Technical Basis Document No. 2: Unsaturated Zone Flow Revision 1 May 2004 1. INTRODUCTION This technical basis document summarizes the conceptual understanding of and the model development for water flow in the Yucca Mountain unsaturated zone, a process affecting the postclosure performance of the high-level radioactive waste repository at Yucca Mountain. This document is one in a series addressing each component of the repository system relevant to predicting its likely postclosure performance. The repository is located in the unsaturated zone approximately 300 m below ground surface and at about the same distance above the water table (Bodvarsson et al. 1999, p. 5). The unsaturated zone is an important natural barrier to future radionuclide transport from the repository. Figure 1-1 illustrates the relationship between unsaturated zone flow paths and other components in Yucca Mountain. Unsaturated zone flow is affected by processes described in Technical Basis Document No. 1: Climate and Infiltration, Technical Basis Document No. 3: Water Seeping into Drifts, Technical Basis Document No. 4: Mechanical Degradation and Seismic Effects, Technical Basis Document No. 13: Volcanic Events, and Technical Basis Document No. 14: Low Probability Seismic Events. Unsaturated zone flow directly relates to water seepage and, therefore, the in-drift environment discussed in Technical Basis Document No. 5: In-Drift Chemical Environment. The unsaturated zone flow paths affect the distribution and quantity of seepage water entering the waste emplacement drifts and potentially contacting waste canisters. Unsaturated zone flow impacts downstream repository system components, including corrosion, waste dissolution, and radionuclide and colloid transport through the engineered barrier system, the unsaturated and saturated zones, and the biosphere. Unsaturated zone flow is thus directly related to unsaturated zone transport, colloidal transport, and saturated zone flow and transport. 1-1 May 2004 No. 2: Unsaturated Zone Flow Revision 1 Figure 1-1. Components of the Postclosure Technical Basis for the License Application This document provides a comprehensive review of the current understanding of flow paths in the unsaturated zone. It first reviews the geologic setting of the unsaturated zone and data collection activities related to flow processes in the unsaturated zone. It then presents the conceptual understanding of unsaturated zone flow paths (based mainly on the data that have been collected in the unsaturated zone) and discusses the development of the site-scale unsaturated zone flow model, including model uncertainties related to total system performance assessment (TSPA). This document also provides responses to the unsaturated zone flow-related Key Technical Issue (KTI) agreements (Table 1-1) between the U.S. Department of Energy (DOE) and the U.S. Nuclear Regulatory Commission (NRC) in the appendices of this report. May 2004 1-2 No. 2: Unsaturated Zone Flow Revision 1 Table 1-1. Unsaturated Zone Flow–Related Key Technical Issue Agreements Addressed in This Document Appendix H A B C E F D B C G I Short Description Providing the technical basis for FEP 1.2.06.00 (Hydrothermal Activity) Providing the technical basis for the portion of fracture flow through the CHn unit (vitric) Providing the analysis of geochemical data used for support of flow field below the repository Updating the calibrated properties model report, incorporating uncertainties from all significant sources Providing Unsaturated Zone Flow and Transport PMR (REV.00, ICN 02), documenting the resolution of issues on page 5 of the Open Item 8 presentation Providing AMR Conceptual and Numerical Models for Unsaturated Zone Flow and Transport (REV.01) and the AMR Analysis of Hydrologic Properties Data (REV.01) Providing an analysis of uncertainties in predicting unsaturated zone flow TSPAI 3.22 AIN-1 under future climate conditions using unsaturated zone models calibrated for the current climate condition Providing the analysis of geochemical and hydrologic data used for support of flow field below the repository Calibrating the unsaturated zone flow model using the most recent data on saturation and water potentials Providing an overview of water flow rates above and below the repository used in different models Documenting the effectiveness of the PTn to dampen episodic flow, including reconciling the differences in 36Cl studies NOTE: aThe responses to these agreements were submitted under separate cover. KTI ENFE 2.03a RT 1.01 RT 3.02 TEF 2.11 TEF 2.12 TEF 2.13 AIN-1 TSPAI 3.24 TSPAI 3.26 TSPAI 3.27 USFIC 4.04a Note Regarding the Status of Supporting Technical Information–This document was prepared using the most current information available at the time of its development. This technical basis document and appendices providing KTI agreement responses that were prepared using preliminary or draft information reflect the status of Yucca Mountain Project scientific and design bases at the time of submittal. In some cases this involved the use of draft analysis and model reports and other draft references whose contents may change with time. Information that evolves through subsequent revisions of the analysis model reports and other references will be reflected in the license application (LA) as the approved analyses of record at the time of LA submittal. Consequently, the project will not routinely update either this technical basis document or its KTI agreement appendices to reflect changes in the supporting references prior to submittal of the LA. May 2004 1-3 No. 2: Unsaturated Zone Flow INTENTIONALLY LEFT BLANK 1-4 No. 2: Unsaturated Zone Flow Revision 1 May 2004 Revision 1 2. GEOLOGIC SETTING AND DATA COLLECTION Yucca Mountain is located in an arid region with limited precipitation. Strong evaporation– transpiration processes limit the amount of water available for infiltration into the ground. Infiltrating water then percolates through the thick unsaturated zone at Yucca Mountain. Percolation represents the processes that redistribute the limited flow through alternating tuff layers with varying densities of fractures and through faults. Subsurface heterogeneity has an important effect on flow paths in the Yucca Mountain unsaturated zone and is largely determined by the corresponding geologic setting (Figure 2-1). Data collected from the unsaturated zone form the basis for understanding unsaturated zone flow paths and for developing unsaturated zone flow models. This section briefly reviews the unsaturated zone geologic setting and the data collected from the unsaturated zone. Source: BSC 2003a, Figure 1.2-4. Figure 2-1. The Yucca Mountain Geologic Setting: Yucca Mountain Ridge, Fran Ridge, and Busted Butte, Viewed from the Southwest across the Solitario Canyon Fault May 2004 2.1 GEOLOGIC SETTING AND HYDROLOGIC UNITS Subsurface formations in the unsaturated zone consist of heterogeneous layers of anisotropic, fractured volcanic tuffs, with alternating welded and nonwelded ash-flow and ash-fall deposits. The cooling history of these volcanic rocks determines their mechanical and hydrologic properties (Bodvarsson et al. 1999, p. 8). Syndepositional processes (such as welding, fracturing, and formation of lithophysal cavities) along with postdepositional activities (such as 2-1 No. 2: Unsaturated Zone Flow Revision 1 hydrothermal alteration, faulting, and additional fracturing) control the heterogeneous distributions of hydrologic properties in the unsaturated zone. The major lithostratigraphic units of the unsaturated zone beneath Yucca Mountain, from top to bottom, are the volcanic tuff formations of the Paintbrush (Tp) Group, the Calico Hills Formation (Tac), and the Crater Flat (Tc) Group. The lithostratigraphic nomenclature divides the Paintbrush Group into the Tiva Canyon (Tpc), Yucca Mountain (Tpy), Pah Canyon (Tpp), and Topopah Spring (Tpt) tuffs. The Crater Flat Group is divided into the Prow Pass (Tcp), Bullfrog (Tcb), and Tram tuffs (Tct). For purposes of hydrogeologic studies, a separate hydrogeologic nomenclature was developed based on the degree of welding and hydrologic property distributions (CRWMS M&O 2000a, Tables 4.7-1 and 8.10-1). The major hydrogeologic units are divided into the Tiva Canyon welded (TCw), the Paintbrush nonwelded (PTn), (consisting primarily of the Yucca Mountain and Pah Canyon members and the interbedded tuffs), the Topopah Spring welded (TSw), the Calico Hills nonwelded (CHn), and the Crater Flat undifferentiated (CFu) units. Based on the hydrogeologic properties of the lithostratigraphic units, Flint (1998) developed a detailed hydrogeologic stratigraphy for use in numerical flow and transport modeling. The correlations between lithostratigraphic units, hydrogeologic units, and corresponding unsaturated zone flow model layers are presented in Table 2-1. Figure 2-2 shows the spatial relationship of the major hydrogeologic units of the unsaturated zone in both perspective and east–west cross-sectional views. Lithostratigraphic units from the geologic framework model (BSC 2002a) are shown in plan view through the repository horizon and along an east–west cross section of the repository in Figure 2-3. The welded units typically have low matrix porosities (about 10%) and high fracture densities, whereas the nonwelded and bedded tuffs have relatively high matrix porosities (about 30%) and low fracture densities (BSC 2003b, Table 7). Portions of these units can be altered to zeolites or clays, depending on their cooling history and the presence of water. Such alteration does not affect porosities greatly, but it does decrease the permeabilities of formations where it occurs (BSC 2003b, Tables 7 and 8). May 2004 2-2 No. 2: Unsaturated Zone Flow Table 2-1. Major Hydrogeologic Units, Lithostratigraphy, and Unsaturated Zone Model Layer Correlation Major Hydrogeologic Unitsa Tiva Canyon welded (TCw) Paintbrush nonwelded (PTn) Topopah Spring welded (TSw) No. 2: Unsaturated Zone Flow Lithostratigraphic Nomenclature b Tpcr Tpcp TpcLD Tpcpv3 Tpcpv2 Tpcpv1 Tpbt4 Tpy (Yucca) Tpbt3 Tpp (Pah) Tpbt2 Tptrv3 Tptrv2 Tptrv1 Tptrn Tptrl, Tptf Tptpul, RHHtop Tptpmn Tptpll Tptpl Tptpv3 Tptpv2 Hydrogeologic Unit c CCR, CUC CUL, CW CMW CNW BT4 TPY BT3 TPP BT2 TC TR TUL TMN TLL TM2 (upper 2/3 of Tptpln) TM1 (lower 1/3 of Tptpln) PV3 PV2 2-3 Revision 1 Unsaturated Zone Flow Model Layer d tcw11 tcw12 tcw13 ptn21 ptn22 ptn23 ptn24 ptn25 ptn26 tsw31 tsw32 tsw33 tsw34 tsw35 tsw36 tsw37 tsw38 tsw39 (vitric, zeolitic) May 2004 Table 2-1. Major Hydrogeologic Units, Lithostratigraphy, and Unsaturated Zone Model Layer Correlation (Continued) Lithostratigraphic Nomenclature b Tacbt (Calicobt) Tcpuv (Prowuv) Tcpuc (Prowuc) Tcpmd (Prowmd) Tcplc (Prowlc) Tcplv (Prowlv) Tcpbt (Prowbt) Tcbuv (Bullfroguv) Tcbuc (Bullfroguc) Tcbmd (Bullfrogmd) Tcblc (Bullfroglc) Tcblv (Bullfroglv) Tcbbt (Bullfrogbt) Tctuv (Tramuv) Tctuc (Tramuc) Tctmd (Trammd) Tctlc (Tramlc) Tctlv (Tramlv) Tctbt (Trambt) and below Tptpv1 Tpbt1 Tac (Calico) Major Hydrogeologic Unitsa Calico Hills nonwelded (CHn) Crater Flat undifferentiated (CFu) Source: Simmons 2004, Table 8-3. NOTE: a Modified from Montazer and Wilson 1984, Table 1. b BSC 2002a. c Flint 1998. d BSC 2003c. No. 2: Unsaturated Zone Flow 2-4 Hydrogeologic Unit c BT1 or BT1a (altered) CHV (vitric) or CHZ (zeolitic) BT PP4 (zeolitic) PP3 (devitrified) PP2 (devitrified) PP1 (zeolitic) BF3 (welded) BF2 (nonwelded) Not Available Not Available Revision 1 Unsaturated Zone Flow Model Layer d ch1 (vitric, zeolitic) ch2 (vitric, zeolitic) ch3 (vitric, zeolitic) ch4 (vitric, zeolitic) ch5 (vitric, zeolitic) ch6 (vitric, zeolitic) pp4 pp3 pp2 pp1 bf3 bf2 tr3 tr2 May 2004 Revision 1 Figure 2-2. Yucca Mountain Site-Scale Hydrogeology in Three-Dimensional Perspective (a) and along an East–West Cross Section (b) Source: CRWMS M&O 2000b, Figure 3.2-1. May 2004 2-5 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2003a, Figure 1.2-3. NOTE: ECRB = Enhanced Characterization of the Repository Block. Figure 2-3. Yucca Mountain Lithostratigraphic Units and Major Hydrogeologic Units in Plan View through the Repository Horizon (a) and East–West Cross Section of Units Intersected by the Repository Horizon (Tptpul, Tptpmn, Tptpll, and Tptpln) (b) May 2004 2-6 No. 2: Unsaturated Zone Flow Revision 1 2.2 HYDROGEOLOGIC DATA The Yucca Mountain site has been characterized in the past two decades. The data, including hydrogeologic data, have been collected at multiple depths from surface-based boreholes and underground tunnels (i.e., the Exploratory Studies Facility (ESF) and Enhanced Characterization of the Repository Block (ECRB) Cross-Drift). Figure 2-4 illustrates the locations of deep boreholes, the ESF, and the ECRB Cross-Drift used in unsaturated zone flow studies (CRWMS M&O 2000b, Figure 2.1-2). Hydrogeologic data have been used for formulating, calibrating, and validating unsaturated zone flow models. Source: CRWMS M&O 2000b, Figure 2.1-2. Figure 2-4. Deep Boreholes and Underground Drifts of the ECRB Cross-Drift and the Exploratory Studies Facility and the Major Faults in the Vicinity of Yucca Mountain May 2004 2.2.1 Hydrogeologic Data for the Rock Matrix Rock core samples collected from the unsaturated zone (Flint 1998, p. 11) were analyzed to determine matrix porosity, bulk density, particle density, and water content, with the number of samples used for different matrix properties in different units shown in Table 2-2. Subsets of samples were measured for water potential determination, saturated hydraulic conductivity (or 2-7 No. 2: Unsaturated Zone Flow Revision 1 permeability), and moisture-retention relations. Matrix permeabilities in nonwelded units are orders of magnitude higher than those in welded units. Table 2-2 shows averaged matrix properties developed from core data for different hydrogeologic units. Deep boreholes (USW NRG#4, USW NRG#5, USW NRG-6, USW NRG-7a, USW SD-7, USW SD-9, USW SD-12, USW UZ-1, USW UZ-4, UE-25 UZ#5, USW UZ-7a) were instrumented in the unsaturated zone to measure in situ matrix water potential using thermocouple psychrometers at multiple depths (Rousseau et al. 1999, p. 77; Rousseau et al. 1997, pp. 18 to 19). Monitoring in these boreholes began in October 1994 and continued to December 2001. As an example, Figure 2-5 shows stabilized in situ water potential measurements at different depths in borehole USW SD-12. The data indicate that matrix water potential is on the order of 1 bar (105 Pa) through the unsaturated zone. In situ matrix water potential data were also collected from the ECRB Cross-Drift (BSC 2004, Section 7.2). Source: BSC 2004, Figure 6.2-4. NOTE: Simulation results are removed from the original figure. Figure 2-5. Observed Water Potentials and Perched Water Elevations for Borehole USW SD-12 May 2004 2-8 No. 2: Unsaturated Zone Flow Hydrogeologic 0.200 0.387 0.428 0.233 0.413 0.498 0.490 0.054 0.157 0.155 0.111 0.131 respectively, in Calibrated Properties Model (BSC 2003b). The hydrogeologic units correspond to those given in Table 2-1. 