1. Bill W. Arnold, Al-Aziz Eddebbarh1, and Scott C. James2
Specification of Recharge and Lateral Boundary Conditions for the Site-Scale Saturated Zone Flow Model Near Yucca Mountain[1]
Bill W. Arnold, Al-Aziz Eddebbarh1, and Scott C. James2
Sandia National Laboratories, Total Systems Performance Assessment, P.O. Box 5800, Albuquerque, NM 87185–0778.
2Geohydrology Department P.O. Box 5800 Albuquerque, NM
1Los Alamos National Laboratory Earth and Environmental Division Los Alamos, NM
Introduction
 The Yucca Mountain Project  The Saturated Zone as a Barrier
Radionuclides potentially released into seepage water contacting breached waste packages in the repository would migrate downward through the unsaturated zone for approximately 210 to 390 m (690 to 1,300 ft) to the water table. At that point, radionuclides would enter the saturated zone and migrate downgradient within the tuff and alluvial aquifers to the accessible environment. At a distance of 15 to 18 km along the travel path from the repository, groundwater flow enters the alluvial aquifer and remains in the alluvium until it is subject to uptake into the accessible environment.
Yucca Mountain in Nye County, Nevada, is the proposed location for a national high-level radio-active waste repository. It is critical to understand the flow of groundwater and the transport of radionuclides that may be potentially released to the saturated zone beneath and downgradient from Yucca Mountain
The Death Valley Regional Flow System Model[2]
The Death Valley regional flow system (DVRFS) model is a MODFLOW-2000 model comprising 194 rows, 160 columns, and 3 (1997) or 16 (2002) layers. It is based on a regional mass balance and calibrated to spring flow data. Cells are 1,500×1,500 m2 (4,921×4,921 ft2) in the horizontal and variable depth in the vertical. Currently, two models for the DVRFS are under consideration for Yucca Mountain: 1) a base-case model developed in 1997 with only three layers and data collected up through the early 90s that is to be submitted for licensing; and, 2) an alternate model (presented here) completed in 2002 (and under continued development) using data collected up to that point. The DVRFS is calibrated using regressions methods in MODFLOW-2000 to best match measured head and discharge data points and the best conceptual hydrostratigraphic model is selected.
Three sources are used to specify the recharge into the site-scale model: 1) the same distributed recharge applied to the DVRFS due to precipitation; 2) the DVRFS distributed recharge is replaced by the flux through the bottom layer of unsaturated zone flow model within the area of its footprint; and, 3) the DVRFS distributed recharge is replaced by the estimated recharge through Fortymile Wash in the corresponding area. The lateral fluxes into the site-scale model are used as target boundary conditions that are adjusted during calibration. They are extracted from the DVRFS cell-by-cell flux file with short utility codes and converted from units of m3/day to kg/s. Because of different grid sizes between the models, these fluxes are extracted from the nearest MODFLOW cell corresponding to a FEHM node.
Yucca Mountain site-scale saturated zone flow and transport model domain
Boundary Conditions for the Yucca Mountain Site-Scale Saturated Zone Flow Model[3]
Distributed recharge that is applied to the alternate SZ site-scale flow model. The three sources of recharge are:
1) Recharge equivalent to that measured for the regional-scale model; 2) Recharge through the bottom of the unsaturated zone flow model; and, 3) Recharge through Fortymile Wash
North
South
East
West
Fluxes extracted from the cell-by-cell flux file (MODFLOW-2000) of the regional-scale saturated zone flow model that most closely correspond to the boundaries of the site-scale saturated zone flow model (FEHM). Negative values indicate flow out of the model domain and positive values are for fluxes into the domain.
[1]BSC, Recharge and lateral groundwater flow boundary conditions for the saturated zone site-scale flow and transport model, ANL-NBS-MD REV 01, Las Vegas, Nevada, 2004.
[2]D’Agnese, F. A., G. M. O’Brien, C. C. Faunt, W. R. Belcher, and C. San Juan, A three-dimensional numerical model of predevelopment conditions in the Death Valley regional ground-water flow system, Nevada and California, USGS Water-Resources Investigations Report 02–4102, [3]BSC, Saturated zone site-scale flow model, MDL-NBS-HS REV02, Las Vegas, Nevada, 2004.