The University of Arizona

Environmental Isotope Geochemistry in Groundwaters of Southwestern Arizona, USA, and Northwestern Sonora, Mexico: Implications of Groundwater Recharge, Flow, and Residence Time in Transboundary Aquifers

Item Type text; Electronic Dissertation

Authors Zamora, Hector Alejandro

Publisher The University of Arizona.

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Download date 11/10/2021 05:25:45

Link to Item http://hdl.handle.net/10150/631319 ENVIRONMENTAL ISOTOPE GEOCHEMISTRY IN GROUNDWATERS OF SOUTHWESTERN ARIZONA, USA, AND NORTHWESTERN SONORA, MEXICO: IMPLICATIONS FOR GROUNDWATER RECHARGE, FLOW, AND RESIDENCE TIME IN TRANSBOUNDARY AQUIFERS

Hector A. Zamora

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

In the Graduate College

THE UNIVERSITY OF ARIZONA

2018 Date: 11/15/2018

Date: 11/15/2018

STATEMENT BY THE AUTHOR

This dissertation has been submitted in partial fulfillment of the requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that an accurate acknowledgement of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

Signed: Hector A. Zamora

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my advisor Karl W. Flessa for the continuous support, patience, and knowledge through my journey as a graduate student, and Jennifer McIntosh for her valuable feedback and motivation during difficult times. My sincere thanks also go to my dissertation committee: Thomas Meixner and David Dettman, for their insightful comments and encouragement that allowed me to view my research from various perspectives. Special thanks to Christopher Eastoe who, even though was not officially part of my graduate committee, always had time to provide invaluable advice and wisdom during the writing of the three different manuscripts here presented, and Benjamin Wilder who made this project became a reality in so many ways.

I would like to extend my gratitude to all the people who have assisted me in any form during the different field work campaigns including Peter Holm, Charles Conner, Ami Pate, and Colleen Filippone with the National Park Service, Federico Godinez Leal, Horacio Ortega Morales with El Pinacate Biosphere Reserve in Mexico, Is Boset Saldaña with Pronatura Noroeste, Eliana Rodriguez Burgeño and Jorge Ramirez Hernandez at the University of Baja California in Mexicali, and the people at Ejido Vicente Guerrero. Additionally, I thank Dee Korich and Peter Chipello at Tucson Water for their support, patience, and flexibility while I was finishing this work. Finally, I also thank my wife, family, and friends for their patience and ongoing support through this journey.

Financial support for this research has come in part from the NSF Graduate Research Fellowship Program, Geological Society of America Graduate Student Research Grant, University of Arizona Water Resource Research Center, University of Arizona Geosciences Department, R. Wilson Thompson Scholarship, ChevronTexaco Fellowship, and the National Park Service Southwest Border Resource Protection Program.

TABLE OF CONTENTS

LIST OF FIGURES ...... 7 LIST OF TABLES ...... 9 ABSTRACT ...... 10 CHAPTER 1: INTRODUCTION ...... 12 CHAPTER 2: PRESENT STUDY...... 16 2.1 Lower Colorado River ...... 16 2.2 Gran Desierto Wetlands ...... 16 2.3 Lower Sonoyta River ...... 17 REFERENCES ...... 19 APPENDIX A: Groundwater origin and dynamics on the eastern flank of the Colorado River delta, Mexico ...... 22 A.1 Abstract ...... 24 A.2 Introduction ...... 24 A.3 Study Area ...... 26 A.4 Previous Studies ...... 30 A.5 Methods ...... 31 A.6 Data ...... 33 A.7 Results ...... 36 A.8 Discussion ...... 39 A.9 Conclusions ...... 48 A.10 Acknowledgements ...... 49 A.11 Figures ...... 50 A.12 Tables ...... 65 A.13 References ...... 72 APPENDIX B: Evaluation of groundwater sources, flow paths, and residence time of the Gran Desierto pozos, Sonora, Mexico ...... 80 B.1 Abstract ...... 82 B.2 Introduction ...... 82 B.3 Study Area ...... 84 B.4 Methods ...... 88 B.5 Data ...... 91

B.6 Results ...... 92 B.7 Discussion ...... 94 B.8 Conclusions and Implications for Regional Groundwaters ...... 99 B.9 Acknowledgements ...... 101 B.10 Figures ...... 102 B.11 Tables ...... 113 B.12 References ...... 117 APPENDIX C: Groundwater isotopes in the Sonoyta River watershed, USA-Mexico: implications for recharge sources and management of the Quitobaquito Springs ...... 126 C.1 Abstract ...... 128 C.2 Introduction ...... 128 C.3 Background ...... 130 C.4 Sample Collection and Methods ...... 134 C.5 Data ...... 136 C.6 Results ...... 137 C.7 Discussion ...... 139 C.8 Conclusions ...... 142 C.9 Acknowledgements ...... 143 C.10 Figures ...... 144 C.11 Tables ...... 158 C.12 References ...... 160

LIST OF FIGURES

0.1 General Study Area ...... 13

A.1 Map of Colorado River Basin and Study Area ...... 50 A.2 Map of Groundwater Levels in 1925 and 2010 ...... 51 A.3 Monthly Mean River Discharge ...... 52 A.4 Map of Sampling Site Locations ...... 53 A.5 Graph of δ2H versus δ18O Values for Endmembers ...... 54 A.6 Graph of δ2H versus δ18O Values for Evaporated Colorado River Waters ...... 55 A.7 Piper Diagram for Surface and Groundwaters ...... 56 A.8 Graph of δ2H versus δ18O Values for Surface and Groundwaters ...... 57 - 2- A.9 Graph of Cl versus SO4 Concentrations for Surface and Groundwaters ...... 58 A.10 Contour Maps for Cl- and δ2H Distribution ...... 59 A.11 Graph of a) Na+ versus Cl- , b) Br- versus Cl-, and c) Cl/Br versus Cl- ...... 60 2+ 2+ A.12 Graph of (Ca + Mg ) versus (SO4 + HCO3) ...... 61 - A.13 Graph of a) Cl/SO4 versus Cl and b) Cl/SO4 versus Mg/Ca ...... 62 A.14 Map of Tritium Distribution in Groundwaters ...... 63 A.15 Conceptual Block Diagram ...... 64

B.1 Map of Colorado River Basin and Study Area ...... 102 B.2 Map of Groundwater Levels in 2010 ...... 103 B.3 Map of Sampling Site Locations ...... 104 B.4 Piper Diagram for Rainfall, Surface, and Groundwaters ...... 105 B.5 Graph of δ2H vs δ18O Values for Rainfall ...... 106 B.6 Graph of δ2H vs δ18O Values for Surface and Groundwaters ...... 107 B.7 Map of a) Tritium and b) Carbon-14 Distribution in Local Waters ...... 108 B.8 Graph of Weighted Average δ18O and δ2H Values by Percentile Range ...... 109 B.9 Graph of a) Br- versus Cl- for Local Waters ...... 110 - B.10 Graph of Cl/SO4 versus log Cl for Local Waters ...... 111 B.11 Graph of Cl/Br versus Cl- for Local Waters ...... 112

C.1 Map of Sonoyta River Watershed ...... 144 C.2 Map of Sampling Site Locations ...... 145 7

C.3 Graph of Spring Discharge at Quitobaquito ...... 146 C.4 Map of Major Geologic Units ...... 147 C.5 Cross-Section of Quitobaquito Spring Area...... 148 C.6 Piper Diagram for Groundwaters in the Upper Sonoyta River Basin ...... 149 C.7 Graph of δ2H versus δ18O Values for Rainfall ...... 150 C.8 Graph of δ2H versus δ18O Values for Tinajas ...... 151 C.9 Piper Diagram for Spring and Well Samples ...... 152 C.10 Graph of δ2H versus δ18O Values for Mean Rainfall, Groundwater, and Spring Samples ...... 153 C.11 Map of Tritium and Carbon-14 Distribution ...... 154 C.12 Map of Quitobaquito Spring Area ...... 155 2- - C.13 Graph of SO4 versus Cl for Local Waters ...... 156 - C.14 Graph of Cl/SO4 versus Cl for Local Waters ...... 157

LIST OF TABLES

A.1 Field Parameters, Major Ion and Stable Isotope Data for Lower Colorado River Area ...... 65 A.2 Average Water Compositions ...... 69 A.3 δ2H vs δ18O Values for Groundwaters in Southwestern Arizona and Northwestern Sonora ...... 70 A.4 δ2H vs δ18O Values for Rainfall at Organ Pipe Cactus National Monument ...... 71

B.1 Field Parameters, Major Ion and Stable Isotope Data for Gran Desierto Area ...... 113

C.1 Field Parameters, Major Ion and Stable Isotope Data for lower Sonoyta River Area ...... 158

ABSTRACT

The usefulness of environmental isotopes (δ18O, δ2H, 3H, and 14C) and other chemical tracers in hydrogeological research was evaluated in three case-studies in two different and adjacent basins located in southwestern Arizona, USA and northwestern Sonora, Mexico. In this dry region, located in the Sonoran Desert, riparian and wetland ecosystems have been replaced by irrigated fields and the local aquifers are prone to overdraft. The remaining functional ecological systems support a variety of terrestrial, avian, and aquatic fauna as well as a diversity of vascular plants that contrast with the surrounding desert. These ecosystems heavily rely on and compete with human settlements for water resources that are expected to decline as climate warms (Barnett, 2008). Detailed hydrogeochemical studies that address the origin of aquifer recharge and groundwater residence time are needed to understand the impacts of increased groundwater use and expected intensified drought in the area (Seager et al., 2007; Ault et al.,

2016).

In Appendix A, we use environmental isotope data and major ion chemistry from surface and groundwaters in the lower Colorado River to understand aquifer recharge mechanisms and the geochemical evolution of groundwaters in this transboundary aquifer. We find that locally- recharged groundwaters are dominated by Na-Cl of meteoric origin and mix with Colorado River

3 14 waters of Ca-SO4 composition beneath the Yuma and San Luis Mesa. Low H and C data are consistent with bulk residence times of 5,700 corrected 14C years B.P.

In Appendix B, we use environmental isotope data and major ion chemistry from surface waters, groundwaters, and precipitation in the Gran Desierto wetlands, Sonora to establish aquifer recharge mechanisms, water origin, flow paths, and groundwater residence time on these enigmatic spring-fed wetlands. We find that local recharge originates as winter precipitation, but

is not the main source of water in the pozos. Instead, the pozos are fed by evaporated Colorado

River water following flow paths created by the Altar Fault. The environment that allowed

recharge to the aquifer feeding the pozos no longer exists, and the pozos are now vulnerable to

major groundwater pumping and development in the area.

In Appendix C, we use environmental isotopes and water chemistry to distinguish water

types, recharge mechanisms, and residence time along several reaches of the Sonoyta River and

Quitobaquito Spring. We find that areas located up gradient from the Sonoyta River are

supported by local recharge which corresponds to water from the largest 30% of rain events

mainly occurring during winter. Quitobaquito Spring is supported by 1) a mix of modern recharge and Pleistocene-aged groundwater or 2) Sonoyta River water supplying water through a suggested fault system connecting the spring to the alluvial aquifer beneath the. Ionic ratios seem to support the latter explanation. Curent drought conditions and groundwater use have reduced or eliminated recharge to the spring.

This work shows that environmental isotopes (δ18O, δ2H, 3H, and 14C) are among the

most useful suite of tracers in groundwater and surface water in southwestern Arizona and

northwestern Sonora. The distinctive isotopic composition derived particularly from altitude effects permits a clear identification of water sources in high-relief basins in the Basin-and-

Range Province. The results demonstrate the dependence of winter recharge in the area and the importance of high elevation recharge in the lower elevations of the basin.

CHAPTER 1: INTRODUCTION

The Colorado River and the Sonoyta River are two transboundary systems shared by

Mexico and the USA. The lower reach of the Colorado River and the Sonoyta River are both

located in the Sonoran Desert (Fig. 1). At first sight, the two systems seem overwhelmingly

different. The Colorado River basin covers an area of 637,137 km2 compared to the 12,628 km2

of the Sonoyta River basin (Harshbarger & Associates, 1978), the Colorado River used to flow

2,334 km from the Rocky Mountains in Colorado to the Gulf of California and the Sonoyta River

length is only 280 km, the Colorado River irrigates 1.2 million ha of farmland only in the

Mexicali and Imperial valleys (Barnett et al., 2009) while the Sonoyta River only irrigates a total

of 15,000 ha (Rosen et al., 2010; Hollet, 1995). Yet, they also share many similarities.

Unknown to most, the Sonoyta River was a tributary of the Colorado River until the

Pleistocene, when the Pinacate lava flows diverted the former connection to its present location

(Donnelly 1974; Miller and Fuiman, 1987). In most recent times, both systems have been subject to a variety of anthropogenic perturbations, including dam construction, flow diversions, heavy use for agriculture, and municipal and industrial development, among others. These anthropogenic activities have severely degraded natural ecosystems within the river and its floodplain.

The Colorado River no longer reaches the lower part of the delta today. Its waters are diverted to support the irrigation of pastures and crops in the San Luis and Mexicali valley.

Riparian, wetland, and estuarine habitats in the delta occupy less than 5% of their original

780,000 ha extent (Zamora-Arroyo and Flessa, 2009). In the Sonoyta River, perennial surface flow and wetlands occurred along several reaches prior to the 19th century, but were significantly

reduced also by the diversion of surface water into irrigated fields (Rosen et al., 2010). Given

the scarcity of rainfall and limited surface-water resources, the area now relies heavily on groundwater for beneficial uses. The combination of increasing groundwater demand for

irrigation and municipal use, on both sides of the border, with drought conditions has contributed

to the decline of water levels in many parts of both aquifers (Ramírez-Hernández et al., 2013;

Minckley et al., 2013).

Figure 1. Map of study area. Dashed blue lines show dry river beds. Black polygon shows the location of Organ Pipe Cactus National Monument (OPCNM).

Besides these problems, the Colorado River delta and the Sonoyta River still support a

series of aquatic ecosystems that are sustained by occasional pulse floods, underflows,

agricultural return flows, or springs (Glenn et al., 1996; Ezcurra et al., 1998; Rosen et al., 2010).

The plant diversity of the remaining aquatic ecosystems contrasts with the surrounding desert

(Felger, 2001). These sites serve as a refuge to a variety of aquatic vertebrates, and terrestrial and

avian fauna some of which have been recognized as endangered or threatened (Minckley et al.,

2013).

The sustainability of groundwater resources, including the life-supporting springs and wetlands fed by natural groundwater discharge, highly depends on the delicate balance of recharge and depletion (Stonestrom, 2008). Groundwater recharge results from a complex interaction of climate, geology, and vegetation across a range of spatial and temporal scales

(Stonestrom, 2008). In this area of the world, water-resource planning relies on identifying the source, timing, and location of recharge and understanding the interacting processes that control it (Stonestrom, 2008).

As part of this study, we use environmental isotopes (δ18O, δ2H, 14C, and 3H) and water

chemistry to distinguish among different water types, timing and location of recharge, potential

groundwater flow paths, and sources of salinity. In basins with high relief, such as those found

in southwestern North America, stable isotopes in recharge preserve the signatures related to the

altitude and seasonality of precipitation and help to establish the origin of natural waters (e.g.

Eastoe and Towne, 2018; Winograd et al., 1998; Cunningham et al., 1998). Radioactive tracers

have been successfully used to estimate groundwater residence times and establish areas of

active recharge (Eastoe et al., 2004; Sanford et al., 2004). Anions such as chloride and bromide

have been useful to evaluate sources of salt in groundwaters (Herczeg et al., 2001), estimate

groundwater recharge (Subyani, 2004), and reconstruct the origin and movement of groundwater

(Davis et al., 1998).

Here, we present a systematic attempt to improve our understanding of groundwater

dynamics in these two basins located in the heart of the Sonoran Desert. These studies become

particularly relevant in this part of the world where ecological systems and human settlements

heavily rely on water resources that are expected to decline due to human-induced climate

change (Barnett et al., 2009). Further insight on recharge dynamics could potentially enhance

our ability to assess and mitigate the susceptibility of aquatic environments and ground-water resources to natural and anthropogenic climatic shifts (Stonestrom, 2008).

CHAPTER 2: PRESENT STUDY

Details of the study area within the lower Colorado River and the Sonoyta River basin, methods, results, and conclusions of this study are presented in the manuscripts appended to this dissertation. The purpose of this chapter is to introduce the objectives and provide a summary of the most important findings of the study. Here, we will use three case studies in southwestern

Arizona and northwestern Sonora as examples to demonstrate the usefulness of environmental isotopes (δ18O, δ2H, 14C, and 3H) and major ion chemistry in solving hydrogeochemical problems.

The objectives of this study include the following: 1) characterize the isotopic and geochemical composition of rainfall, surface water, and groundwaters, 2) identify sources of aquifer salinity, 3) identify seasonality of local recharge, 4) determine groundwater flow paths and groundwater origins, and 5) estimate residence time of groundwaters. The specific objectives of every case study are:

2.1 Lower Colorado River

a) Investigate the isotopic and geochemical composition of surface and groundwaters within

the study area, and nearby localities;

b) Characterize the isotopic evolution of evaporated Colorado River water;

c) Map the spatial distribution of δ2H values and dissolved Cl-;

d) Identify the major sources of salinity in groundwater, and possible geochemical processes

that might affect its ionic composition;

2.2 Gran Desierto Wetlands

a) Characterize the isotopic and geochemical composition of rainfall, surface water, and

groundwaters;

b) Identify the isotopic composition and seasonality of local recharge;

c) Investigate groundwater recharge sources in the Gran Desierto aquifer and the pozos;

d) Provide an estimate of groundwater residence times;

e) Evaluate groundwater dynamics and flow paths throughout the aquifer

2.3 Lower Sonoyta River

a) Identify seasonality, isotopic composition, and geochemical characteristics of local

recharge;

b) Estimate isotopic composition of high elevation recharge;

c) Provide an estimate of groundwater residence times;

d) Investigate flow paths and sources for aquifer recharge at the Quitobaquito Spring system

located within the Organ Pipe Cactus National Monument.

The three case studies are interrelated in the following manner: (1) the isotopic and geochemical evolution of surface and groundwaters within the lower Colorado River can help to identify possible sources of aquifer recharge in the Gran Desierto wetlands, (2) the seasonality and isotopic composition of local recharge seem to be very uniform throughout the lower Colorado

River and the lower Sonoyta River basin, and (3) both areas have been subjected to anthropogenic activities that have degraded natural ecosystems. Yet, they still support a series of aquatic environments of ecological importance in need of future management strategies.

The conclusions of this study show the usefulness of the combination of environmental isotopes and major ion chemistry to identify the seasonality and composition of recharge at a given location, sources and origin of aquifer recharge, groundwater flow paths and residence time, sources of aquifer salinity, and groundwater mixing in the aquifer. These tools and the

information they provide are particularly useful in high-relief basins where waters of different origins mix.

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Science, 319: 1080-1083

Barnett, T.P., Pierce, D.W., & Gleick, P. (2009). Sustainable water deliveries from the Colorado

River in a changing climate. Proceedings from the National Academy of Science. Vol.

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Cunningham, E., Long, A., Eastoe, C., & Bassett, R. (1998). Migration of recharge waters

downgradient from the Santa Catalina Mountains into the Tucson basin aquifer, Arizona,

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Davis, S.N., Whittemore, D.O., & Fabryka-Marin, J. (1998). Uses of chloride/bromide ratios in

studies of potable water. Ground Water. Vol. 36, 338-350.

Donnelly, M.F. (1974). Geology of the Sierra el Pinacate volcanic field, northern Sonora,

Mexico, and southern Arizona, U.S.A. Unpublished Ph.D. dissertation, Stanford

University, Stanford, CA.

Eastoe, C., & Towne, D. (2018). Regional zonation of groundwater recharge mechanisms in

alluvial basins of Arizona: Interpretation of isotope mapping. Journal of Geochemical

Exploration, 194: 134-145.

Felger, R. (2001). Flora of the Gran Desierto and Rio Colorado of Northwestern Mexico.

Tucson: The University of Arizona Press.

Glenn, E.P., Lee, C., Felger, R., & Zengel, S. (1996). Effects of water management on the

wetlands of the Colorado River delta, Mexico. Conservation Biology. Vol 10, 1175-1186.

Harshbarger & Associates, Inc. (1979). Overview report of groundwater basins along

international boundary Arizona, U.S. and Sonora, Mexico. Inter. Boundary & Water

Comm., U.S. Section, Preliminary Report, Pr-236-79-1, 124 p.

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eremus, a new subspecies of pupfish from Organ Pipe Cactus National Monument,

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Aquiatic Vertebrates: Conservation and Management in the Río Sonoyta Basin, Sonora,

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APPENDIX A:

Groundwater origin and dynamics on the eastern flank of the Colorado River delta,

Mexico.

Groundwater origin and dynamics on the eastern flank of the Colorado River delta,

Mexico.

Hector A. Zamora1, Christopher J. Eastoe1,4, Jennifer C. McIntosh2, Karl W. Flessa1, & Jeffrey

Welker3.

1. University of Arizona, Department of Geosciences, 1040 E. 4th Street, Tucson, Arizona,

85721, USA.

2. University of Arizona, Department of Hydrology and Atmospheric Sciences, 1133 E.

James E. Rogers Way, Tucson, Arizona, 85721, USA.

3. University of Alaska Anchorage, Department of Biological Sciences, 3211 Providence

Drive, Anchorage, Alaska, 99508, USA.

4. Retired.

A.1 Abstract

Environmental isotope data and major ion chemistry from groundwaters in the lower

Colorado River area were used to understand aquifer recharge mechanisms and the geochemical

evolution of groundwater in a transboundary aquifer vulnerable to overdraft and salinization.

Local recharge originates as precipitation and occurs during winter through pathways of

preferential infiltration along the base of the Gila Range. Locally-recharged groundwater is dominated by Na-Cl of meteoric origin and is highly concentrated due to the dissolution of soluble salts accumulated in the surface. Hydrochemical evolution in the irrigated floodplain is controlled by the original Ca-SO4-type Colorado River water, but salinity is increased through

evapotranspiration, precipitation of calcite that leads to dissolution of gypsum by the common

ion effect, dissolution of accumulated soil salts, dedolomitization, and exchange of aqueous Ca2+

for adsorbed Na+. The Na-Cl dominated local recharge flows southwest from the Gila Range,

and mixes with the Ca-SO4 dominated floodplain waters beneath the Yuma and San Luis Mesa.

Low 3H concentrations suggest that recharge within the Yuma and San Luis Mesa occurred

before the 1950’s, and 14C data are consistent with bulk residence times up to 5,700 corrected 14C

years before present. This local flow system is not actively recharged or recharge occurs at a rate

that is significantly lower than what is being withdrawn leading to aquifer overdraft and

deterioration.

A.2 Introduction

The Colorado River is a strictly managed river. Water regulatory practices are

implemented to provide vital water resources to seven states in the U.S. and two states in Mexico

along its 2500 km course from the Rocky Mountains (Fig. 1). In the lower Colorado River basin,

which begins south of the Utah-Arizona border at Lee’s Ferry, more than 27 million people

depend on the river for sustenance. Nearly 1.2 million ha of farmland are irrigated with

Colorado River water in the fertile and productive fields of the Mexicali and Imperial Valleys

(Barnett et al., 2009).

