GEOCHEMISTRY OF GROUND WATER

IN ALLUVIAL BASINS OF ARIZONA

AND ADJACENT PARTS OF NEVADA,

NEW MEXICO, AND CALIFORNIA

NEW MEXICO

.___1

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1406-C
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Geochemistry of Ground Water 

in Alluvial Basins of Arizona 

and Adjacent Parts of Nevada, 

New Mexico, and California

By FREDERICK N. ROBERTSON

REGIONAL AQUIFER-SYSTEM ANALYSISSOUTHWEST ALLUVIAL BASINS, ARIZONA AND ADJACENT STATES

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1406-C

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1991
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government

Library of Congress Cataloging in Publication Data

Robertson, Frederick N.

Geochemistry of ground water in alluvial basins of Arizona and adjacent parts of Nevada, New Mexico, and California.

(Geological Survey professional paper ; 1406-C)

Bibliography: p.

Supt. of Docs, no.: I 19.16:1406-C

1. Water, Underground West (U.S.) 2. Water, Underground Southwest, Nev. 3. Geochemistry Southwest, Nev.

4. Sedimentary basins Southwest, Nev. I. Title. II. Series: U.S. Geological Survey professional paper ; 1406-C. 

GB1022.R63 1991 551.49'09791 88-607930

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started in 

1978 following a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States. The RASA Program 

represents a systematic effort to study a number of the Nation's most 

important aquifer systems, which in aggregate underlie much of the country 

and which represent an important component of the Nation's total water 

supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political 

subdivisions to which investigations have often arbitrarily been limited in the 

past. The broad objective for each study is to assemble geologic, hydrologic, 

and geochemical information, to analyze and develop an understanding of the 

system, and to develop predictive capabilities that will contribute to the 

effective management of the system. The use of computer simulation is an 

important element of the RASA studies, both to develop an understanding of 

the natural, undisturbed hydrologic system and the changes brought about in 

it by human activities, and to provide a means of predicting the regional 

effects of future pumping or other stresses.

The final interpretive results of the RASA Program are presented in a series 

of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number, 

and where the volume of interpretive material warrants, separate topical 

chapters that consider the principal elements of the investigation may be 

published. The series of RASA interpretive reports begins with Professional 

Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available. ^-7

Dallas L. Peck 

Director

CONTENTS

Foreword........................................................................ Ill

Abstract.......................................................................... Cl

Introduction..................................................................... 2

Purpose and Scope...................................................... 2

Approach .................................................................. 2

Methods of Data Collection and Analysis ........................ 2

Previous Water-Quality and Geochemical Investigations.... 3

Acknowledgments....................................................... 3

Physiography and Drainage.......................................... 4

Climate..................................................................... 4

Well-Numbering System.............................................. 6

Geologic Setting............................................................... 6

Pre-Cenozoic Rocks..................................................... 6

Cenozoic Rocks .......................................................... 7

Basin-Fill Deposits...................................................... 7

Lower Basin Fill.................................................. 7

Upper Basin Fill................................................... 9

Stream Alluvium.................................................. 10

Regional Hydrology.......................................................... 10

Occurrence of Ground Water ........................................ 10

Ground-Water Flow System ......................................... 12

Recharge to the Alluvial Aquifers ........................... 12

Discharge from the Alluvial Aquifers....................... 12

Chemical Character of the Ground Water ............................. 12

Sources of Dissolved Species ........................................ 13

Calcium............................................................... 13

Magnesium.......................................................... 13

Sodium............................................................... 14

Potassium............................................................ 14

Chloride.............................................................. 14

Sulfate................................................................ 14

Carbon................................................................ 14

Silica.................................................................. 14

Geochemical Environment............................................ 14

Dissolved Oxygen................................................. 15

Total Organic Carbon............................................ 16

Depositional Environment............................................ 16

Solubility Effects of Oxygenated Environments............... 16

Occurrence and Distribution of Minor and Trace

Elements....................................................... 16

Fluoride.............................................................. 16

Chromium........................................................... 17

Arsenic............................................................... 20

Boron................................................................. 20

Nitrate............................................................... 21

	Page

Chemical Character of the Ground Water Continued 

Occurrence and Distribution of Minor and Trace

Elements Continued

Selenium............................................................. C21

Barium ............................................................... 22

Characterization of Ground Water in Recharge and

Discharge Areas................................................... 22

Variation with Depth .................................................. 23

Chemical Models............................................................... 23

Mineralogy of the Basin Fill......................................... 24

Chemical Analysis of the Basin Fill................................ 25

Aqueous Speciation and Saturation Data ........................ 25

Carbon Isotopic Composition ........................................ 25

Chemical Evolution of the Ground Water.............................. 29

Evolution of the Ground Water in Recharge Areas........... 29

Movement of Water and Reactions in the Unsaturated

Zone............................................................ 35

Evolution of the Ground Water in Selected Basins ........... 36

Vekol Valley........................................................ 36

Lower San Pedro Valley........................................ 41

Ranegras Plain..................................................... 47

McMullen Valley................................................... 53

Willcox Basin....................................................... 60

Basins Along the Colorado River............................. 67

Movement of Water into the Piedmont Areas...... 67

Evolution of the Ground Water......................... 69

Sensitivity Analysis .......................................................... 71

Clay Mineralogy............................................................... 72

Mass Transfer of Oxygen and Silica............................... 75

Mass Transfer of Kaolinite ........................................... 76

Ion Exchange................................................................... 77

Sodium..................................................................... 77

Potassium.................................................................. 78

Magnesium................................................................ 78

Ion Exchange in Ranegras Plain ................................... 79

Comparison of Mass Transfer Among Basins......................... 80

Distribution Patterns of Mass Transfer .......................... 81

Basin Categorization and Information Transfer...................... 81

Basin Categorization................................................... 81

Open Basins......................................................... 81

Closed Basins....................................................... 83

Projection of Ground-Water Chemistry .......................... 83

Open Systems...................................................... 83

Closed Systems.................................................... 84

Summary........................................................................ 85

Selected References.......................................................... 87
VI CONTENTS

ILLUSTRATIONS

Page

FIGURE 1. Map showing area of study and water provinces................................................................................................... C5

2, 3. Sketches showing:

2. Well-numbering system in Arizona......................................................................................................... 6

3. Generalized structure and stratigraphy of a typical alluvial basin in the study area ........................................ 8

4. Block diagram showing generalized position and extent of the coarse-grained and fine-grained facies in the lower

basin fill.................................................................................................................................................. 9

5. Map showing area of study divided into 72 basins................................................................................................. 11

6. Block diagram showing recharge, movement, and discharge of ground water............................................................ 13

7. Graph showing dissolved-oxygen concentrations and distance along flow paths for selected basins................................ 15

8. Map showing occurrence and distribution of hexavalent chromium .......................................................................... 18

9. Graph showing distribution of plagioclase for selected basins.................................................................................. 25

10-13. Graphs showing X-ray diffraction patterns of:

10. Less than 2-micrometer fractions of basin fill from well (D-9-l)13bbd in Vekol Valley.................................... 26

11. Less than 2-micrometer fractions of basin fill from well (C-5-3)29aaa in the Bosque basin, Gila Bend Plain........ 27

12. Less than 2-micrometer fractions of basin fill from well (D-2-8)26caa in East Salt River Valley ....................... 28

13. 200-micrometer fractions of basin fill from the unsaturated zone.................................................................. 34

14, 15. Maps showing:

14. Basins for which data were collected and for which models were developed................................................... 37

15. Generalized geology, wells, and ground-water movement in Vekol Valley..................................................... 39

16. Graphs showing changes in concentration of dissolved chemical constituents along the flow path in Vekol Valley............ 40

17. Trilinear plot showing relative compositions of ground water in Vekol Valley ........................................................... 42

18. Graphs showing changes in concentration of dissolved chemical constituents along flow paths in selected basins in the

Basin and Range lowlands.......................................................................................................................... 44

19. Map showing generalized geology, wells, and ground-water movement in lower San Pedro Valley................................ 46

20. Graphs showing comparison of ground-water compositions along the flow paths in lower San Pedro Valley.................... 47

21. Map showing generalized geology, wells, and ground-water movement in Ranegras Plain........................................... 49

22. Trilinear plot showing relative compositions of ground water in Ranegras Plain ........................................................ 50

23-25. Graphs showing:

23. Changes in concentration of dissolved chemical constituents along the flow paths in Ranegras Plain.................. 52

24. Changes in concentration of dissolved chemical constituents along the flow path to wells Ranegras 2 and

Ranegras 3................................................................................................................................... 54

25. Comparison of ground-water compositions in Vekol Valley and Ranegras Plain.............................................. 56

26. Map showing generalized geology, wells, and ground-water movement in McMullen Valley......................................... 57

27. Trilinear plot showing relative compositions of ground water in McMullen Valley...................................................... 58

28, 29. Graphs showing:

28. Changes in concentration of dissolved chemical constituents along the flow path in lower McMullen Valley ........ 60

29. Changes in concentration of dissolved chemical constituents along the flow path in upper McMullen Valley........ 61

30. Map showing generalized geology, wells, and ground-water movement in the Willcox basin ........................................ 63

31. Graphs showing changes in concentration of dissolved chemical constituents along the flow path in the Willcox basin...... 64

32. Trilinear plot showing relative compositions of ground water in the Willcox basin...................................................... 66

33. Maps showing basins and wells sampled along the Colorado River .......................................................................... 68

34-37. Graphs showing:

34. 8D and 8180 content of Colorado River water and of ground water in basins along the Colorado River.............. 69

35. Measured and model-derived concentrations of dissolved oxygen from wells in selected basins ......................... 75

36. Relation of average silica concentrations in ground water from selected basins to silica minerals and

temperature .................................................................................................................................. 76

37. Relation of potassium to sulfate for selected basins ................................................................................... 79

38. Sketches showing idealized distribution of mass transfer ....................................................................................... 82

TABLES

Page

TABLE 1. Mean, standard deviation, and concentration range of selected trace and minor elements in ground water ......................... C17

2. Analytical data for selected wells in recharge and downgradient areas ......................................................................... 23

3. Phases used for derivation of mass transfer ............................................................................................................. 31

4. Summary of mass transfer for recharge waters......................................................................................................... 31
CONTENTS VII

	Page

TABLE 5. Major oxides calculated from derived mineral phases................................................................................................. C31

6. Chemical analyses of basin fill from Vekol Valley and the Bosque basin and average compositions of selected rock types...... 32

7. Maximum, minimum, and average saturation indices of the recharge waters with respect to various mineral phases............ 32

8. Distribution of calcite and dolomite in the unsaturated zone........................................................................................ 33

9. Chemical analyses of selected streams in Arizona...................................................................................................... 35

10. Analytical data for selected wells in Vekol Valley, lower San Pedro Valley, Ranegras Plain, McMullen Valley, and the

	Willcox basin................................................................................................................................................ 38

11. Summary of mass transfer for Vekol Valley............................................................................................................. 41

12. Saturation indices of the ground water in Vekol Valley with respect to various minerals................................................. 41

13. Summary of mass transfer for lower San Pedro Valley .............................................................................................. 48

14. Saturation indices of the ground water in lower San Pedro Valley with respect to various minerals .................................. 48

15. Summary of mass transfer for Ranegras Plain.......................................................................................................... 53

16. Saturation indices of the ground water in Ranegras Plain with respect to various minerals.............................................. 55

17. Summary of mass transfer for McMullen Valley........................................................................................................ 59

18. Saturation indices of the ground water in McMullen Valley with respect to various minerals............................................ 62

19. Summary of mass transfer for the Willcox basin....................................................................................................... 65

20. Saturation indices of the ground water in the Willcox basin with respect to various minerals ........................................... 65

21. Percentage of Colorado River water in wells............................................................................................................ 70

22. Weighted-average analysis of Colorado River water and analysis of well Q in the Yuma area........................................... 70

23. Clay phases considered in the mass transfer............................................................................................................. 72

24. Sensitivity analysis and summary of mass-transfer calculations ................................................................................... 73

25. Chemical analyses of four clay samples from selected basins ....................................................................................... 74

26. Structural formulas and location of four clay samples................................................................................................. 74

27. Chemical analyses of the bulk-rock fraction of core from selected wells in East Salt River Valley..................................... 74

28. Mass transfer derived using structural formulas of four clay samples ........................................................................... 75

29. Average major ion concentrations in dilute downgradient waters................................................................................. 84

30. Analyses of well-cuttings leachate for chloride, sulfate, nitrate, and fluoride from wells in Vekol Valley and the Bosque

	basin (Gila Bend Plain)................................................................................................................................... 85

METRIC CONVERSION FACTORS

For readers who wish to convert measurements from the inch-pound system of units to the metric 

system of units, the conversion factors are listed below:

Multiply inch-pound units By To obtain metric units

inch (in)

foot (ft)

mile (mi)

square mile (mi2 )

acre-foot (acre-ft)

cubic foot per second (ft3/s)

gallon per minute (gal/min)

degree Fahrenheit (°F)

25.4 

0.3048 

1.609 

2.590 

0.001233 

0.02832 

0.06309 

°C=(temp °F-32)/1.8

millimeter (mm) 

meter (m) 

kilometer (km) 

square kilometer (km2) 

cubic hectometer (hm3) 

cubic meter per second (m3/s) 

liter per second (L/s) 

degree Celsius (°C)

ALTITUDE DATUM

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD 

of 1929) a geodetic datum derived from a general adjustment of the first-order level nets of both the 

United States and Canada, formerly called Sea Level Datum of 1929.

REGIONAL AQUIFER-SYSTEM ANALYSIS-SOUTHWEST ALLUVIAL BASINS, 

ARIZONA AND ADJACENT STATES

GEOCHEMISTRY OF GROUND WATER IN ALLUVIAL BASINS OF

ARIZONA AND ADJACENT PARTS OF NEVADA,

NEW MEXICO, AND CALIFORNIA

By FREDERICK N. ROBERTSON

ABSTRACT

Chemical and isotope analyses of ground water from 28 basins in the 

Basin and Range physiographic province of Arizona and parts of 

adjacent States were used to evaluate ground-water quality, determine 

processes that control ground-water chemistry, provide independent 

insight into the hydrologic flow system, and develop information 

transfer. The area is characterized by north- to northwest-trending 

mountains separated by alluvial basins that form a regional topography 

of alternating mountains and valleys. On the basis of ground-water 

divides or zones of minimal basin interconnection, the area was divided 

into 72 basins, each representing an individual aquifer system. These 

systems are joined in a dendritic pattern and collectively constitute the 

major water resource in the region.