2-9 No. 2: Unsaturated Zone Flow ƒÓ Unit CCR & CUC CUL & CW CMW CNW BT4 TPY BT3 TPP BT2 TC TR TUL TMN TLL TM2 & TM1 PV3 PV2a PV2v BT1a BT1v CHV CHZ BTa BTv PP4 PP3 PP2 PP1 BF3/TR3 BF2 0.241 0.088 0.103 0.043 0.275 0.229 0.285 0.331 0.346 0.322 0.271 b 0.321 0.318 Source: BSC 2003b, Table 8. May 2004 0.221 0.297 0.175 0.234 NOTE: (a) BT1a was used as an analog for permeability because only one permeability data point is available for PV2a. (b) BT1v was used as an analog for porosity, residual saturation, and permeability because only one sample is available for BTv. (c) PP1 was used as an analog for permeability because only one measurable permeability data point is available for BF2. k is permeability; ƒÐ is standard deviation; n is number of samples; ƒÓ is porosity; nd is number of samples with nondetected permeability measurements; ƒ¿ and m are fitting parameters for the van Genuchten water potential relationship; SE is standard error; Sr is residual liquid saturation; ƒÅ and upscaled k are defined in Equations 29 and 34, Table 2-2. Matrix Properties Developed from Core Data upscaled k [m2] upscaled log(k) [log(m2)] ƒÐlog(k) 4.7 ~ 10.15 6.4 ~ 10.20 -14.33 -19.20 1.8 ~ 10.16 4.0 ~ 10.14 -15.74 -13.40 4.1 ~ 10.13 1.3 ~ 10.15 -12.39 -14.90 1.3 ~ 10.13 1.1 ~ 10.13 -12.87 -12.96 6.7 ~ 10.13 4.4 ~ 10.17 -12.17 -16.36 3.2 ~ 10.16 2.8 ~ 10.17 -15.50 -16.56 4.5 ~ 10.19 3.7 ~ 10.17 -18.34 -16.44 -19.63 -17.54 a 2.3 ~ 10.20 2.9 ~ 10.18 a 4.3 ~ 10.13 -12.37 -16.45 3.5 ~ 10.17 2.1 ~ 10.13 -12.67 -11.81 1.6 ~ 10.12 5.2 ~ 10.18 -17.28 -18.08 b 8.2 ~ 10.19 b 1.5 ~ 10.16 6.4 ~ 10.15 -15.81 -14.20 5.4 ~ 10.17 8.1 ~ 10.17 -16.27 -16.09 1.1 ~ 10.15 c -14.95 c nnd n SElog(k) 0.27 0.43 0.97 0.65 0.43 0.46 0.31 0.12 0.24 0.91 0.14 0.23 0.09 0.19 0.46 0.37 a 0.34 0.87 0.19 0.24 0.08 0.50 b 0.97 0.11 0.19 0.29 0.58 c 0 25 1 0 0 0 1 0 0 5 1 12 35 24 42 2 a 0 1 0 0 17 8 b 2 0 3 1 1 c 3 15 5 10 11 2 11 11 21 6 46 37 74 51 21 16 a 16 9 35 46 99 9 b 6 51 34 27 7 c 0.47 2.74 2.38 2.05 1.41 0.64 1.09 0.39 1.12 3.02 0.94 1.61 0.97 1.65 3.67 1.57 a 1.38 2.74 1.11 1.62 0.91 2.05 b 2.74 0.75 1.18 1.52 1.64 c 1/ƒ¿ [Pa] 8.27 ~ 104 5.46 ~ 105 2.50 ~ 105 2.03 ~ 104 4.55 ~ 103 7.63 ~ 104 8.90 ~ 103 2.12 ~ 104 1.74 ~ 104 2.71 ~ 105 9.43 ~ 104 1.75 ~ 105 1.40 ~ 106 6.01 ~ 104 3.40 ~ 106 1.00 ~ 106 2.17 ~ 105 1.94 ~ 104 4.72 ~ 106 1.35 ~ 104 3.39 ~ 103 4.45 ~ 105 6.42 ~ 106 5.04 ~ 104 5.00 ~ 105 1.32 ~ 105 6.22 ~ 105 1.13 ~ 105 8.94 ~ 104 8.46 ~ 106 log(1/ƒ¿) [log(Pa)] SElog(1/ƒ¿) m 0.388 0.280 0.279 0.178 4.918 5.737 0.259 0.245 0.188 0.199 5.398 4.308 0.219 0.247 0.174 0.379 3.658 4.883 0.182 0.300 0.088 0.104 3.950 4.325 0.126 0.218 0.170 0.310 4.239 5.432 0.290 0.283 0.116 0.111 4.974 5.244 0.317 0.216 0.108 0.521 6.147 4.779 0.442 0.286 0.097 0.278 6.532 6.000 0.059 0.293 0.156 0.042 5.336 4.287 0.349 0.240 0.183 0.049 6.674 4.131 0.158 0.257 0.094 0.094 3.530 5.649 0.499 0.147 0.043 0.207 6.808 4.703 0.474 0.407 0.401 0.084 5.699 5.120 0.309 0.272 0.147 0.234 5.794 5.052 4.951 6.927 0.193 0.617 0.931 0.032 Sr SEm ƒÅ 0.02 0.20 0.31 0.24 0.13 0.07 0.14 0.06 0.05 0.21 0.07 0.12 0.19 0.12 0.20 0.42 0.36 0.13 0.38 0.06 0.06 0.26 0.36 b 0.29 0.08 0.10 0.30 0.11 0.21 3.47 12.29 0.085 0.045 6.08 -2.58 0.042 0.032 -0.26 3.46 0.019 0.064 -0.56 0.26 0.008 0.023 -2.64 6.14 5.00 7.06 0.013 0.054 0.025 0.024 10.90 6.27 0.042 0.061 14.48 9.04 0.073 0.065 5.03 -0.19 7.39 0.007 0.011 0.073 0.008 -2.07 -3.80 8.30 7.13 3.37 6.69 6.05 3.11 8.86 0.117 0.070 Revision 1 0.008 0.022 11.87 -0.87 0.036 0.020 0.224 0.031 0.041 0.036 Revision 1 In the 1980s, temperature measurements were made in numerous boreholes within the central block of Yucca Mountain and in the surrounding area (Sass et al. 1988) as part of a regional heat flow study. More recently, in situ temperature data were made as part of the Yucca Mountain instrumented borehole monitoring program (Rousseau et al. 1999, p. 77; Rousseau et al. 1997, pp. 18 to 19). Thermal properties (including rock grain density, dry and wet rock thermal conductivities, and rock grain specific heat capacity) were also measured for rock samples collected from surface-based boreholes (BSC 2003b, Section 6.3). 2.2.2 Permeability Data for Fractures In order to compile data regarding matrix and fracture permeability, fracture geometry data (density, trace length, dips, and strikes) were obtained through detailed line surveys along the ESF tunnel walls. Fracture frequency data also has been collected from surface-based boreholes. As previously mentioned, fracture densities in welded units are much higher than those in nonwelded units (BSC 2003b, Table 7). Air injection tests have been used to characterize fracture permeabilities from surface-based and underground boreholes associated with the ESF (LeCain et al. 2000; BSC 2003b, Section 6.1; BSC 2003a, Sections 6.1 and 6.11). In a welded unit, fracture permeability is orders of magnitude higher than the corresponding matrix permeabilities. Matrix and fracture permeabilities are roughly on the same order of magnitude for a nonwelded unit (BSC 2003b, Tables 7 and 8). Temporal air pressure fluctuations on the ground surface propagate through the unsaturated zone (BSC 2003b, Section 4). Air pressure (pneumatic) data, as a function of time, were collected using sensors installed in a number of surface-based boreholes (including UE-25 NRG-5, USW NRG-6, USW NRG-7a, USW SD-7, USW SD-12, and USW UZ-7a) (Rousseau et al. 1999, p. 77; Rousseau et al. 1997, pp. 18 to 19). This data set is useful for understanding airflow in the unsaturated zone and for inferring large-scale fracture permeabilities. Air permeabilities have been measured in ESF boreholes (BSC 2003a, Sections 6.1 and 6.11). More than 3,500 separate pneumatic injections have been undertaken to systematically characterize air permeability in locations throughout the ESF and ECRB Cross-Drift. Table 2-3 summarizes mean permeabilities and standard deviations in different areas of the underground facilities. May 2004 2-10 No. 2: Unsaturated Zone Flow Table 2-3. Comparison of Geometric Means and Standard Deviations of Air Permeability Measurements Collected in Niches and Alcoves in the Yucca Mountain Exploratory Studies Facility NOTE: Niche 1 is also referred to as Niche 3566; Niche 2 is also referred to as Niche 3650; Niche 3 is also referred to as Niche 3107; Niche 4 is also referred to as Niche 4788; Niche 5 is also referred to as Niche Type of Site Intersects brecciated zone Predominantly within brecciated zone Moderately fractured welded tuff Postexcavation welded tuff Moderately fractured welded tuff Postexcavation welded tuff Moderately fractured welded tuff Highly fractured welded tuff Postexcavation welded tuff Highly porous lithophysal cavities; holes on side of excavation Highly porous lithophysal cavities; holes on side of excavation Highly porous lithophysal cavities; holes above of excavation Highly porous lithophysal cavities; holes above of excavation Discretely faulted and fractured nonwelded tuff Highly fractured postexcavation welded tuff Transition from upper lithophysal to welded fractured nonlithophysal in near vertical boreholes Borehole Cluster Niche 1 Preexcavation Niche 1 Radial Niche 2 Preexcavation Niche 2 Postexcavation Niche 3 Preexcavation Niche 3 Postexcavation Niche 3 Radial Niche 4 Preexcavation Niche 4 Postexcavation Niche 5 Preexcavation side Niche 5 Postexcavation side Niche 5 Preexcavation overhead Niche 5 Postexcavation overhead Alcove 4 Alcove 6 Alcove 8 Source: BSC 2003a, Table 6.1.2-5. 1620. 2.2.3 Hydrologic Data for Faults Although faults may serve as important flow paths within the unsaturated zone (Section 3), hydrologic data collected from the unsaturated zone for faults are limited. To obtain fault properties, air injection testing and tracer testing were conducted in the northern Ghost Dance fault alcove constructed off the ESF (LeCain et al. 2000, p. 2). The goals of the fault testing were to determine air permeability, porosity, and gaseous tracer transport characteristics (transport porosity and longitudinal dispersivity) in the volcanic rocks (tuff) that comprise the fault zone, footwall, and hanging wall of Ghost Dance fault. Permeability, porosity, and tracer transport characteristics of these tuffs control fluid movement in Yucca Mountain; quantified values of these parameters are needed for fluid flow numerical modeling in the unsaturated zone. Air injection testing was also conducted in southern Ghost Dance fault and Bow Ridge fault. In general, fracture permeabilities in fault zones are higher than nonfault zones (LeCain et al. 2000, Summary). No. 2: Unsaturated Zone Flow 2-11 Revision 1 log(k) (m2) Mean Standard Deviation 0.92 -13.0 0.66 -11.8 0.81 -13.4 0.88 -11.8 0.70 -13.4 0.82 -12.4 0.92 -13.8 0.85 -13.0 0.78 -11.9 0.77 -11.4 0.73 -11.2 1.14 -11.4 1.27 -11.0 0.93 -13.0 0.67 -11.9 1.29 -13.1 May 2004 Revision 1 2.3 GEOCHEMICAL DATA Geochemical analyses offer additional information over large scales, both spatially and temporally, for evaluating the flow processes of the Yucca Mountain unsaturated zone. Different fluid samples analyzed in Yucca Mountain studies include pore waters, gases from the unsaturated zone, and perched water. This section synthesizes available geochemical information within the unsaturated zone relevant to unsaturated zone flow paths. Figure 2-6 summarizes the available geochemical and isotopic information related to unsaturated zone flow and transport. Brief discussions on the data follow. This section does not present a comprehensive review of all available geochemical information, but rather focuses on those related to unsaturated zone flow processes. Source: BSC 2001, Figure 6.2-1. Figure 2-6. Geochemical Information Related to Unsaturated Zone Flow Paths May 2004 2-12 No. 2: Unsaturated Zone Flow Revision 1 2.3.1 Geochemical Composition of Pore Water The chemical composition of unsaturated zone pore water provides corroborative evidence of water flow rates and pathways in the unsaturated zone. The pore water composition is determined by rock–water interactions. Pore water samples were extracted from unsaturated core samples recovered from dry drilled boreholes. Pore water extracted from the PTn are calcium chloride or calcium sulfate-type water. In terms of relative portions of anions, the chemical composition of TSw pore water falls between those of the PTn and CHn units. Pore water extracted from the CHn unit is sodium carbonate bicarbonate-type water. Sodium concentration increases as depth increases in the CHn. This shift in dominance from divalent to monovalent cations primarily reflects ion exchange reactions with zeolites in the CHn unit (Figure 2-6). Deviations from vertical trends in ion concentrations (chloride, sulfate, and sodium) suggest that at least some component of lateral flow exists within the unsaturated zone (BSC 2002b, pp. 121 to 122). Pore water strontium data have been obtained from several boreholes. The data obtained from borehole USW SD-7 show that the ratio of 87Sr/86Sr increases with depth from ground surface to the repository horizon. This increase is steeper within the nonwelded units of the PTn (BSC 2002b, Figure 38), suggesting an enhanced water–rock interaction within the PTn (Figure 2-6). This is consistent with the current conceptual model in which water flow within the PTn occurs mainly in the matrix (Section 3.1.2). Observed pore water strontium concentrations from selected boreholes were also used to verify the site-scale unsaturated zone flow model (Section 4.3). Chloride concentration data for pore water from the TCw, PTn, TSw, and CHn hydrogeologic units were compared against other available data to elucidate general trends (BSC 2002b, Section 6.5.3). The smaller concentration of chloride (which behaves as a conservative tracer) in perched water implies that pore water and perched water have distinctly different histories of geochemical evolution, undergoing different degrees of evaporation and water–rock interaction. This observation has been used for developing the conceptual understanding of fracture–matrix interaction in the TSw unit (Section 3.2.1). Apparent infiltration rates were estimated by the chloride mass balance method, using chloride concentrations from pore water samples (BSC 2002b). PTn samples beneath deep alluvium had infiltration rates between 0.6 and 3.3 mm/yr. The overall average infiltration rate for pore water samples collected along the ESF main drift and the ECRB Cross-Drift is about 6 mm/yr. Observed pore water chloride concentration distribution along the ECRB Cross-Drift (Figure 2-7) is used to verify the site-scale unsaturated zone flow model (Section 4.3). 2-13 May 2004 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2004, Figure 6.5-4. NOTE: Simulation results were removed from the original figure. 2.3.2 Ages of Pore Water The 14C dating method was used to estimate the ages of pore water (BSC 2002b, Section 6.6.4.1). In general, 14C activities in pore waters from both the PTn and CHn yield apparent ages ranging from present-day to 5,200 years (Figure 2-6). Apparent ages are based on the assumptions that the initial 14C activity is 100 pmc (percent modern carbon) and that geochemical processes have not significantly altered the carbon isotopic composition of the sample. Changes relative to the initial atmospheric 14C activity are assumed to be solely the result of radioactive decay. To be accurate, radiocarbon age estimates must be corrected for contaminants like those potentially introduced to the pore water by CO2 from drilling air. Contamination with this CO2 would shift the isotopic signature to younger age estimates, causing the pore water to appear to have erroneously smaller residence times (Yang 2002, Section 4.1.2). The 14C age estimates presented here were estimated based on the approach summarized by Yang (2002), which considered this correction for drilling air CO The 14C values for perched water indicate an apparent age of 3,500 to 11,000 years (BSC 2002b, Section 6.9.3). 36Cl analyses of perched water indicate that its age ranges from 2,000 to 12,000 years for the different perched water bodies sampled, which are in general agreement with 14C–based ages (BSC 2002b, Section 6.6.3.6). 14C age data are used for verifying the site-scale unsaturated zone flow model (Section 4.3). Figure 2-7. Observed Pore Water Chloride Concentration Distribution along the ECRB Cross-Drift 2. May 2004 2-14 No. 2: Unsaturated Zone Flow Revision 1 36Cl and Tritium Cl/Cl ratios and tritium data were used to provide evidence for potential fast path signals in unsaturated zone flow. There are ongoing 36Cl validation studies being conducted by the U.S. Geological Survey (USGS), Lawrence Livermore National Laboratory (LLNL), and the Desert Research Institute (BSC 2003a, Section 6.14.2.1). Elevated 36Cl/Cl ratios were reported from Yucca Mountain at the depth of the repository horizon during the late 1990s (Fabryka-Martin et al. 1996; 1997; 1998). The 36Cl/Cl values above 1,250 × 10-15 (the upper limit for Pleistocene meteoric input) were attributed to atmospheric nuclear testing in the Pacific Ocean and interpreted as an indication that at least some meteoric water is capable of percolating rapidly through the unsaturated zone to depths of 300 m below the surface in the last 50 years (Fabryka-Martin et al. 1996; 1997; 1998; Levy et al. 1999). Current flow and transport models used for TSPA include potential transport pathways that, under present-day climate conditions, result in a small fraction of the modeled flow paths acting as fast flow paths (BSC 2003d, Figure 6.9-1). The inclusion of these fast flow paths in unsaturated zone flow modeling is consistent with the results indicated by 36Cl measurements reported by Fabryka-Martin et al. (1996). Because of the fast path hydrologic implications for the repository, the validation study was initiated in late 1999 to independently verify the presence of bomb-pulse 36Cl in the ESF. The study primarily entailed analyses of core from 50 new boreholes drilled across two zones, the Sundance fault and the Drill Hole Wash fault, where significant 36Cl/Cl bomb-pulse ratios were identified by Fabryka-Martin et al. (1996). Fabryka-Martin et al. (1996) reported numerous samples from the ESF with elevated levels of Cl in a 165-m-wide zone associated with the Sundance fault (Figure 2-8). Ratios of 36Cl/Cl near or above 1,250 × 10-15 were obtained for 11 of 16 samples between ESF Stations 34+28 and 35+93 (36Cl/Cl ratios from 1,339 × 10-15 to 4,105 × 10-15). Also, 9 of 15 samples from Niche 1, also associated with the Sundance fault, had 36Cl/Cl values from 1,235 × 10-15 to 2,038 × 10-15. Validation study sampling targeted these zones to maximize the probability of reproducing the Cl signal. 