et al, 2003).…”

UZ Flow Models and Submodels

“…Wu et al (2002 [161058], p. 35-11) analyzed the capillary barrier capacities in unsaturated units and indicated that, on average, it took several thousands years for water to travel through the PTn. Both Wu et al (2000 [154918]; 2002 [161058]) and Flint et al (2003 [163967]) analyzed the implications of capillary barrier development in subunits of the PTn for lateral diversion of flow in the PTn. Along sloping layers, strong capillary barrier capacities promote lateral diversions.…”