The lower Colorado River basin has seen extensive land-use changes in the last century.

Native vegetation has been replaced by pastures and crops, and surface Colorado River water has been diverted for irrigation. These changes led to a massive loss of natural habitat in the

Colorado River delta (termed “delta” below). The river no longer reaches the lower part of the delta today, and riparian, wetland, and estuarine habitats occupy less than 5% of their original

780,000 ha extent (Zamora-Arroyo and Flessa, 2009). The delta still supports a few aquatic ecosystems of international relevance which are sustained by occasional pulse floods, underflows, and agricultural return flows (Glenn et al., 2013).

The aim of this study is to establish sources of solutes, sources of aquifer recharge, groundwater residence time, and geographic variation of major ion chemistry in groundwater on the eastern flank of the Colorado River delta. This is accomplished by using new and historical environmental isotope data (δ18O, δ2H, 3H, and 14C) and major ion chemistry. In the over- allocated Colorado River system, distinguishing the different sources of water and salt becomes increasingly important for the long-term management and protection of water resources and the natural and semi-natural habitats that depend on them.

The results of this investigation are used to evaluate groundwater dynamics and geochemical process of this transboundary aquifer along the U.S.-Mexico border between

Arizona and Sonora. An improved understanding of hydrogeochemical processes that control groundwater sources and its chemical evolution provides information that is vital for the effective management and use of groundwater resources in this aquifer vulnerable to overdraft

and salinization. This information also serves as a baseline for understanding the effects of

groundwater pumping in natural areas further south.

A.3 Study Area

A.3.a Geography

The Colorado River delta lies along the western boundary of the Sonoran Desert and

within the Salton Trough geologic region. The delta extends from the confluence of the

Colorado and Gila River near Yuma, Arizona to the Gulf of California, and covers an area of

more than 600 km2 (Fig. 1; Valdés-Casillas et al., 1998). The delta is delimited to the west by

the Sierra Cucapah in Baja California, Mexico. The Gran Desierto de Altar (Altar Basin), the

largest area of active sand dunes in North America is located to the east of the study area in the

state of Sonora, Mexico, and covers an area of 5500 km2 (Lancaster et al., 1987). The Gila River

historically flowed around the Gila Range, an arid and rugged, northwest-southeast trending range with its highest elevation at 960 meters above sea level (Fig. 1), before joining the

Colorado River. Today, the Gila River rarely reaches the area under normal conditions. The lower Colorado River marks the western boundary for the purposes of this study.

A.3.b Climate

The area’s climate is warm and arid. Records from 1948 to 2009 near Morelos Dam in

Baja California show maximum temperatures reaching 50 °C (CONAGUA, 2012). Annual

evaporation rates range between 3.2 and 1.9 m, and annual precipitation averages only 52 mm

(CONAGUA, 2012). Precipitation occurs as sporadic events during winter and summer seasons

as cyclonic and convective events, respectively (Alvarez-Borrego, 1975). Southeast winds coming from the Gulf of California are predominant during spring and summer, and generally bring high temperatures and humidity (Flessa et al., 2012). Northwest winds are typical during

fall and winter seasons, and are often accompanied by cold fronts and lower temperatures (Flessa et al., 2012).

A.3.c Geology

The San Luis, Mexicali, and Yuma valleys are part of a graben with a basement at a depth of 3200 m that consists of igneous and metamorphic rocks of Cretaceous age (Payne et al.,

1979). The valley fill contains marine sediments deposited during transgressions of the Gulf of

California, and sedimentary deposits of continental origin (Van Der Kamp, 1973). Sediment exposed at the surface is of alluvial type, and represents the most recent cycle of deposition from the Colorado River and the Gila River (Olmsted et al., 1973). The most abundant material in the area is fine sand with silt, and thin layers of clay. These clay units are found along the Yuma

Valley and beneath the Yuma Mesa, and extend south into the Altar Basin in Mexico (Dickinson et al., 2006; CONAGUA, 2008).

The escarpment that divides the western edge of the Altar Basin and the delta was formed by the Cerro Prieto Fault, the southernmost extension of the San Andreas Fault system. The fault passes through the study area and continues into the Gulf of California (Fig. 1; Merriam, 1965).

The eastern boundary of the Altar Basin is the seismically inactive Altar Fault (Fig. 1; Pacheco et al., 2006). The Cerro Prieto and Altar Faults trend are NW-trending and dip to the west. Both faults are inferred to have significant dextral offset, and both drop the southwestern side down.

The Altar Fault affects groundwater movement and offsets the water table, as observed in the

Yuma Mesa, as the result of either juxtaposition of materials with different permeability, or the presence of fault gouge and mineralization along the fault zone (Fig. 2; Olmsted et al., 1973).

Strike-slip movement along the Cerro Prieto fault system is estimated to be as high as 60 mm/year (Curray and Moore, 1984; Gastil et al., 1975).

A.3.d Colorado and Gila River Discharge

Prior to the development of major river engineering projects (1930s – 1960s), such as

Hoover Dam and Glen Canyon Dam, the Colorado River flowed along its natural course and discharged into the Gulf of California. Natural annual flows ranged between 1.6 x 1010 m3 and

1.8 x 1010 m3 at Lee’s Ferry (Meko and Woodhouse, 2007). Peak flows occurred from April to

June when late-spring snowmelt from higher elevations entered the area (Fig. 3). The Gila River contributed an estimated 0.16 x 1010 m3 per year to Colorado River discharge at its confluence

near Yuma before upstream diversions dewatered the river (U.S. Bureau of Reclamation, 1952).

The entire flow of the Colorado is now captured and used before reaching the river’s

mouth, and south of the U.S.-Mexico border no water flows, except during unusually wet years

and during engineered environmental flows resulting from water treaties between the U.S. and

Mexico (Morrison et al., 1996; Flessa et al., 2013). Climatic anomalies arise from El Niño

Southern Oscillation, and affect the entire Colorado River catchment. When upstream storage

reservoirs are full, high precipitation during El Niño years can increase river discharge. This was

observed at the U.S. - Mexico border where daily discharges peaked at 935 m3/s during the mid-

1980s (Valdés-Casillas et al., 1998).

A.3.e Hydrology

The aquifer of the delta is formed by the upper fine - and medium - grained sediment of

the younger alluvium (Ariel Construcciones, 1968; Diaz Cabrera, 2001; Olmsted et al., 1973).

Flood plain sediments are highly permeable with transmissivity values of up to 10,788 m2/day

(Metzger and Loeltz, 1973). Direct infiltration from the Colorado River and overbank flooding

were the main source of recharge to the aquifer prior to major agricultural development

(Ramirez-Hernandez et al., 2013; Dickinson et al., 2006). Today, the aquifer has an annual

recharge of 755 x 106 m3 by infiltration from unlined irrigation canals supplied with Colorado

River water, and groundwater coming from the Colorado River Basin upstream of Yuma, north

of the U.S.-Mexico boundary (Payne et al., 1979; Orozco-Durán et al., 2015). Diffuse recharge,

or direct infiltration and percolation after precipitation, is assumed to be negligible. The

extensive alluvial fans observed at the base of the Gila Range suggests that mountain system

recharge (MSR) and focused recharge in ephemeral streams is likely to occur just like in other

semi-arid basins in southern Arizona (Wahi, 2008; Meixner et al., 2016).

Regional groundwater flowed in a northeast-southwest direction from the junction of the

Colorado River and Gila River near Yuma, Arizona to the northern Gulf of California (Diaz

Cabrera, 2001; Fig. 2). Unlined canals and groundwater pumping have disturbed the source and

sink patterns of water movement to and from the aquifer within the delta (Dickinson et al.,

2006). Regional groundwater flow direction has remained generally constant, but in areas where

long-term surface irrigation has occurred (e.g. Yuma Valley), groundwater levels are higher now

than during pre-development time (Fig. 2). Several reaches along the river now act as drains for

groundwater where groundwater levels are high, but less than 3.5 x 107 m3 (4.6%) of the yearly

Colorado River flow discharges as groundwater into the Gulf of California (CONAGUA 2006,

2007, 2010). Cones of depression have formed along the Arizona-Sonora border where pumping rates are high and recharge rates are low (Ramirez-Hernandez et al., 2013). Inputs of industrial and urban wastewater, as well as cattle and farming activities, have affected the quality of ground and surface water in the delta (Orozco-Durán et al., 2015).

A.3.f Ciénega de Santa Clara

The Ciénega de Santa Clara (Ciénega, below) is one of the few brackish or freshwater habitats that remain along the old course of the Colorado River in Mexico. The Ciénega lies along a shallow depression on the eastern edge of the delta, along a formerly active arm of the

Colorado River, and covers an area of 6,000 ha mostly dominated by Typha dominguensis (Fig.

1, Glenn et al., 1996). It is an “off-channel” wetland, its water does not come directly from the

Colorado River. The most important source of water for the Ciénega is brackish groundwater

(>2.6 ppt) derived from the Wellton-Mohawk Irrigation Drainage District of Arizona (WMIDD).

The Wellton and Mohawk valleys are irrigated with Colorado River water. Excess reflux is transported to the Ciénega by a concrete-lined canal, the Wellton-Mohawk Drain, which delivers

1.3 x 108 m3/y. The Riito Drain, which transports wastewater from Mexican agriculture, supplies approximately 1.4 x 107 m3 to the Ciénega (Flessa et al., 2012).

A.4 Previous Studies

Isotope data (δ18O, δ2H, 3H, and 14C) and major ion chemistry have been previously used in the lower Colorado River valley and in the delta to identify water and salinity sources (Guay et al., 2006; Makdisi et al, 1982; Payne et al., 1979). As a result of fractionation, waters develop distinctive isotopic compositions that indicate their sources or the processes to which they have been exposed (Clark and Fritz, 1997). Payne et al., 1979 identified groundwater in the study area originating from pre-dam Colorado River water (water recharging the aquifer prior to Hoover

Dam completion in 1936), and modern or post-dam Colorado River water. Pre-dam Colorado

River water in the Mexicali Valley has average δ18O and δ2H values of -14.6‰ and -112‰, respectively (Payne et al., 1979). These values are similar to average δ18O and δ2H values for

Colorado River headwaters that plot on or near the global meteoric water line (GMWL; Craig,

1961; Guay et al., 2006). Post-dam Colorado River water has higher average δ18O and δ2H

values than old Colorado River water, and plots away from the GMWL due to evaporation while

stored in the upstream reservoir lakes, particularly while stored at Lake Mead just upstream of

Hoover Dam (Guay et al., 2006).

Tritium (3H) and Carbon-14 (14C) provide estimates of the time since groundwater

recharge occurred. Tritium concentrations for modern Colorado River water were 5.3 tritium

units (TU) by 2017 and higher in previous years. Carbon-14 values for Colorado River water

near Yuma, Arizona range between 98 and 104 percent modern carbon (pMC) in 2009 (Haber,

2009). The atmosphere in southern Arizona contained 14C at a level of 105 pMC between 2009

and 2012 - slightly higher than pre-bomb levels (Chris Eastoe, University of Arizona, unpublished data, 2017).

A.5 Methods

A.5.a Field Methods

Surface water samples were collected from the Ciénega, Wellton-Mohawk Drain, and

Colorado River at Yuma, Arizona during May 2013, October 2013, and July 2014 (Fig. 4).

These surface water samples were used to establish evaporation trends of Colorado River water.

Groundwater samples were collected from the Minute 242 well field along the Arizona- Sonora

border in October 2016. These groundwater samples were used to evaluate potential water

sources besides Colorado River water. Additionally, two bulk sediment samples from the San

13 Luis Mesa were analyzed for δ CCaCO3 to attempt to correct 14C ages.

Temperature, pH, dissolved O2, and electrical conductivity (EC) levels were measured in

the field after each parameter had stabilized. Samples for oxygen, hydrogen, and carbon (DIC)

stable isotopes were filtered with a 0.45-μm nylon filter and kept in capped glass vials with no

3 14 headspace. Unfiltered water samples were collected for H and C analysis in rinsed 1-L HDPE

and amber borosilicate glass bottles, respectively. Samples for ions and alkalinity were filtered

with a 0.45-μm nylon filter and kept in HDPE bottles. Cation samples were preserved with

concentrated optima grade HNO3. All samples were kept on ice while in the field and then

refrigerated at 4 °C prior to analysis. Alkalinity was determined by the Gran-Alkalinity titration

- method (Gieskes and Rogers, 1973) within 12 hours of collection, but is expressed as HCO3

assuming dominance of this anion at the observed pH values, and to be consistent with units used

in previous studies.

A.5.b Laboratory Methods

δ18O and δ2H values were measured at the Environmental Isotope Laboratory,

Department of Geosciences, University of Arizona. δ18O and δ2H values were determined on a

Finnigan Delta-S mass spectrometer with automated CO2 equilibration and Cr reduction

attachments. Analytical precisions (1σ) for these techniques are 0.08 % for δ18O and 0.9% for

δ2H. δ18O and δ2H data are reported in delta notation , where R is

the ratio of the heavier over the lighter isotope in the sample, and Rstd is the isotope ratio of

Vienna Standard Mean Ocean Water (VSMOW).

13 δ CDIC values were measured on a Thermo-Finnigan Delta Plus XL continuous-flow gas-

ratio mass spectrometer coupled with a Gasbench automated sampler. Samples were reacted for

>1 hour with phosphoric acid at room temperature in Exetainer vials previously flushed with He gas. Standardization is based on NBS-19 and NBS-18 and precision is + 0.30 ‰ or better (1σ).

All δ13C values are expressed in delta notation relative to the Vienna Pee Dee Belemnite (VPDB)

standard.

Tritium values were measured by liquid scintillation counting on electrolytically enriched water in a Quantulus 1220 Spectrophotometer with a detection limit of 0.5 tritium units (TU) for

9-fold enrichment and 1,500 min of counting. One TU is equivalent to one tritium atom in 1018 atoms of hydrogen. Carbon-14 was measured as liberated CO2 reduced to graphite at the NSF-

Arizona Accelerator facility. These results are reported as percent modern carbon (pMC) relative to NBS standards Oxalic Acid I and II.

Anion concentrations were determined in the Department of Hydrology and Water

Resources at the University of Arizona using a Dionex Ion Chromatograph model 3000 with an

AS23 analytical column (precision + 2 %). The analyses for cations were performed by the

Arizona Laboratory for Emerging Contaminants (ALEC) at the University of Arizona using a

Perkin-Elmer Elan-II Inductively Coupled Plasma – Mass Spectrometer (precision + 2 %).

A.6 Data

Data previously reported were included in the final dataset for this study (Table A1).

This dataset contains published and unpublished results for water samples from agricultural wells in the San Luis Valley (Palomares-Ramirez, 2011), Yuma Mesa (USGS, 2016; Dickinson et al.,

2006), and lower Colorado River floodplain (Makdisi et al., 1982; Payne et al., 1979). USGS data is available online at https://waterdata.usgs.gov/nwis from the USGS National Water

Information System (USGS, 2016).

Average δ18O, δ2H, and solute chemistry values for the different potential water sources in the area were determined from other existing databases or publications, and are used for comparison with our results in the study area (Table A2). These endmembers include pre-dam

Colorado River water, post-dam Colorado River water, agricultural discharge, Gila River, and

local recharge. Pre-dam Colorado River water ion concentration and δ18O and δ2H values were

approximated using data from USGS station 9380000 at Lee’s Ferry. This station was used

because it is located upstream from Lake Mead, where enrichment by evaporation occurs.

Colorado River water near Lee’s Ferry is assumed to be a good representation of water reaching

the delta prior to major development along the lower Colorado River. For post-dam Colorado

River water δ18O, δ2H, and ion values were calculated using data from USGS station 9522000 at

Morelos Dam. For agricultural discharge δ18O, δ2H, and ion values were calculated using data from USGS station 9529300 at the Wellton-Mohawk Drain, Towne (2017), and Payne et al.,

(1979). Values of δ18O, δ2H, and ion concentrations in the upper Gila River, near Safford, were

calculated using well data from Towne (2009). Data for these USGS stations is also available

online (USGS, 2016).

Locally-recharged δ18O and δ2H groundwater values were calculated using well data from USGS (2016), Towne (2017), and the authors near the study area (Table A3). These wells

are all located in small catchments in southwestern Arizona or northwestern Sonora, and their

respective aquifer receives recharge from mountain ranges with peak elevations around 1200

meters or less. The average δ18O and δ2H values are assumed to be a good representation local

groundwaters located away from the influence of major streams. This endmember is referred to as “local recharge” throughout the text and figures.

Finally, multi-year rainfall isotope data collected at Organ Pipe Cactus National

Monument (OPCNM) was used to estimate long-term δ18O and δ2H average values or

precipitation, and also the seasonality of recharge in the area (Table A4). These data represent

individual events collected between 1990 and 2016 through the United States Network for

Isotopes and as part of this study (USNIP; Welker, 2012). OPCNM is located 175 kilometers

east of the study area at an elevation of 515 masl which is similar to the average elevations in the

Gila Range.

All δ18O and δ2H single values for the different endmembers are shown in Fig. 5A, and

their average value is shown in Fig. 5B.

A.6.a Evaporated Colorado River Water

A method described by Clark and Fritz (1997) was used to model the δ18O and δ2H

values of Colorado River water at different degrees of evaporation. Average pre-dam Colorado

River water is used as a starting point (-15‰ and -115‰). A displacement of data to the right of the GMWL reflects evaporative loss. Average humidity is assumed to be 60% to obtain an evaporation slope between 5 and 6 which is characteristic of evaporated Colorado River water in the area (Robertson, 1991; Guay et al., 2006). Equilibrium and kinetic fractionation factors for

18O and 2H are calculated using the following equations (Majoube, 1971; Gonfiantini 1986):

(1)

(2)

(3)

(4)

In Eqns. 1 and 2, T is the mean annual temperature (K) and α is the fractionation factor. A

temperature of 298 Kelvin is assumed for calculation purposes. This temperature is nearly

identical to the average temperature at Yuma, Arizona (296 Kelvin; AZMET, 2017). In Eqns. 3

and 4, h is the relative humidity (0.60).

The enrichment factor ε is calculated using equation 5.

(5)

The evaporative enrichment for δ18O and δ2H values can be modelled, according to a Rayleigh

distillation, by assuming different residual water fractions ( ) in the following equation (6):

(6)

18 18 ε Ototal is the overall enrichment for O in this case. The overall enrichment for evaporation

under the specified conditions is +15.06‰ for 18O, and +84.51‰ for 2H. The result of Equation

6 is added to the average δ18O and δ2H values of old Colorado River water to model the

evolution of Colorado River water under different degrees of evaporation. The results of these

calculations are shown in Fig. 6.

A.7 Results

A.7.a Major Ion Trends

The distribution of predominant anions and cations shows that surface Colorado River water evolves from a Ca-HCO3 dominated water type in its headwaters into Na-Ca-Cl-SO4

dominated waters as it travels downstream, and reaches the international border with Mexico

(Fig. 7). Colorado River floodplain samples are divided between Ca-SO4 and Na-Cl water types

within the study area. Samples belonging to the Yuma and San Luis Mesa and Gila Range

foothills groups show Na+ and Cl- as the predominant ions (Fig. 7).

The saturation indices of water samples were calculated using the hydrogeochemical

equilibrium model of PHREEQC (Parkhurst and Appelo, 1999). All groundwater samples in the

floodplain, Yuma and San Luis Mesa, and Gila Range foothills are undersaturated with respect to

+ - 2+ -2 halite, gypsum, and anhydrite (SI < 0). This allows Na , Cl , Ca , and SO4 concentrations to

increase along the flow paths. The majority of the groundwater samples are supersaturated or

close to saturation with respect to dolomite, calcite, or both (SI between -1 and 1) indicating a strong presence of these two minerals in the aquifer system.

A.7.b Stable Isotopes

A.7.b.1 Endmembers and Evaporation Calculation

A summary for the average (δ18O, δ2H) values of the different endmembers is shown in

Table A2, and Fig. 5B. Pre-dam Colorado River water values are -15‰ and -115‰, post-dam

Colorado River water values are -12‰ and -97‰, agricultural discharge values are -10.8‰ and -

89‰, Gila River water values are -9.9‰ and -71‰, and local recharge values are -7.5‰ and -

53‰. Rainfall isotope data yielded average values of -7.1‰ and -45‰ for winter, -5.1‰ and -

33 for summer, -6.1‰ and -39‰ overall, and -7.5‰ and -50‰ for the 30% wettest events.

Surface water samples from the Ciénega (SURF in Fig. 6) have δ18O values between -

10.6‰ and +6.0‰, and δ2H values between -88‰ and +8‰. The highest δ18O and δ2H values

are located in the southern part of the Ciénega near the tidal flats (sites 76 and 77). All water

samples fall to the right of the GMWL (Fig.6). For the Ciénega, the regression line of δ18O and

δ2H values has a slope of 5.8 in the δ18O vs δ2H plot and is referred to as the Colorado River

Evaporation Trend (CRET) in all δ18O vs δ2H plots (Fig. 6). The slope is within the range of

other studies in the area (5-6) and is characteristic of evaporated Colorado River water

(Robertson, 1991; Guay et al., 2006). The modelled evaporation calculation shows that some of

the samples in the Ciénega have gone through a high degree of evaporation (some exceeding

50%).

A.7.b.2 Study Area Data

Stable isotope data for the study area is shown in Figs. 8 (A and B). The (δ18O, δ2H)

values of Colorado River (CR) collected at Yuma, Arizona were -11.8‰ and -95‰, respectively

(Table A1). Wellton-Mohawk Drain discharge (WMD) had (δ18O, δ2H) values of -10.6‰ and -

87‰, respectively, at the time of sampling. Water samples from wells in the Colorado River floodplain (CRFP), on both sides of the border, have δ18O values between -9.4‰ and -14.7‰,

and δ2H values between -75‰ and -112‰. Water samples from wells in the Yuma and San Luis

Mesa (MESA) have δ18O values between -7.9‰ and -14.9 ‰, and δ2H values between -60‰ and

-114‰. Water samples from four wells near the Gila Range (G. RANGE) have δ18O values between -7.6‰ and -8.7‰, and δ2H values between -55‰ and -67‰.

A.7.c 3H and 14C

Tritium and 14C concentrations for Colorado River water at Yuma, were ~5 TU in 2017

and ~101 (between 98 and 104) pMC in 2009, respectively (Table A1). However, values for 3H

and 14C were higher during the previous decades when more bomb-pulse 3H and 14C were

present in the atmosphere. Water delivered by the Wellton-Mohawk drain is derived mainly

from Colorado River water, so the concentration in agricultural discharge is assumed to at least

similar to post-dam Colorado River water values: > 5 TU. Colorado River floodplain samples

range between 5 and 16 TU, and San Luis Mesa samples range between <0.1 and 15 TU. Three

14C measurements from the San Luis Mesa have 59, 29, and 26 pMC, corresponding to

uncorrected 14C ages between 4,800 and 11,500 14C years before present. Corrected 14C ages,

13 using the Fontes-Garnier model (Fontes and Garnier, 1979), and assuming a δ CCaCO3 value of 0

13 -2 14 ‰, δ Csoil-CO2 value of -23 ‰, partial pressure of CO2 of 10 , and a Co of 105 pMC, ranged

between 4,300 and 7,000 14C years before present. The two bulk sediment samples from the San

13 Luis Mesa analyzed for δ CCaCO3 yielded an average of -5.6 ‰ (-5.9 and -5.4 ‰). The change

13 14 in the δ CCaCO3 value from 0 ‰ to -5.6 ‰ in the Fontes-Garnier model results in corrected C

ages between 2,200 and 5,700 14C years before present.