Geochemical models were developed to identify reactions and mass 

transfer responsible for the chemical evolution of the ground water. On 

the basis of mineralogy and chemistry of the two major rock associations of the area, a felsic model and a mafic model were developed to 

illustrate geologic, climatic, and physiographic effects on ground-water 

chemistry. Two distinct hydrochemical processes were identified: (1) 

reactions of meteoric water with minerals and gases in recharge areas 

and (2) reactions of ground water as it moves down the hydraulic 

gradient. Reactions occurring in recharge and downgradient areas can 

be described by a 13-component system. Major reactions are the 

dissolution and precipitation of calcite and dolomite, the weathering of 

feldspars and ferromagnesian minerals, the formation of montmorillonite, iron oxyhydroxides, and probably silica, and, in some basins, ion 

exchange.

The geochemical modeling demonstrated that relatively few phases 

are required to derive the ground-water chemistry; 14 phases 12 

mineral and 2 gas consistently account for the chemical evolution in 

each basin. The final phases were selected through analysis of X-ray 

diffraction and fluorescence data, aqueous speciation and saturation 

data, and mass-balance and isotopic constraints and through chemical 

models developed from mineral combinations among the 27 phases that 

were considered realistic in these geologically and mineralogically 

complex basins. X-ray diffraction of basin-fill sediments confirm the 

presence of the postulated minerals and their weathering sequences.

Manuscript approved for publication April 23, 1987.

High partial pressures of soil C02 and large concentrations of 

dissolved C02 in recharge areas, and the rapid depletion of C02 

downgradient, accompanied by high weathering rates of the silicates 

which also decrease downgradient, indicate that carbonic acid is the 

impetus in the weathering process. Reactions in the soil zone and the 

unsaturated zone are influential and, in some instances, are as important as the mineralogy of the source rock in determining ground-water 

compositions.

The basins can be divided geochemically into two general categories closed systems, which evolve under closed hydrologic conditions, 

and open systems, which are open to C02 and other constituents along 

the flow path. The ground-water chemistry of the unconfined aquifers 

in the eastern part of the study area and of the aquifers underlying the 

flood plain along the Colorado River generally evolves under open 

conditions. The ground-water chemistry of most basins in the central 

and western parts and of the confined aquifers in the eastern part 

evolves under closed conditions. The factors that determine whether a 

basin is an open or closed system are the amount of and the spatial and 

seasonal distribution of annual precipitation and the presence or 

absence of fine-grained confining units.

The basins along the Colorado River are unique among basins in the 

region. Virtually all ground water underlying the flood plain originated 

as seepage or overbank flow from the Colorado River. Initial deuterium 

content of about 120 per mil is indicative of precipitation from the 

central part of Colorado. Using chemical models derived from other 

basins, weathering and dissolution reactions were defined but not with 

the same consistency, suggesting the presence of other mineral phases 

or reactions. The commonly observed increases in sulfate and chloride 

concentrations and the lower sulfate to chloride ratios of the ground 

water relative to the river water, however, probably are not a function 

of evapotranspiration and sulfate reduction, as is proposed in the 

literature. The chemical and isotope data indicate that dissolution of 

halite and gypsum is the major cause of changes in water chemistry in 

these basins along the river.

The oxidizing nature of the ground water greatly increases the 

solubility of several trace elements. Fluoride, hexavalent chromium, 

arsenic, boron, barium, selenium, lead, and nitrogen as nitrate may 

occur in concentrations that exceed State and Federal maximum 

contaminant levels. Fluoride, hexavalent chromium, arsenic, nitrate, 

and boron present environmental problems in several basins. Sources

Cl
C2 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

and controls for these elements were determined or postulated through 

statistical and graphic techniques, geologic information, and equilibrium models.

Through the use of the geochemical models, climatic and geologic 

data, and analytical data of the basin-fill sediments, information 

transfer was developed for prediction of water quality and geochemistry throughout most of the area.

INTRODUCTION

The U.S. Geological Survey, in response to a congressional decision to appraise the Nation's ground-water 

resources, began the Regional Aquifer-System Analysis 

(RASA) Program in 1978 to study and define the 

Nation's most important aquifer systems. The RASA 

program represents a systematic effort to evaluate the 

ground-water resources that underlie most of the Nation 

and are a major component of the Nation's total water 

supply. Previous hydrologic investigations of regional 

aquifers commonly have been limited in scope because of 

political subdivisions and the physical enormity of many 

hydrologic systems. The RASA program was designed 

to circumvent such difficulties by providing sufficient 

personnel and time for a comprehensive analysis of the 

respective hydrologic system.

The Southwest Alluvial Basins (Swab) RASA was 

implemented to evaluate the ground-water resources in 

the alluvial basins of Arizona and parts of those basins 

that extend into California, Nevada, and New Mexico. 

Objectives of the overall study were to define the extent 

of the ground-water system, the amount of water in 

storage, the amount of recharge to the ground water, the 

quality of the ground water, and the effects of groundwater development (Anderson, 1980). The regional aquifer of the Swab/RASA study area differs from most other 

regional aquifers in that the hydrologic system comprises 

72 independent alluvial basin aquifers. These 72 separate 

aquifers do not form a single, areally extensive aquifer 

system; they nonetheless constitute the major regional 

water resource when considered collectively. Ground 

water, the most important source of water in this region, 

occurs mainly in these alluvial basins.

PURPOSE AND SCOPE

The purposes of the geochemistry part of the Swab/ 

RASA investigation were to (1) evaluate overall quality 

of the ground water, (2) determine probable sources of 

dissolved species, (3) determine spatial distribution of 

major solutes, (4) determine occurrences and solubility 

controls of trace and minor elements, (5) quantify geochemical processes that control ground-water chemistry, 

(6) examine a variety of chemical and isotope data 

to provide insight into the hydrologic systems, and

(7) provide information transfer from basins that have 

been more intensively studied to basins that are undeveloped or have complex geochemical histories.

This report summarizes onsite investigations and geochemical sampling programs that were conducted 

throughout the study area and describes the general 

geology and hydrology of the area, with emphasis on 

their impact on the regional geochemistry. The chemistry and mineralogy of the rocks that form the mountains, 

and of the basin fill in several basins, are described and 

related to the chemical evolution of the ground water. 

Geochemical models were developed to define the reactions responsible for evolution of the ground-water chemistry and were used to evaluate the geochemistry in 

basins of different hydrologic and physiographic settings. This report is directed primarily to geochemists 

but also is intended for hydrologists and other scientists 

and engineers concerned with water quality.

APPROACH

The approach of the study was to collect pertinent 

geochemical and water-quality data in selected basins for 

interpretation of the geochemistry and definition of the 

water quality. A large part of the geochemical study was 

the acquisition of new data. Chemical and isotope data 

were collected in 28 basins from the Arizona-New Mexico 

border to the Colorado River. A comprehensive sampling program was initiated because of deficiencies in 

existing field measurements, trace-element data, and 

isotope data and because of diversity in physiography, 

geology, and water chemistry among basins. The basins 

selected were those having unique physiographic, geologic, hydrologic, or geochemical characteristics or those 

representative in these characteristics of numerous 

other basins. Geochemical and statistical computer codes 

were used for data interpretation. Through analysis of 

the newly acquired data, geochemical similarities and 

differences were defined. Definition of the geologic, 

hydrologic, and geochemical differences and similarities 

was essential for the development of information transfer because of the large number of basins within the 

study area. This transfer of information the projection 

of geochemistry or of water quality was subsequently 

used to define a larger number of basins.

METHODS OF DATA COLLECTION AND ANALYSIS

Water samples were collected from more than 500 

wells in 28 basins throughout the study area. All samples 

for major ion, trace-element, and isotope analysis were 

collected and preserved using procedures suggested 

by the U.S. Geological Survey Central Laboratory in
INTRODUCTION C3

Denver, where the samples were subsequently analyzed. 

Temperature, pH, and alkalinity were determined in the 

field; fixed end-point titration was used for alkalinity 

determination. (See Barnes, 1964; Brown and others, 

1970.)

The isotopic compositions presented in this paper are 

reported by 8 notation:

8=-

_ D

^

standard

Rstandard

xlO3 , (1)

where R is either the D/H, 180/ 160, or 13C/ 12C isotope 

ratio and all values are reported in per mil (%o). Positive 

values show the sample to be enriched in the heavy 

isotope species, and negative values show the sample to 

be depleted in the heavy isotope species relative to the 

standard. Deuterium (8D) and oxygen (8180) ratios are 

reported relative to Standard Mean Ocean Water 

(SHOW), and the carbon isotope ratios relative to Chicago Peedee Belemnite (PDB). The 8D, S180, 813C, and 

tritium (reported in tritium units, TU) were analyzed by 

the U.S. Geological Survey Research Group in the 

northeastern region, Reston, Va. Precision of the 8D, 

8180, and 813C analyses was ±1.0%o, ±0.1%o, and 

±0.1%e, respectively. Carbon-14 (14C) isotope samples 

were analyzed by the Radiocarbon Dating Laboratory, 

Laboratory of Isotope Geochemistry, University of 

Arizona, and reported in percent modern carbon. 

Samples for the 14C analysis were precipitated in the 

field or delivered to the laboratory within 24 hours for 

precipitation.

The Center for Meteorite Studies, Arizona State University, analyzed selected well cuttings and well cores 

for bulk mineralogy and chemistry. The Soils, Water, 

and Engineering Department, University of Arizona, 

analyzed the clay fraction of the cuttings and cores for 

clay minerals. The computer program WATEQ2 (Ball 

and others, 1980) was used to calculate equilibrium 

relations of the aqueous and mineral phases. Major ion 

and trace-element data were analyzed by linearregression analysis to determine statistical significance 

between variables (Nie and others, 1975).

PREVIOUS WATER-QUALITY AND 

GEOCHEMICAL INVESTIGATIONS

Water-quality studies dealing primarily with 

dissolved-solids distributions or single constituents have 

been done in several basins in Arizona. Kister and Hardt 

(1966) investigated the salinity of ground water in western Pinal County. Kister prepared dissolved-solids maps 

for the Tucson area (1974b) and the Phoenix area (1974a). 

Kister and others (1966) prepared maps showing the 

fluoride content and salinity of ground water in the

Willcox basin, and Kister (1973) showed the dissolvedsolids content of ground water and selected fluoride 

concentrations for the lower Colorado River region, 

Arizona, Nevada, New Mexico, and Utah. Smoor (1967) 

determined the hydrochemical facies of ground water in 

the Tucson basin. Osterkamp (1974) mapped the chemical quality of ground water for public supply in the 

Phoenix area, and Osterkamp and Laney (1974) mapped 

the chemical quality of water for public supply in the 

Tucson area. Laney evaluated the quality of water along 

the Gila River in Graham County (1970) and the chemical 

quality of water in the Tucson basin (1972). Laney (1977) 

studied the effects of phreatophyte removal on water 

quality in the Gila River area, Graham County. Simpson 

and others (1970) determined the seasonal recharge to 

ground water in the Tucson basin area by deuterium 

analysis. Winograd and Robertson (1982) investigated 

deep oxygenated ground water in the Basin and Range 

province of Nevada and Arizona, and Robertson (1975) 

evaluated the occurrences of hexavalent chromium in 

Paradise Valley. Olmsted and others (1973), Metzger and 

Loeltz (1973), and Metzger and others (1973) investigated the water resources and water quality in a comprehensive study along the Colorado River between 

Hoover Dam and the international boundary. Wallick 

(1973) used radiocarbon to date the ground water in the 

Tucson basin, and Parada (1981) determined the isotopic 

content of soil carbon dioxide in the Tucson basin. 

Schmidt (1979) evaluated ground-water quality in the 

major basins of Maricopa County, and Robertson (1984, 

1986) determined the solubility controls of trace elements 

in alluvial basins of Arizona. Robertson and Garrett 

(1988) mapped the distribution of fluoride in ground 

water in alluvial basins of Arizona and parts of adjacent 

States.

ACKNOWLEDGMENTS

The author gratefully acknowledges the many individuals who provided assistance and cooperation throughout 

the investigation. The author is particularly grateful to 

Donald C. Thorstenson and L. Niel Plummer for their 

excellent counsel in the geochemical modeling and their 

technical support thoughout the study. Special appreciation is extended to Blair F. Jones, whose early interest 

and support assisted the study, and to Isaac J. Winograd 

for valuable technical discussions and constructive suggestions. Review of the manuscript and many helpful 

comments by John D. Hem, Ivan Barnes, and Donald C. 

Thorstenson were greatly appreciated and improved the 

paper. Thanks are extended to Carlton B. Moore, 

Arizona State University, and David Hendricks, University of Arizona, who provided the X-ray and chemical 

analysis of the well-cuttings samples. The author also
C4 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

thanks the hundreds of farmers, ranchers, and other well 

owners, whose cooperation and assistance made possible 

the field investigations and collection of data.

PHYSIOGRAPHY AND DRAINAGE

The study area encompasses about 82,000 mi2 (square 

miles) of the Basin and Range physiographic province 

(Fenneman, 1931) in Arizona and small parts of the 

adjacent States of California, Nevada, and New Mexico. 

The area was subdivided on the basis of hydrologic 

characteristics into the Central highlands water province 

and the Basin and Range lowlands water province (U.S. 

Geological Survey, 1969) (fig. 1).

The study area is characterized by generally north- to 

northwest-trending mountains separated by wide alluvial basins that form a regional topography of alternating 

mountains and valleys. Altitudes range from 150 ft (feet) 

above sea level near Yuma to more than 11,000 ft above 

sea level in the Central highlands. Although a common 

geologic structure prevails throughout the Basin and 

Range province, physical differences exist between the 

Central highlands and the Basin and Range lowlands.

The valleys in the central and southwestern parts of 

the Basin and Range lowlands between Phoenix and 

Yuma are more extensive than in other parts of the study 

area, encompassing about 80 percent of the local terrain. 

The mountain ranges are small, deeply dissected, and 

rimmed by large pediments that form typically large oval 

basins. The valley floors range in altitude from less than 

200 to about 2,000 ft above sea level, and the mountains 

bounding the valleys rise an additional 1,000 to 2,000 ft 

above the valley floor. The valleys in the northwestern 

and southeastern parts of the Basin and Range lowlands 

are characterized by long north-trending valleys and 

mountains that are approximately equal in areal extent. 

