Integrated Use of Data and Numerical Models for Site Conceptual Model Development In Complex Hydrogeologic Systems
Tuesday, September 24, 2019: 10:10 a.m.
Rainier Mesa is a tuffaceous plateau located on the Nevada National Security Site that has been subjected to a numerous subsurface nuclear tests conducted in a series of tunnel complexes. The tunnels are constructed near the middle of an 800 m thick Tertiary volcanic sequence of faulted, low-permeability welded and non-welded bedded, vitric and zeolitized tuff units that overlie a regional groundwater flow system within densely fractured carbonates. This presentation will focus on the multi-year development and iterative testing of a comprehensive site conceptual model that forms a basis for fluid flow and radionuclide transport simulations in support of the monitored natural attenuation strategy employed by the U.S. DOE. The hydrogeologic complexity of the site necessitated a comprehensive analysis of all available data to first explain and then properly develop numerical models that reasonably represent the dominant flow characteristics of the site which include an upper perched zone of saturated overlying a regional flow system separated by a thin unsaturated zone, and perplexing patterns and trends of fluid drainage from faults and fractures observed during tunnel excavation and operation. Emphasis will be placed on how seemingly disparate datasets and numerical modeling exercises were critical for successful site conceptualization.