2.3.3 36 36 36 May 2004 2-15 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2003a, 6.14.2-1. NOTE: Although the main trace of the Sundance fault (shaded broad dashes) is exposed at a distance of 3,593 m from the ESF North Portal, the entire zone between about 3,400 and 3,650 m is pervasively fractured. Analytical errors (2ó) are shown as vertical lines if they are larger than the size of the symbol. USGS-LLNL validation study samples are shown as filled circles. Previously published LANL data are shown as open squares. Figure 2-8. 36Cl/Cl Ratio Plotted against Sample Location in the Exploratory Studies Facility In addition to 36Cl/Cl ratios, tritium was also used as a potential indicator of percolation rates. The analytical uncertainty for tritium analysis is 4 tritium units, based on counting statistics. All values above 25 tritium units lie outside the range of the population of background samples (BSC 2002b, Sections 6.2.6 and 6.6.2.2). The limitation of this approach is that background, in this case, includes postbomb waters that have decayed to prebomb tritium levels. This limitation contributes to uncertainty in tritium analyses. Bomb-pulse levels of tritium (greater than 25 tritium units) have been observed in the Bow Ridge fault zone in ESF Alcove 2 and in about 6% of the pore waters extracted from core samples from 11 surface-based boreholes. These detections occur within the TCw, PTn, and TSw, and also in some samples from the CHn as deep as the Prow Pass member of the CFu (BSC 2002b, Section 6.6.2.3). In addition to the Bow Ridge fault location in the ESF above the PTn unit, samples with high tritium values are present in the south ramp of the ESF, where the PTn units are faulted and offset, and from 750 to 950 m in the ECRB in the upper lithophysal unit of the Topopah Spring Tuff. The occurrences in the Bow Ridge fault and in the south ramp of the ESF are clearly May 2004 2-16 No. 2: Unsaturated Zone Flow Revision 1 linked to the absence of the PTn units or the inability of these units to impede downward percolation of young water at those locations. In the ECRB, it is unclear what features may provide the pathways for the percolation of young water. The investigation into the presence of tritium in pore water from the ECRB is in progress (BSC 2003a, Section 7.14.2.2). 2.3.4 Uranium Isotopes At Yucca Mountain, uranium isotopic ratios have been used to evaluate the prevalence and frequency of fracture flow through the unsaturated zone and the issue of local recharge to the water table. Substantial differences in 234U/238U activity ratios between pore water and fracture water (from 234U/238U in fracture minerals as well as perched water) imply minimal liquid exchange between fracture and matrix flow pathways (BSC 2002b, Section 6.6.7). Groundwater from the saturated zone beneath Yucca Mountain contains elevated 234U/238U activity ratios (between 6 and 8) compared to water from wells in adjacent areas to the south (between 1.5 and 4 (BSC 2002b, Figure 42)). Groundwater obtained from Paleozoic carbonate rocks at depth beneath Yucca Mountain (UE-25 p#1) has a much smaller 234U/238U ratio of 2.32, typical of the regional carbonate aquifer and is indicative of the stratification of shallow and deep aquifers at the site (Figure 2-6). The anomalous uranium isotopic compositions of shallow saturated zone water beneath Yucca Mountain are similar to the 234U/238U compositions measured for deep unsaturated zone fracture minerals and perched water bodies in the welded TSw. This similarity supports the concept of recharge through the thick unsaturated zone at Yucca Mountain and that much of the local recharge derives from flow through fracture pathways in the welded units of the unsaturated zone rather than from percolation through the matrix column in these units (BSC 2002b, Section 6.6.7). Interpretations of the uranium isotopic data are used for developing a conceptual model of water flow within the unsaturated zone (Section 3). 2.3.5 Fracture-Lining Minerals Deposits of calcite and opal lining fractures and cavities in the ESF contain spatial and temporal information on past water migration through the Yucca Mountain unsaturated zone. These mineral coatings provide a record of past water percolation through the connected fracture network in areas where solutions exceed chemical saturation with respect to various mineral phases. Calcite is abundant in the calcic soils at Yucca Mountain, leading to rapid saturation of infiltrating water with respect to calcite. The volcanic rocks are calcium-poor, so infiltrating water is essentially the only source of calcium available to form the calcite in the unsaturated zone. Therefore, the calcite that has formed in a cavity can be related directly to the amount of water required to transport that amount of calcium into the cavity. The calculated amount of water would be a minimum estimate of the amount of water that actually seeped into the cavity, unless the water evaporated completely within the cavity. The total percolation flux for the whole unsaturated zone, based on measurements of total calcite abundance in the ESF, indicates a flux of about 2 mm/yr, which is within an order of magnitude of the estimated long-term infiltration flux at the surface (BSC 2002b, Section 6.10.3.5). Field observations also indicate that less than 10% of all fractures and open spaces contain coatings of calcite and opal (BSC 2002b, Section 7.6). May 2004 2-17 No. 2: Unsaturated Zone Flow Revision 1 Geochronological data indicate that calcite and opal have grown in the fractures of deep unsaturated zone at extremely slow and relatively uniform rates (1 to 5 mm per million years) over the last 10 million years (Neymark et al. 2002). This uniform mineral growth rate implies that fracture flow in the deep unsaturated zone at Yucca Mountain did not vary substantially despite climate variations. In addition, 230Th/U ages do not show clustering that can be correlated with cycles of pluvial-interpluvial climate over the last 200,000 years. The 234U/238U ratios calculated for opal and calcite deposited within the repository horizon also imply that deep unsaturated zone fracture flow at Yucca Mountain was low in volume or infrequent (BSC 2002b, Section 7.6). The fracture coating data are used for verifying the site-scale unsaturated zone flow model (Section 4.3). 2.4 IN SITU FIELD TESTING This section briefly reviews selected in situ field tests that are important for understanding flow paths within the unsaturated zone, and for developing and verifying unsaturated zone flow models. More comprehensive discussions of in situ field tests can be found in In Situ Field Testing of Processes (BSC 2003a). 2.4.1 In Situ Borehole Testing As discussed in Section 2.2, a number of deep boreholes were instrumented in the Yucca Mountain unsaturated zone to measure in situ pneumatic pressure, water potential, and temperature at multiple depths (Rousseau et al. 1999, p. 77; Rousseau et al. 1997, pp. 18 to 19). The data collection also included measurements of gas phase 14C from selected boreholes. The data collected from in situ borehole testing are used for developing and validating the site-scale unsaturated zone flow model (Section 4). 2.4.2 Busted Butte Tests The Busted Butte test facility is located 8 km southeast of the Yucca Mountain repository area. It was chosen as a test location because of its readily accessible exposure of the vitric CHn (Figure 2-9) (CRWMS M&O 2001, Sections 6.8.1.1 and 6.8.1.2). May 2004 2-18 No. 2: Unsaturated Zone Flow Source: BSC 2003a, Figure 6.13.1-1. NOTE: This schematic of the Busted Butte unsaturated zone transport test site shows the relative locations of the different experiment phases and borehole locations. Figure not drawn to scale. Figure 2-9. Busted Butte Unsaturated Zone Transport Test 2-19 No. 2: Unsaturated Zone Flow Revision 1 May 2004 Revision 1 Busted Butte tests include several phases. Tracer tests there indicate that strong capillary forces in the rock matrix of the vitric CHn are likely to modulate fracture flow from overlying units, thereby damping pulses of infiltrating water. This conclusion was partially derived from observed fluorescein tracer plumes (occurring in the matrix only) from phase 1A test (Figure 2-10). The phase 1B experiment of the Busted Butte tests also showed that, even when injection occurs immediately adjacent to a fracture, water appears to be imbibed quickly into the surrounding matrix. Under the injection rate of 10 mL/hr, the transport times for the nonsorbing tracers were observed to be on the order of 30 days over a distance of about 28 cm, whereas pure fracture flow would have resulted in transport times of minutes to hours at this flow rate. Based on field observations from the Busted Butte site, water flow is considered to occur only in the matrix within the vitric CHn unit in the unsaturated zone flow models (BSC 2003b, p. 29). Source: BSC 2003a, Figure 6.13.2-1. Figure 2-10. Fluorescein Plume at Each of the Four Phase 1A Mineback Faces at Busted Butte May 2004 2-20 No. 2: Unsaturated Zone Flow Revision 1 2.4.3 Drift Scale Test The Drift Scale Test (DST) is the second underground thermal test to be carried out in the ESF at Yucca Mountain, Nevada. The purpose of the test is to evaluate the coupled thermal, hydrologic, chemical, and mechanical processes that take place in unsaturated fractured tuff over a range of temperatures (approximately 25°C to 200°C). The DST provides important data for studying the coupled processes in the unsaturated zone. Data derived from the DST were used for developing models describing the coupled processes and their effects on water flow in the unsaturated zone (BSC 2003e, Section 7). Details regarding the DST layout, borehole orientations, operation of the test, and measurements performed (as well as their uncertainties) are discussed in Section 6.3 of the Thermal Testing Measurements Report (BSC 2002c). In brief, the DST consists of an approximately 50-m-long drift that is 5 m in diameter. Nine electrical floor canister heaters were placed in this drift (the heated drift) to simulate nuclear waste-bearing containers. Electrical heaters were also placed in a series of horizontal boreholes (wing heaters) drilled perpendicularly outward from the central axis of the heated drift. These heaters were emplaced to simulate the effect of adjacent emplacement drifts. The DST heaters were activated on December 3, 1997, with a planned period of 4 years of heating, followed by 4 years of cooling. After just over 4 years, the heaters were switched off on January 14, 2002, and since that time the test area has been slowly cooling. Figure 2-11 shows a the test layout with the main heater tunnel, the wing heaters, and the array of observation boreholes for monitoring temperature as well as chemical, mechanical, and hydrologic variables. The data on the evolution of gas phase composition and aqueous speciation, isotopic composition, mineralogical alterations and associated porosity and permeability changes, pH evolution, changes in water content and air permeabilities, and rock deformations have been collected during the DST. May 2004 2-21 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2002c, Figure 6.3-2. Figure 2-11. Three-Dimensional Perspective of the As-Built Borehole Configuration of the Drift Scale Test May 2004 2.4.4 Alcove 1 Tests and Alcove 8–Niche 3 Cross-Over Tests The Alcove 1 test site is located near the North Portal of the ESF (Liu et al. 2003). Alcove 1 was constructed for measuring seepage originating from the surface infiltration. The alcove is about 5.5 m high and 5.8 m wide. Rocks between the ground surface and the alcove are within the intensely fractured TCw unit. A bulkhead was installed near the face of the alcove in order to isolate the end of the alcove from the ESF. The bulkhead was intended to raise the relative humidity in the end of the alcove and reduce evaporation from the wall of the alcove. This allowed observation of the wetting front arrival through observation of dripping from the alcove ceiling and walls. During the infiltration test, water was applied at the ground surface directly over the end of the alcove. The size of the infiltration plot was 7.9 m × 10.6 m. During the late stage of Alcove 1 tests, a tracer, bromide, was introduced into the infiltrating water. Seepage into the alcove was collected as a function of time. Tracer concentrations were obtained by analyzing the seepage water. Infiltration and tracer transport tests have also been conducted at the cross-over test site, where Alcove 8 in the ECRB Cross-Drift is about 20 m directly above Niche 3 (also referred to as Niche 3107) in the ESF main drift (Figure 2-12) (BSC 2003a, Section 12). The test site is located in the upper lithophysal and middle nonlithophysal subunits of the TSw unit. The upper 2-22 No. 2: Unsaturated Zone Flow Revision 1 lithophysal subunit contains lithophysal cavities. Liquid water both without and with tracers was released along a fault (line-release) and from a large 3 m by 4 m plot (areal release) on the floor of Alcove 8. Seepage rate and tracer concentration data are collected from Niche 3. The tests generate data sets that are useful for understanding flow behavior within a fault, the importance of the matrix diffusion within the unsaturated zone, and other important flow and transport processes. These tests provide important data for studying seepage and water flow behavior in the unsaturated zone. Model results are compared with the experimental observations collected from both Alcove 1 and Alcove 8–Niche 3 tests to enhance understanding of flow and transport processes within the unsaturated zone and for validating the numerical approaches used in unsaturated zone models (BSC 2004, Section 7; Liu et al. 2003). Source: BSC 2003a, Figure 6.12.1-2. Figure 2-12. Location of Test Bed between the ECRB Cross-Drift and Exploratory Studies Facility Main Drift May 2004 2-23 No. 2: Unsaturated Zone Flow INTENTIONALLY LEFT BLANK 2-24 No. 2: Unsaturated Zone Flow Revision 1 May 2004 Revision 1 3. CONCEPTUAL UNDERSTANDING OF UNSATURATED ZONE FLOW A conceptual understanding of unsaturated zone flow processes is numerically modeled to predict flow paths under future conditions. The development of the models is based mainly on the geologic setting of the unsaturated zone and the variety of data collected from the unsaturated zone, in addition to theoretical and numerical studies conducted in the last two decades. This section briefly presents the model representations of unsaturated zone flow processes, as illustrated in Figure 3-1. Figure 3-1. Overall Water Flow Behavior in the Unsaturated Zone, Including the Relative Flux Magnitudes of Fracture and Matrix Flow Components in the Different Hydrogeologic Units May 2004 3.1 OVERALL FLOW PATTERN WITHIN THE UNSATURATED ZONE Figure 3-1 schematically shows the overall conceptualized water flow behavior in the unsaturated zone, including the relative importance of fracture and matrix flow components in the different hydrogeologic units, as described in Section 2. The characteristic flow behavior in each of the major hydrogeologic units is described in the following sections. Precipitation infiltrates through the soil and then percolates through the unsaturated zone, where this model 3-1 No. 2: Unsaturated Zone Flow Revision 1 illustrates the geologic control over water flow. Water flows through the densely fractured Tiva Canyon welded (TCw) unit mainly through fractures. Within the more porous Paintbrush nonwelded (PTn) unit, most of the water flows through the matrix, where the high storage capacity promotes the dampening of infiltration pulses. Small amounts of flowing water preferentially pass through faults that cut through this unit. Accurate determination of the flow components is especially important for chemical transport processes. Flow in fractures (fracture flow) is typically much faster than flow in the matrix (matrix flow), leading to much shorter transport times for radionuclides and other chemicals in fractures compared to the matrix. 