The mountain ranges are moderately dissected, and the 

valleys have pronounced slopes that extend from the 

mountains to the basin axis. The altitude of the valley 

floors ranges from about 2,500 ft in Detrital Valley in the 

northwest (location of valleys and other geographic areas 

shown in fig. 5) to about 4,000 ft in the Willcox basin in 

the southeast, with adjacent mountains rising as much as 

6,000 ft above the valley floors.

In contrast to the Basin and Range lowlands, the 

Central highlands are mountainous and contain shallow 

inter-montane basins that occupy only a small part of the 

total area. With the exception of Verde and Big Chino 

Valleys, the basins commonly contain little more than a 

thin veneer of alluvium along a stream channel.

Six major river systems the Colorado, Salt, Gila, 

Santa Cruz, San Pedro, and Verde Rivers drain the 

area from east to west (fig. 1). The Colorado River 

originates in Colorado and flows southwestward across

Utah and Arizona and southward along the Arizona 

border to the Gulf of California. The Gila River and its 

tributary the Salt River originate in the highlands near 

the New Mexico-Arizona border and flow westward to 

join near Phoenix. The Gila River is tributary to the 

Colorado River near Yuma. The Verde River originates 

in the highlands and joins the Salt River near Phoenix. 

The San Pedro River originates in Mexico and flows 

northward to join the Gila River. The Santa Cruz River 

originates near the international boundary and flows 

northward to join the Gila River west of Phoenix.

CLIMATE

The climate of the area is arid to semiarid, and 

precipitation generally is dependent on altitude, which 

increases in a northeasterly direction. Mean annual precipitation ranges from less than 3 in (inches) near Yuma 

to more than 30 inches in the higher altitudes of the 

Central highlands and of the eastern part of the area. 

Precipitation is highly variable and in any given year 

may depart greatly from the mean. Mean annual lake 

evaporation ranges from less than 60 inches in the higher 

altitudes of the Salt, Verde, and Gila Rivers to about 86 

in along the Colorado River near Yuma (Anderson and 

others, written commun., 1986).

Two periods of precipitation are related to atmospheric flow patterns and pressure systems that draw 

quantities of moisture over the area. One system originates to the northwest over the Pacific Ocean providing 

winter precipitation, and the other originates to the 

south over the Gulf of Mexico and the Gulf of California 

providing summer precipitation. Winter precipitation 

generally is the result of regional storms of low intensity 

and long duration. Summer precipitation occurs as highintensity short-duration thunderstorms that are localized 

and frequent. The amounts of precipitation for each 

period differ depending on location. Summer precipitation supplies about two-thirds of the total precipitation in 

the eastern part of Arizona but decreases to less than 

one-third in the western part (University of Arizona, 

undated). The amount of precipitation is highly variable 

within a basin; areas near mountains receive precipitation more frequently and in larger amounts than the 

central parts of the basin.

In contrast to rainfall, mean temperatures in the study 

area are consistent and vary little from year to year. 

Mean temperatures in July range between 80 and 90 °F 

in the northwestern and southeastern parts of the Basin 

and Range lowlands and between 90 and 95 °F in the 

southwestern part. Mean temperatures in January range 

between 40 and 50 °F in the northwestern and southeastern parts and between 50 and 60 °F in the southwestern 

part. Mean temperatures in July range between 70 and
a, SI 8 

I

OO01
C6 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

80 °F in the Central highlands and in January are 

generally below 40 °F (Dunbier, 1970).

WELL-NUMBERING SYSTEM

Well identification and location are indicated by the 

well-numbering system used by the U.S. Geological 

Survey and the U.S. Bureau of Land Management in 

Arizona (fig. 2).

GEOLOGIC SETTING

The present-day physiography characterized by mountains and basins was formed primarily by large-scale 

normal faulting during the Basin and Range disturbance 

14 to 6 million years ago (Scarborough and Peirce, 1978). 

Structural movement resulted in a series of upthrown 

and downthrown blocks throughout a large region in 

Arizona and nearby States. Block faulting altered previous drainage patterns, and the subsiding areas became 

depositional sites for locally derived sediments. Concurrent with this faulting, erosion and subsequent deposition of basin fill sculptured the mountains and formed the 

basins and valleys that now exist. The mountains consist 

mainly of consolidated intrusive and metamorphic rocks 

of Precambrian, Mesozoic, and Tertiary age. Volcanic 

and sedimentary rocks of Tertiary age also are common 

in many ranges. Volcanic and sedimentary rocks of 

Paleozoic and Mesozoic age occur in parts of a few ranges 

but are less abundant. The basins in the study area 

contain alluvial deposits and volcanic rocks of late Cenozoic age (Wilson and others, 1969).

PRE-CENOZOIC ROCKS

The pre-Cenozoic rocks that form the major mountain 

ranges are divided broadly into intrusive, metamorphic, 

and volcanic rocks. Intrusive rocks of Precambrian age 

include granite, quartz monzonite, granodiorite, quartz 

diorite, and, locally, gabbro. Metamorphic rocks of Precambrian age include phyllite, mica schist, chlorite 

schist, amphibolite, and gneiss. These rocks are most 

abundant in the Central highlands and adjacent areas 

where they partially bound several major basins and 

form the largest ranges having the highest altitudes 

(Wilson and others, 1969). Intrusive and volcanic rocks of 

Mesozoic age are scattered throughout the central, western, and southwestern parts of the study area. Paleozoic 

rocks of predominantly marine limestone and mudstone 

crop out along the Colorado Plateau in the Central 

highlands and locally in the southeastern part of the 

study area. Isolated outcrops of Paleozoic rocks occur in 

west-central Arizona (Shafiqullah and others, 1980). Sed-

.= 0

RIVER MERIDIAN

GILA AND SALT RIVER BASE LINE

R.1 E

R.5 E.

R.6E.

WELL {D-4-5)19caa

J.,

R.5E.

T. 

4 

S.

6

7

18

 19

30

31

5

8

17 

r- ^

20

29

32

4

9

s

,16

21

28

33

3

^

15

22

27

34

2

^ -11

14

23

26

35

I/

12

13

24

25

36

_I j_

 bSec. 19

19-

_a .-b 

FIGURE 2. Well-numbering system in Arizona. The well numbers used by the U.S. Geological Survey in Arizona are in 

accordance with the Bureau of Land Management's system of 

land subdivision. The land survey in Arizona is based on the 

Gila and Salt River meridian and base line, which divide the 

State into four quadrants. These quadrants are designated 

counterclockwise by the capital letters A, B, C, and D. All land 

north and east of the point of origin is in A quadrant, that north 

and west in B quadrant, that south and west in C quadrant, and 

that south and east in D quadrant. The first digit of a well 

number indicates the township, the second the range, and the 

third the section in which the well is situated. The lower case 

letters a, b, c, and d after the section number indicate the well 

location within the section. The first letter denotes a particular 

160-acre tract, the second the 40-acre tract, and the third the 

10-acre tract. These letters also are assigned in a counterclockwise direction, beginning in the northeast quarter. If the 

location is known within the 10-acre tract, three lowercase 

letters are shown in the well number. In the example shown, 

well (D^4-5)19caa designates the well as being in the 

NEV4NEV4SWV4 sec. 19, T. 4 S., R. 5 E. Where more than one 

well is within a 10-acre tract, consecutive numbers beginning 

with 1 are added as suffixes.

imentary rocks of Mesozoic age of continental origin are 

found in places in southeastern Arizona. The rocks were 

structurally deformed, intruded by magmas, and overlain by younger sedimentary rocks during the late Mesozoic and early Cenozoic (Paleocene) as a result of the
GEOLOGIC SETTING C7

Sevier and Laramide orogenies. Rocks formed during 

the Laramide orogeny are areally extensive and include 

rich porphyry copper deposits in parts of south-central 

Arizona (Wilson and others, 1969; Shafiqullah and others, 1980).

CENOZOIC ROCKS

Three major geologic events followed the Laramide 

orogeny during the Cenozoic Era: (1) the Mid-Tertiary 

orogeny, which was accompanied by calc-alkaline volcanism and plutonism and the accumulation of sediments; (2) 

the Basin and Range disturbance, which was accompanied by andesitic-basaltic volcanism, block faulting, basin 

formation, and sedimentation; and (3) maximum basin 

filling, which was followed by uplift, valley erosion, and 

development of terraces along major rivers (Scarborough and Peirce, 1978).

The Mid-Tertiary orogeny was characterized by 

crustal melting, plutonism, and extrusion of intermediate to silicic lavas. The volcanic rocks formed during the 

orogeny are areally extensive and are predominantly 

andesitic, rhyolitic, and dacitic in composition. Rhyolitic 

volcanic rocks form most of the Chiricahua Mountains 

and parts of the Galiuro and Winchester Mountains in the 

southeast, the Superstition Mountains near Phoenix, and 

the Tucson and Roskruge Mountains near Tucson. Intermediate volcanic rocks, transitional in chemical composition between mafic and felsic rocks, form large parts of 

the Galiuro and Winchester Mountains and are predominant in the Sand Tank Mountains near Vekol Valley and 

adjacent areas of the Papago Indian Reservation in 

south-central Arizona. (See Scarborough and Peirce, 

1978.)

Following the Mid-Tertiary orogeny, thick sequences 

of pre-Basin and Range sedimentary rocks of Tertiary 

age were deposited in places. These rocks of continental 

origin range from boulder conglomerate to mudstone and 

generally are well indurated. Their areal distribution and 

clast type indicate depositional environments unrelated 

to the present-day geology of the surrounding mountains. The rocks are highly deformed, faulted, and tilted 

and generally are found at the basin margins in fault 

contact with older and younger rocks. These pre-Basin 

and Range sedimentary rocks are present in many basins 

beneath the basin fill (Laney and Hahn, 1987).

Most of the rocks in the alluvial basins in the study 

area were deposited after the Basin and Range disturbance and are Quaternary to late Tertiary in age. The 

basins were created in response to an east-northeast to 

west-southwest regional extension field that resulted in a 

network of north- to northwest-trending horsts and 

grabens bounded by high-angle normal faults. The subsidence of these grabens and subsequent erosion resulted

in late Cenozoic basin-fill deposits that exceed thicknesses of 8,300 ft (Scarborough and Peirce, 1978).

BASIN-FILL DEPOSITS

Basin-fill deposits include clastic sediments, evaporites, interbedded volcanic rocks, stream alluvium, and 

flood-plain deposits. The deposits occur as distinct beds 

of sand and gravel and as poorly sorted interbedded 

clayey silt, sand, and gravel locally interbedded with 

volcanic rocks. Deposits near the basin margins commonly are fanglomerate or fluvial sand and gravel that 

grade laterally into silt, clay, and evaporites near the 

center of the basin. The amount of fine-grained silt and 

clay material (less than 0.0625 mm (millimeter) in diameter) in the deposits near the basin margins ranges from 

about 10 to 50 percent but may increase toward the 

center of the basin to more than 90 percent. Fine-grained 

deposits and evaporites that lie in basin centers accumulated in internal or restricted drainage environments.

Deposition of the basin fill was controlled primarily by 

tectonic processes. Erosion of upthrown areas and concurrent deposition of sediments resulted in the basinward increase in fine-grained sediments and occurrences 

of the thick, fine-grained deposits in the deepest structural parts of the basins. The transition from coarsegrained to fine-grained deposits generally occurred near 

fault boundaries but varied from place to place because of 

such factors as size of the drainage area, amount of 

precipitation on the drainage area, relief of the depositional area, structural movement, and shifting of the 

stream channels during deposition. Gradual development 

of integrated or through-flowing drainage also influenced 

grain-size distribution. The generalized structure and 

stratigraphy of a typical alluvial basin in the study area 

are shown in figure 3.

On the basis of source area, particle size, age, composition, and degree of cementation, the basin-fill deposits 

are informally divided into three general units: (1) lower 

basin fill, (2) upper basin fill, and (3) stream alluvium 

(Pool, 1986). These general units represent a depositional sequence that has been observed in many basins.

LOWER BASIN FILL

The lower basin fill was deposited under internal 

drainage conditions and consists of the finest grained and 

most indurated basin-fill sediments. The unit was deposited during active tectonism of the Basin and Range 

disturbance and consequently is the most highly faulted 

and deformed. The lower basin fill is subdivided into 

older and younger parts on the basis of content 

and texture.
C8 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

NOT TO SCALE 

EXPLANATION

STREAM ALLUVIUM

CLAY, SILT, AND EVAPORITES

FINE SAND

SAND

COARSE SAND AND GRAVEL

GRAVEL

PRE-BASIN AND RANGE SEDIMENTARY ROCKS OF TERTIARY AGE

INTRUSIVE, METAMORPHIC, AND VOLCANIC CONSOLIDATED ROCKS, 

LOCALLY SEDIMENTARY ROCKS

FIGURE 3. Generalized structure and stratigraphy of a typical alluvial basin in the study area.

The older part of the lower basin fill consists of 

siltstone, evaporites, and intercalated basalt sequences 

and is moderately to well cemented with calcium carbonate. Massive evaporite sequences that are thousands of 

feet thick occur in places in the older unit. Thick evaporites in the older part have been penetrated by drilling in several deep basins adjacent to the Central 

highlands 4,000 ft of halite near Red Lake in Hualapai 

Valley, 3,600 ft of halite in West Salt River Valley, 5,985 

ft of anhydrite in the Eloy area, and 600 ft of gypsum in 

the Safford basin. In addition to these massive deposits, 

1,500 ft of interbedded gypsum and mudstone is present 

in the Douglas basin, 1,300 ft of interbedded gypsum,

silt, and clay in lower San Pedro Valley, and several 

hundred feet of interbedded silt, clay, and gypsum in 

Paradise Valley (Eberly and Stanley, 1978; Scarborough 

and Peirce, 1978). Fine-grained gypsiferous deposits of 

the lower basin fill also occur at depths of less than 1,000 

ft in Ranegras Plain and McMullen Valley.

The younger part of the lower basin fill consists of clay 

and silt with lenses of sand and gravel; it is less consolidated and contains fewer volcanic rocks than the older 

part. The coarse sand and gravel facies are found in 

several basins at depths as shallow as 500 to 700 ft below 

land surface. Coarse-grained facies underlie Vekol Valley, Ranegras Plain, Harquahala Plain, McMullen Val-
GEOLOGIC SETTING C9

'^^ss»^^v5bjo^»^^Q^i%! &'*^^°^&Q£S3^»re ''

EXPLANATION

CLAY AND SILT 

SAND

GRAVEL

COARSE SAND AND GRAVEL

PRE-BASIN AND RANGE SEDIMENTARY ROCKS OF TERTIARY AGE

INTRUSIVE, METAMORPHIC, AND VOLCANIC CONSOLIDATED ROCKS, 

LOCALLY SEDIMENTARY ROCKS

FIGURE 4. Generalized position and extent of the coarse-grained and fine-grained fades in the lower basin fill.

rr ~-

' #: 

D -." 

