CONTENTS

Abstract
Introduction 

Purpose and Scope
Hydrogeology
Acknowledgments

Water-Level Measurements

Quality-Assurance Flags 
Temperature Effects 

Sources of Water-Level Fluctuations 

Precipitation
Barometric Pressure and Earth Tides
Seismic Events and Underground Nuclear Tests
Pumpage

Analysis of Water Levels
Summary
References Cited
Appendix 1
Appendix 2

INTRODUCTION

Pahute Mesa is located in Nye County, southern Nevada (fig. 1). Areas 19 and 20 in the northern part of the Nevada Test Site (NTS) cover the eastern half of Pahute Mesa (fig. 2). In these two areas, 77 of 85 nuclear tests were detonated near¹ or below the water table (Laczniak and others, 1996, p. 51). Accurate ground-water levels beneath Pahute Mesa are necessary to determine the flow paths of ground water containing or potentially containing radioactive contaminants from these tests. Quality-assured water-level data can be used to construct flow maps, calibrate steady-state and transient ground-water flow models, and locate sites for future remedial monitoring. In addition, ground-water levels that vary with time due to different factors can be analyzed for trends. Determining the causes of existing trends can aid in understanding factors influencing the flow system.

Purpose and Scope

The purpose of this report is to compile and quality assure water-level data at wells throughout Pahute Mesa and vicinity. As part of the quality assurance, ancillary data pertinent to computing hydraulic heads in wells are compiled and analyzed. These include well completion and measuring-point data, and other information related to hole completion and water properties (such as water temperature). Quality-assured water-level data at each well were analyzed for variability and for hydrologically significant trends. For those wells with significant water-level trends, an attempt was made to identify the cause.

Water-level data from 1963 to 1998 were compiled for 65 observation wells on Pahute Mesa and vicinity in the northwestern part of the Nevada Test Site (fig. 2). Well information including altitudes, depths, open intervals, and stratigraphy of the open interval are provided in table 1.

Hydrogeology

Pahute Mesa is an elevated plateau ranging from about 5,000 to 8,000 ft above sea level. Four hydrographic areas (HA)² comprise the plateau--Buckboard Mesa HA to the south, Oasis Valley HA to the southwest, Gold Flat HA to the northwest, and Kawich Valley HA to the northeast. The geographic features for which the HA's are named (fig. 1) generally define their location.

Pahute Mesa is composed of Miocene-age volcanic rocks. Multiple eruptions from several calderas at or near Pahute Mesa have produced volcanic deposits that are thousands of feet thick. No borehole drilled in Pahute Mesa has yet penetrated the entire section of volcanic rocks³, although many holes from 4,000 to as deep as 13,686 ft deep have been drilled (Blankennagel and Weir, 1973, table 2). Because of the vast thickness of volcanic rocks underlying Pahute Mesa, these rocks form the principal aquifers and confining units in the area and are the primary control on the ground-water flow system. Principal stratigraphic units in open intervals of wells on Pahute Mesa that occur below the water table are listed in table 2.

The hydraulic properties of the volcanic rocks underlying Pahute Mesa were described by Blankennagel and Weir (1973). Most of the ground-water flow in the volcanic rocks is fracture flow. Rhyolite lavas and partly to densely welded ash-flow tuffs are the principal volcanic-rock aquifers. Rhyolite lavas generally have the highest permeabilities but, in a regional sense, may be restricted areally and in thickness. Welded ash-flow tuffs are slightly less permeable than the lavas but are widespread and thick; therefore, they may provide lateral continuity for water to move through the regional flow system. Nonwelded ash-flow and ash-fall tuffs are generally considered confining units, especially when they are zeolitized. These nonwelded tuffs have low fracture porosity and permeability because they are less likely to fracture than the welded tuffs, and when they do, the fractures are likely to reseal.

The primary sources of recharge to Pahute Mesa are infiltration of precipitation and subsurface inflow (Blankennagel and Weir, 1973). An estimated 3,150 acre-ft/yr of water is recharged to Pahute and Rainier Mesas through precipitation, with the higher altitudes of the mesas receiving greater amounts of recharge (Blankennagel and Weir, 1973, table 6). Subsurface inflow from the north, derived from ranges such as the Kawich Range and other ranges further north and west, contributes an estimated 5,500 acre-ft/yr (Blankennagel and Weir, 1973, p. B20).

Ground water flows south-southwest across most of Areas 19 and 20 (fig. 2)⁴. Ground water flowing past the southern margins of Pahute Mesa ultimately discharges at either Oasis Valley (about 25 mi to the southwest), Alkali Flat (about 70 mi to the south), or Death Valley (about 60 mi to the southwest) (Laczniak and others, 1996, p. 40).

Horizontal ground-water gradients across the study area are typically 25 to 100 ft/mi (O'Hagan and Laczniak, 1996). Vertical ground-water gradients generally change from variable or no gradient to downward in most of Area 19 beginning at depths greater than 2,500 ft below the top of the saturated zone. In the western part of Area 19 and in Area 20, ground-water gradients are generally upward below these same depths (Blankennagel and Weir, 1973, fig. 10). In some areas of Pahute Mesa, anomalously high water levels (relative to regional water levels) are common in shallow wells (O'Hagan and Laczniak, 1996).

Acknowledgments

This study was funded by the U.S. Department of Energy under Interagency Agreement DE-A108-96NV11967. This report benefited greatly from earlier work under the direction of David B. Wood and Randell J. Laczniak who compiled, verified, and entered water-level and well-construction data for Pahute Mesa into the U.S. Geological Survey's (USGS) National Water Information System database. The stratigraphy of the open intervals in the wells was obtained from a geologic database of wells on Pahute Mesa provided by Richard Warren of Los Alamos National Laboratory. Meteorological data were provided by Douglas Soule' of the National Oceanic and Atmospheric Administration. Sarah Ryker, USGS, ran the statistical water-level trends used in this report.

¹ "Near" is defined as less than two cavity radii from the nuclear test to the top of the water table. The cavity is the void that results from instantaneous meltdown of the rock after a nuclear test.

² Formal hydrographic areas in Nevada were delineated systematically by the U.S. Geological Survey and Nevada Division of Water Resources in the late 1960's (Rush, 1968; Cardinalli and others, 1968) for scientific and administrative purposes. The official hydrographic-area names, numbers, and geographic boundaries continue to be used in Geological Survey scientific reports and Division of Water Resources administrative activities.

³ Well ER-19-1-1, which penetrates below the volcanic rocks, is located between Rainier and Pahute Mesas.

⁴ The limitations of the data used to construct the ground-water flow map presented in figure 2, such as constraints on the lateral and vertical distribution of wells, are discussed by O'Hagan and Laczniak (1996). In general, the flow map was intended to represent a "coherent surface from which to generalize the regional occurrence and movement of ground water" (O'Hagan and Laczniak, 1996).