UNLV Retrospective Theses & Dissertations
1-1-1995
Ground water - surface water interactions in the lower Virgin River area, Arizona and Nevada
Lynn Metcalf University of Nevada, Las Vegas
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Repository Citation Metcalf, Lynn, "Ground water - surface water interactions in the lower Virgin River area, Arizona and Nevada" (1995). UNLV Retrospective Theses & Dissertations. 502. http://dx.doi.org/10.25669/z90f-mtsv
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A Bel! & Howell Information Company 300 North Zeeb Road. Ann Arbor. Ml 48106-1346 USA 313/761-4700 800/521-0600
Ground Water - Surface Water Interactions
in the Lower Virgin River Area
Arizona and Nevada
by
Lynn Metcalf
A thesis submitted in partial fulfillment of the requirements for the degree of
Master of Science
in
Water Resources Management
Department of Geoscience University of Nevada, Las Vegas August 1995 UMI Number: 1376193
UMI Microform 1376193 Copyright 1995, by UMI Company. All rights reserved.
This microform edition is protected against unauthorized copying under Title 17, United States Code.
UMI 300 North Zeeb Road Ann Arbor, MI 48103 The thesis of Lynn Metcalf for the degree of Master of Science in Water Resources Management is approved.
h ! L ^ . / j C / ) a a aJ2 Q srC\. Chairperson, Neil L. Ingraham, Ph.D.
xamining Committee Member, David K. Kreamer, Ph.D.
— f e n v ; <>}J Examining Committe^Mem^Sr, Steve A.Mizell, Ph.D.
Grp^uate Faculty Representative, James E. Deacon, Ph.D.
Interim Dean of the Graduate College, Cheryl L. Bowles, Ed. D.
University of Nevada, Las Vegas August 1995 ABSTRACT
This study assessed interactions between ground water and surface water in the lower Virgin River, Arizona and Nevada, using stable isotopes and major ions as environmental tracers. The study reach starts at the Virgin River
Gorge in Arizona and terminates at Lake Mead in Nevada, 80 km downstream.
Discharge data showed that major sources of water for the lower Virgin River were discharge from the upper Virgin River and the Littlefield Springs, with minor amounts from Beaver Dam Wash, a perennial tributary. No strong evidence was found for ground-water seepage into the channel. In general, dissolved solids concentrations increased downstream because of water loss due to evapotranspiration. From the environmental tracer results, three general water types were identified: (1) local water including springs and streams in surrounding mountains, surface water from Beaver Dam Wash, and ground water from the Beaver Dam Wash floodplain aquifer; (2) Virgin River water consisting of surface water from the Virgin River, ground water from the
Virgin River floodplain, and flow from the Littlefield springs; and (3) ground water from deep aquifers underlying the floodplain aquifers. The local water is the most isotopically enriched (SD values between -91 and -70%o) and has the lowest dissolved solids concentrations of all the waters sampled. Deep ground water is the most isotopically depleted of the waters with 6D values ranging from -106 to -99%o. Virgin River water is isotopically intermediate (5D from -97 to -87%o) but has the highest dissolved solids of all waters sampled. TABLE OF CONTENTS
ABSTRACT...... iii
LIST OF TA B LE S...... viii
LIST OF FIGURES...... ix
ACKNOWLEDGMENTS...... xi
CHAPTER 1 INTRODUCTION...... 1 Previous W ork ...... 5 Investigations in the Study A re a ...... 5 Ground Water-Surface Water Interactions ...... 9 Stable Isotopes as Tracers ...... 9 Chlorides as Conservative Tracers ...... 12
CHAPTER 2 ENVIRONMENTAL SETTING...... 14 Physical Characteristics of the Study Area ...... 15 Physiographic Setting ...... 15 Geographic Location ...... 15 Physiographic Location ...... 15 C lim ate ...... 18 Geologic Setting ...... 21 Stratigraphy ...... 21 Geologic History ...... 23 S tructures ...... 25 Hydrogeology ...... 28 Unconsolidated Rocks ...... 29 Younger Alluvium ...... 30 Beaver Dam Wash Aquifer ...... 31 Older Alluvium ...... 32 Muddy Creek Formation ...... 32 Consolidated Rocks ...... 34 Carbonate R o ck s ...... 34 Noncarbonate Rocks ...... 36
iv Surface Water Hydrology ...... 37 Inflows ...... 37 Diversions ...... 40 Water Quality - Previous Work ...... 40 Ground W ater ...... 40 S prings ...... 40 Wells ...... 43 Surface W a te r ...... 44
CHAPTER 3 METHODS...... 46 Physical Methods ...... 46 Approach ...... 46 Rationale ...... 46 Methods ...... 48 Discharge Measurements ...... 48 Water Quality Sampling ...... 51 Flash Flood Sampling on Tributaries ...... 54 Analytical Methods ...... 56 Discharge and Related Calculations ...... 56 Error Calculations ...... 56 Stable Isotope Analysis ...... 58 Chemical Analysis ...... 59
CHAPTER 4 RESULTS ...... 61 Surface Water Discharge ...... 61 Discharge During Sampling Events ...... 61 Discharge During Seepage Studies ...... 66 Stable Isotopes ...... 69 Precipitation ...... 69 Stable Isotopes in Springs ...... 73 Virgin River Gorge Springs ...... 73 Virgin River Valley Springs ...... 76 Virgin Mountain Springs and Streams ...... 76 Northern B a sin ...... 76 Stable Isotopes in W e lls ...... 76 Stable isotopes in Surface W a te r ...... 80 Chemistry...... 84 Chemistry of Springs ...... 84 Virgin River Gorge Springs ...... 86 Virgin River Valley Springs ...... 86 Virgin Mountain Springs and Streams ...... 88 Chemistry of Wells ...... 88 Chemistry of Surface W a te r ...... 92
v CHAPTER 5 DISCUSSION ...... 97 Surface W ater ...... 98 Surface Water Discharge ...... 98 Discharge Variability with Time ...... 98 Discharge Changes With P osition ...... 100 Surface Water Quality ...... 102 Stable Isotopes in the Lower Virgin River ...... 102 Chloride Concentrations in the Virgin R iv e r...... 109 Summary of Surface Water Quality D a ta ...... 119 GroundWater ...... 122 Springs ...... 122 Stable Isotopes in S p rings ...... 122 Chlorides in Springs ...... 127 Dissolved Solids in Springs ...... 127 Water Type of Springs ...... 128 W e lls ...... 128 Stable Isotopes in W ells ...... 128 Chlorides and Dissolved Solids in W e lls ...... 132 Water Types in W ells ...... 133 Ground Water Summary ...... 134 Conceptual Model of Hydrogeology in the Lower Virgin River Basin ...... 140 Local W aters ...... 142 Virgin River W aters ...... 145 Ground Water in the Lower Virgin River Area ...... 146
CHAPTER 6 CONCLUSIONS ...... 150 Suggestions for Future Research ...... 151
APPENDIX A ...... 155
APPENDIX B ...... 160
APPENDIX C ...... 162
APPENDIX D ...... 167
REFERENCES...... 174
vi LIST OF TABLES
Table 1: Precipitation data for sites in and near the lower Virgin R iv e r 19 Table 2: Historical discharge data for US Geological Survey gaging station on Lower Virgin River near Littlefield , AZ ...... 39 Table 3: Summary of chemical and isotopic analyses of springs in the lower Virgin River study area ...... 41 Table 4: Surface water sampling rationale ...... 47 Table 5: Summary of surface water samples collected ...... 49 Table 6: Laboratory methods used for major ion analyses ...... 60 Table 7: Discharge Data from Sampling Events ...... 62 Table 8: Standard Errors for Discharge Measurements Made During Sampling Events ...... 64 Table 9: Results of seepage study on lower Virgin R iver ...... 67 Table 10: Stable Isotopes in precipitation and flood w ater ...... 70 Table 11: Stable isotopes in lower Virgin River area springs ...... 74 Table 12: Stable isotopes in water from wells in Virgin River study area .... 78 Table 13: Stable isotopes in the lower Virgin River and Beaver Dam Wash 81 Table 14: Summary of chemical data for springs and streams in the lower Virgin River area ...... 85 Table 15: Summary of major ion and field data for wells ...... 89 Table 16: Ground Water Summary ...... 135
vii LIST OF FIGURES
Figure 1: Map showing location of lower Virgin River study a re a ...... 2 Figure 2: Lower Virgin River Study Area Map ...... 3 Figure 3: Schematic cross-section across the lower Virgin River valley .... 17 Figure 4. Location of precipitation sites near the lower Virgin River study area 20 Figure 5: Generalized stratigraphy of the lower Virgin River area ...... 22 Figure 6: Map showing approximate locations of major structural features in the lower Virgin River study area ...... 26 Figure 7: Trilinear diagram of relative ion percentages showing samples collected by the U.S. Geological Survey at two sites on the lower Virgin River ...... 45 Figure 8: Plots of discharge of the lower Virgin River during low-flow surface water sampling events ...... 65 Figure 9: Plot of 6180 and SD for precipitation and flash-flood waters from a convective storm in the Virgin Mountains, Nevada ...... 72 Figure 10: Plot of 6180 and SD for springs and streams in the lower Virgin River a re a ...... 75 Figure 11: Plot of 6180 - 5D composition of water from wells in the lower Virgin River study area ...... 79 Figure 12: Plot showing 6180 - 6D composition of the lower Virgin River and Beaver Dam Wash during periodic sampling events between June 1992 and October 1993 ...... 83 Figure 13: Trilinear diagram of relative major ion percentages for springs in the lower Virgin River area ...... 87 Figure 14: Trilinear diagram of relative major ion percentages for wells in the lower Virgin River study a re a ...... 91 Figure 15: Trilinear diagram of relative major ion percentages for Virgin River and Beaver Dam W a s h ...... 93 Figure 16: Plots showing variation in EC of the lower Virgin River as a function of distance downstream ...... 94 Figure 17: Plots showing variation in major ion composition of the lower Virgin River as a function of distance downstream...... 95 Figure 18: Plot of monthly mean discharge for the Virgin River from USGS stream gage at Littlefield, A rizo na...... 99
v iii Figure 19: Plot of 5D variation over time at five sites on the lower Virgin River ...... 103 Figure 20: Plots of S1B0 and 5D at sites on the lower Virgin River during low streamflow sampling events ...... 106 Figure 21: Comparison of chloride concentrations in the lower Virgin River from samples collected for this study with historical data ...... 111 Figure 22:Plots of variation in chloride concentration with Virgin River discharge ...... 113 Figure 23: Plot of variation in chloride concentration over time at five sites on the lower Virgin River ...... 114 Figure 24: Plot showing variation in chloride concentration in the lower Virgin River with distance downstream ...... 116 Figure 25: Plots showing chloride and 5D variation in the'lower Virgin River in October 1992 and October 1993 ...... 120 Figure 26: Plots showing chloride and 5D variation in the lower Virgin River in July 1993...... 121 Figure 27: Plot showing stable isotopes in springs and streams in the lower Virgin River area ...... 123 Figure 28: Plot of stable isotopes in all ground water sampled ...... 129 Figure 29: Plots comparing chloride and 6D of waters in the lower Virgin River area ...... 138 Figure 30: Plot of <5180 and 6D that shows the types of water found in the lower Virgin River study area ...... 141
ix ACKNOWLEDGMENTS
First of all, I would like to thank my thesis committee - Dr. Neil Ingraham for having the patience to let me figure things out at my own pace; Dr. Steve
Mizell for helpful discussions and detailed editing of this document; Dr. Dave
Kreamer for getting me started and helping me find funding; and Dr. James
Deacon, for his insights into the Virgin River system.
Generous funding for this research was provided by the Las Vegas
Valley Water District. I appreciate both the financial support and the technical support provided by the District's research department. I am especially indebted to Terry Katzer and Kay Brothers for not turning me away four years ago when I came knocking on their doors looking for funding, and for subsequently sharing with me their knowledge of the Virgin River system. I would also like to thank Mike Johnson and Dave Donovan.
For the four years that I have been working on this thesis, I have been employed at the U.S. Geological Survey. I would like to thank all the hydrologic technicians with whom I have worked who taught me how to collect the data I needed to collect for this study. I would also like to thank all my supervisors over the years who have kindly worked around my thesis. Many people at the USGS have given me lots of help; in particular, Doug Trudeau, Jim Thomas, Robin Sweet, Barb Lewis and Dede Shoemaker have helped me
get to this point. Special thanks to Randy Laczniak, for making me learn how
to use arc/info and answering my questions about arc/info and hydrogeology;
Robert Vega, for procuring coverages and answering my daily onslaught of
questions about arc/info and what's wrong with the printer with patience and
good humor; Marsha Hilmes for constantly reminding me that I could and
would finish my thesis; and Gary Dixon, for encouraging me to find a thesis
project I liked and believing in me when everyone else thought I was a 'loose
cannon'.
Many people from various agencies gave me data and let me join them
in field work. Thanks to: Martin Einert from the Bureau of Reclamation who got
me started studying the Virgin River and introduced me to the joys of kayaking the Virgin River in mid-summer; the Nevada and Arizona Strip Districts of the
BLM, Randall Wilson from the SCS, the Arizona Department of Water
Resources, the Utah District of theUSGS, and the Virgin Valley Water District.
Several landowners were kind enough to let me sample their wells; thanks to the Linetti's, the Wilson's, Verl Frehner, and the Sunset Acres and Lucky-U
Ranch well owners.
Many good friends helped with sample collection including Darren
Reeves, Kim Zukosky, Gary Icopini, Bob Knowles, Julie Miller, and Marsha and
Rocky Hilmes. I'd also like to thank Vickie DeWitt in the Geoscience
Department who answered all my funding questions with great patience.
xi I would like to thank my parents, Judy and Lee Metcalf, and my brothers and sister, Lee, Todd and Bess, for their unconditional support in all my endeavors. In particular, I thank my brother, Lee, for much-needed reality checks and humor breaks.
Finally, I am most grateful to John Schmeltzer (the one person who could never refuse to go out in the field with me) for giving up weekends and holidays to be my sherpa on the Virgin River; for running seven miles back to the truck in mid-July because I didn’t know the Mesquite diversion was so far upstream; for letting me be the first to scratch his truck; and occasionally reminding me that a thesis is a temporary, six-credit thing; but mostly, for his unconditional love and support.
x ii CHAPTER 1
INTRODUCTION
The Virgin River is a highly saline tributary to the Colorado River that originates north and west of Zion National Park in Utah and flows through the northwestern corner of Arizona to the Overton Arm of Lake Mead in southern
Nevada. The total dissolved solids (TDS) concentration of the river ranges between 1000 and 2500 mg/L (USGS QWDATA database, 1994).
The general location of the study area is shown on Figure 1. This study concentrated on the lower Virgin River, which is identified as the reach from the
Virgin River Gorge east of Littlefield, Arizona, downstream to Lake Mead in
Nevada. The approximate surface water drainage boundaries and geographical features of the lower Virgin River drainage basin are shown in
Figure 2.
In recent years, the water resources of the entire Virgin River have been the subject of many development scenarios by municipalities in Utah, Arizona, and Nevada, in spite of elevated dissolved solids. An understanding of both ground and surface water in the drainage area of the Virgin River is crucial to ensure sustainable development of these resources and to protect ecosystems that depend on the Virgin River. 2
Tule Desert Subbasin Lower Virgin River Utah B a s in ^ Arizona
.Virgin River
Las Vegas
Lake Mead
Colorado ~~ River
40 eo MILES
40
Figure 1: Location of Lower Virgin River Study Area 4 30 4 00 1 113°45' ~ r __
3 7 0 3 0' 3 70 30' % J l Mountains
i I Dam Wash
3 7 0 0 01 3 7G 0 01 - East Mormon Mountains
Littlefield
Mesquite
o Bun veistqe nkerville Riversid t \ Diversion \
Lower ^ s Virgin River
Lake Mead ? 20MILES 3 6 0 30 3 60 3 0' - 20 KILOMETERS
1 1 4 0 3 0‘ 15' 1140 0 0’
Figure 2: Map of the lower Virgin River study area The purpose for this study was to determine what the relationship
between ground water and surface water resources is in the lower Virgin River
area. After reviewing previous studies done in the area (described below),
more specific research questions were identified. First, how much ground
water discharges to the lower Virgin River and how does discharging ground
water affect water quality and salinity in the river? Secondly, does the lower
Virgin River contribute significant amounts of recharge to ground water in the
basin? Finally, what is the origin of the ground water used in the basin?
After reviewing several studies of ground water-surface water
interactions (discussed below), the method of investigation chosen was to look
at natural isotopic and chemical tracers in ground water and surface water in the study area. The data collection plan was to focus first on the lower Virgin
River, looking for changes in river discharge, tracer concentration, and major ion chemistry over time and along the reach studied. Ground water from wells and springs in the study area was sampled, as were sources of recharge to the lower Virgin River that had been identified in previous research.
Measurements of river discharge and concentration of isotopic and chemical tracers in the river were used to determine the relative importance of the various sources of discharge. Stable isotope ratios and major ion chemistry were used to assess water quality changes due to the various sources of recharge as well as physical processes occurring along the river reach. Additionally, concentrations of tracers and major ion chemistry in ground water were compared throughout the study area to get an idea of which aquifers were connected and where ground-water recharge might be occurring.
This paper summarizes the results of background research and analytical results for the data collected, and presents interpretations and conclusions drawn from this data. This report is divided into six chapters. The remainder of Chapter 1 discusses previous investigations into both the hydrology of the lower Virgin River area and the nature of ground water-surface water interactions in other locations. Chapter 2 presents a detailed description of the physical setting of the lower Virgin River area based on geologic, hydrologic and water quality investigations performed by previous investigators. Chapter 3 describes the methods of investigation. Chapter 4 is a presentation of the results of sample analysis and measurements made.
Chapter 5 is an analysis of the data presented in the Chapter 4. Chapter 6 presents the conclusions reached by analysis of all the different types of data collected and suggests possibilities for further study.
Previous Work
Investigations in the Study Area
Previous investigations of the hydrology of the lower Virgin River area were studied to determine what was known about the basic hydrology of the area, the nature of ground water-surface water interactions, and the chemical nature of water in the study area. Previous studies in the lower Virgin River drainage basin include a water balance for the hydrologic basin, studies of
ground water quality and quantity, investigations into the nature and origin of
major springs in the area, and studies of the nature and origin of salinity in the
Virgin River. One of the earliest sources of water quality data for the Virgin
River was a report by Hardman and Miller (1934). They collected samples for
major ion analysis from the Virgin River, Beaver Dam Wash and some of the
major springs that flow into the river in the Virgin River Gorge upstream of
Littlefield, Arizona (commonly known as the Littlefield Springs). Hardman and
Miller (1934) discussed the water supply for the Virgin River and the source of the springs that enter the river from the Virgin River Gorge to the town of
Littlefield, Arizona. They concluded that the source for the Littlefield springs must have been an influent reach of the river upstream of the Virgin River
Gorge.
As part of a reconnaissance series for the state of Nevada, Glancy and
Van Denburgh (1969) prepared an appraisal of water resources for the lower
Virgin River drainage basin in Nevada. Glancy and Van Denburgh (1969) used existing surface water, precipitation, and ground water data to estimate a water budget for the lower Virgin River area. With this information, the authors assessed the available water supply for development in the basin.
Trudeau (1979) studied the hydrogeology and hydrogeochemistry of the
Littlefield Springs, a group of more than 90 saline springs that discharge into the Virgin River upstream of Littlefield, Arizona. He collected geologic data, inventoried the springs, made field measurements, and collected samples of
the springs for laboratory gross chemistry and tritium analysis. He concluded
that the source for the Littlefield springs was a combination of local
precipitation and water that had infiltrated from the Virgin River approximately
16 kilometers upstream of the first spring.
In 1981 the Soil Conservation Service (SCS) published a study of the
sources of salinity in the lower Virgin River in Nevada. They concluded that
86% of the salt load was derived from sources above Littlefield, Arizona. The
salinity in the lower Virgin River was also examined by Woessner and others in
1981. They prepared salinity and water budgets for the river system and found
that salt and water inflow to the basin exceeded outflow at Lake Mead by 35%
and 37%, respectively. They determined that consumptive use by
phreatophytes and agriculture was the dominant mechanism to remove salt and water from the system. Woessner and others (1981) also examined the
relationship between ground water and surface water in the lower Virgin River;
specifically, they attempted to locate gaining and losing reaches of the river.
The study identified the reach from Littlefield, Arizona to the Mesquite irrigation diversion (13 km in length) as a gaining reach during the winter months. They calculated that the total ground water discharge to the Virgin River was in excess of 43.7x10® m3/yr; however, because most of the ground water was consumed by evapotranspiration in the floodplain, gains in river discharge were only measurable in the winter months. 8
Black and Rascona (1991) studied ground water conditions in the
Arizona portion of the lower Virgin River basin. They identified four aquifers in
their study area based on geologic and hydrogeologic properties and sampled
some of the wells. Using a computer model of ground-water flow in the
carbonate rock province of the Great Basin, Burbey and Prudic (1993)
calculated ground-water flow in the northern part of the lower Virgin River
valley. Burbey and Prudic (1993) estimated that of about 17.4 x 106 m3/yr of
recharge to the basin, approximately 60 percent of which originates in the
northeastern portion of the drainage basin and the remainder comes from
adjacent basins and subbasins. The model simulated that 6.2 x 106 m3/yr of
ground water discharges to the lower Virgin River.
The Las Vegas Valley Water District (LWWD) (Brothers and others,
1992, 1993) published a summary of previously collected ground and surface water data to support their water rights applications for ground and surface water in the lower Virgin River basin. The report included a three-dimensional ground and surface water model of the basin designed to predict the effect of several water withdrawal scenarios. The model simulations predicted that withdrawals proposed by LW W D would have no impacts on existing water rights and minimal impacts on the ecosystem. 9
Ground Water-Surface Water Interactions
An understanding of the interactions between ground water and surface
water is important to any assessment of the water resources of an area. In the
lower Virgin River area, the need for understanding such interactions is critical
because both surface and ground water are being used currently and will likely
be developed to a greater degree in the future.
A variety of techniques exist for determining flow between ground water
and surface water bodies including the following: the determination of
piezometric contours and examination of the resulting gradients; baseflow
separation on long-term and storm hydrographs; the use of chemical and
isotopic tracers; and the solution of mass balance equations. The latter two
techniques, the use of isotopic and chemical tracers, and mass balance
equations, have been used in basins where extensive hydrologic data is not
available. Extensive hydrologic data is not available for most of the lower
Virgin River hydrologic basin because much of the area is uninhabited.
Isotopic and chemical tracers were chosen to develop an understanding of the
interaction between surface and ground water with minimal expense.
Stable Isotopes as Tracers
The tracers commonly referred to as ‘stable isotopes’ are oxygen-18
(abbreviated as 180) and hydrogen-2 ( usually referred to as deuterium and abbreviated as D). Stable isotopes are measured as the ratio of the heavier, 10
less-abundant isotope to the lighter, more abundant isotope, such as, oxygen-
18 to oxygen-16 (180/160) and deuterium to hydrogen-1 (D/1H). Results of
stable isotope analyses are reported in delta (5) notation as the per mil (%o)
deviation of the sample from a standard. The standard used is Standard Mean
Ocean Water (SMOW) which has a delta value of zero for both isotopes.
Water samples that have isotopic values that are greater than SMOW have a
greater concentration of the heavy isotope relative to the light isotope than
SMOW and are said to be 'isotopically enriched' relative to SMOW; while those
which have a negative delta value have a smaller concentration of the heavy
isotope relative to the light isotope than SMOW and are referred to as
'isotopically depleted' relative to SMOW.
Stable isotopes of oxygen and hydrogen can be used as tracers
because waters from different sources generally have different isotopic
compositions due to different condensation and evaporation histories. The
factors which most affect the stable isotopic value of precipitation are the
temperature of condensation and the composition of the parent vapor
(Dansgaard, 1964). The ratio of heavy to light isotopes produced by a phase
change (e.g. evaporation or condensation) is governed by the temperature at which it occurs. Phase changes at low temperatures produce greater fractionation between heavy and light isotopes than phase changes at high temperatures. In terms of precipitation from an air mass, condensation at lower temperatures, whether due to altitude or latitude, produces precipitation that is 11
more depleted in heavy isotopes than that produced at higher temperatures.
As an air mass rises to higher altitudes, heavy isotopes are progressively
"rained-out" and the remaining vapor is gradually depleted of the heavy
isotopes. Due to this rain-out (orographic effect), precipitation falling at high
altitudes is more depleted than that falling at lower altitudes in the same
general area (Dansgaard, 1964).