3.1.1 Flow through the TCw Unit The high density of interconnected fractures and low matrix permeabilities in the TCw unit (BSC 2003b, Tables 7 and 8) are considered to give rise to significant water flow in fractures and limited matrix imbibition (water flow from fractures to the matrix). Thus, episodic infiltration pulses are expected to move rapidly through fracture networks, with little attenuation by the matrix. This is partially supported by pneumatic data in the TCw showing little attenuation of the barometric signal in monitoring boreholes relative to the barometric signal observed at the land surface (CRWMS M&O 2000c, Section 6.1.2). 3.1.2 Flow through the PTn Unit The relatively high matrix permeabilities and porosities and low fracture densities of the PTn unit (BSC 2003b, Tables 7 and 8) convert the predominant fracture flow in the TCw to dominant matrix flow within the PTn (CRWMS M&O 2000c, Section 6.1) (Figure 3-2). The dominance of matrix flow is supported by field tests conducted in the PTn unit. Salve et al. (2003) performed water release tests along a fault within the PTn unit. Water was released under constant-head conditions from a 0.3-m interval within a borehole that crosses the fault. A total of 193 L of water during seven distinct events was released over two weeks between October 21, 1998, and November 5, 1998. Between November 30, 1999, and December 2, 1999, an additional 136 L of water were introduced into the same interval during three distinct events lasting 4 to 7 hours (Salve et al. 2003). It was observed that during the first release test, the wetting front advanced slowly as a result of significant matrix imbibition. It was also found that water that imbibed into the matrix was retained for periods extending to at least a few months for the given test conditions. Based on these observations and considering that water release rates used in the tests were much larger than water percolation rates under ambient conditions, it is concluded that the dry porous PTn matrix is capable of attenuating episodic percolation fluxes in localized areas (such as around faults) where fast flow would otherwise be expected to dominate (Salve et al. 2003, p. 282). This conclusion is consistent with Busted Butte field observations of the vitric CHn, another nonwelded unit (Section 2.4.2). May 2004 3-2 No. 2: Unsaturated Zone Flow Revision 1 Figure 3-2. Water Flow Behavior within the PTn Characterized by Dominant Matrix Flow and a Few Fast Flow Paths The PTn layer has the potential capacity to attenuate infiltration pulses, given that matrix flow dominates this layer and that the relatively low saturation (under ambient conditions) in this high-porosity layer provides available pore space for water storage. This is also consistent with the test results of Salve et al. (2003, p. 269) mentioned above. The modeling study of Wang and Narasimhan (1993, pp. 354 to 361) demonstrates that this will result in water flow below the PTn being approximately at steady state. Faults (or geologic structures) may cut through the entire PTn unit at some locations, leading to potential fast flow paths when the localized tuff matrix is not dry enough. As will be discussed in Section 3.2.4, the fast flow paths, if existing, would only carry a small amount of water and should not affect the overall flow paths in the unsaturated zone significantly. The issue regarding the effectiveness of the PTn to dampen episodic flow will be further addressed in the response to KTI USFIC 4.04. May 2004 3-3 No. 2: Unsaturated Zone Flow Revision 1 In an early conceptual model of Yucca Mountain, Montazer and Wilson (1984, pp. 45 to 48) indicated that significant lateral flow occurs within the PTn unit. The contrast in hydraulic properties at internal layer contacts within the PTn could cause lateral flow. Furthermore, the transient flow of water from the TCw unit to the PTn unit promotes air entrapment, which could further reduce the vertical liquid flux into the PTn. Montazer and Wilson (1984, p. 47) also showed that vertical heterogeneities within the PTn unit may result in a much larger effective permeability of the unit in the direction of dip, compared with the effective permeability in the direction normal to the bedding plane. They argued that the combination of this factor and capillary barrier effects might introduce considerable lateral flow within the unit. A fine grid simulation reported by Wu et al. (2002) supports the existence of lateral flow within the PTn resulting from capillary barrier effects. This conceptual model was further supported by the fact that the site-scale unsaturated zone flow model, considering lateral flow mechanisms within the PTn, better matches the observed chloride data within the unsaturated zone (BSC 2004, Section 6.5.2), as shown in Figure 3-3. Source: BSC 2004, Figure 6.5-3. NOTE: The simulations were performed with the site-scale unsaturated zone flow model with and without lateral flow mechanism in the PTn. Some simulation results were removed from the original figure. Figure 3-3. Comparison between Observed and Simulated Chloride Concentration Distribution for Borehole USW SD-9 May 2004 3-4 No. 2: Unsaturated Zone Flow Revision 1 3.1.3 Flow through the TSw Unit Unsaturated flow in the TSw unit occurs primarily through fractures (Figure 3-1). Supporting evidence for this comes from the magnitude of matrix hydraulic conductivity values of the TSw relative to the estimated average infiltration rate. The maximum matrix percolation rate is equal to the matrix hydraulic conductivity if the hydraulic gradient is assumed to be unity under unsaturated conditions. Because the matrix hydraulic conductivity is much lower than the average estimated infiltration rate (CRWMS M&O 2000c, Section 6.1.2), the remainder of the flow must be distributed in the fracture network (Pruess et al. 1999, p. 283). Other evidence for fracture flow comes from calcite coating data, which are signatures of water flow history and indicate that most of the deposition is found within the fractures in the welded units (Paces et al. 1998, p. 37). As discussed in Conceptual and Numerical Models for UZ Flow and Transport (CRWMS M&O 2000c, Section 6.1.2), 14C ages of the perched water bodies below the TSw unit also suggest fracture dominant flow within the TSw. These ages, ranging approximately from 3,500 to 11,000 years (Yang et al. 1996, p. 34), are much younger than if the matrix were the major water flow path within the TSw. Therefore, fracture flow is considered the dominant water flow mechanism within the TSw. 3.1.4 Flow below the Repository Flow behavior below the repository is especially important for modeling radionuclide transport from the repository horizon to the water table, because transport paths follow the water flow pattern. The main hydrogeologic units below the repository are the CHn and CFu. Both of these units have vitric and zeolitic components that differ in their degree of hydrothermal alteration and subsequent hydrologic properties. The zeolitic rocks have low matrix permeability and some fracture permeability; consequently, a relatively small amount of water may flow through the zeolitic units, with most of the water flowing laterally in perched water bodies and then vertically down along faults (Figure 3-1). One distinctive feature below the repository is the existence of perched water zones, which have been reported in a number of boreholes within the lower portion of the TSw unit and within the upper portion of the CHn unit (BSC 2004, Section 6.2). These perched water bodies were found primarily in the northern part of the repository area, where lower permeability, sparsely fractured zeolitic rock units predominate. The occurrence of perched water suggests that certain layers of the lower TSw (e.g., the basal vitrophyre) and the upper CHn (zeolitic portion) serve as barriers to vertical flow. Perched water is further discussed in Section 3.2.5. On the other hand, similar to the PTn unit, the vitric units have relatively high matrix porosity and permeability; therefore, porous medium-type flow dominates (Figure 3-1). This is supported by the test results within the CHn at the Busted Butte underground facility, as previously discussed in Section 2.4. The results showed that water flow and tracer transport occur mainly within the matrix of the CHn, where fracture flow is believed to be limited. As discussed in Section 2.3 of this chapter, different kinds of geochemical data have been collected below the repository. Perched water analysis has yielded residence ages ranging from 3,500 to 11,000 years (14C data) (Figure 2-6). As previously indicated, this supports the idea that May 2004 3-5 No. 2: Unsaturated Zone Flow Revision 1 water flow within the TSw unit (including its portion below the repository) is mainly vertical and occurs in fractures. As a result of ion exchange reactions with clays and zeolites along the flow paths, chemical compositions of pore water extracted from the CHn are generally found to be similar within a given stratigraphic unit and markedly different between different host lithologies in a given borehole, implying significant lateral flow within the zeolitic portion of the CHn unit (BSC 2002b, pp. 121 to 122). Chloride data and strontium concentration data below the repository were also collected from several deep surface-based boreholes. The consistency between concentration data and three-dimensional simulation results using the site-scale unsaturated zone flow model further supports the conceptual understanding of the flow pattern below the repository, as shown in Figure 3-1 (BSC 2004, Section 7). The issue regarding the flow field below the repository will be further addressed in the responses to KTIs RT 1.01, RT 3.02, and TSPAI 3.24. 3.2.1 Fracture–Matrix Interaction Fracture–matrix interaction refers to flow and transport (or mass exchange) between fractures and the matrix. Owing to their different hydrologic properties, distinct flow and transport behavior occurs in each component. The extent of fracture–matrix interaction is a key factor in determining flow and transport processes in the unsaturated zone. Modeling results and field observations show limited fracture–matrix interaction within welded units at Yucca Mountain (CRWMS M&O 2000c, Section 6.1.3). Chloride concentrations within the shallow TCw unit are generally lower than 10 mg/L; they range from 30 to 80 mg/L in the nonwelded PTn unit, and decline again to 5 to 10 mg/L in the deep perched water bodies. These chloride concentration data indicate that perched water is recharged mainly from water moving through fractures, with only a small degree of interaction (or mixing) with matrix water (Section 2.3.1 of this document; CRWMS M&O 2000c, Section 6.1.3; Yang et al. 1996, p. 55). Uranium isotopic ratios have been used to address the prevalence and frequency of fracture flow through the unsaturated zone and the issue of local recharge to the water table. Substantial differences in 234U/238U activity ratios between pore water and fracture water (from 234U/238U in fracture minerals as well as perched water) imply that there may be minimal liquid exchange between these two types of flow pathways (Section 2.3.4). Furthermore, studies by Ho (1997, pp. 401 to 412) show that the match between numerical simulations and observed matrix saturation and water potential data is improved by significantly reducing the amount of fracture– matrix interaction. The occurrence of limited fracture–matrix interaction is also supported by observations from Rainier Mesa and other analog sites. The Rainier Mesa site is characterized by a thick sequence of alternating welded and nonwelded unsaturated tuffs, similar to those at Yucca Mountain. Thordarson (1965, pp. 6 to 7 and pp. 75 to 80) noted that typically only portions of fractures 3.2 SPECIFIC ASPECTS OF UNSATURATED ZONE FLOW PATTERN The overall flow pattern within the unsaturated zone was described in Section 3.1. The following section is devoted to some specific issues important for conceptual understanding of flow paths within the unsaturated zone. 3-6 May 2004 No. 2: Unsaturated Zone Flow Revision 1 carried water and that the chemical composition of water obtained from fractures was substantially different from that of water samples extracted from the nearby rock matrix at that site. At a field site in the Negev Desert in Israel, man-made tracers were observed to migrate with velocities of several meters per year across a 20- to 60-m-thick unsaturated zone of fractured chalk (Nativ et al. 1995, pp. 253 to 261). Such high velocities could only occur under conditions of limited fracture–matrix interaction. The concept of limited fracture–matrix interaction at the Yucca Mountain site is further supported by many other independent laboratory tests as well as theoretical and numerical studies (CRWMS M&O 2000c, Section 6.1.3). In a number of laboratory experiments without considering matrix imbibition, Glass et al. (1996, pp. 6 to 7) and Nicholl et al. (1994, pp. 2533 to 2546) demonstrated that gravity-driven fingering flow is a common flow mechanism in individual fractures. This can reduce the wetted area in a single fracture to fractions as low as 0.01 to 0.001 of the total fracture area (Glass et al. 1996, pp. 6 to 7). However, consideration of matrix imbibition can increase wetted areas of fingering flow patterns in individual fractures (Abdel-Salam and Chrysikopoulos 1996, pp. 1537 to 1538). The wetted area in a fracture under unsaturated flow conditions is generally smaller than the geometric interface area between fractures and the matrix (Wang and Narasimhan 1993, pp. 329 to 335), even in the absence of fingering flow. This results from the observation that liquid water in an unsaturated fracture occurs as saturated segments that cover only a portion of the fracture–matrix interface area. Liu et al. (1998, p. 2645) suggested that in unsaturated, fractured rocks, fingering flow occurs at both a single fracture scale and a connected fracture network scale (Figure 3-4). This concept is supported by the field observations from the Rainier Mesa site, as discussed above, and by a numerical study of Kwicklis and Healy (1993, pp. 4097 to 4099). This study found that a large portion of the connected fracture network played no role in conducting water flow. Studies have also shown that fracture coatings can either reduce or increase the extent of fracture–matrix interaction. Thoma et al. (1992, pp. 1357 to 1367) performed experiments on coated and uncoated tuff fractures and observed that the low-permeability coatings inhibited matrix imbibition considerably. In contrast, fracture coatings may in some cases increase the fracture–matrix interaction when microfractures develop in the coatings (Sharp et al. 1996, p. 1331). At this point, coating effects on fracture-matrix