Quasi-three dimensional ground-water modeling of the hydrologic influence of paleozoic rocks on the ground-water table at Yucca Mountain, Nevada

Abstract

The proposed high-level radioactive waste repository site at Yucca Mountain, Nevada, has created a need to understand the, ground-water system at the site. One of the important hydrologic characteristics is a steep gradient on the ground-water table north of the repository site. This study investigates the cause of the steep gradient, based on the possible influence by Paleozoic rocks under the Yucca Mountain area. A quasi-three dimensional, steady-state, finite-difference model of the groundwater flow system of the Yucca Mountain Site and vicinity, was developed using a manual trial-and-error calibration technique to model the ground-water table. The ground-water system in the model was divided into a two layers, which consist of Cenozoic volcanic rocks and Paleozoic carbonate rocks. The carbonate rocks were defined to be a confined aquifer. The model simulates vertical flow from the volcanic rocks to the underlying carbonate rocks in an area where the Eleana Formation, a Paleozoic clastic aquitard, is absent. The model requires a vertical hydrologic connection in a particular region and a large difference in hydraulic heads between the volcanic rocks and the carbonates to create the steep gradient north of the repository site. The regions of different hydraulic gradient on the water-table surface could be simulated by spatial variations of the horizontal hydraulic conductivity in the volcanic rocks.