Discrete Fracture Network Modeling and Simulation of Subsurface Transport for the Topopah Spring Aquifer at Pahute Mesa

This paper describes the development of discrete fracture network (DFN) models for the Topopah Spring Aquifer (TSA) beneath Pahute Mesa on the Nevada National Security Site, formerly the Nevada Test Site. Pahute Mesa was the site of 85 underground nuclear tests conducted between 1965 and 1992. The TSA, a fractured tuff with multiple cooling zones and varying intensity of fracturing, is one of the principal aquifers within and immediately downgradient of the main weapons testing areas beneath Pahute Mesa. This work will improve the representation of radionuclide transport processes in large-scale, regulatory-focused models by providing a basis for estimating upscaled flow and transport parameters that simultaneously consider smaller scale processes like fracture orientation, aperture variability, and flow channeling that affect transport behavior through the network. Twenty realizations of the DFN models were used to generate upscaled parameter distributions for permeability and permeability anisotropy, volumetric and effective transport porosity, and longitudinal dispersivity.