interaction are not totally clear. Therefore, the coating effects are not considered in the unsaturated zone flow models. Based on the above discussion, it is believed that fingering flow in fractures is a common flow mechanism in unsaturated fractured rocks and a major reason for limiting fracture–matrix interaction (Figure 3-4). To incorporate the effects of fingering flow into modeling flow and transport in unsaturated fractured rocks, the active fracture model was developed (Liu et al. 1998, pp. 2633 to 2646; CRWMS M&O 2000c, Section 6.4). In the model, only a portion of connected fractures are considered to actively conduct liquid water as a result of fingering flow at a fracture network scale. Figure 3-5 shows the only observed in situ fracture flow under ambient conditions from the underground tunnels. While the scarcity of the observed fracture under ambient conditions partially results from the drying effects during and after the excavations, it is consistent with the conceptual idea that only a portion of fractures conducts water. May 2004 3-7 No. 2: Unsaturated Zone Flow Revision 1 Figure 3-4. Water Flow in Fractures Characterized by Fingering Flow at Different Scales May 2004 3-8 No. 2: Unsaturated Zone Flow Source: BSC 2003a, Figure 6.2.1-1. Figure 3-5. Flow Paths Observed During Niche Excavations: Ambient Flow Path at Niche 1 (a); Blue Dyed Flow Path at Niche 1 (b); Pink Dyed Flow Path at Niche 5 (c); and Pink Stain on the Floor of a Lithophysal Cavity at Niche 5 (d) 3-9 No. 2: Unsaturated Zone Flow Revision 1 May 2004 Revision 1 3.2.2 Effect of Major Faults Different kinds of faults with varying amounts of displacement exist at Yucca Mountain. Fault properties are variable and generally controlled by rock type and stratigraphic displacement. Because major faults may have the potential to significantly affect the flow processes at Yucca Mountain, they are important features of the unsaturated zone. A fault is considered to serve as a localized, fast flow conduit for liquid water, especially below the repository (Figure 3-1). Above the repository, transient water flow may occur within the fault as a result of temporally variable infiltration. At Yucca Mountain, major faults cut through the PTn unit, which may (in light of its high porosity and storage capacity) attenuate transient flow events within faults. Alternatively, the attenuating effect of the PTn on transient flow may be significantly reduced within the faults as a result of low-permeability mineral coatings along fracture walls within fault zones, or if the adjacent rock matrix has been altered to low-permeability clays or zeolites. However, fast flow along major faults above perched water bodies is expected to carry only a small amount of water and may not contribute significantly to the total liquid flow above the repository horizon in the unsaturated zone, as will be discussed in Section 3.2.4 of this chapter. Below the repository, low-permeability layers (or perched water zones) at the base of the TSw and in the CHn may laterally divert a considerable amount of flow to major faults, which may vertically focus flow to the water table (BSC 2004, Section 6.2). However, it is also possible that alteration within or along faults in the CHn and CFu causes them to be of low permeability, slowing water transport times from the TSw to the water table. Nevertheless, to be conservative faults below the repository have been treated as localized fast flow paths in the current unsaturated zone flow model. 3.2.3 Transient Flow Temporal variation in the infiltration rate drives the time-dependent or transient nature of flow in the unsaturated zone. The temporal variation of the infiltration may be short-term because of weather fluctuations that drive episodic flow, or long-term because of climate change. As discussed in Section 3.1.2, the PTn is believed to greatly attenuate episodic infiltration pulses such that water flow below the PTn is approximately steady. This is supported by the modeling studies of Wang and Narasimhan (1993, pp. 354 to 361) and Wu et al. (2000, pp. 30 to 32, and 39 to 41), and the test results of Salve et al. (2003, p. 269). However, water flow in the southern part of Solitario Canyon may be transient because the PTn is completely offset by the Solitario Canyon fault in this area. Some transience is also expected for liquid flow through isolated fast flow paths that cut through the PTn because of the lack of a significant attenuation mechanism. 3.2.4 Flow Focusing and Fast Flow Paths Flow focusing refers to the occurrence of significant water flow through a very small area or zone. The potential for flow focusing below the PTn unit has been demonstrated using numerical studies by Bodvarsson et al. (2003), which support the conceptual model of unsaturated flow focusing illustrated in Figure 3-6. In this conceptual model, infiltration is dispersed in the near surface and also in the low effective permeability PTn layer. From there, 3-10 May 2004 No. 2: Unsaturated Zone Flow Revision 1 water begins to be focused in the TSw layer and continues into fewer discrete flow pathways on through the CHv and CHz layers. These discrete flow pathways, which likely occur in fractures, as shown in Figure 3-4, deliver percolating water to the groundwater. Whereas the focused flow pattern below the PTn unit has a critical impact on the percolation distribution above the repository and radionuclide transport from the repository to water table, details of this focused flow pattern are not very well understood. While focused flow occurs in the TSw and below, the distribution of focused flow paths is relatively uniform and not limited to major faults, as supported by many field observations (CRWMS M&O 2000c, Section 6.1.7; Bodvarsson et al. 1999, p. 13). Average measured matrix saturations suggest relatively uniform values for most of the units, and in situ water potential measurements also show little variability within the TSw for different boreholes. It was also observed that temperatures within the TSw unit are fairly uniform (CRWMS M&O 2000c, Section 6.1.7). All these observations also support the use of the continuum approach in the unsaturated zone flow models. NOTE: Grey zones correspond to saturated zone and perched water bodies. Processes and geologic layers not represented to scale. Figure is an illustration only. Figure 3-6. Schematic Conceptual Model of Unsaturated Zone Flow Focusing in Yucca Mountain May 2004 3-11 No. 2: Unsaturated Zone Flow Revision 1 3.2.5 Perched Water Perched water is defined as saturated zones that are above or not directly connected to the groundwater table. Such phenomena may occur when large permeability differences exist between geologic units. Perched water zones at Yucca Mountain have been detected in a number of boreholes (USW UZ-14, USW NRG-7a, SD-7, USW SD-9, UE-25 SD-12, and USW G-2), occurring in the lower portion of the TSw and the upper portion of the CHn (BSC 2004, Section 6.2) (Figure 3-7). Hydraulic testing indicates that the volume of the perched water bodies at Yucca Mountain varies greatly, depending on borehole location (CRWMS M&O 2000c, Section 6.1.4). Source: BSC 2003a, Figure 6.2-2. Figure 3-7. Observed Matrix Liquid Saturations and Perched Water Elevations for Borehole USW UZ-14 The presence of perched water has important implications for the transport times and flow paths of water through the Yucca Mountain unsaturated zone. First, 14C age data for perched water suggest dominant fracture flow in the TSw unit (see Section 3.1.4). Second, the occurrence of perched water bodies indicates that certain layers of the TSw and the CHn serve as barriers to vertical flow and cause lateral flow diversion (Figure 3-8). These conceptual points are included in the unsaturated zone flow model. May 2004 3-12 No. 2: Unsaturated Zone Flow Revision 1 3.2.6 Effects of Thermal Processes The discussions above have focused on unsaturated zone flow under ambient conditions. After a geologic repository is constructed at Yucca Mountain, the emplaced radioactive wastes will emit a significant amount of heat as a result of the radioactive decay of the wastes (CRWMS M&O 2000c, Section 6.3). This heat will influence hydrologic, mechanical, and chemical conditions in both the near field (drift scale) and far field (mountain scale). This section discusses the potential effects of thermal processes and the corresponding coupled processes—thermal-hydrologic, thermal-mechanical, and thermal-chemical—on flow within the Yucca Mountain unsaturated zone. Figure 3-8. Flow Patterns within and near a Perched Water Body Characterized by Strong Lateral Flow within the Perched Water Body and the Associated Fault-Dominated Flow May 2004 3-13 No. 2: Unsaturated Zone Flow Revision 1 The potential thermal-hydrologic response of the unsaturated, fractured tuff to decay heat involves a number of key processes (CRWMS M&O 2000c, Section 6.3.1). Conceptually, when formation temperatures increase sufficiently around the waste packages, pore water will boil and vaporize. Most of the vapor generated in the matrix will move into the fractures, where it will become highly mobile. When the vapor encounters cooler rock away from the repository drifts, it will condense, and fracture saturation will increase locally. Part of the condensate may then imbibe into the matrix. The amount of imbibition will depend on the fracture–matrix interaction. Some portion of the condensate will remain in the fractures, becoming mobile, and potentially flowing back toward the boiling zone. However, because capillary forces are relatively weak in the fractures, a substantial amount of liquid may drain by gravity (Figure 3-9). The stronger the vapor flux away from the drifts and the reflux toward the drifts, the more obvious the heat pipe conditions (vapor–liquid counterflow with phase change) in the temperature fields. Where heat pipe conditions exist, the temperature will remain at the nominal boiling point. Eventually, the heat output will decrease and become small enough so as not to affect the liquid flow field, and the flow field will return to steady state. Steady-state flow will not be reestablished until the dryout zones near the drifts are resaturated. The emplacement of heat-generating wastes in the repository likely will alter large-scale flow processes associated with the mountain as well (CRWMS M&O 2000c, Section 6.3.1). Heat-driven features at this scale potentially include the development of large-scale, gas phase, buoyant convection cells and thermally altered liquid phase flow fields, both above and below the repository. Heat transfer from emplaced wastes will increase temperatures of the host rock surrounding the repository, resulting in mechanical changes to physical properties of the rock within the unsaturated zone (CRWMS M&O 2000c, Section 6.3.2). Expansion of the rock matrix caused by heating will create stress in the rock and induce changes in the fracture apertures. This may also change unsaturated zone flow pattern at different scales. Chemical effects in response to a high thermal load in the repository may alter material hydrologic properties (CRWMS M&O 2000c, Section 6.3.3). Above the repository level, the temperature is projected to be sufficiently high to vaporize the water, resulting in precipitation of minerals in fractures. The condensate, being out of equilibrium with the rock, may dissolve mineral phases from the walls of fractures and then flow back into zones where chemical precipitation occurs. Below the repository, the processes are not completely the same because the liquid within the fractures, under the influence of gravity, can migrate away from the heat source and leave the two-phase flow system. This dissolution of minerals at one point in the fracture network and their redeposition at another, could lead to the formation of precipitate zones over the drifts. In a precipitate zone, the porosity and permeability of fractures and portions of the matrix near the fractures may be reduced. When thermal perturbation is considerably decreased, the precipitate may start to be dissolved by water from ambient, downward percolation. Because the process of chemical dissolution is much slower than the process of chemical precipitation, the precipitate zone may become virtually permanent after the thermal perturbation. Therefore, these changes will influence the flow field not only while the thermal process is active, but afterwards as well. The effects of thermal processes and the resultant coupled processes on unsaturated zone flow will be further discussed in Section 4.4. May 2004 3-14 No. 2: Unsaturated Zone Flow Revision 1 NOTE: For the current repository design concept, the dryout zone around drifts will extend to a distance of approximately 5 to 10 m from the drift wall. Figure 3-9. Effects of Thermal Processes on Flow Pattern near Drifts 3.3.1 Climate and Infiltration Net infiltration is the ultimate source of percolation flux within the unsaturated zone and provides the water for flow and transport mechanisms that may move radionuclides from the repository to the water table. Net infiltration is spatially and temporally variable because of the nature of the storm events that supply precipitation and the variation in soil cover and topography. Infiltration is believed to be high on side slopes and ridge tops where bedrock crops out, and fracture flow in the bedrock is able to move moisture away from zones of active evapotranspiration. 3.3 RELATIONS BETWEEN UNSATURATED ZONE FLOW AND OTHER PROCESSES As indicated in Section 1, water flow process within the unsaturated zone is closely related to infiltration, water seepage into drifts, and radionuclide transport within the unsaturated zone. While detailed discussions of these related processes are given in the corresponding technical basis documents, this section provides a brief description of these related processes. May 2004 3-15 No. 2: Unsaturated Zone Flow Revision 1 Net infiltration depends on precipitation. The global records show that climate is cyclic over 400,000-year periods. Sharpe (2003, Table 6-1) suggests the existence of a long-term, modern interglacial climate state for at least the last 9,000 years. Climatic data forecasting indicates that during the next 10,000 years at Yucca Mountain, the modern-day climate should persist for 400 to 600 years, followed by a warmer and much wetter monsoon climate for 900 to 1,400 years. The monsoon climate will be followed by a cooler and wetter glacial transition climate for the remaining 8,000 to 8,700 years of the regulatory period. The upper-bound precipitation values for the monsoon and glacial transition climates exceed the upper bounds of the region’s modernday climate by about 100 mm. The glacial transition climate’s lower bound exceeds the modern lower bound values by about 150 mm. Thus the future climate is wetter, but not substantially wetter than the modern climate. A detailed discussion of climate and infiltration process is provided in Technical Basis Document No. 1: Climate and Infiltration. 