*."»*

£:V

\ f X

VN«

-=-:~F

:' ' ' .

o .»"_

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f » >

f '

^N»*C

ley, Butler Valley, and Bullard Wash and commonly are 

overlain by fine-grained sediments of the lower basin fill. 

Coarse-grained fades of the lower basin fill also are 

found in the eastern part of the study area in the lower 

San Pedro and San Simon Valleys and Safford basin 

(Pool, 1986). The position and extent of these fades in the 

lower basin fill are generalized in figure 4.

UPPER BASIN FILL

The upper basin fill was deposited near the end of the 

Basin and Range disturbance and thus is considerably 

less deformed, tilted, and faulted than the lower basin

fill. The unit generally is coarser grained and less 

cemented than the lower basin fill and is characterized by 

grain-size distributions that are less coincident with fault 

boundaries. Fine-grained facies where they occur are 

less areally extensive than those in the lower basin fill. 

Moderately coarse grained facies of the upper basin fill 

are as much as 1,000 ft thick in several basins in the 

central and eastern parts of the area (Brown and Schumann, 1969; Davidson, 1973a; Laney and Hahn, 1987). 

The upper basin fill also occurs in the western part of the 

study area but generally is less than 100 ft thick. 

Fine-grained facies of the upper basin fill also occur in
CIO REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

southeastern basins as lacustrine deposits of clay, silt, 

sand, and some limestone (Pool, 1986).

The sediments of the upper basin fill were laid down, 

for the most part, in closed-basin depositional environments, as were those of the lower basin fill. In some 

basins, however, grain-size distributions represent complex depositional environments that may include 

through-flowing drainage from one basin to another. 

Depositional features less coincident with the basin configuration and fault boundaries, the absence of evaporites, and the abundance of rounded clasts unrelated to 

adjacent mountains indicate that sediments of the upper 

part of the upper basin fill may have accumulated at 

a time when through-flowing drainage conditions were 

prevalent (Laney and Hahn, 1987).

STREAM ALLUVIUM

Stream alluvium occurs as narrow bands of alluvial 

deposits generally less than 100 ft thick along most 

streams that drain the study area. Thick sections of 

stream alluvium occur only along the Salt, Gila, Santa 

Cruz, Verde, San Pedro, and Colorado Rivers. The 

deposits are as much as 300 ft thick along the Salt and 

Gila Rivers in the Phoenix area (Laney and others, 1978; 

Laney and Hahn, 1987) and as much as 700 ft thick along 

the Colorado River. They range in width from several 

hundred feet to a maximum of about 10 mi (miles) along 

the Colorado River. The thick and areally extensive 

flood-plain deposits along the Colorado River have been 

divided into older and younger alluviums (Metzger and 

Loeltz, 1973; Metzger and others, 1973; Olmsted and 

others, 1973). These units overlie the Bouse Formation, 

which is a marine deposit of Miocene and Pliocene age 

formed in an embayment of the Gulf of California. The 

Bouse Formation was incised and overlain by stream 

alluvium when the Colorado River established its 

present configuration during the Pliocene (Shafiqullah 

and others, 1980).

REGIONAL HYDROLOGY

The regional ground-water flow system closely parallels the direction of the surface-water drainage. Ground 

water in areas north and south of the Gila River generally flows toward the Gila and along the Gila River 

drainage. Ground water in basins that adjoin the Colorado River flows toward the river, and ground water in 

a few individual basins along the international boundary 

flows southward into Mexico. The intrusive and metamorphic rocks and most of the volcanic rocks that separate the basins generally are impermeable and transmit 

little, if any, water. Where basins are hydraulically 

interconnected, ground water moves from the basin of

higher altitude to the basin of lower altitude. Basins are 

hydrologically interconnected where the alluvium overlying the consolidated or other impermeable rocks along 

ground-water divides is saturated. Most of the basins in 

the study area are connected hydraulically; only the 

Willcox basin is internally drained. On the basis of 

ground-water divides or zones of minimal hydrologic 

basin interconnection, the study area was divided into 72 

basins or individual alluvial aquifers that are joined in a 

dendritic pattern and constitute the regional aquifer 

system (fig. 5).

OCCURRENCE OF GROUND WATER

Ground water occurs in the study area in rocks of all 

principal types: intrusive, metamorphic, and volcanic 

rocks; pre-Basin and Range sedimentary rocks of Tertiary age; and basin-fill sediments. The water-bearing 

properties of these rocks, however, vary widely. Well 

yields range from a few gallons per minute (gal/min) to 

more than 6,000 gal/min.

The water-bearing characteristics of the intrusive, 

metamorphic, and volcanic rocks are dependent on localized joint fractures and weathering characteristics. In 

general, well yields of intrusive and metamorphic rocks 

are limited to a few gallons per minute. Volcanic rocks, 

which can be more porous and fractured, may yield more 

water and serve as local sources, but, like intrusive 

rocks, their water-yielding properties generally are limited. Volcanic rocks are the source for the municipal 

water supply for the city of Kingman, Ariz. (Gillespie and 

Bentley, 1971), and ground-water flow through fractured 

volcanic rocks has been reported on the Papago Indian 

Reservation (Heindl, 1967). Although the intrusive, metamorphic, and volcanic rocks generally contain little 

water, they may indirectly influence the hydraulic conductivity or recharge of the alluvial aquifers. Intrusive 

granitic rocks are more likely to weather to permeable 

quartz-rich sandy soils, whereas volcanic rocks are more 

likely to weather to tight clayey deposits. The weathering process therefore may form deposits of differing 

hydraulic conductivity or deposits that may enhance or 

preclude infiltration.

The pre-Basin and Range sedimentary rocks of Tertiary age that underlie some of the basins locally may yield 

copious water to wells. Because the rocks often lie 

at great depth and are well indurated, structurally 

deformed, and discontinuous throughout the area, 

however, they generally are not an important waterbearing unit.

The permeable, coarse-grained basin-fill deposits contain vast amounts of recoverable ground water and are 

the main water-bearing units. The high porosity and 

permeability of sand and gravel allow water to move
113

REGIONAL HYDROLOGY

111° 109°

Cll

' DurangoQ

COLORADO

GULF OF 

( CALIFORNIA

EXPLANATION

ALLUVIAL DEPOSITSLocally may include 

evaporite deposits 

and volcanic rocks

CONSOLIDATED 

ROCKS

BASIN BOUNDARY

33

31

50

I

100 MILES

1 Detrital Valley

2 Lake Mohave area

3 Mohave Valley

4 Chemehuevi Valley

5 Parker Valley

6 Palo Verde and Cibola Valleys

7 Yuma area

8 Lechuguilla Desert

9 Castle Dome Plain

10 La Posa Plain

11 Bill Williams River area

12 Sacramento Valley

13 Hualapai Valley

14 Truxton basin

15 Big Sandy basin

16 Santa Maria River area

17 Bullard Wash area

18 Butler Valley

19 McMullen Valley

50 100 KILOMETERS

20 Ranegras Plain

21 Harquahala Plain

22 Palomas and Sentinel Plains

23 King Valley and

San Cristobal Wash area

24 Mohawk Valley

25 Tule Desert, La Abra Plain, and 

Sonoyta Valley area

26 Growler Wash area

27 Cuerdade Lena Wash area

28 San Simon Wash area

29 Vekol Valley

30 Gila Bend Plain

31 Rainbow Valley

32 Lower Hassayampa basin

33 Upper Hassayampa basin

34 Skull Valley

35 Little Chino Valley

36 Big Chino Valley

37 Verde Valley

38 Agua Fria River area

39 West Salt River Valley

40 Paradise Valley

41 East Salt River Valley

42 Maricopa-Stanfield area

43 Santa Rosa Valley

44 Tecolote Valley

45 Baboquivari Wash area

46 Altar Valley

47 Viopuli Wash area

48 Avra Valley

49 Eloy area

50 Donnelly Wash area

51 Lower Verde River area

52 Tonto Creek area

53 Upper Salt River area

54 San Carlos basin

55 Dripping Springs basin

56 Lower San Pedro Valley

57 Upper Santa Cruz area

58 Sonoita basin

59 Upper San Pedro Valley

60 Benson area

61 Douglas basin

62 San Bernardino Valley

63 Willcox basin

64 San Simon Valley

65 Aravaipa Valley

66 Safford basin

67 Bonita basin

68 Black River basin

69 White River basin

70 Upper Gila River basin

71 San Francisco River basin

72 Duncan basin

FIGURE 5. Area of study divided into 72 basins.
C12 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

freely from areas of higher to areas of lower hydraulic 

head. In contrast, the fine-grained sediments contain 

water, but the small grain size and consequently the 

molecular attraction restrict the flow of water. Water 

stored in fine-grained units of low permeability may be 

recovered if the units lie adjacent to permeable sand and 

gravel deposits.

GROUND-WATER FLOW SYSTEM

Depth to the water table varies widely throughout the 

study area but in all basins is greater near the mountain 

fronts. Depth to water near the mountains is typically 

300 to 800 ft or more and decreases basinward to about 5 

to 400 ft. The hydraulic gradient of the water table 

resembles that of the land surface but is somewhat 

gentler. Ground water moves perpendicular to the mountain fronts toward the basin axis, where it either discharges or blends with existing underflow. Groundwater velocities are on the order of a few feet to a few 

tens of feet per year. Velocities generally are greater 

near the mountain fronts, where gradients are steeper 

and permeabilities are higher than those in the interior of 

the basins.

Ground water generally occurs under unconfined conditions, but confined conditions may exist where extensive fine-grained facies of the basin-fill deposits are 

present and sufficient recharge occurs. Confined conditions and flowing wells, some of which discharge more 

than 600 gal/min, are found extensively in the eastern 

part of the study area in the Safford basin, the Benson 

area, lower San Pedro Valley, San Bernardino Valley, 

and San Simon Valley. Confined conditions also exist in 

Verde Valley (Owen-Joyce and Bell, 1983).

RECHARGE TO THE ALLUVIAL AQUIFERS

Ground water is recharged to the alluvial aquifers as 

(1) infiltration of precipitation and runoff along mountain 

fronts (mountain-front recharge), (2) infiltration along 

streams that generally traverse the central axis of basins 

(basin-center recharge), and (3) underflow from adjacent 

basins of higher altitude. The source of most of the 

ground water is precipitation that falls in the study area. 

Only basins along the Colorado River are recharged by 

water that does not originate within the study area.

Of the three modes of recharge, mountain-front 

recharge accounts for most of the ground water in 

storage. Mountain-front recharge occurs within an indeterminate zone that begins at the hardrock-alluvium 

contact and extends into the basin. A large part of the 

recharge probably occurs near the contacts in the numerous arroyos and coarse-grained alluvial fans that drain 

the mountains.

Infiltration along streams recharges the aquifers in 

several basins. The basins along the Colorado River 

receive virtually all their water from infiltration of river 

water directly into the alluvium. Along the Salt and Gila 

Rivers near Phoenix, surface water infiltrates the 

coarse-grained stream alluvium adjoining the rivers. 

Basin-center recharge probably occurs to some extent in 

other basins that contain major streams. The recharge 

provided by the smaller streams and washes probably is 

insignificant.

Underflow of ground water from adjacent basins of 

higher altitude contributes to the recharge of several 

basins in the south-central part of the study area. 

Underflow from Avra Valley (Moosburner, 1972) and the 

upper Santa Cruz area (Anderson, 1972) contributes to 

the recharge of the Eloy and Maricopa-Stanfield areas, 

and underflow from the southern part of the Willcox 

basin contributes to the recharge of the adjoining Douglas basin (Brown and Schumann, 1969). Ground-water 

underflow may be a small recharge component in some 

basins in the west-central part.

DISCHARGE FROM THE ALLUVIAL AQUIFERS

Ground-water discharge from basins occurs through 

(1) discharge to streams, (2) evapotranspiration from 

areas where the water table is shallow, (3) ground-water 

underflow out of the basin, and (4) ground-water pumping. Discharge to streams and discharge by evapotranspiration occur in basins that have gaining streams or 

shallow depths to water. Ground water that discharges 

by underflow moves downvalley and exits through a 

narrow alluvium-filled gap between the mountains. 

Because the shallowest water levels within a basin are 

found downvalley near this gap, evapotranspiration as 

well as underflow may occur in these areas. These 

natural processes of discharge have been altered in many 

basins by ground-water pumping, which in some basins is 

the dominant discharge mechanism. Pumping has lowered water levels as much as several hundred feet in 

some basins near Phoenix (Laney and others, 1978). The 

ground-water hydrology of a basin aquifer characterized 

by mountain-front recharge, ground-water movement, 

and natural ground-water discharge is illustrated in 

figure 6.

CHEMICAL CHARACTER OF THE 

GROUND WATER

The ground water in the study area, as indicated by 

dissolved-solids and trace-element content, generally is 

of suitable chemical quality for most purposes but is not 

without some problems. Dissolved-solids concentrations 

ranged from an average of less than 500 mg/L
CHEMICAL CHARACTER OF THE GROUND WATER C13

Consolidated rocks

FIGURE 6. Recharge, movement, and discharge of ground water.

(milligrams per liter) in 11 of the 28 basins sampled 

during this study to more than 3,000 mg/L in basins along 

the Gila and Colorado Rivers (see section entitled 

"Chemical Evolution of the Ground Water"). Ground 

water in most of the basins sampled contained concentrations of less than 1,000 mg/L. Ground water in most of 

the 72 basins generally contains average concentrations 

of less than 1,000 mg/L (Kister, 1973). The ground water 

in most basins can be classified (Durfor and Becker, 1964) 

as hard (>120 mg/L) to very hard (>180 mg/L). The 

average hardness of the ground water sampled was 314 

mg/L as calcium carbonate and ranged from a low of 35 

mg/L in lower San Pedro Valley to a high of 629 mg/L in 

West Salt River Valley. In several basins, the ground 

water contains large concentrations of the trace elements fluorine, chromium, arsenic, lead, boron, or selenium that exceed standards established for public supply or certain agricultural uses.

SOURCES OF DISSOLVED SPECIES

CALCIUM

Calcium, as well as the other alkaline earths, ionizes 

readily and forms a considerable number of moderately

soluble compounds such as carbonates, fluoride, hydroxides, and phosphates. Its ionic radius of 0.99 A (angstrom) allows calcium to enter into eightfold coordination 

with tectosilicates, producing stable structures with both 

ferromagnesian and nonferromagnesian minerals. 

