Abstract
The Nevada Yucca Mountain site is being investigated to determine if it is a suitable site for the construction of a high-level nuclear waste repository. A feature of concern north of the selected site is an abrupt rise in the water table. This high gradient of 0.15 is flanked to the north by a moderate gradient of 0.015 and to the south by a very small gradient of 0.0001. Since the mechanisms creating this feature have the potential to cause changes in the position and configuration of the water table, they must be understood so risk analysis of the site may be performed. The three distinct gradient regions found at the site may be related to the Cenozoic volcanics, the Paleozoic clastic aquitard, and the Paleozoic carbonates. The large hydraulic gradient regionally corresponds with the northern limit of the Paleozoic carbonates, at the contact of the Eleana Formation, a Paleozoic aquitard. This study investigates, using finite difference modeling, the relationship between the steep hydraulic gradient and hydraulic conductivity contrasts. The site was modeled with flow boundaries to investigate the effects of variable gradient input to the flow balance calculation. A model was run with differential volcanic hydraulic conductivity zones with regulated flow into the carbonates. Constant head boundaries were implemented in models to investigate the effect of both a confined and open carbonate zone and with vertical barriers above the argillite/carbonate contact. The results of the study found that vertical and horizontal hydraulic conductivity contrasts do not fully account for the steep gradients, although the vertical contrasts marginally increase the gradient from horizontal contrasts. The confined carbonate zone model produced results that do not correlate with field data. The vertical barrier model did successfully reproduce steep gradients with gradient steepness related to flow restriction. Through the use of flow boundaries the steep gradient was reproduced successfully with a contrast of 0.8 orders of magnitude by allowing flow into the carbonate zone.
Davidson, Timothy Ross (1994). Finite - difference modeling of the Yucca Mountain, Nevada Area: a study of the regional water table gradients based on hydraulic conductivity contrasts. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1994 -THESIS -D2536.