3.3.2 Water Seepage into Drifts Water enters the unsaturated zone from above as net infiltration, then percolates downward through the host rock under the impetus of gravity. When the percolation encounters an opening such as an emplacement drift, capillary forces will tend to divert water around the opening. This phenomenon is known as the capillary barrier effect (BSC 2003f, Section 6.3.1) and is illustrated in Figure 3-10. The barrier effect leads to a local saturation build up in the host rock immediately above, and adjacent to the drift opening. If capillary and permeability properties of the fracture network in this region are sufficiently large (or strong), some or all of the incident percolation is diverted around the drift. Locally, however, the saturation may increase (and capillarity decrease) such that water can enter the drift as seepage (BSC 2003f, Section 6.3.1). Seepage is defined as the movement of liquid water into an underground opening, and does not include movement of water vapor or condensation within openings. The seepage threshold is the critical percolation flux below which seepage in the opening is unlikely to occur. Field tests have demonstrated the existence of a seepage threshold in the host rock. May 2004 3-16 No. 2: Unsaturated Zone Flow Revision 1 Figure 3-10. Phenomena and Processes Affecting Drift Seepage Source: BSC 2003f, Figure 6.3-1. For an emplacement drift of a given shape, the seepage threshold—and the amount of seepage once this threshold is exceeded—depend on the local flow conditions in the near field (Figure 3-10). These conditions are mainly influenced by the local percolation flux reaching the opening, and by the local hydrologic properties of the host rock (principally the capillary strength and relative permeability of the fracture network) (BSC 2003f, Section 6.3.1). Smallscale heterogeneity of hydrologic properties (i.e., fracture characteristics) increases the likelihood of seepage (Birkholzer et al. 1999). In addition, intermediate scale and mountain scale variability in hydrologic properties and flow paths within Yucca Mountain produces spatial variability in the percolation flux. The drift opening cross section shape and size affect seepage, in the manner originally discussed by Philip et al. (1989). As discussed in Section 3.2.6, seepage into drifts is also affected by thermal process and the resultant coupled processes. A detailed discussion of seepage process is provided in Technical Basis Document No. 3: Water Seeping into Drifts. 3.3.3 Unsaturated Zone Transport Radionuclide transport within the unsaturated zone is strongly related to unsaturated zone flow through the advection, and advective transport pathways are consistent with flow pathways discussed in Section 3.1. (Advective transport (advection) refers to the movement of dissolved or colloidal materials because of the bulk flow of fluid.) In the welded units, advection through May 2004 3-17 No. 2: Unsaturated Zone Flow Revision 1 fractures is expected to dominate transport behavior, mainly because liquid water largely flows through fracture networks in these units. Advection is also an important mechanism for transport between fractures and matrix, especially at interfaces between nonwelded and welded units. At these interfaces, transitions occur between dominant fracture flow and dominant matrix flow. Liquid water flow paths below the repository horizon will be critical to the radionuclide transport resulting from advection, particularly in perched water bodies, where lateral transport of radionuclides is likely to occur. Dominant fault and fracture flow in the zeolitic part of the CHn provides relatively short transport times for transport to the water table, whereas dominant matrix flow in the vitric part of the CHn provides much longer transport times. In addition to advection, radionuclide transport within the unsaturated zone is affected by several other mechanisms, such as matrix diffusion, sorption, colloid-facilitated transport, and decay. Matrix diffusion refers to solute transport from fracture networks to surrounding matrix blocks resulting from molecular diffusion (CRWMS M&O 2000c, Section 6.2.2). Mass transfer between fractures and tuff matrix may play an important role in transport within Yucca Mountain. Because flow velocity in the matrix is much slower than in fractures, transfer of radionuclides from fractures to the matrix can significantly retard the overall transport of radionuclides to the water table. Sorption is an important mechanism in reactive chemical and radionuclide transport (CRWMS M&O 2000c, Section 6.2.3). It is used to describe a combination of chemical interactions between dissolved solutes and the solid phases (immobile rock matrix or colloids), including adsorption, ion exchange, surface complexation, and chemical precipitation. Radionuclide transport in the unsaturated zone also involves a colloid-facilitated transport mechanism (CRWMS M&O 2000c, Section 6.2.4). Colloids are particles small enough to become suspended (and thus transportable) in a liquid. They can interact with radionuclides through sorption mechanisms. Unlike sorption of radionuclides to the rock matrix, however, radionuclides sorbed on colloids are potentially mobile. Therefore, colloids can facilitate radionuclide transport through the unsaturated zone at a faster rate than the aqueous phase alone. Radioactive decay is a transformation process that affects the concentration of radionuclides during transport through the unsaturated zone (CRWMS M&O 2000c, Section 6.2.5). For simple decay, radionuclide concentration decreases exponentially with time, creating stable decay products. Chain decay adds additional complexity because of the ingrowth of new radionuclides created from the decay of a parent radionuclide. One aspect of potential significance with respect to chain decay is that daughter products may have significantly different sorption behavior than the parent radionuclide, therefore exhibiting different transport behavior. A detailed discussion of radionuclide transport process is provided in Technical Basis Document No. 10: Unsaturated Zone Transport. May 2004 3-18 No. 2: Unsaturated Zone Flow Revision 1 4. DEVELOPMENT OF THE SITE-SCALE FLOW MODEL The site-scale unsaturated zone flow model has been developed based on the geology of the site, conceptual understanding of flow paths within the unsaturated zone, and field data. The unsaturated zone flow model is used to generate unsaturated zone flow fields used directly by TSPA. While the details of the unsaturated zone flow model are documented in UZ Flow Models and Submodels (BSC 2004), this section provides a brief introduction to development and validation of the site-scale unsaturated zone flow model. 4.1 NUMERICAL APPROACH AND RELATED ISSUES In the site-scale unsaturated zone flow model, simulation of the complex flow and transport processes occurring within the unsaturated zone requires simplification of the real-world system. Among the many issues related to this simplification, this section discusses three important ones: selection of the numerical approach; treatment of heterogeneity; and the active fracture model used to deal with fingering flow in fracture networks. 4.1.1 Selection of the Numerical Approach Several approaches are available in the literature for modeling flow and transport in unsaturated fractured rocks. When classified according to the manner in which fracture networks are treated in the model structure, these approaches mainly fall into one of two categories: the continuum approach and the discrete fracture network approach. Reviews on these approaches, which have been developed and used in different fields (including oil reservoir engineering, groundwater hydrology, geothermal engineering and soil physics), can be found in Flow and Contaminant Transport in Fractured Rock (Bear et al. 1993) and National Research Council (1996). In the continuum approach, fractures are considered to be sufficiently ubiquitous and distributed in such a manner that they can be meaningfully described statistically (Bear et al. 1993). The role of individual fractures in fractured media is considered to be similar to that of individual pores in porous media. Therefore, one can describe average fracture properties as macroscopic and those associated with individual fractures as microscopic. In the continuum approach, connected fractures and rock matrix are viewed as two or more overlapped interacting continua. In other words, at a “point,” two or more continua are considered to coexist. In this case, the continuum mechanics formulations, such as those used for porous media, can be used to describe flow and transport in each continuum. Coupling of processes between different continua is determined by their interaction mechanisms at a subgrid scale. Depending on the number of continua and the methodology used to treat fracture–matrix interaction, continuum models can be classified further as effective-continuum, dual-continuum, and multiple-interacting-continua models (Figure 4-1). (In Figure 4-1, solid lines between node points represent potential flow paths.) The effective-continuum model replaces fractures and rock matrix with a single effective continuum. Dual-continuum (dual-porosity and dual-permeability) models treat fractures and the matrix as two separate, yet interacting, continua. The multiple-interacting-continua model further subdivides the matrix into more than one continuum to consider the nonequilibrium of flow and transport within the matrix. 4-1 May 2004 No. 2: Unsaturated Zone Flow Revision 1 Figure 4-1. Effective-Continuum Model (a); Dual-Porosity with One Matrix Gridblock (b); Dual-Permeability with One Matrix Gridblock per Fracture Gridblock (c); and Multiple- Interacting-Continua Model with Three Matrix Gridblocks per Fracture Gridblock (d) May 2004 4-2 No. 2: Unsaturated Zone Flow Revision 1 The discrete fracture network approach involves the generation, by computer simulation, of synthetic fracture networks and the subsequent modeling of flow and transport in these networks. The approach has been extensively used for single phase flow and transport, with deterministic, stochastic, artificial, or site-specific fracture networks (e.g., Bear et al. 1993; National Research Council 1996). This approach has been adapted for use in unsaturated zone studies at Yucca Mountain (CRWMS M&O 2000c, Section 6.4). Both continuum and fracture network approaches have advantages and disadvantages. While the fracture network approach is useful as a tool for concept evaluation or model-based process studies, it has several limitations. First, the approach requires geometric parameters that may strongly impact flow and transport, such as fracture apertures and conductivity, but typically cannot be well constrained from field observations. Second, it is difficult to separate the conductive fracture geometry from the nonconductive fracture geometry. Third, flow and transport models based on the approach can be complex and computationally intensive for realistic fracture densities. Fourth, so far, studies based on the fracture network approach have rarely considered fracture–matrix interaction (flow and transport between fractures and the matrix), because of computational intensity for unsaturated flow and transport. Fracture–matrix interaction has important effects on flow and transport processes in unsaturated fractured rocks. Because the continuum approach is relatively simple and straightforward to implement, it is preferred for most applications encountered in practice (National Research Council 1996). For example, because the number of fractures is estimated over the site on the order of 109 at Yucca Mountain (Doughty 1999), it is practically impossible to construct and calibrate a discrete fracture network site-scale model with so many fractures, considering data availability and computational feasibility. Therefore, the dual-continuum approach has been used as the baseline approach for modeling flow and transport within Yucca Mountain. The use of the continuum approach is also supported by the study of Finsterle (2000), who demonstrated that the continuum approach can capture flow behavior generated from a discrete feature model. A discussion of the other issues related to the selected numerical approach for the unsaturated zone flow model, such as integrated finite difference method, upstreaming, weighting, and grid refinement, can be found in UZ Flow Models and Submodels (BSC 2004). 4.1.2 Treatment of Subsurface Heterogeneity Heterogeneities exist at different scales within both fracture and matrix continua in the unsaturated zone at Yucca Mountain. Treatment of subsurface heterogeneity is important for modeling flow and transport processes. A geology-based deterministic approach, in which an entire model layer is assigned uniform properties, is used mainly for representing subsurface heterogeneity. The key justification for the above approach is that the overall behavior of site-scale flow and transport processes is determined mainly by relatively large-scale heterogeneities associated with the geologic stratification and tectonic features (e.g., faults) of the mountain. Within the same geologic unit, hydrologic properties are relatively uniformly distributed because of the intrastrata homogenization induced by tuff depositional environments. This justification is also consistent with a field observation that matrix saturation distribution is relatively uniform within a given geologic unit (CRWMS M&O 2000c, Section 6.4.3). Displacement of strata along faults can 4-3 May 2004 No. 2: Unsaturated Zone Flow Revision 1 result in units with different hydrologic properties being placed against each other across the fault, resulting in lateral heterogeneities within the unsaturated zone. Zhou et al. (2003) recently demonstrated the validity of this approach in dealing with subsurface heterogeneity. They used a two-dimensional vertical cross section in the east–west direction through borehole USW UZ-14 to investigate the effect of multiscale heterogeneity on unsaturated flow and transport within the unsaturated zone. Specifically, they generated and used random fields of three selected properties: matrix permeability, km; matrix van Genuchten parameter, ám; and fracture permeability, kf (Zhou et al. 2003, Figure 13). For comparison, they employed different sets of rock property distributions in three cases. In these cases, mean rock properties for a given geological layer are the same. In Case A, the layered approach mentioned above was used. In Case B, stochastically generated kf variability was considered within a geological layer. In Case C, variabilities of all the selected rock properties were included, with the variabilities determined from the measured hydrologic properties. Thus, Case A only considers large-scale heterogeneity, while Cases B and C include small-scale heterogeneity within a geological unit. Figure 4-2 shows comparisons of vertical water fluxes within the matrix and fractures along the repository zone for three cases. Although relatively large differences exist for the water flux in the matrix, distributions of water fluxes in fractures are very similar for these three cases. Because the matrix flux corresponds to only a small percentage of total water flux, the three cases essentially provide similar water flow fields at the site scale. Zhou et al. (2003, Figure 17) also compared simulated results for tracer transport from the repository to water table, and again found that the results are similar for the three cases. In summary, the study of Zhou et al. (2003) demonstrates that heterogeneities within each geological unit have only a minor effect on the site-scale flow processes. May 2004 4-4 No. 2: Unsaturated Zone Flow Revision 1 Source: Zhou et al. 2003, Figure 16. 