A.8 Discussion

A.8.a General Patterns

Colorado River headwaters are initially Ca-HCO3 dominated as a result of lower pH, and

the dissolution of primary silicate and carbonate minerals. These waters evolve into Ca-SO4 type

in the upper Colorado River in part due to the interaction with the local geology, anthropogenic

activities (e.g. mining and farming), and evaporative concentration where extensive irrigation of

land occurs (Fig. 7). Salts (halite and gypsum) dissolved from the Eagle Valley Evaporite,

Paradox Formation, Mancos Shale, Chinle Formation, and their associated soils, account for

approximately half of the total solutes in this part of the river (Tuttle et al., 2007; Repenning et

al., 1969).

Agriculture dominates the floodplain in the lower Colorado River area. Here, the

proportion of Cl- and Na+ in Colorado River water increases due irrigation reflux, marine salt

input, and/or halite evaporites in the lower Colorado River basin. Evaporation at Lake Mead and

mixing with return flow is evident from Fig. 5A where post-dam Colorado River waters plots to the right of the GMWL, and overlap agricultural discharge in some cases. Nearly 30% of the total river surface discharge has been lost to evapotranspiration by the time Colorado River water enters Morelos Dam, as suggested by the δ18O and δ2H values of post-dam Colorado River water

(Figs. 5 and 6).

The majority of Colorado River floodplain groundwater samples are Ca-SO4 dominated,

but a few samples fall within the Na-Cl facies. The opposite is true for the Yuma and San Luis

Mesa and Gila Range foothills where groundwater samples are Na-Cl dominated, but a few

samples fall within the Ca-SO4 domain (Fig. 7). The influence of SO4-rich Colorado River water

in the floodplain is evident in Fig. 9. While most of the groundwaters in the floodplain have

-2 2- SO4 values > 200 mg/L, those in the Mesa and Gila Range foothills have SO4 values < 200

mg/L.

Evaporated Colorado River water would fall along the 1:2 line in Fig. 9, which represents

- the Cl/SO4 mass ratio in post-Dam Colorado River water. There is an excess of Cl , relative to

-2 -2 SO4 , in virtually every water sample plotted. Bacterial SO4 reduction could drive water

- -2 samples to plot to the left of the evaporation line, creating an excess of Cl . However, SO4

-2 concentrations are relatively high, oxic conditions prevail in the unconfined aquifer, and SO4

reduction has only been noted in a few wells within the floodplain where organic matter is more

readily available (Olmsted et al., 1973). Thus, evaporation by itself does not explain the

- -2 - observed relationship between Cl and SO4 , and there are additional sources of Cl . These

additional sources of Cl- result in a wide range of Cl- concentrations which tend to be lower near

the Colorado River floodplain than in the Mesa (Fig. 10A).

The (δ2H, δ18O) values for groundwater in the study area are also quite diverse. Fig. 10B

shows the differences in δ2H in groundwaters. Within the floodplain, the observed range of δ2H

corresponds to mixtures of average pre-dam river water with δ2H values near -114 ‰ and post-

dam river water with average values near -97 ‰ (compare Fig. 8A). Mixtures with a high

proportion of pre-dam water (<-105 ‰) dominate groundwater beneath a broad area of the

floodplain in Mexico. The area is poorly constrained to the south, and this pattern may extend

further south than indicated in Fig. 10B.

The average local recharge (δ2H, δ18O) values (-7.5‰ and -53‰) are slightly lower than

the average winter precipitation values at the OPCNM rain gauge (-7.1‰ and -45‰), but are

consistent with the 30% wettest events (-7.5‰ and -50‰) which mainly occur during winter when precipitation is higher than evapotranspiration allowing infiltration and percolation into the water table (Fig. 5B; Eastoe and Towne, 2018; Jasechko and Taylor, 2015). Recharge at the base of the Gila Range is likely dominated by MSR and focused recharge along major washes similar to other semi-arid basins in southern Arizona (Wahi, 2008). Groundwater flows from these recharge zones in the mountain front margins of the Gila Range west into the Yuma and San

Luis Mesa. Local recharge is evidently present in at least one of the samples located at the base of the Gila Range (Fig. 8B, -7.6‰ and 55‰).

The groundwater levels (Fig. 2), location of the samples (Fig. 4), chemical and isotopic composition (Figs. 7 and 8) suggest that Na-Cl dominated groundwaters from the Yuma and San

Luis Mesa and the Gila Range (local recharge) are moving westward, and mixing with Ca-SO4-

dominated waters from the floodplain (Colorado River). This idea is illustrated in Fig. 10 which

shows higher Cl- and δ2H over the eastern side of the study, relative to groundwaters in the

floodplain, and intermediate values between them. Payne et al. (1979) and Makdisi et al. (1982)

suggested that the Na-Cl dominated waters along the border represent Gila River water.

Presence of Gila River water is not discarded, however, the provenance of the endmember with

higher δ2H and δ18O values than the floodplain waters is attributed to local recharge. Prior to

major development lower Gila River waters likely had δ2H and δ18O values consistent with high

elevation recharge (Gila River samples plotting closer to the GMWL [δ18O = -12 to -10‰] in

Fig. 5A). There seems to be an evaporation trend in the Gila River samples plotted in Fig. 5A that overlap the local recharge cluster. However, this is likely influenced by modern irrigation

and the infiltration and percolation of evaporated agricultural return which did not occur prior to major development in the floodplain. It is reasonable to believe that some degree of evaporation could have occurred along the Gila River prior to reaching the study area, however, it is difficult to know with certainty. Historical hydrochemical data along the lower Gila River is scant, and recent data show that today groundwater up to 90 kilometers upstream from the Colorado and

Gila River confluence is dominated by Colorado River chemistry (Towne, 2017). Peak flows in the Colorado River occurred from April to June when late-spring snowmelt arrived in the area and replenished the aquifer. Historical Colorado River streamflow was at least two orders of magnitude larger than the Gila River during high flow season. It is very likely that the two rivers mixed, even before their confluence, resulting in waters dominated by Colorado River chemistry, and a pure Gila Range endmember would be hard to find west of the Gila Range.

Independently of the origin of groundwaters in the eastern side of the study area, the low

3H levels indicate that recharge within the Yuma and San Luis Mesa occurred at least before the

1950’s, prior to the detonation of thermo-nuclear devices for most groundwater samples (Fig.

14), and the 14C data are consistent with bulk residence times of thousands of years (2,200 and

5,700 14C years before present). The combination of old water and limited modern recharge across the Mesa suggests that the aquifer is vulnerable to overdraft.

A.8.b Source of Solutes

A.8.b.1 Na+ and Cl-

Most waters from the Colorado River floodplain, Yuma and San Luis Mesa, and Gila

Range foothills have a Na/Cl equivalent ratio close to the trends corresponding to halite dissolution and seawater dilution (Fig. 11A). A Na/Cl equivalent ratio higher than one indicates release of Na+ from silicate weathering reactions (Meybeck, 1987). Silicate dissolution is

+ - unlikely a source of Na because where this occurs, HCO3 is the most abundant ion (Herczeg et

al. 2001), and this is not the case for any of the samples analyzed in this study.

Halite beds likely exist in the delta as a result of marine transgression/regression cycles

and seawater evaporation, but within the study area there is no evidence of them in well log data

(Olmsted et al., 1973). Partial dissolution of evaporite deposits explains high salinity in

groundwaters in the western part of the Colorado River delta (Portugal et al. 2005). Here, halite

and sylvite associated with lacustrine clayey sediments have been identified by x-ray diffraction,

- -2 and severely affect Cl and SO4 concentrations in groundwater. These clays are also likely

found in the study area.

Groundwater levels in the sampled areas are several meters higher than the high tide

levels in the nearby coastline of the Gulf of California (Figs. 2 and 4). Isotope data for the Mesa

could be interpreted as indicating mixing of Colorado River water in one end and seawater in the

other (Fig. 8B), but based on the elevation of the water table and the location of the groundwater

samples, this is physically impossible.

Since no halite-bearing strata are known within the study area, and no evidence exists for

seawater intrusion, the possible sources of Cl- are 1) Cl- bearing clays, 2) irrigation water, 3) precipitation, and 4) dry deposition and eventual dissolution of marine-derived salts. The ions

Cl- and Br- provide a useful tracer combination to identify the source of salinity in groundwater.

Bromide is rejected during the process of halite precipitation, and the Cl/Br mass ratio of solid

NaCl is usually 2-3 orders of magnitude higher than in the original waters (~ 5,000; Braitsch,

1971). The Cl/Br mass ratio of sea water is about 290 and is preserved in precipitation occurring near the sea (Davis et al. 1998).

The Cl/Br mass ratio of post dam Colorado River water upstream of Lake Mead (USGS

Station 09404200) appears consistent with a trend line resulting from the dissolution of halite plotting very close to the Cl- axis because of the low Br- content in the mineral (Fig. 11B, inset).

Closer inspection of Cl/Br data in the study area (Fig. 11C) provides an alternative explanation of Br- content in the river water. Figure 11C indicates large ranges in both Cl- (1 – 60 mg/L) and

Cl/Br (1000-3000) in river water. The figure shows a mixing line for seawater with a river water composition chosen as 100 mg/L Cl-, and a Cl/Br = 2000. Other mixing lines are possible for alternative choices of river water composition. The range of Cl- could be explained in part by changes in the amount of dilution of salt input from upstream evaporites. However, the prominent linear data array to the right of the mixing line is better explained by very small additions of seawater or sea-salt to river water. The Cl/Br equivalent ratio of local precipitation follows a trend line resulting from the dilution of seawater (Fig. 11B). The Cl/Br mass ratio in local precipitation ranges between 150 and 274, which is similar to the marine Cl/Br mass ratio and is consistent with marine-derived aerosols (Fig. 11C). A single sample from the Gila Range and a few samples from the floodplain and the Yuma and San Luis Mesa plot near the marine

Cl/Br mass ratio (Fig. 11C). Most of the samples have intermediate Cl/Br equivalent ratios.

- This indicates mixing between Colorado River water having irrigation and halite-derived Cl , and local recharge having Na+ and Cl- originating from seawater aerosols.

Groundwaters within the floodplain and the Yuma and San Luis Mesa have Cl- concentrations between 132 and 1000 mg/L (Table A1). It is important to emphasize that some of the variability in Cl- concentration is likely explained by the spatial and temporal distribution of the sample collection. Water samples were obtained from wells with depths between 40 and

242 meters from the surface. Shallower wells are more likely to be disturbed by anthropogenic

activity, such as irrigation. The historical data used in this study are for samples collected

between 1962 and 2016. Older samples could reflect a chemical composition more closely

related to pre-dam Colorado River water with evaporation and less anthropogenic sources of

solutes, and newer samples could be more similar to post-dam Colorado River water.

2+ 2+ -2 - A.8.b.2 Ca , Mg , SO4 , and HCO3

2+ 2+ -2 A charge balance should exist between cations and anions if Ca , Mg , SO4 , and

- HCO3 are derived from dissolution of calcite, dolomite, and gypsum (Fisher and Mullican,

1997). There is an approximate 1:1 relationship for groundwaters in the study area with a slight

2+ 2+ -2 - deficiency of (Ca +Mg ) relative to (SO4 +HCO3 ), particularly in groundwater samples from

the floodplain (Fig. 12). The excess negative charge is balanced by Na+ likely derived from old

groundwater discharging into the river through the exchange of Ca2+ or Mg2+ for Na+ with clay

minerals. The exchange explains the excess Na+ relative to Cl- observed in Fig. 11A, and causes

floodplain groundwaters to plot above the 1:1 halite dissolution trend.

The highest Cl/SO4 mass ratio in the Gila Range samples (~6) approaches the same ratio

in seawater (~7.4, Fig. 13). This further supports the idea that local recharge originates from

marine vapor and is carried inland where it falls as precipitation, but preserves its marine

chemical ratios. As locally recharged groundwaters having a high Cl/SO4 molar ratio move

-2 westward, they mix with SO4 dominated-endmember (Colorado River water) as illustrated by

the Mesa samples in Figs. 13A and 13B (dashed line).

The Mg/Ca mass ratio for the Gila Range samples varies between 0 and 1 (Fig. 13B).

Samples 43 and 82, both located in the Mesa, have the lowest δ18O and δ2H values of all the

samples (-14.9 and -114, and -14.8‰ and -111, respectively), and characterize pre-dam Colorado

River water. We assume Mg/Ca mass ratio range of these two samples to be representative of

pre-dam Colorado River water (0.2 - 0.5). Within the floodplain, Mg/Ca molar ratios range

between 0.2 and ~3. Some degree of evaporation is observed in floodplain samples (Figs. 8A

and 13A), but the Mg/Ca mass ratios would remain constant if this was the only process

occurring in the floodplain, and would plot in the dashed circle in Fig. 13B. Three additional

processes are believed to affect Colorado River floodplain groundwaters, along with evaporation

1) precipitation of solid phases such as calcium carbonate, 2) de-dolomitization of Mg-bearing carbonates, and 3) exchange of Ca2+ or Mg2+ for Na+ in the vadose zone, as previously discussed.

A.8.c Hydrochemical Evolution

Features of the regional flow system, the relations between major solutes, and stable

isotope data suggest that the next set of reactions is responsible for the hydrochemical evolution

of groundwater in the study area:

2+ 2- Ca + CO3  CaCO3

2+ 2- CaSO4  Ca +SO4

2+ 2+ 2- Ca(Mg)CO3  Ca + Mg + CO3

Ca2+ + 2Na-X = Ca-X + 2Na+

In the last reaction, X represents an ion exchange site occupied by two monovalent or one

divalent cation. Evolution of groundwaters in the study area occurs as described next.

Precipitation having ionic ratios similar to those of sea water recharges the aquifer along

the Gila Range. Most precipitation events undergo evaporation and transpiration by water-

efficient native vegetation, leading to the accumulation of meteoric salts near the surface.

These readily soluble salts are dissolved during the most intense and infrequent events, and

+ 2+ - -2 contribute with Na , Ca , Cl , and SO4 to groundwater when excess precipitation reaches the aquifer. Average local recharge in the region plots near the GMWL. This suggests that infiltration occurs during winter, when evaporation is low, as MSR and through preferential pathways along the major washes draining the Gila Range. The concentration of locally-

recharged groundwater is remarkably higher than the rainfall it was derived from, as observed in

the Gila Range samples, and is dominated by Na-Cl. This groundwater flows towards the southwest, and mixes with Ca-SO4-dominated Colorado River water along the Yuma and San

Luis Mesa (Fig. 8).

Mineral-water equilibria suggest that dissolution-precipitation of calcite and dolomite, dissolution of halite and gypsum, and exchange of aqueous Ca2+ for adsorbed Na+ control the

concentrations of solutes in the floodplain. Groundwater pumping draws sulfate-rich

groundwater used for flood-irrigation in the Yuma and San Luis Valley. Soil water is subjected

2+ 2- to evapo-transpiration, and Ca and CO3 are removed by precipitation of solid phases such as

calcium carbonate. Precipitation of calcium carbonate allows further dissolution of gypsum by

the common ion effect. In the special case where groundwater is in equilibrium with calcite and

dolomite, the dissolution of dolomite (dedolomitization) increases Mg2+ concentrations as

observed in Fig. 13B (Appelo and Postma, 2005).

Montmorillonite, is the most abundant clay in the study area, and has considerable

capacity for cation exchange (Olmsted. 1973). As soil water moves through the soil, Na+ is released for Ca2+ during the cation-exchange process. This affects the Mg/Ca ratio in floodplain

2+ 2+ -2 2- samples (Fig. 13), and explains the deficit of Ca + Mg relative to SO4 and HCO3 (Fig. 12),

which is balanced by the excess Na- observed in Fig. 11A. The groundwater produced by this set

of reactions is enriched by readily soluble salts left behind by evapotranspiration of irrigation

water, and contribute to the salinization of the aquifer when excess irrigation infiltrates and reaches the water table (Fisher and Mullican 1997). Once in the aquifer, the enriched solution mixes with unaltered Ca-SO4 groundwater, and Na-Cl groundwaters derived from local recharge

(Fig. 15).

A.9 Conclusions

This study investigated the origin and geochemical evolution of groundwaters beneath the unsaturated zone of an unconfined alluvial aquifer adjacent to the lower Colorado River near the U.S.-Mexico border. Environmental isotope data and major ion chemistry from groundwaters in the lower Colorado River area were used to understand aquifer recharge mechanisms, and the geochemical evolution of groundwater in a transboundary aquifer vulnerable to overdraft and salinization. The results shown here are typical of conditions in other alluvial aquifers within the arid southwestern United States, and in other arid areas around the world where agricultural regimes have contributed to the salinization and deterioration of groundwater quality (Fisher and Mullican, 1997; Herczeg et al., 2001; Oren et al., 2004).

Seawater intrusion is not physically possible as groundwater levels in the sampled areas are several meters higher than the high tide levels. Local recharge occurs during the wetter 30% of the winter rainfall events as MSR and through preferential infiltration pathways along the major washes draining the Gila Range. The other 70% of the rainfall events undergo evaporation and transpiration by water-efficient native vegetation, and cause the precipitation and accumulation of meteoric salts with seawater chemical ratios near the surface. Soluble salts accumulated on the surface are dissolved during the most intense and infrequent events and the solution infiltrates and percolates into the water table. The solute concentrations of locally-

recharged groundwater is remarkably higher than the rainfall it derived from, it is dominated by

+ 2+ - 2 Na-Cl, and contributes with Na , Ca , Cl , and SO4- to the aquifer.

Hydrochemical evolution in the irrigated floodplain is mostly controlled by the original

Ca-SO4 type Colorado River water. Mineral saturation states, ionic relations, and stable isotopes indicate that salinity is augmented by evapotranspiration, precipitation of calcite that leads to dissolution of gypsum by the common ion effect, dissolution of accumulated soil salts,

dedolomitization, and exchange of aqueous Ca2+ for adsorbed Na+. The Na-Cl dominated local

recharge flows southwest from the Gila Range, and mixes with the Ca-SO4 dominated floodplain

waters beneath the Yuma and San Luis Mesa. Low 3H concentrations suggest that recharge

within the Yuma and San Luis Mesa occurred before the 1950’s, and 14C data are consistent with

bulk residence times of thousands of years (4,800 and 11,500 14C years before present).

This study shows that geochemical processes in the surface and unsaturated zone, and ion

exchange severely affect groundwater composition over large areas. These natural processes

along with anthropogenic activities such as land-use change, water diversion, and extensive

irrigation can result in the deterioration and salinization of an aquifer. Residence time of groundwaters in the Yuma and San Luis Mesa suggest that local recharge occurs over relatively long time scales and is not sustainable. Added to this, the lack of Colorado River flow and constant input of ions derived from atmospheric deposition are likely to worsen the aquifer salinization problems.

A.10 Acknowledgements

This research was supported by a National Science Foundation Graduate Research Fellowship

Grant (DGE-1143953), a University of Arizona Water Sustainability Program Fellowship, and a

University of Arizona Geosciences Department R. Wilson Thompson Scholarship to H.A.Z.

A.11 Figures

Figure A1: Colorado River Basin (top). Study Area: lower Colorado River, Colorado River delta, and major geographical and geological features (bottom).

Figure A2: Groundwater levels in 1925 (top) and 2010 (bottom). Contours represent water table elevation in meters above sea level. Based on data from Olmsted et al., (1973), CONAGUA (2010), and the Arizona Department of Water Resources Well Registry (2015).

Figure A3: Monthly mean discharge in m3/sec for Colorado River at Lee’s Ferry (USGS Station 09380000) and Gila River as the combination of the Gila (USGS Station 09474000), Verde (USGS Station 09510000) and Salt River (USGS Station 09497500). All data available online through the National Water Information System at https://waterdata.usgs.gov/nwis.

Figure A4: Location of sampling sites in study area. Symbols for adjacent locations overlap in some cases.

Figure A5: A. δ2H vs δ18O values for the water endmembers in the area compared to the Global Meteoric Water Line (GMWL), and the Colorado River evaporation trend (CRET, dashed line). B. Average values for each endmember. This figure also includes winter, summer, overall, and wettest 30% events for rainfall based on the rain gauge located at Organ Pipe Cactus National Monument (Table A4).

Figure A6: Plot of δ2H vs δ18O showing modeled evolution of Colorado River water evaporating under 60% relative humidity (Mod. Evaporated C.R.). The percent shown at the bottom of the graph shows how much evaporation has occurred according to our modelled calculations. Also shown are data for surface water from the Cienega (SURF), average pre-dam Colorado River water, average post-dam Colorado River water, the Global Meteoric Water Line (GMWL), and the Colorado River evaporation trend (CRET).

Figure A7: Piper diagram showing data for surface water and groundwater in the study area (CRFP, MESA, G. RANGE) and in the Colorado River Basin including data for headtwaters (USGS Station 09196500), Green River, Utah (USGS Station 09315000), Lee’s Ferry (USGS Station 09380000), Hoover Dam (USGS Station 09421500), U.S.-Mexico Border (USGS Station 09522000), and Gila River near Dome, Arizona (USGS Station 09520500). All data available online through the National Water Information System at https://waterdata.usgs.gov/nwis.

Figure A8: A. δ2H vs δ18O values of surface water and groundwater from A: Colorado River floodplain (CRFP). B. San Luis Mesa (MESA and Gila Range [G. RANGE]) relative to the average endmembers, Global Meteoric Water Line (GMWL), and Colorado River evaporation trend (CRET).

- 2- Figure A9: Cl and SO4 concentrations (mg/L) for surface water and groundwater (CRFP, MESA, G. RANGE) in the study area.

Figure A10: A. Contour maps showing spatial patterns for Cl-. B. δ2H in the study area. Filled circles represent sampling locations. SLRC: San Luis Rio Colorado, Mexico. WMD: Wellton- Mohawk Drain.

Figure A11: A. Na+ vs Cl- (meq/L), B. Br- (mg/L) vs Cl- (mg/L), and C. Cl-/Br- (mass) vs Cl- (mg/L) for floodplain (CRFP), Yuma-San Luis Mesa (MESA), Gila Range (G.RANGE), surface water from the Cienega (SURF), local rainfall (RF), and Colorado River (CR). CR data obtained from USGS Station 09404200. Horizontal line in C shows the seawater (SW) Cl/Br ratio.

Figure A12: Ca + Mg vs SO4 + HCO3 (meq/L) for groundwater from Yuma-San Luis Mesa (MESA), Gila Range (G.RANGE), and Colorado River floodplain (CRFP).

- -2 - Figure A13: A. Cl /SO4 (mass ratio) vs Cl (mg/L) values for floodplain (CRFP), Yuma-San Luis Mesa (MESA), Gila Range (G.RANGE), and surface water from the Cienega (SURF). B. - -2 Cl /SO4 vs Mg/Ca (mass ratio) values for floodplain (CRFP), Yuma-San Luis Mesa (MESA), and Gila Range (G.RANGE) samples.