Although all air masses originate from the ocean and start out with the
same isotopic composition, the composition of a cloud becomes more depleted
in heavy isotopes each time precipitation condenses out of the cloud. In this way, air masses which are farther from their source (i.e. farther inland) will
produce precipitation that is more depleted than clouds which are undergoing their first condensation from ocean air vapor. This trend is known as the continental effect and was described by Dansgaard (1964) based on observed patterns of isotopic values in meteoric waters. Craig (1961) further described the isotopic composition of meteoric waters in terms of the relationship between deuterium and oxygen-18. Craig (1961) found that on a plot of the 5D and
5180 compositions of meteoric waters from around the world, the points defined a line with the equation 5D = 85180 + 10, which is commonly referred to as the global meteoric water line (MWL). In general terms, waters which plot on the meteoric water line have not undergone significant evaporation, while those which plot to the right and groups of samples which define a line of lower slope than the meteoric water line have undergone significant evaporation. 12
The significance of both temperature and continental effects in ground
water-surface water studies is that rivers which originate farther inland or at
higher elevations than the ground water with which they interact often have different, i.e. more depleted, isotopic compositions than local waters. Large river systems such as the Colorado River (Smith and others, 1992; Payne and others, 1979) retain the stable isotopic composition of their headwaters and are relatively unaffected by inflows in their lower reaches. This difference allows stable isotopes to be useful as tracers in such waters.
Stable isotopes of oxygen and hydrogen have been used in tracer and mass balance investigations to solve a variety of problems including the infiltration of rivers to ground water (Brown and Taylor, 1974; Payne and
Shroeter, 1979; Hussain et al, 1992); the discharge of ground water to rivers
(Ingraham and Taylor, 1989; Space et al, 1991; McKenna et al, 1991; Simpson and Herczeg, 1991); the interactions between lakes and ground water
(Krabbenhoft et al, 1990); and the determination of ground-water flow paths in a regional carbonate aquifer (Thomas and Welch, in press; Winograd and
Friedman, 1972).
Chlorides as Conservative Tracers
The element chlorine is most commonly found in natural waters as the chloride ion (Cl'). Chloride salts are easily soluble and thus, chloride is ubiquitous in natural waters (Feth, 1981). Chloride is useful as a tracer 13 because i t "... is not removed or supplied significantly by interaction with rocks and is not precipitated as a salt until very high salinities are reached (Drever,
1988, p.242)." Simpson and Herczeg (1991) used chlorides and stable isotopes as tracers in a study of the causes of increases in salinity along the length of the River Murray in Australia. Simpson and Herczeg (1991) compared changes in chloride concentration downstream with changes in stable isotope composition. Using the two types of tracers, they were able to identify areas where inflow of small amounts of highly saline ground water was the means to increase salinity, and where evaporation was the mechanism increasing salinity. CHAPTER 2
ENVIRONMENTAL SETTING
This study examines the ground water-surface water interactions in the
lower Virgin River area using stable isotopic and chemical signatures as
tracers. Before new tracer data could be evaluated, the physical setting and
existing tracer data for the study area were investigated. In the following
section, the physical setting of the study area is discussed and previously-
collected data on chemistry and stable isotopes of ground and surface water in
the study area are introduced.
Physical characteristics, such as geography, climate, geology,
hydrogeology, and hydrology, are critical to any interpretation because these factors constrain the availability and movement of water. The use of isotopic
and chemical tracers often provides solutions to ground water-surface water
interactions that are not unique, that is, tracer data may point to more than one
possible solution. Knowledge of the physical setting can limit the number of
solutions.
Tracer data from previous studies in and near the study area provides
baseline information on water quality and a point of comparison for analyses collected for this study.
14 15
Physical Characteristics of the Study Area
Physiographic Setting
Geographic Location
The lower Virgin River surface-water drainage basin is an area of approximately 4500 km2 located roughly between latitudes of 36°30' and
37°38', and longitudes of 113°45' and 114°30\ The basin is located largely within the southeast corner of Nevada with smaller portions in northwestern
Arizona and southwestern Utah as depicted on Figure 1. The Tule Desert area is the only sub-basin within the lower Virgin River drainage basin (Glancy and
Van Denburgh, 1969). The Tule Desert sub-basin, located in the western part of the drainage area, is surrounded by highlands and drains south toward the lower Virgin River.
Physiographic Location
The lower Virgin River drainage area is located in the transition zone between the Basin and Range physiographic province and the Colorado
Plateau province. Major ranges and highlands bounding the lower Virgin River drainage basin include the Virgin Mountains, an arcuate range located to the south and east, the Beaver Dam Mountains to the east, the Bull Valley
Mountains to the northeast, the Clover Mountains to the north and northwest, the Mormon Mountains to the west and Mormon Mesa to the southwest.
Highlands within the drainage basin include the Tule Springs Hills located east 16
of the Tule Desert and the East Mormon Mountains located east of the Mormon
Mountains and west of the Tule Springs Hills. Figure 2 shows the major
topographic features of the basin.
Elevations in the lower Virgin River drainage area range from 2442
meters at Mount Bangs in the Virgin Mountains to 400 m at the Virgin River floodplain at Lake Mead. River channel elevation ranges from 700 m at the top of the reach in the Virgin River Gorge to 400 m at the bottom of the reach at
Lake Mead.
The general geomorphology of the drainage area is shown on Figure 3, a generalized cross-section through the study area from north to south. South and east of the river, the Virgin Mountains are flanked by alluvial fans which are then cut by floodplain alluvium of the lower Virgin River. North of the river, the mountain ranges are farther from the river and the deposits consist of alluvial fans flanking the mountains, and valley fill material cut by floodplain alluvium of the Virgin River and its ephemeral and perennial tributaries.
The Virgin River is a tributary to the Colorado River that has its confluence at Lake Mead. The only perennial tributary to the lower Virgin River is Beaver Dam Wash which has its confluence with the Virgin River east of
Littlefield, Arizona. There are numerous ephemeral drainages on both sides of the river, including Sand Hollow Wash, Toquop Wash and Half Way Wash on the north side of the river and Nickel Creek on the south side of the river. 17
LU CO
z (modified from Woessner and from others, (modified 1981) Figure vlaley 3: Schematic River cross-section the Virgin across Figure lower
$ O JS 18
Many of the tributaries trend roughly north-south (perpendicular to the river),
and may be fault-controlled (Las Vegas Valley Water District, pers.comm.,
1994).
Climate
The climate in the study area is arid in the valleys and semi-arid in the
mountains (Glancy and Van Denburgh, 1969). Long term climate data is
available for ten stations near the study area in Nevada and Utah (U.S.
Department of Commerce, 1992). Location, elevation, average annual
precipitation, average high and low temperature, and number of degree days
for each station are listed on Table 1. Sites are shown on Figure 4.
The lower Virgin River valley floor receives 100 mm of precipitation in an
average year (SCS, 1981). Data from Table 1 show a range in average annual
precipitation data from 105 to 340 mm, with the greater amounts of precipitation found in areas of higher elevation to the north and east of the study area.
Winter precipitation occurs primarily as regional storms of low intensity and long duration. Summer precipitation typically occurs as localized, high-
intensity, short duration convectional storms. The moisture source for winter storms is usually from the west, while the source for summer storms is generally from the south and southeast (French, 1983). Table 1: Precipitation data for sites in and near the lower Virgin River CO 'O .2 z UJ & •v s ‘5i ■o CL < ■n < d) c CO CO E m d) o c 3 d> fi c “ . c n a r « c OJ * o>>*
o O E E Si: 35 Q > o. 75 CO CO o CO CO CM CO ■M- co CM r~ in o 00 CO CO CO CM o o >* 0 o )’ > CO O < CL CO CO T“ to T— T- ) O CD ) O CD tf) 10 0) 2 o £ CO in uo CO T~ O 5 0 CM 00 CM CO 05 * w 3 0) o £L CL Z J a CO r- o CO co CO Is- CO* o o> CO CO CD CD CD C O) CD (D 05 CD O 75 CO CM co r- . ^ r co co m 0) c Q> CO co r^- CO > sz 0- LO >. 5 0) o o 0) O 3 0) U> UJ ' .12 N- M CD in r- O CO CO CO CM CM CO CM C 0) . Q
Lower Virgin River Pioche Basin Modena Enterprise-Betyl _Caliente
_Veyo Powerhouse Pahranagat Wildlife Refuge ' Gunlock Powerhouse Utah Arizona
Valley of Fire . State Park
Las Vegas
Modena Precipitation Data Collection Site
60 MILES
Figure 4: Location of precipitation sites near the lower Virgin River study area 21
Average temperatures in the lower Virgin River valley range from -1 to
16°C in winter and from 21 to 410 C in July (SCS, 1981). Temperatures at
higher elevations are likely to be lower than those provided for the valley floor.
Average annual free surface evaporation is greater than 2000 mm per year
(Woessner, 1981) on the valley floor.
Geologic Setting
A basic overview of the geologic history of Nevada is provided in
Stewart (1980). Regional geologic mapping that covers the study area
includes work by Longwell and others (1965), Moore (1972), Hintze (1986) and
Bohannon (1983, 1984). Other studies of the region include Armstrong (1968),
Wernicke and others (1988), Axen and others (1990), Axen (1993), Bohannon and others (1993), and Anderson and Barnhard (1993).
Stratigraphy
A generalized stratigraphic section of the study area (adapted from
Bohannon and others, 1993) is shown on Figure 5. In addition to the stratigraphic column, Figure 5 also shows which stratigraphic units are exposed in the mountain ranges surrounding the study area. Naming conventions in
Figure 5 follow those of Bohannon and others (1993). 22
CRETACEOUS
*-1000 Feci Meiers Tank Fm.
JURASSIC
Muddy Creek TRIASSIC Formation
Toroweap Red — iFm.i— sandstone 10 6 unit PERMIAN Esplanade Ss. Lovoll Wash Member
Bird Bitter PENN Spnng Ridgo Ls, SYLVANIAN Fm. Member 13 2 Monte MISSISSIPIAN Cnsto Fm.
DEVONIAN onoovicifltf Thumb Momber
Muav Fm —lUnghtL- Angql Sh Tapoats Ss Gr A GnPRECAMBR1AN
After Bohannon and otheis (1993). with Beaver Dam Mountains data from Hintze, 1986 Figure 5: Generalized stratigraphy of the lower Virgin River area 23
Geologic history
Precambrian rocks are exposed along the west flank and core of the
Virgin Mountains (Moore, 1972), in the East Mormon Mountains and Tule
Springs Hills (Axen and others, 1990), and in the Beaver Dam Mountains
(Hintze, 1986). Precambrian rocks in the study area include intrusive igneous
and high-grade metamorphic rocks such as granite, gneiss, schist, and amphibolite. Pegmatites are locally abundant. Precambrian rocks are not named except in the Beaver Dam Mountains where they are referred to as an equivalent of the Vishnu Schist of the Colorado Plateau (Hintze, 1986).
An erosional unconformity separates Precambrian crystalline rocks from
Lower Cambrian rocks in the study area. By the end of the Precambrian (650 ma), southern Nevada was situated along the western margin of a continent. A westward-thickening prism of shelf sediments, known as the miogeosynclinal belt of the Cordilleran geosyncline, was forming (Stewart, 1980). The study area was at the eastern edge of that miogeosynclinal belt in an environment where terrigenous detrital sediments were deposited on a continental shelf. To the west of the study area, a thicker section of miogeosynclinal sediments, in places 12,000 m thick, was deposited. During the Early Cambrian, sandstones, quartzites and shales were deposited in the study area. By the
Middle Cambrian, deposition changed from detrital sediments to carbonates
(Stewart, 1980). From Middle Cambrian through Late Pennsylvanian, a thick section of carbonates was deposited in southern Nevada. During the Permian, 24
deposition shifted to carbonates interbedded with terrigenous sediments.
Mesozoic rocks in the study area indicate that during the Early Triassic
period, marine carbonates and siltstones were deposited in a shallow basin.
Upper Triassic continental sandstones, conglomerates and claystones indicate
that by Late Triassic, the study area was to the east of the continental margin.
Deposition of terrigenous sedimentary rocks continued throughout the
Mesozoic Era. During the middle- to late Mesozoic Sevier Orogeny, a north -
northeast trending belt of east-vergent thrusts developed in southern Nevada
and western Utah. In the north and west portion of the lower Virgin River
hydrographic basin, upper Precambrian and lower Paleozoic rocks were thrust
over upper Paleozoic, and Mesozoic rocks.
Early Cenozoic rocks are not found in the study area. Middle - to late -
Tertiary-aged rocks in the study area indicate continental deposition of
sediments and tuffs with localized formation of carbonate and evaporite
sequences likely formed in inland lakes. In the late Cenozoic era, the region was deformed by crustal extension that resulted in normal faulting and produced basin and range features characteristic of the region. Late Cenozoic basins underwent sedimentation that produced alluvial fan, valley fill and playa deposits. Basin-filling sedimentation continued until the beginning of the
Quaternary (2 Ma) when the downcutting of the Colorado River and its tributaries (including the Virgin River) began (Hoover and others, 1992), 25
Structures
The lower Virgin River drainage basin is located in a transition zone
between the Basin and Range and the Colorado Plateau physiographic
provinces. The dominant structural features in the lower Virgin River area
include the Virgin River depression, a structural basin; the Virgin Mountain
Anticline and flanking faults that allowed uplift and exposure of the
Precambrian core of the Virgin Mountains; and Sevier orogeny thrusts to the
north and west and the subsequent detachment faults that developed along those thrusts. Figure 6 depicts the major structural features of the lower Virgin
River valley area.
The Virgin River depression is a mid-Tertiary-aged structural basin that coincides with the southern portion of the lower Virgin River surface-water drainage basin. Bohannon and others (1993) estimated that the Virgin River depression has subsided 3 to 6 km relative to the adjacent Virgin Mountains and Colorado Plateau. It is bounded on the south by northeast-trending strike slip faults of the Lake Mead fault system. The depression consists of two sub basins, the Mormon Sub-Basin (roughly centered around Mormon Mesa) and the Mesquite Sub-Basin (roughly centered around Mesquite, Nevada) separated by a complex buried ridge (Bohannon and others, 1993). The
Mormon Sub-Basin is bounded on the north, east and southeast by normal faults. The Mesquite Sub-Basin is also bounded by normal faults including the
Piedmont fault to the east and the East Ridge fault to the west. 13° 45 26
3 7 0 3 0 ’ 37 0 30 Mountains
T„le S p r i n j s £■ T^rv*f *
^Aofry>o»% TbrUS 4"
37 0 0 0 ' 37 “00 ast Lower Mormon Virgin Mountains
Virgin Rive-*- ^epr-tssif^i
e sq u it
&uWb«S' Mormon Sukrb<*6ir\ 2 0 MILES 36 0 30 1 Lake 36°30 ' Mead 20KILOMETERS 4 °00 Figure 6: Map showing approximate locations of major structural features in the lower Virgin River study area 27 The Piedmont fault is a listric normal fault that dips to the west along the western flank of the Virgin Mountains, and the East Ridge fault is an east- dipping, high angle normal fault that separates the Mesquite Sub-Basin from the buried ridge that separates the two sub-basins. To the south and east of the Virgin River depression, the Virgin Mountains experienced uplift and denudation of Mesozoic and Paleozoic strata. The uplift was bounded on the west by the Piedmont fault and on the east by the Sullivan's Canyon (Bohannon and others, 1991) and the Front faults (Bohannon and Lucchitta, 1991; Bohannon, 1991) and other related faults that separate the Colorado Plateau from the eastern Great Basin. The uplift and subsequent denudation exposed Precambrian rocks at the center of the Virgin Mountain anticline. The Virgin Mountain Anticline plunges to the north and trends roughly N30E for most of its length and turns abruptly N10E at its northern end. In the northern and western part of the drainage area, late Mesozoic Sevier Orogeny thrusts were cut and rotated by Tertiary extensional features (Axen and others, 1990). Sevier orogeny structures in the drainage area are characterized by east-vergent, large-displacement thrusts. From west to east, the nearby thrusts are the Glendale Thrust (located in the North Muddy Mountains to the southwest of the study area, not shown on Figure 6), the Mormon Thrust (located in the central Mormon Mountains); and the Tule Springs Thrust (located in the Tule Springs Hills). These thrusts placed Middle 28 Cambrian and younger carbonates over Triassic to Jurassic rocks. The Mormon and East Mormon Mountains, and Tule Springs Hills were highly extended during Neogene Times by movement on (from west to east) the Mormon Peak, Tule Springs and Castle Cliff detachments (Axen and others, 1990). The Mormon Peak detachment is a west-dipping, low-angle normal fault that cuts the Mormon Thrust plate, the Tule Springs Thrust plate, and autochthonous rocks on the west side of the Mormon Mountains. The Tule Springs detachment cuts the Tule Springs thrust allochthon, Mesozoic and Paleozoic strata, and Precambrian crystalline basement. A number of other faults are present in the northern part of the lower Virgin River drainage basin. These faults are primarily normal faults that are located in the East Mormon Mountains and the Tule Springs Hills. They generally trend north- south and include both faults which are synchronous with the Tule Springs detachment and those which post-date it (Axen, 1993). Hydrogeology Hydrogeologic units in the lower Virgin River area include both unconsolidated and consolidated rocks, although the primary aquifers in the study area are within unconsolidated basin fill. Differentiable unconsolidated units in the study area include Younger Alluvium of the lower Virgin River, Beaver Dam Wash Alluvium, Older Alluvium, and the Muddy Creek Formation. Consolidated rocks include carbonate and noncarbonate rocks. 29 Unconsolidated Rocks In the study area, the Virgin River structural depression is filled with sediments to depths of several thousand meters (Bohannon and others, 1993). Hoover and others (1992) performed detailed mapping of the surficial geology of the Riverside quadrangle and recognized four categories of deposits ranging in age from Quaternary to Tertiary. However, for the purposes of discussing the water bearing properties of the sediments, hydrogeologic units described by Glancy and Van Denburgh (1969), Woessner and others (1981), Black and Rascona (1991), and Brothers and others (1992) will be used. Aquifers within unconsolidated basin fill in the lower Virgin River basin are the Younger Alluvium, Beaver Dam Wash floodplain alluvium, Older Alluvium, and the Muddy Creek Formation. Most authors (Glancy and Van Denburgh, 1969; Woessner and others, 1981; Black and Rascona, 1991; Brothers and others, 1992) have discussed the Older Alluvium and the Muddy Creek Formation together as one aquifer system. The probable reason for this is that the thickness of Older Alluvium is not well understood and in wells spudded in Older alluvium, the boundary between the two is commonly unclear from drilling logs. In the following discussion, each is described as to its occurrence and physical nature separately. Then, aquifer properties of the two are discussed the together. 30 Younger Alluvium Glancy and Van Denburgh (1969) classified Younger Alluvium as deposits in alluvial areas where recent deposition has been the dominant process and where the thickness of the deposits renders them hydrogeologically significant. In the study area, younger alluvium is found in the Virgin River and the Beaver Dam Wash floodplains; however, younger alluvium in the Beaver Dam Wash floodplain is considered a separate aquifer from that of the Virgin River floodplain (Black and Rascona, 1991) and is described below. Younger Alluvium of the lower Virgin River (YA) ranges in thickness from 1 to 30 m (United States Bureau of Reclamation [USBOR], 1989) and consists of unconsolidated lenses of gravel, sand, silt and clay deposited in a fluvial setting. In some places it is underlain by Muddy Creek sediments (USBOR, 1989) and in others by Older Alluvium. The Younger Alluvium aquifer is unconfined and ground water yields vary from less than one to several hundreds of L/m (Glancy and Van Denburgh, 1969). Depth to ground water in the Younger Alluvium ranges from 1 to approximately 45 m, and generally increases with distance from the active channel. In 1978, USBOR performed pumping tests on a 45 m deep pumping well in the Virgin River floodplain near Riverside, Nevada. The well was drilled in approximately 20 m of floodplain alluvium underlain by at least 25 m of Muddy Creek Formation. Woessner and others (1981) used the results of this test to calculate a transmissivity of 8.6 x 10'3 m2/s and hydraulic conductivity of 3.4 x 31 10‘4 m/s for the Younger Alluvium. Seven wells (YA-1 through YA-7) in the Younger Alluvium aquifer were sampled for stable isotopes and major ion chemistry in the present study. Well construction data for the wells sampled (when available) is presented in Appendix A. Drilling logs for these wells indicate that the wells are drilled in clay, sand, and gravel. Water levels reported (Brothers and others, 1993) for wells sampled in the Younger Alluvium were 8 to 10 meters below land surface. Beaver Dam Wash Aquifer Black and Rascona (1991) described the Beaver Dam Wash floodplain aquifer (BDW) as an unconfined aquifer consisting of unconsolidated silts, sands, and gravels. These relatively permeable materials were deposited within a channel incised by Beaver Dam Wash into relatively less permeable Older Alluvium. Depths to ground water range from less than 3 to 20 m below land surface. Wells yield from less than one to several hundred liters per second (Black and Rascona, 1991). Woessner and others (1981) estimated transmissivity and hydraulic conductivity values for the Beaver Dam Wash floodplain aquifer with data from drilling logs; they calculated ranges of 2x1 O'3 to 6.5x10"3 m2/s for transmissivity and 2x1 O'4 to 8.5x1 O'4 m/s for hydraulic conductivity. Four wells sampled for the present study were located in the Beaver Dam Wash floodplain aquifer. According to drilling logs for three of the four 32 wells (no log was examined for the fourth), the wells were completed in gravel, or sand and gravel. Completion depths were 31 m or less. Static ground water levels were between 4 and 5 m below land surface. Older Alluvium Older Alluvium is present in areas where erosion and deformation dominate over deposition (Glancy and Van Denburgh, 1969). Older Alluvium consists of unconsolidated to consolidated deposits of boulders, gravel, sand, silt and clay. Older alluvium is present in the study area in alluvial fans and terraces that flank the mountain ranges. Thickness of Older Alluvium varies from meters to greater than 100 m. Older Alluvium is thickest along the valley axis and thins at the edge of the mountain ranges (Glancy and Van Denburgh, 1969). Muddy Creek Formation The Muddy Creek formation is the oldest of the valley fill aquifers in the lower Virgin River drainage basin. The Muddy Creek Formation is Miocene in age and was deposited between 10 and 4 million years ago (Bohannon and others, 1993). The Muddy Creek underlies both Younger and Older Alluvium in the study area. It consists of conglomerate, sandstone, siltstone, and claystone that is generally poorly-consolidated but in places may be well- indurated (Las Vegas Valley Water District, 1992). Thickness of the Muddy Creek in the lower Virgin River drainage area varies widely. Muddy Creek 33 sediments are thickest (2000-2200 m) below the Mesquite structural sub-basin adjacent to the Piedmont Fault and thin gradually to the north and west along the axis of the Virgin River (Bohannon and others, 1993, Figure 8A). In the northern portion of the drainage basin the Muddy Creek is inferred to be meters to hundreds of meters thick (Bohannon and others, 1993), Older Alluvium/Muddy Creek Formation aquifer Most wells completed in the lower Virgin River drainage basin are completed in the either the Older Alluvium or the Muddy Creek Formation or, most commonly, in both. One well sampled for this study (OA-12) was reported by Black and Rascona (1991) to be completed in the Muddy Creek Formation alone. The older alluvium and at least the upper parts of the Muddy Creek act as aquifers where saturated and are generally unconfined. Yield to wells varies from less than one to several hundred liters per second (Glancy and Van Denburgh, 1969). Transmissivities and hydraulic conductivities were calculated from drilling logs by Woessner and others (1981, Appendix D). Transmissivities range from 1.3x1 O'4 to 6.2x1 O'3 m2/s and hydraulic conductivities range from 3.8x1 O'6 to 3.7x1 O*4 m/s. Seventeen of the wells sampled for the present study (OA-1 through OA -17) were located in the Older Alluvium/Muddy Creek Formation. The wells were located between 0.8 and 3.2 kilometers from the active river channel. Ground water levels were available for 14 of those wells (Brothers and others, 34 1993), and ranged from 5 to 91 m below the surface. Two wells sampled (OA -1 and OA-2) had water levels of less than 10 m below the surface, yet the screened interval of each well was at least 150 m below land surface. For OA- 12, completed solely in Muddy Creek Formation, Black and Rascona (1991) reported a water level altitude of 451 m (134 m below land surface) and a yield of about 25 Us. Consolidated Rocks Carbonate Rocks Carbonate rocks that yield water in the lower Virgin River drainage basin include Paleozoic carbonates that occur in the northern portion of the drainage basin in the Mormon, East Mormon, and Beaver Dam Mountains and the Tule Springs Hills and underlie the valley fill of the Tule Desert. Carbonate rocks also outcrop in the Virgin Mountains and the Virgin River Gorge and are associated with spring flow in these areas. In the carbonate rock aquifers, storage and transmission of water may occur where solution cavities have formed along fractures and other zones of weakness (Glancy and Van Denburgh, 1969). Though little specific data is available for carbonate aquifers in the study area, study of carbonate aquifers elsewhere in the Basin and Range province indicates that regional carbonate aquifers in the south and central Great Basin store significant amounts of high-quality (suitable for most uses) water (Dettinger, 1989). The most productive regional carbonate aquifers occur in a thick section of Paleozoic to Mesozoic-aged carbonates located along a north- south corridor in south and central Nevada. The study area lies to the east of this corridor, in a region generally characterized by thin or isolated carbonate rocks that may contain water of a quality unsuitable for most uses. Factors which may affect the quality of water produced by carbonate rocks in this region include the presence of salt-bearing minerals, the connectedness to regional carbonate aquifers, and the presence of low transmissivity rocks that can serve as obstructions to flow (Dettinger, 1989). However, since no wells in carbonate aquifers were sampled for this study, the exact nature of carbonate rocks in the study area is unknown. The Littlefield Springs and springs in the Virgin Mountains, East Mormon Mountains and the Tule Springs Hills issue from carbonate bedrock and alluvium. Trudeau (1979) inventoried the Littlefield Springs. He concluded that the springs could be divided into three types, Virgin River Gorge, Virgin Valley and an intermediary source. Springs in the Virgin River Gorge issue from fractured Paleozoic limestone mapped by Bohannon and others (1991) as Monte Cristo Limestone. Trudeau characterized springflow in the gorge as locally-semiconfined to unconfined. Seven of the Littlefield springs in the Virgin River Gorge (VG-1 to VG-7) were sampled for this study. Springs sampled were issuing from fractures and small caverns in the limestone and alluvium in the gorge. 