4.1.3 Active Fracture Model A traditional continuum approach assumes uniformly distributed flow patterns at a subgrid scale. Therefore, such an approach cannot be used for representing gravity driven fingering flow and transport in fracture networks, resulting from subsurface heterogeneities and nonlinearity involved in unsaturated flow. To incorporate this flow behavior into the continuum approach, the site-scale unsaturated zone flow model uses the active fracture model. The active fracture model was developed within the context of the dual-continuum approach (Liu et al. 1998). While the details of the model can be found in Liu et al. (1998), a brief description of the model is provided here for convenience. The active fracture concept is based on the reasoning that, because of fingering flow, only a portion of fractures in a connected, unsaturated fracture network contribute to liquid water flow, while other fractures are simply bypassed. The connected fractures that actively conduct water are called active fractures. In other words, the active fracture model uses a combination of the volume-averaged method and a simple filter to deal with fracture flow and transport. Inactive fractures are filtered out in modeling fracture– matrix interaction, flow, and transport in the fracture continuum. Figure 4-2. Comparison of Simulated Matrix and Fracture Flux (m/s) at the Repository Horizon in Cases A, B, and C May 2004 4-5 No. 2: Unsaturated Zone Flow Revision 1 The conventional, capillary equilibrium-based, fracture water distribution model assumes that liquid water occupies first fractures with small apertures, and then fractures with relatively large apertures, as water potential (or water saturation) increases. In contrast, the active fracture model presumes gravity-dominated, nonequilibrium, preferential liquid water flow in fractures, which is expected to be similar to fingering flow in unsaturated porous media. A liquid finger can bypass a large portion of a porous medium, which does not necessarily correspond to large pores. The above discussion is valid for large-scale flow processes and not inconsistent with the possible validity of a capillary equilibrium-based fracture water distribution concept at relatively small scales, corresponding to an individual flow path or a single flow finger. Flow and transport conditions and fractured rock properties determine the fraction of active fractures in a connected fracture network. In the active fracture model, this fraction is expressed as a power function of fracture saturation. Analysis of Hydrologic Properties Data (BSC 2003g, Section 6.7) showed that, in theory, this expression is consistent with fractal flow patterns often observed in unsaturated systems. It also demonstrated that the active fracture model-based simulation results are generally consistent with 14C age and fracture coating data, although some uncertainty may exist in interpreting these data sets (BSC 2003g, Section 7). This study concludes that simulated distributions of large-scale water flux, matrix saturation, and water potential are not sensitive to active fracture model parameter values (BSC 2004, Section 6.8), although the water velocity distributions are (Liu et al 1998). A further discussion of effects of active-fracture-model parameter values on solute transport process may be found in Technical Basis Document No. 10: Unsaturated Zone Transport. 4.2.1 Boundary Conditions The three-dimensional unsaturated zone model domain and the numerical grid for this study is shown in plan view in Figure 4-3 and encompasses approximately 40 km2 of the area over the mountain. This three-dimensional model grid uses a refined mesh in the vicinity of the repository, located near the center of the model domain, covering the region east of the Solitario Canyon fault through the Ghost Dance fault and north to beyond Pagany Wash fault. The model domain is selected to focus on the repository area and to investigate the effects of different infiltration scenarios and major faults on moisture flow around and below the repository. In the model grid, vertical or inclined 30-m-wide zones represent modeled faults. 4.2 OVERVIEW OF THE SITE-SCALE UNSATURATED ZONE MODEL The site-scale unsaturated zone flow model is a three-dimensional, dual-permeability unsaturated flow model. This section discusses some key elements of the unsaturated zone flow model, including the boundary conditions, property calibration, and simulation scenarios for TSPA. 4-6 May 2004 No. 2: Unsaturated Zone Flow Revision 1 Figure 4-3. Plan View of the Three-Dimensional Unsaturated Zone Flow Model Domain Source: BSC 2004, Figure 6.1-1. The ground surface of the mountain with exposed tuff or the tuff–alluvium contact in areas with significant alluvial cover is taken as the top model boundary, and the water table is treated as the May 2004 4-7 No. 2: Unsaturated Zone Flow Revision 1 bottom model boundary. Spatially variable surface infiltration is applied into fractures from the top boundary. All lateral boundaries (see Figure 4-3) are treated as no flow (closed) boundaries and allow flow only along vertical planes. This treatment should be reasonable for the eastern boundary, which is along or near the Bow Ridge fault, because high vertical permeability and lower capillary forces are expected within the faults (see fault properties estimated in Calibrated Properties Model (BSC 2003b, Section 6.3.4)). The southern, western, and northern lateral boundaries are farther away from the repository and, thus, have little effect on unsaturated flow within or near the repository. For simulations involving airflow and nonisothermal conditions, both the top and bottom boundaries of the model are treated as possessing Dirichlet-type conditions with specified constant but spatially varying gas pressure and or temperature. 4.2.2 Model Calibration and Uncertainties The site-scale unsaturated zone flow model is a three-dimensional dual-permeability model. In the model, 32 layers with different hydrologic properties are used to represent the stratigraphic units encountered at the site. Each layer is assigned homogeneous hydrologic properties, with the exception of the layers in CHn, which are assigned hydrologic properties for either vitric or zeolitically altered rock types. Because field observations from the Busted Butte test sites (Section 2.4.1) clearly indicated that flow occurs in the matrix only within the vitric CHn, the fracture flow component within the vitric CHn is not considered in the site-scale unsaturated zone flow model. Directly measured fracture permeability and matrix hydrologic properties are available for most of the model layers. However, the unsaturated zone flow model cannot directly use these properties. This is because measurement scales are generally inconsistent with the property resolution scales in the model. Also, not all the needed hydrologic properties (such as fracture van Genuchten parameters) are measured. Consequently, model calibration is needed to develop hydrologic properties for the site-scale unsaturated zone flow model. Model calibration involves using numerical models for the unsaturated zone to predict unsaturated zone conditions and then comparing them to observations of these conditions from field measurements (such as saturation data and gas pressure data). Model parameters are adjusted (calibrated) so that the difference between the model predictions and the observed data is minimized. Model calibration involves a series of increasingly complex models. Because model calibration needs a great number of forward simulation runs, computationally efficient one-dimensional models are used for developing preliminary calibrated hydrologic properties. Using one-dimensional models, data from multiple boreholes are inverted simultaneously to estimate layer average properties. Matrix saturation, in situ water potential data, and pneumatic pressure (barometric pumping response) data are used to estimate the hydrologic properties in one-dimensional calibrations. Two-dimensional cross-sectional models are employed for estimating fault properties. In a two-dimensional model, a fault is modeled perpendicular to the plane of the model. Again, matrix saturation, water potential, and pneumatic pressure data are inverted. 4-8 May 2004 No. 2: Unsaturated Zone Flow Revision 1 One major limitation of the one-dimensional models is that they can not capture the important three-dimensional flow behavior, such as lateral flow, because of capillary barrier effects in the PTn and effects of the perched water. To resolve this issue, three-dimensional models are used for limited trial-and-error calibrations and verification of simulation results against many different data sets, including data sets used for one- and two-dimensional calibrations (BSC 2004). Note that automatic calibrations have been used for one- and two-dimensional cases. Additional data sets used for model calibration at this stage include perched water elevations at several surface-based boreholes and pore water chloride concentration data from surface-based boreholes and the ECRB Cross-Drift. The latter data are closely related to spatial distribution of percolation within the unsaturated zone. The model is also calibrated against the temperature data observed from deep boreholes, which are also related to the percolation process. Property adjustments are needed to calibrate the three-dimensional unsaturated zone flow model against the data mentioned above. For example, Figures 4-4, 4-5, and 4-6 present some calibration (simulation) results compared to corresponding field observations. Source: BSC 2004, Figure 6.4-1. Figure 4-4. Comparison of Simulated and Observed Gas Pressure at Borehole USW SD-7 in a 60-Day Period May 2004 4-9 No. 2: Unsaturated Zone Flow Source: BSC 2004, Figure 6.3-2. Figure 4-5. Comparison of Simulated and Observed Ambient Temperature Profiles for the Five Boreholes under the Present-Day, Mean Infiltration Rate No. 2: Unsaturated Zone Flow Revision 1 May 2004 4-10 Revision 1 Source: BSC 2004, Figure 6.2-3. NOTE: In the figure, preq_mA, monq_mA, and glad_mA correspond to present-day mean infiltration, monsoon mean infiltration, and glacial transition mean infiltration rates. Infiltration Rates Model calibration is subject to uncertainty. (The uncertainties are handled in the TSPA by considering a number of unsaturated zone flow fields generated by different property sets.) A major source of parameter uncertainty is the conceptual model described in Section 3. Infiltration rate uncertainty also contributes to parameter uncertainty, because flow processes in the unsaturated zone are largely determined by top boundary conditions. To capture this uncertainty, three infiltration scenarios (including present-day mean, upper-bound, and lower-bound infiltration maps) are used for the parameter calibration. In addition, scale effects are a well-known source of parameter uncertainty. This is especially true for determination of the unsaturated zone model parameters. For example, matrix parameters are measured in the unsaturated zone at core scale on the order of several centimeters, whereas in the unsaturated zone flow model, numerical gridblocks are on the order of a few meters to hundreds of meters. Scale-dependence of hydrologic parameters has been widely recognized in the scientific Figure 4-6. Comparison of Simulated and Observed Matrix Liquid Saturation and Perched Water Elevations for Borehole UE-25 SD-12, Using the Simulation Results for the Three Mean May 2004 4-11 No. 2: Unsaturated Zone Flow Revision 1 community (e.g., Neuman 1994). Although upscaling is partially considered in developing uncalibrated matrix properties, the calibrated matrix permeabilities are, on average, higher than uncalibrated ones for the three infiltration scenarios, which is consistent with findings reported in the literature (e.g., Neuman 1994) and implies that the scale effects were captured by the model calibration. Fracture permeability values calibrated against pneumatic pressure data are also about two orders of magnitude higher than averaged permeability values measured from air injection tests with injection intervals of several meters. Furthermore, calibrated properties are not unique because of data limitation. Different property sets may match the data equally well. Considering the difficulties in accurately quantifying parameter uncertainty, standard deviations of measured data are directly used for the parameter uncertainty of the calibrated parameter sets (BSC 2003h, Section 6.4). The parameter uncertainty of the uncalibrated property sets (measured data) is largely a result of small-scale spatial variability. Because the degree of spatial variability decreases with scale (subgrid scale (or high frequency) spatial variability is removed at a large scale), this treatment is likely to provide the upper limits of uncertainty on calibrated parameters for the given conceptual model and infiltration rates. As previously discussed, the unsaturated zone flow model is based on the continuum (dualpermeability) approach that conceptualizes a fracture network as a continuum. The continuum approach is commonly used for modeling large-scale flow and transport problems (National Research Council 1996) and has been shown to be able to account for important flow and transport processes observed from a number of field tests in the ESF and the ECRB Cross-Drift at Yucca Mountain (e.g., Finsterle 2000). However, a continuum model is not designed to capture complex discrete-flow behavior in unsaturated fractured rock. In a discrete fracturenetwork study, Liu et al. (2002) suggest from the model results that the average spacing of active flow paths (in unsaturated fracture networks) within a layered system increases with depth, based on fracture network connectivity characteristics. Fracture network connectivity is not a characteristic considered by the current continuum approach. The unsaturated zone flow model uses van Genuchten (1980) relationships, developed especially for porous media, for describing unsaturated flow in fractures. This constitutive relationship model largely determines the corresponding simulation results. However, the applicability of van Genuchten relationships for fractured rocks is questioned (Liu and Bodvarsson 2001, Glass et al. 1996). Liu and Bodvarsson (2001) reported that van Genuchten (1980) relationships underestimate fracture relative permeability for a large range of water saturation values and argued that an improved relationship model is needed for fractures. The uncertainty in the fracture constitutive relationship is fundamental to unsaturated zone models. The unsaturated zone flow model uses the active fracture model of Liu et al. (1998) to deal with fingering flow and transport in unsaturated fractures. The active fracture model has been theoretically shown to be consistent with fractal flow patterns (common in different unsaturated systems), and simulation results based on the model can represent field-scale observations from different sources reasonably well (BSC 2003g, Sections 6.7 and 7). Considering the complexity of unsaturated flow in fractures at different scales, the tests for active fracture model validation require that detailed fracture–matrix interactions, partitioning of fracture flows from matrix flows, and water balances be taken into account (BSC 2003g, Sections 6.7 and 7). May 2004 4-12 No. 2: Unsaturated Zone Flow Revision 1 The issues regarding model calibration and related uncertainties are further addressed in responses to KTIs TEF 2.11, and TSPAI 3.26. 4.3 CONFIDENCE BUILDING ACTIVITIES In addition to being consistent with the conceptual understanding of water flow in the unsaturated zone, the unsaturated zone flow model is validated by checking for the consistency between modeling results with hydrologic and temperature data, geochemical data, and in situ field test data. These data are not used for model calibrations. 