Determination of fracture apertures via calibration of three-dimensional discrete-fracture-network models: application to Pahute Mesa, Nevada National Security Site, USA

摘要

The Pahute Mesa Corrective Action Unit on the Nevada National Security Site (USA) contains several fractured aquifers that can potentially provide high-permeability pathways for migration of radionuclides away from underground nuclear testing locations. Though geometric properties of fractures such as length and orientations, can generally be obtained by geophysical methods and borehole image analyses, their hydraulic properties (primarily influenced by their apertures) are often unknown or have a high degree of uncertainty. This study presents a novel approach to determine fracture apertures at a site by integrating numerical models of flow with data of certain geometrical fracture attributes and hydraulic response of the system because of long-term pumping. Discrete fracture network (DFN) models were used in this study to construct a three-dimensional (3-D) flow model for lava-flow aquifers of Western Pahute Mesa (WPM). Models built using a 3-D DFN code, dfnWorks, were calibrated to hydraulic drawdown observations recorded at the site of a forced-gradient experiment (BULLION FGE). Multiple conceptualizations of boundary conditions, fracture aperture distributions, and realizations of the fracture network were considered to simulate flow and migration of particles between an injection well and a pumping well. Calibrating the DFN flow models to hydraulic drawdown data constrained the ranges of aperture values and helped develop a realistic description of the properties of fractured rocks in WPM. The aperture values resulting from this study are expected to enhance understanding of radionuclide transport in WPM and support the development of large-scale flow and transport models.