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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5185

Pre- and Post-Reservoir Ground-Water Conditions and Assessment of Artificial Recharge at Sand Hollow, Washington County, Utah, 1995-2005

By Victor M. Heilweil, David D. Susong, Philip M. Gardner, and Dennis E. Watt

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ABSTRACT

Sand Hollow, Utah, is the site of a surface-water reservoir completed in March 2002, which is being operated by the Washington County Water Conservancy District primarily as an aquifer storage and recovery project. The reservoir is an off-channel facility receiving water from the Virgin River, diverted near the town of Virgin, Utah. It is being operated conjunctively, providing both surface-water storage and artificial recharge to the underlying Navajo aquifer. The U.S. Geological Survey and the Bureau of Reclamation conducted a study to document baseline ground-water conditions at Sand Hollow prior to the operation of the reservoir and to evaluate changes in ground-water conditions caused by the reservoir.

Pre-reservoir age dating using tritium/helium, chlorofluorocarbons, and carbon-14 shows that shallow ground water in the Navajo Sandstone in some areas of Sand Hollow entered the aquifer from 2 to 25 years before sample collection. Ground water in low-recharge areas and deeper within the aquifer may have entered the aquifer more than 8,000 years ago. Ground-water levels in the immediate vicinity of Sand Hollow Reservoir have risen by as much as 80 feet since initial filling began in March 2002. In 2005, ground water was moving laterally away from the reservoir in all directions, whereas the pre-reservoir direction of ground-water flow was predominantly toward the north.

Tracers, or attributes, of artificial recharge include higher specific conductance, higher dissolved-solids concentrations, higher chloride-to-bromide ratios, more-depleted stable isotopes (Snake2H and Snake18O), and higher total-dissolved gas pressures. These tracers have been detected at observation and production wells close to the reservoir. About 15,000 tons of naturally occurring salts that previously accumulated in the vadose zone beneath the reservoir are being flushed into the aquifer. Except for the shallowest parts of the aquifer, this is generally not affecting water quality, largely because of the large saturated thickness of the Navajo aquifer. Since the initial filling of Sand Hollow Reservoir, arsenic concentrations have risen to exceed U.S. Environmental Protection Agency standards only in some shallow observation wells. These increases in arsenic concentration are likely caused by increasing pH associated with artificial recharge beneath the reservoir, rather than flushing of previously accumulated salts in the vadose zone. There has been no trend of increasing arsenic concentration in deeper production wells.

Estimated evaporation rates for Sand Hollow Reservoir, calculated by the Jensen-Haise method with data from the Sand Hollow weather station, range from about 55 to 61 inches per year and result in a total evaporative loss of about 6,000 acre-feet of water from March 2002 to September 2004. Rates of artificial recharge of ground water beneath Sand Hollow Reservoir have ranged from about 0.02 to 0.44 feet per day, with an average rate excluding the initial 3-month wetting period of about 0.06 feet per day. A total of about 28,000 acre-feet of recharge to the underlying Navajo aquifer occurred from March 2002 to September 2004.

This report is contained in the following files:

SIR2005_5185.pdf (6.2 mb)

The files are readable with Adobe Acrobat Reader. The reader is available for free download from Adobe Systems Incorporated.

CONTENTS

Abstract

Introduction

Purpose and Scope

Previous Studies

Acknowledgments

Hydrogeologic Setting

Methods

Water-Level Data

Ground-Water Samples

Surface-Water Sample

Chemical Analyses

Meteorology and Precipitation Data

Temperature of Reservoir Water

Evaporation and Artificial Recharge

Pan Evaporatioration

Penman Evaporation

Jensen-Haise Evaporation

Turbulent-Transfer Evaporation

Viscosity Corrections for Recharge Rates

Pre- and Post- Reservoir Ground-Water Conditions

Water Levels

Water Quality

Precipitation

Pre-Reservoir Ground-Water Quality

Apparent Age of Pre-Reservoir Ground Water

Virgin River and Sand Hollow Reservoir Surface-Water Quality

Flushing of Naturally Accumulated Vadose-Zone Salts

Effects of Artificial Recharge on Ground-Water Quality

Field Parameters

Geochemical Parameters

Assessment of Artificial Recharge

Comparison of Evaporation Rates

Artificial Recharge Estimates

Summary

References Cited


Send questions or comments about this report to the author, Victor M. Heilweil at heilweil@usgs.gov, 801.908.5042.

For more information about USGS activities in Utah, visit the USGS Utah District home page.

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