USING A THREE-DIMENSIONAL GEOLOGIC FRAMEWORK TO INVESTIGATE A CURIOUS MODELING RESULT FOR THE DEATH VALLEY REGIONAL GROUNDWATER FLOW SYSTEM

The Death Valley regional groundwater flow system encompasses a proposed site for a high-level nuclear waste repository of the United States of America and the Nevada National Security Site (NNSS), where nuclear weapons were tested. It is an ideal site for which to consider the utility of three-dimensional hydrogeologic frameworks to the construction of groundwater models because of system complexity and the many stakeholders involved, including four United States (U.S) federal agencies (U.S. Department of Energy, National Park Service, Bureau of Land Management, and U.S. Fish and Wildlife Service) and a number of local counties, towns, and residents.

The curious result of interest here is caused by difficulties in identifying sources of water for the regional discharge areas in valleys near the center of the region and what this implies about system hydraulic properties. The problem is that given what seems to be likely rock characteristics and geometries, insufficient water is simulated to reach the discharge areas. It has been suggested that this issue draws the entire idea of using three-dimensional hydrogeologic frameworks to construct a groundwater model of this area into question, and that a two-dimensional model simulated using transmissivities would be better. Of interest is how well the goal of increased system understanding and development of model adequacy and trust by stakeholders is served by considering this “surprise” in the context of a three-dimensional hydrogeologic framework or a vertically averaged representation of the system. A related issue is how to use pumping-test derived transmissivities in model development. Here we explore these fundamental questions. Results suggest that the transmissivity measurements need to be used carefully regardless of how models are constructed because wells in this system are never fully penetrating. In addition, the difficulty in simulating sufficient flow to the discharge areas suggests the preconceived notions about subsurface properties at depth are inadequate and that the actual subsurface distribution is not accurately characterized with the available data, information, and interpretations. Alternatives that to some degree violate preconceived notions and simulate sufficient water to the discharge areas are discussed.