Weathering of plagioclase and dissolution of calcite and 

probably dolomite are widespread sources of calcium in 

all basins. The ferromagnesian minerals of the pyroxene 

and amphibole groups also are minor sources. Gypsum 

and anhydrite, although not as widespread, are the most 

prolific sources where they occur in playas or evaporites.

MAGNESIUM

Magnesium, also an alkaline earth, behaves geochemically differently from calcium. Because of its smaller 

ionic radius, 0.66 A, magnesium occurs mainly in sixfold 

coordination and is not found in feldspars but is incorporated in ferromagnesian minerals during early crystallization of magmas. Thus, in primary rock genesis, magnesium resembles iron or manganese more than calcium 

(Rankama and Sahama, 1950). Silicate sources of magnesium are the ferromagnesian minerals augite, hornblende, and biotite. Dolomite, if present, may be a major 

source of magnesium.
C14 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

SODIUM

Sodium, together with lithium, potassium, rubidium, 

and cesium, is part of the alkali metal group. Sodium 

compounds are highly water soluble because the large 

ionic size and monovalent charge decrease the attractive 

forces within the crystal below those of the water 

dipoles. The ionic radius of sodium, 0.98 A, is close to 

that of calcium, and therefore sodium is also found with 

calcium in eightfold coordination in silicate minerals. 

Because of its smaller charge, however, sodium does not 

tend to be incorporated in the higher temperature ferromagnesian minerals. The most widespread source of 

sodium is the weathering of feldspars. Halite (NaCl), 

where present in the basin fill, can cause large concentrations of solute sodium.

POTASSIUM

Potassium, because of its larger ionic radius, 1.33 A, 

and its correspondingly high coordination number of 8 to 

12, forms minerals that are the last to crystallize from 

cooling magmas. Potassium feldspar is formed near the 

final stages of crystallization and is the major source of 

potassium. Micas and feldspathoids also contain potassium and may be additional sources. Although potassium 

is nearly as abundant as sodium in the rocks, unlike 

sodium, it is present in ground water only in small 

concentrations.

CHLORIDE

Chlorine is a member of the halogen family, a highly 

electronegative group that forms monovalent ionic compounds or salts. Chloride, the ionized form, is enriched in 

the last fraction of cooling magmatic liquids and vapors; 

consequently, alkali rocks, pegmatites, volcanic emanations, and hydrothermal solutions are characterized by 

large concentrations of chloride (Rankama and Sahama, 

1950). Ore deposits or hydrothermal deposits are sources 

of chloride in most basins. Chloride in precipitation and 

chloride in airborne particles probably are insignificant 

sources. Owing to the high solubility of most chloride 

compounds, particularly halite, the compounds are concentrated in evaporites or playa deposits. Deposits of 

halite, whether they predate the basins or were formed 

during basin deposition, if present, are the source of all 

large chloride concentrations.

SULFATE

Sulfur has properties of a nonmetal and forms binary 

covalent compounds with metals and oxides with oxygen. 

Sulfur occurs in the elemental state and in oxidation 

states of -2 to +6 and can be divided into two general 

classes the reduced form as sulfides and the oxidized

form as sulfates. In high-temperature volcanic and 

hydrothermal environments, sulfur is present as sulfides, but when exposed to the atmosphere or oxygenated water during weathering, the sulfides are oxidized 

to sulfates. The sulfates are moderately soluble and can 

be concentrated in evaporites and playa deposits as 

gypsum (CaS04 -2H20) or anhydrite (CaS04). These 

deposits are the sources of the large sulfate concentrations found in some basins. Ore deposits or hydrothermal 

deposits probably are sources of sulfate in most basins.

CARBON

Carbon is a nonmetal which, through sharing of the 

four electrons in its outer shell, forms strong covalent 

bonds and exists in oxidation states of 4, 0, and +4. It 

occurs in the reduced form as methane and organic 

carbon and in the oxidized form as carbon dioxide. 

Carbon dioxide is the stable form in the study area, 

where it occurs primarily as the bicarbonate ion, which is 

the dominant ion in dilute waters. The principal source of 

carbon is soil-gas carbon dioxide produced by respiration 

of plants. Paleozoic carbonate rocks, which occur locally, 

may be additional sources. The parameters of total 

inorganic carbon (CT), which is the summation of all 

dissolved carbon species in solution, and of the partial 

pressure of carbon dioxide (PCo2) are calculated using 

field-measured pH, temperature, and alkalinity.

SILICA

Silicon is a metalloid showing properties of both metals 

and nonmetals. The metalloid nature is observed in the 

silicon-oxygen bond, as it is neither ionic nor covalent but 

intermediate in nature. Silicon unites with oxygen and 

forms stable silica tetrahedra consisting of four oxygen 

atoms about a silicon atom. These tetrahedra may exist 

as separate units or may join in single or double groups, 

chains, or sheets to form an enormously large family of 

generally insoluble silicate minerals. Plagioclase followed 

by potassium feldspar and the ferromagnesian silicate 

minerals are the major sources of dissolved silica in the 

ground water. Quartz, because of its low solubility, and 

volcanic glass, because of a general paucity in the basin 

fill, are not major sources.

GEOCHEMICAL ENVIRONMENT

The functional relation between the geologic environment and the stability of minerals has profound influence 

on the coexisting chemical composition of the water. To 

provide critical information on weathering sequences and 

stability of minerals, the ability of the hydrochemical 

system to oxidize or reduce an element commonly 

called the redox potential, or Eh must be estimated,
CHEMICAL CHARACTER OF THE GROUND WATER C15

o

7

| 6

_i

CC 

LU

Q_

(A 

^ 5 

<

QC 

<D

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2 4

2 

LU

CD 

X 0

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LU

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O

C/5

C/5 2

Q

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I I I I I I I I I

340

308

 308 

 A

 ?15

~640 ~

640 ftC

16 1729

345

345

 ?50 ^ 1975 JH 754 L

354 436 230_ 354 A 900 

354 "g^f m^ 33 502 Flowing

30 _G ~37~*E .H H J 31 nN 590 

1005 * 700 900 u ~^

r 160_ 128*U74 1^

^ 128 390 ^ AM1485 

AK 33 558 38 Flowing

1-^°5 39 1280

ill 1T

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29

 

I I I I I I I I I

A

A

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700

128

128

33

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B

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E

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G

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J

K

L

M

N

EXPLANATION

WELL LOCATION

Vekol Valley

Ranegras Plain

Butler Valley

San Pedro Valley

San Simon Wash 

WELL DATA-Numbers indicate, in feet,

from top to bottom, well depth,

depth to water, depth to top of

aquifer, and water temperature,

in degrees Celsius

10

WELL NUMBER 

(D-7-2)18aba

B (D-8-1)31cbc 

(D-7-1)10cbc2 

(D-8-1)14baa 

(D-9-1)13bbd 

(D-8-1)35abd 

(B-4-15)18bbb 

(B-6-16)33aaa 

(B-6-16)26aad 

(B-3-15)2dab

K (B-8-14)29cdd 

(D-9-17)24dcc 

(D-8-17)32daa

N (C-16-1)10cca

DISTANCE FROM MOUNTAIN-FRONT RECHARGE AREA, IN MILES

FIGURE 7. Dissolved-oxygen concentrations and distance along flow paths for selected basins.

computed, or measured. The redox potential, expressed 

in volts or pe, represents the chemical behavior of a 

redox couple with respect to known oxidizing or reducing 

agents. The geochemical environment of an aquifer in 

relation to the weathering of rocks and the formation of 

secondary minerals, and to the subsequent solute chemistry, is a function of the redox potential and the hydrogen ion concentration or pH of the system.

DISSOLVED OXYGEN

Oxidizing environments were confirmed throughout 

the study area. Field determinations of dissolved oxygen 

at the wellhead revealed that significant concentrations 

(>3 mg/L) of dissolved oxygen are present in ground 

water in all the alluvial basins, regardless of geologic

setting or location of wells within a basin. Dissolved 

oxygen was detected in water from wells nearly 2,000 ft 

deep and in waters that are older than 10,000 years as 

indicated by hydraulic and 14C data. Dissolved-oxygen 

concentrations for several basins plotted as a function of 

distance along a flow path are shown in figure 7. The 

distance along the flow path generally was defined as the 

distance from the nearest major mountain front, which 

presumably receives the greatest precipitation, to the 

well. Well C may be an exception, as it apparently 

receives some recharge along lower Vekol Wash. Most of 

the basins sampled are agriculturally undeveloped and 

generally are overlain by a confining layer.

The deepest well sampled, 1,975 ft, is in Vekol Valley. 

The basin-fill sequence is described in the section on 

Vekol Valley. Twelve-inch steel casing is perforated
C16 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

between 603 and 1,973 ft. A depth to water of 436 ft, the 

presence of a thick confining layer, and the lack of 

ponding conditions preclude percolation of overlying 

oxygen-rich water. The distance to the recharge area is 

about 8 or 9 mi, assuming that recharge along the Sand 

Tank Mountains supplies the major recharge to the basin 

(Wilson, 1979). This assumption is probably valid owing 

to the greater precipitation that falls on these mountains. 

The dissolved-oxygen concentration of the ground water 

is about 70 percent of saturation.

The validity of the sampling and analytical techniques 

is best demonstrated by the 1,485-ft-deep flowing well 

in lower San Pedro Valley that taps the regional confined 

aquifer. The aquifer is situated approximately at the 

1,370- to 1,485-ft interval and may be overlain by as 

much as 1,200 ft of gypsiferous clay and silt. The 

confining layer, the current hydraulic heads of about 20 ft 

above the land surface, and the flow rates on the order of 

50 to 75 gal/min preclude mixing of recent recharge with 

water in the confined aquifer.

The sample bottles were suspended down the well 

casing, purged, and sealed under water, precluding any 

atmospheric contamination during sampling. Purge tests 

involving introduction of dye into the sample bottle prior 

to filling indicated that within 20 minutes the flowing 

water completely purged the bottle. A Hack model 

OX-2P field kit was used for the analyses, which were 

verified at other sites by a YSI model 57 dissolvedoxygen meter. Identical values of 3.75 mg/L for five 

replicates attest to the validity of the sampling and 

analytical methods.

TOTAL ORGANIC CARBON

The total organic carbon content of the ground water is 

small. The content in 13 of 31 samples analyzed was 

below the detection limit of 0.1 mg/L. The concentrations 

in the remaining 18 samples ranged from 0.1 to 2.7 mg/L, 

and the average and median values were 0.68 and 0.4 

mg/L, respectively. Contamination by pump lubricating 

oil is suspected in many cases to be the cause of the 

higher values. Oil was observed frequently on the 0.45- 

or 0.10-|xm (micrometer) pore-size filters. The small 

concentrations of total organic carbon are consistent with 

the oxygenated waters.

DEPOSITIONAL ENVIRONMENT

The oxygenated ground water can be explained by the 

depositional history and hydrology of a basin. The basinfill sediments were transported into the basin and deposited under oxidizing conditions. Presumably, the detritus 

remained in an oxidized state during burial either above 

or below the water table. The absence of carbon or

sulfide minerals in drill cores attests to the oxidizing 

environment. The oxidized sediments also indicate that 

deposition was not accompanied by the existence of 

perennial lakes, which are characterized by reducing 

conditions. Ponding conditions, if present, were probably 

limited to shallow oxygenated ephemeral lakes or playas. 

Surface runoff saturated with dissolved oxygen before 

infiltration remains oxygenated because little organic 

matter is available in the sediments.

SOLUBILITY EFFECTS OF OXYGENATED ENVIRONMENTS

The oxidizing nature of the alluvial aquifers is noteworthy and geochemically significant (Winograd and 

Robertson, 1982) because most of the literature depicts 

ground water as basically reducing. Solubility and movement of many elements are dependent on the redox 

potential of the ground water. In the relatively narrow 

pH range of 7-9, a small change of the redox potential 

may mobilize metals. The mobilities of arsenic, chromium, molybdenum, nitrogen, sulfur, selenium, vanadium, and uranium are greatly increased by oxidizing 

conditions if converted to the oxyanion form in their 

highest oxidation state. Iron and manganese, on the 

other hand, form insoluble metal oxides and are immobile 

in their higher oxidation states but become soluble in 

their lower states under reducing conditions.

OCCURRENCE AND DISTRIBUTION OF MINOR AND 

TRACE ELEMENTS

Ground water in selected basins was routinely sampled 

and analyzed for aluminum, arsenic, barium, beryllium, 

boron, cadmium, chromium, cobalt, copper, iron, lead, 

lithium, manganese, mercury, molybdenum, selenium, 

strontium, uranium, vanadium, and zinc. The mean, 

standard deviation, and range of concentrations of these 

elements are shown in table 1. Because of the oxidizing 

nature of the ground water and the composition and 

texture of the alluvium, concentrations of several of 

these elements are larger than would be expected in 

these ground-water systems and in many samples exceed 

maximum contaminant levels for drinking water established by the U.S. Environmental Protection Agency 

(1977). Fluoride, chromium, arsenic, nitrate, and boron, 

in particular, present environmental problems in several 

basins.

FLUORIDE

Fluoride is widely distributed in the ground water in 

the study area. The maximum contaminant level for 

fluoride is between 1.4 and 1.8 mg/L and is based on 

annual average maximum daily air temperatures ranging
CHEMICAL CHARACTER OF THE GROUND WATER C17

from 45 to 90 °F. Twenty-five percent of the 546 samples 

analyzed exceeded 1.8 mg/L. Because of the prevalence 

of large fluoride concentrations that exceed the maximum contaminant level, obtaining potable ground water 

for public supply is a major problem in many basins. The 

occurrence and variability of fluoride appears to be 

related to several controls: (1) mineral-equilibrium reactions, (2) sorption or desorption reactions, and (3) availability of the fluoride ion in the rocks.

During the rock-forming process, the fluoride ion 

generally is concentrated in residual fluids of cooling 

magmas, as evidenced by its occurrence in fumaroles, 

pneumatolytic and hydrothermal deposits, pegmatites, 

and volcanic glass (Goldschmidt, 1958). The acidic and 

intermediate volcanic rocks are a suspected major source 

of fluoride, as these types of rocks commonly are found 

throughout the study area and occur to some extent in 

most basins. In volcanic rocks, fluoride may reside in the 

glass phase as well as in the rock minerals. Fluoride in 

hornblende and micas is probably an additional source, as 

these minerals are found in most intrusive and metamorphic rocks.