4.3.1 Consistency with Hydrologic and Temperature Data Water potential data were collected from calibrated heat dissipation probes installed in the tunnel wall (at a depth of 2 m) along the ECRB Cross-Drift inside the ESF. As part of the three-dimensional flow and transport modeling validation process, modeled results of water potentials (for present-day, mean infiltration rates) were compared to field observations (Figure 4-7) (BSC 2004, Section 7.2). Most of the observed water potential data were distributed between 0.1 (104 Pa) and 1 (105 Pa) bars, with a maximum of 3.4 bar. The model predicted approximately 1 bar for the same section of tunnel. Even though the data available for comparison from the ECRB Cross-Drift are limited, the unsaturated zone model generally predicted the range of water potential data from in situ measurements. Three-dimensional model simulation results (for the present-day, mean infiltration rates) were also compared with other sets of hydrologic and temperature data. The comparisons with perched water and matrix water potential data collected within the CHn hydrogeologic unit from borehole USW WT-24 are shown in Figure 4-8; the comparisons with gas pressure data from boreholes UE-25 SD-12 and USW UZ-7a are shown in Figure 4-9; and the comparisons with ambient temperature data from boreholes USW H-5, USW H-4, and UE-25 WT#18 are shown in Figure 4-10. The model was found to be consistent with these sets of data (BSC 2004, Section 7). 4-13 May 2004 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2004, Figure 7.2-1. NOTE: The preq_mA refers to simulation results for the present-day, mean infiltration rates. Figure 4-7. Comparison of Predicted and Measured Water Potential along the ECRB Cross-Drift Using the Present-Day, Mean Infiltration Rates May 2004 4-14 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2004, Figure 7.3-1. NOTE: The preq mA refers to simulation results for the present-day, mean infiltration rates. Figure 4-8. Comparison of Predicted and Measured Matrix Water Potentials and Perched Water Elevations at Borehole USW WT-24, Using the Present-Day, Mean Infiltration Rate (preq_mA) May 2004 4-15 No. 2: Unsaturated Zone Flow Source: BSC 2004, Figures 7.4-1 and 7.4-2. Figure 4-9. Comparison of Three-Dimensional Pneumatic Prediction to Observation Data from Boreholes USW UZ-7a and UE-25 SD-12 No. 2: Unsaturated Zone Flow Revision 1 May 2004 4-16 Revision 1 Source: BSC 2004, Figures 7.7-1 to 7.7-3. Figure 4-10. Comparison of Simulated and Observed Temperature Profiles for Boreholes USW H-5, USW H-4, and UE-25 WT#18 May 2004 4.3.2 Consistency with Geochemical Data Because geochemical data provide independent insights into flow and transport processes in the unsaturated zone, various geochemical data sets are used for verifying the unsaturated zone flow model. 4-17 No. 2: Unsaturated Zone Flow Revision 1 4.3.2.1 Model Validation with 14C 14C data were collected from perched water, pore water, and gas samples from the Yucca Mountain unsaturated zone (BSC 2002b, Section 6.6.4). 14C data from gas samples are considered to be most representative of in situ conditions (Yang 2002, Section 4.1.2). 14C is also considered to be the most sensitive isotope for measuring pore water age within the Yucca Mountain unsaturated zone, owing to its half-life time (which is in the same order of magnitude as the pore water age in the unsaturated zone) and its detectable abundance. Gas samples were collected from different kinds of boreholes, including open and instrumented surface-based boreholes. The data from instrumented boreholes, USW SD-12 and USW UZ-1, provide more reliable indicators of in situ matrix pore water ages (BSC 2002b, Section 6.6.4.3). 14C ages (BSC 2002b, Table 20) calculated using the data from these two boreholes are used for validating the unsaturated zone model. Gas phase 14C ages are interpreted to represent the ages of the in situ pore water. This interpretation is based on the rapid exchange of gas phase CO2 (reaching equilibrium in hours to days) with dissolved CO2 and HCO3 - in pore water. Water transport times simulated with the site-scale unsaturated zone model are within the range of field observations, indicating the consistency between simulation results and the data (BSC 2004, Section 7.5). As an example, Figure 4-11 shows the simulation results compared to age data for borehole USW UZ-1. 4.3.2.2 Model Validation with Chloride Chloride is hydrologically very mobile and chemically inert, and a nearly ideal natural tracer for the study of water movement in the liquid phase. A chloride model was developed to validate the unsaturated zone model by testing it with data not used in the development or calibration of the unsaturated zone model. The chloride model simulates large-scale unsaturated zone chloride transport processes. It uses the three-dimensional flow fields calculated by the unsaturated zone model and incorporates chloride-in-precipitation data to model advective and diffusive chloride transport in the unsaturated zone (BSC 2004, Section 7.8). The simulated pore water chloride concentration is compared with analysis of samples collected along the ESF. Chloride concentrations in the ESF using the three infiltration scenarios are plotted in Figure 4-12. The criterion for validation is that the range of the simulated chloride concentration falls within the range of measured concentrations (BSC 2002d, Attachment I-1-2-3, BSC 2004, Section 7.1). The range of the simulated chloride concentration of the base-case flow field (preq_mA) in the ESF generally fall within the range for measured concentrations. The figure also indicates that the trend of measured chloride concentrations in samples is preserved in the calculated chloride concentrations. Note that measured chloride data are clustered around three areas with distances of about 1,000, 3,600, and greater than 6,800 m. For the first two locations, at 1,000 and 3,600 m, the simulated (preq_mA) results are either within or at the range of measurements. For the last portion, however, the simulations are well within the range of measurement for greater than 7,000 m and are close to (but a little higher than) the measurement. The figure also shows simulation results for the present-day, upper-bound infiltration scenario (preq_uA), present-day, lower-bound infiltration scenario (preq_lA), and glacial transition mean infiltration scenario. May 2004 4-18 No. 2: Unsaturated Zone Flow Source: BSC 2004, Figure 7.5-1. Figure 4-11. Comparison between Simulated (the Curves with gamma = 0.6 and gamma = 0.4) and Observed Water Residence Ages in the Matrix for Borehole USW UZ-1 No. 2: Unsaturated Zone Flow Revision 1 May 2004 4-19 Revision 1 Source: BSC 2004, Figure 7.8.1. 4.3.2.3 Model Validation with Calcite Modeling calcite deposition provides additional confidence for validation of the unsaturated zone flow model. A one-dimensional reactive transport model (an unsaturated zone submodel) is used to constrain the infiltration flux, by comparing the simulation results with calcite abundances from a deep surface-based borehole (USW WT-24). The percolation flux in the unsaturated zone is largely determined by infiltration rates corresponding to the top boundary conditions of the unsaturated zone flow model. Simulation results indicate that observed calcite abundance from USW WT-24 is consistent with a range of infiltration rates used in the unsaturated zone flow model near the borehole USW WT-24 (BSC 2004, Section 7.9) (Figure 4-13). Figure 4-12. Comparison of Chloride Concentration (mg/L) Profiles under Present-Day Infiltration Rates with Simulation Results for Mean (preq_mA), Upper (pre_uA), and Lower (preq_lA) Bounds, and Glacial Transition Infiltration Rates (glaq_pmA) at the Exploratory Studies Facility May 2004 4-20 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2004, Figure 7.9-3. Figure 4-13. Comparison between Measured and Simulated Total (Fracture plus Matrix) Calcite Abundances (in ppmV or 10-6 volume fraction) in the WT-24 Column for Different Infiltration May 2004 Rates (Curves for Cases with Millimeters per Year) Strontium concentrations and the 87Sr/86Sr ratio in pore fluids and secondary minerals can provide important constraints on infiltration rates, flow paths, residence times, and degrees of water rock and fracture–matrix interaction at Yucca Mountain. Simulation results based on three-dimensional flow fields from the unsaturated zone flow model are also found to be generally consistent with strontium concentrations observed from boreholes USW SD-9 and UE-25 SD-12 and from the ECRB Cross-Drift (BSC 2004, Section 7.10) (Figure 4-14). 4-21 No. 2: Unsaturated Zone Flow Source: BSC 2004, Figure 7.10-1. Figure 4-14. Comparison of Measured and Modeled Strontium Concentrations as a Function of Elevation for the Surface-Based Boreholes USW SD-9 (a) and UE-25 SD-12 (b) No. 2: Unsaturated Zone Flow Revision 1 May 2004 4-22 Revision 1 4.3.3 Consistency with In Situ Test Results Comparisons between simulation results and field observations from Alcove 8–Niche 3 tests are used to evaluate the methodology (continuum approach) used in the site-scale unsaturated zone flow model. A three-dimensional submodel for the test site (based on the continuum modeling approaches used by the unsaturated zone flow model) is developed, and modeling analysis uses both model calibration and prediction (BSC 2004, Section 7.6). The Alcove 8-Niche 3 tests are briefly described in Section 2.3. Infiltration rate, seepage rate, and tracer concentration data from the tests are used to corroborate model simulations. It was found that the field observations are well represented by the modeling results. For example, Figure 4-15 shows a comparison between simulated and observed seepage rate as a function of time. A similar model evaluation approach was used for Alcove 1 tests (described in Section 2.3). Again, modeling results are found to be consistent with field observations (e.g., Figure 4-16). More detailed discussions of these model evaluation activities are given in UZ Flow Models and Submodels (BSC 2004, Section 7.6) and Liu et al. (2003). Source: BSC 2004, Figure 7.6-8. Figure 4-15. Comparison between Simulated Seepage Rates as a Function of Time and Field Observations Collected from Alcove 8–Niche 3 Tests May 2004 4-23 No. 2: Unsaturated Zone Flow Revision 1 Source: Liu et al. 2003, Figure 4. Figure 4-16. Comparison between Simulated Seepage Rates as a Function of Time and Field Observations Collected from Alcove 1 Tests 4.4 MODELING STUDIES OF THERMAL EFFECTS ON UNSATURATED ZONE FLOW Heat released by the emplaced waste induces coupled changes in thermal, hydrologic, chemical, and mechanical processes both at the drift scale near the waste and at the site scale over Yucca Mountain. The site-scale thermal-hydrologic, thermal-hydrologic-chemical, and thermalhydrologic- mechanical model results (BSC 2003e) are described in the following sections. May 2004 4.4.1 Site-Scale Thermal-Hydrologic Effects The site-scale thermal-hydrologic model estimates the unsaturated zone responses to the repository thermal load in the Yucca Mountain unsaturated zone system under present and future climates, as well as the effect of ventilation (BSC 2003e, Sections 6.2 and 6.3). The dual-permeability model is used for all the site-scale coupled process models, including the thermal-hydrologic model. In general, thermal loading at the repository results in significant changes in the temperature conditions (Figure 4-17) and moisture distribution, both at the repository and in the zone directly above and below the repository, which have a large impact on fluid flow near repository drifts. The moisture conditions under repository heating become “drier” (experiencing reduced liquid saturation in fracture and matrix systems) than the ambient conditions. Both the base case with 50 years of ventilation and the alternative case without ventilation were evaluated (BSC 2003e, Sections 6.2.1 and 6.2.2). This drying effect with ventilation is found to reach its maximum between 100 and 500 years. Without ventilation, the drying effect lasts 1,000 years. At large mountain scales, the thermal-hydrologic models predict that no extensive dryout zones develop from the repository thermal load under the future 4-24 No. 2: Unsaturated Zone Flow Revision 1 climates and the current thermal load scheme. The most significant dryout occurs in fractures within several meter regions immediately surrounding the drifts. Source: BSC 2003e, Figure 6.2-1a. Figure 4-17. Contours of Temperature in the Two-Dimensional North–South Cross Section of the Unsaturated Zone Model Grid at 100 Years with Base-Case Thermal Loading Thermal loading at the repository has a significant impact on percolation fluxes near repository drifts for times less than 1,000 years. Strong liquid and gas flow fields, in particular for the nonventilation case, are developed in local areas surrounding repository drifts. These flow fields, especially flow through the fractures surrounding drifts, are enlarged many times (with ventilation) to one order of magnitude (without ventilation) higher than the ambient conditions at the repository drifts and at earlier times (100 to 500 years). Furthermore, simulation results indicate that no fracture or matrix liquid fluxes flow directly into drifts and that no seepage occurs during the entire thermal loading period, even with much higher infiltration rates imposed from climate changes. The thermal-hydrologic model results predict that repository heating will have only a small impact on flow through the pillar regions between two drifts. This is because boiling does not occur in these pillar areas, and moisture conditions there are not much changed from ambient status. Simulated vertical fluxes in the pillar regions show little variation in thermal activity at drifts. In fact, flow through the pillar regions is more affected by surface infiltration or climate changes than by repository heating. The thermal-hydrologic model results also predict that repository heating with ventilation will have in general only a limited impact on far-field flow fields. In this case, thermally enhanced flux zones extend no more than 30 m in the regions directly above or below the repository blocks. Without ventilation, on the other hand, thermally impacted flux zones above the May 2004 4-25 No. 2: Unsaturated Zone Flow Revision 1 repository grow to as thick as 100 m. In both modes of thermal operations, the thermal-hydrologic model predicts a much stronger thermal-hydrologic effect along highly permeable columns of faults that intersect repository blocks, because of the stronger vapor flow and condensation. 4.4.2 Site-Scale Thermal-Hydrologic-Chemical Effects The site-scale thermal-hydrologic-chemical model was used to evaluate the coupled thermal-hydrologic-chemical processes and their effects on unsaturated zone flow (BSC 2003e, Section 6.4). A two-dimensional cross section of the repository was selected for a series of thermal-hydrologic and thermal-hydrologic-chemical calculations having increasing complexity in terms of processes and chemical components. The results of the simulations indicate that mineral precipitation and dissolution will not significantly affect the hydrologic properties and the percolation flux compared to the effects caused by thermal-hydrologic processes alone. Changes to water chemistry, mineralogy, and hydrologic properties in the ambient temperature regions are minimal. For example, Figure 4-18 shows a comparison of simulation results (for vertical water flux) with thermal-hydrologic and thermal-hydrologic-chemical models. Very little difference was found between the results from the two models. 4-26 May 2004 No. 2: Unsaturated Zone Flow Revision 1 Source: BSC 2003e, Figur