36 The Littlefield Springs in the Virgin Valley area issue from a cliff-forming limestone at the edge of the Virgin River floodplain near Littlefield, Arizona. Trudeau (1979) described this limestone as a fresh-water limestone. Samples were collected from eight springs in this region (W-1 to W -8). Springs in the Virgin and East Mormon Mountains and the Tule Springs Hills issue from alluvium in areas where carbonate rocks are prominent. Most of these springs have been developed for stock watering and the original spring orifice is not visible. Springs of this type sampled for this study include four springs in the Virgin Mountains, and one in the Tule Springs Hills. Noncarbonate Rocks Noncarbonate rocks that may yield water include fractured Precambrian crystalline rocks in the Virgin Mountains and Mesozoic clastic rocks in the southern and eastern portion of the basin, and volcanic rocks in the northernmost portion of the drainage basin. Noncarbonate rocks yield small amounts of water primarily through fractures. Springs are the primary discharge points for noncarbonate aquifers in the study area. Two springs in the Virgin Mountains that issue from noncarbonate bedrock were sampled. The springs issue from Precambrian metamorphic rocks in the Virgin Mountains. 37 Surface Water Hydrology The Virgin River originates at an altitude of 2600 m in southern Utah, flows southwest through the northwest corner of Arizona and southeastern Nevada, and discharges to Lake Mead. The drainage is divided into an upper and lower basin. The boundary of the upper and lower basin historically has been drawn at the Virgin River Gorge, approximately 22 km east of Littlefield, Arizona. The annual runoff of the lower Virgin River consists of upper basin discharge, Littlefield Springs inflow, Beaver Dam Wash discharge, and to a lesser extent, surface runoff from flood events in the drainage basin, and seepage of ground water to the channel. Flow is diminished in the basin by irrigation diversions, evaporation from the water surface, and transpiration from phreatophytes in the flood plain (Glancy and Van Denburgh, 1969). Peak flows occur during the winter as a result of winter storms in the upper basin and in the spring as a result of snowmelt in the upper basin. Inflows Information on the quantity of discharge from the upper Virgin River basin is available from a U.S. Geological Survey (USGS) stream gaging station on the lower Virgin River at Littlefield, Arizona. Discharge at the Littlefield gage is a useful estimator of the upper basin discharge because inflows between the upper basin and the Littlefield gage (Beaver Dam Wash and the 38 Littlefield Springs) are well quantified. The Littlefield gage has been in service for 65 years, and average annual discharge for the period of record is 215 million m3 (Emett and others, 1994). Monthly mean discharges for the period of record and the duration of the present study are shown below in Table 2. The work of Trudeau (1979) and Glancy and Van Denburgh (1969) suggests that the Littlefield Springs contribute a total of 1.7 to 1.8 m3/s of constant discharge to the lower Virgin River. Trudeau’s estimates for the Littlefield Springs were based on six years of data from the Littlefield, Arizona and St. George, Utah USGS gages. Beaver Dam Wash is the only perennial tributary to the Virgin River. Trudeau (1979) estimated that the average flow of Beaver Dam Wash was 0.09 m3/s based on 17 miscellaneous measurements made by the USGS between the years of 1946 and 1968. The USGS recently (1993) installed a stream gage on Beaver Dam Wash approximately 2 km above its confluence with the Virgin River, however at this time no data on annual runoff is available. Several ephemeral tributaries in the lower Virgin River basin contribute flow during runoff events, but the contribution of ephemeral tributaries to annual runoff is estimated to be minor (Glancy and Van Denburgh, 1969). Ground water seepage to the lower Virgin River has been estimated by various authors using different methods to be 6x10e m3/yr (Glancy and Van Denburgh, 1969; Burbey and Prudic, 1993) and 44x106 m3/yr ( Woessner and others, 1981) which is equivalent to a constant discharge of 0.2 and 1.4 m3/s, respectively. 39 Table 2: Historical discharge data for US Geological Survey gaging station on Lower Virgin River near Littlefield , AZ1 ______Month Monthly Monthly Standard Years of Mean Mean Deviation Record2 1930-1993 1993 1930-1993 m*/s October 4.10 5.46 2.54 64 November 5.38 5.63 2.22 64 December 6.37 5.60 4.23 64 January 6.71 21.34 3.54 64 February 9.17 36.68 9.43 64 March 10.10 33.56 9.96 64 April 11.86 36.56 10.83 64 May 12.08 39.51 14.67 64 June 3.88 9.45 4.42 64 July 3.03 2.82 2.00 64 August 5.21 3.51 4.83 64 September 4.16 3.12 3.65 64 1. Source: Emett, D.C., Hutchinson, D.D., Jonson, J.A., and O’Hair, K.L., 1994, Water Resources Data for Nevada, Water Year 1993: U.S. Geological Survey - Data Report N V-93-1. 2. Period of record spans October 1,1930 to September 30,1993, inclusive. The U.S. Geological Survey defines a Water Year as starting on October 1 and ending on September 30 of the following year. 40 Diversions Portions of the flow of the lower Virgin River are diverted for flood irrigation in Mesquite, Bunkerville and Riverside, Nevada. The approximate location of each diversion is shown on Figure 2. The Mesquite irrigation diversion is located 25 km below the top of the study reach and consists of a diversion structure and a concrete-lined canal with a design-capacity of 1,56 m3/s. The Bunkerville diversion, located 8 km downstream, is designed to convey 1.13m3/s. The Riverside canal, an unlined structure, has a design capacity of 0.25 m3/s. A small diversion structure on Beaver Dam Wash diverts about .03 m3/s for use in Littlefield, Arizona (Woessner and others, 1981). Water Quality - Previous Work Ground Water Springs A number of springs in the lower Virgin River basin have been sampled for stable isotopes and water chemistry in the past as presented in Table 3. The Littlefield Springs have been sampled several times. The Las Vegas Valley Water District (pers. comm., 1993) collected samples from two of the Littlefield Springs for analysis of stable isotopes of oxygen (180) and hydrogen (2H), and the radioisotopes tritium (3H) and carbon isotopes (13C and 41 Table 3: Summary of chemical and isotopic analyses of springs in the lower ______Virgin River study area ______Spring Name Locality Water Type 51sO fiD Source of <%») ( % o ) Data1 Littlefield Springs Littlefield Area Ca S 04 —— 1,2 Petrified Spring Littlefield Area Ca S 04 - - 3 Petrified Spring Littlefield Area — -12.4 -97.0 4 Littlefield Spring Littlefield Area — -12.3 -97.0 4 North Spring Virgin Mts — -12.2 -91.0 4 Juanita Spring Virgin Mts mixed H C 03 -11.6 -87.0 5 Government Virgin Mts - -8.7 -75.0 4 Spring Tule Spring Tule Springs Hills — -10.6 -84.0 4 Snow Spring Tule Springs Hills - -10.0 -79.5 5 Peach Spring East Mormon Mts — -10.4 -76.5 6 Gourd Spring East Mormon Mts — -10.6 -77.5 6 Garden Spring Clover Mts Ca H C 03 — — 7 Sheep Spring Clover Mts Ca H C 03 -12.0 -87.0 5 North Ella Spring Clover Mts — -11.6 -86.5 5 1. Stable isotopes are reported as the per mil deviation of the ratio of heavy to light isotopes in the sampie to that of the standard (standard mean ocean water). 2. Sources of Data: 1. Trudeau,1979 2. Hardman and Miller, 1934 3. Glancy and Van Denburgh, 1969 4. Las Vegas Valley Water District, unpublished data 5. Thomas, J.M., Lyles, B.F., and Carpenter, L.A., 1991 6. Thomas, J.M., and Welch, A.H., in press 7. Welch, A.H., and Williams, R.P., 1987 42 14C). The stable isotope results are provided on Table 3 and show that the two springs have similar composition. The radioisotope data for the springs can be used to estimate an age of recharge for the spring water. The combination of the three isotopes yields an estimated date of recharge of between several hundred and several thousand years before present. Trudeau (1979) also sampled the springs for tritium and estimated a minimum age of recharge of 22 years. Trudeau (1979) also sampled the major ion chemistry of the springs and found that they were of a calcium sulfate composition with high concentrations of dissolved solids (greater than 2500 mg/L). Trudeau’s analytical results were similar to those of previous investigators (Hardman and Miller, 1934; Glancy and Van Denburgh , 1969). Three springs in the Virgin Mountains were sampled for stable isotopes by earlier investigators (Thomas and others, 1991; Las Vegas Valley Water District, pers. comm., 1993). Stable isotope values ranged from -12.2 to -8.7%o in 6180 and from -91 to -75%o in 5D. One of the springs, Juanita Spring, was also sampled for major ion chemistry and had a mixed cation, bicarbonate composition. Glancy and Van Denburgh (1969) also sampled a few springs in the Virgin Mountains for major ion analysis (not shown on Table 3) and the springs were all mixed cation, bicarbonate waters with low dissolved solids concentrations (less than 500 mg/L). Stable isotopic analyses were available for four springs in the central 43 part of the lower Virgin River study area ((Tule and Snow springs in the Tule Springs Hills and Peach and Gourd Springs in the East Mormon Mountains). The isotopic compositions were similar, ranging from -10.6 to -10.0%o in 5 180 and -84 to -76.5%o in 5D. Two springs in the Clover Mountains (Sheep and North Ella) were sampled for stable isotopes and had isotopic values that were -12 and -11.6 %o in 5180 and -87.0 and -86.5%o in 5D. Sheep Spring and a third spring in the Clover Mountains, Garden Spring, were also sampled for major ion chemistry; both springs were of a calcium, bicarbonate composition. Wells Information on the major ion composition of ground water from wells was assembled from Glancy and Van Denburgh (1969), and Woessner and others (1981), Welch and Williams (1987), and Thomas and others (1991). Available chemical data are summarized in Appendix B. Wells previously sampled include wells along the axis of the Virgin River and a few in the northern part of the study area. All wells included in Appendix B were drilled into alluvium. The major ion composition of wells sampled along the axis of the Virgin River ranged from a sodium sulfate water to a mixed- cation, sulfate water. In general, well water in the floodplain of the Virgin River (YA) tended to be the mixed-cation, sulfate type while water from wells in older alluvium (OA) above the floodplain had a sodium to mixed cation, sulfate to 44 sulfate plus chloride composition. Concentration of dissolved solids (as measured by specific electrical conductance) in the wells along the Virgin River ranged from less than 1000 to greater than 3000 microsiemens per centimeter (uS/cm). Surface Water Surface water chemical data is available for the Virgin River from the following sources: miscellaneous sites sampled by Hardman and Miller (1934); USGS data from the Littlefield gage site from 1950 to the present and another gage operated in the 1970's on the Virgin River above Halfway Wash; and five sites sampled by Woessner and others (1981) on the Virgin River. The lower Virgin River is of a calcium to mixed cation, sulfate plus chloride composition. U.S. Geological Survey data from 40 years (USGS, 1965-1993; USGS QWDATA database, 1994) demonstrate that the basic composition of the water in the river has not changed much in that time, as shown on Figure 7, trilinear diagrams showing percentages of the major ions in samples from each of the sites (447 from Littlefield and 52 from above Halfway Wash). Median specific electrical conductance (EC) values at the Littlefield gage during this period range from 2600 uS/cm in the month of February to 3300 uS/cm in the month of August. <$p *h 100 (j S" ■ ifr % <£> 'b 4 « CALCIUM PERCENT CHLORDE Virgin River at USGS at Littlefield, Arizona (1952-1993) P P d3 $> c>y.ciuM PERCENT CHLORIDE Virgin River above Halfway Wash, Nevada (1978-1986) Figure 7: Trilinear diagram of relative ion percentages showing samples collected by the U.S. Geological Survey at two sites on the lower Virgin River CHAPTER 3 METHODS Physical Methods Approach The basic approach of the sampling plan was to sample both the river and its potential sources. Because the river was the focus of the study, it was sampled along the entire reach in several different flow conditions. The goal in sampling the river was to determine where changes in discharge or chemical quality occur along the reach. The goal in sampling the potential sources of recharge or discharge was to determine if the sources of the river could be discerned based on isotopic and chemical properties. Rationale Surface water sampling locations on the Virgin River and Beaver Dam Wash were chosen based on proximity to known or suspected sources of water (e.g. springs, seeps and tributaries) and accessibility. Plate 1 shows surface water sampling locations. Table 4 lists the sampling rationale for each Virgin River and Beaver Dam Wash surface water site. The Virgin River and Beaver 46 47 Table 4: Surface water samplinc rationale Km from Site Sampling Location Top of Rationale Name Reach VIRGIN RIVER MAIN STEM Virgin River at Virgin River Gorge SW1 0 Top of reach to be studied Rest Area Below Virgin River Gorge, above SW2 19.5 Evaluate flow and springs Beaver Dam Wash, below gorge Below Beaver Dam Wash SW3 19.8 Evaluate discharge and chemical effects of wash USGS Gage at Littlefield, Arizona SW4 20.9 Below Littlefield Springs at location that has continuous discharge data Below springs from Littlefield SW5 22.5 Assess chemical effects of Formation Littlefield Springs Near Big Bend Wash SW6 32.2 Look for changes in relatively undisturbed (no surface inflows or diversions) reach Above Mesquite Diversion SW7 35.4 Above irrigation diversions; look for changes along Littlefield reach At former USGS gage site at SW8 45.1 Close to wells sampled Bunkerville Bridge At USGS gage below Riverside SW9 59.5 Continuous discharge data Bridge At Meadow Valley Farms SW10 64.4 Monitor discharge and chemical changes below diversions About 2 km above Halfway Wash SW11 70.8 Continue monitoring at USGS site discharge and chemical changes Above Lake Mead near Duck SW12 77.2 Above Lake Mead Club . . TRIBUTARIES Beaver Dam Wash at confluence TR1 19.8 Only perennial tributary with Virgin River Nickel Creek TR4 60 Flows during flash floods Unnamed wash TR5 65 Flows during flash floods 48 Dam Wash sites were sampled on seven occasions between June 1992 and October 1993. The June 1992, July and October 1993 sampling events were performed to characterize the isotopic composition of the lower Virgin River during low-discharge conditions. The January, March and June of 1993 samples were collected to determine how winter and spring discharge differed from low-flow conditions. Discharge measurements were made concurrently with sampling when discharge conditions allowed (June 1992, July and October 1993). Other information collected in the field included pH, EC, and temperature. Laboratory analyses included stable isotopes of oxygen and hydrogen, and major ions (calcium, magnesium, sodium, potassium, sulfate, chloride, bicarbonate, nitrate and silica). Not all field and laboratory analyses were performed at each site in each sampling round. Table 5 summarizes the field measurements and samples collected during each sampling event. Three ephemeral streams in the drainage basin (TR-2, TR-3 and TR-4) were sampled one time during the spring and summer of 1993. Two ephemeral tributaries (TR-4 and TR-5) were sampled during a flash flood event. Springs and wells were sampled once. Methods Discharge Measurements Surface water discharge was measured at each sampling site on the Virgin River and Beaver Dam Wash during low flow events. Discharge 49 Table 5: Summary of surface wafer samples collected Sampling Location Site Data Collected1 ID Jun Oct Jan Mar Jun Jul Oct Virgin River at Virgin SW1 FQI FQGI FGI FGI FI FQGI FQGI River Gorge rest area Below Virgin River Gorge SW2 — — FGIFGI — FQGI — Below Beaver Dam SW3 - - FGI FI —— - Wash USGS gage at SW4 - FQGIFGI FGI FI FQGIFQGI Littlefield ,AZ Below Littlefield Springs SW5 FQI — FGI FI — FQGI — Near Big Bend Wash SW6 — — FGI FI — FQGI — Above Mesquite SW7 — FGI FGIFI — FQGIFGI diversion At Bunkerville Bridge SW8 FQI FQGI FGI FI — FQGI — Below Riverside Bridge SW9 FQI FQGIFGI FGI FI FQGI FQGI at USGS gage At Meadow Valley Farms SW10 -— FGI FI — FQGI — Above Halfway Wash SW11 FQI FQGI FGI FI FI FQGIFQGI Above Lake Mead, near SW12 — FQGIFGIFGI - — -- duck club Beaver Dam Wash TR1 — FGI FGI FGI - FQGI — 1. F - Field Measurements including pH, temperature and specific electrical conductance Q - Discharge G - Major ions including Ca, Mg, Na, K, Cl, SO*, H C 0 3, NOa and S i0 2 I - Stable isotopes including 180 and Deuterium 2. Two samples collected, one grab sample and one composite was not measured during flooding and seasonal high flows (January, March and June, 1993). Discharge measurements were made by measuring width, depth and velocity at intervals across the stream and summing the resultant discharge (Q) according to Equation 1 (Sauer and Meyer, 1992), shown below. Q - £ ( V 4 XV/) i-i ( 1) Where: Q Total Discharge bi width of im increment d| depth of the ith increment Vj velocity in the i**1 increment N number of increments Each cross-section was divided into approximately 20 increments so that each increment represented approximately five percent of the total discharge. Stream width and interval width were measured using a calibrated line that was extended across the stream, perpendicular to the flow direction. Depth was measured on a calibrated rod. Velocity was measured using a Price AA (for the Virgin River main stem) or a pygmy (Beaver Dam Wash) current meter with a standard rating. Velocity for measurements in water of depth 2.5 ft or less was measured at a point below the water surface that was 0.6 times the total depth. In depths of greater than 2.5 ft, velocity was measured at two points in each interval, at 0.2 and 0.8 times the depth and an average velocity was used. 51 Seepage studies On two occasions, discharge measurements were used to try to determine seepage of ground water to the channel of the lower Virgin River. The reach selected for this experiment was an 11 km reach starting at SW4 (Littlefield stream gage) and ending at SW6 (downstream of Big Bend Wash). This reach was selected because there are no surface water diversions or contributions and thus, gains in flow can be attributed to ground water seepage to the channel. Discharge measurements were made and calculated as described above. To further constrain the discharge estimate made by the standard method, two measurements were made, in succession, at each site. River stage was monitored for the duration of the seepage run from records of stage recorded at the USGS gage at SW4. Water quality sampling Surface water Surface water data was collected on the Virgin River and Beaver Dam Wash on seven occasions between October 1992 and October 1993. See Table 5 for a list of specific samples collected during each sampling event. Surface water samples collected at high flow (October 1992, January and March of 1993) were grab samples collected within 3 m of the bank. At lower flows, samples were collected according to the equal-width-increment (EWI) method of the USGS (Edwards and Glysson, 1988). Samples were composed of discrete, depth-integrated samples collected at equal-width increments across the stream and homogenized in a polyethylene churn splitter. Temperature was measured in the field using a hand-held mercury thermometer with 0.5°C gradations. Specific electrical conductance (EC) was measured with a Fischer Scientific probe with automatic temperature compensation according to an internal thermistor. The pH probe used was an Orion model 250A with automatic temperature compensation and temperature read-out. Meters for both EC and pH were calibrated before and after each measurement with standards that bracketed the expected sample values and were of a similar temperature. Samples collected for cation analysis were filtered through a 0.45 micron filter and preserved with nitric acid. The early samples for anion analysis (October 1992, January and March of 1993) were not filtered due to a miscommunication with the laboratory, while the later ones were filtered. Anion samples were preserved by chilling them to 4°C. Samples for stable isotope analysis were collected in 125 ml glass bottles fitted with a polyseal lid. The samples were not filtered. Springs Ground water was sampled at a variety of locations in the basin. Emphasis was placed on sampling near the river when possible and sampling possible sources of local recharge such as springs. To this end, 21 springs 53 and 29 wells were sampled between November 1992 and February 1994. Field data was collected from twenty-one springs in the Virgin River drainage basin. Spring samples were collected from three regions of the basin, the Virgin River Gorge (VG), the Virgin River Valley (W ) and the Virgin Mountains (VM). Samples were collected for major ion analysis from thirteen of the springs and for stable-isotope analysis from seventeen of the springs. In collecting spring samples, every effort was made to find (and obtain the sample from) the orifice of the spring. This technique was not possible in many of the Virgin Mountain Springs (those designated VM) as most of the springs are developed and sometimes piped great distances for stock watering. In situations where sampling from the origin was not possible, the sample was collected from flowing water. In general, procedures for the collection of field data and collection and preparation of samples for laboratory analysis are identical to those described above for the collection of surface water samples. Due to the remote locations of many of the springs, sample preparation such as filtration and addition of preservatives was in most cases done several hours after collection. Samples for major ion analysis collected before April 1993 were not filtered, while those collected after that time were filtered. Cation samples were preserved with nitric acid and anion samples were preserved by chilling to 4°C. Field measurements (temperature, EC and pH) were performed in the field at the time of collection. 