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

SUMMARY

Periodic water-level data from 1963 to 1998 were compiled and quality assured for 65 observation wells and test holes throughout Pahute Mesa. Additionally, hourly water-level fluctuations from 1996 to 1998 were examined in well PM-2 using a pressure transducer and electronic data logger. As part of the quality assurance of all water levels, ancillary data pertinent to computing hydraulic heads in wells were 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). The 65 wells were chosen for inclusion in this report based on two arbitrary criteria. The criteria were (1) a minimum of three water-level measurements for a well and (2) the first and last measurement had to span at least 1 month.

Quality-assured water levels that were not necessarily in error but that did not represent static heads in the regional aquifer system or required some other qualification were flagged. Water levels flagged included those affected by pumping or well construction, water levels elevated above or depressed below regional ground-water levels by more than 75 ft, anomalous values, and water levels affected by nuclear tests or borehole deviations. Water levels affected by pumping typically occurred in water-supply wells. Water levels affected by well construction may show a rising or falling trend that can last for hours to years before equilibration with the head in the aquifer. Elevated water levels, which are common in wells on some areas of Pahute Mesa, may result from perched or semi-perched conditions, upward or downward hydraulic gradients, or local conditions near the well. Static water levels in 15 of the 65 wells for this report are greater than about 75 ft above or below regional water levels.

Because of insufficient data on temperature distribution and zones of inflow for most wells on Pahute Mesa, water levels were not flagged to indicate temperature effects. However, a cursory examination of about 30 wells with available water-level and temperature data indicate that water levels in most wells would not be significantly affected by temperature if corrected to 95^oF. Wells with large corrections (greater than 10 ft) are those with long water columns (greater than 1,500 ft of water above the assumed point of inflow) in combination with mean water-column temperatures exceeding 105^oF.

Water-level fluctuations in wells on Pahute Mesa are caused by several natural or human factors. Natural factors include infiltration of precipitation, barometric pressure, Earth tides, and earthquakes. Human factors include ground-water pumpage and underground nuclear testing.

On Pahute Mesa, no observed water-level fluctuations were attributed to an earthquake caused by tectonic activity. The magnitude and duration of changes in water levels caused by nuclear tests under Pahute Mesa are affected by the size of, and distance from, a test. In the immediate vicinity of a detonation, water-level changes can be large (hundreds of feet). Further from a detonation, water levels are affected less but changes still can be relatively large (tens of feet). Because most of the wells on Pahute Mesa do not have water-level records going back to the 1960's and 1970's, determining water levels which might be affected by past nuclear tests is difficult.

Seven wells on or near Pahute Mesa have been used for water withdrawal at the NTS. Between 1983 and 1998, approximately 2.29 billion gallons (7,030 acre-feet) of water were withdrawn from volcanic aquifers on or near Pahute Mesa. Water use generally rose from 1983 to 1989 and declined through 1998. Three wells, WW-8, UE-19c WW, and U-20 WW, have supplied most of the water for Pahute Mesa activities.

Water levels from 25 wells were analyzed for variability and for hydrologically significant trends. Some of the trends were quantitatively or qualitatively correlated with potential factors causing the trends. The wells selected for trend analysis have multiple years of water-level record.

Thirteen wells had at least one hydrologically significant trend. The largest change in water levels (1,029 ft in 25 years) occurred in well U-19v PS 1D as a result of the Almendro nuclear test. Likely explanations for trends in most of the wells are changes in precipitation patterns that affect recharge rates to the ground-water system, pumping effects from water-supply well U-20 WW, or a combination of these two factors.

Well U-20 WW had a 27-ft decline in water level from 1985 to 1995 due to pumping followed by about a 5-ft increase in level from 1996 to 1998 during a period of no pumping. Water levels in six observation wells show water-level trends that may be influenced by pumping in well U-20 WW. These wells range from about 0.85 mi to 1.8 mi from well U-20 WW. Four wells with at least 8 years of record show declining water levels through 1995 or 1996 followed by stable or rising water levels.

Small-magnitude changes in water level occurring over periods of several months to several years commonly are masked by the effects of cyclic changes in air pressure and Earth tides. These small-scale changes in water level, which are difficult to distinguish with periodic data, can sometimes be seen with continual water-level data.