Figure A14: Tritium (TU) data for groundwater samples in the study area. Shading indicates topography (see Fig. 1).

Figure A15: Conceptual block diagram model of the study area.

A.12 Tables T SS Type Date Latitude Longitude Group pH E.C Ca Mg Na K Cl SO4 NO3 Br HCO3 δ18O δ2H δ13C 14C 3H Source (°C) 1 Surface Oct-13 32.732 -114.638 CR 1.7 220 351 4.2 0.4 -11.8 -95 -12.0 101 7 * Surface Feb-17 32.732 -114.638 CR -11.4 -95 5.3 * 2 Surface May-13 32.728 -114.634 WMD 3.9 639 821 20.5 1.0 -10.6 -87 * 3 Well Jun-67 32.748 -114.533 CRFP 7.3 3.0 170 62 660 238 312 1 Well May-80 CRFP 200 68 410 8 660 370 367 -10.5 -84 1 4 Well May-80 32.715 -114.595 CRFP 313 120 520 6 930 650 563 -11.6 -92 1 5 Well Mar-63 32.714 -114.561 CRFP 22 7.7 2.2 127 43 408 272 1.2 329 1 Well May-80 CRFP 22 150 50 310 6 540 290 244 -10.3 -78 1 6 Well Apr-05 32.709 -114.551 CRFP 22 7.5 3.3 180 69 414 7 640 410 0.1 362 -9.4 -75 5 2 7 Well Apr-05 32.709 -114.605 CRFP 25 7.4 4.5 218 89 719 7 927 831 0.1 398 -10.6 -86 16 2 8 Well Apr-05 32.679 -114.539 CRFP 24 7.6 1.9 56 20 377 4 175 399 8.0 317 -11.6 -96 13 2 9 Well Apr-05 32.678 -114.426 G.RANGE 27 7.7 1.6 92 30 179 6 354 90 0.1 2.8 148 -8.5 -67 2 2 10 Well Aug-87 32.678 -114.430 G.RANGE 2.4 75 33 370 5 620 110 0.4 172 -8.5 -66 1 11 Well Aug-87 32.677 -114.409 G.RANGE 1.4 60 34 160 6 270 82 8.8 196 -7.6 -55 1 12 Well Apr-05 32.666 -114.644 CRFP 27 7.3 3.7 221 68 499 8 531 879 0.2 293 -11.1 -90 -9.0 70 14 2 13 Well May-80 32.635 -114.764 CRFP 22 7.9 2.2 200 51 220 6 230 550 0.6 434 -12.8 -104 1 14 Well Sep-05 32.620 -114.505 G.RANGE 7.7 2.1 57 20 331 5 432 181 3.5 224 -8.7 -67 0.2 2 15 Well Apr-05 32.620 -114.651 CRFP 27 7.3 2.2 117 34 285 5 327 377 1.5 258 -10.5 -84 10 2 16 Well May-62 32.585 -114.796 CRFP 7.6 1.3 107 30 143 160 300 1 Well Aug-62 CRFP 7.2 1.2 105 29 132 175 295 1 Well Oct-62 CRFP 7.2 1.2 96 28 137 150 288 1 Well Apr-64 CRFP 7.4 1.3 118 26 150 185 304 1 Well May-65 CRFP 7.7 1.3 118 31 168 175 318 1 Well Feb-67 CRFP 7.7 1.4 116 34 172 175 320 1 Well Mar-67 CRFP 7.7 1.4 118 33 168 183 308 1 Well May-80 CRFP 21 8.2 1.9 161 45 170 6 260 320 0.0 0.7 296 -14.3 -108 1 Well Jul-88 CRFP 22 7.6 2.0 160 50 190 4 270 340 0.4 328 1 17 Well Sep-05 32.557 -114.676 MESA 24 7.5 1.9 87 32 280 4 195 425 3.0 310 -11.8 -97 14.1 2 Well Sep-05 MESA 7.6 2.0 92 33 265 4 206 433 2.7 -11.8 -97 15.9 2 Well Sep-05 MESA 7.7 1.9 91 29 262 4 172 416 3.5 -11.8 -97 16.1 2 Well Sep-05 MESA 8.1 1.8 90 30 258 4 158 419 3.6 -11.8 -99 15 2 18 Well Aug-87 32.549 -114.593 MESA 7.9 2.5 140 46 300 5 570 220 11.4 144 -8.0 -65 1 19 Well Aug-87 32.545 -114.684 MESA 27 7.5 2.2 87 40 320 4 430 330 2.2 184 -8.5 -76 1 20 Well Apr-05 32.487 -114.753 MESA 26 7.5 1.7 166 38 162 4 242 294 0.1 262 -13.8 -108 8 2

21 Well Apr-05 32.469 -114.692 MESA 27 7.7 2.4 148 37 288 7 529 226 0.6 152 -13.0 -101 -11.0 59 <0.1 2 22 Well Apr-05 32.445 -114.596 MESA 32 8.0 1.6 44 12 230 6 371 71 0.1 95 -7.9 -62 -9.0 29 0.1 2 23 Well Apr-09 32.490 -114.809 CRFP 22 7.2 1.8 32 69 243 4 358 390 182 -12.0 -96 3 24 Well Apr-09 32.487 -114.791 CRFP 25 7.7 1.3 50 71 191 3 229 220 255 -14.3 -108 3 25 Well Apr-09 32.474 -114.897 CRFP 22 7.1 1.8 44 88 189 4 270 380 343 -12.4 -99 3 26 Well Apr-09 32.473 -114.902 CRFP 21 7.1 2.3 68 100 254 4 380 520 370 -11.8 -94 3 27 Well Apr-09 32.468 -114.852 CRFP 20 7.0 2.0 76 75 336 4 353 450 309 -12.0 -95 3 28 Well Apr-09 32.467 -114.886 CRFP 22 7.4 1.9 42 67 270 3 210 360 295 -11.9 -96 3 29 Well Apr-09 32.466 -114.917 CRFP 22 6.9 2.9 109 116 270 5 364 500 342 -13.3 -103 3 30 Well Apr-09 32.465 -114.830 CRFP 22 7.1 2.4 54 73 358 4 355 450 391 -11.8 -95 3 31 Well Apr-09 32.457 -114.895 CRFP 24 6.6 2.7 66 98 237 5 394 430 366 -14.0 -108 3 32 Well Mar-15 32.455 -114.690 MESA -11.0 -83 -10.1 * 33 Well Apr-09 32.448 -114.920 CRFP 22 7.0 1.3 34 54 194 2 220 190 300 -14.7 -112 3 34 Well Apr-09 32.447 -114.824 CRFP 24 7.3 1.6 28 76 222 3 281 290 313 -14.2 -108 3 35 Well Apr-09 32.446 -114.878 CRFP 22 7.0 2.9 76 68 316 4 434 404 420 -13.4 -105 3 36 Well Apr-09 32.445 -114.908 CRFP 21 6.9 3.7 108 71 452 5 646 500 466 -13.9 -107 3 37 Well Apr-09 32.444 -114.884 CRFP 21 7.0 2.5 78 91 296 4 401 530 361 -14.0 -109 3 38 Well Apr-09 32.443 -114.884 CRFP 23 7.6 0.9 40 54 128 2 133 110 295 -14.5 -111 3 39 Well Apr-09 32.434 -114.834 CRFP 21 7.1 2.3 76 78 288 4 341 450 372 -13.8 -106 3 40 Well Dec-74 32.432 -114.615 MESA 65 23 239 433 82 107 -9.1 -71 4 41 Well Apr-09 32.429 -114.853 CRFP 20 7.5 1.5 62 69 218 3 233 280 318 -14.2 -103 3 42 Well Apr-09 32.428 -114.928 CRFP 20 7.2 3.8 60 113 434 5 688 620 281 -13.4 -104 3 43 Well Dec-74 32.378 -114.687 MESA 64 21 156 188 124 243 -14.9 -114 <0.5 4 44 Well Dec-74 32.415 -114.622 MESA 62 35 240 458 87 114 -9.3 -73 <0.5 4 45 Well Apr-09 32.423 -114.921 CRFP 23 7.2 2.5 96 97 205 4 401 410 354 -14.3 -110 3 46 Well Apr-09 32.422 -114.841 CRFP 22 7.0 3.5 82 91 322 5 553 470 347 -13.5 -105 3 47 Well Apr-09 32.416 -114.845 CRFP 23 7.2 2.4 66 53 155 3 235 290 284 -14.1 -109 3 48 Well Apr-09 32.414 -114.940 CRFP 24 7.2 2.8 78 98 312 4 501 560 325 -13.3 -105 3 49 Well Apr-09 32.413 -114.859 CRFP 23 7.2 2.4 76 79 245 4 397 370 356 -14.0 -107 3 50 Well Dec-74 32.420 -114.636 MESA 100 33 269 533 136 110 -10.2 -79 4 51 Well Apr-09 32.410 -114.904 CRFP 25 7.1 1.2 52 56 137 3 199 200 290 -14.3 -110 3 52 Well Apr-09 32.409 -114.881 CRFP 22 7.2 1.4 58 57 173 3 247 290 290 -13.9 -108 3 53 Well Apr-09 32.405 -114.929 CRFP 20 7.5 1.0 22 27 137 5 156 90 209 -14.4 -110 3 54 Well Apr-09 32.397 -114.910 CRFP 22 7.3 1.9 50 60 221 3 312 310 309 -14.2 -109 3 55 Well Apr-09 32.397 -114.940 CRFP 21 7.0 1.5 52 60 139 3 260 220 281 -13.9 -108 3 56 Well Dec-74 32.397 -114.748 MESA 43 14 155 184 105 170 -14.3 -111 <0.5 4

57 Well Apr-09 32.394 -114.888 CRFP 21 7.7 1.6 54 58 219 3 314 290 343 -13.6 -104 3 58 Well Dec-74 32.381 -114.637 MESA 80 25 289 507 136 107 -10.4 -81 4 59 Well Apr-09 32.387 -114.929 CRFP 21 7.1 1.0 52 57 114 2 160 130 391 -14.6 -110 3 60 Well Apr-09 32.387 -114.950 CRFP 22 7.1 2.7 60 61 315 4 455 310 384 -13.8 -107 3 61 Well Apr-09 32.386 -114.881 CRFP 21 7.3 1.6 58 56 185 3 305 260 252 -14.2 -109 3 62 Well Dec-74 32.399 -114.696 MESA 47 14 238 306 166 120 -13.4 -104 4 63 Well Dec-74 32.371 -114.539 MESA 27 11 239 298 84 161 -8.4 -64 4 64 Well Apr-09 32.375 -114.883 CRFP 25 7.4 2.3 66 52 257 4 422 220 265 -14.0 -107 3 65 Tap Mar-15 32.371 -114.761 MESA -13.9 -105 -10.2 * 66 Well Apr-09 32.359 -114.929 CRFP 23 7.7 1.4 36 35 208 3 257 220 262 -14.2 -109 3 67 Well Oct-13 32.136 -114.924 CRFP 3.7 1006 298 0.0 2.7 -9.6 -76 * 68 Surface May-13 32.062 -114.904 SURF 5.4 1149 827 10.8 2.3 -9.6 -81 * 69 Surface Jul-14 32.057 -114.899 SURF 30 7.9 2.9 558 782 22.9 1.0 305 -10.6 -88 -8.7 * Surface May-13 SURF 3.8 603 837 17.7 1.0 -10.6 -87 * 70 Surface Oct-13 32.039 -114.822 SURF 3.6 975 251 0.0 2.5 -6.7 -66 * 71 Surface Jul-14 32.039 -114.908 SURF 35 7.6 4.9 906 1028 0.0 1.5 318 -8.6 -77 -7.9 * Surface May-13 SURF 5.9 1259 1263 1.5 2.1 -7.4 -73 * 72 Surface Jul-14 32.038 -114.895 SURF 35 7.6 3.6 550 771 19.2 1.0 297 -10.5 -88 -8.1 * 73 Surface Jul-14 32.025 -114.872 SURF 27 7.3 2.3 654 868 3.8 1.1 348 -10.2 -85 -10.3 * Surface May-13 SURF 6.0 1184 1323 0.0 2.2 -8.1 -74 * 74 Surface Jul-14 32.015 -114.868 SURF 28 7.6 3.6 728 944 0.0 1.2 346 -9.8 -83 -7.9 * 75 Surface May-13 31.988 -114.827 SURF 6.0 1182 1346 0.0 2.5 -7.3 -69 * Surface Jul-14 SURF 37 8.2 27.9 7425 4960 10.5 16.9 324 -0.4 -28 -3.8 * 76 Surface May-13 31.965 -114.814 SURF 12.5 2848 2597 0.0 5.8 -1.1 -39 - * Surface Jul-14 SURF 28 7.5 144 44933 8249 0.0 181.8 341 0.7 -17 -5.5 * 77 Surface May-13 31.958 -114.811 SURF 146 54492 9653 0.0 119.9 6.0 8 * 78 Well Jan-82 32.350 -114.531 MESA 32 7.3 34 10 270 6 270 270 0.5 150 -10.5 -77 -8.1 1.6 5 79 Well Jan-82 32.378 -114.688 MESA 30 7.7 57 17 210 6 340 140 0.5 120 -12.7 -93 -10.8 0.7 5 80 Well Apr-78 32.428 -114.599 MESA 28 7.9 49 13 220 9 400 79 0.6 98 -8.6 -63 -9.7 0 5 81 Well Apr-78 32.387 -114.718 MESA 31 7.3 43 11 170 10 250 130 0.2 130 -14.1 -104 -11.2 0.5 5 82 Well Apr-78 32.387 -114.718 MESA 29 7.4 45 10 120 7 180 130 0.5 160 -14.8 -111 -11.5 1.4 5 83 Well Apr-78 32.457 -114.690 MESA 28 7.7 110 27 230 5 430 160 1.5 160 -12.5 -94 -11.3 0.1 5 84 Well Oct-16 32.457 -114.690 MESA 7.3 1.8 93 23 224 5 416 157 0.0 0.5 328 -12.7 -98 * 85 Well Oct-16 32.450 -114.674 MESA 7.5 2.1 100 28 230 7 516 142 0.0 0.0 185 -10.9 -85 * 86 Well Oct-16 32.446 -114.660 MESA 29 7.5 1.9 92 27 216 7 511 112 0.0 0.0 198 -9.4 -72 * 87 Well Oct-16 32.441 -114.645 MESA 29 7.5 1.8 88 20 216 6 464 99 0.0 0.0 104 -9.8 -75 *

88 Well Oct-16 32.437 -114.630 MESA 32 7.7 1.6 69 14 204 5 422 89 0.0 0.4 109 -9.3 -72 <0.5 * Well Oct-16 MESA 30 7.7 1.5 58 14 217 5 398 81 0.3 0.6 64 -8.7 -65 * 89 Well Oct-16 32.423 -114.585 MESA 31 7.6 1.4 36 9 203 4 346 76 0.0 0.4 73 -7.9 -60 -9.4 26 <0.4 * 90 Well Oct-16 32.419 -114.570 MESA 30 7.6 1.3 31 9 202 3 319 69 0.0 0.4 144 -8.0 -60 * 91 Well Oct-16 32.343 -114.321 MESA 33 7.6 3.6 91 7 549 4 840 491 2.4 1.7 110 -8.6 -63 * Rain Apr-16 RF 9 6 0.1 0.1 -7.0 -39 * Rain Apr-16 RF 8 5 0.1 0.0 -4.4 -24 * Rain Apr-16 RF 60 11 0.3 0.2 -3.7 -21 * Table A1. 1 USGS National Water Information System (USGS, 2016) E.C. measured in mS/cm 2 Dickinson et al. 2006 Ions measured in mg/L 18 2 3 Palomarez-Ramirez, 2011 δ O and δ H relative to V-SMOW (‰) 13 4 Payne et al. 1979 δ C relative to V-PDB (‰) 14 5 Makdisi et al. 1982 C expressed in percent modern carbon (pMC) 3 * This study H expressed tritium units (TU)

Type Year pH Ca Mg Na K HCO3 SO4 Cl NO3 Br δ18O δ2H Source Pre-Dam C.R. 1967-2015 7.8 89 30 95 5 190 290 65 2.16 0.05 -15.0 -115 1 S.D. 35 12 44 2 35 137 35 5.00 0.04 0.3 2 Post-Dam C.R. 1980-2015 8.2 89 32 146 5 197 302 149 0.37 0.31 -12.0 -97 1 S.D. 12 4 29 1 15 49 35 0.17 0.04 0.6 5 Ag. Discharge 1961-1995 7.8 271 117 881 9 392 941 1514 2.13 0.62 -10.8 -89 1, 2, 3 S.D. 72 37 83 1 45 97 570 0.56 0.01 0.4 4 Gila River 2004-2005 8.0 56 19 342 4 255 262 348 3.64 - -9.9 -71 3 S.D. 50 14 352 5 137 257 411 4.92 - 1.3 20 Local Recharge 1981-2017 7.8 -7.5 -53 1 S.D. 0.3 2 Table A2. 1. USGS National Water Information System (USGS, 2016) Ions measured in mg/L 18 2 2. Payne et al., 1979. δ O and δ H relative to V-SMOW (‰) 3. Towne, 2017

Type Location Name Date Latitude Longitude Elevation δ18O δ2H 14C 3H Source Well C-06-02 27DCD1 Mar-81 32.871 -112.443 480 -7.4 -51 1 Well C-06-01 31DAD1 Mar-81 32.859 -112.383 534 -7.6 -52 1 Well D-07-02 18ABA Mar-81 32.827 -112.199 562 -7.3 -55 0.9 1 Well D-07-01 22ACA Mar-81 32.807 -112.244 554 -8.0 -54 1 Well D-08-01 14BAA Mar-81 32.729 -112.222 601 -7.9 -57 1 Well D-08-01 31 CBD Mar-81 32.685 -112.303 689 -7.4 -52 1 Well C-10-01 36CAA Apr-81 32.513 -112.317 687 -7.2 -54 1 Well D-11-01 14CAA2 Apr-81 32.468 -112.241 681 -7.3 -58 1 Well D-12-03 27BCC Apr-81 32.353 -112.059 556 -7.0 -53 1 Well C-16-07 27BAD Jul-83 32.009 -112.975 413 -7.5 -53 1 Well C-14-07 35AAC Jan-85 32.169 -112.951 414 -7.5 -52 1 Well C-03-07 13AAB Feb-13 33.172 -112.930 309 -7.5 -49 2 Well C-01-11 03BDB Oct-13 33.371 -113.382 453 -7.3 -51 2 Well C-01-12 15BDD Oct-13 33.339 -113.483 386 -7.5 -52 2 Well C-02-12 12AAC Oct-13 33.276 -113.437 342 -8.0 -56 2 Well C-01-11 25BAD Oct-13 33.317 -113.342 409 -7.3 -51 2 Well C-02-09 01DBB Nov-13 33.283 -113.134 345 -7.2 -52 2 Well C--02-08 21DBC Apr-14 33.237 -113.083 293 -7.4 -53 2 Well C-13-05 25CCB Feb-16 32.261 -112.743 545 -8.0 -59 * Well C-14-07 35AAC Mar-16 32.169 -112.951 414 -7.3 -53 * Well POZO NUEVO Oct-16 31.825 -113.719 77 -7.6 -51 87 <0.4 * Well C-16-04 6ABD Mar-17 32.066 -112.715 729 -7.8 -56 80 1.0 * Table A3 1 USGS National Water Information System (USGS, 2016) 2 Towne, 2017 δ 18O & δ2H relative to V-SMOW (‰) * This study 14C percent modern carbon (pMC) Elevation in meters above sea 3H expressed triitium units (TU)

Date Rainfall (mm) δ18O δ2H Date Rainfall (mm) δ18O δ2H

2-Oct-90 14.0 -7.3 -43 14-Sep-04 12.4 -7.6 -63 6-Feb-96 4.3 -5.0 -34 26-Oct-04 19.1 -6.5 -36 27-Feb-96 4.3 -5.2 -25 9-Nov-04 12.2 -5.7 -36 19-Mar-96 8.4 -7.4 -47 23-Nov-04 5.6 -3.8 -11 9-Jul-96 3.6 -2.8 -17 7-Dec-04 21.8 -10.5 -61 16-Jul-96 20.8 -9.0 -60 4-Jan-05 56.4 -10.4 -70 7-Jan-97 6.1 -9.4 -65 25-Jan-05 4.3 -7.8 -60 7-Jan-97 6.1 -9.6 -62 1-Feb-05 2.8 -7.3 -52 14-Jan-97 8.1 -3.2 -13 15-Feb-05 28.4 -14.2 -107 14-Jan-97 8.1 -3.2 -17 22-Feb-05 14.5 -4.1 -23 28-Oct-97 17.3 -6.8 -36 1-Mar-05 3.6 -4.6 -30 17-Mar-98 21.1 -5.8 -34 8-Mar-05 13.7 -8.5 -58 31-Mar-98 26.2 -6.0 -30 26-Apr-05 1.3 -6.7 -54 25-Aug-98 11.4 -1.6 -11 26-Jul-05 10.2 -1.4 -8 8-Dec-98 8.4 -4.0 -12 2-Aug-05 16.8 -8.7 -65 13-Jul-99 17.0 -9.6 -65 9-Aug-05 29.5 -3.7 -23 20-Jul-99 6.6 -7.0 -59 6-Sep-05 2.5 -1.3 -2 3-Aug-99 62.2 -5.5 -38 18-Oct-05 13.7 -4.4 -17 26-Jun-01 25.4 -3.9 -31 14-Mar-06 15.7 -5.5 -22 14-Aug-01 12.5 0.5 7 12-Jul-06 3.3 -3.0 -27 20-Aug-01 20.1 6.1 18 18-Jul-06 31.2 -2.6 -17 30-Jul-02 5.8 -7.1 -57 9-Aug-06 25.1 -6.3 -43 29-Oct-02 2.3 -5.3 -24 15-Aug-06 23.4 -5.3 -36 24-Dec-02 3.3 -7.3 -43 22-Aug-06 25.1 -3.4 -20 14-Jan-03 7.9 -5.4 -33 6-Sep-06 19.6 -8.3 -64 18-Feb-03 18.3 -9.7 -73 17-Mar-16 2.4 -5.0 -41 26-Feb-03 3.1 -3.0 -10 25-Mar-16 2.7 -7.4 -50 18-Mar-03 10.7 -5.9 -33 24-Apr-16 25.0 -6.5 -38 15-Apr-03 5.3 -5.8 -32 25-Apr-16 9.0 -7.0 -39 22-Jul-03 3.6 1.1 4 26-Apr-16 40.0 -4.4 -24 29-Jul-03 6.4 -0.3 1 26-Apr-16 15.0 -5.7 -45 5-Aug-03 4.1 -2.1 -16 26-Apr-16 7.0 -4.4 -35 19-Aug-03 2.3 1.2 7 26-Apr-16 4.5 -3.7 -21 26-Aug-03 33.3 -3.3 -23 17-Jun-16 25.0 -3.4 -26 2-Sep-03 2.0 -1.2 -19 1-Oct-16 50.0 -3.2 -21 24-Feb-04 25.1 -7.7 -48 1-Oct-16 33.0 -2.3 -9 9-Mar-04 13.0 -10.8 -75 1-Oct-16 7.0 -5.2 -46 6-Apr-04 48.5 -7.6 -49 16-Oct-16 50.0 -4.2 -31 13-Jul-04 6.6 -1.4 -19 16-Oct-16 25.0 -6.2 -45 27-Jul-04 20.3 -3.8 -22 16-Oct-16 10.0 -5.5 -41 10-Aug-04 22.4 -2.1 -13 1-Jan-17 100.0 -6.9 -51 17-Aug-04 18.5 -3.9 -16 1-Jan-17 30.0 -7.6 -53 Table A4. δ18O & δ2H relative to V-SMOW (‰)

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APPENDIX B:

Evaluation of groundwater sources, flow paths, and residence time of the Gran Desierto

pozos, Sonora, Mexico

Evaluation of groundwater sources, flow paths, and residence time of the Gran Desierto

pozos, Sonora, Mexico.