54 Wells Municipal production wells sampled were purged at least 30 minutes prior to sampling unless the well was in use in which case the amount of purging was minimal. Samples from domestic and community (neighborhood wells shared by two or more homes) wells were collected by running outside taps until the well started pumping and then sampling from the well head. Field measurements of temperature, EC and pH in ground water were performed using the same equipment as for surface water. Samples for laboratory analysis were collected directly from the wellhead or from the closest tap to the wellhead. Samples for stable isotope analysis were collected in the same manner as were surface water samples. Flash flood sampling on tributaries Flash flood sampling of ephemeral tributaries was performed to determine the nature of flash flood waters originating in the basin. Concurrent precipitation sampling was performed to determine if flash flood waters were composed of storm precipitation or local ground water. Flash flood water collectors were placed in two ephemeral tributaries at the downstream end of the study reach. Precipitation collectors were set up at four locations upstream of the collectors in the tributaries. Each flood water collector was constructed of a 1L glass bottle with a 2.2 cm diameter pvc coupling taped to the opening to 55 extend it approximately five cm above the bottle. Seventy-five to one hundred mL of mineral oil was poured into the bottle to prevent sample evaporation. A spherical sponge approximately 3 cm in diameter was also placed in the bottle and coated with mineral oil. The sponge was intended to prevent overflow of the sample and to seal the bottle with mineral oil to prevent evaporation. The collectors were buried in the active channel of each wash so that only the top centimeter of pvc coupling was above ground. Two collectors were placed in the channel at each sampling location (FLTR-4 and FLTR-5) shown on Plate 1. Precipitation collectors were constructed of 1L glass bottles into which eight-cm by ten-cm plastic funnels were inserted and secured with duct tape. Seventy-five to one hundred mL of mineral oil was poured into the bottle to prevent sample evaporation. The bottles were buried up to the neck to prevent breakage. Collectors were placed at four sites upstream of the flood water collectors. Each flood collector and precipitation collector was located on a topographic map, and its position recorded with a Sony portable Geographic Positioning System (GPS) receiver. The precipitation collectors were checked when flood events or convective storms were known to have occurred, and approximately once a month, otherwise. When a flash flood occurred in the flash flood study area, the samplers were checked as soon as possible after the event. The flood water collectors were located and unearthed. Sample in the bottle was poured off into a 125 mL glass bottle fitted with a polyseal lid and saved for isotopic analysis. The 56 remaining sample and mineral oil were disposed, and the mineral oil was replaced and the bottle was re-buried. Precipitation collectors were also checked at this time. The amount of precipitation was recorded and the precipitation was poured into a 125 mL glass bottle fitted with polyseal lid and saved for isotopic analysis. The mineral oil was replaced and the bottle was buried again. Analytical Methods Discharge and Related Calculations Discharge was measured as described above and individual discharge measurements were computed as the sum of incremental discharge measurements made in the center of discrete sections across the stream. Before assessments of gain or loss of discharge could be made, some estimate of the uncertainty for individual discharge measurements was needed. The error estimates described below were made for all discharge measurements made during low-flow periods. Error calculations A method of computing error in individual discharge measurements was presented by Sauer and Meyer (1992). Their method quantified the many possible sources of uncertainty in an individual discharge measurement made with standard USGS procedures (as the discharge measurements made for this 57 study were). Errors computed by their method are standard errors at the 68 percent level of significance. Sauer and Meyer identified possible sources of uncertainty as the following: Errors in Cross-Sectional Area - including width and depth errors; Errors in Mean Stream Velocity - including instrument errors, pulsation errors, vertical velocity distribution errors, oblique flow errors, and stream turbulence errors; Errors in Computational Methods - errors inherent in the methods of computing the horizontal and vertical distribution of depth and velocity and flow between two consecutive verticals; and Systematic Errors and Other Uncertainties - random errors in any portion of the measurement, systematic errors caused by improper use or calibration of measuring equipment, general uncertainties resulting from boundary effects, ice, flow obstructions, and wind. The method described by Sauer and Meyer (1992) consists of determining errors attributable to the above described causes and combining them into a root-mean-square error. Their equation is provided below: 58 ( s i • s,2) 2 a2 o«2 0.2 o»2 a2 a2 Sf + *6 * * 6b * + VN N (2) Where: Sq = Standard Error for individual discharge measurement Sd = Depth measurement errors St = Pulsation errors Sj = Instrument Errors Ss = Vertical distribution of velocity errors Ssb = Systematic errors in width measurement Sstj = Systematic errors in depth measurement Ssv = Systematic errors in velocity measurement Inherent in the use of this equation are the assumptions that the error terms independent of each other and the stream cross-section is uniform enough to allow the use of average depths and velocities across a section (Sauer and Meyer, 1992). Methods for estimating each of the individual errors listed above are provided in Appendix C. Stable Isotope Analysis Stable isotope analysis for oxygen-18 and deuterium was performed at the Environmental Isotope Laboratory of the Water Resources Center of the Desert Research Institute in Las Vegas, Nevada. Oxygen isotope ratios were determined by the quantitative conversion of a 10-microliter aliquot of sample to C 02 gas using guanidine hydrochloride (Dugan and others, 1985). The C02 gas was then directly injected into a 59 Finnigan-Mat delta E mass spectrometer. Hydrogen isotope ratios were determined by the quantitative conversion of a 5-microliter aliquot of sample to hydrogen gas using zinc as the reducing agent (Kendall and Coplen, 1985). The hydrogen gas was then introduced into a mass spectrometer. All results are reported in the standard delta notation as a per mil (%o) variation from standard mean ocean water (SMOW): 5D 5 180 - p . -iooo Standard (3 ) Where Rsampie and Rstandard are the deuterium to hydrogen-1 and oxygen-18 to oxygen-16 ratios in the sample and standard, respectively. The precision for S180 analysis is ± 0.2 %o while that for 6 D analysis is ±1%o. Chemical Analysis Chemical analysis for major ions was performed at the Water Analysis Laboratory of the Water Resources Center of the Desert Research Institute in Reno, Nevada. Specific methods of analysis for each of the constituents are listed on Table 6. 60 Table 6: Laboratory methods used for major ion analyses ANALYTE ANALYTICAL METHOD NOTES Alkalinity electrometric titration, automated 1 Chloride Colorimetric, automated ferricyanide 2 Sulfate Ion Chromatography 2 Sodium Atomic Absorption, direct aspiration 2 Potassium Atomic Absorption, direct aspiration 2 Calcium Atomic Absorption, direct aspiration 2 Magnesium Atomic Absorption, direct aspiration 2 Silica colorimetric, molybdate blue, automated 1 Nitrate Colorimetric, automated, cadmium reduction 2 Notes: 1. Techniques of Water-Resources Investigations of the United States Geological Survey, Methods for the Determination of Inorganic Substances in Water and Fluvial Sediments. Book 5, Chapter A im 1979, Editors, Marvin W Skougstad, Marvin J. Fishman, Linda C. Friedman, David E, Erdmann, and Saundra S. Duncan. U.S. Government Printing Office, Washington D.C. 20402, pages 519, 520. 2. Methods for Chemical Analysis of Water and Wastes, EPA-600/4-79-020 March 1979, Environmental Monitoring and Support Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati, Ohio 45268, Method 325.1. CHAPTER 4 RESULTS Surface Water Discharge Discharge During Sampling Events Discharge was measured at each sampling site during each sampling event when possible. Measured discharge for each sampling event is shown on Table 7. Due to flood conditions, discharge was not measured during the January, March and June of 1993 sampling events. For these events USGS discharge measurements and estimates derived from stage-discharge relationships at USGS gages in Littlefield and Riverside are included to illustrate river stage at the time of sampling. The greatest discharge during sampling occurred during March of 1993 when the discharge at the USGS gage at Littlefield, Arizona (SW4) was 50.2 m3/s {Emett and others, 1993). The least discharge occurred during the summer months, with values at Littlefield of 1.53 m3/s in June 1992 and 3.37 m3/s in July of 1993. In addition to providing a measure of the river stage at the time of sampling, discharge measurements at successive intervals downstream can provide information about river gains and losses. In this study, sufficient 61 62 Table 7: Discharg e Data During Sampling Events SITE Jun-921 Oct-922 Jan-933 $i/lar-93 Jun-935 Jul-936 pct-93 m3/s SW1 0.10 3.91 11.40 NM NM 1.74 1.27 SW2 NS NSNMNMNS 2,90 NS SW3 NS NS NM NM NS NS NS SW4 NS 4.41 7.42 8 5 0 .2 3 9 12.908 3.37 3.00 SW5 1.53 NS NM NMN 3.20 NS SW6 NS NS NM NMNS 3.32 NS SW 7 NS NMNMNMNS 2.86 NM SW8 0.82 3.06 NM NMNS 1.76 NS SW9 0.47 5.15 7.47 8 31.108 10.96 8 1.23 1.14 SW10 NS NS NM NMNS 1.41 NS SW11 0.41 3.93 11.15 NM NM 1.35 1.36 SW 12 NS 0.68 NMNMNSNS NS BDWNS NM 0.276 2.193 NS 0.200 NS NOTES: 1. Low flow; river stage steady 2. Measurements made over period of 4 days; river stage fluctuation of over 3 feet during that period. 3. Measurements made over period of 7 days; stage fluctuated, and was flooding during that period. 4. Both Virgin River mainstem and Beaver Dam Wash flooding. 5. River flooding, no measurements made, abbreviated sample run 6. Low flow, river stage steady. 7. Low flow, river stage steady, abbreviated sample run 8. Discharge estimate by USGS, using stage-discharge relationship. 9. Discharge measurement by USGS. NM - Sample collected but no measurement made. NS - Not sampled or measured. BDW - Beaver Dam Wash 63 information was only available to estimate gains and losses under low-flow conditions. For each low-flow measurement, a standard measurement error to a 68% level of significance was calculated. This error estimate can be used to determine if a change in discharge calculated between two measuring points is actually a gain or loss in discharge, or if it is error due to the method of measurement or conditions under which a measurement was taken. Discharge measurements for the June 1992, July 1993 and October 1993 sampling events, the calculated standard error and one standard deviation for each are listed on Table 8. Discharge measurements made for the October 1992 sampling event were made before (SW4, 8, 9, 11 and 12) and during (at SW1) a flood, so comparisons for the purpose of estimates of gains and loss are likely not valid, and therefore are not included in Table 8. Error calculation worksheets are included in Appendix C. Plots of discharge against distance downstream from the top of the reach (SW1) for each of the low-flow sampling events are shown as Figure 8. Each discharge measurement is bracketed by one standard deviation calculated from the standard error. The July 1993 measurements are the most complete representation of discharge changes from the top to the bottom of the study reach. In July 1993, there was a 1.63 m3/s increase in flow between SW1 and SW4 located 21 kilometers downstream. During the June 1992 and October 1993 sampling events, similar increases in discharge of 1.43 m3/s (between SW1 and SW5, 64 Table 8: Standard Errors for Discharge Measurements Made During Sampling Events SITEMILES Q 1 Q -E R R 2 DIFFERENCE 3 m3/s ±m 3/s June 1992 SW1 0 0.099 ±0.006 SW5 22.5 1.528 ±0.067 1.429 SW8 45 0.821 ±0.043 -0.708 SW9 59.5 0.467 ±0.023 -0.354 SW11 70.8 0.413 ±0.021 -0.054 July 1993 SW1 0 1.738 ±0.078 SW2 19.5 2.904 ±0.127 1.166 SW 4 20.9 3.368 ±0.152 0.464 SW5 22.5 3.192 ±0.147 -0.175 SW 6 32.2 3.317 ±0.156 0.125 SW7 35.4 2.858 ±0.134 -0.458 SW8 45 1.759 ±0.091 -1.099 SW9 59.5 1.228 ±0.092 -0.531 SW 10 64.4 1.407 ±0.073 0.178 SW11 70.8 1.347 ±0.068 -0.059 BDW 19.8 0.200 ±0.010 October 1993 SW1 0 1.274 ±0.054 SW4 13 3.000 ±0.140 1.726 SW9 37 1.143 ±0.061 -1.856 SW11 44 1.361 ±0.066 0.218 I Notes: 1. Q = discharge in cubic meters per second 2. Q ERR = % standard error of the measurement x Q (from column 3); that is, 68% of the measurements made under similar conditions are expected to differ from the true value by QERR (Sauer and Meyer, 1992) 3. DIFFERENCE = Difference in Q (column 3) from preceding upstream station to downstream station. Positive values indicate gains in flow, negative values, losses. 65 June 1992 October 1992 T I I £ 4 E QJ S’ 3 i CO a 2 I I I 0 20 40 60 80 0 20 40 60 80 Km from Top of Reach (SW1) Km From Top of Reach (SW1) 4 July 1993 October 1993 3 I, u> if) « E E, Q) S’ 2 - XCO CO o o C fl CO z .Z 5 r 1 0 i i i i 0 20 40 60 80 0 20 40 60 80 Km from Top of Reach (SW1) Km From Top of Reach (SW1) Figure 8: Plots of discharge (in m3/s) of the lower Virgin River during low-flow surface-water sampling events. Error bars show calculated standard error to a 68% level of significance. 66 22.5 km downstream) and 1.73 m3/s, respectively, were recorded. In July of 1993, there was a reduction in discharge in excess of 1m3/s from SW7 to SW8. Both the June 1992 and the October 1993 discharge measurements record steady reductions in flow from SW4 to SW9, and in the case of June 1992, to SW11. The July 1993 data includes four discharge measurements made between the Littlefield USGS gage (SW4) and the Mesquite irrigation diversion (below SW7). Changes in discharge between sites in this sub-reach were not greater than measurement error. Discharge During Seepage Studies In addition to measurements made during sampling events, discharge was measured at the top (SW4) and bottom (SW6) of a eleven kilometer reach of river during two seepage runs performed in September of 1993 and February of 1994. Stable isotope samples were also collected at the top and bottom of the study reach. Discharge and calculated standard measurement error are shown below in Table 9. Standard measurement errors ranged from ± 0.14 to 0.16 m3/s in September 1993 and from ±0.51 to ±0.57 m3/s in February 1994. Because two discharge measurements were made at each site, it was necessary to calculate the net change in discharge from upstream (SW4) to downstream (SW6) taking into account all measured values. Calculations of net change in discharge (before measurement error was evaluated) were made by subtracting the 67 Table 9: Results of seepage study on lower Virgin River SITE DISCHARGE STD ERROR STD ERROR 0 18O/QD (m3/s) (%) (m3/s) (%o) SEPTEMBER 1993 SW -4 (a) 3.17 4.52% ±0.14 __ SW -4 (b) 3.04 4.45% ±0.14 _ SW -6 (c) 3.29 4.69% ±0.15 ___ SW -6 (d) 3.34 4.69% ±0.16 ___ Net Change in Ranges from 0.12 m3/s [ SW6(c) - SW4(a] to 0.3 m3/s [SW6(d) - Discharge SW4(b)] FEBRUARY 1994 SW -4(e) 5.49 9.68% ±0.53 SW-4(f) 5.36 9.46% ±0.51 -12.8/-95 SW-6(g) 5.65 10.0% ±0.57 -12.6/-94 Net Change in Ranges from 0.16 [SW6(g)-SW4(e)] to 0.29 [SW6(g) - +0.2/+1.0 Discharge SW4(f). 68 higher value for the upstream site from the lower value for the downstream site to come up with an estimate of lowest possible gain indischarge, and by subtracting the lower value from the upstream site from the higher value from the downstream site to get the greatest possible gain in discharge. The calculated gains in discharge are shown on Table 9. In September 1993, the calculated gain in discharge from SW4 to SW6 was between 0.12 (less than measurement error) and 0.3 m3/s ( twice measurement error). By subtracting an average measurement error (0.15 m3/s) from the calculated gains in discharge, it is estimated that the gain in discharge downstream was between 0 and 0.15 m3/s at this time. An additional factor to be considered in assessing changes in discharge downstream is that the river stage dropped 0.003 m (0.01 ft) between the first and second measurements at SW4 (Littlefield USGS stream gage which has a staff plate to measure river stage). According to the USGS rating curve (a measure of the relationship between river stage and discharge) for the Littlefield gaging station (SW4), a change in river stage of this magnitude is equivalent to a change in discharge of0.06m 3/s (2 ft3/s). No stage information was collected for the downstream measuring site, SW6, so it is difficult to assess whether this site experienced a change in river stage between measurements also. Therefore, because of the variability of measured values and changes in river stage in the duration of the seepage study, the apparent gain in discharge downstream cannot be considered strong evidence of gains in discharge due to ground water inflow. 69 In February 1994, the calculated gain in discharge from SW4 to SW6 was between 0.16 and 0.29 m3/s, both of which are less than measurement error. Because the measured gain in discharge was not greater than measurement error, the apparent gain cannot be considered evidence of ground water seepage into the channel between these two sites at this time. Stable Isotopes Samples for stable isotopic analysis were collected from precipitation, springs, wells and surface water in the lower Virgin River basin. The following sections present the results of those analyses. Precipitation Precipitation and the resultant flash flood water samples were collected from a convective storm that occurred on August 26, 1993. Results of individual sample data including elevation, volume, and isotopic values are provided in Table 10. Locations of the precipitation and flash flood collectors are shown on Plate 1. Precipitation was collected at four locations, and at each location, precipitation from the entire storm was collected. Elevations of the precipitation collectors ranged from 980 to 1 140 meters and between 32 and 95 ml were collected. Values of 510O in the samples ranged from -10.5%o to -7.9%o with a volume-weighted average of -9.3 %<>; 5D values ranged from - 73%o to -58 %0 with a volume-weighted average of -66.3 %o. A volume- 70 Table 10: Stable Isotopes in Precipitation and Flood Water SITE ID ELEVATIONVOLUME 51eO 0D (meters) (mL) %0 %0 PR1 1120 32 -10.5 -73 t o PR2 1140 95 *o -70 PR3 1000 64 -7.9 -58 PR4 980 32 -9.1 -65 AVG —- -9.4 -66.5 VOLUME ***- — -9.3 -66.3 WEIGHTED AVG Flood V\fatet Collectors Nickel Creek 550 -9.3 -68 Unnamed 570 -9.4 -69 Wash 71 weighted average was used to account for differences in amount of precipitation collected at each site. Flash flood waters were collected from two ephemeral tributaries to the Virgin River during the same storm. The collection devices (described in Chapter 3) were in the ephemeral tributary channels for the duration of the runoff event and were full at the time of retrieval. The two tributaries, Nickel Creek and an unnamed wash, originate along the northwestern flank of the Virgin Mountains above Riverside, Nevada. Nickel Creek originates at 1950 meters and is perennial in wet years down to an elevation of approximately 1000 meters. The collector (VRTR4A) on Nickel Creek was located at an elevation of 550 meters. The unnamed wash (VRTR5) originates at 980 meters and the collector (VRTR5C) was located at an elevation of 570 meters. The values of 5180 in flood waters collected from Nickel Creek (VRTR4A) and the unnamed wash (VRTR5C) were -9.3%0 and - 9.4%o, respectively; and values of 6D were -68%o and -69%o, respectively. A plot of 6D versus 5180 for both precipitation and flood waters from the August 26 convective storm is shown on Figure 9. Local ground water (from springs VM-5 and VM-6) are shown on the plot for comparison. The isotopic composition of the floodwaters is intermediate between the compositions of the individual precipitation collectors and is more depleted in heavy isotopes than local ground waters. 72 -55 -60 co E -65 M I ' 70 ro ® -75 ■a E -80 a) o . Q -85 t o -90 -13 -12 11 -10 -9 8 •7 18 5 0 (per mil deviation from SMOW) Legend • Precipitation ■ Flood Water + Local gTound water MWL - Global Meteoric Water Line (Craig, 1961) Figure 9: Plot of 6 and 6D for precipitation and flash-flood waters from a convective storm that occurred on August 26, 1993 in the Virgin Mountains, Nevada 73 Stable Isotopes in Springs The spatial distribution and stable isotopic compositions of individual springs and wells is shown on Plate 2. Twenty-four springs in the lower Virgin River basin were sampled for stable isotopic analysis. The springs were located in four physiographic regions in the study area, the Virgin River gorge (VG), the Virgin River floodplain near Littlefield, Arizona (Virgin Valley or W ), the Virgin Mountains (VM) and the northern part of the study area (TD). The sampled spring locations are shown in Plate 1. The results of stable isotope analyses for all spring samples are shown on Table 11. A plot of 5D versus S180 for all springs sampled is shown on Figure 10. Figure 10 shows that the Virgin Gorge and Virgin Valley springs (collectively known as the Littlefield Springs) have a distinct isotopic signature from the other springs in the study area. This plot also shows that the springs and streams in the Virgin Mountains have a wide range of stable isotope values. Virgin River Gorge Springs Six springs (VG-1 through VG-7) were sampled in the Virgin River gorge, a bedrock narrows through which the Virgin River passes at the upstream end of the study area. Springs sampled in the Virgin River gorge were located adjacent to the river and flowed directly into the river. Samples were collected in October of 1993 when the river stage was low. 74 Table 11: Stable isotopes in lower Virgin River area springs SAMPLEDATESOURCE 5 1sO 5D ID TYPE1 %0 %0 VIRGIN RIVER GORGE SPRINGS VG-1 10/2/93 CARB -12.8 -96 VG-2 10/2/93 CARB -12.9 -96 VG-3 10/2/93 CARB -12.9 -97 VG-4 10/2/93 CARB -13 -97 VG-5 10/2/93 CARB -13 -95 i VG-7 10/2/93 CARB -13.1 CO O) MEAN ■12.95 -96.17 STDEV 0.084 0.84 VIRGIN RIVER VALLEY SPRINGS W -1 10/24/92 CARB/OA -12.6 -96 W-2 1/9/93 CARB/OA -12.6 -95 W - 4 1/9/93 CARB/OA -12.5 -96 W - 5 1/9/93 CARB/OA -12.5 -96 W - 7 1/9/93 CARB/OA -12.6 -96 MEAN ■12.56 -95.8 STDEV 0.055 0.447 VIRGIN MOUNTAIN SPRINGS AND STREAMS VM-1 4/2/93 OA -9.8 -80 VM-1 12/17/93 OA -9.6 -78 VM-2 6/26/93 OA -12.2 -88 VM -3 6/26/93 IG -12 -87 VM -4 6/26/93 IG -12.4 -87 VM -5 8/6/93 OA -9 -75 VM -6 8/21/93 OA -8.8 -73 VM -6 6/21/93 OA -8.7 -75 TR -2 04/02/93 -10.6 -80 TR-3A2 07/31/93 -12.4 -88 TR-3B2 07/31/93 -12.4 -89 T R -3C 2 07/31/93 TR-4 08/06/93 -10.4 -76 MEAN ■10.7 -81.3 STDEV 1.51 6.24 1. CARB - carbonate bedrock or alluvium; AL - alluvium; XL - crystalline bedrock 2. TR -3 samples are from one stream at 3 different sites within 100 feet of each other 75 -70 o 2 -75 (0 E £ -80 c o ro -85 0)> T3 -90 E i— 0) -95 CL o c o m Q CD 10 -100 -14 -13 -12 -11 -10 9 8 5 180 (per mil deviation from SMOW) Legend O Virgin Gorge □ Virgin Valley + Tule Springs Hills A Virgin Mountains (springs and streams) MWL - Global Meteoric Water Line (Craig, 1961) Figure 10: Plot of 8 ^ 0 and 5D for springs and streams in the lower Virgin River area. 76 Values of 6180 of the samples ranged from -13.1%o to -12.8%o, while 5D values ranged from -97%o to -95%o. In general, the downstream springs were more depleted than the upstream springs. Virgin River Valley springs Samples for stable isotopic analysis were collected from five springs (W 1 ,2,4,5 and 7) on the edge of the Virgin River floodplain near Littlefield, Arizona. Values for 5180 ranged from -12.6%o to -12.5%o, and 5D values were - 96%o and -95%o. Virgin Mountain springs and streams Samples for stable isotopic analysis were collected from six springs (VM-1 to VM-6) and three flowing streams (TR-2,3 and 4) in the Virgin Mountains. Values for 6180 in the springs ranged from -12.4%o to -9.0%o and SD values ranged from -89%o to -75%o. Values of stable isotopes in the flowing streams ranged from -12.4 to -10.4%o for 6180 and from -89 to -76%o for SD. Northern Basin One spring in the Tule Springs Hills was sampled. The stable isotopic composition was -10.2 and -78%o in 5180 and 5D, respectively. Stable Isotopes in Wells Samples were collected from twenty-nine wells between November 1992 and February 1994 for this study; well locations