USGS

CHARACTERIZATION OF HYDROGEOLOGIC UNITS USING MATRIX PROPERTIES, YUCCA MOUNTAIN, NEVADA

by Lorraine E. Flint

Water-Resources Investigations Report 97-4243

A pdf is available for this report.

Abstract

Determination of the suitability of Yucca Mountain, in southern Nevada, as a geologic repository for high-level radioactive waste requires the use of numerical flow and transport models. Input for these models includes parameters that describe hydrologic properties and the initial and boundary conditions for all rock materials within the unsaturated zone, as well as some of the upper rocks in the saturated zone. There are 30 hydrogeologic units in the unsaturated zone, and each unit is defined by limited ranges where a discrete volume of rock contains similar hydrogeologic properties. These hydrogeologic units can be easily located in space by using three-dimensional lithostratigraphic models based on relation- ships of the properties with the lithostratigraphy. Physical properties of bulk density, porosity, and particle density; flow properties of saturated hydraulic conductivity and moisture-retention characteristics; and the state variables (variables describing the current state of field conditions) of saturation and water potential were determined for each unit. Units were defined using (1) a data base developed from 4,892 rock samples collected from the coring of 23 shallow and 8 deep boreholes, (2) described lithostratigraphic boundaries and corresponding relations to porosity, (3) recognition of transition zones with pronounced changes in properties over short vertical distances, (4) characterization of the influence of mineral alteration on hydrologic properties such as permeability and moisture-retention characteristics, and (5) a statistical analysis to evaluate where boundaries should be adjusted to minimize the variance within layers. This study describes the correlation of hydrologic properties to porosity, a property that is well related to the lithostratigraphy and depositional and cooling history of the volcanic deposits and can, therefore, be modeled to be distributed laterally. Parameters of the hydrogeologic units developed in this study and the relation of flow properties to porosity that are described can be used to produce detailed and accurate representations of the core-scale hydrologic processes ongoing at Yucca Mountain.

CONTENTS

Abstract

Introduction

Site Description

Location and Geologic Setting

Lithologic Features

Hydrologic Implications of Lithologic Stratification

Transition Zones

Capillary and Permeability Barriers

Methods

Sample Collection

Laboratory Core Processing

Moisture-Retention Characterization

Correction of Water-Content and Water-Potential Data

Characterization of Hydrogeologic Units

Phase 1:  Porosity Profiles

Tiva Canyon Tuff

Nonwelded Paintbrush Tuff and Base of Tiva Canyon Tuff

Topopah Spring Tuff

Calico Hills Formation

Prow Pass and Bullfrog Tuffs

Phase 2:  Alteration, Microfractures, and Permeability

Phase 3:  Moisture Retention and Predictions of Field Water Potentials

Phase 4:  Statistical Analysis to Produce Mean Values for Modeling Parameters

Spatial Distribution of Porosity

Summary

References

Appendix: Frequency Distributions of Porosity for Hydrogeologic Units

 


Laboratory measurements for 4,892 rock samples collected from the coring of 23 shallow and 8 deep boreholes at Yucca Mountain, Nevada are included in two spreadsheets. Core Properties.xls includes measurements of bulk density, porosity, particle density, field water content and saturation, and saturated hydraulic conductivity. Moisture Retention.xls includes moisture-retention characteristics for a subset of samples. Hydrogeologic unit is included for each sample as described in Flint (1998).


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U.S. Department of the Interior, U.S. Geological Survey
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