Solute calcium may be a control of fluoride in ground 

water through equilibrium reactions with the mineral 

fluorite. Fluorite is thought to be a control of fluoride in 

basins that contain concentrations greater than ~5 

mg/L. Under equilibrium conditions, small concentrations of calcium permit large fluoride concentrations in 

solution:

or

^-l2 [F-r=Ksp,

(2)

(3)

where

Ca = calcium, 

F = fluoride,

CaF2 ^fluorite, 

Ksp = solubility product, and

[ ] represent thermodynamic activity. 

Concentrations of less than ~5 mg/L do not appear to be 

controlled by equilibrium with fluorite or other minerals 

(Robertson, 1986).

A strong correlation of fluoride with pH significant at 

the 0.01 percent level (p<0.0001) indicates that pHdependent exchange or sorption reactions are important 

controls of fluoride concentrations. During weathering of 

rocks and release of fluoride ions into solution, the ions 

may initially exchange for hydroxyl groups on montmorillonitic clays. As pH increases downgradient, greater 

concentrations of hydroxyl ions may effect an exchange 

of hydroxyl for fluoride ions, thereby increasing the 

concentrations of fluoride. An assumption of pHdependent exchange of fluoride solely on the basis of the

pH-fluoride correlation may be tenuous, however, 

because the correlation is assumed not to be an indirect 

effect of other reactions, specifically pH increases and 

calcium decreases downgradient. The assumption is 

strengthened by a weaker calcium-fluoride correlation 

and the fluorite saturation data. A much stronger correlation coefficient of pH and fluoride compared with that 

of calcium and fluoride (p<0.001) and the fact that the 

correlation occurs in ground waters that are undersaturated with respect to fluorite support a direct effect of 

hydroxyl-fluoride exchange.

Leachate analyses of basin-fill sediments also indicate 

that sorption-desorption is occurring. Analyses show 

that leachable fluoride is present in amounts generally 

twentyfold more than concentrations found in the ground 

water and thus is available for dissolution to the aqueous 

phase. Because the solubility product of fluorite is not 

reached in the ground water of most basins, exchange or 

sorption-desorption reactions appear to be a major control (Robertson, 1984).

CHROMIUM

Chromium in the +6 oxidation state, or hexavalent 

state (Cr+6), is a naturally occurring constituent in the 

ground water throughout much of central and western 

Arizona (fig. 8). Five percent of 436 samples contained 

Cr+6 concentrations above the maximum contaminant 

level of 50 |xg/L (micrograms per liter). Large concentrations of Cr+6 may be more prevalent than indicated by

TABLE I. Mean, standard deviation, and concentration range of

selected trace and minor elements in ground water

[In micrograms per liter]

Element

Aluminum ..........

Arsenic .............

Barium .............

Beryllium ...........

Boron. ..............

Cadmium ...........

Chromium ..........

Cobalt ..............

Copper .............

Iron ................

Lead. ...............

Lithium. ............

Manganese. .........

Mercury ............

Molybdenum. .......

Selenium. ...........

Strontium. ..........

Uranium............

Vanadium. ..........

Zinc ................

of

samples

. . . . 363

, . . . 467

, . . . 314

, . . . 296

. . . . 453

, . . . 336

.... 436

... 298

.... 369

.... 485

... 371

.... 295

... 494

.... 387

.... 347

.... 434

... 311

30

... 293

379

Mean

9.8

15.7

95.9

00.0

539.8

00.0

10.3

.1

2.3

19.2

3.1

136.2

6.0

0.1

7.7

1.9

1,590.6

10.3

18.5

57.5

Standard 

deviation

20.1

31.6

464.7

00.0

905.4

00.0

30.7

.7

7.4

59.2

10.9

221.1

40.2

0.2

16.6

3.1

2,481.3

7.3

19.0

189.7

Range

0-300

0-1,300

0-8,100

0-2

7-11,000

0-.2

0-300

0-6

0-70

0-810

0-110

7-2,600

0-720

0-0.2

0-150

0-30

0-2,100

1.2-27

0-130

0-2,900
CIS REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

115° 113° 111° 109°

yjTM f.* ^'^ 'fiw|-'^s^%»v ;;;\e r-if A% st, -Ofltelj^-, rw-.sfster

j O^^./h tPl^ » -^j^-^, A4^; C" ^ ^ ^ rfi^%%. ^- ^r ^* ^iil ^^^^^ -*^ 1 "-* fc f^- iim?ig!f+~ <%" ' '^-jfe*' 1^' iTO ftf'i* \ V^ Yi^x.--rf/ ~'"& J 'i^- 1 I \ ^^^^_"> fr-?*~ * .^fi^*^-" ''XJ .?* "* *i J--J f£ * C" -f-^ \ "\^ ^" >*;'lv :*\V-I^-^f ' ""I*.

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CALIFORNIA

33

31°

ALLUVIAL DEPOSITS-Locally may 

include evaporite deposits and 

volcanic rocks

CONSOLIDATED ROCKS 

BASIN BOUNDARY

I T 

50 100 KILOMETERS

EXPLANATION

CONCENTRATIONS OF HEXAVALENT CHROMIUM, 

IN MICROGRAMS PER LITER

0-10

10 -50

MORE THAN 50

NO DATA

FIGURE 8. Occurrence and distribution of hexavalent chromium.
CHEMICAL CHARACTER OF THE GROUND WATER C19

the sampling program, however, because a relatively 

large part of western Arizona is undeveloped and all 

areas could not be evaluated.

The source of Cr+6 is the trivalent chromium (Cr+3) in 

the basin fill. Chromium is one of the most widely 

dispersed elements and occurs in rocks of all principal 

types (Ahrens, 1965). Chromium occurs primarily in the 

mineral chromite (FeCr204), the economic source of 

chromium, and in silicate minerals of clinopyroxenes, 

amphiboles, and micas as replacement of ferric iron. 

Trivalent chromium is the normal form in the weathering 

environment and is highly insoluble and immobile, but 

under oxidizing and existing pH conditions, it may be 

oxidized to Cr+6 and form the soluble chromate ion 

(CrO^2). The dissolved oxygen probably is the oxidant of 

the Cr+3. The oxidation of the mineral chromite in an 

oxygenated environment may proceed by

FeCr204 +1.7502(aq)+2.5H20^

2Cr04 2+FeOOH+4H+ . (4)

The presence of Cr+6 in the ground water is closely 

related to the geology, hydrology, and geochemical environment of the aquifer. Occurrence and magnitude are 

associated with source rock type and with areas in basins 

that contain ground water that is old, oxidizing, and high 

in pH. Larger Cr+6 concentrations, those greater than 50 

|xg/L, occur most commonly in basins bounded by volcanic rocks, particularly intermediate volcanic rocks. 

Cr+6 is also found in ground water of basins bounded by 

intrusive rocks, but concentrations there generally are 

smaller (Robertson, 1984). The basins that contain the 

largest concentrations are those that receive the least 

recharge, resulting in ground water having long residence time. Concentrations of Cr+6 range from 0 |xg/L in 

lower San Pedro Valley and other basins in the eastern 

part of the study area to 60 |xg/L in Vekol Valley and 300 

|xg/L in Ranegras Plain. Cr+6 does not occur in the 

ground water in the eastern part of the study area where 

precipitation and recharge are significantly higher.

Cr+6 correlates positively (p<0.0001) within the study 

area with pH, fluoride, vanadium, and uranium. Correlation with fluoride may be an indirect effect of a 

pH-fluoride-dependent exchange control. The correlation with vanadium and uranium may be due to their 

geochemical similarity; vanadium and uranium, like chromium, are oxidized to their highest, most soluble oxidation state.

The oxidation of Cr+3 is a pH-dependent reaction and 

is favored by high pH conditions. High pH conditions 

occur basinward as a result of silicate hydrolysis, which, 

under closed hydrochemical conditions, proceeds more 

rapidly than solution C02 uptake or availability. Conditions for C02 removal are optimal under high reaction

rates associated with hydrolysis of intermediate volcanic 

and pyroclastic rocks (Jones, 1966) and probably account 

for the larger Cr+6 concentrations associated with rocks 

of these types. The reactions and concomitant increases 

in pH are reflected in the distribution of Cr+6 where the 

distribution is characterized by small concentrations 

near the basin margin that increase basinward to a 

maximum near the basin center (Robertson, 1984).

Because chromium solubility is related to the pH and 

the redox potential of the ground water, the solubility of 

chromium species was examined thermodynamically by 

Eh-pH measurements using the equipment of Back and 

Barnes (1961). Although Eh measurements are quantitatively questionable, as the solubilities derived from 

them do not always agree with theoretical values derived 

from the Nernst equation, the measurements proved 

useful in describing the oxidizing environment and 

explaining the Cr+6 occurrences. The occurrences are 

concluded to be a result of oxidation of the Cr+3 within 

the basin fill, as the measured potentials using a platinum 

electrode and the pH values give equilibrium solubilities 

close to those observed. The Cr+6 appears to be in 

equilibrium with Cr+3 oxyhydroxide. Thus, the Cr+3 

species is the electron donor in the half-cell reaction. The 

oxidation may be unique in the sense that, assuming that 

the electrode is not responding to dissolved oxygen and 

that other redox-controlling couples are absent, the 

electrode may be responding to the unique Cr+3-Cr+6 

couple. Calculated equilibrium solubilities of Cr+3 in the 

ground water under existing pH and redox conditions 

should be on the order of 10~ 7 to 10~8 M (molar), and the 

observed Cr+6 concentrations are on the order of 10~5 to 

10~6 M. Reliable measured potentials may be possible in 

the 10~5 to 10~6 M concentration range (Stumm and 

Morgan, 1981, p. 491) under favorable conditions, particularly in the absence of other redox couples or mixed 

potentials. The existence of a disequilibrium system 

(Thorstenson, 1984) involving iron or manganese and 

probably other redox couples may not be critical 

because reactions involving these elements may have 

gone to completion. As discussed later, model results 

show that minerals containing iron and manganese 

weather increasingly slower or not at all downgradient, 

thus diminishing the importance of kinetic reactions at 

the electrode. The absence of interfering redox potentials could readily explain the large concentrations of 

Cr+6 in older waters and the absence of Cr+6 in younger 

waters. Because Eh measurements were made in only 

two basins during the study, it is not known if the 

measurements are predictive throughout the area. On 

the basis of the few measurements, the Cr+6 concentrations appear to be strictly a function of partial heterogeneous equilibrium of the ground water with the trivalent 

chromium solid.
C20 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

ARSENIC

Nearly 7 percent of the 467 samples analyzed for 

arsenic exceeded the maximum contaminant level of 

50 jxg/L. Notably large arsenic concentrations were 

found in ground water in Verde Valley and on the Papago 

Indian Reservation in south-central Arizona. Concentrations exceeded 200 jxg/L in both areas, and average 

concentrations were 59 and 62 jxg/L, respectively. Verde 

Hot Springs contained an arsenic concentration of 

1,300 jxg/L.

All five outer shell electrons of the arsenic atom 

participate in the formation of compounds, and, consequently, arsenic occurs in oxidation states ranging from 

As"2 to As+5. The most common compounds of arsenic 

are the sulfides in the reduced form and arsenates in the 

oxidized form. The numerous oxides of arsenic form 

when sulfide and arsenide deposits are exposed to oxygen of the atmosphere or with oxygenated waters. The 

arsenates, arsenic as As+5, are the predominant oxide 

minerals and the most stable species in oxygenated 

waters. Arsenic also occurs as arsenite (As 4"3) in mildly 

reducing waters (Hem, 1985). Arsenic speciation of several samples from the reservation confirmed that arsenic 

occurs as As+5.

The source of the arsenic in the ground water is 

oxidized arsenic compounds in the basin fill, which are 

suspected to be derived from hydrothermal sulfide and 

arsenide deposits in the surrounding mountains. Arsenic 

presumably is oxidized to As 4" 5 before transport into the 

basin or, if after transport, before burial. The occurrences of arsenic, probably in the arsenate form 

HAsO^2 , and the circumstances of its removal are 

related to several factors the oxidation state of the 

arsenic, the Eh and pH of the water, and the nature of 

the surface (soil and sediment) that adsorbs or immobilizes arsenic. In oxidizing environments, As+5 is strongly 

retained on fine and medium sands relative to As+3; the 

retention may be related to the interaction between As+5 

and Fe+3 (Ferguson and Davis, 1972; Gulens and others, 

1979). Consequently, arsenic is enriched in the basin-fill 

sediments relative to its crustal abundance of about 2 

ppm (parts per million). The arsenic content of samples 

of basin fill analyzed ranged between 2 and 88 ppm. 

The highest arsenic values were found in Verde Valley 

7 to 88 ppm from well cuttings and outcrop samples of 

the Verde Formation of Miocene and Pliocene age. The 

lower values were found in clean limestones, and the 

higher values in clayey limestones and red clay beds 

near Lake Montezuma (Owen-Joyce and Bell, 1983; 

Robertson, 1986).

Arsenic in the ground water correlates most strongly 

(p<0.01) with lead, molybdenum, selenium, vanadium, 

and fluoride. Except for lead and fluoride, all elements 

occur as negative oxyanions like arsenate. Fluoride,

although not an oxyanion, also is a negative ion. 

Although arsenic shows no significant correlation with 

dissolved iron (correlation coefficient, r, =0.09), arsenic 

concentrations and arsenic removal appear to be controlled partly by iron in the solid phase. Water from the 

village well at Pisinimo on the Papago Indian Reservation contained, at different times, arsenic concentrations 

between 0 and 50 jxg/L and concentrations exceeding 100 

|xg/L. A time-series analysis indicated that the arsenic 

concentrations were directly related to the pumping 

time, to the redox potential, and to the dissolved-oxygen 

content and inversely related to the iron concentrations 

in the ground water. The well, after sitting idle for a 

period of weeks, initially produced water high in iron and 

low in arsenic and dissolved oxygen.

Coprecipitation or adsorption of the arsenate ion on 

ferric hydroxide (or ferric oxyhydroxide) was considered 

the simplest and most plausible mechanism for the 

removal of arsenic from solution. The pH of the ground 

water (8.2) remained constant throughout the test, 

which, along with the trend in dissolved oxygen, suggests that Eh is a significant factor (Robertson, 1986).

Mineralogic controls appear to have little or no effect 

on arsenic solubility in the oxygenated environment. 