are shown on Plate 1. The 77 results of stable isotopic analyses of well samples are shown on Table 12. Results presented in Table 12 are divided into Virgin River basin floodplain alluvium (Younger Alluvium, YA), older alluvium (OA) which includes alluvial fan, and valley fill material and Beaver Dam Wash alluvium (BDW), based on the general location of the well. A plot of 6D - 6180 for all wells is shown on Figure 11. Wells in Older Alluvium (OA-1 to OA-17) showed the greatest range of values: from -14 to -9.9%o in 6180 with an average of -12.4%o, and from -106 to -76%o in 6D with an average o f -94.3%o. Wells in Younger Alluvium (YA-1 to YA-8) ranged from -13.1 to -11.4%o in 6180 with an average of -12.4%o, and from -95 to -90%o in 6D with an average of -93.1%o Four of the wells sampled were located in Beaver Dam Wash alluvium (BDW-1 to BDW-4). The oxygen- isotope composition of the well water ranged from -12.1 to -1 1,4%o in 6 180 with an average of -11.8%o. The stable hydrogen-isotope composition ranged from - 86 to -84%o in 6D with an average of -84.8%o. Figure 11 shows that most of the OA and YA wells plot just to the right of the meteoric water line, while 3 of the 4 BDW wells plot on the meteoric water line. 78 Table 12: Stable isotopes in water from wells in Virgin River study area SAMPLE I.D. DATE 51bO 6D %„ %a Beaver Dam Wash Aquifer BDW-1 6/15/93 -11.4 -84 BDW-2 6/16/93 -12.1 -86 BDW-3 6/16/93 -11.7 -84 BDW-4 6/18/93 -11.8 -85 MEAN -11,9 -85 STDEV 0.21 1 Younger Alluvium YA-1 11/12/92 -12.2 -94 YA-2 4/9/93 -11.4 -90 YA-3 4/9/93 -12.5 -93 YA-4 4/9/93 -12.4 -94 YA-5 4/13/93 -12.1 -91 YA-6 6/17/93 -13.1 -95 YA-7 6/16/93 -12.9 -95 YA-8 2/11/94 -12.3 -93 MEAN -12.4 -93.1 STDEV 0.52 1.81 Older Alluvium .. OA-1 11/12/92 -13.5 -102 OA-2 11/12/92 -13.5 -104 OA-3 11/12/92 -12.8 -96 OA-4 11/12/92 -13.9 -104 OA-4 8/18/94 -13.9 -102 OA-5 11/12/92 -11.9 -92 OA-6 11/12/92 -14 -106 OA-7 11/12/92 -12.1 -93 OA-8 11/12/92 -10.4 -84 OA-9 11/12/92 -13.4 -102 OA-10 3/29/93 -13.2 -99 OA-10 4/1/94 -13.5 -99 OA-11 4/13/93 -12.7 -95 OA-12 4/13/93 -12.8 -101 OA-13 6/5/93 -11.7 -89 OA-14 6/5/93 -11.8 -90 OA-15 6/5/93 -9.9 -76 OA-16 6/5/93 -10.6 -78 OA-17 6/17/93 -12.7 -93 MEAN -12.5 -95 STDEV 1.23 8.65 -70 -75 w -80 c -85 o CO -90 *> □ ■ •oa> jf -100 5 180 (per mil deviation from SM OW ) Legend + Beaver Dam Wash Aquifer Wells ■ Younger Alluvium Wells □ Older Alluvium Wells MWL - Global Meteoric Water Line (Craig, 1961) Figure 11: Plot showing 5180 - 5D composition of water from wells in the lower Virgin River study area. 80 Stable isotopes in surface water The Virgin River was sampled for stable isotopic analysis on seven occasions between June, 1992 and October, 1993. All of the 55 samples collected were analyzed for 5D values, while only 41 were analyzed for 5180. Values of 5180 and 6D for all Virgin River surface water samples are presented in Table 13. Figure 12 is a plot of 5D - 5180 for all surface water samples collected from the Virgin River. The samples range from -13.1%o to -9.7%o, in 5180 and average -12.1%o. The river samples ranged from -95%o to -85%o in 5D with an average of -93%o. Figure 12 shows that samples collected during the January, March and June of 1993 sampling events are the most isotopically depleted in heavy isotopes relative to light isotopes and have the least variation between samples of the same event. Figure 12 also shows that Beaver Dam Wash (TR-1) has a stable isotopic composition that is more enriched in heavy isotopes relative to light isotopes than the lower Virgin River. Although twelve sampling sites were identified, only four sites (SW- 1,4,9, and 11) were sampled during all the sampling events. At SW1 at the top of the study reach, 5180 values ranged from -13.0%o to -9.7%o with a mean of 12.0%o. The values for 5D at SW1 ranged from -95%o to -85%o with a mean of - 92.1%o. 81 r". cn in m *- z oj oi m' s cc UJ m o- T“ r* CO OI T~* CO o- in CO > UJ O) OJN. Tf o in CDCM Q © ro o OI in q O) OI I- T— o d d d d d d d CD o O in o CO CO o O OI o in < o in w CM 0 04 1 - N a) 0>i Co a 01 C\l o i o i t -' CO o o 4->o o O w _QJ CM o - co in o i 0 ) o> J d O) I I I o i o' i o 4->re CO CO i2 iy XI 0 4 CO in co CO O) ° !I r- UJ 3 SUJ P Q a> 5 I § 5 Uj CO CO CO CO CO CO CO CO CO CO CO (/) £/j (/j “ re I- UJ UJ CO CO 00 o CO CM O) O) o> o> oCO o O) o CM O) CO Oi i n CO co a» i n co co CM CO T t o oo i n O) 0)0 0 co ooo> CO CO o> CO O) CO Co UJ CO CO CO 83 -80 -82 CO £ -84 -86 ^c o to -88 o □ '>to •o -90 o o □ E -92 q; S .9 4 Q “= -96 -14 -13 -12 -11 -10 •9 5 180 (per mil deviation from SMOW) Legend O June 1992 □ October 1992 A January 1993 v March 1993 © June 1993 O Juty 1993 + October 1993 e Beaver Dam Wash MWL - Global Meteoric Water Line (Craig, 1961) Figure 12: Plot showing 5 ^ 0 - 5D composition of the lower Virgin River and Beaver Dam Wash during periodic sampling events between June 1992 and October 1993. 84 The 5180 values at SW4, located 21 km downstream from the top of the reach (SW1), vary from -13.0%o to -1 2.1 %o with an average of -12.5%o. The 6D of the river water at SW4 ranged between -95%o and -92%o with an average of 93.7%o. The third station (SW9) located 60 km below the top of the reach (SW1) had 5180 values ranging from -13.1%o to -11,2%o and a mean of -12%o. The 5D values for station SW9 range from -95%o to -88%o and average 91.3%o. Samples collected from station SW11, located 71 km downstream from SW 1, had 5180 values between -12.4%o and -10.7%o with an average of 11,6%o and a standard deviation of 0.8%o. The 5D values of river water collected at SW 11 ranged from -95%o to -87%o with a mean of -90.8%o. Chemistry Chemistry of Springs Twenty-four springs in the lower Virgin River basin were sampled for field parameters and seventeen were analyzed for gross chemistry. The springs were located in three physiographic regions in the study area, the Virgin River gorge (VG), the Virgin River floodplain (W ), and the Virgin Mountains (VM). Spring locations are shown on Plate 1. Gross chemistry data for all springs is provided in Appendix D, and Table 14 lists the mean, standard deviation, minimum and maximum values for each parameter in each group of springs. The chemical data for each spring was also plotted on a trilinear 85 Table 14: Summary of chemical data for springs and streams in the lower ______Virgin River area ______UNITSMEANSTDEV MIN MAX Virgin River Gorge Springs 3 Samples E C 12 uS/cm 3405 97.7241 3300 3570 TEMP12 C 25.3 0.76 24.5 26.5 pH12 6.9 0.28 6.7 7.43 S i02 mg/l 16.0 0.64 15.5 16.7 Cl mg/l 374.7 12.90 364 389 S 0 4 mg/l 1190 34.64 1170 1230 H C 0 3 mg/l 431.3 9.29 425 442 N mg/l 0.3 0.05 0.2 0.3 Ca mg/l 383.3 16.20 373 402 Mg mg/I 115.3 3.06 112 118 Na mg/l 261.7 10.60 252 273 K mg/l 31.3 1.95 29.9 33.5 Virgin River Valley Springs 5 Samples E C 1'2 uS/cm 3620.3 117.21 3510 3850 T E M P 12 C 23 3.23 15.5 25.5 pH1'2 7.1 0.32 6.79 7.74 S i0 2 mg/l 15.9 0.30 15.6 16.4 Cl mg/l 402.4 7.99 393 415 S 0 4 mg/l 1204 23.02 1180 1240 H C 0 3 mg/l 446.6 2.41 444 450 N mg/l 0.21 0.09 0.1 0.4 Ca mg/l 396.4 6.15 391 407 Mg mg/l 117.6 1.34 116 119 Na mg/l 276.2 7.05 271 288 K mg/l 33.86 0.87 33.2 35.3 Virgin Mountain Springs and Streams 9 Samples E C 2 uS/cm 754.7 151.39 530 1063 T E M P 2 C 22.7 5.59 16.5 30 p H 2 7.8 0.65 6.97 8.71 S i02 mg/l 30.4 3.89 24.6 38.1 Cl mg/l 32.4 13.30 16 59.4 S 0 4 mg/l 81.8 44.56 31.8 163 H C 0 3 mg/l 320 59.47 237 425 N mg/l 0.3 0.70 0 2.1 Ca mg/l 64.2 17.93 29.1 85.1 Mg mg/l 29.2 8.78 18.5 45.1 Na mg/l 48.5 22.76 31 94.6 K mg/l 5.8 5.51 2 19.2 1. Includes samples for which only field data were collected 2. Measured in the field 86 diagram and shown as Figure 13. The trilinear diagram of major ion composition shows that the springs and streams in the Virgin Mountains are markedly different from the springs in the Virgin Gorge (VG) and Virgin Valley (W ), while the Virgin Gorge and Virgin Valley springs are very similar in chemical composition. Virgin River Gorge Springs The six spring sample collected in the Virgin River Gorge revealed that the springs had similar chemical and physical properties. Field-measured values of specific electrical conductance, temperature and pH averaged 3405 uS/cm, 25.3°C, and 6.9, respectively. Three of the Virgin Gorge springs (VG- 1,3 and 6) were sampled for major ions and were shown to have a calcium sulfate composition. Virgin River Valley Springs Field measurements performed on eight springs in the Virgin Valley region showed that the springs had similar values for EC, temperature, and pH. Average values for EC, temperature and pH were 3620 uS/cm, 23°C and 7.1, respectively. Major ion analysis of five of the springs (W -1,2,4,5 and 7) showed that they were nearly identical in chemical composition, ail being calcium sulfate type waters. 87 Figure 13: Trilinear diagram major diagram of Trilinear relative ion for percentages the in springs Virgin River Figure 13: lower study area 88 Virgin Mountain Springs and Streams Samples were collected from five springs (VM-1,2,3,4 and 5) and three streams in the Virgin Mountains (TR-2,3 and 4) for field parameters and laboratory analysis of major ions. Specific electrical conductance values of the samples ranged from 530 to 1063 uS/cm, with an average of 755 uS/cm. Sample temperatures ranged from 16°to 30°C with an average of 22.7°C. Values for pH ranged from 6.97 to 8.71 and averaged 7.8. Unlike the other sampled spring waters, bicarbonate was the dominant anion of springs in the Virgin Mountains. The dominant cation varied from calcium near Lime Kiln Canyon (VM-2, VM-3, VM-4, TR-3), to magnesium in Nickel Creek (TR-4), to an even mixture of calcium, magnesium and sodium (TR-2 and VM-1 and VM-5) farther south. Chemistry of Wells Twenty-eight wells in the study area were sampled for major ion analysis. Well locations are shown on Plate 1. The wells are divided into Younger Alluvium (YA), Older Alluvium (OA), and Beaver Dam Wash Alluvium (BDW) based on the type of material the well is screened in as determined from geographic location and well driller’s logs, where available (see Appendix A). Field and chemistry data for each well are provided in Appendix D, and Table 15 shows a summary for each ion and field measurement. Plate 2 shows the spatial distribution of water types and Plate 3 shows the spatial distribution Table 15: Summary of major ion and field data for wells VARIABLE UNITS MEAN STANDARD MIN MAX DEVIATION Younger Alluvium 8 samples . EC1 uS/cm 3436 450.6 3040 4430 TEMP1 C 22.8 1.67 20.5 26 pH1 7.28 0.33 6.9 7.85 Si02 mg/l 25.1 8.0 15 40.2 Cl mg/l 436 88.9 359 634 S04 mg/l 1173 183.4 911 1406 HC03 mg/l 277 52.2 222 369 N mg/l 0.58 1.518 0.01 4.34 Ca mg/l 318 57.7 241 410 Mg mg/l 125 22.1 90.8 152 Na mg/l 310 45.3 249 381 K mg/l 26 3.8 23 33 Older Alluvium 17 samples EC1 uS/cm 1283 844 445 3200 TEMP1 C 24.8 1.7 21.5 27 pH1 7.63 0.30 6.8 7.98 SI02 mg/l 24.4 5.7 17.4 34.4 Cl mg/l 111.6 111.7 13.9 413 S04 mg/l 355.8 319.1 38.5 1030 HC03 mg/l 195.8 56.4 143 366 N mg/l 1.16 1.43 0.01 5.82 Ca mg/l 91,1 74.2 29.7 279 Mg mg/l 44.5 30.3 15.5 122 Na mg/l 117.8 79.4 36 276 K mg/l 8.99 5.72 3.2 21.5 Beaver Pam Wash Alluvium 3 samples EC1 uS/cm 636.7 15.28 620 650 TEMP1 C 20.3 0.58 20 21 pH1 7.5 0.06 7.4 7.5 Si02 mg/l 37.7 2.08 36 40 Cl mg/l 19.3 0.58 19 20 S04 mg/l 99.3 1.15 98 100 HC03 mg/l 245.7 7.23 241 254 N mg/l 0.301 0.0520 0.271 0.361 Ca mg/l 68.7 1.53 67 70 Mg mg/l 20.7 1.15 20 22 Na mg/l 34 1.7 33 36 K mg/l 4.3 0.12 4.2 4.4 * Field Measurements 90 of EC and chloride ion concentration. A trilinear diagram summarizing the chemistry of all wells is shown as Figure 14. Eight of the wells sampled were located in Younger Alluvium. All of the wells (YA-1 to YA-8) had high concentrations of dissolved solids. Average values for EC, temperature and pH were 3436 uS/cm, 23°C and 7.3, respectively. Chloride ion concentrations averaged 436 mg/L. The well waters were a mixed cation, sulfate plus chloride type. Seventeen of the wells sampled were located in Older Alluvium. Older alluvium wells had a wide range of water quality. Average values for EC, temperature and pH were 1283 uS/cm, 25°C and 7.6, respectively. Mean chloride ion concentration was 112 mg/l. Older Alluvium well waters fall into to basic water types: mixed cation, bicarbonate waters (OA-1 ,OA-2,OA-13,14, 15 and 16); and sodium to mixed cation, sulfate waters (OA-3 -12 and OA -17). Water samples from the three wells completed in Beaver Dam Wash alluvium (BDW-1,2 and 3) had similar chemical characteristics. Values for EC, temperature and pH averaged 637 uS/cm, 20.3°C and 7.5, respectively. Waters from all three wells in Beaver Dam Wash alluvium were a calcium bicarbonate type. 91 Figure 14: Trilinear diagram Trilinear major diagram of relative ion for percentages Figure study 14: wells in the River area Virgin lower Chemistry of Surface Water Major ion chemistry samples were collected from the Virgin River and Beaver Dam Wash on five occasions between October 1992 and October 1993. Field and laboratory data for each event are provided in Appendix D. The chemical composition of water in the Virgin River main stem was a mixed cation (calcium, sodium and magnesium), sulfate plus chloride water during all sampling events, as shown on Figure 15, a trilinear diagram. During each sampling event, downstream stations showed greater EC than upstream stations, as shown on Figure 16. The increase in EC downstream was more pronounced during sampling events at lower discharges. Plots of cation and anion variation downstream for the July and October 1993 sampling events (Figure 17) show that five of the eight major ions (S042', Cl", Ca2+, N a\ and Mg2+) increase in concentration with distance downstream. Figure 16 also illustrates that the salinity of the river, as measured by EC, varied widely over the course of the study, ranging from 1100 uS/cm in March of 1993 to 5000 uS/cm in June of 1992. Individual ion concentrations show a similar pattern. Chloride ion concentrations, for example, ranged from 66 to 482 mg/L. The lowest concentrations occurred during high streamflows in March of 1993 and the highest values occurred in October of 1993 during the lowest streamflow conditions sampled. 93 m o o o CO UJ O o o O' LU Q_ O a Figure 15:Trilinear diagram of relative major ion percentages for Virgin River and Beaver Dam and Wash, Beaver major Dam River for of ion diagram percentages Virgin 1992-1993 relative Figure 15:Trilinear 94 E* 6000 _o 5) 5000 o0> c ~ 4000 10/92 3 ■a R 3000 10/93 S 2000 a>o UU o 1000 E 4)o OT 0 0 20 40 60 80 100 Distance from Top of Reach (SW1), in km Figure 16: Plots showing variation in EC of the lower Virgin River as a function of distance downstream (a) JULY 1993 in /bauj l | o u o o u o a flu e n o u j) u a b l/ ( u) U|BJ3U0 U0| UO|JBiJU3OU00 ) iu s b / l ( o noin o in o ” CO ” _ o o CO in o CO a Or l— a: JZ CO 5 .fc q § .c CO w ro c o 0) ca o o E a. o a) E Figure 17: Plots showing variation in major ion composition of the lower Virgin River as a function of distance downstream. Plot (a) shows variation during July 1993 sampling, and (b) shows variation during October 1993 sampling event. 95 Beaver Dam Wash was sampled during the October 1992, January, March and July of 1993 sampling events. A summary of Beaver Dam Wash field and chemistry data is included in Appendix D. Water quality of Beaver Dam Wash was markedly different from that of the Virgin River. Field measurements show values of EC, temperature and pH averaging 682 uS/cm, 19°C, and 8.0, respectively. Major ion analyses indicated a calcium bicarbonate type water. CHAPTER 5 DISCUSSION The purpose of this study was to determine what changes in quantity and quality occur in the lower Virgin River from the top of the reach at the Virgin River Gorge to the bottom of the reach above Lake Mead, and to determine if the changes (if any) were caused by interaction between ground water and surface water. Methods used to evaluate this interaction included measurements of water quantity and chemistry (stable isotopes and major ions) over time and along the study reach. Changes were noted during the study and were evaluated to see if they were the result of ground water/surface water interaction. The first section describes changes in surface water quantity and quality noted during the study. The second section summarizes ground water types in the basin using stable isotopic and chemical tracers. The third section is a conceptual model of ground and surface water in the lower Virgin River area based on data collected for the present study and data presented in earlier investigations. 97 98 Surface Water Surface Water Discharge Discharge Variability with Time Discharge during sampling events ranged from flood discharges in excess of 50 m3/s to discharges of less than 3 m3/s in the summer seasons of 1992 and 1993. Discharge data collected over a 64-year period (1930 to 1993, inclusive) at the USGS stream gage at Littlefield, Arizona was used to make judgments about whether the study period (June 1992 to October 1993) was typical of discharge conditions for the river. Study of the long term, seasonal variation in monthly mean discharge at the Littlefield streamgage shows that the greatest discharges occur in April and May and the lowest discharges occur in July. Figure 18 presents the monthly mean discharge for the 1993 water year (October 1, 1992 to September 30, 1993) compared to the mean monthly discharge for the period of record (1930 to 1993) at the Littlefield gage. Figure 18 shows that discharges for the 1993 water year were above average for all months except December (1992), August and September (1993). Annual runoff during the 1993 water year was 2.5 times the average annual runoff and the majority of that discharge occurred in January - June. Although January - May have the highest monthly mean discharges in any year, discharges for January - May during the 1993 water year were between 2 and 3 times greater than the monthly mean discharges for the period of record. In summary, discharges for the study period were above the 64-year average. 99 1600 45 to to S' 1400 40 cT"* E 35 0 1200 •a' 0 L_CD \ O) to 30 coi - . c 1000 o -Co to \1993 25 to 800 c 20 c to \ 1930-1993 to 0 600 0 15 400 4- 10 c c o 200 5 o M r Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Month Source; Emett, D.C,.Hutchinson, D.D., Jonson, J.A., and O’Hair, K.L., 1994, Water Resources Data for Nevada, Water Year 1993:U.S. Geological Survey-Data Report NV-93-1 Figure 18: Plot of monthly mean discharge for the Virgin River from USGS streamgage at Littlefield Arizona, showing monthly mean discharge for the 1993 Water Year and for the period of record (1930-1993) of the gage. Error bars on the 1930-1993 data points represent ± one standard deviation 100 Discharge Changes With Position Surface water discharge was measured along the study reach during low-flow sampling events and during two seepage runs. Variations in discharge along the study reach are presented in Table 7 and shown graphically on Figure 8. Changes in discharge (in excess of measurement error) measured during sampling events can be attributed to irrigation diversions, and the inflow of Beaver Dam Wash and the Littlefield Springs. The reduction in discharge between sitesSW7 and SW8 is due to diversion of part of the flow for irrigation at the Mesquite diversion located approximately 5 km downstream from site SW7. Below SW7, it was difficult to assess gains or losses because of irrigation diversions and return flows from the Mesquite diversion, as well as the Riverside and Bunkerville irrigation diversions. An increase of 1.5 to 1.7 m3/s between sampling sites SW1 and SW4 can be explained by the inflow of the Littlefield Springs and Beaver Dam Wash. Discharge measurements made during the July 1993 sampling event showed a total gain of 1.63 m3/s, of which 0.2 m3/s (the measured discharge of Beaver Dam Wash) could be attributed to Beaver Dam Wash inflow and the remainder, 1.43 m3/s, to discharge of the Littlefield Springs. The measured gain between SW1 and SW4 in July 1993 is of a similar magnitude to that measured in June 1992 and October 1993. The increase in discharge is also consistent with the work of Trudeau (1979), who estimated that the total discharge of the Littlefield 101 Springs above the Littlefield USGS gage was 1.75 m3/s and the average flow of Beaver Dam Wash was 0.09 m3/s. To further investigate ground water seepage to the channel in the reach of river between the Littlefield Springs and the Mesquite irrigation diversion, detailed discharge measurements were made and stable isotope samples were collected (in February only) at the top and bottom of an 11 km reach that started at the Littlefield gage and ended at SW6, approximately 6 km above the Mesquite diversion. The measurements were made in September and February to look at low-flow-with-evaporation and higher-flow-without- evaporation conditions, respectively. No increase in discharge in excess of measurement error was detected in either seepage run (see Table 9). The result of this seepage investigation was contrary to previous studies (Glancy and Van Denburgh, 1969; Woessner and others, 1981; Burbey and Prudic, 1993), which found evidence of ground water inflow within this reach of the Virgin River. The reason for the discrepancy between the studies may be above-normal discharge (2.5 times the mean annual discharge) during the sampling period of this study. The sheer volume of water present during winter and spring sampling events may have masked subtle changes in discharge caused by ground water inflow at these times. Winter flooding may have also increased bank storage which was slowly released throughout the summer and fall, precluding any true measurement of low flow conditions. 102 Surface Water Quality Water sources contributing to the lower Virgin River include upper basin discharge, the Littlefield Springs, Beaver Dam Wash and, possibly, ground water seepage to the channel. The purpose of looking at isotopic and chemical tracers in surface water over time and along the study reach, was to determine the relative importance of those sources, and in particular, to identify areas where ground water and surface water interact. Stable Isotopes in the Lower Virgin River The conservative nature of stable isotopes limits the reasons why changes in isotopic composition can occur, so that if changes occur, in either space or time, we know that either the sources of water are changing, or some physical process like evaporation is altering the isotopic composition of water in the river. The stable isotopic composition of the river at low stream flow (summer and fall) is more likely to reveal the relative importance of inflow of waters with different isotopic composition. The stable isotopic composition of the river at high streamflow (winter and spring) conditions indicates the nature of storm and seasonal runoff. In addition, the stable isotopic composition of winter and spring discharge is important because surface water discharge to ground water may occur during high stream flows. Figure 19 shows the seasonal differences in deuterium values at six sampling sites on the lower Virgin River and Beaver Dam Wash. The isotopic 103 Q. CO Q. > c >, n>* 0 1 “3 o> o re re ra m* cn o O) W 2 -j to 2 z CO 0) CT> o CO O Date Legend ♦ SW1 • SW4 *•••■ SW7 ♦ SW9 T- T SW11 Beaver Dam W ash Littlefield Springs Figure 19: Plot of variation in 8D over time at five sites on the lower Virgin River. The 5D composition of Beaver Dam Wash over time and the 8D composition of the Littlefield Springs are pictured for comparison. Location of sampling sites is shown on Platel. 104 data illustrated in Figure 19 emphasizes the difference between low discharge, summer and fall isotopic composition and high discharge, winter and spring isotopic composition. During the summer (June and October 1992, and July and October 1993), relative deuterium concentration increased by as much as 8 % o from top to bottom of the reach, while during the winter (January, March and June of 1993), deuterium (and oxygen-18, not shown) concentrations remained essentially constant throughout the study reach. Stable isotopes during summer and fall The stable isotopic composition of the lower Virgin River at any point is a quantity-weighted sum of the isotopic composition of its sources. During low- discharge conditions, the influence of individual sources on the overall isotopic composition of the river is most apparent. Data from this and earlier studies were used to estimate a representative isotopic composition for each of the possible sources. The isotopic composition of upper basin discharge is represented by samples from this study collected at SW1 located at the upstream end of the study reach. The isotopic composition of upper basin discharge (as measured at SW1 and shown on Table 13) ranged from -13.0 to -9.7%o in 5180 and -95 to -85%0 in 6D during the study. Eleven of the Littlefield Springs (VG-1,2,3,4,5 and 7; W - 1,2,4,5 and 7, shown on Table 11) were sampled for stable isotopes during this study and little variation was noted between springs and between spring 105 samples collected at different times. The mean stable isotopic composition of the Littlefield Springs sampled was -12.8%0 in 5180 and -96.0%o in 5D. Four samples were collected from Beaver Dam Wash during the course of this study (TR-1 on Table 13), and the isotopic composition averaged -11.6 and -84.8%o in 5180 and 5D, respectively, with little variation in either isotope. The isotopic composition of ground water from wells in the lower Virgin River Basin varies widely (see Table 12), even among wells located at similar distances from the river such as OA-2 and YA-3, which have 5D values of -104 and -93%o, respectively. So, a representative composition for ground water was not estimated. Figure 20 shows plots of 5180 and 6D in the lower Virgin River during each low flow sampling event. The potential sources are shown on each plot. The plots shown in Figure 20 illustrate that for all four sampling events, upper basin discharge has a more enriched isotopic composition than the average isotopic values of the Littlefield Springs. In addition, at SW4 and SW5, located below the majority of the Littlefield Springs (VG1-7 and W 1-7 on Table 11), the river has an isotopic composition that is more depleted in heavy isotopes than its composition at SW1 as shown in Figure 19. Assuming that the isotopic composition of the Littlefield Springs is relatively constant, the observed depletion in heavy isotopes at SW4 relative to those at SW1, should be directly proportional to the percentage of water that is derived from the springs. Cu cc O (per mil) 5 O (per mil) CO cm 5 0 CO CM o O) ' o ' GO (MOWSuopi.CAapmojj |iiu (MOIAIS (MOIAIS opcococococncnao p c m n c n c p c o c o c © ^ ( O c o o f M ^or t c M o « c o o - ( o e o o < M 5 uiojj 00 uopejAep in □B O COCOCO CM O CO o» O O O O 0 0 CO CO CO CO N CD f T □ B 05 o> N O CN 1 o a. Z> a 5 / c ‘ CL a) c a. fl) Figure 20: Plots of 5^®0 and 5D at sites on the lower Virgin River during four low streamflow sampling events: (a) June 1992; (b) October 1992; (c) July 1993;and (d) October 1993. 