Introduction The ground water hydrology and possible radionuclide transport pathways from the potential underground high- level nuclear waste repository located near Yucca Moun- tain, Nevada, are currently being evaluated. The Yucca Mountain Project uses geologic and hydrologic information gathered from many sources (Czarnecki et al. 1997). This information is used to model ground water flow and poten- tial radionuclide transport (Civilian Radioactive Waste Management System Management and Operating Contrac- tor 2000; Luckey et al. 1996; Eddebbarh et al. 2003; Zyvoloski et al. 2003). The construction of computational grids to reflect geologic structure and stratigraphy for flow and transport numerical models can be a formidable task. The quality of these grids with respect to numerical trunca- tion error and accuracy of representation of hydrogeology is of foremost importance to the Yucca Mountain Project. In fact, an understanding of the quality of the grids is nec- essary to having a defensible model. One of the challenges facing a researcher in a project of this magnitude is under- standing the tradeoff between a high-resolution model that represents hydrostratigraphy with a high degree of fidelity and a lower resolution model that is more suited to making the large number of calculations necessary for repository safety or performance assessment evaluation. The errors resulting from using a model with too low of a resolution for heterogeneous aquifers are described in detail by Hait- jema et al. (2001). The modeling challenge can be met in various ways including accepting some amount of error, using the telescopic mesh refinement approach (Ward et al. 1987), or using large computational resources. For exam- ple, Frind et al. (1988) use a dual scale (local vs. aquifer) approach to solve for the large-scale flow field, as well as the local dispersive processes. In a study of the Macrodis- persion Experiment site on the Columbus Air Force Base in Mississippi (Feehley et al. 2000), statistical methods are Abstract Three-dimensional grids representing a heterogeneous, ground water system are generated at 10 different reso- lutions in support of a site-scale flow and transport modeling effort. These grids represent hydrostratigraphy near Yucca Mountain, Nevada, consisting of 18 stratigraphic units with contrasting fluid flow and transport properties. The grid generation method allows the stratigraphy to be modeled by numerical grids of different resolution so that com- parison studies can be performed to test for grid quality and determine the resolution required to resolve geologic structure and physical processes such as fluid flow and solute transport. The process of generating numerical grids with appropriate property distributions from geologic conceptual models is automated, thus making the entire process easy to implement with fewer user-induced errors. The series of grids of various resolutions are used to assess the level at which increasing resolution no longer influences the flow and solute transport results. Grid resolution is found to be a critical issue for ground water flow and solute transport. The resolution required in a particular instance is a function of the feature size of the model, the intrinsic properties of materials, the specific physics of the problem, and boundary conditions. The asymptotic nature of results related to flow and transport indicate that for a hydrologic model of the heterogeneous hydrostratigraphy under Yucca Mountain, a horizontal grid spacing of 600 m and verti- cal grid spacing of 40 m resolve the hydrostratigraphic model with sufficient precision to accurately model the hypo- thetical flow and solute transport to within 5% of the value that would be obtained with much higher resolution. 122 Grid Resolution Study of Ground Water Flow and Transport by Kathleen M. Bower 1 , Carl W. Gable 2 , and George A. Zyvoloski 3 1 Geology/Geography Department, Eastern Illinois University, 600 Lincoln Ave., Charleston, IL 61920–3099; cfkmb1@ eiu.edu 2 Corresponding author: Earth and Environmental Sciences, MS T003, Los Alamos National Laboratory, Los Alamos, NM 87545; gable@ lanl.gov 3 Earth and Environmental Sciences, MS T003, Los Alamos National Laboratory, Los Alamos, NM 87545; gaz@ lanl.gov Received February 2002, accepted April 2004. Copyright © 2005 by the National Ground Water Association. Vol. 43, No. 1—GROUND WATER—January–February 2005 (pages 122–132)