Speciation and saturation data show the ground water to 

be very highly undersaturated with respect to the arsenate and arsenite minerals As205, AlAs04-2H20, 

Ca3(As04)2 -4H20, Ca3(As04)2 -6H20, FeAs04 -2H20, 

Mn3(As04)2 -8H20, Pb3(As04)2 , Zn3(As04)2 -2.5H20, and 

As203 (arsenolite and claudetite). Judging from the concentrations of arsenic and metals found in the ground 

water, it is unlikely that other metal-arsenic compounds 

are mineralogic controls of arsenic.

A major control of arsenic occurrence and removal 

appears to be the coprecipitation or adsorption of 

HAsO^2 on active ferric oxide or oxyhydroxide surfaces. 

The removal process would also explain the large arsenic 

concentrations found in the red clay beds near Lake 

Montezuma, in which the red color is due to the presence 

of ferric oxides (Robertson, 1986).

BORON

Boron concentrations normally found in drinking 

water and food are not toxic to humans, but plants may 

be affected by small concentrations. Citrus trees are 

particularly sensitive to boron and may begin to show 

toxicity symptoms if the concentrations exceed 1,000 

jxg/L (Gough and others, 1979). Boron concentrations in 

the ground water ranged from 7 to 11,000 jxg/L, and 

concentrations in about 13 percent of the samples 

exceeded 1,000 jxg/L. Boron concentrations tend to be 

lower in the southeastern basins and higher in the central 

and western basins, suggesting that aridity may play
CHEMICAL CHARACTER OF THE GROUND WATER C21

some role. Average boron concentrations range from 40 

p-g/L in the Willcox basin to 1,200 fig/L in Ranegras 

Plain.

Boron is enriched in the residual portions of magmas 

and hydrothermal solutions and crystallizes as independent minerals only during the hydrothermal or pegmatite 

stage. The small ionic radius of trivalent boron, the only 

oxidation state, of 0.23 A prevents its substitution in 

late-crystallizing hydroxyl-bearing minerals. Tourmaline 

is the most abundant of the boron silicate minerals. 

Boron replaces silicon in the tetrahedral structure, but 

the substitution is small because of considerable differences in the ionic radii of the two elements (Si=0.42 A). 

The total amount concealed in the silicate structure is 

geochemically significant, however, as the crustal abundance of boron is only about 5 ppm.

In addition to boron substitution in silicate tetrahedral 

minerals and a few late magmatic borosilicate minerals, 

boron is found in volcanic emanations. Boron from these 

emanations as soluble borate may form important economic deposits if it accumulates in closed basins as 

evaporites or playa deposits. The borate deposits in the 

playas of southern California and Nevada are the world's 

most abundant source of boron and contain the minerals 

kernite (Na2B407-4H20), borax (Na^CV 10H20), ulexite (NaCaB509 -8H20), and colemanite (Ca2B6011 -5H20) 

(Rankama and Sahama, 1950).

Boron correlates positively (p<0.0001) with chloride, 

sulfate, dissolved solids, nitrate, and molybdenum. The 

positive correlations of boron with generally soluble 

constituents suggest that evaporites in the basin fill are 

sources of the large concentrations of boron in the ground 

water. The large concentrations in Ranegras Plain probably are due to the presence of the evaporites in the 

lower basin fill. The boron in the evaporites may be 

hydrothermal in origin or may have resulted from weathering of borosilicate minerals, including the silicate tetrahedral minerals in the rocks surrounding the basin. 

The overall larger boron concentrations in the western 

basins may be a function of aridity through the reworking of the secondary boron compounds in the soil or 

unsaturated zone.

NITRATE

Nitrogen could be considered a major element but is 

included in this section with the minor and trace elements because of its environmental implications. About 

12 percent of the 480 ground-water samples analyzed for 

nitrate exceeded the maximum contaminant level of 45 

mg/L as nitrate. Average concentrations range from 0.5 

mg/L in the basins along the Colorado River to 41.4 mg/L 

in West Salt River Valley. The sources of nitrogen are 

numerous and include natural aerosols, precipitation,

fixation by microorganisms, decaying organic matter, 

fertilizers, animal and industrial wastes, and weathering 

of consolidated and unconsolidated rocks. The nitrogen 

content of igneous rocks and sedimentary rocks as 

nitrate is about 46 and 510 ppm, respectively (Rankama 

and Sahama, 1950).

The large nitrate concentrations in West Salt River 

Valley have multiple sources. Hem (1970) cites an analysis of water from a well in an intensively irrigated and 

fertilized area in West Salt River Valley that contained a 

nitrate concentration of 83 mg/L, which probably is due, 

at least in part, to the leaching of fertilizers. In the 

north-central part of the valley, concentrations of more 

than 90 mg/L occur over a large area. A time-series 

analysis using historical data indicates that this highnitrate area has been in existence at least since 1920 and 

possibly since the turn of the century (Schmidt, 1979). 

The spatial distribution of the concentrations and the 

time-series analysis indicate a natural or geologic origin 

of the nitrate. A thick, fine-grained unit of the upper 

basin fill that overlies the aquifer also supports a geologic 

origin. Nitrate occurrences of natural origin were also 

observed in the ground water of Vekol Valley, where 

concentrations ranged between 30 and 40 mg/L in the 

central part of the basin. Because the basin is undeveloped and the aquifer is overlain by a thick silt-clay unit, 

the concentrations are associated with a natural origin. 

Hem (1970) cites an analysis of water from a well 12 mi 

south of Gila Bend that contained a nitrate concentration 

of 68 mg/L. The large concentration probably was related 

to a natural source, as the area was undeveloped at the 

time of sampling.

Nitrate does not seem to have any controls other than 

the availability of nitrogen in the environment. Nitrate 

salts are highly soluble and, on the basis of leachate 

analyses discussed in the section on "Projection of 

Ground-Water Chemistry," do not sorb readily on the 

basin-fill sediments. Some natural sources of nitrogen in 

desert areas are rocks, animals, precipitation, atmospheric aerosols, dust, the legumes species of desert 

vegetation, caliche, and playa deposits (Feth, 1966). If 

precipitation were not sufficient to leach the soluble 

nitrate salts from soils depositional rates exceed 

removal rates an eventual increase in nitrate concentrations from these sources would occur in the ground 

water.

SELENIUM

About 2 percent of the 434 samples tested for selenium 

contained concentrations above the maximum contaminant level of 10 M-g/L. The geochemistry of selenium is 

similar to that of sulfur also in group VI of the periodic 

table but in some respects it is even more similar to
C22 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

that of arsenic. Selenium occurs in four oxidation states, 

and its solubility is affected by redox potential, pH, and 

probably coprecipitation and sorption reactions. Under 

oxidizing conditions, selenium occurs as the selenite and 

selenate anions SeO^2 and SeO^2 , respectively, and 

under reducing conditions, as selenides, Se~2 (Lakin, 

1972). The soluble species of selenium in ground water 

are the selenite and selenate forms (Hem, 1985).

Selenium correlates positively (p<0.01) with the elements arsenic, boron, calcium, chloride, magnesium, 

potassium, dissolved solids, sodium, nitrate, strontium, 

and sulfate and negatively with pH. The correlations of 

selenium with mostly soluble components may be 

explained by the higher solubility of the oxidized anion 

forms, and, like boron and nitrate, its occurrence can be 

associated with evaporites or playa deposits. A strong 

negative correlation with pH tends to discount EhpH-dependent mineral phases as a control in the oxygenated environment.

Availability of selenium in the basin fill could be a 

control, judging from the small crustal distribution. The 

abundance of selenium in the Earth's crust and in igneous 

rocks is only about 0.09 and 0.05 ppm, respectively 

(Lakin, 1972). The correlation of selenium with arsenic 

and the strong correlations of selenium with fluoride, 

molybdenum, and arsenic in Verde Valley, where the 

highest selenium concentrations occurred, however, suggest that selenium analogous to arsenic may be controlled by sorption on ferric oxide surfaces. If the chemical behavior of selenium parallels that of arsenic, 

enrichment of selenium in the basin fill could occur, 

discounting the crustal distribution and favoring the 

control of the dissolved species by sorption reactions. 

Chemical analyses of the basin fill for selenium are not 

available to define the distribution in relation to the 

dissolved phase.

BARIUM

About 1 percent of the 314 samples analyzed for 

barium exceeded the maximum contaminant level of 

1,000 |ULg/L. Concentrations ranged from 0 to 8,100 |mg/L; 

however, most values did not exceed 500 jutg/L. Barium 

does not correlate in the ground water with any other 

element (p<0.05). In several individual basins, however, 

barium correlates positively (p<0.01) with strontium, 

calcium, lithium, and potassium. Barium correlates most 

significantly and frequently with potassium, which, 

along with the correlations with the alkaline earths and 

lithium, indicates that potassium feldspar and possibly 

alkali basalt are the likely sources of barium.

Dissolved barium apparently is controlled by the solubility product of the highly insoluble mineral barite 

(BaS04):

Ba+2 +S072;

-BaSO,

or

k-2 [SC>4 ]=Ksp barite.

(5)

(6)

Large amounts of barium, however, also are removed 

from solution by ion exchange or adsorption by secondary clay minerals. Barium is preferentially absorbed 

relative to sodium, calcium, strontium, and magnesium 

(Nesbitt and others, 1980). Although barium is preferentially retained, its occurrence in ground water nonetheless appears to be one of an inorganic solubility 

control. Saturation indices (defined in section entitled 

"Aqueous Speciation and Saturation Data") of barite 

for 12 basins range from 0.392 to -0.375 and average 

-0.09, which indicates that the ground water is in 

equilibrium with respect to barite. The saturation data, 

the narrow concentration range of barium, and a poor 

correlation with sulfate, discounting gypsum or anhydrite as sources, indicate that mineral equilibrium is the 

sole control of barium (Robertson, 1984).

CHARACTERIZATION OF GROUND WATER IN 

RECHARGE AND DISCHARGE AREAS

The water chemistry of the recharge areas and the 

downgradient areas is summarized in table 2. In the 

recharge areas, the ground water is typically a calcium 

bicarbonate type. Recharge areas generally contain 

large concentrations of bicarbonate ( 5.0 mmol/L (millimole per liter)) and calcium ( 2.0 mmol/L) and relatively 

small concentrations of magnesium, sodium, chloride, 

sulfate, and fluoride (all <1.0 mmol/L). Chloride and 

sulfate concentrations are variable, but in general the 

lowest concentrations are found in the recharge areas. 

Ground-water temperatures generally range between 15 

and 30 °C. The pH of the ground water generally is near 

neutral (6.9-7.4), and dissolved oxygen is always present 

and near saturation (~9 mg/L). Sulfide species, ferrous 

iron, organic carbon, and other reducing agents are 

absent. Concentrations of trace elements are small, 

commonly below detection limits.

In the downgradient areas, the composition evolves 

into either a sodium bicarbonate or sodium mixed-anion 

type or, as dissolved solids increase, into a sodium 

chloride or sodium calcium sulfate type. Total inorganic 

carbon, Pco2» an^, in basins containing the more dilute 

waters, calcium and magnesium decrease markedly. Sulfate and chloride concentrations and, in some cases, 

calcium and magnesium increase depending on the availability of gypsum and halite within the basin fill. Temperatures increase to as high as 45 °C, and pH values 

increase to as high as 9.5. Dissolved oxygen is invariably 

present in concentrations typically ranging from 50 to 80
CHEMICAL MODELS C23

TABLE 2. Analytical data for selected wells in recharge and downgradient areas

[In millimoles per liter unless otherwise indicated; values in parentheses are in milligrams per liter; meq/L, milh'equivalents per liter. Dashes indicate no value

was calculated]

Recharge area (7 basins)

pH (units) .....................

Temperature (°C) ..............

Mg"1"2

Na+ ...........................

cr ............................

so4 -2 ..... .....................

Alkalinity (meq/L) .............

F-................... ..........

Si02 ...........................

Dissolved solids. ...............

2

Number 

of 

wells

....... 9

....... 9

....... 13

....... 13

....... 13

....... 8

....... 13

....... 13

....... 13

....... 11

....... 8

....... 11

....... 11

....... 9

Mean

7.21

25.8 

1.948 (78) 

.590 (14) 

.970 (22) 

.052 (1.9) 

.567 (20) 

.216 (21) 

4.810 (294) 

.013 (0.26) 

.490 (30) 

.720 (44) 

- (495) 

.60

Standard 

deviation

0.20 

3.80 

.45 (18) 

.29 (7) 

.69 (16) 

.02 (.8) 

.45 (16) 

.19 (18) 

.67 (41) 

.01 (.2) 

.65 (40) 

.10 (6) 

 (68)

Downgradient area (23 basins)

Number 

of 

wells

547 

521 

556 

555 

554 

543 

554 

501 

554 

546 

544 

554 

554 

521

Mean

7.61 

27.130

1.986 (80) 

1.118 (27) 

8.025 (185) 

.125 (4.9) 

6.921 (245) 

2.040 (196) 

3.407 (208) 

.080 (1.5) 

.457 (28) 

.525 (32) 

 (938)

Standard 

deviation

0.46 

5.29 

2.39 (96) 

1.52 (37) 

9.30 (214) 

.08 (3.0) 

10.45 (371) 

3.36 (323) 

1.80 (111) 

.10 (1.9) 

.74 (46) 

.20 (12) 

 (949)

Calculated.

percent of saturation, and concentrations of several trace 

elements increase basinward. Total dissolved solids of 

 400 mg/L in the recharge area may decrease to less 

than 200 mg/L downgradient owing to precipitation 

reactions or may increase to several thousand milligrams 

per liter owing to dissolution reactions.

VARIATION WITH DEPTH

The dissolved-solids concentration in the ground water 

in some alluvial aquifers, roughly the upper 2,000 ft, may 

show slight increases or decreases with depth, but the 

trends are not statistically significant (p<0.05). 

Although data and well control are not complete in all 

basins, the following trends were observed: increasing 

dissolved-solids content with depth in Ranegras Plain, 

Verde Valley, and the Big Sandy and upper Santa Cruz 

basins and in the artesian aquifer in lower San Pedro 

Valley, and decreasing dissolved-solids content with 

depth in East and West Salt River Valleys, Harquahala 

Plain, Rainbow, McMullen, Hualapai, Sacramento, and 

Altar Valleys, and the Willcox basin, and in the watertable aquifer along the San Pedro River. In most of these 

basins, concentrations of magnesium, bicarbonate, and 

silica decrease with depth and pH, temperature, and the 

majority of trace elements increase with depth. Calcium 

generally, but not invariably, also decreases with depth.