106 107 Isotopic mass balance calculations made using discharge and stable isotope data from July 1993 show that the isotopic values observed at SW4 can be accounted for by a combination of Upper Basin (SW1), Littlefield Springs (VG and W-samples), and Beaver Dam Wash (TR-1) discharges. From these calculations, it appears that Beaver Dam Wash also has an influence on the isotopic composition of the river at Littlefield. Although Beaver Dam Wash is volumetrically minor its isotopic composition is enriched relative to the river upstream and the Littlefield Springs. For example, in July 1993, the SD of Beaver Dam Wash was -83%o, while that of the river upstream (at SW1) was -91 %o and the Littlefield Springs were -96%o. The inflow of Beaver Dam Wash, though it was 6 % of the total discharge, resulted in a 1%o enrichment in 6D. Between SW4 and SW7, an area that Woessner and others (1981) identified as a reach that was gaining ground water, more frequent sampling was performed during the July 1993 sampling event to try to detect ground water recharging into the channel. The 5D composition fluctuated between -92 and -91 %0 in the samples collected at SW4, SW5, SW6 and SW7. As described in Chapter 4, the change in the stable isotopic composition was not greater than analytical precision (±1%o). So, no stable isotopic evidence for the inflow of ground water was found. The first irrigation diversion (Mesquite diversion) on the lower Virgin River is located approximately 5 kilometers below SW7, and the next sampling site downstream, SW8, was located below both the Mesquite and Bunkerville irrigation diversions. From SW8 to SW12, the isotopic composition of the river is progressively more enriched in heavy isotopes in all sampling events. In general, the enrichment is greater for 510O than for 6D, following a line that has a lower slope than the global meteoric water line (Craig, 1961 ). Craig (1961) and Dansgaard (1964) identified this type of trend as indicative of evaporation, exclusive of transpiration which does not cause fractionation of the stable isotopes. Given the warm, arid climate of southern Nevada and the irrigation diversions from the Virgin River, evaporation is a likely mechanism for isotopic enrichment in this reach. Stable isotopes in Winter and Spring Stable isotope samples collected in the winter and spring months (January, March and June - the river was still flooding at this time) of 1993 show that river water had a lower concentration of heavy isotopes than in samples collected in the summer and fall and was a uniform isotopic composition at all sampling points within the study reach (see Table 13 and Figure 19). Samples collected in March 1993 were the most depleted in heavy isotopes, having mean 5 180 and SD values o f -13 and -95%o, respectively. In addition, the isotopic concentrations of the March 1993 samples were the least variable with position (5D values of samples collected at 11 of the 12 sites on the Virgin River were -95%o). The isotopic composition of the river during 109 January was also relatively constant, exhibiting isotopic variations for 5180 and 5D that were 0.3 and 3%o, respectively. Figure 19 shows that the isotopic compositions exhibited at all sites during the winter and spring of 1993 (see Table 13) were slightly more enriched than the Littlefield Springs and were the same as the isotopic composition at SW1. Isotopic concentrations sampled at SW1 and shown on Figure 19 indicate that upper basin discharge is the primary source of water during January through June of 1993. Discharge data supports the isotopic evidence that the primary source of flow was from the upper Virgin River basin during this period. Discharges from USGS gages on the Virgin River in both the upper and lower basin indicate that the majority of the discharge at the Littlefield, Arizona USGS gage during the winter and spring sampling events was accounted for at two upper basin gages (at Bloomington and St. George, Utah) immediately upstream of the study reach (ReMillard and others, 1994). Chloride Concentrations in the Virgin River The chloride concentration in surface water is derived from the chloride concentrations of sources recharging the river. Although ionic concentrations are available for seven major ions, chloride is discussed in depth because it is considered to be the most conservative of the major ions within the conditions encountered ; thus, it can be used as an environmental tracer. In addition, because 30 years of chloride data exist from the USGS gage at Littlefield, 110 Arizona, a comparison between historical data and study data can be made to establish whether the chloride concentrations in 1993 water year are typical. Changes in chloride concentration over time Figure 21 is a box and whisker plot showing both the range of chloride values for each month for the thirty years of record at the USGS gage at Littlefield, Arizona (SW4) and the values collected for this study in each month that samples were collected. The chloride concentrations of samples collected during high streamflow of January and March 1993 were among the lowest concentrations ever measured during these two months at the Littlefield gage. The sample collected in July 1993 had a chloride concentration that was lower than 75% of the historical July samples collected. This low concentration may be the result of unusually high streamflow (2 to 3 times the average streamflow during these months) during the preceding winter and spring. During the winter and spring of 1993, chloride-depleted water overtopped the channel banks and infiltrated in the floodplain of the river. Once the river stage receded, this chloride-depleted water drained back into the river. Another possibility is that upper basin discharge was more dilute than in most years. 111 0t/j y o so o'gS® >-< . 05 o gy s gy 8 8 t p££ JZ"cn i i f i i t i i t l l I! g 52 az - j a . I 1 l * * t I I x UJ Sh O O O x x x xxx gg o # jequjsoaQ jequueAON XX ja q o p o o . .O' o jequuejdes 4.sn6nv XX A|np OX X OO aunp ft Adiaj Arizona at Littlefield, River Virgin Lower rvj HJdv qojD(/\j AJDnjqej With historical data (1952-1993) collected collected by the Geological data historical With U.S. (1952-1993) Sun/ey a ■x" Ajonuof Comparison 21: Figure collected concentrations of this chloride for study (1993) u n o •o CO o O/beuu) epuoino 112 Seasonal variation in chloride ion concentration As with stable isotope samples, chloride concentrations varied seasonally. The lowest concentrations occurred during high streamflows in March of 1993 and the highest concentrations occurred in October of 1993 during the lowest discharge conditions sampled. Figure 22 is a plot of chloride concentration against discharge for samples collected during this study as well as samples collected by the USGS at Littlefield, Arizona from 1953 to 1993. This plot shows that there is an inverse relationship between chloride concentration and discharge. In the Virgin River system, higher flows are related to either snowmelt or flash floods, both of which tend to be more dilute than the river water, and this relationship is expected. Figure 23 shows chloride variation over the course of the study at selected sampling points. The most variation in chloride concentration between sampling sites occurred during periods of low flow in the summer and fall. The least variation in chloride concentration occurred in periods of high streamflow, especially in March of 1993. This pattern is similar to that seen in stable isotopes and can be explained as follows. During high streamflow, the river changes little from site to site because the volume entering the lower Virgin River basin from upstream is much greater than any inflow that occurs within the basin. 113 600 — J g 500 c •M.2 400 1_(0 o£ 300 c o o 200 d) •a i_ _o 100 -C O 0 10 20 30 40 50 60 70 80 Discharge (m Is) 600 2 400 ^ 200 o 100 0 10 20 30 40 50 60 70 80 Discharge (m3/s) Figure 22: Plots of variation in chloride concentration with Virgin River discharge for (a) samples collected at all sites for the present study; and (b) samples collected at Littlefield, Arizona by the U.S. Geological Survey between 1953 and 1993 (USGS QWDATA database, 1994) 114 500 _J "5) 400 E_ c. ,o ’■ J 3 300 u.CD c O(D 200 C o O 100 *oCD 'i_ V o SI 0 o u O ° DATE ° Legend —t— SW1 SW4 - D- SW7 SW9 — Figure 23: Plot of variation in chloride concentration over time at five sites on the lower Virgin River. The chloride concentration of Beaver Dam Wash over time and the average chloride concentration of the Littlefield Springs sampled are shown for comparison. Sampling locations shown on Plate 1. 115 Chloride variation with distance downstream The variation in chloride concentration with distance downstream from the top of the reach during each sampling event is illustrated in Figure 24. In general, downstream samples have greater concentrations of chloride than upstream samples. Samples collected in July of 1993 show an increase in chloride ion concentration from the top to the bottom of the reach. Samples collected in October of 1993 show an overall increase in chloride concentration from the top to the bottom of the reach, but between sampling sites, there are decreases from upstream sites to downstream sites. For example, in October of 1993, the chloride concentration dropped from 448 mg/L at SW1 to 373 mg/L at SW4, 21 kilometers downstream. The decrease in chloride concentration was accompanied by an increase in discharge of 1.73 m3/s that can be attributed to the inflow of the Littlefield Springs (1.65 m3/s) and Beaver Dam Wash (0.08 m3/s). Chloride ion concentrations of the eight Littlefield Springs sampled averaged 392 mg/L; and concentrations for Beaver Dam Wash averaged approximately 30 mg/L. Chloride values for the Littlefield Springs agree with published values by Trudeau (1979). The mass balance equation below can be used to determine if what we know about the sources of input above SW4 can explain the decrease in chloride ion concentration. 116 600 t 500 E, .1 400 '*-3 +*E o5 300 c o « 200 XJ o .c 100 o 0 20 40 60 80 100 Distance from top of reach (SW1) in km Legend * —■ October 1992 A—A January 1993 f i - f i March 1993 July 1993 V - V October 1993 Figure 24: Plot showing variation in chloride concentration in the lower Virgin River with distance downstream from the top of the study reach (SW1) for each sampling event. 117 Q4C4 ■ Q ici * Q/Pir * Qbcb (4) where: Q4 = discharge at SW4 c4 = chloride ion concentration at SW4 Q! = discharge at SW1 c, = chloride ion concentration at SW1 Q,f = discharge of the Littlefield Springs above SW4 clf = chloride ion concentration of Littlefield Springs Qb = discharge of Beaver Dam Wash cb = chloride ion concentration of Beaver Dam Wash Solving the equation for c4 yields a chloride concentration of 405 mg/L. Since the observed concentration at SW4 (373 mg/L) is less than the calculated value by more than 30 mg/L, and chloride is considered to be essentially conservative in the setting described, either the chlorides in the river were diluted by a source that has not been accounted for or some source of uncertainty in the numbers used in the mass-balance equation is causing the discrepancy between observed and calculated values. One possible source of error in the mass balance is the estimate of discharge of the Littlefield Springs, which was made by calculating the 118 difference when all other sources of water (SW1, SW4 and Beaver Dam Wash) had been accounted. Errors in discharge measurements could cause miscalculations of the amount of discharge contributed by the springs. The discharge estimate for Beaver Dam Wash was taken from USGS daily discharge estimates (ReMillard and others, 1994) from a stream gage located on Beaver Dam Wash 2 kilometers above the confluence with the Virgin River. The USGS classified discharge estimates from this gage as poor record, meaning that 95% of the daily discharge values were greater than 15 percent from their true value (Emett and others, 1994). Standard errors calculated for the discharge measurements made at SW1 and SW4 may account for differences of .05 and .14 m3/s, respectively. Of the possible errors in discharge measurements, errors in the measurements of Beaver Dam Wash are considered the most important because the chloride ion concentration of Beaver Dam Wash is less than ten percent that of any other source. To address uncertainties in the volume of water contributed by Beaver Dam Wash, Equation 4 can be rearranged to solve for Qb, and Qlf, the discharge of the Littlefield Springs, can be expressed as Qif— Q4 - Qi ~ Qb and using discharge values for October 1993, Q,f = 1.73 - Qb Then, Equation 4 can be rearranged as follows: 119 q _ Q4 C4 - Q^., - 1.73c lf ( ° t - c ,t ) (5) Using chloride concentration and discharge data from October 1993, the discharge of Beaver Dam Wash required to produce the chloride concentration seen at SW4 is 0.35 m3/s, approximately four times the original estimate. This mass balance calculation suggests that larger volumes of streamflow than originally anticipated, or ground water inflow from the Beaver Dam Wash floodplain aquifer could be a source of low chloride water. The dilution of Virgin River water observed in October of 1992 and 1993 (but not in July of 1993) may be the result of increasing amounts of water entering the river (as ground or surface water) in October because less water is used for irrigation in the Beaver Dam Wash valley. Summary of Surface Water Quality Data A summary of stable isotopic and chloride ion tracers in surface water is shown on Figures 25 and 26. Stable isotopic concentrations in the river respond to the inflow of the Littlefield springs which have a more depleted isotopic composition than upper basin discharge at low flows; while chloride concentrations in the river were diluted by smaller amounts of low concentration water from Beaver Dam Wash. At low flows, the inflow of 120 (|iiu jad) tunuajnaa 9 D) 00 E, o o c CO o (J>O) ^ C L c o u0) _ O h c oo a) T i o .c O —I-----1-----1-----1 to o o o o o r-. co a> o (i;ui jad) iunud)ndc 9 o Cl E, 0 □ e in ££ 0 1 c oil) c o O !5a) o of CO CM t - (l/Giu) uoijBJjuaouoo apuomo (l!lu jad) wnuajnaa 9 Figure 25: Figure showing Plots variation (a) the both in chloride concentration Virgin the lower in and 5D from the top of River the reach study (SW1) for October and October the 1992 relationship (b) 1993; for chloride between several and 5D sampling sites the Virgin on lower River October and in 1992 October Sampling 1993. locations shown on Plate 1. 121 E a> 4 00 a. E .2 a> 3 a> Q ."5 340 to t— i— i— i— t— i— r 0 10 20 30 40 50 60 70 80 Kilometers from Top of Reach (SW1) -86 = -87 — SW11 — SW9 koiF -88 0> a . -89 — SW 10 | -90 <5 -91 — SW1 SW7 SVBWB 3 0) -92 — SWB ^ -93 — SW2 -94 I I I I I (b ) 320 340 360 380 400 420 440 Chloride Concentration (mg/L) Figure 26: Plots showing (a) the variation in both chloride concentration and 5D in the lower Virgin River along the study reach for July 1993; (b) the relationship between chloride and 5D for several sampling sites on the lower Virgin River in July 1993. Sampling sites are shown on Plate 1. 122 even a small amount of isotopically enriched Beaver Dam Wash water was detectable, but it only changed the isotopic composition of the river by about 1%o. In general, both tracers are affected by evaporation below the Mesquite diversion; stable isotopic composition becomes more enriched in the heavy isotope and chloride concentration increases. As with discharge measurements, inflow of ground water to the channel below the Littlefield Springs and Beaver Dam Wash was not evident from environmental tracer data. Ground Water Springs Ground water sampled for this study includes springs in the mountains and hills surrounding the study area and wells in unconsolidated material within the basin. Most of the springs and streams sampled were from the Virgin Mountains in the southern part of the study area. Beaver Dam Wash is included in the discussion of springs as representative of recharge to the northeastern part of the basin. Stable Isotopes in Springs Figure 27 shows S180 and 5D values for springs and streams in the study area including both those collected for this study and in previous studies 123 -70 5 S° -75 7 R CO -80 c o > TJ01 -90 E k. cu CL -95 7 7 7 7 7 a W to -100 -14 -13 -12 11 -10 -9 8 5 180 (per mil deviation from SMOW) Legend V Littlefield Springs □ Beaver Dam Wash + Virgin Mountains springs and streams o Northern Basin MWL - Global Meteoric Water Line (Craig, 1961) Figure 27: Plot of 8^®0 and 8D for springs and streams in the lower Virgin River area. Data plotted include samples collected as part of the present study and those collected by previous investigators (primarily samples from the Northern Basin). 124 (see Table 3). Virgin Mountains springs and streams cluster into two distinct groups, one that is characterized by isotopic compositions that plot on the meteoric water line and another group that plots below and to the right of the meteoric water line. Springs in the northern part of the study area can also be described this way. All of the Littlefield Springs, both Virgin Gorge and Virgin Valley springs, had similar isotopic compositions. Six springs and streams (VM-2, VM-3, VM-4, TR-3 [2 samples], Juanita Spring and North Spring) cluster together on or near the meteoric water line. The isotopic values of the springs range in S180 from -12.4 to -11 .6%o and in 6D from -91 to -87%o. The springs are all located in the central part of the Virgin Mountains except for Juanita Spring which is located in the southern Virgin Mountains. The position of the springs on and near the meteoric water line suggests that they have not been significantly affected by evaporation. The range of isotopic values found in these springs and streams is consistent with springs found at similar altitudes elsewhere in the southern Basin and Range province by Thomas and Welch (in press). For example, springs in the southern Spring Mountains which reach elevations of 2600 meters have 5D compositions of -90%o, while mean 5D values for the Sheep Range are -93%0 for elevations ranging from 1700 to 2560 meters, and average deuterium composition of ground water in the Meadow Valley Wash system (adjacent to the west of the northern part of the drainage basin) is -87%o. The remaining seven spring and stream samples collected in the Virgin 125 Mountains were more enriched in heavy isotopes than the samples found in the central Virgin Mountains (discussed in the preceding paragraph). Five samples from three springs in the southern Virgin Mountains (VM-1, VM5, VM-6) showed stable isotopic values that plotted below and to the right of the meteoric water line. Such a position relative to the meteoric water line indicates evaporation at some point in the water cycle (Dansgaard, 1964). All three springs are developed and impounded for stock watering, thus increasing the exposure of the spring water to the atmosphere and the potential for evaporation. While none of the springs was actually sampled from standing water, each may have been exposed to the atmosphere long enough for evaporation to have altered the water and the enrichment in heavy isotopes is evidence of that exposure. The remaining two Virgin Mountains samples are stream samples from Nickel Creek (TR-4) and an unnamed stream located to the northeast of Nickel Creek (TR-3). Both samples plot to the right of the meteoric water line. The stream water may have undergone some evaporation from the water surface, but much less than the developed springs in the southern Virgin Mountains. Seven spring samples from the northern part of the study area, one collected for this study (TD-1) and six collected previously (Peach, Gourd, Tule, Snow, Sheep and North Ella Springs) are also shown on Figure 27. Spring localities include East Mormon Mountains, Tule Springs Hills, and Clover Mountains, but all are labeled as Northern Basin Springs on Figure 27. 126 As indicated on Figure 27, northern basin springs are more enriched than the samples in the central Virgin Mountains, and similar to the spring and stream samples in the southern Virgin mountains. TD-1, Tule, Peach, Gourd and Snow springs, located in the Tule Springs Hills/ East Mormon Mountains area, plotted to the right of the MWL, but in a position that is similar to that of the streams in the southern Virgin Mountains (Nickel Creek and TR-2). Gourd Spring is collected in a shallow cistern that is covered by a board, so it is likely that some evaporation occurs from the water surface. Tule Springs and TD-1 are impounded and developed for stock watering and evaporation is the likely mechanism for enrichment in heavy isotopes. Sheep and North Ella Springs, located in the Clover Mountains have isotopic compositions that are more depleted than any of the other northern basin springs sampled, and, accordingly, they plot closer to the samples from the central Virgin Mountains on Figure 27. Beaver Dam Wash surface water and ground water from the floodplain of Beaver Dam Wash ( Wells BDW-1,2,3 and 4) can be considered typical of recharge to the northeastern part of the study area. On Figure 27, Beaver Dam Wash surface water and wells plot on or near the global meteoric water line and are only slightly (from 2 to 5 per mil) more enriched in heavy isotopes than central Virgin and Clover Mountain waters. The eleven Littlefield Springs sampled in the Virgin River Gorge (VG- 1,2,3,4,5 and 7 on Table 11) and Virgin Valley (W -1,2,4,5 and 7 on Table 11) 127 regions had a mean isotopic composition of -12.8%o in S180 and -96.0%o in 5D with little variation between springs. Chlorides in Springs Plate 3 shows the chloride concentration in ground waters sampled for this study. Chloride concentrations from springs in the mountains surrounding the study area are generally less than 100 mg/L. Virgin Mountain springs have chloride concentrations that are somewhat higher (18 to 59 mg/L) than two springs (Garden and Sheep Springs) in the mountains and hills to the north of the Virgin River that have chloride concentrations of less than 20 mg/L. Surface water samples from Beaver Dam Wash, which drains the northeastern part of the lower Virgin River surface water drainage area, had chloride concentrations ranging from 11 to 34 mg/L. Littlefield Springs (VG and W springs on Table 14 and in Appendix D) had chloride concentrations ranging from 375 (VG springs) to 400 mg/L (W springs). Dissolved solids in springs The average EC value for the Virgin Mountains springs was 756 uS/cm. Samples collected from flowing streams and springs had lower EC values than springs that were impounded. Of the springs in the northern basin, EC values were available for three springs. The two springs located in the Tule Springs/ East Mormon Mountains area (Garden Spring and TD-1) had EC values similar to those in the Virgin Mountains, while Sheep Spring in the Clover Mountains 128 had an EC value of 140 uS/cm. Specific conductance values for Beaver Dam Wash above its confluence with the lower Virgin River averaged 640 uS/cm with little variation. The eight Littlefield Springs sampled (VG and W samples in Appendix D) for this study had EC values that ranged from 3300 to 3800 uS/cm. Virgin Gorge (VG) samples had lower average EC values than Virgin Valley (W ) samples. Water Type of Springs Springs and streams in the Virgin Mountains are calcium to mixed cation, bicarbonate waters. Data from the northern part of the study area shows that Sheep and Garden springs were calcium bicarbonate and calcium, bicarbonate plus sulfate waters, respectively (Welch and Williams, 1986; Thomas and others, 1991). Beaver Dam Wash water is of a calcium bicarbonate composition. The Littlefield Springs (both VG and W springs) have a calcium sulfate composition. Wells Stable Isotopes in Wells Stable isotopic composition of water from wells sampled (presented in Table 12 and Figure 11) has a wide range, from -14.0 to -9.9%o in 5180 and from -106 to -76%o in 6D. Figure 28 is a plot of stable isotopes in well samples. Stable isotopic compositions of potential sources of ground water recharge 129 -75 V) -80 E □ ♦ 2 -85 O£ 0) Q. -100 a to -110 -15 14 -13 -12 11 -10 9 8 8 1sO (per mil deviation from SMOW) Legend ■ Younger Alluvium Wells O Virgin Mts springs ♦ Older Alluvium Wells □ Northern Basin + Beaver Dam Wash Aquifer Wells A Littlefield Springs © Beaver Dam Wash • Virgin River v Upper Basin Discharge M W . - Global Meteoric Water Line (Craig. 