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Water-Resources Investigations Report 2002–4141

Prepared in cooperation with the U.S. DEPARTMENT OF ENERGY, Under Interagency Agreement DE–AI08–92NV10874

Results of Hydraulic Tests in Miocene Tuffaceous Rocks at the C-Hole Complex, 1995 to 1997, Yucca Mountain, Nye County, Nevada

By Arthur L. Geldon, Amjad M.A. Umari, Michael F. Fahy, John D. Earle, James M. Gemmell, and Jon Darnell

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Abstract

Four hydraulic tests were conducted by the U.S. Geological Survey at the C-hole complex at Yucca Mountain, Nevada, between May 1995 and November 1997. These tests were conducted as part of ongoing investigations to determine the hydrologic and geologic suitability of Yucca Mountain as a potential site for permanent underground storage of high-level nuclear waste.

The C-hole complex consists of three 900-meter-deep boreholes that are 30.4 to 76.6 meters apart. The C-holes are completed in fractured, variably welded tuffaceous rocks of Miocene age. Six hydrogeologic intervals occur within the saturated zone in these boreholes—the Calico Hills, Prow Pass, Upper Bullfrog, Lower Bullfrog, Upper Tram, and Lower Tram intervals. The Lower Bullfrog and Upper Tram intervals contributed about 90 percent of the flow during hydraulic tests.

The four hydraulic tests conducted from 1995 to 1997 lasted 4 to 553 days. Discharge from the pumping well, UE-25 c #3, ranged from 8.49 to 22.5 liters per second in different tests. Two to seven observation wells, 30 to 3,526 meters from the pumping well, were used in different tests. Observation wells included UE-25 c #1, UE-25 c #2, UE-25 ONC-1, USW H-4, UE-25 WT #14, and UE-25 WT #3 in the tuffaceous rocks and UE-25 p #1 in Paleozoic carbonate rocks.

In all hydraulic tests, drawdown in the pumping well was rapid and large (2.9–11 meters). Attributable mostly to frictional head loss and borehole-skin effects, this drawdown could not be used to analyze hydraulic properties. Drawdown and recovery in intervals of UE-25 c #1 and UE-25 c #2 and in other observation wells typically was less than 51 centimeters. These data were analyzed.

Hydrogeologic intervals in the C-holes have layered heterogeneity related to faults and fracture zones. Transmissivity, hydraulic conductivity, and storativity generally increase downhole. Transmissivity ranges from 4 to 1,600 meters squared per day; hydraulic conductivity ranges from 0.1 to 50 meters per day; and storativity ranges from 0.00002 to 0.002.

Transmissivity in the Miocene tuffaceous rocks decreases from 2,600 to 700 meters squared per day northwesterly across the 21-square-kilometer area affected by hydraulic tests at the C-hole complex. The average transmissivity of the tuffaceous rocks in this area, as determined from plots of drawdown in most or all observation wells as functions of time or distance from the pumping well, is 2,100 to 2,600 meters squared per day. Average storativity determined from these plot ranges is 0.0005 to 0.002. Hydraulic conductivity ranges from less than 2 to more than 10 meters per day; it is largest where prominent northerly trending faults are closely spaced or intersected by northwesterly trending faults.

During hydraulic tests, the Miocene tuffaceous rocks functioned as a single aquifer. Drawdown occurred in all monitored intervals of the C-holes and other observation wells, regardless of the hydrogeologic interval being pumped. This hydraulic connection across geologic and lithostratigraphic contacts is believed to result from interconnected faults, fractures, and intervals with large matrix permeability. Samples of UE-25 c #3 water, analyzed from 1995 to 1997, seem to indicate that changes in the quality of the water pumped from that well are probably due solely to lateral variations in water quality within the tuffaceous rocks.

First posted December 7, 2009

For additional information contact:


Chief, Yucca Mountain Project Branch
U.S. Geological Survey
Box 25046, Mail Stop 421
Denver Federal Center
Denver, CO 80225–0046
303-236-5050

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Suggested citation:

Geldon, A.L., Umari, A.M.A., Fahy, M.F., Earle, J.D., Gemmell, J.M., and Darnell, J., 2002, Results of hydraulic tests in Miocene tuffaceous rocks at the C-hole complex, 1995 to 1997, Yucca Mountain, Nye County, Nevada: U.S. Geological Survey Water-Resources Investigations Report 2002-4141, 58 p.



Contents

Abstract

Introduction

Purpose and Scope

Previous Work

Acknowledgments

Hydrogeologic Setting

Instrumentation

Packers

Transducers

Barometers

Pumps

Flowmeters

Data Acquisition and Instrument Control

Hydraulic Tests

Analytical Methods

Earth Tides and Barometric Effects

Flow Distribution in the C-holes

Monitoring Network

Description of Tests

Hydraulic Properties

Calico Hills Interval

Prow Pass Interval

Upper Bullfrog Interval

Lower Bullfrog Interval

Upper Tram Interval

Miocene Tuffaceous Rocks

Paleozoic Carbonate Rocks

Analytical Uncertainty

Chemistry of Water Samples Collected During Hydraulic Tests

Background Chemistry of UE-25 c #3 Water

Changes in UE-25 c #3 Water Chemistry During Pumping

Summary and Conclusions

References


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