A deep Paleozoic carbonate aquifer is of special concern for the proposed high-level nuclear repository at Yucca Mountain, Nevada. It provides a potential pathway for groundwater transport of contaminants from the repository back to the biosphere. The map of groundwater temperature at the water table has been interpreted by several investigators to indicate upwelling groundwater along major fault zones both east and west of Yucca Mountain. Of particular concern is the permeability of these fault zones. The deep carbonate aquifer has been penetrated by only one test hole in the vicinity of Yucca Mountain: UE-25pl. In this borehole the hydraulic head in the deep carbonate aquifer was 20 m higher than head in the overlying tuff aquifer. Observations in UE-25pl show good Earth tide water-level fluctuations in the deep carbonate aquifer. Both the large 2-cm amplitude of the tidal signal and the fact that it is in phase with the tidal potential indicate that the deep aquifer is well confined by a layer of low hydraulic conductivity. An estimate of the maximum hydraulic permeability of a fault zone that penetrates the confining is made by simulating the tidal response. Given a maximum permeability and the head difference, one can calculate the upward flow from the deep carbonate aquifer into the overlying tuffs. This analysis supports the interpretation of upward flow the deep carbonate to the overlying tuffs. This is a favorable condition for the proposed repository. The upward potential for flow from the deep carbonate aquifer protects it from the downward movement of contaminants. Further deep drilling is necessary to confirm that the high head persists beneath the entire proposed repository. Nothing should be done either through construction of the repository or through groundwater development to reduce heads in the carbonate aquifer; the higher heads protect the carbonate aquifer.¿ 1997 American Geophysical Union