1961) Figure 28: Plot 8^®0 and 5D for all ground water sampled in the lower Virgin River study area. 130 including springs in the surrounding mountains, Beaver Dam Wash, and the Virgin River are also shown. In general, wells in floodplain alluvium (BDW and YA wells) have stable isotopic compositions that are similar to the surface water (either Beaver Dam Wash or the Virgin River). Wells in the older alluvium (OA wells) have a wide range of isotopic compositions, but most do not resemble the composition of either the Virgin River or Beaver Dam Wash. The four wells sampled that were located in Beaver Dam Wash alluvium (BDW-1,2,3 and 4) have stable isotopic compositions that are similar to surface water of Beaver Dam Wash which had mean isotopic compositions of -11.6 and -85%o in 5180 and 5D, respectively . Six wells located in younger alluvium of the Virgin River (YA-1,2,3,4,5 and 8) exhibited isotopic compositions that were similar to the average of the lower Virgin River samples collected (-12.1 and - 92.4%o in 61sO and 5D, respectively). All six wells are located in the younger alluvium of the Virgin River floodplain, and all but one (YA-4) for which well completion data is available were completed to depths of less than 200 feet. Four wells (YA-6, YA-7, OA-11 and OA-17) have isotopic compositions that plot between Virgin River water and Littlefield Springs water. All four wells are located near both the river and the Littlefield Springs and may be in a position to (or due to pumping) intercept either water that would otherwise discharge at the springs or Virgin River water or both. The former is more likely for wells located in older alluvium above the floodplain (OA-11 and OA- 17), while a combination of both sources is most likely in the two wells in the 131 floodplain of the Virgin River (YA-6 and YA-7). The most isotopically depleted waters occurred in seven wells (OA-1,2,4 [2 samples],6,9,10 [2 samples] and 12) located south of the Virgin River between the town of Riverside and the Arizona-Nevada state line. Stable isotopic compositions of waters from these wells ranged in 5180 from -13.9 to - 12.8%o and in SD from -106 to -99%o. Each of the wells had screened intervals in either the older alluvium/Muddy Creek of the alluvial fans or in deep older alluvium below younger alluvium of the Virgin River floodplain. A group of four wells in Older Alluvium south of Mesquite, Nevada (OA- 13,14,15 and 16) have isotopic compositions that are the same or more enriched than Virgin Mountain springs and plot on Figure 28 below and to the right of the meteoric water line. Isotopic compositions that are the same or more depleted than a potential source of recharge (precipitation falling on the Virgin Mountains) may indicate any or several of the following: the aquifer was recharged by local recharge, the aquifer was recharged by local recharge that underwent evaporation either before recharge or after pumping from the well, or it was recharged by water from another source that experienced evaporation before recharge. Two of these wells serve as community wells and are attached to large storage tanks. In sampling these wells it was not possible to get a sample directly from the well head and the samples were collected from the tanks. The water sampled from the tanks had been sitting there for an undetermined length of time and evaporation from the tank is likely. 132 Chlorides and Dissolved Solids in Wells Chloride and dissolved solids concentrations in wells show similar patterns and are combined in the following discussion. Complete chloride and EC data for wells are presented in Table 15 and Appendix D. Plate 3 shows the spatial distribution of chloride concentration and EC. Chloride concentrations in wells varied from 14 to 634 mg/L and dissolved solids as measured by EC varied from 445 to 4450 uS/cm. As with stable isotopes, the wells in recent floodplain alluvium (BDW and YA wells) had chemical compositions that were similar to the nearby surface water (Beaver Dam Wash and Virgin River, respectively). Wells in Older Alluvium (OA wells) had a wide range of chloride and EC values, but, with the exception of two wells (OA-11 and OA-17) concentrations of chloride and dissolved solids (EC) were consistently less than wells in Younger Alluvium. Chloride concentration and EC were the lowest and most consistent in wells in the Beaver Dam Wash alluvium (BDW-1,3 and 4). Wells in the Younger Alluvium (YA-1 to YA-8) had the highest concentrations of both chlorides and dissolved solids with chloride concentrations ranging from 372 to 634 mg/L and EC values of 3040 to 4430 uS/cm. Water from wells in the Older Alluvium (OA-1 to OA-17) had a wide range of chloride and dissolved solids concentrations. Chlorides varied from 14 to 413 mg/l and EC varied from 445 to 3200 uS/cm. The wells fall into three 133 groups based on chlorides and dissolved solids. The first group consists of two wells with high chlorides and dissolved solids, similar to wells in Younger Alluvium. The two wells in this group are located in older alluvium near Littlefield, Arizona. The chemical concentrations as well as their stable isotopic compositions are similar to the Littlefield springs and they may consists of some combination of Littlefield Springs water and water from the Older or Younger alluvium aquifers. The remaining wells in Older Alluvium fall into two broad groups: water with low chloride and dissolved solids water and wells with intermediate compositions (relative to all the wells sampled for this study). The low chloride group consists of four wells, two in the deep floodplain (OA-1 and OA-2) and two on the alluvial fan south of Mesquite (OA-15 and OA-16) which had chloride concentrations of less than 40 and EC values of less than 650 uS/cm. The intermediate group consists of the remaining wells in Older Alluvium, eleven wells with chloride concentrations greater than 39 and less than 125 and EC values of between and 600 and 1500. These wells are located in the alluvial fans north and south of Mesquite, Nevada. Water Types in Wells Ground water sampled in the study area can be grouped into three general types: calcium to mixed cation - bicarbonate water, sodium to mixed cation - sulfate water, and calcium to mixed cation - sulfate plus chloride water. 134 Calcium to mixed cation - bicarbonate waters, the least chemically evolved of the waters, are found in wells in the Beaver Dam Wash alluvial aquifer (BDW1.3 and 4), in deep wells in the Virgin River floodplain (OA-1 and OA-2), in four wells located south of the river near the state line(OA-13 to OA-16), and in springs and streams in the Virgin Mountains(VM-1,2,3,4,5 TR-2, TR-3 and TR-4). Calcium to mixed cation, sulfate plus chloride waters are found in lower Virgin River surface water, the Littlefield Springs, wells in younger alluvium in the floodplain of the river (YA-1 toYA-8) and in wells in older alluvium near Littlefield, Arizona. Sodium to mixed cation, sulfate water was sampled from wells in older alluvium north and south of the Virgin River including OA- 3,4,5,6,7,8,9,10 and 12. Ground Water Summary Table 16 provides a summary of the isotopic and chemical tracer data for the wells and springs sampled. As discussed above for individual tracers, wells in the floodplain of both Beaver Dam Wash and the lower Virgin River have isotopic and chemical compositions that are similar to the adjacent surface water bodies. This similarity is the most obvious example of ground water-surface water interaction in the study area; surface water recharging ground water in the floodplain. Of the remaining ground water sampled, springs and streams in the surrounding mountains have consistent Table 16: Ground Water Summary Sample ID 018 D Dominant Dominant Cl EC Cation1 Anion Beaver Dam Wash Aquifer Wells BDW-1 -11.4 -84 Ca HC03 19 650 BDW-2 -12.1 -86 BDW-3 -11.7 -84 Ca H C 03 20 640 BDW-4 -11.8 -85 Ca H C 03 19 620 Younger Alluvium Wells YA-1 -12.2 -94 Na +Ca S04 + Cl 459 3040 YA-2 -11.4 -90 Ca S 0 4 + Cl 634 4430 YA-3 -12.5 -93 Na +Ca S 04 + Cl 446 3220 YA-4 -12.4 -94 Ca +Na S04 + Cl 372 3510 YA-5 -12.1 -91 Ca S 0 4 + Cl 380 3000 YA-6 -13.1 -95 Ca S 0 4 + Cl 390 3350 YA-7 -12.9 -95 S 04 + Cl 450 3580 YA-8 -12.3 -93 Ca S 0 4 + Cl 359 3360 Older Alluvium OA-1 -13.5 -102 Ca H C 03 28.8 630 OA-2 -13.5 -104 Ca +Na HC03 13.9 576 OA-3 -12.8 -96 Ca + Na S 04 75.6 1070 OA-4 -13.9 -104 Ca S04 102 1350 OA-4 -13.9 -102 OA-5 -11.9 -92 Na S 04 104 1000 OA-6 -14 -106 Na S 04 125 2150 OA-7 -12.1 -93 Na + Ca S04 106 1000 OA-8 -10.4 -84 Na S04 39.3 960 OA-9 -13.4 -102 N a S 04 121 1370 +Ca+Mg OA-10 -13.2 -99 Na S 04 129 2520 OA-10 -13.5 -99 OA-11 -12.7 -95 Ca +Na S 0 4 + Cl 413 3200 OA-12 -12.8 -101 Na S 04 77.4 957 OA-13 -11.7 -89 Na HC03+S0 70.6 702 4+CI OA-14 -11.8 -90 Na H C 03 57.4 605 OA-15 -9.9 -76 Ca+Na HC03 35.3 556 OA-16 -10.6 -78 Na+Ca+Mg HC03 29.4 445 OA-17 -12.7 -93 Ca+Na S 04 + Cl 370 2720 136 Table 16: Ground Water Summary Sample ID 018 D Dominant Dominant Cl EC Cation1 Anion SPRINGS ^ VM-1 -9.8 -80 Na+Ca HC03 59 982 VM-1 -9.6 -78 VM-2 -12.2 -88 Ca H C 0 3 45 840 VM -3 -12 -87 Ca H C 0 3 25 673 VM -4 -12.4 -87 Ca H C 0 3 22 724 VM -5 -9 -75 Ca+Na+Mg HCQ3 39 760 VM-6 -8.8 -73 VM -6 -8.7 -75 TR -2 -10.6 -80 Mg+Ca+Na HC03 30 854 TR -3 -12.4 -88 Ca H C 0 3 16 530 TR-4 -10.4 -76 Mg H C 0 3 30 673 W -1 -12.6 -96 Ca S04 + Cl 415 3520 W - 2 -12.6 -95 Ca S04 + Cl 393 3630 W - 4 -12.5 -96 Ca S 0 4 + Cl 401 3600 W - 5 -12.5 -96 Ca S 0 4 + Cl 400 3600 W - 7 -12.6 -96 Ca S 0 4 + Cl 403 3620 VG-1 -12.8 -96 Ca S 0 4 + Cl 371 3300 V G -2 -12.9 -96 V G -3 -12.9 -97 Ca S04 + Cl 389 3470 V G -4 -13 -97 V G -6 -13 -95 Ca S04 + Cl 364 3360 V G -7 -13.1 -96 137 chemical and isotopic compositions, the Littlefield Springs (W and VG springs) have a relatively homogeneous isotopic and chemical composition, and wells in older alluvium have a wide range of chemical and isotopic compositions. Few wells in the older alluvium yield water that resembles the Virgin River, but there are similarities between wells in the older alluvium. Wells in alluvium of the floodplain of Beaver Dam wash resemble Beaver Dam Wash surface water both isotopically and chemically. Previous investigators {Black and Rascona, 1991) concluded that the alluvium of Beaver Dam Wash was a completely separate aquifer that discharged water from the northeastern part of the basin. The stable isotopic and chemical composition of both wells and surface water from Beaver Dam Wash is similar to springs and streams at equal altitudes elsewhere in the region and it is likely that Beaver Dam Wash is recharged within the basin. Ground water from shallow wells in the floodplain aquifer (YA-1 through YA-8) is similar in composition to river water sampled during this study. Figure 29 shows 5D plotted against chloride concentration for all wells and potential sources of recharge (a) and the Virgin River (b). Figure 29(a) shows that all wells in younger alluvium have 5D values of between -95 and -90%o and chloride values between 350 and 450 mg/L, except YA-2, which had a chloride concentration in excess of 600 mg/L. In Figure 29(a), the wells in younger alluvium, along with two wells in Older Alluvium (OA-11 and OA-17) (a) ground water and (b) Virgin River Virgin (b) and for water area ground River (a) Virgin lower the in of 8Dwaters and Chloride 29: Figure 5 5 D (per mil deviation from SMOW) -110 -105 -100 -110 -105 -100 -95 -90 -85 -80 -75 -70 -95 -90 -85 -80 -75 -70 o A one luimWls LittlefieldSprings • Alluvium Younger Wells □ + Virgin River ■ Beaver Dam Wash Dam Beaver ■ River Virgin Older Alluvium Wells * Northern Springs Northern * AlluviumOlder Wells Beaver Dam Wash Wells + Virgin Mountains springsMountains Virgin + Wells Wash Dam Beaver Virgin River Water River Virgin Ground Water Ground I I I L I I I I I J ChlorideConcentration (mg/L) 10 0 30 0 50 0 700 600 500 400 300 200 100 0 10 0 30 0 50 0 700 600 500 400 300 200 ChlorideConcentration 100 (mg/L) 0 Z A “A A A “ AAAZA . ..a Legend A ft? * (b) 139 near Littlefield and the Littlefield Springs, plot separately from all other ground water in the study area. Figure 29(b) plots 5D against chloride concentration for lower Virgin River surface water. The Virgin River samples have a variety of compositions, but generally occupy the same position on Plot 29(b) as wells in Younger Alluvium on Plot 29(a). Four wells located upstream of the confluence Beaver Dam Wash and the Virgin River have stable isotopic and chloride ion concentrations that are similar to both the lower Virgin River and the Littlefield Springs. Two of the wells, however, are drilled into older alluvium north of the Virgin River. Since the Littlefield springs emerge from older alluvium both north and south of the river in this area, it is possible that the wells intercept water that would otherwise discharge at the springs. Aside from the Beaver Dam Wash wells, only a few wells in the basin resemble local recharge sampled in the basin. Four wells in the older alluvium south of Mesquite, Nevada (OA-13,14,15 and 16) have chloride ion concentrations and water types that are similar to local recharge and stable isotopic compositions that are similar to the more evaporated samples of local recharge. For the most part, the remaining wells in the basin are more depleted isotopically than any known source of recharge in the basin. Chemically and spatially, the wells fall into two groups: wells drilled into older alluvium on the alluvial fans north and south of the Virgin River near Mesquite which yield a 140 sodium sulfate water with chloride concentrations between 50 and 150 mg/L (OA-4,6,9,10 and 12); and wells located in the floodplain of the lower Virgin River, but screened far below the bottom of the Virgin River floodplain alluvium aquifer which have a mixed cation bicarbonate water with chlorides of less than 25 mg/L. Conceptual Model of Hydrogeology in the Lower Virgin River Basin Meteoric water in the lower Virgin River basin may be separated into three broad categories based on stable isotopic and chemical compositions, which can be confirmed by occurrence. The first type of water may be considered local water which includes modern local precipitation and recharge. This water has 6D values between -91 and -58 per mil. The second type of water includes the river proper and bank storage ground water in the shallow floodplain as well as the Littlefield Spring water from which much of the river water originates. These waters range between -96 and -85%o in 6D. The third type of water may be identified as deeper ground water in the older alluvium. This water has 5D values between -106 and -99 per mil. These waters will be discussed individually and discussed in terms of a conceptual model of the hydrogeology of the lower Virgin River basin. Figure 30 is a plot of oxygen 18 and deuterium of meteoric water in the lower Virgin River study area on which these types of water have been delineated. Figure 30: Plot of 8^80 and 5D that summarizes the types of water found inarea. study found River water of types Virgin lower the summarizes 5Dthat and 8^80 of Plot 30: Figure 5 D (per mil deviation from SMOW) -110 -105 -100 -95 -85 -80 -90 -75 -70 1 -4 1 -2 1 -0 9 -8 -9 -10 -11 -12 -13 -14 -15 ♦ e ■ + Beaver Dam Vtfash Dam Beaver Wells Alluvium Older Younger Alluvium Wells Younger Beaver Dam Wash Aquifer Wells Wash Dam Beaver 5 180 (per mil deviation from SMOW) from deviation mil (per 180 5 lblMtocWtrLn Cag 1961) (Craig, Line Water Meteonc Global - . W M y»«IID> v Upper Basin Discharge Basin Upper v 'groundwater Isotopically depleted Legend O Virgin Mts springsMts Virgin O a o Northern Basin Northern o • Virgin River Virgin • D ♦. Littlefield Springs Littlefield Virgin River water River Virgin Littlefield Springs and Springs Littlefield ground water, YA 142 Local Waters Identifying the composition of local recharge is important in defining how much of the water used in the basin is actually being recharged in the basin. Local waters in the study area include precipitation in the surrounding mountains, local mountain springs and streams, and both ground and surface water of Beaver Dam Wash. Typically, precipitation samples are used to represent the isotopic composition of modern recharge in a region (Ingraham and others, 1990; Smith and others, 1992). However, due to the brevity of this study and the large size of the lower Virgin River study area, a collection program could not be implemented that would provide a representative sample of local precipitation. Nor was there available isotopic data for precipitation in the study area from previous studies. Precipitation from a single summer convective storm and subsequent runoff were collected, the stable isotopic composition of which indicated a direct hydrologic relationship. Ephemeral precipitation events and subsequent runoff may serve to recharge the basin, however, they are infrequent and of variable intensity; thus, the total quantity is difficult to determine. Therefore, in this analysis, samples from mountain springs and streams are assumed to represent modern recharge for the following reasons: (1) Regardless of their origin, it appears that many of the springs and streams sampled (TR-2,3,4 and VM-3 and 4) are actively recharging local alluvium. These streams and springs were observed flowing over coarse 143 sands and gravels and at some point each became influent (i.e., there was no longer surface flow), which would indicate recharge to local alluvium. (2) The springs sampled fit previous descriptions of springs fed by local precipitation. Previous authors (Mifflin, 1968; Glancy and Van Denburgh, 1969; Winograd and Thordarson, 1975; Burbey and Prudic, 1993) have described the nature of local springs in the Basin and Range Province. These authors characterize local springs as perched ground water emerging from consolidated rock or thin alluvium at the head of ephemeral drainages. Such springs are fed by local precipitation and characterized by variable discharge and temperatures. Most springs and mountain streams sampled for the present study fall into this category. Springs and ephemeral streams emerged either from bedrock or hillside alluvium. Sparse data on variations in discharge is available as most sampling sites were quite remote and were only observed once. Of the sampling sites that were observed more than once, all of the springs appeared to be perennial and it was difficult to assess discharge variations due to impoundments at the springs. Variations in discharge over time were observed in two of the streams sampled in the Virgin Mountains, Nickel Creek (TR-4) and an unnamed stream in the central part of the range, TR-3. (3) The isotopic compositions of springs and streams sampled agrees with published isotopic compositions for local meteoric water at similar 144 elevations in the region. Thomas and Welch (in press) reported average deuterium compositions (excluding samples that showed significant evaporation) of local springs in the southern Spring Mountains (altitude: 2600 m) of -90%0; in the Sheep Range (altitude of springs 1700 to 2560 m) of -93%o; and in the Meadow Valley Wash drainage of -87%o. Deuterium compositions of springs and streams in the Virgin Mountains (altitude 2000 to 2400 m) range from -91 to -73%o with a mean of -82%o; however if the springs that show significant evaporation (using the same criteria as Thomas and Welch [in press]) are removed from the sample set, the range is -91 to -76%o and the mean is -86%o. In addition to the local recharge evidenced by springs and ephemeral streams, the only perennial tributary, Beaver Dam Wash, is also a source of local recharge. Beaver Dam Wash and the Beaver Dam Wash alluvial aquifer convey water from the northeastern part of the lower Virgin River drainage basin (Burbey and Prudic, 1993). Surface water from the wash and wells in the floodplain have 5D values of between -86 and -83%o and show little isotopic evidence of evaporation. Based on published, high hydraulic conductivity values for the Beaver Dam Wash alluvial aquifer and a stable isotopic composition that is similar to modern recharge in the region, it is likely that both the ground and surface water of Beaver Dam Wash represent modern recharge to the lower Virgin River drainage area. In fact, Beaver Dam Wash is the only source of local recharge that one can conclusively say reaches the lower Virgin 145 River. Local recharge within the drainage basin is isotopically more enriched than the other sources of water for the lower Virgin River and does not appear to be the source of high salinity to the river. However, aside from extreme ephemeral events and Beaver Dam Wash surface water flow, the contribution of local waters to the river appears to be minimal. Virgin River Waters The Virgin River is chemically and isotopically distinct from both local recharge and deep ground water in the lower Virgin River drainage basin. The Virgin River carries the isotopic and chemical signature of the Upper Virgin River and that of the Littlefield Springs in direct proportion to the amount each contributes. Isotopically, the two sources are similar except during the summer months when discharges from the upper basin tend to be more isotopically enriched, likely due to irrigation diversions of river water. Chemically, upper basin discharge and the Littlefield Springs are similar, both having calcium plus sodium, chloride plus sulfate waters. Discharges from the Littlefield Springs appear to have a constant chemical (Hardman and Miller, 1934) (Trudeau, 1979) and stable isotopic composition, although only limited isotopic data from this study and two other unpublished values are available to assess changes in isotopic composition over time. Recharge sources for the Littlefield Springs are not fully understood, 146 though the body of isotopic and chemical data suggests either a large, homogenized source or one that has been constant for at least the last sixty years (time elapsed since Hardman and Miller’s 1934 samples were collected). The mean 6D composition of the Littlefield Springs sampled was -96%o. This value is more depleted in deuterium than any local recharge within the lower Virgin River drainage basin and, therefore, it is unlikely that local, modern recharge to mountains surrounding the lower Virgin River drainage basin contributes significantly to the recharge of the Littlefield Springs. Below the inflow of the Littlefield Springs, the lower Virgin River does not appear to be affected by the hydrogeology of the lower Virgin River drainage basin; that is, the river appears to flow through the basin unchanged by waters in the basin. Processes in the basin such as evaporation and transpiration change the isotopic composition and amount of water in the river, but evidence of major sources of water were not found in this study. It does appear that the river recharges nearby ground water as seen in six wells (YA-1,2,3,4,5 and 8) located in the shallow Virgin River floodplain sediments. These wells have chemical and isotopic compositions that are similar to the river. Ground Water in the Lower Virgin River Area A group of seven wells drilled in Older Alluvium in the basin exhibit isotopic signatures that are more depleted than any other water in the surface water drainage area. These wells have 5D values between -106 and -99%o, a 147 range which is 4%o more depleted than the most depleted of the river samples, 8%o more depleted than the most depleted local spring and 13%o more depleted than recharge from Beaver Dam Wash, the most active site of modern recharge. Chemically, the wells vary from a group of two calcium-bicarbonate wells (OA-1 and OA-2) located in the deep floodplain aquifer near Bunkerville, to sodium-sulfate wells (OA-4,6,9,10 and 12) located in the older alluvium/Muddy Creek aquifer of the alluvial fans north and south of the river near Mesquite and one well south of Riverside, Nevada. The isotopic evidence alone suggests clearly that the water in these wells is not the product of modern recharge in the basin, at least not any that was sampled for this or any of the other studies cited in this paper. Possible sources for this isotopically depleted water include water that was recharged in the basin during different climatic conditions or water from outside the lower Virgin River surface-water drainage basin. Climatic conditions that would produce more isotopically depleted waters include cooler temperatures, increased precipitation, and changes in air mass trajectories. Recharge during the cooler, wetter climates of the late Pleistocene could have produced waters that were similarly depleted in heavy isotopes. Alternatively, some ground water in the lower Virgin River area may have originated outside the basin in a region that produces more isotopically depleted waters. Glancy and Van Denburgh (1969) suggested that