Hector A. Zamora1, Benjamin Wilder2, Christopher J. Eastoe1,5, Jennifer C. McIntosh3, Karl W.

Flessa1, and Jeffrey Welker4.

1. University of Arizona, Department of Geosciences, 1040 E. 4th Street, Tucson, Arizona,

85721, USA.

2. University of Arizona, Desert Laboratory on Tumamoc Hill, 1401 E. University Blvd.,

Tucson, Arizona, 85721, USA.

3. University of Arizona, Department of Hydrology and Atmospheric Sciences, 1133 E.

James E. Rogers Way, Tucson, Arizona, 85721, USA.

4. University of Alaska Anchorage, Department of Biological Sciences, 3211 Providence

Drive, Anchorage, Alaska, 99508, USA.

B.1 Abstract

Environmental isotopes and water chemistry distinguish water types, aquifer recharge

mechanisms and flow paths in the Gran Desierto and Colorado River delta aquifer. The aquifer

beneath the Gran Desierto supports a series of spring-fed wetlands, locally known as pozos,

which have provided vital water resources to diverse flora and fauna and to travelers who visited

the area for millennia. Stable isotope data shows that local recharge originates as winter

precipitation, but is not the main source of water in the pozos. Instead, Colorado River water

with substantial evaporation is the main component of water in the aquifer that feeds the pozos.

Prior to infiltration, Colorado River water was partially evaporated in a wetland environment

similar to what is now the Ciénega de Santa Clara. Groundwater then followed flow paths,

created by the Altar Fault, into the current location of the pozos at Bahía Adair. Mixing with seawater is observed at the pozos located near the coast of the Gulf of California. Agricultural consumption of Colorado River water significantly reduced recharge to the aquifer feeding the pozos. The wetlands that allowed recharge to the aquifer feeding the pozos no longer exist, and

the pozos are now vulnerable to major groundwater pumping and development in the area.

B.2 Introduction

The combination of environmental isotopes (δ18O, δ2H, 3H, and 14C) and water chemistry

has been useful in the study of alluvial aquifers in the Basin and Range Province in western

North America. The effects of altitude, latitude, evaporation and paleoclimate on isotopes along

with solute chemistry of waters have been used to characterize water zones, sources of recharge,

and flow paths (Eastoe et al., 2004; Guay et al., 2006; Hopkins et al., 2014). These tools are

particularly useful in areas where surface and groundwater of different origins mix, such as the

Colorado River delta (Fig. 1).

Observations and studies in the early twentieth century along the Colorado River and its

delta documented wetland and riparian ecosystems swarming with life, sustained by the muddy

waters of the then mighty Colorado (Sykes 1937; Leopold 1949). Today, water-management practices, including dams and the over-allocation of Colorado River water, have drastically reduced these once vast ecosystems. The river no longer reaches the lower part of the delta, but a string of coastal wetlands that harbor oases of plant and animal communities, and an underground aquifer of unknown extent still exist in the area (Ezcurra et al., 1998; Glenn et al.,

2006).

Adjacent to the upper Gulf of California and the Colorado River Delta is the most extensive sand dune field of North America, the Gran Desierto de Altar. On the westward coastal fringe of this massive dune field are a series of wetlands and fresh water springs, or pozos as they are locally known. These oases represent one of the most critical biological and cultural resources in this dry borderland region of the Sonoran Desert (Felger 2000; Felger and Broyles,

2007). The spring-fed water bodies support a diversity of flora that contrasts with the surrounding desert, due to the water’s availability and relatively low salinity (Glenn et al., 1996).

The pozos support at least 26 species of vascular plants (Ezcurra et al., 1998), and a variety of terrestrial and avian fauna congregates around them.

The remote freshwater springs were also destinations of multi-day pilgrimages along the historic Salt Trails where indigenous inhabitants navigated through the largest dune field in

North America to the sea to collect salt (Lumholtz, 1912; Underhill et al., 1997). Oxygen isotope ratios in shells from prehistoric middens and carbon-14 (14C) ages from buried charcoal

suggest that people visited the northern Sonoran coast during the fall and winter to harvest fish

and shellfish soon after the stabilization of the early Holocene sea level rise around 7000 yrs.

B.P. (Douglas et al., 2015; Dettman and Huckleberry, 2008), if not before (Bowen 1998; Hayden

1998). Pioneering explorers seeking an overland route to California also used the pozos as a watering stop along their expeditions. Observations relative to early explorers’ descriptions and photos from the early nineteen hundreds suggest that the pozos have not changed drastically during the recent centuries (Lumholtz, 1912).

Although few in number, previous studies in the area have attempted to decipher the source of recharge of the pozos using major ion chemistry (Ezcurra et al., 1988). Nonetheless, the underpinning questions of age and origin of the pozos water have remained unsolved. In this study, we use environmental tracers to advance our understanding on these enigmatic oases. Our initial hypotheses were that (1) the systematic study of oxygen and hydrogen isotopes would refine the understanding of groundwater sources in the Gran Desierto aquifer and the pozos; (2) the radioactive isotopes tritium (3H) and 14C would provide an estimate of groundwater residence times; and (3) the combination of these environmental isotopes, major ion chemistry, and the features of the local hydrology would provide information to evaluate groundwater dynamics

(e.g. flow paths) throughout this transboundary aquifer. This information is relevant because of anticipated severe droughts and water shortages in the region (Seager et al., 2007; Ault et al.,

2016), and is an important contribution needed to manage the hydrological, biological, and cultural resources of the Gran Desierto wetlands.

B.3 Study Area

B.3.a Climate

The Gran Desierto is one of the driest and hottest deserts in North America. Data provided by the National Park Service and El Pinacate Biosphere Reserve (Charles Conner, NPS, personal communication, 2017) from two meteorological stations located at Estación Lopez-

Collada (31.7119, -113.9904) and Sierra El Rosario (32.0995, -114.1807) show maximum temperatures reaching 46 °C, and a mean annual temperature of 23 °C. Southeast winds from the Gulf of California are predominant during spring and summer, and bring high temperatures and humidity. Northwest winds are typical during fall and winter seasons, and they are accompanied by cold fronts and lower temperatures (Flessa et al., 2012).

Precipitation occurs as sporadic, cyclonic events during winter, and rare localized convective events during summer. The annual average ranged between 53 mm and 57 mm at the two meteorological stations. The proportion of winter precipitation (November – April; 37 mm to 39 mm) was higher than summer precipitation (May – October; 16 mm to 18 mm) over a ca. ten-year period. Detailed statistical studies of rainfall patterns in the Gran Desierto have shown that the proportions of winter and summer precipitation are equal over longer timescales, but the amount of rainfall and the frequency of rainy periods decrease westward towards Yuma (Ezcurra and Rodrigues, 1984). Hurricane-derived rainfall is sporadic, occurs during September and

October, but might be as important as rainfall from winter fronts and monsoon events (Eastoe,

2014; Eastoe, 2016).

B.3.b Geology

The geology and hydrogeology of the Colorado River delta and the geology of the Altar

Basin have been described by Olmsted et al., (1973), Dickinson et al., (2006), and Pacheco et al.,

(2006). The area is bounded to the northeast by northwest-southeast mountain ranges consisting of igneous and metamorphic rocks of Cretaceous age (Gila Range, Tinajas Altas, and Sierra El

Rosario), and to the east by basaltic flows of Pleistocene to late Holocene age (Sierra El

Pinacate). The Altar Basin covers the area between San Luis Rio Colorado and Bahia Adair

(Fig. 1). This area was once part of the delta complex, but now is an inactive, subsidiary basin

above the level subject to flooding by the Colorado River (Pacheco et al., 2006).

The structural basement in the delta is composed of igneous and metamorphic rocks of

Cretaceous age, and is overlain by sedimentary sequences up to 5 km in thickness that show the

delta’s progradation during the last ~5 Ma (Payne et al., 1979; Pacheco et al., 2006). The lower

part of the sequence consists of a shale unit deposited under open marine conditions, and grades

upward into a thick series of interstratified mudstone, siltstone, and sandstone representing the

sub-aqueous part of the delta. The latter unit grades upward into poorly consolidated sand

representing the sub-aerial (fluvio-deltaic) parts of the delta (Pacheco et al., 2006).

Alluvial sediment exposed at the surface of the delta represents the last cycle of

deposition from the Colorado River and the Gila River (Olmsted et al., 1973). The most

abundant material is fine sand with silt and thin layers of clay. These clay units are found along

the Yuma Valley and extend into the mesas of Yuma and San Luis (Dickinson et al., 2006). The

mesas are fluvial terraces built in the Late Pleistocene during periods of intense alluviation and

sea-level highstands. These plateau-forming structures rise < 20 meters above the floodplain’s elevation (Colletta and Ortlieb, 1984), and are covered by the sand dunes of the Gran Desierto.

The Late Pleistocene dune field extends from the eastern end of the Colorado River delta to the

Sierra El Pinacate (El Pinacate), and is fed by sand from nearby sources, ultimately derived from sediments deposited by the Colorado River (Lancaster et al., 1987; Beveridge et al., 2006).

The escarpment that divides the western edge of the Altar Basin from the Colorado River delta was formed by the Cerro Prieto Fault, a southern extension of the San Andreas Fault system. The fault traverses the study area and continues into the Gulf of California (Fig. 1;

Merriam, 1965). Strike-slip movement along the Cerro Prieto fault system is estimated to be as

much as 60 mm/year (Curray and Moore, 1984; Gastil et al., 1975). The eastern side of the Altar

Basin is limited by the seismically inactive Altar Fault, which is collinear with the Algodones

Fault (Fig. 1; Pacheco et al., 2006). Both faults are NW-trending, dip to the west, have significant dextral offset, and drop the southwestern side down. The Algodones Fault affects groundwater movement and offsets the water table, as observed in the Yuma Mesa, as the result of either juxtaposition of materials with different permeability, or the presence of fault gouge and mineralization along the fault zone (Olmsted et al., 1973). The Cerro Prieto Fault is assumed to have the same effect on groundwater movement as its eastern counterpart.

B.3.c Hydrology

The principal aquifer beneath the Colorado River valley and the mesas of Yuma and San

Luis occurs within a gravel complex of different ages (Olmsted et al., 1973). The gravel zone reaches a thickness of more than 30 meters and lies at an average depth of 45 meters from the surface in the valley and 55 meters from the surface in the Yuma Mesa. Most water extraction wells in the area penetrate this unit due to its high transmissive properties (Dickinson et al.,

2006). The alluvial aquifer becomes confined towards the coast as a result of the presence of nearly impermeable clay layers with hydraulic conductivity of 0.001 cm/hr, and artesian pressure develops (Ezcurra et al., 1988). Thick and extensive coastal salt flats (sabkhas) barren of vegetation, and believed to be of paleo-deltaic origin (May, 1973), cover several areas along the coast particularly north west of Bahía Adair. Along these coastal salt flats, pozos develop in locations where the permeability of the clay increases because of desiccation cracking, flocculation due to ion exchange, or digging by local fauna in the search of freshwater (Ezcurra et al., 1988). This exposes a sandy layer containing groundwater with a vertical head gradient of

~0.40 m/m, and a hydraulic conductivity 243 cm/hr (Ezcurra et al., 1988).

Under natural conditions, the Colorado and the Gila rivers were losing streams and the main source of recharge to the regional aquifer (Ramirez-Hernandez et al., 2013; Olmsted et al.,

1973). Groundwater flowed in a northeast-southwest direction from the confluence of the

Colorado and the Gila Rivers near Yuma, Arizona, to the Gulf of California (Fig. 2; Diaz

Cabrera, 2001). During the last few decades, the aquifer has had an annual recharge of 75.5 x

107 m3 by infiltration from unlined irrigation canals supplied with Colorado River water, and groundwater coming from the Colorado River Basin upstream of Yuma, Arizona (Payne et al.,

1979; Orozco-Durán et al., 2015). However, the Colorado River groundwater discharge into the

Gulf of California is now less than 3.5 x 107 m3/y (Comisión Nacional del Agua, 2010).

Locally-derived groundwater recharge likely occurs at the base of el Pinacate as mountain system recharge (MSR), and focused recharge in ephemeral streams as in other semi- arid basins in southern Arizona (Wahi, 2008; Meixner et al., 2016). Direct infiltration and percolation of rain water to the water table is considered negligible. Groundwater at the base of

El Pinacate is assumed to flow westwards, although groundwater level data is unavailable to confirm flow patterns.

B.4 Methods

Several sources of aquifer recharge are found within the watershed including, but not limited to, Colorado River water, local recharge, and agricultural discharge. In order to further investigate the origin and dynamics of the groundwater system feeding the pozos, we collected surface and groundwater samples on several trips to the area between 2013 and 2017, and during different times of the year (Figure 3). Thirty four surface water samples were collected from the pozos and ten from the Ciénega de Santa Clara (Ciénega). The Ciénega is a wetland supported by brackish groundwater derived from the Wellton-Mohawk Irrigation Drainage District of

Arizona (see Fig. 1 for location of the Ciénega). The Wellton-Mohawk valleys are irrigated with

Colorado River water. The wetland was sampled because it represents the composition of

evaporated Colorado River water and can be compared with the pozos.

The pozos were drained using a hand bucket and allowed to refill to obtain a

representative sample where this was possible. Groundwater samples were collected from the

two wells available in the area. Two water samples were obtained from auger holes adjacent to

the pozos in order to minimize evaporation. These holes were hand-drilled in the clay layer beneath the surface. The pozos were divided into two different sub-groups. Those located along

the Altar Fault in the vicinity of Bahía Adair were designated as Bahia Adair (Adair) pozos, and

those located along the escarpment dividing the delta from the San Luis Mesa were designated as

Cerro Prieto Fault (Cerro Prieto) pozos. Water samples collected from auger holes and wells are included with the Adair pozos, but are differentiated in Table B1. Sampling site 14 (locally known as Pozo Nuevo), is at the base on the west side of El Pinacate at an elevation of 78 masl, and is separated from both groups (see Fig. 3). Rainfall samples were collected using rain gauges scattered throughout the region. Three tap water samples were obtained from El Golfo de

Santa Clara’s supply because wells were unavailable for sampling.

B.4.a Field Methods

Temperature, pH, and electrical conductivity were measured in the field after each

parameter had stabilized using a YSI 556 Multiparameter System sonde calibrated with standard

pH buffer (4, 7, and 10) and conductivity (1413 μS) solutions. Samples collected for stable

isotopes analysis (O, H, and C) were filtered through a 0.45-μm nylon filter and kept in 20 mL

glass vials with no headspace. Unfiltered water samples were collected for age tracers (3H and

14C) using rinsed 1-L HDPE and amber borosilicate glass bottles, respectively. Samples for ions

and alkalinity were filtered with 0.45-μm nylon filters into 30 mL HDPE bottles. Cation samples

were preserved by adding two drops of concentrated optima grade HNO3. All samples were kept

on ice during field collection and then refrigerated at 4 °C. A thin layer of mineral oil was added

to the rain-collecting devices to prevent evaporation. The accumulated rainfall was recovered

from all stations during field visits in late October and late March during two years (2015 and

2016).

B.4.b Analytical Methods

Values for δ18O and δ2H were determined in the Environmental Isotope Laboratory,

Department of Geosciences, University of Arizona, using a Finnigan Delta-S mass spectrometer

18 2 with automated CO2 equilibration and Cr reduction attachments. The δ O and δ H values are reported in delta notation where . R is the ratio of the heavier over

the lighter isotope in the sample, and Rstd is the isotope ratio of Vienna Standard Mean Ocean

Water (VSMOW). Calibration followed the method of Coplen (1995), using international

standards VSMOW and Standard Light Antarctic Precipitation (SLAP). The analytical

precisions (1σ) for these techniques are 0.9 % for δ2H and 0.08 % for δ18O.

13 Values for δ CDIC were measured on a Thermo-Finnigan Delta Plus XL continuous-flow gas-ratio mass spectrometer coupled with a Gasbench automated sampler. Samples were reacted for more than 1 hour with phosphoric acid at room temperature in Exetainer vials flushed with

He gas. Standardization is based on NBS-19 and NBS-18 and precision is + 0.30 ‰ or better

(1σ). Values for δ13C are expressed in delta notation relative to the Vienna Pee Dee Belemnite

(VPDB) standard.

Tritium values were measured by liquid scintillation counting on electrolytically enriched

water in a Quantulus 1220 Spectrophotometer. The detection limit was 0.5 TU for 9-fold

enrichment and 1,500 min of counting. One tritium unit (TU) is equivalent to one tritium atom

18 in 10 atoms of hydrogen. Carbon-14 was measured as liberated CO2 reduced to graphite at the

NSF-Arizona AMS facility. Carbon-14 results are reported as percent modern carbon (pMC)

- relative to NBS standards Oxalic Acid I and II. Anion concentrations (excluding HCO3 and

2- CO3 ) were determined using a Dionex Ion Chromatograph model 3000 with an AS23 analytical

column (precision + 2 %) at the University of Arizona Department of Hydrology and Water

Resources. Alkalinity was determined by the Gran-Alkalinity titration method (Gieskes and

- 2- Rogers, 1973) within 24 hours of collection. The HCO3 and CO3 concentrations were

determined using the PHREEQC speciation model (Parkhurst and Appelo, 1999). The analyses

for cations were performed using an Elan DRC-II Inductively Coupled Plasma – Mass

Spectrometer at the University of Arizona Laboratory for Emerging Contaminants (ALEC).

B.5 Data

New (this study) and published isotope and solute chemistry data for groundwater and

surface water samples in the Colorado River delta and the Altar Basin is compiled and presented

in Table B1. Rainfall data collected as part of this study is insufficient to calculate long-term mean δ18O and δ2H values. For this reason, rainfall stable isotope data were obtained from the

United States Network for Isotopes (USNIP) station located at Organ Pipe Cactus National

Monument (OPCNM, Welker, 2012). These data are a collection of individual events registered

between 1990 and 2006 (Table A4). OPCNM is located at an elevation of 515 meters above sea

level and ~100 km northeast from the study. OPCNM’s elevation is similar to the average

elevations seen at El Pinacate where recharge into the local aquifer is likely to occur at its base.

Additionally, major ion chemistry for rainfall was obtained from the National

Atmospheric Deposition Program (NADP, 2016) station also located at OPCNM. These data

represent individual events collected between 1980 and 2017. Bicarbonate is not reported in the

- NADP samples. In these cases, HCO3 was estimated by balancing the charge among the major

ions, assuming it to be the missing anion. Then, these samples were plotted on a Piper Plot for

hydrochemical characterization (Fig. 4).

B.6 Results

B.6.a Rainfall

There is a wide distribution of solutes, but in general, rainfall seems to be dominated by

the Ca-SO4 and Na-Cl facies. Only a few samples plot within the Ca-HCO3 facies.

Data for winter precipitation (November to April) follows a trend with a slope of 7.9 and

a y-intercept of 9.89 (R2 = 0.91, Fig. 5A); almost identical to the Global Meteoric Water Line

(GMWL; Craig, 1963). Data for summer precipitation (May to October) follows a trend with a slope of 6.43 and a y-intercept of -2.11 (R2 = 0.89, Fig. 5A). The weighted mean (δ18O, δ2H)

values are -5.7 ‰ and -38 ‰ for yearly rainfall, -7.2 ‰ and -47 ‰ for winter rainfall, and -4.3

‰ and -29 ‰ for summer precipitation (Fig. 5B).

B.6.b Ciénega de Santa Clara

Surface water samples from the Ciénega have Cl- concentrations between 550 mg/L and

54,492 mg/L, and SO4 concentrations between 251 mg/L and 9,653 mg/L. The concentrations of

these solutes increase southward towards the Gulf of California (Table B1). The δ18O values range between -10.6‰ and +6.0‰, and the δ2H values range between -88‰ and +8‰ (Fig. 6A).

The highest (δ18O, δ2H) values are also located in the southern part of the Ciénega near the tidal

flats (sites 76 and 77). Most of the water samples fall to the right of the GMWL. The regression

line (δ18O and δ2H) has a slope of 5.8 (Fig. 6A). The slope is within the range observed by other

studies in the area (slopes between 5 and 6; Eastoe and Towne, 2018) and is characteristic of

evaporated Colorado River water (Robertson, 1991; Guay et al., 2006). This evaporation line is

shown in Figure 6 as the Colorado River Evaporation Trend (CRET).

B.6.c Colorado River Floodplain and San Luis Mesa

In general, Colorado River floodplain samples are divided between Ca-SO4 and Na-Cl

facies (Fig. 4). Groundwaters in the eastern side of the San Luis Mesa show Na+ and Cl- as the

predominant ions, and evolve into a Ca-SO4 facies as they move westward and mix with

Colorado River water. These groundwaters are undersaturated with respect to halite, gypsum,

+ - 2+ -2 and anhydrite allowing Na , Cl , Ca , and SO4 concentrations to increase along the flow paths.

The δ18O values for Colorado River floodplain groundwaters range between -11.8 ‰ and -14.7

‰, and δ2H values between -94 ‰ and -112 ‰. The (δ18O, δ2H) values for the San Luis Mesa

groundwaters range between -7.9 ‰ and -12.7 ‰, and between -60 ‰ and -98 ‰, respectively

(Fig. 6).

B.6.d Pozos of the Gran Desierto and El Golfo Groundwater

In general, the waters of the pozos are characterized as Na-Cl facies, with the exception of sites 9 and 14 which have a Na-HCO3 chemistry (Fig. 4). Site 14 (Pozo Nuevo), at the base of

El Pinacate and away from any major stream, yielded (δ18O, δ2H) values of -7.6‰ and -51‰.

The (δ18O, δ2H) values for the Adair pozos range between -3 ‰ and -8 ‰, and between -40 ‰

and -62 ‰, respectively (Fig. 6). The (δ18O, δ2H) values for the Cerro Prieto pozos and the El

Golfo samples show less variability, excluding sample 19 (highly evaporated), and range between -6.3 ‰ and -7.1 ‰, and -62 ‰ and -67 ‰ (Fig. 6), respectively.

Tritium activities were below detection levels for all pozos samples including the Adair and Cerro Prieto locations (Fig. 7A). Carbon-14 values varied between 10 and 90 pMC in the

Adair pozos, including site 14 at Pozo Nuevo which had a value of 87 pMC (Fig. 7B). The 14C

values in the Cerro Prieto pozos ranged between 3 and 25 decreasing from north to south. There

is no clear correlation between 14C (or 3H) and δ18O values.

