Development and Calibration of Dual-Permeability Models in Complex Hydrogeologic Settings: An Example from the T-Tunnel Complex, Rainier Mesa, Nevada National Security Site

A dual-permeability flow model of the T-tunnel nuclear testing complex and surrounding area was developed to facilitate predictions of radionuclide migration from the tunnel tests through a thick sequence of variably-saturated, faulted, low-permeability volcanic tuff units to an underlying regional flow system. The hydrogeologic complexity necessitated a multi-stage calibration effort to capture the dominant flow characteristics including: laterally and vertically extensive saturation of an upper perched interval with simulated pressure heads supporting water level measurements, a thin unsaturated zone situated between two saturated zones, and fracture saturations congruent with field observations that range from fully saturated to dry. Moreover, the tunnel complex served as a drain to the perched flow system during operation with a total estimated water volume of 1.36×10 m exiting the portal over a 24 year period, and a quasi steady-state portal discharge rate of 1.01×10 m/s prior to portal sealing. A dual-permeability model with discontinuous fracture networks reconstructed from site-specific data was able to reproduce the salient hydrologic features identified above, including only a 15% discrepancy between simulated and estimated total portal discharge volume and a near exact match to the quasi steady-state portal discharge rate. This excellent reproduction of field observations builds confidence that the numerical model captures the relevant flow characteristics of the site.