considerable amounts of water may be transmitted through carbonate rocks in the study area north of the Virgin River southward toward the river. The abundance of north-south trending faults in this area (Axen and others, 1990; Axen, 1993;) may have enhanced the capability of the carbonate bedrock to transmit water from north to south. If the carbonate rocks in the northern part of the study area are hydrologically connected to the regional carbonate aquifer, the regional aquifer would be a source of isotopically depleted ground water. In their ground water flow model, Burbey and Prudic (1993) estimated that ground water flow from the White River flow system to the north was contributing recharge to the lower Virgin River area through carbonate rocks at depth. Thomas and Welch (in press) calculated mean 6D values from this region to be -109%o. Flow from this region or a similar region could produce the isotopically depleted ground waters found in the lower Virgin River valley. The Virgin River, then, is primarily water that originates in the upper Virgin River basin and is transported to the lower basin as river flow and as subsurface flow that emerges at the Littlefield Springs. The high salinity is the result of the high salinity source of the Littlefield Springs and upper basin discharge. Local recharge, however lower in salinity, is not sufficient to dilute the river and reduce the salinity. In fact, minimal effect on the river by local recharge is observed due to the small amount of local recharge available. Deeper ground water in the area is also lower in salinity than the surface water as controlled by the Littlefield Springs and also displays an isotopic composition indicative of a different climate/recharge area. This water does 149 not seem to be hydrologically active in the lower Virgin River and does not discharge to the river in quantities large enough to affect the river’s composition. Deeper ground water appears to be hydrologically separate from recent local recharge water or even older recharge from the Virgin River itself, as the chemical composition of the deeper, older ground water is not similar to that of the river or Littlefield Springs. The water in the lower Virgin River originates in the upper basin and is released as surface flow and as recharge to the Littlefield Springs. The river is minimally affected by local recharge or local ground water, and is simply transported through the river valley to Lake Mead. CHAPTER 6 CONCLUSIONS Surface water and ground water in the lower Virgin River surface water drainage basin were sampled to study ground water - surface water interactions in this area. Data collected included discharge measurements and environmental tracers (stable isotopes and major ions) in the lower Virgin River, and environmental tracers in ground water in the drainage basin. The isotopic and chemical composition of the lower Virgin River is controlled by the composition of its primary sources: discharge from the upper Virgin River basin and the Littlefield Springs. The isotopic signature of the river resembles that of each of the sources in direct proportion to the amount of discharge each source provides. The chemical composition of the river behaves in a similar manner except that Beaver Dam Wash may dilute the river water more than expected. No evidence was found of ground water inflow into the channel below the Littlefield Springs. Discharge during the tenure of this study was two to three times the average discharge, primarily due to seasonal flooding that originated in the upper Virgin River basin. Discharges of this magnitude tend to mask subtle changes, like those due to ground water inflow, and thus, the question of the amount and chemical composition of ground water inflows into the Virgin River could not be resolved by data collected during the period of 150 151 this study. From ground water sampling of wells in the lower Virgin River drainage basin, it is evident that shallow wells in the floodplain of the river are hydrologically connected to the river. Other ground water in the basin is dissimilar from both the river and local recharge as represented by springs and perennial streams in the surrounding mountains and highlands. Suggestions for Future Research Though much of the ground water sampled appears to have no hydrologic connection to the Virgin River, information about the chemical and isotopic nature of ground and surface water in the study area collected for this study will be useful in future investigations of water resources in the region. In particular, trends identified in this initial survey of isotopic and chemical composition of water resources in the lower Virgin River area could be verified and explored further in future investigations. Topics for future investigations include isotopic evidence for ground-water recharge under different climatic conditions (paleowaters); the effect of geology on ground water movement in the lower Virgin River area; separation of evaporation from transpiration in the lower Virgin River using stable isotopes and chlorides; and relationship between the composition of precipitation and runoff produced by convective storms. Among the wells sampled for this study were seven wells that produce 152 water with an isotopic composition that is more depleted in heavy isotopes than any source of modern local recharge in the study area. The source of that water is of great interest because several of these wells produce water that is used for municipal supply of drinking water in Bunkerville and Mesquite, Nevada. One explanation for the presence of this isotopically depleted water is that it is paleowater, that is, it recharged the aquifer in the past when a cooler and wetter climate prevailed. One line of evidence that has been used to identify paleowaters is the relationship between 6180 and 5D as expressed by the meteoric water line. In many cases, paleowaters exhibit a 5180 and 5D realtionship that has the same slope as the modern, global meteoric water line (Craig, 1961), but a lower Y-intercept (Fontes, 1981; Issar and others, 1986). The isotopically-depleted ground waters identified in this study have a 5180- 5D relationship that is similar to paleowaters identified with both stable and radio-isotopes in other regions. More data from ground water in the study area and possible source areas outside the study area is needed to assess this possibility. In investigating the source of recharge for isotopically-depleted ground waters in the study area, the importance of sources of recharge outside the surface water drainage basin cannot be ignored. The study area is located in a transition zone between the Great Basin and Colorado Plateau physiographic provinces, both of which have productive regional aquifers. The influence of geology on the movement of ground water from outside the lower Virgin River 153 surface-water drainage area should be examined further as part of the definition of ground-water flow paths in this region. Within the study area, both stable isotopes and chloride concentrations can be used to separate the effects of evaporation from transpiration in the lower Virgin River. The stable isotopic composition of a water is altered by evaporation, but not by transpiration; while the chloride concentration of a water is affected (increased) by both processes. By looking at the change in discharge (loss of water by evaporation and transpiration) in concert with the change in both stable isotopic composition and chloride concentration, gross rates of evaporation and transpiration could be determined. The relationship between the isotopic composition of precipitation and runoff in arid climates was the subject of a limited experiment performed as part of this thesis. The isotopic composition of flash flood waters from two channels in the alluviual fans flanking the Virgin Mountains was compared with four precipitation samples from the convective storm that produced the runoff. The isotopic composition of the waters in the two channels was virtually identical (within analytical precision), and was also similar to the volume-weighted average of the four precipitation samples, suggesting that the flood waters were composed solely of precipitation rather than ground water. This result conflicts with that found for more humid areas (Sklash, Farvolden and Fritz, 1975;Sklash and Farvolden, 1979; Ingraham and Taylor, 1989) in which stormwater runoff was found to contain a large component of ground water. The lack of ground water contribution observed in ephemeral channels in the study area is a microcosm of the lower Virgin River as a whole; that is, the lower Virgin River appears to flow from Littlefield, Arizona (below the Littlefield Springs) to Lake Mead relatively unaffected by inflows within the basin. APPENDIX A WELL CONSTRUCTION AND AQUIFER DATA 155 WELL IDENTIFICATION NOTES DATE LAB ID1 SAMPLED SITE ID: A2401505-01 6/15/93 BDW-1 A2411529-01 6/16/93 BDW-2 AZ401505-02 6/16/93 BDW-3 AZ411532-01 6/18/93 BDW-4 111292-MF04 11/12/92 YA-1 NV146923-01 4/9/93 YA-2 NV146923-02 4/9/93 YA-3 NV146923-03 4/9/93 YA-4 AZ391602-01 4/13/93 YA-5 AZ411534-01 6/17/93 YA-6 AZ401503-01 6/16/93 YA-7 NV146912-01 2/11/94 YA-8 111292-BCW01 11/12/92 OA-1 111292-BCW02 11/12/92 OA-2 111292-MF02 11/12/92 OA-3 111292-MF05 11/12/92 OA-4 111292-MF07 11/12/92 OA-5 111292-MF09 11/12/92 OA-6 111292-MF15 11/12/92 OA-7 111292-MF16 11/12/92 OA-8 111292-MF22 11/12/92 OA-9 NV146913-01 3/29/93 OA-10 AZ411533-01 4/13/93 OA-11 041393-MF20 4/13/93 OA-12 AZ391611-01 6/5/93 OA-13 AZ391611-02 6/5/93 OA-14 AZ391616-01 6/5/93 OA-15 AZ391616-02 6/5/93 OA-16 AZ411533-02 6/17/93 OA-17 vJotes 1. Original identification number assigned when sample was collected and used on sample bottles sent to laboratories for analysis. 2. Identification number assigned to well according to type of Tiaterial well was drilled in and location of well. 157 Table of local names for springs and streams sampled. Source of local names is U.S. Geological Survey 7.5 minute topographic maps. Spring or Stream Date Sampled Local Name Sample I.D. VM-1 4/2/93 South Key West Spring VM-5 8/6/93 North Key West Spring VM-6 8/21/93 Government Spring TR-4 8/6/93 Nickel Creek 158 Tf 03 co CO CD O CO CM O LO 0 0 CM CO y T3 >v ro O) 0) CO TJ _ c CD O) (0CO >(0 O) *U £CO h £ r 03 C/3 03 CO 03 o > in 0 3 co 00 CO in cm CO O) o > oo CO CD 00 in r^ - in CO oo CO CD in in c r in o 03 CD co 0 0 CD CO "Tf CO o > CM CO CO CO CO m CM CO CO X j- co O CO cm co 00 oo 0 3 co in O CD 00 T— 00 m * m in in co CO CO to CO c o CM t - r O CO in O) C N § o CO CD CO O ) CO CO CM * - CM CO r - 00 CO CO T f o o co in o o O ) CO CM CM O CO oo CD CD 0 3 Ui ^ ^ in in m in in in in m UJ LLI co co M- in in o > o CO o o > CM CM CM CO CM CM CM CM CM CO CO CO LU UJ co in UJ UJ UJ UJ UJ £ « o o N § N g q: z q: tr co QC w W O CO co o CO CO ( 0 O CO O CO *o CO > CO 2 2 co g co § CO > h F z hw CO in 00 CO in CO 03 ^ O CO CO O ) r in T t CO T - co ▼— CO CO co D) O oS 03 * U “D >> O) O) O) O) in co in O) (D co CO h - o> O ) co CM CO 0 3 0 3 CO in CD h - in in m in co o> co CO c o .a CO CO m - in in 4 -1 U CO in in in in o w J! in 0 3 CM in O) in eg co co CO CD h- M* in in in in CO CO 0 3 CM O 00 CO t - CM CO oo co in cm c m co in CM CO 0 3 o co co CO Is- CM co r- h- in in in in in in in uj rc in in co CD < CD CO 0 3 m CM CO 0*1 m co .&*-“I lc u O i - CM CO £ £ £ ODD o o OQ CO DQ APPENDIX B WATER CHEMISTRY DATA FOR WELLS IN THE LOWER VIRGIN RIVER AREA FROM EARLIER STUDIES 160 Appendix B: Background isotopic and chemical data from wells in the lower Virgin River area. Z CO o X O o c CO co J U o f— O CO O O O X D o z 0 o co CD . Q E . a CD a) cu Z o a* E ) O (0 E 0) £ co m OO o CM o - x co n i CN o CN oo in o c o> o oo t N C O O v t ^ O ( ° ? ® ? N CO N - h o n i o o o 0 - CD t- ^ 00 m N tv- - i r - D T I ' r 1 - T ' ^ ' l f ' f N (M ® r (M (NN m * ^ & - 00 * h T7 00 T7 T7^’ S P coa5fe2cNT7 o o X- - n m in r- £ * 2 - t co lO o D T 2 ) O 5 N N 00 CN CN 05 CO t O)CD IT) J 52 OCO CN CO CO n i o c n i o cn - - I I v I I I I I I I I I t I I I J I pvj I t I I i i i - 3 ' ' N ( ) 5 L L L —I LL - U LL LL CO r f r > > 1 0 - T O in t t x— x— OC OCO CO CO CO Oin CO CO CD *~ r~ CO NO CN CO o z : z o zz oo o ) o H CO s o - 7 i i /—i O „ , , , * r 01 in t t CN - h OC (O CO CO in CN T“ o - y < Y Y 7 - ■x— N CN CN CN O > > 01 x i n $ no^r( CO (O r ^ o in J— r x o a CD CO o CO o CO 00 - 1 01 T“ OO CO v X- CD in TT CN 5CO 05 CO CO NCN CN * r o CO - i t i o o o o CN CN -U ^LU UJ ^ I- LU CO CN O x T** in OC OCO CO ^ CO r CO '- f CN in CN v-* CM (N x— “ r T~ >i h»» O in CO o> CO - t -r- rt i- d - < - - — x- T— T- T- CN >i LU fX o O CO ~ x - h CO ni n i in in ■N- —T— T— T^r ^ TT NCN CN NCN CN n 1 CO co co in CN * a w o NCN CN s- Is - h in $ 00 ^ r T“ CO O) CO CN nC o CO in in CO m • | - ai n i T*“ CN CO CN o in CO •—■»SP i s ? 1 GO CO CD o> 00 ) O CN CD h*- - h T“ " r CO Oin CO O) t - to v 1 CD - - r*> ID t m o in yp u XT' rvJ 00 00 00 o $5 in in o CN ^3- CD 00 CO CO CN - t - 5 .t/J £ J T *o 5 g> 5 Q > *-» iS > _ =J aj O U CO > ) D c CO W ? U D 'C S ^.s ^.s CO a> co a> Urn u) a =j c . a O (15 * N CO CN z> CO § ^ X— W M X3 STANDARD DISCHARGE MEASUREMENT ERROR WORKSHEETS 162 163 O CD CO CO T** o co oo oo h - CN cd CN CO 00 o w O CD CN CN CO O ) CO CO N CM CN ID O) h - CD o o o o o T - T - T“ CM t - O o o o o d d d d d o o' o b o' b o o o o o d o d d 07 CD 00 N - 07 c o CN CM CN (O in CD 00 CN ID CO CN h * t T CO 00 0 7 T— 07 CD O ) CO 0) t- O ) CO r r CN CO cn ,„ ^ h - CN CO CN r - T - i n 00 T r r CO r N . CO o ID CO O ’ O ) CO CO CN Q . ' O CO d o d o ^ ^ CO ID O T-* CO CO CO CO CN T - d r ) T“ o CO CN T“ r t- CD ID CO 00 o O ) CN CD CD CO ID o T— N 0 3 CO p* fO CO N CN CO (N o ID 00 -N- O) t O r q (D d V d d d CO ^ CO ID co d ^ (N CO CO CO CN T~ d CO CD 00 CO h - r O ) 0 ^ N CO t- CJ) CJ) o o 00 r - o * CO CO CN 00 o (O co ^ in in o ro in cq <0 co in CN o T— CO cd ID'N* in ^ t ^ in ID ID 07 07 CO ID in ID CM co CN O ) O r ID CO CO o O) o O) > ID T** t— CN o O? 07 07 00 00 00 C N C N V - 07 O O CN CN CN h - i— CO CN CN CN CN LU cd CN c d CN CN CN CN CN LU X to -C 0 CO 3 00 CT) CN o C- N- t- r - CN 00 n . o o -^r 00 CN CO 0 1 00 CN c q CO CN CO CN CN CO CN ID CL CO cd cd to cd cd c d c d cd CO CO CO CO CO CO cd c d CO o CN ID N- O O CO O CD ID N G (D CN OO ID CO CD CO CN h - 00 CO 00 03 CO CO CO CO CD •N- in ID m CO *— 5 cn CN d d T“ o o o o o o d d o o o o o T— -r_ Z 3 O)i o c — I 3 < CN CO 00 ID ID 00 CD 07 03 CO ID If) CO OO ID CN ID oo in CJ CO CO CD CD N. 0 s K N CO N ID (O in CD N. h- h-CD cq CO h -. ir ID in ID in in to in in in to in in i d ID in 0 q : q ; CO CO CN i n o lu CD o * a o CN t - - CN i ^ - 00 o (N CN CN CN CN T“ CO r ^ Q CO o o CO 0 1 CN < D z CN CN ^r o CO N N1 N1 (O lO CO CN N - CO CO CM o <0 2 T— CN CN CN CN (N CN CN CN CN CN CN CN T~ CD O) ID CN oo m O) O) _ CO (O CN CN > °R cd co ^ T CD p ID 00 N 03 . CD O) > 00 CD CO N- (O CO CO CO CN CD O ) ^ r ID oo h - CD 03 •N- r r ^3- ^ r N* 'N’ 'N* N- •N- ■*3* ^ r ■N- •N- 0 7 ID CO c o CO 00 ID r f N. 07 N (O N1 fO fO CO CN CN ’O’ in > o> 0 7 ^ r ID TT CN CD O i n in CN 0 0 N- CO (0 d d D d d o d t -* d ^ d T“* d d d d CD GO CN ID ’N - h - CD 00 if) ID in CO GO CD 00 CN O) 0 7 07 co m o co co CN CN o cd 07 o cd ID CN cd o T”“ T~ CN (D T- ^ r N a *— CD LU i - in 00 07 co N" o CO N- in 05 CO in to CN CO T— N- N. 5> 05 1“ CO T“* CO CO c b cb in in in ’ r)Cl ' r^ - O CO CO O) 00 in TJ* CN O CO o CN CO N- CO CD CO T_ CO 00 CO cb in' in in ^r 00 CN in O) CD (O 00 T~ c r CN CO CO 00 in n ; CO CO in co in cn N* N" ib N* N- N- N- N* r r in T" 00 CO CO 05 05 > CO CO o in i— co o T— o CN O) 00 o o T— CN r - LU CN c\i CN CN CN CN CN CN CN ILU T— O0 3 cn ------< LU o N- o 00 N- o o •O’ CN o: tn CN CN CN CN T— T— CN CN CN CO in CL cn CO CO* CO CO CO CO CO cb cb cb cb 0 3 co O .05 in CO O •ss in CO o O T» CO < j cn i n CO CO ] g CO CO CO CO CO CO CO o ’ b o o CO b o ’ b o ’ b o ’ o ' 03 c o O)1 05 o u. Q> Q..Q _ i L> D) <13 0) CQ C/3 LL < o (N 00 in 00 Q. i n in 00 CN in (N CN o co >O) CO 00 in in 00 05 CO CO O) 00 CO N- CO h - CO lf) h » 00 § N 1 N* N* T f N* ^ r in 0 3 00 CO i n in CO 05 CN 00 h- in 5 T“| CO CO T~ CN N; CO in 05 r — o T~ b b T_ N" h - CN N- CO 00 05 42 m Q ' Y“ cb n ! cb 03 o> O oo T— o T“* 05 0 0 05 N ' *” T“ T— r ~ T- LU N" CT) TT CO CO CD b i § § i § § 5 co cn cn cn C/3 cn cn cn cn cn C/3 to 165 Table C-2. Notes of calculations of Sd and S, Site Streambed sd Meter Meter Meter Meter Material 7/93 10/93 10/92 6/92 SW-1 Sand & Silts-even 2 % AA2 AA AA p3 SW-2 Sand & Silts-even AA SW -4 Stable, some cobbles AAAAAA 1 2 n 2D SW-5 Stable, sand-even AA AA SW-6 mobile stream bed 10 % AA SW-7 mobile stream bed 10% AA SW-8 stable w/ cobbles AAAA AA 2 N 2D SW -9 stable w/ cobbles PAA P 2D SW- stable w/cobbles P 10 2D SW- mobile 10% PAA P 11 SW- mobile 10% P P 12 TR-1 even w/ small gravel P 1 * (2D ^ >2 1 where D is average depth2 AA-current meter; S, (AA)= 0.7V 13 P-pygmy meter; S, = 1,8V °3 166 Appendix C Table . Equations for Calculation of Standard Measurement Error (s j+ s 2) [ — 1 £— ] *S 2*S 2* S 2*S 2+0 . 7 5 V \ i N i S h V Where: Sq = standard error in discharge (Q) measurement Sd = error caused by depth measurement, which is estimated by the following equation: 10,2 1*( — > 2D Where: D = average depth S, = Error caused by pulsation of velocity St = 16.6T'28 Where: T = time of exposure, in seconds (use average time over which revolutions of meter were counted) N = number of vertical sections S, = Insturment error for meter S, = 0.7v 1 (for AA meter) S| = 1.83v'0"3 (for pygmey meter) Where: v = average velocity of all sections Ss= Vertical distribution errors (for 1-point method) 1 2 0 - 4-+5.0 2 SA I AT Sh= oblique angle flow error Sh~1 % Sv = error caused by horizontal distribution of depth and velocity s = 32 N'0 88 APPENDIX D CHEMICAL DATA FOR ALL SAMPLES 167 cd in o p CM CM P O CN oo CN M- cq in p Tt cq r»- CM o O) co o' in co co cd m - M CN o i CN CO o> t\i co co co co co co co co CN N. o O O o o p p o CD CD p N- CO cn 00 h- re o' CO CM 00 T— ■<— Z (D N. in 00 S N s s Tf ai cb i o in CM CM CM CM CM CM CM CM O) in CO in CO CN CO CO O O o o o o o si o 00 in CN N; CM iq t—; CD 0 0 ffl N CD S O) cb o i cb CD cb CO oo CO in E CO CMCN CN COCN M1 O O O o p o p in O) o in T“ o> -r- to re i n c m c o h»’ ib ib T— o* in ib N-’ oo’ ai CO* ib ai 0 O N - O f - O o> O) o oo 00 CO 1^. 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CO CO CO CO CO o o o o CO 10 r N ^ IO S *— CM CO in CD CM mw oco ^S CL (D O O E E E S E E o: oc a: £ £ > > > > > > > > > H h* H < CO FIELD AND MAJOR ION DATA FROM WELLS IN THE LOWER VIRGIN RIVER AREA SITE ID DATE Field Measurements IONS (mg/L) z EC TEMP pH Si02 Cl SQ4 HCQ3 N Ca re I O) co® co co D i o e n i o - T N O i M Sb>a>a)5>9>9>9> CM noni n in n o in M cotj C M O O O O O O O O O CMT-COtOO(MCMCM » , i n i n t o i n ai n- in in ui r uJcoMscooinin O CMCMiCM CM C M o c q. 0 C 0 M i-C M C M C M M -C M ■ M’COMCOOCOM'CO M O M O T (M TT CO CM COCM J O n o CO-M-^. 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CO ra CO CD CD tn CO CD in CO o CD o xt CO < cd CM CM CM CM CM CM CM CM CO CM CO CO CM CM Q i ^ CD tn in OO in h- xt Xt Tt- ■M- N- T~ CM < a 00 CO in CM CO CO CM p p CM CM CO p p O d d d d 2 E d T“ CM xt < in CD h- 00 O) o Xt CD o o O o oxt o O o o o o o o o 1 i LU cc t £ £ 5 5 £ t $ fc i— (0 co co CO to CO CO CO CO CO 0) C/Jai CO co co CC X cc cc Q. cc XX XX X DC DC cc X > > > > > > > > > > > > > > > > > REFERENCES Anderson, R.E., and Barnhard T.P., 1993, Aspects of three-dimensional strain at the margin of the extensional orogen, Virgin River depression area, Nevada, Utah, and Arizona: Geological Society of America Bulletin, v. 105, p. 1019-1052. Armstrong, R.L., 1968, Low-angle (denudation) faults, hinterland of the Sevier orogenic belt, eastern Nevada and western Utah: Geological Society of America Bulletin, v. 83, p. 1729-1754. 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UMI Approximate Boundary of Lower Virgin River Surface-Water Drainage Basin Tule Desert Subbasin 7 ' 30 52'30 - 22130 Tula Desert 7' 30 TD-1 Toquop Wash San Holl Was 52*30 FLTR-4A SW10 Nickel \Creek \ swn 7'30 Beaver Dam JWash UT 370 00 lash AZ Big Bend Wash swi Sand Holtov TR1 Wash SI iD W -4 o \ OA-11—V BDW-3 O, I BDW-t ! SWA-5 5.6 SW5 iW6 \ SW7 OA-14 lA-15 R-4A \ Nickel \C reek VM-2 7'30 SW1 LEGEND 52' 30 Surface Water Drainage A/ Basin Boundary N Ephemeral Stream N fterennial Stream Interstate Highway H a Secondary Road N State Line SAMPLING SITES -® Spring o Well San Holl Was 52*30 ; OA-2. ^V-IO ^FLTO'4A SW10 iYA-4 \ Nickel \C r e e k *. SW11 JR-2 V SW12 VM-5 A/M-t VIRGIN RIVER' 360 30 22*30 sand Hollow- Wash - 5.6 iW5 52'30 iW6 SW7 'AS * - OA-14 IA-15 [L7R-4A \ Nickel \C neek VM-2 -o J R -2 JR v 37' 30 VM-5 A/M-1 6MILES 6 KILOMETERS 7 130" 1 1 40 00' 113°52‘30“ SW1 LEGEND 52'30" Surface Water Drainage N Basin Boundary N Ephemeral Stream N Fterennial Stream Interstate Highway N Secondary Road N State Line 45' SAMPLING SITES -e Spring o Well ▼ Surface Water ♦ Flash Flood / 37'30" Precipitation Collector Plate 1: Sampling Locations in Lower Virgin River Study Area 360 30' 9 7 PLEASE NOTE: Oversize maps and charts are filmed in sections in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, WITH SMALL OVERLAPS The following map or chart has been refilmed in its entirety at the end of this dissertation (not available on microfiche). A xerographic reproduction has been provided for paper copies and is inserted into the inside of the back cover. Black and white photographic prints (17" x 23") are available for an additional charge. UMI 1 140 22'30" 15' 7' 30" T — T" 37°37'30" F 30' NORTH ELLA SPRING ^jSHEEP SPRING 22'30" ,GARhEN SPRING \ 15' _^NOW SPRINC Tule 7' 3 0" 1 1 4 0 0 0 ' 52'30" 113° 45' 370 37130" 30' 22'30" 15 Tula Desert 7 130 SUMMIT SPRING jTULE SPRING TD-1 (ABE SPRING) Toquop Wash Nickel -.Creek _^>NOW SPRING 7 ' 30 " Beaver Dam JWash UT 3 70 00 .Wash AZ swi ‘c Big Bend i J L i Wash \ 52'30" li \v i \ 45' \ Nickel \C re e k LEGEND A / Surface Water Drainage * V Basin Boundary /'./ Ephemeral Stream N Perennial Stream //^ J Interstate Highway N Secondary Road N State line SAMPLING SITES -® Spring ° Well v Surface Water -o Spring sampled by others Composition of wells, springs and streams C ations Antons N a tK CkF Co HC03+C03 Mg ijL SG3 60MEQ/l_ i I i I 60MEQ/L 8 180 8D (p e r m il) (p e r m il) Teachspring \ 5 2 ' 30 " \ ) K ,V > K I V f: . A ■ A\V\\ ^ / K j \ ( w1S a. ; ^ ri i %ii« \ i i • » « 45' \ l ! / 3 P \\ swti ■j w ~»J 'M. i \ i f. - \ Nickel .j a s s r ? A % \K V \Creek ■ \ ~ X r \ V U ^ X x \ I A\ X Q^&RtjWENTSWjlNG^VM-6~~*' ] V JUANITA S>FHNG 37130" SW12, ' K r P ! VIRGIN RIVER 360 30' 1 1 40 30 22'30" 15' 7'3d 4 5 2 ' 30 " 4 - 45 * \ Nickel p> \C re e k s V V ) rENTSt*S)ING_VM-6 KH 37'30" NORTH SPRING SMILES _r------J 6 KILOMETERS 360 30' 7'30" 1 140 0 0 ' 1 1 30 52'30" T V Secondary Road A f State Line 52'30" SAMPLING SITES Spring Well Surface Water Spring sampled by others 45' Composition of wells, springs and streams Cations A n to n s Na tK CWF C o HC03+C03 Mg UL S04 60MEQ/L 60MEQ/L -wV* m -■ '-I10 8 180 BD (p e r mil) (p e r mil) Composition of Virgin River or Beaver Dam Wash In July 1993 SW11 37‘30" & Plate 2: Major Ion and Stable Isotope Compositions of Water in the 36*30' Lower Virgin River Study Area PLEASE NOTE: Oversize maps and charts are filmed in sections in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, W ITH SMALL OVERLAPS The following map or chart has been refilmed in its entirety at the end of this dissertation (not available on microfiche). A xerographic reproduction has been provided for paper copies and is inserted into the inside of the back cover. Black and white photographic prints (17" x 23”) are available for an additional charge. UMI 1 140 2 2'30" 15' T nr ~ T" 3 70 37'30" 30' 22130" 15 T u le D e s e rt 7' 30" 114 °00' 52'30" 113° 45' 22130 Tule Desert 7 130" 3 70 00' Toquop Wash 3 )\ M I* q.: 52'30" vs*, y * } < *932J \ K 104 : I ( '°n I \ \ ’ ’, 76 1070 V ' 8 0 \ ( ;S \ ) l i! \ Vi \ Y -\ \ 14 -OMOOC 660 1 0 2 1I 0>l 6 6 0 1320 12 125, 2000 45 " \ i i h 359 / y 3380 y 634 244Q V 4450' \ 372 \ \ j 3470 \ Nickel \ Halfway Jh \ \ Creek Wash \ \ Y\. '-I.. 30 16 \ ) _ R 5 4 Beaver Dam Wash UT Wash AZ Big Bend 3 5 4 389 413 370 3360 3470 2720 390 ,^£''3580 3300 <^401 ■'■'3600 393 ■415 3630 3520 52'30 932 104 : \ I. 303i ,605 660 } 0 2 \ Nickel \ Creek ,840 854 530 LEGEND Surface Water Drainage Basin Boundary Ephemeral Stream Perennial Stream Interstate Highway Secondary Road State Line SAMPLING SITES Springs Wells Surface Water —chloride concentration X ) \i 52'30" \ V : ) 104 \ 101C C \ 459 U V (121 ~\ \ 14 V 560. F rr •. s ■. ; / 45 x i i m 359 / y 3380 634 V 4450' 244Q \ v 372 4.70 Halfway J \ Nickel \ Wash ^ ^ 1 9 0 Creek \ '-i.. 30 16 .854 530- 30 ^673 39 \ ^760 V 37130" '59 VIRGIN RIVER 360 30‘ 1140 3 0 22‘30" 15* 7 130" / 620\ \ 20 ^^3580 3300 \ <*401 ‘i 640^ ■—3600 650. 393 415 3630 3520 52'30 932 104 303i , —'*■ X. >1351 ,605 560 660 ; s . \ Nickel \ Creek ,840 854 530. 760 371 30 724 -® 59 6 M ILES 6 KILOMETERS 7'30" 1140 0 0 ' 1130 52‘30 a m Surface Water Drainage /y Basin Boundary N Ephemeral Stream N Perennial Stream Interstate Highway {fS jj Secondary Road /\ State Line SAMPLING SITES -® Springs ° Wells * Surface Water 29 —chloride concentration °445 ^ sp e c ific electrical conductance Plate 3: Chloride and Specific Electrical Conductance (EC) Values at Sampling Locations in the Lower Virgin River Study Area -fl)Cffl n 0> s'0 0® *'3 3!! Uo I* Mffl W ®p *5 SI .,-d tQ oo o o o o