B.7 Discussion

B.7.a Rainfall and Local Recharge

The chemistry of regional rainfall is variable and is not restricted to a particular hydrochemical composition. However, the Na-Cl and Ca-SO4 facies seem to predominate (Fig.

4). The cation portion of the Piper Plot shows a linear trend of rainfall samples plotting between

Ca2+ and Na+. One possible explanation of this pattern could be related to the source of the water

vapor for precipitation. Rainfall formed from vapor originating near the Pacific coast will have

similar ion ratios to those of sea water, through release and dissolution of marine aerosols

(Herczeg and Edmunds, 2000), and will plot in the Na-Cl facies of the Piper Plot. Rainfall moving inland from more distant sources (e.g. Gulf of Mexico, East Pacific) into the area might

2+ -2 + - have higher concentrations of Ca and SO4 , relative to Na and Cl , as marine-derived solutes

fallout near the coast and continental dust containing gypsum is rapidly acquired and dissolved

(Herczeg and Edmunds, 2000).

The (δ18O, δ2H) values observed at site 14 (Pozo Nuevo) plot near the GMWL and

represent locally-recharged groundwaters (-7.6‰ and -51‰). Similar groundwater values have been observed in other catchments located in the lower Gila basin and the Western Mexican

Drainage in southwestern Arizona, and adjacent to the study area (Towne, 2017; Towne, 2018).

The local recharge value is slightly lower than the weighted average (δ18O, δ2H) values for winter rainfall at the OPCNM rain gauge (-7.2‰ and -47‰, Fig. 6).

Long-term records of (δ18O, δ2H) values in precipitation and groundwater in tropical areas show that groundwater recharge is biased to heavy monthly rainfall exceeding the ~70th

intensity percentile (or the 30% most intense events; Jasechko and Taylor, 2015). In the Sonoran

Desert, the amounts and stable isotope data of rainfall show that events with totals above the 60th percentile usually have lower (δ18O, δ2H) values than the weighted winter averages (e.g. Fig. 8).

In fact, the average δ18O and δ2H values for the 30% of the most intense rainfall events (-7.5 ‰, and -50 ‰), are almost identical to locally-recharged groundwaters (-7.6‰ and -51‰, site 14).

Based on these observations, it is reasonable to conclude that recharge at the base of El

Pinacate is limited to the top 30% of the most intense rainfall events. During these events, which occur during winter, the amount of water is large enough and the environmental conditions are optimal for infiltration and percolation of moisture into the water table. This pattern has been observed in other regions of southwest Arizona (Eastoe and Towne, 2018).

B.7.b Origin of the Pozos of the Gran Desierto

Water samples collected from the pozos and nearby wells along the coast have different

(δ18O, δ2H) values relative to groundwaters in the lower Colorado River floodplain and in the

San Luis Mesa (Fig. 6A). Ezcurra et al., (1988) hypothesized that water in the pozos was derived as local recharge flowing from El Pinacate towards the coast (Fig. 1). While site 14 is consistent with local recharge as discussed in the previous section, the pozos (both Adair and Cerro Prieto), well, and tap water samples from El Golfo plot far from the GMWL, and require further explanation (Figs. 6A and 6B).

Most of the Cerro Prieto pozos plot in a cluster with δ18O values between -6.3‰ and -

7.1‰ along the CRET. The association of the pozos samples with the CRET suggests that they consist of evaporated Colorado River water. The plotting position of this cluster approaches

Colorado River water that has undergone ~40% evaporation at 60% relative humidity according to calculations made by Zamora (Fig. A6 in Appendix A) using the method described by Clark

and Fritz (1997, pages 87-88). Some of the Adair pozos samples plot near the Cerro Prieto pozos

cluster, but there is significantly more scatter among the Adair pozos, and they also seem to

follow a different trend away from the CRET (dashed polygon in Fig. 6B). The water table

elevation of the Adair pozos is no higher than 1.5 masl, and these pozos seem to be located near the fresh groundwater and seawater interface. There is a possibility that some seawater intrudes into the fresh groundwater system. Higher than average tide levels during spring tides and tidal

surges caused by hurricanes would certainly force seawater inland. Seawater intrusion would

cause some of the east Adair pozo samples to plot along a trend from evaporated Colorado River

water to seawater as shown in Figure 6B.

The monovalent ions Cl- and Br- provide a useful tracer combination to identify salinity

sources in groundwater. Bromide is rejected during the process of halite precipitation, and the

Cl/Br mass ratio of solid NaCl is usually 2-3 orders of magnitude higher than in the original waters (~ 5,000; Braitsch, 1971). Available data from the Colorado River water near Yuma,

Arizona (USGS Stations 09429600 and 09521100; USGS, 2017) are consistent with a trend line resulting from the dissolution of halite (Figs. 9A and 9B). Water samples from the pozos seem

to plot close to a seawater dilution or mixing trend (Figs. 9A and 9C). There are two alternatives

that explain this observation. Given the location of the pozos near the coast, it is likely that there

is a percentage of seawater in the pozos. This is consistent with the pozo-seawater trend observed in the stable isotope data and supports field observations made by May (1973) who hypothesized the intrusion of seawater into the pozos closest to the coast. The percentage of

seawater in the solution would depend on location, but mass balance calculations using δ18O

suggest that it would not exceed 25%.

The other possibility is the dissolution of marine-derived aerosols. The Cl/Br mass ratio

in local precipitation ranges between 150 and 274. These values are similar, but lower than the

marine Cl/Br mass ratio of 290 (Davis et al., 1998), and are likely due to the presence of

organobromine compounds including CH3Br and C2H4Br2. These compounds could have been

originated as agricultural pesticides in the extensive Mexicali or San Luis valleys or as natural

metabolites in marine organisms (Gribble, 1999). Constant dissolution of marine-derived aerosols with marine signature would cause samples to plot along the seawater dilution trend.

However, this mechanism by itself would not explain the observed mixing trend between evaporated Colorado River water and seawater observed in Figure 6B. Freshwater-seawater

- - -2 mixing and dissolution of marine aerosols not only affect Cl and Br , but also SO4

concentrations. Figure 10 shows Cl/SO4 ratios of waters in the study area seem to have a tendency to plot towards the seawater ratio likely also as a direct result of freshwater-seawater mixing and dissolution of marine aerosols.

A single mixing relationship between Colorado River water and seawater (in terms of Cl-

and Br-) does not explain the variability observed in the pozos samples. Figure 11 shows

different mixing lines for seawater with diverse river water compositions. The chemical

compositions of the pozos samples are likely the result of mixing between Colorado River water with variable starting chemical compositions, and seawater. The variability in the starting composition of Colorado River water, in terms of Cl-, could be explained in part by changes in

the amount of salt input from evaporites, and evaporation. As previously discussed, water in the

pozos seems to have undergone up to 40% evaporation from its original Colorado River

composition, affecting the original Cl- concentration.

B.7.c Conceptual Model

Recharge to the pozos did not occur directly from the Colorado River channel, but rather, it was subjected to an evaporative process at the surface prior to infiltration and percolation into the water table. Evaporation causes the pozos samples to plot along the CRET. This process is assumed to occur prior to infiltration, because groundwater samples collected from wells (sites 9 and 12), and tap water samples (municipal supply; sites 21, 22, 23) plot along the CRET and care was taken during collection to prevent evaporation. In modern times, the (δ18O, δ2H) values for the pozos plotting near the CRET (-6.8 and -67; Fig. 8B) are only found along the Colorado

River in highly evaporative environments such as the Ciénega, and Topock Marsh (Guay et al.,

2006). Clay layers across the floodplain and the San Luis and Yuma Mesa deposited in a low- energy environment could be vestigial surfaces showing the possible location of such settings

(Olmstead et al., 1973; Dickinson, et al., 2006). These environments are no longer found along the highly engineered floodplain of the Colorado River.

After infiltration and percolation, the direction of groundwater flow was influenced by the major faults in the study area. Faults are known to affect flow patterns in groundwater aquifers (Bense et al., 2013). The pozos are conspicuously located along or near the Cerro Prieto and the Altar faults. It seems that groundwater flow and discharge was focused along the faults.

The Ciénega is also situated along the Cerro Prieto Fault, in an abandoned meander within the historical alluvial plain of the Colorado River. The presence of low permeability material, such as fault gouge, mineralization, and clay smearing might hamper flow across the fault, and develop a strongly anisotropic permeability near the Altar Fault zone (Dickinson, et al., 2006).

The combination of a strong hydraulic gradient generated in the former recharge zones near

Yuma, where a wetland environment could have been located in the past, and the fault-related

permeability allowed groundwater to flow for tens of kilometers likely following old alluvial channels now buried under the dunes of the Gran Desierto (Ezcurra et al., 1988; Fig. 4).

Age tracers suggest that none of the recharge for groundwater discharging at present in the pozos occurred in the last ~65 years. Tritium activities are below detection levels in all the pozos samples (Fig. 9A). This is different from modern Colorado River water near Yuma which has a value of ~5 TU and even higher values in the past decades as a result of nuclear testing

(Payne et al., 1979). Carbon-14 values are also lower in the pozos water samples than Colorado

River water at Yuma (> 100 pMC, Fig. 9A). Corrected 14C ages were calculated for sites 13 and

20 using the Fontes-Garnier model (Fontes and Garnier, 1979). For these two sites, water is constantly running, the point of discharge is free of any vegetation that would contribute modern

DIC, and both plot near the CRET, being Colorado River water of the original source. This reasoning lead us to believe that, at least for these two sites, there are no additional sources of

DIC and there is no mixing with other endmembers (such as seawater) which would invalidate

13 the assumptions for the Fontes-Garnier model. Using a δ CCaCO3 value of -5.6 ‰ (bulk

13 -2 14 sediment), δ Csoil-CO2 value of -23 ‰, partial pressure of CO2 of 10 , and a pre-bomb Co of 84 pMC (Goodfriend and Flessa, 1997) the model yielded corrected ages of 6,400 and 18,400 14C years for sites 13 and 20, respectively. Groundwaters with δ18O and δ2H values and low 14C pMC, similar to what is observed today at the pozos, are not found around the hypothesized recharge zone near Yuma. This suggests that this older water may have been flushed out of the system along the present-day river channel by more contemporary, less-evaporated Colorado

River water.

B.8 Conclusions and Implications for Regional Groundwaters

The stable isotope data for this study lead to considerations that are relevant to other

areas where waters of different sources and ages mix. Weighted average values for winter,

summer, and annual precipitation helped to characterize isotopic signature and seasonality of

local recharge in the region. Local recharge tends to be easier to recognize away from major riverbeds, which carry water from distant sources, when they are running. In the study area, the

local recharge endmember is observed along the base of El Pinacate. Modern Colorado River

waters plotting closer to the GMWL were identified near the floodplain. Evaporated Colorado

River waters plotting along the CRET were identified at the Ciénega, and at both pozos

locations.

Stable and radioactive isotope data suggest that locally-derived recharge is not the main source of water supporting the pozos, as suggested by previous studies. Instead, it seems that evaporated water derived from the Colorado River is a major component of the pozos recharge.

Recharge near the axis of the Colorado River channel occurs with minor evaporation, but in

prehistorical times, recharge also occurred at the floodplain margins after being exposed to

highly evaporative conditions. Wetland environments where standing water would persist due to

the presence of fine-grained material are likely to have played a significant role in the aquifer

recharge and are likely to have experienced significant evaporation.

After infiltration and percolation, groundwater flow direction was influenced by the

major faults in the study area. The aquifer underlying the pozos was supplied by evaporated

Colorado River water and followed preferential flow paths determined by the local faults. The

presence of low permeability material along the fault obstructs flow across the fault, leading to

the development of a strongly anisotropic system near the Altar Fault zone that favors

100

groundwater flow parallel to the fault. Groundwater flow direction was likely enhanced by the

strong hydraulic gradient generated at the recharge zone.

The Adair pozos are located at the freshwater and seawater interface near the coast.

Here, fresh groundwater appears to be mixing with seawater prior to reaching the surface as

suggested by stable isotope and water chemistry data. Tidal surges created during hurricane- powered storms likely intensify mixing by allowing further inland intrusion of seawater.

The pozos of the Gran Desierto have remained resilient over the centuries despite major

landscape changes and water over use in the Colorado River delta. The aquifer feeding the pozos

was recharged by evaporated pre-dam Colorado River water. Flow from the river to the pozos

occurred at least prior to the 1960’s, but perhaps thousands of years ago as suggested by the 14C

data. The environment where recharge occurred is no longer present and the pozos are

vulnerable to major groundwater pumping and development in the area.

B.9 Acknowledgements

This material is based upon work supported by the National Park Service Southwest Border

Resource Protection Program and a University of Arizona ChevronTexaco Geology Fellowship.

The authors express their gratitude to the staff at El Pinacate and Gran Desierto de Altar

Biosphere Reserve, Organ Pipe Cactus National Monument, and Ejido Vicente Guerrero for their

logistics support and assistance during fieldwork.

101

B.10 Figures

Figure B1: Top: Colorado River Basin. Bottom: lower Colorado River, Colorado River delta, Altar Basin, and major geographical and geological features. Dashed red line separates the floodplain (west) from the Yuma and San Luis mesas (east). Dry bed of the Gila River shown as dashed line in inset.

102

Figure B2: Groundwater levels in 2010. Contours represent water table elevation in meters above sea level. Based on data from Dickinson et al., (2006), the Arizona Department of Water Resources (2015), and CONAGUA (2010).

103

Figure B3: Location of sampling sites in study area. Detail box: floodplain samples from Palomares-Ramirez (2011). Red dashed line separates floodplain (west) from the San Luis and Yuma mesas (east).

104

Figure B4: Piper Diagram showing data for rainfall samples collected at Organ Pipe Cactus National Monument (National Atmospheric Deposition Program, 2016), Colorado River floodplain well samples (Palomares-Ramirez, 2011), San Luis Mesa well samples and pozos water samples for this study.

105

Figure B5: A. δ2H vs δ18O values for individual precipitation events collected at Organ Pipe Cactus National Monument between 1990 and 2006. B. Weighted average δ18O and δ2H values for overall, winter, summer precipitation, and 30% wettest events for samples in 5A. Average seawater Gulf of California from Dettman et al., (2004).

106

Figure B6: A. δ2H vs δ18O values of surface and groundwater from Colorado River floodplain, San Luis Mesa, Cienega, mean winter rainfall (RF), 30% wettest rainfall events at OPCNM rain gauge, and pozos relative to the Global Meteoric Water Line (GMWL), and Colorado River evaporation trend (CRET). B. Detailed view showing the pozos samples relative to the GMWL and the CRET. Red dashed triangle shows the mixing trend that would be followed by mixing evaporated Colorado River water (δ18O = -6.8 ‰, δ2H = -67 ‰), local recharge, and seawater (SW).

107

Figure B7: A. Tritium (TU) data for water samples in the study area. B. Carbon-14 (pMC) for water samples in the study area. Shading indicated topography as shown in Figure 1. Additional 14C and 3H data along the U.S.-Mexico border not shown in Table B1 from Dickinson et al. (2006).

108

Figure B8: Weighted average δ18O and δ2H values according to percentile range (10%). As an example, the points for 80% correspond to an amount-weighted average of the largest 20% of rain events. Dashed line points to δ18O value of local recharge in the region (site 14). Dotted line points to δ2H value of local recharge in the region (site 14). Plot based on Table A4 for precipitation at OPCNM.

109

Figure B9: A. Br-vs Cl- (mg/L) values for water samples in the study area. B. Detailed view of the lower end of graph A. C. Detailed view for the upper end of graph A. Colorado River water (C.R.) at Yuma obtained from USGS Station 09404200 (USGS, 2016).

110

- 2- - Figure B10: Cl /SO4 (mass) vs log Cl (mg/L) for surface and groundwaters used in this study. Dashed lines show hypothetical mixing lines between seawater (SW) – or whole salt as aerosols– in one end, and Colorado River water at the other. Colorado River floodplain samples from Palomares-Ramirez (2011).

111

Figure B11: Cl-/Br-(mass) vs log Cl- (mg/L) for surface and groundwaters used in this study. Dashed lines show hypothetical mixing lines between seawater – or whole salt as aerosols– in one end, and Colorado River water with different Cl- concentrations at the other. Colorado River water (C.R.) at Yuma obtained from USGS Station 09404200 (USGS, 2016).

112

B.11 Tables

T SS Type Lat Long Date pH E.C. Na Mg K Ca Cl SO Br Alk HCO CO δ18O δ2H δ13C 3H 14C Source (°C) 4 3 3 1 B. Adair 31.505 -113.652 Jun-16 26 9.1 14.1 3658 2 76 2 4535 726 20.5 57.1 1945 86 -6.4 -53 -5.1 <0.7 88 1 2 B. Adair 31.645 -113.848 Jun-16 25 9.4 5.3 1298 11 13 5 1488 720 3.6 32.9 1001 106 -5.0 -45 -3.7 <0.5 50 1 B. Adair 31.645 -113.848 Jan-17 1035 11 11 7 1132 299 3.2 -6.9 -53 1 B. Adair Oct-16 -5.2 -45 1 B. Adair Jan-83 9.0 5.4 1182 5 16 8 1064 480 11.3 567 60 2 B. Adair Jan-83 10.1 6.1 1343 15 16 16 1099 624 14.4 708 84 2 3 B. Adair 31.696 -113.948 Jun-16 25 8.8 11.3 2827 14 36 9 4183 260 15.7 16.6 694 17 -4.1 -43 -3.4 <0.5 33 1 B. Adair Jan-17 2430 13 31 6 3977 257 11.0 -6.2 -53 1 B. Adair Jun-82 8.8 5.8 1308 30 8 1383 19.0 570 2 4 B. Adair 31.608 -114.015 Jun-16 25 8.2 5.5 1170 36 24 59 1813 398 5.1 6.7 338 7 -5.7 -55 -2.6 <0.8 63 1 5 B. Adair 31.524 -114.129 Jun-16 25 7.8 4.7 1071 24 21 38 1338 242 3.6 944 5 -2.8 -40 -11.3 <0.8 91 1 B. Adair Jan-16 787 24 24 52 1035 317 2.4 -5.9 -55 1 B. Adair Jan-17 861 22 18 52 1136 380 2.5 -5.9 -55 1 B. Adair Jan-83 8.7 4.1 851 13 23 39 890 427 5.7 348 2 B. Adair Jun-82 8.6 4.1 839 27 27 14 851 480 4.8 293 2 6 B. Adair 31.516 -114.126 Jan-16 1079 26 30 92 1382 493 4.0 -6.3 -58 1 B. Adair Jan-16 946 25 24 50 1240 370 3.3 -6.3 -58 1 7 B. Adair Jan-16 5382 22 44 23 8631 1604 14.6 -3.0 -40 1 8 B. Adair Jan-17 513 16 10 34 666 225 1.7 -6.8 -61 1 9 Well 31.560 -113.718 Jun-16 25 8.1 5.8 1571 8 7 6 1079 386 2.7 74.8 2600 27 -5.8 -51 -8.4 <0.9 72 1 Well 31.560 -113.718 Oct-16 30 8.0 4.2 807 4 4 8 774 244 2.8 37.6 1270 10 -5.9 -50 1 10 Auger 31.645 -113.848 Jun-16 25 9.5 17.1 4785 2 33 1 3856 4866 7.3 155.9 4576 434 -6.1 -53 1 11 Auger 31.696 -113.948 Jun-16 25 8.6 18.9 -6.1 -54 1 12 Well 31.713 -113.990 Jun-16 25 7.9 3.7 789 8 3 18 1068 272 1.9 15.6 596 0 -7.4 -62 -9.6 <0.8 54 1 13 B. Adair 31.513 -114.136 Jun-16 25 7.8 2.7 562 14 14 28 734 213 1.7 6.5 351 7 -6.8 -62 -6.1 <0.5 12 1 B. Adair Jan-16 -6.6 -59 1 B. Adair Jan-17 474 13 12 37 638 183 1.6 -6.6 -61 1 14 Pozo Nuevo 31.825 -113.719 Oct-16 31 7.6 0.5 90 4 4 17 12 15 7.0 348 0 -7.6 -51 -12.6 <0.4 87 1

113

Pozo Nuevo Nov-82 8.5 0.6 97 5 8 18 18 19 4.2 256 2 15 C. Prieto 31.959 -114.744 Jul-14 32 7.8 378 149 0.9 2.9 178 -7.1 -66 -4.6 <0.5 25 1 C. Prieto 31.959 -114.744 Oct-13 1.8 384 153 0.9 -6.9 -66 1 16 C. Prieto 31.958 -114.754 Oct-13 2.0 429 146 1.2 -6.7 -66 1 17 C. Prieto 31.947 -114.749 Jul-14 32 7.7 527 161 1.5 3.3 207 -6.9 -65 -4.1 <0.9 12 1 C. Prieto 31.947 -114.749 Oct-13 2.2 544 167 1.5 -7.0 -66 1 18 C. Prieto 31.944 -114.742 Jul-14 30 7.2 559 147 1.2 4.4 279 -6.7 -64 -8.0 1 C. Prieto 31.944 -114.742 Oct-13 2.6 565 170 1.4 -6.3 -64 1 19 C. Prieto 31.937 -114.732 Jul-14 30 8.0 2558 546 7.7 6.1 382 -0.3 -28 -5.6 1 C. Prieto 31.937 -114.732 Oct-13 6.3 1720 356 5.4 -4.6 -54 1 20 C. Prieto 31.853 -114.638 Jul-14 36 7.4 2.8 692 144 2.3 3.0 186 -7.0 -67 -4.3 <0.5 3 1 C. Prieto 31.853 -114.638 Oct-13 2.6 671 139 2.0 -7.1 -67 1 C. Prieto 31.853 -114.638 Jun-16 25 7.8 2.6 535 10 4 35 753 190 1.9 -7.0 -66 1 21 Golfo 31.687 -114.499 Oct-13 -6.9 -62 1 22 Golfo 31.686 -114.501 Oct-13 -7.0 -66 1 23 Golfo 31.686 -114.500 Oct-13 -6.9 -62 1 24 Mesa 32.457 -114.690 Oct-16 7.3 1.8 224 23 5 93 416 157 0.5 6.7 328 -12.7 -98 1 25 Mesa 32.450 -114.674 Oct-16 7.5 2.1 230 28 7 100 516 142 3.7 185 -10.9 -85 1 26 Mesa 32.446 -114.660 Oct-16 29 7.5 1.9 216 27 7 92 511 112 4.0 198 -9.4 -72 1 27 Mesa 32.441 -114.645 Oct-16 29 7.5 1.8 216 20 6 88 464 99 1.4 104 -9.8 -75 1 28 Mesa 32.437 -114.630 Oct-16 32 7.7 1.6 204 14 5 69 422 89 0.4 2.4 109 -9.3 -72 <0.5 1 Mesa Oct-16 30 7.7 1.5 217 14 5 58 398 81 0.6 0.9 64 -8.7 -65 1 29 Mesa 32.423 -114.585 Oct-16 31 7.6 1.4 203 9 4 36 346 76 0.4 1.2 73 -7.9 -60 -9.4 <0.4 26 1 30 Mesa 32.419 -114.570 Oct-16 30 7.6 1.3 202 9 3 31 319 69 0.4 1.6 144 -8.0 -60 1 31 Mesa 32.343 -114.321 Oct-16 33 7.6 3.6 549 7 4 91 840 491 1.7 110 -8.6 -63 1 32 Floodplain 32.490 -114.809 Apr-09 22 7.2 1.8 243 69 4 32 358 390 182 -12.0 -96 3 33 Floodplain 32.487 -114.791 Apr-09 25 7.7 1.3 191 71 3 50 229 220 255 -14.3 -108 3 34 Floodplain 32.474 -114.897 Apr-09 22 7.1 1.8 189 88 4 44 270 380 343 -12.4 -99 3 35 Floodplain 32.473 -114.902 Apr-09 21 7.1 2.3 254 100 4 68 380 520 370 -11.8 -94 3 36 Floodplain 32.468 -114.852 Apr-09 20 7.0 2.0 336 75 4 76 353 450 309 -12.0 -95 3 37 Floodplain 32.467 -114.886 Apr-09 22 7.4 1.9 270 67 3 42 210 360 295 -11.9 -96 3

