Previous studies [1] of the groundwater system near Yucca Mountain, Nevada identified significant spatial lateral variability in the groundwater temperature measured at or near the water table. In particular, the groundwater temperatures in the vicinity of the Paintbrush Fault were found to be elevated by several degrees compared to those in the surrounding region. Upward movement of water from a warmer underlying carbonate aquifer has been suggested as one explanation for this temperature variability. A non-isothermal groundwater flow model was constructed to test the hypothesis of upwelling along the Paintbrush Canyon fault zone. The MULTIFLO code [2] was used to model heat transport and density dependent flow in the fractured tuff aquifers near Yucca Mountain, Nevada. The non-uniform logically rectangular grid of size 30x25x20 conforms to a coarse (four layer) hydrostratigraphic framework for the region. A zone of enhanced permeability along the Paintbrush Canyon fault zone was required to match measured hydraulic heads. However, calculated groundwater temperatures match measured temperatures without including the postulated upward movement of water from the underlying carbonate aquifer. The calculated elevated temperatures above the Paintbrush Fault in this model were caused by buoyancy driven upward flow above a structural high in the underlying carbonate aquifer. These results suggest that groundwater temperature variations in the Yucca Mountain region may be due simply to buoyancy driven flow initiated by the variable thickness of the tuff aquifers.

This abstract documents work performed in part by the Center for Nuclear Waste Regulatory Analyses under contract No. NRC–02–97–009. The report is an independent product and does not necessarily reflect the regulatory position of the NRC.

References:

1. J. H. Sass, A. H. Lachenbruch, W. W. Dudley Jr., S. S. Priest and R. J. Munroe, 1988, Temperature, thermal conductivity, and heat flow near Yucca Mountain, Nevada: Some tectonic and hydrologic implications, US Geol. Surv. Open-File Rep., 87-649, 118 pp.

2. P. Lichtner, M. S. Seth, and S. Painter, 2000, MULTIFLO User's Manual, Multiflo Version 1.2: Two-Phase Non-isothermal Coupled Thermal-Hydrologic-Chemical Flow Simulator. San Antonio, Texas: Center for Nuclear Waste Regulatory Analyses.