FAULT PERMEABILITY NEAR YUCCA MOUNTAIN

A deep Paleozoic carbonate aquifer is of special concern for the propo sed high-level nuclear repository at Yucca Mountain, Nevada. It provid es a potential pathway for groundwater transport of contaminants from the repository back to the biosphere. The map of groundwater temperatu re 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 t hese fault zones. The deep carbonate aquifer has been penetrated by on ly one test hole in the vicinity of Yucca Mountain: UE-25pl. In this b orehole the hydraulic head in the deep carbonate aquifer was 20 m high er than head in the overlying tuff aquifer. Observations in UE-25pl sh ow good Earth tide water-level fluctuations in the deep carbonate aqui fer. Both the large 2-cm amplitude bf the tidal signal and the fact th at it is in phase with the tidal potential indicate that the deep aqui fer is well confined by a layer of low hydraulic conductivity. An esti mate of the maximum hydraulic permeability of a fault zone that penetr ates the confining is made by simulating the tidal response. Given a m aximum permeability and the head difference, one can calculate the upw ard flow from the deep carbonate aquifer into the overlying tuffs. Thi s analysis supports the interpretation of upward flow from the beep ca rbonate to the overlying tuffs. This is a favorable condition for the proposed repository. The upward potential for flow from the deep carbo nate aquifer protects it from the downward movement of contaminants. F urther deep drilling is necessary to confirm that the high head persis ts beneath the entire proposed repository. Nothing should be done eith er through construction of the repository or through groundwater devel opment to reduce heads in the carbonate aquifer; the higher heads prot ect the carbonate aquifer.