114

38 Floodplain 32.466 -114.917 Apr-09 22 6.9 2.9 270 116 5 109 364 500 342 -13.3 -103 3 39 Floodplain 32.465 -114.830 Apr-09 22 7.1 2.4 358 73 4 54 355 450 391 -11.8 -95 3 40 Floodplain 32.457 -114.895 Apr-09 24 6.6 2.7 237 98 5 66 394 430 366 -14.0 -108 3 41 Floodplain 32.448 -114.920 Apr-09 22 7.0 1.3 194 54 2 34 220 190 300 -14.7 -112 3 42 Floodplain 32.447 -114.824 Apr-09 24 7.3 1.6 222 76 3 28 281 290 313 -14.2 -108 3 43 Floodplain 32.446 -114.878 Apr-09 22 7.0 2.9 316 68 4 76 434 404 420 -13.4 -105 3 44 Floodplain 32.445 -114.908 Apr-09 21 6.9 3.7 452 71 5 108 646 500 466 -13.9 -107 3 45 Floodplain 32.444 -114.884 Apr-09 21 7.0 2.5 296 91 4 78 401 530 361 -14.0 -109 3 46 Floodplain 32.443 -114.884 Apr-09 23 7.6 0.9 128 54 2 40 133 110 295 -14.5 -111 3 47 Floodplain 32.434 -114.834 Apr-09 21 7.1 2.3 288 78 4 76 341 450 372 -13.8 -106 3 48 Floodplain 32.429 -114.853 Apr-09 20 7.5 1.5 218 69 3 62 233 280 318 -14.2 -103 3 49 Floodplain 32.428 -114.928 Apr-09 20 7.2 3.8 434 113 5 60 688 620 281 -13.4 -104 3 50 Floodplain 32.423 -114.921 Apr-09 23 7.2 2.5 205 97 4 96 401 410 354 -14.3 -110 3 51 Floodplain 32.422 -114.841 Apr-09 22 7.0 3.5 322 91 5 82 553 470 347 -13.5 -105 3 52 Floodplain 32.416 -114.845 Apr-09 23 7.2 2.4 155 53 3 66 235 290 284 -14.1 -109 3 53 Floodplain 32.414 -114.940 Apr-09 24 7.2 2.8 312 98 4 78 501 560 325 -13.3 -105 3 54 Floodplain 32.413 -114.859 Apr-09 23 7.2 2.4 245 79 4 76 397 370 356 -14.0 -107 3 55 Floodplain 32.410 -114.904 Apr-09 25 7.1 1.2 137 56 3 52 199 200 290 -14.3 -110 3 56 Floodplain 32.409 -114.881 Apr-09 22 7.2 1.4 173 57 3 58 247 290 290 -13.9 -108 3 57 Floodplain 32.405 -114.929 Apr-09 20 7.5 1.0 137 27 5 22 156 90 209 -14.4 -110 3 58 Floodplain 32.397 -114.910 Apr-09 22 7.3 1.9 221 60 3 50 312 310 309 -14.2 -109 3 59 Floodplain 32.397 -114.940 Apr-09 21 7.0 1.5 139 60 3 52 260 220 281 -13.9 -108 3 60 Floodplain 32.394 -114.888 Apr-09 21 7.7 1.6 219 58 3 54 314 290 343 -13.6 -104 3 61 Floodplain 32.387 -114.929 Apr-09 21 7.1 1.0 114 57 2 52 160 130 391 -14.6 -110 3 62 Floodplain 32.387 -114.950 Apr-09 22 7.1 2.7 315 61 4 60 455 310 384 -13.8 -107 3 63 Floodplain 32.386 -114.881 Apr-09 21 7.3 1.6 185 56 3 58 305 260 252 -14.2 -109 3 64 Floodplain 32.375 -114.883 Apr-09 25 7.4 2.3 257 52 4 66 422 220 265 -14.0 -107 3 65 Floodplain 32.359 -114.929 Apr-09 23 7.7 1.4 208 35 3 36 257 220 262 -14.2 -109 3 68 Cienega 32.062 -114.904 May-13 5.4 1149 827 2.3 -9.6 -81 1 69 Cienega 32.057 -114.899 Jul-14 30 7.9 2.9 558 782 1.0 4.8 305 -10.6 -88 -8.7 1 May-13 3.8 603 837 1.0 -10.6 -87 1

115

70 Cienega 32.039 -114.822 Oct-13 3.6 975 251 2.5 -6.7 -66 1 71 Cienega 32.039 -114.908 Jul-14 35 7.6 4.9 906 1028 1.5 5.1 318 -8.6 -77 -7.9 1 May-13 5.9 1259 1263 2.1 -7.4 -73 1 72 Cienega 32.038 -114.895 Jul-14 35 7.6 3.6 550 771 1.0 4.7 297 -10.5 -88 -8.1 1 73 Cienega 32.025 -114.872 Jul-14 27 7.3 2.3 654 868 1.1 5.5 348 -10.2 -85 -10.3 1 May-13 6.0 1184 1323 2.2 -8.1 -74 1 74 Cienega 32.015 -114.868 Jul-14 28 7.6 3.6 728 944 1.2 5.5 346 -9.8 -83 -7.9 1 75 Cienega 31.988 -114.827 May-13 6.0 1182 1346 2.5 -7.3 -69 1 Jul-14 37 8.2 27.9 7425 4960 16.9 5.1 324 -0.4 -28 -3.8 1 76 Cienega 31.965 -114.814 May-13 12.5 2848 2597 5.8 -1.1 -39 - 1 Jul-14 28 7.5 144 44933 8249 181.8 5.4 341 0.7 -17 -5.5 1 77 Cienega 31.958 -114.811 May-13 146 54492 9653 119.9 6.0 8 1 78 Seawater 8.0 47.9 10752 1295 390 416 19345 2701 66.0 145 0.6 3 4 Table B1

Sources: E.C. measured in mS/cm 1 This study Ions measured in mg/L 18 2 2 Ezcurra et al., 1988 δ O and δ H relative to V-SMOW (‰) 3 Palomarez-Ramirez, 2011 δ13C relative to V-PDB (‰) 4 Dettman et al., 2004 14C expressed in percent modern carbon (pMC) 3 H expressed tritium units (TU)

116

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APPENDIX C:

Groundwater isotopes in the Sonoyta River watershed, USA-Mexico: implications for

recharge sources and management of the Quitobaquito Springs

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Groundwater isotopes in the Sonoyta River watershed, USA- Mexico: implications for

recharge sources and management of the Quitobaquito Springs.

Hector A. Zamora1, Christopher J. Eastoe1,5, Benjamin Wilder2, Jennifer C. McIntosh3, Karl W.

Flessa1, and Jeffrey Welker4.

1. University of Arizona, Department of Geosciences, 1040 E. 4th Street, Tucson, Arizona,

85721, USA.

2. University of Arizona, Desert Laboratory on Tumamoc Hill, 1401 E. University Blvd.,

Tucson, Arizona, 85721, USA.

3. University of Arizona, Department of Hydrology and Atmospheric Sciences, 1133 E.

James E. Rogers Way, Tucson, Arizona, 85721, USA.

4. University of Alaska Anchorage, Department of Biological Sciences, 3211 Providence

Drive, Anchorage, Alaska, 99508, USA.

5. Retired.

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C.1 Abstract

Environmental isotopes and water chemistry were used to distinguish water types,

recharge mechanisms, and residence time along several reaches of the Sonoyta River and

Quitobaquito Spring located near the U.S.- Mexico border. Areas located up gradient from the

Sonoyta River, such as the Puerto Blanco Mountains and La Abra Plain are supported by local

recharge which corresponds to water from the largest 30% of rain events mainly occurring

during winter. For Quitobaquito Spring, the (δ18O, δ2H) values are too low to be derived from local recharge. Two possible explanations are given for the origin of water 1) a mix of modern recharge and Pleistocene-aged groundwater and 2) Sonoyta River water supplied water through a suggested fault system connecting the spring to the alluvial aquifer beneath the river. The latter explanation is supported by Cl/SO4 mass ratios. Groundwater resources are finite and the

riparian and wetland areas are vulnerable to aquifer overdraft and unregulated groundwater use.

C.2 Introduction

Alluvial aquifers in the Basin and Range Province in western North America have been

extensively studied through the use of environmental isotopes (δ18O, δ2H, 3H, and 14C) and solute

chemistry. Waters develops distinctive isotopic and chemical compositions as a result of

fractionation resulting from the hydrological, biological, and chemical processes that occur at the

surface, in the vadose zone, and in saturated zones of the aquifer systems. This geochemical

characterization of waters provides the means to distinguish sources of aquifer recharge, identify

preferential flow paths, and estimate groundwater residence time. The method is particularly

effective in high-relief basins that receive recharge from bounding mountain ranges and from

rivers with distant headwaters such as the Sonoyta River watershed (Fig.1). Here, in the heart of

the Sonoran Desert, along the U.S. - Mexico border, ecological systems and human settlements

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heavily rely on and compete for water resources that are expected to decline as climate warms

(Barnett, 2008).

Perennial surface flow and cienegas (wetlands) occurred along several reaches of the

Sonoyta River prior to the 19th century, but were significantly reduced by the diversion of surface water into irrigated fields (Rosen et al., 2010). Livestock grazing during the early 20th century severely degraded the river channel. Over recent decades, the combination of drought conditions with increasing groundwater demand for irrigation and municipal use on both sides of the border has contributed to the decline of water levels in the alluvial aquifer (Minckley et al.,

2013). Intensive groundwater use started in 1952, and peaked in the 1980s, when approximately

1.32 x 108 m3/year of groundwater were used to irrigate 13,000 ha in the Sonoyta Valley in

Mexico, and 2,000 ha in the Tohono O’odham Nation (Murguia, 2000; Rosen et al., 2010;

Hollett, 1995). The volume of water used for irrigation is estimated to have exceeded natural recharge by 0.97 x 108 m3 during this time period (Rosen et al., 2010; Hollett, 1995).

Today, surface water resources at Organ Pipe Cactus National Monument (OPCNM, Fig.

1) and the lower Sonoyta River are limited to bedrock depressions (known as tinajas) that collect seasonal runoff, springs such as Quitobaquito and Dripping springs (see Fig. 2 for location), and a few perennial reaches along the river (Hendrickson and Varela Romero 1989; Miller and

Fuiman, 1987). The largest perennial reach occurs 1.6 km south of Quitobaquito Spring

(referred to as Quitobaquito below), near the privately-owned Rancho Agua Dulce (Fig. 2), where granitic bedrock is exposed along a narrow river channel. The perennial reaches of the river are vestiges of a once intact riparian ecosystem in one of the more arid portions of the

Sonoran Desert. The pockets of surface water that persist serve as refuge to a diversity of native

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and endemic aquatic vertebrates recognized as endangered or threatened (e.g. Desert Pupfish;

Minckley et al., 2013).

The efforts of this study focus around Quitobaquito and the reaches of the Sonoyta River adjacent to OPCNM, and within the Sonoyta Valley and La Abra Plain (Fig. 2). Long-term

spring flow measurements at Quitobaquito show a reduction in discharge during the last 25 years

(Peter Holm, National Park Service [NPS], Personal Communication; Fig. 3). Detailed

hydrogeochemical studies that address the origin of groundwater recharge and residence time are

needed to understand the impacts of increased groundwater use and expected intensified drought

in these aquatic ecosystems. This study uses chemical and isotopic tracers combined with

available stratigraphic and piezometric information to assist in these efforts. Our initial

hypotheses were that (1) a hydraulic connection exists between the local aquifer originating in

the Puerto Blanco Mountains, believed to be the main source of groundwater supporting

perennial flow at Quitobaquito, and the regional aquifer of the Sonoyta River, and (2)

unregulated groundwater use will negatively impact in the status of Quitobaquito and the

perennial reaches of the Sonoyta River. To test these hypotheses, we use stable isotopes (δ18O

and δ2H), radioactive tracers (14C and 3H), and major ion chemistry of groundwater. In

southwestern North America, stable isotopes in recharge preserve the signatures related to the

altitude and seasonality of precipitation, thus, helping to discriminate between waters of different

origin (e.g. Eastoe and Towne, 2018; Winograd et al., 1998; Cunningham et al., 1998).

Radioactive tracers can be used to estimate groundwater residence times and identify areas of

active recharge (Eastoe et al., 2004; Sanford et al., 2004). Improved knowledge of recharge

dynamics in the study area can help to manage water resources in this water-stressed region.

C.3 Background

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C.3.a Study area

The Sonoyta River watershed is located in the Basin and Range Province (Fig. 1). It is a

transboundary watershed in southern Arizona, United States and northern Sonora, Mexico, that

covers an area of 12,618 km2 (Harshbarger & Associates, 1978). Major watercourses within the

watershed include the Sonoyta River originating near the Pozo Verde Mountains in Mexico, and

the Vamori, Sells, and San Simon washes draining the Tohono O’odham Nation west of the

Baboquivari Mountains and south of Cimarron Peak. The drainage network converges south of

the international border, 40 km east of the town of Sonoyta, Mexico. From here, the Sonoyta

River continues parallel to the border, just south of OPCNM, through irrigated lands in Mexico,

then turns southwards near the Sierra El Pinacate and reaches the Gulf of California.

C.3.b Climate

Climate is arid to semiarid and precipitation follows a bimodal pattern. Summer

precipitation (June – early September) consists of localized, intense, convective precipitation

generated in the tropical eastern Pacific Ocean and Gulf of California by the North American

Monsoon, and accounts for about half of the annual precipitation (Hereford 1993; Wright et al.,

2001). Winter precipitation consists of widespread and long-lived frontal systems generated in temperate regions of the eastern Pacific (Sellers and Hill, 1974; Wright et al., 2001). Weather stations maintained by the NPS staff at OPCNM show that total annual rainfall increases with elevation (NPS, Personal communication). The Sonoyta Valley, located 400 meters above sea level (masl), receives 180 mm/yr of rainfall. Ajo Peak, the highest point within OPCNM at 1465 masl, receives ~300 mm/yr of rainfall. The highest elevations of the watershed are at the crest of

the Baboquivari Mountains, about 2,300 masl. Precipitation data are available at the Kitt Peak

National Observatory, 2096 masl, where the annual average is about 560 m, of which 457 mm is

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snow (WRCC, 2017). Average air temperature ranges between 13.1 - 32.6 °C at the lower elevations, and 5 - 21 °C at the higher elevations.

Studies in Tucson Basin (180 km east of the study area) have established that summer and winter precipitation have distinctly different amount-weighted mean values of δ18O and δ2H, reflecting seasonal changes in condensation temperature and moisture source (Eastoe and

Dettman, 2016; Wright et al., 2001; Wright, 2001). An isotope altitude effect is also well- established in Tucson Basin where lapse-rates of 1.6‰ per 1,000 m for δ18O, and 11‰ per 1,000 for δ2H have been measured (Wright, 2001). Such distinctions, extrapolated to surrounding basins with few or no isotope data for precipitation, provide evidence of the sources and seasonality of recharge in the region (e.g. Eastoe and Towne, 2018; Wahi et al., 2008).

C.3.c Geology and Hydrogeology

The Basin-and-Range Province of western North America is characterized by graben and horst physiography consisting of long, narrow mountain ranges separated by alluvium-filled valleys suitable for groundwater storage. The geology and hydrogeology of the OPCNM, including the Sonoyta Valley, La Abra Plain, and surrounding areas, have been described by

Haxel et al. (1984), Gray et al. (1988), and Bezy et al. (1992). Principal geological units are shown in Fig. 4. The study area is bounded to the north and northwest by mountain ranges consisting of Mesozoic igneous and metamorphic rocks exposed by late Cretaceous thrust faulting and tectonic sliding (Quitobaquito Hills, Puerto Blanco Mountains; Haxel et al., 1984).

Granite gneiss, schist, granite, and other metamorphic rocks of sedimentary origin are the main lithological components (Gray et al., 1988). To the northeast, the study area is bounded by igneous rocks of late Cretaceous and Tertiary age, include andesite, rhyolite, latite, and granodiorite complexes in the Ajo Range (Brown, 1992).

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A detailed description of the basin fill deposits in the study area is provided by Carruth

(1996). In general, basin fill is mainly unconsolidated to weakly consolidated gravels, sands, and

silts that cover the pediments on the lower slopes, and were deposited as overlapping and

interconnecting lenses of slope wash and alluvium along the mountain fronts. These basin fill

deposits extend from the base of the mountain fronts as bajadas, or alluvial plains, and slope

down towards the Sonoyta River Valley where their depths may exceed 300 meters. Stream- channel deposits occur in incised channels within the less permeable basin-fill deposits. These deposits are left behind by ephemeral streams (e.g. Aguajita Wash, Fig. 2) that originate in the nearby mountains and merge with the Sonoyta River.

Most irrigation wells are drilled in the basin-fill, but groundwater also occurs in stream- channel deposits and fractured granitic rocks on the southwest side of the Quitobaquito Hills where a series of fault-controlled springs are found (Quitobaquito; Fig. 5, Carruth, 1996).

Anderson et al., (1990), reported hydraulic conductivities ranging between 3.5 x 10-6 and 3.5 x

10-4 m/s for alluvial basin fill in southern Arizona. Hydraulic conductivity is low in volcanic,

crystalline, and metamorphic rocks, but can be enhanced if weathering and/or fracturing is

extensive (Trainer, 1988). Woloshun (1989) estimated hydraulic conductivities ranging between

2.2 x 10-8 and 1.6 x 10-7 m/s for fractured igneous and metamorphic rocks in southern Arizona.

Recharge in the regional Sonoyta River basin is dominated by mountain system recharge

(MSR) occurring at the basin margins as in other semi-arid basins in southern Arizona (Wahi,

2008). MSR includes mountain runoff infiltration at the mountain front, known as mountain- front recharge (MFR), and subsurface percolation from fractured mountain bedrock into basin alluvium, known as mountain-block recharge (MBR). Additional contributions include focused recharge in ephemeral streams, and irrigation reflux along the Sonoyta River. Diffuse recharge,

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defined as the direct infiltration of precipitation followed by percolation to the water table, is assumed to be negligible. On the basis of isotope evidence, recharge in southern Arizona originates mainly as precipitation from the wettest 30 % months, when the amount of water is large enough and environmental conditions are optimal for infiltration and percolation into the water table (Eastoe and Towne, 2018; Jasechko and Taylor, 2015).

The chemical character of groundwaters depends on the location. Recharge areas tend to be dominated by Ca-HCO3 water types and low concentrations of total dissolved solids.

Downgradient areas in alluvial basins tend to be dominated by Na-HCO3-Cl water types

(Robertson, 1991; see Fig. 6).

C.4 Sample Collection and Methods

Groundwater samples were collected from shallow monitoring wells and agricultural wells, which range in depth from 3 to 300 meters. Wells were purged for three well volumes, or bailed, unless they were in continuous use, except for site 13. For this reason, the (δ18O, δ2H)

values for site 13 collected in March 2016 are left out of the discussion. Spring samples were

collected in March 2016 from Dripping Springs (site 1), located near the top of the Puerto

Blanco Mountains at an elevation of ~650 masl, and Quitobaquito, located less than 200 meters

north of the international border (Fig. 2, sites 3 and 4). Rainfall samples were collected between

2015 and 2017 from rain gauges scattered through OPCNM (adjacent to sites 1, 3, 9, 12, and 16)

prepared with a thin layer of mineral oil to prevent evaporation. The accumulated rainfall was

recovered during several visits to the study area in late October and late March of the respective

years.

For groundwater and surface water samples, pH, temperature, and electrical conductivity

were measured in the field using an YSI 556 Multiparameter System sonde calibrated with

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standard pH and conductivity buffers. Samples collected for ions and alkalinity analysis were

filtered through 0.45-μm nylon filters into 30 mL HDPE bottles. Cation samples were preserved

by adding two drops of concentrated optima grade HNO3. Water samples collected for stable

isotope analysis (O, H, and C) were also filtered through 0.45-μm nylon filters, and were kept in

20 mL glass vials with no headspace. For age tracers (3H and 14C), we collected unfiltered water

samples using rinsed 1-L HDPE and amber borosilicate glass bottles, respectively. All samples

were kept on ice during field collection and then refrigerated at 4 °C.

Stable O and H isotopes were measured on a gas-source isotope ratio (Finnigan Delta-S)

mass spectrometer with automated CO2 equilibration and Cr reduction attachments at the

University of Arizona’s Environmental Isotope Laboratory. Values for stable O and H are

reported in delta notation relative to VSMOW (Vienna standard mean ocean water) with standardization based on international reference materials SLAP (standard light Antarctic precipitation) and VSMOW. Precision is 0.9 ‰ or better for δ2H and 0.08 ‰ or better for δ18O.

Values for stable C were measured on a continuous flow gas-ratio (Thermo-Finnigan Delta Plus

XL) mass spectrometer coupled with a Gasbench automated sampler. Samples were reacted with

phosphoric acid at room temperature in He-flushed vials. Tritium (3H) was measured by liquid

scintillation spectrophotometry on electrolytically enriched samples mixed 1:1 with Ultimagold

Low Level Tritium cocktail. The detection limit was 0.5 TU for 1,500 min of counting using a

Quantulus 1220 Spectrophotometer at the University of Arizona. Calibration for 3H was relative

to NIST SRM 4361, and results are presented as tritium units (TU). For radiocarbon (14C),

carbon was extracted as CO2 from 1-liter unfiltered water samples and reduced to graphite. The

product was measured by accelerator mass spectrometry on a National Electrostatics Pelletron

AMS at the NSF-Arizona AMS facility. Calibration for 14C was relative to IAEA Oxalic Acids I

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- and II, and results are presented as percent modern carbon (pMC). Anion (excluding HCO3 ) and

cation concentrations were determined using a Dionex DX-600 Ion Chromatography system, and

an Elan DRC-II Inductively Coupled Plasma – Mass Spectrometer (precision + 2), respectively,

at the University of Arizona Laboratory for Emerging Contaminants (ALEC). Alkalinity was

obtained using the Gran-Alkalinity method (Gieskes and Rogers, 1973), and the results were

- used in a PHREEQC speciation model to estimate HCO3 in mg/L (Gieskes and Rogers, 1973).

C.5 Data

New data and published isotope and ion chemistry for the Sonoyta River basin from the

Water Quality Portal (WQ Portal; http://www.waterqualitydata.us) are presented in Table C1.

Water Quality Portal Data, which includes data from the USGS National Water Information

System (NWIS) and EPA’s Storage and Retrieval Data Warehouse (STORET), has been

published by Carruth (1996), Goodman (1992), and Hollet (1985) in their respective studies.

Table C1 includes all individual values for sites with multiple data.

Rainfall data collected as part of this study are insufficient to estimate long term mean

(δ18O, δ2H) values. Additional rainfall stable isotope data were obtained from the United States

Network for Isotopes (USNIP) station (Table A4; Welker, 2012). Using δ18O and δ2H data for

individual rainfall events (Fig. 7A), primarily collected at a rain gauge located at the OPCNM

visitor center (510 masl), long-term seasonal amount-weighted means were calculated (Fig. 7B).

Stable isotope data from tinaja samples collected by NPS service during March and April of

2011 are used to estimate the evaporation trend in the area. Throughout the text, we use the

terms “locally sourced” groundwater or “local recharge” to refer to groundwaters recharged near

OPCNM, or at similar elevations in the lower parts of the basin, in contrast to “high elevation”

recharge which refers to water sourced at the headwaters or margins of the Sonoyta River basin.

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C.6 Results

C.6.a Rainfall

Data for rainfall collected during the winter months (November to April) closely matches

the Global Meteoric Water Line (GMWL; Craig, 1963), and follows a trend with a slope of 7.9

and a y-intercept of 9.89 (R2 = 0.91, Fig. 7A). Data for summer rainfall (May to October) plots

slightly below the GMWL, and follows a trend with a slope of 6.43 and a y-intercept of -2.11 (R2

= 0.89, Fig. 7A). The amount-weighted mean δ18O and δ2H values are -4.3 ‰ and -29 ‰ for summer precipitation, -7.2 ‰ and -47 ‰ for winter rainfall, and -5.7 ‰ and -38 ‰ for all rainfall events (Fig. 7B). If we apply the mean isotopic altitude effects of the Tucson Basin (1.6

‰ per 1,000 m for δ18O and 11 ‰ per 1,000 for δ2H) to the OPCNM means, higher elevations of

the Sonoyta River basin (assuming an elevation difference of 1,490 meters between the rain

gauge location at OPCNM and the Baboquivari Mountains), we estimate (δ18O, δ2H) values of -

9.6‰ and -63‰ for average winter precipitation, and -6.7 ‰ and -45 ‰ for average summer precipitation (Fig. 7B). For comparison, the average (δ18O, δ2H) values at an elevation of 2,000 for the Catalina Mountains, near Tucson, Arizona are -10.5‰ and -66‰ for winter, and -8.0‰

and -53‰ for summer (Wright, 2001).

C.6.b Tinajas

Tinajas had δ18O values ranging between -4.9 and +7.7 ‰, and δ2H values ranging between -30 and +10 ‰. The data plot on an evaporation line of slope 3.16 (R2 = 0.82, Fig. 8).

C.6.c Springs

Dripping Springs (site 1) samples tend to have a Na-HCO3 chemistry, while Quitobaquito

samples are Na-HCO3-Cl water types (Fig. 9). In terms of stable isotopes, there is a clear

difference between the two systems. For Dripping Springs, δ18O values range between -4.5 and -

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5.3 ‰, and δ2H values range between -38 and -45 ‰ (Fig.10; Group B). For Quitobaquito, δ18O

values range between -8.3 and -8.5 ‰, and δ2H values range between -59 and -64 ‰, within group C in Fig. 10. The samples from both sites plot below the GMWL (Fig. 10). A Dripping

Spring sample had a 3H value of 1.8 TU, and 14C value of 103 pMC. For Quitobaquito, the 3H

value was 1.2 TU, and the 14C value was 62 pMC (Fig. 11). Williams Spring (site 2) is adjacent

to Quitobaquito, and plots within Group B.

C.6.d Groundwater

Groundwaters in the Sonoyta River basin transition from Ca-HCO3 type near the headwaters adjacent to the Baboquivari Mountains to Na-HCO3 type upgradient of the

international border. Hollett (1985) determined that degradation of water quality is associated

with the presence of lakebed-clay deposits near Papago Farms in (Fig. 1). Within the study area,

only groundwaters from sites 12 and 13 are Ca-HCO3 type, and the rest are Na-HCO3-Cl and Na-

Cl types (Fig. 9). Two groups of groundwater isotope data are present (Fig. 10). For samples in

Group A, δ18O values range between -7.0 and -7.8 ‰, and δ2H values range between -51 and -

54 ‰. This group includes samples from sites 12, 13, 16, 23, 24, 25, 27, and 28. For samples in

Group C, δ18O values range between -7.8 and -8.7 ‰, and δ2H values range between -59 and -64

‰. This group includes samples from sites 6, 7, 8, 9, 10, 11, 14, 15, 17, 18, 22, 26, and overlaps

the data for the Quitobaquito system. Samples from the lowest reaches of the river (sites 20 and

21, Fig. 1) plot next to group C, but more evaporated.

The groups defined by isotope chemistry have a specific geographic distribution. Group

B is limited to the vicinity of the Puerto Blanco Mountains (Fig. 2). Group A occurs away from the main stem of the Sonoyta River mainly along small tributaries draining low-elevation mountain ranges located near or within OPCNM. Group C is found close to the Sonoyta River.

138

Levels of 3H and 14C, are lower near the Sonoyta River than at sites near the Puerto Blanco

Mountains (except for site 11, 25 pMC; Fig. 11). Site 13, located along the Aguajita Wash had

the highest values for both radioisotopes (2.6 TU and 107 pMC; Fig. 11).

C.7 Discussion

C.7.a Recharge Sources

Sites 12, 28, and 27 are located away from the Sonoyta River (Fig. 1). The (δ18O, δ2H) values for these three sites plot along the GMWL, and represent regional local recharge (around -

7.5‰, and -52‰, Fig. 10). Similar values have been observed in catchments located in the lower Gila River basin, adjacent to the study area (Towne, 2017). For areas located up gradient from the Sonoyta River, such as the Puerto Blanco Mountains and La Abra Plain, local recharge is the only possibility. Group A on Fig. 10 occurs within this area, and corresponds closely to regional local recharge. Local regional recharge also corresponds to water from the largest 30% of rain events, dominated by winter recharge (-7.5‰ and 50‰; Fig. 10). Group B must also be locally sourced, and appears to be the evaporated equivalent of Group A, or in one case of evaporated mean winter precipitation.

Waters of other isotope composition, in Group C, must have a different source. These occur in the Sonoyta River flood plain, including sites ~ 20 km upstream of Quitobaquito, where they cannot originate from the spring area. Such water is best explained as originating in the

Baboquivari Mountains because they resemble the isotope composition in site 22. The (δ18O,

δ2H) values (-8.6‰ and -61‰) of site 22 are (1) close to the value estimated for mean winter

precipitation at 2000 masl in the Baboquivari Mountains; and (2) in agreement with other

localities in southern Arizona where groundwater may originate as precipitation from >1500

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masl, such as Patagonia Town where (δ18O, δ2H) values range between -10.5‰ and -8.2‰ for

oxygen and -7.3‰ and -58‰ for hydrogen (average -8.7‰ and -62‰; Gu et al. 2008).

Group C water also occurs at Quitobaquito, where its origin is not so clear, but the

possibilities include: a) Sonoyta River, if a plausible flow path can be identified (discussed

further below), b) hurricane rain, but this is unlikely to affect only the spring (Eastoe et al. 2015),

and c) Late Pleistocene precipitation which had lower (δ18O, δ2H) values in most or all of North

America (Jasechko et al. 2015). In southern Arizona, the shift from modern to lower values

occurred at 13-14 ka (Wagner et al. 2010), corresponding to about 18 pMC. Paleobotanical

remains from packrat middens in the area dating back to 22,000 - 11,000 years B.P. suggest a

precipitation decrease ranging from 16% to 70% from the wetter Pleistocene to Holecene

conditions (amount of change depending on the elevation), with the Pleistocene dominated by

winter rain, and with temperatures 8 – 11°C cooler than today (Van Devender, 1985; Van

Devender et al., 1990).

The 14C content at Quitobaquito, 62 pMC, could be explained as an approximate 1:1

mixing between modern local recharge (100 pMC) and water recharged 13,500 years ago (18

pMC). If the same mixing ratio is used on δ18O data, the local recharge value (-7.5‰) and the

Quitobaquito value (-8.4‰) can be used to calculate the value for Pleistocene water which would have been ~ -9.4‰, and consistent with the 2‰ shift proposed by Wagner et al., (2010) and

Jasechko et al., (2015). The fault zone near Quitobaquito contains local recharge represented by

the Williams Spring sample with a temperature of 14°C (see Table C1). This, and the 1.2 TU at

Quitobaquito seems consistent with mixing of modern and older waters in the fault zone.

The volume of groundwater stored in the basin fill for the local flow system around the fault zone is estimated to range between 2.11 x 1010 L and 3.66 x 1010 L (Carruth, 1996). If we

140

assume that this volume is discharged in its entirety through the Quitobaquito system at a rate of

1.7 x 105 L/day, this would yield a residence time ranging between 340 and 590 years. This calculation suggests that the fault zone cannot accommodate enough paleowater to sustain the spring for 13,000 years, or that a larger volume of water is stored in the fractured granite.

A hydraulic connection between the local flow system and the regional flow system along the Sonoyta River might exist (Carruth, 1996). The fault zone from which the springs discharge may extend southeast to the Sonoyta River based on vegetation anomalies and the presence of a former perennial reach located south of the Santo Domingo Hills (Rosen et al.,

2010). A southeast continuation of the fault could intersect the river where the 335 m topographic contour crosses the channel (Fig. 12). When the river had perennial water in the past, it would have ben capable of supplying water to the suggested fault zone, and into

Quitobaquito. Upward flow in the fault zone at Quitobaquito may be explained by heating at

2- - depth (Goodman, 1992). The dissolved SO4 and Cl concentrations at Quitobaquito are higher than local recharge and more similar to Sonoyta River waters (Fig. 13). Similarly, the ratios of

Cl/SO4 at Quitobaquito and Sonoyta River water seem to plot closely to each other suggesting a common origin (Fig. 14).

In summary, there are two possible explanations for the origin of water at Quitobaquito

Spring 1) paleorecharge, although estimated groundwater storage volume seems too small to accommodate Pleistocene-aged waters, and 2) Sonoyta River water supplying water through the suggested fault zone. If paleorecharge is the source, current drought conditions may be affecting hydraulic head in the fault zone due to lowered recharge fluxes. If water is derived from the

Sonoyta River or the allivial aquifer, present groundwater use has probably already eliminated

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recharge to the spring. The latter explanation seems to be more plausible based on the ionic

- 2- mass ratios of Cl and SO4 .

C.7.b Other Data

Samples 20 and 21 plot outside of groups A and C (Fig. 10). However, these two

- 2- 14 samples are slightly different in terms of Cl and SO4 , and C. The concentrations for both

anions are higher in these two samples than in most of the samples in groups A and C (Fig. 13)

and 14C pMC are the lowest among all samples (16 and 27 pMC). At a first glance, these

samples can be interpreted as analogous to Group C, but evaporated prior to recharge. Both

samples are located in the southernmost locations along the Sonoyta River and were obtained

from wells near irrigated lands (Fig. 1). The high degree of evaporation could reflect the return

of water used for surface irrigation to the aquifer, although the low 3H and 14C pMC values do

not suggest recent exposure to the atmosphere.

C.8 Conclusions

Areas located up gradient from the Sonoyta River, such as the Puerto Blanco Mountains

and La Abra Plain which include Group A and Group B, are supported by local recharge. Local

regional recharge corresponds to water from the largest 30% of rain events, dominated by winter

events (-7.5‰ and 50‰). Groundwaters in the Sonoyta River floodplain (Group C), ~20 km

upstream of Quitobaquito, originate in the Baboquivari Mountains where recharge occurs at a

higher elevation. For Quitobaquito, the values (δ18O, δ2H) are too low to be derived from local recharge. Two possible explanations for the origin of water at Quitobaquito include 1) a mix of modern recharge and Pleistocene-aged groundwater and 2) Sonoyta River water supplying water through a suggested fault system connecting the spring to the alluvial aquifer beneath the river.

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The latter seems to be supported by the similarity of Cl/SO4 ratios of waters in Quitobaquito and the Sonoyta River floodplain.

The conclusions here presented have important implications for the management of water resources within the Organ Pipe Cactus National Monument and the lower Sonoyta River. Older groundwaters found in the study area are vestiges of a geologic past when recharge rates where higher and likely occured during a cooler and wetter environment than the modern climatic regime. These resources are finite and the riparian and wetland areas they support are vulnerable to overdraft and unregulated groundwater use. Additionally, future climate scenarios predict declines in recharge of varying magnitudes in the southwest region of the United States (Meixner et al., 2016). Binational collaboration is needed to conserve water resources for the sustainable development of human communities, and the preservation of natural resources in this arid part of the Sonoran Desert.

C.9 Acknowledgements

This material is based upon work supported by the National Park Service Southwest Border

Resource Protection Program and a Graduate and Professional Student Council Grant. The authors express their gratitude to Peter Holm, Charles Connor, Rick Morawe, and Ami Pate at

Organ Pipe Cactus National Monument, Colleen Filippone, and Douglass Towne for their resource support, logistics, and assistance during fieldwork.

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C.10 Figures

Figure C1. Sonoyta River Watershed (delimited by red-dotted line) and major geographical features in the region. Black-dashed line shows limits of the Tohono O’odham Nation (TON). Black bold line shows the limit of the Organ Pipe Cactus National Monument.

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Figure C2. Detail of study area showing location of water sample collection. Pink line shows the watershed boundary, gray-dashed line show water table elevation in meters above sea level. Water levels on Mexican side from Goodman (1992).

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Figure C3. Discharge (liters per minute) at Quitobaquito Spring from 1973 to 2017 (Data from Peter Holm, NPS, Personal Communication).

146

Figure C4. Geology of study area. Map includes data from Carruth (1996), and from the Servicio Geológico Mexicano (Mexican Geological Survey) geologic map H12-A14 in Sonoyta Sonora found at their website (www.sgm.gob.mx/cartas).

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Figure C5. Cross-section of the Quitobaquito Spring area. See Figure 4 for cross-section trace location. Modified from Carruth (1996).

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Figure C6. Piper Diagram showing data for groundwater samples along the Sonoyta River course. “Headwater” samples were collected near the Baboquivari Mountains, “Valleys” samples were collected along the floodplains downstream, and “Papago Farms” samples were collected within the irrigated fields of the Tohono O’odham Nation (see Fig.1 for location). Data from Water Quality Portal (www.waterqualitydata.us).

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Figure C7. A. δ2H and δ18O values for individual precipitation events collected at Organ Pipe Cactus National Monument between 1990 and 2016 (Table A4). B Mean weighted values for winter, summer, yearly and the 30% most intense events from data in 6A. 2000M Winter and 2000M Summer calculated using the mean isotopic altitude effects in Tucson described by Wright (2001).

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Figure C8. δ2H and δ18O values for water samples collected by the NPS from tinajas during March and April 2011 (Colleen Filippone, NPS, Personal Communication).

151

Figure C9. Piper Diagram showing major ion chemistry data for springs and well water samples in the study area.

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Figure C10. A: δ18O and δ2H values for mean rainfall (winter, summer, yearly, and other values shown in Fig 7B), springs, and wells. Red line shows expected evaporation trend in the area with a slope of 3.16 obtained from evaporated tinaja water samples.

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Figure C11. Tritium (TU), and Carbon-14 data (pMC) for water samples in the study area.

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Figure C12.Detailed view of the Quitobaquito Spring area. Dotted lines labeled 1, 2, and 3 show areas with sudden changes in vegetation likely related to the same fault system that created Quitobaquito and extends into the Sonoyta River channel.

155

-2 - Figure C13. SO4 vs Cl concentrations for spring and well samples. Some of the samples plotting below the solid line have the lowest pMC concentrations and are believed to have been affected by sulfate reduction.

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- Figure C14. Cl/SO4 vs Cl concentration for local recharge (sites 1, 12, 13, 27 and 28), Quitobaquito (sites 3 and 4), and Sonoyta River upstream of Quitobaquito (sites 7, 8, 10, 17, 18 and 19).

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C.11 Tables

18 2 13C 13 3 14 Site Type Description Lat. Long. Elev. Date T pH E.C. Ca Mg Na K Cl SO4 HCO3 δ O δ H δ δ C H C Source

1 Spring Dripping Spr. 32.024 -112.892 737 Mar-16 14 6.6 0.28 2 0 69 3 21 29 195 -5.3 -45 -6.5 -8 1.8 103 1 Feb-17 14 8.2 0.24 5 0 76 4 21 26 207 -4.5 -38 2 Mar-11 -5.4 -44 2 Oct-03 22 6.8 0.33 2 0 64 2 25 36 63 2 May-03 7.2 0.30 1 0 61 2 22 32 53 2 Dec-02 13 7.5 0.41 3 0 79 3 22 25 118 2 2 Spring Williams Spr. 31.958 -113.024 347 Dec-76 14 8.3 1.10 33 8 190 4 140 87 307 2 Mar-11 -4.5 -44 2 3 Spring Quitobaquito 31.944 -113.019 336 Feb-17 25 7.6 1.15 38 11 200 5 150 97 305 -8.4 -59 2 Mar-16 28 7.4 1.15 36 12 213 5 170 101 480 -8.3 -64 -8.3 -9.5 1.2 62 1 -8.5 -61 2 4 Pond Quitobaquito 31.944 -113.019 332 Jul-88 30 7.7 1.15 33 10 190 5 150 87 306 3 Jan-85 25 7.7 1.17 38 11 200 5 160 97 311 -8.3 -61 3 Aug-82 28 7.9 1.06 36 10 41 5 150 93 313 -8.4 -63 3 Dec-81 25 8.3 1.14 37 11 210 5 150 92 305 3 Nov-76 26 7.9 1.15 36 10 200 5 150 97 311 2 5 Well Pozo Salado 31.926 -112.940 380 Jul-83 25 8.4 2.52 30 18 500 3 460 230 432 -7.0 -54 -7.3 3 6 Well Aguajita Wash 31.940 -113.010 338 Jan-85 25 7.7 1.19 39 11 210 5 170 98 318 -8.4 -61 3 Jun-83 25 7.7 1.17 40 11 210 9 160 110 337 -8.5 -62 -11.1 3 Dec-76 14 8.2 1.30 40 11 210 6 160 110 315 2 7 Well Lukeville 31.881 -112.816 Feb-17 28 8.0 0.83 14 5 160 3 100 68 195 -8.6 -62 4 8 Well Gringo Pass 31.881 -112.815 Feb-17 26 8.1 0.95 22 8 180 4 140 70 195 -8.7 -62 4 9 Well Headquarters #4 31.950 -112.802 512 Feb-17 32 7.7 0.78 41 10 103 4 83 52 198 -8.3 -60 4 511 Mar-16 31 5.8 0.82 25 11 119 4 104 62 205 -8.2 -62 -9.6 -11.5 <0.7 47 1 Mar-89 33 8.1 0.86 30 13 120 4 110 64 198 2 Jul-88 33 7.8 0.82 30 13 120 5 110 60 198 2 10 Well C. Dos Republicas 31.882 -112.799 432 Mar-16 28 6.0 0.86 11 5 173 4 106 70 284 -8.7 -64 -7.8 -8.4 <0.5 21 1 11 Well CBP Well 32.132 -113.085 342 Mar-16 38 6.8 4.25 129 7 578 18 868 372 118 -7.9 -62 -4.8 -7.8 <0.5 24 1 12 Well Bates Well #2 32.169 -112.950 419 Mar-16 84 13 13 3 5 2 450 -7.3 -53 -9.0 1 Jan-85 16 7.3 0.57 89 12 12 2 6 15 349 -7.5 -52 2 13 Well Bonita Well 32.009 -112.975 438 Mar-16 23 7.0 0.70 84 16 54 3 12 2 452 -5.9 -44 -3.0 -5 2.6 107 1 Jan-83 25 8.0 0.45 60 12 39 1 7 18 322 -7.5 -53 2

158

14 Well Corner Well 31.962 -113.085 308 Mar-16 30 7.3 2.05 81 24 438 16 517 148 678 -8.0 -61 -2.0 -5.5 <0.5 73 1 Jul-83 31 7.5 1.85 48 10 330 7 280 200 340 -8.0 -61 -9.9 3 15 Well Hocker Well 31.953 -113.056 300 Jul-83 24 7.9 2.35 29 10 540 5 350 240 650 -8.1 -64 -7.8 3 16 Well Alamo Canyon 32.066 -112.715 737 Mar-16 21 6.9 0.83 75 17 110 2 104 59 455 -7.8 -56 -12.2 -11.6 0.8 99 1 17 Well Sonoyta Well 1 31.870 -112.802 423 Apr-16 29 7.2 1.47 39 15 288 6 265 191 279 -8.6 -63 -7.6 -9.2 <0.7 49 1 18 Well Sonoyta Well 2 31.877 -112.818 429 Apr-16 28 7.5 1.16 28 11 226 4 190 127 257 -8.7 -64 -7.5 <0.8 1 19 Well Los Papagos 31.887 -112.883 391 Apr-16 24 7.1 1.76 46 12 372 5 265 189 643 -8.0 -60 -8.2 -10.2 <0.7 80 1 20 Well Ejido Kennedy 31.583 -113.312 94 Apr-16 32 8.0 1.59 15 4 363 5 385 146 232 -7.8 -60 -5.5 -8 <1.0 27 1 21 Well Col. Ortiz Garza 31.482 -113.380 56 Apr-16 31 7.9 1.93 23 16 409 5 473 164 438 -7.6 -59 -5.3 -7.7 <0.5 16 1 22 Well Baboquivari 31.824 -111.532 1116 Jun-81 24 7.3 0.58 72 17 30 1 15 66 268 -8.6 -61 2 23 Well Ventana 32.468 -112.241 681 Jan-78 25 8.0 0.58 5 4 114 44 48 165 2 Jan-81 25 8.2 0.58 9 4 110 2 44 40 207 -7.3 -58 2 24 Well Santa Rosa 32.353 -112.059 555 Sep-78 8.4 23 15 109 3 15 61 250 2 Apr-81 8.0 0.81 30 20 120 2 34 100 305 -7.0 -53 2 25 Well Kaka 32.513 -112.317 687 Mar-78 31 7.7 0.53 24 16 61 31 23 222 2 Apr-81 0.53 26 15 60 5 34 22 220 -7.2 -54 2 26 Well Why 32.261 -112.743 544 Feb-16 7.8 0.9 27 5 137 5 124 114 127 -8.0 -59 2 27 Well Pozo Nuevo 31.825 -113.719 77 Oct-16 7.6 0.5 17 4 90 4 12 15 348 -7.6 -51 <0.4 1 28 Well Papago Well 32.099 -113.267 300 Feb-17 27 7.7 0.549 45 7.4 60 2 31 17 268 -7.5 -52 4 Table C1.

Sources: Temperature in °C δ13C relative to V-PDB (‰) 1 This study E.C. measured in mS/cm 14C expressed in percent modern carbon (pMC) 2 WQ Portal (www.waterqualitydata.us) Ions measured in mg/L 3H expressed tritium units (TU) 3 Goodman (1992) δ18O and δ2H relative to V-SMOW (‰) 4 Towne, 2018

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