The extent of mixing of waters within the aquifers is 

not known, but the chemistry of waters in the recharge 

and discharge areas suggests that vertical variations are 

less important than areal variations. The variation in the 

water chemistry within basins appears to be a function

more of the flow path than of mixing. The saturation 

state of the ground water is supporting evidence, as close 

equilibrium with respect to several minerals, particularly calcite, fluorite, and barite, suggests little mixing of 

waters of vastly different compositions.

CHEMICAL MODELS

A goal of chemical modeling is to identify probable 

reactions responsible for the evolution of the ground 

water and any chemical changes that occur within the 

aquifer as a result of these reactions. In addition to 

identification of reactions, the mass transfer the 

amounts of all pertinent minerals or gases that dissolve 

or precipitate in the course of these reactions should be 

defined. Determination of the mass transfer is possible 

for a specified chemical system if the number of dissolved 

chemical constituents equals the number of phases 

selected for reactants and products. For example, in a 

simplified system showing weathering of rocks,

a(unweathered rocks)+b(water)

=c(clays)+d(dissolved species), (7)

it is possible in theory to solve for the reaction coefficients a, b, c, and d, which define the mass transfer of 

each respective phase. However, the number of possible 

mineral and gas phases normally far exceeds the number 

of constituents. The large number of possible phases 

necessitates the use of many likely candidates and the 

elimination of the less likely. In a comprehensive treatment of all reactant and product phases, a "best fit" 

model is developed. A best fit model is one whose
C24 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

reactions are most compatible with observed changes in 

water chemistry and with rock minerals (Plummer and 

others, 1983).

For this study, the computer code BALANCE 

(Parkhurst and others, 1982) was used to facilitate the 

calculation of the reaction coefficients. A series of simultaneous equations using initial and final water compositions and selected mineral or gas phases define the 

changes in molality of each constituent along a flow path 

by determining reaction coefficients for the specified 

mineral stoichiometry or gas compositions. Mass-balance 

calculations are constrained by relations of conservation 

of mass and electrons and are defined by the equation 

(Parkhurst and others, 1982)

mT,k(initial) = (8)

P=i

for each element k=1 to /, where P is the number of total 

reactant and product phases in the net reaction, ap is the 

calculated net mass transfer of the pth phase, bp>k 

denotes the stoichiometric coefficient of the kth element 

in the pth phase, mT<k is the total molality of the kth 

element in solution, 1 is the total number of species, and 

/ is the number of elements included in the calculation. 

The mass transfer is defined as the number of millimoles 

per kilogram H20 of reactant or product phases that 

enter or leave the aqueous phase1 . The values of mT are 

obtained from the analytical data. If the number of 

phases equals the number of elements, a mass balance for 

the selected minerals and gases can be obtained by the 

simultaneous solution of k equations.

As redox reactions are involved in the reaction models, 

it is also necessary to conserve electrons. The redox state 

for a solution is defined as

(9)

where RS is the redox state of the solution, mz is the 

molality of the ith aqueous species, and v{ is the operational valence of the ith species. The operational valence 

is a set of conventions to handle the oxidation states of 

the various elements in the reaction. Equations 8 and 9 

define the mass transfer that occurs in the chemical 

system along the flow path. Although the reactions occur 

along the flow path, they are not based on hydraulic 

principles. The reaction models are not to be confused 

with solute-transport models, which are based on 

hydraulic principles. For a detailed discussion of mass1 For brevity, the reaction coefficients are referred to simply as millimoles, or 

mmol.

transfer calculations, see Parkhurst and others (1982), 

Plummer and others (1983), and Plummer (1984).

The reactant and product phases used in the chemical 

models were selected on the basis of published mineralogical information or were postulated through analysis of 

several types of data: (1) XRD (X-ray diffraction) analysis of the basin fill, (2) XRF (X-ray fluorescence) 

analysis of the basin fill, (3) speciation and saturation 

data, and (4) mass balance and isotopic constraints. The 

selection of the proper phases is a critical aspect of 

chemical modeling and requires significant knowledge 

and understanding of the hydrologic system and of the 

chemistry and mineralogy of the aquifer. The methodology and criteria for the selection of the final phases is 

discussed in later sections.

MINERALOGY OF THE BASIN FILL

The composition of the ground water is determined by 

reactions of water with minerals present in the soil and 

basin fill; hence some knowledge of the mineralogy is 

required. Detailed information on the mineralogy is 

limited, as most previous hydrologic investigations did 

not address mineralogic studies. As part of this study, 

169 samples of drill core or well cuttings of alluvium from 

21 wells in seven basins were analyzed. One-hundred 

seven samples of the less than 62-|xm particle-size fraction were analyzed by XRD for bulk mineralogy and 

by XRF for bulk chemistry, and 62 samples of the less 

than 2-|xm fraction were analyzed by XRD for clay 

mineralogy.

Quartz, plagioclase, calcite, potassium feldspar, muscovite, and clay were the most common minerals in the 

bulk fraction. Pyroxenes and amphiboles were detected 

in a few samples, and dolomite was detected in a few 

samples from the unsaturated zone. The distribution of 

plagioclase, one of the most abundant minerals in the 

bulk fraction, is shown in figure 9.

Samples for the clay-mineralogy analysis were separated into less than 2-|xm fractions, mounted on slides, 

and analyzed using standard procedures (Dixon and 

Weed, 1977). Samples were scanned at a rate of 2°26 per 

minute using copper Ka with X=1.542 A radiation. 

Patterns were made after each of the following procedures: (1) treated with magnesium solution for exchangesite saturation and air dried; (2) treated with ethylene 

glycol; and (3) treated with potassium solution for 

exchange-site saturation and heated at 110, 300, and 500 

°C. Peaks of the patterns that moved from near 15 to 17 

A upon ethylene glycol treatment and to 10 A upon 

heating were identified as smectite. Nominal 10 A peaks 

that showed no variation upon heating were identified as 

mica. Invariant 7 A peaks were identified as kaolinite.
CHEMICAL MODELS C25

PLAGIOCLASE FREQUENCY OFOCCURRENCE INSAMPLED WELLS

M N-> _>

O U1 (J1 

/

1 1

U.BITE

Vek

SSW

BOS

WSR

use

ESR

OLIGOCLASE

WSR

RBW

ESR

BOS

Vek

use

Vek

ANDESINE

Vek

use

LABRADORITE

WSR

ESR

BOS

use

SSW

Vek

|

BYTOWNITE

WSR

SSW

ESR

use

|

ANOR1 Ml 1 1

Vek

SSW

U_§£.

WSR

10 30 50 70

PERCENT ANORTHITE

90 100

EXPLANATION

BASIN ABBREVIATION AND NUMBER OF WELLS SAMPLED 

Vek, Vekol Valley (4) 

BOS, Bosque basin, Gila Bend Plain (2) 

RBW, Rainbow Valley (1) 

WSR, West Salt River Valley (2) 

ESR, East Salt River Valley (4) 

SSW, San Simon Wash area (1) 

USC, Upper Santa Cruz area (7)

FIGURE 9. Distribution of plagioclase for selected basins.

The results of the ethylene glycol treatment alone 

suggest that the samples are predominantly smectite or 

interstratified minerals with smectite layers. All patterns upon treatment show swelling to spacings near 17 

A that collapse to near 10.0 A upon heating. The analysis 

also shows that the clay mineralogy of the basin fill is 

markedly uniform throughout the area and that along 

with smectite, trace to small amounts of muscovite and 

kaolinite are present. About 20 percent of the samples 

contained mixed layered clays, generally smectite interstratified with mica or vermiculite as components. Otherwise, vermiculite and chlorite are absent. The relative 

abundances of the clay minerals in the basin fill are 

indicated by the XRD patterns in figures 10, 11, and 12.

The clay mineralogy of the soil zone, less than 5 ft 

below land surface, also appears to be similar among 

basins in southern Arizona (U.S. Soil Conservation Service, 1974). Smectite generally is the dominant clay, but 

mica and kaolinite are commonly more abundant in soils 

than in the basin fill.

CHEMICAL ANALYSIS OF THE BASIN FILL

The chemical composition of the basin fill imposes an 

additional constraint on the selection and the composition 

of the mineral phases. The phases selected for the

modeling must agree with the chemistry as well as with 

the mineralogy of the respective source rocks because 

the chemical composition of the ground water is controlled in part by the chemical composition of the rocks in 

the soil, unsaturated, and saturated zones. Thus, selection of the correct mineral species also requires specification of the mineral stoichiometry, for which the chemical composition of the basin fill can establish limits. 

Caution must be exercised in interpreting chemical data 

singularly, however, as a combination of incorrect minerals may also have acceptable chemical compositions.

AQUEOUS SPECIATION AND SATURATION DATA

The selection of phases is based partly on the saturation state of the ground water with respect to the 

possible minerals, as the derived mass transfer must be 

consistent with thermodynamic or solubility constraints 

placed on the reactants and products. The derived minerals, dissolved or precipitated, should agree with 

those observed and those determined by the aqueous 

speciation.

The speciation of the solution supplies the activities 

and concentrations of the dissolved species, including 

some of those not analyzed, calculates the partial pressure of several gases, and determines the mineral saturation indices. The saturation indices describe the saturation state of the dissolved species with respect to 

selected mineral phases. The saturation state is defined 

in terms of the saturation index (SI):

IAP (10)

where IAP is the ion activity product and K is the 

equilibrium constant. The term IAP/K defines the ratio 

of the aqueous ion activity product to the solubility 

products in equilibrium with a given mineral phase. If 

the SI is less than zero, the ground water is undersaturated with respect to the mineral and the mineral will 

dissolve. If the SI is greater than zero, the water is 

saturated or supersaturated with respect to a mineral 

and the mineral may precipitate. Other factors, such as 

its stability in the presence of other minerals and kinetic 

factors, may also determine whether or not a mineral 

precipitates. Input data required for the speciation are a 

chemical analysis, field pH, temperature, alkalinity, and 

a specified Eh.

CARBON ISOTOPIC COMPOSITION

The carbon isotopic composition of the ground water is 

a critical constraint on the selection of mineral phases 

and identification of reactions, as the isotopic composi-
C26 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

Potassium 

+ 110 t

Potassium 

+ 110 °C 

Potassium 

air dried

22 20 18 16 14 12 10 8 6 4 22 20 18 16 14 12 10 8 6 4 22 20 18 16 14 12 10 8 6 4

50 FEET 910 FEET 2,100 FEET

26, IN DEGREES

FIGURE 10. X-ray diffraction patterns of less than 2-micrometer fractions of basin fill from well (D-9-l)13bbd in Vekol Valley.

Depth of sample, in feet below land surface, given beneath figure.

tion is determined by reactions of gaseous(g) and aqueous(aq) C02 with carbonate and silicate minerals. When 

minerals or gases are dissolved or precipitated, the 

isotopic composition of the water may change but should 

agree with the isotopic composition specified by the mass 

transfer and with known compositions of all carbon 

phases. The derived mass transfer in turn is determined 

by the selected phases and their stoichiometry. The 

isotopic composition of the carbon in solution determined 

from the general isotope mass-balance equation is

where

(11)

(12)

and the subscript T denotes total carbon.

The isotopic composition of the various carbon species 

is not fully defined for all geologic and hydrologic
CHEMICAL MODELS C27

in o oo

Potassium 

+ 500 °C

Potassium 

+ 300 °C

Ethylene glycol

Magnesium 

saturated

22 20 18 16 14 12 10 8 6 4

480 FEET

Potassium 

+ 500 °C

Potassium 

+ 300 °C

Potassium 

+ 110 °C

Ethylene glycol

Magnesium 

saturated

22 20 18 16 14 12 10 8 6 4

800 FEET 

20, IN DEGREES

Potassium 

+ 500 °C

Potassium 

+ 300°C

Potassium 

air dried

Ethylene glycol.

Magnesium 

saturated

22 20 18 16 14 12 10 8 6 4

1,700 FEET

FIGURE 11. X-ray diffraction patterns of less than 2-micrometer fractions of basin fill from well (C-5-3)29aaa in the 

Bosque basin, Gila Bend Plain. Depth of sample, in feet below land surface, given beneath figure.
C28 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

Potassium 

+ 500 °C

Potassium 

+ 300t

Potassium 

air dried

Potassium 

+ 110 °C

Ethylene glycol

22 20 18 16 14 12 10 8 6 4

375 FEET

Potassium 

+ 500 °C

Potassium ,. / 

+ 300 t ^-A^

Potassium 

+ 110 °C

Potassium 

air dried

Ethylene glycol

22 20 18 16 14 12 10 8 6 4

802 FEET 

20, IN DEGREES

Potassium 

+ 300°C

Potassium 

+ 110 °C

Potassium 

air dried

Ethylene glycol

22 20 18 16 14 12 10 8 6 4

1,800 FEET

FIGURE 12. X-ray diffraction patterns of less than 2-micrometer fractions of basin fill from well (D-2-8)26caa in East Salt 

River Valley. Depth of sample, in feet below land surface, given beneath figure.
CHEMICAL EVOLUTION OF THE GROUND WATER C29

environments, but published measurements for arid and 

semiarid areas allow reasonable estimates. In a study by 

Wallick (1973), the 818C isotopic content of carbonates in 

soil and well core in the Tucson basin ranged from about

-2 to -6%c and averaged about -4%e. The 818C content 

of the few carbonate samples determined during this 

study ranged from 1.8 to -7.0%e and also averaged 

about -4.0%e. Published 818C measurements of C02 gas 

are fewer but indicate a soil-gas composition range of

-15 to -20%c. Wallick (1973, p. 122) determined a mean 

value of -15.12±2.88%e for five samples near Tucson, 

Ariz. Parada (1981) observed that 813C of C02 in desert 

soil near Tucson averaged 18%o and in river beds 

averaged -20%c. Pcc>2 values reported in that study 

ranged from 0.001 to 0.05 atmospheres. Pearson and 

Hanshaw (1970) also reported values of -15 to -19%c in 

the soil of arid west Texas.

CHEMICAL EVOLUTION OF THE 

GROUND WATER

In determining the chemical evolution of the ground 

water in the individual basins, two distinct hydrochemical processes were identified: (1) reactions of meteoric 

water (pure water) as it infiltrates the soil zone and 

percolates through the unsaturated zone to the water 

table in the recharge area and (2) reactions of ground 

water as it moves from the recharge area downgradient 

to discharge areas.

A great deal of emphasis was placed on the evolution of 

the water in the recharge area, as many of the reactions 

are distinctly different from subsequent reactions along 

the flow paths. Major processes controlling recharge 

chemistry are the weathering of silicates and the dissolution of carbonate minerals in an acid environment. Both 

reactions consume protons. Reactions downgradient 

occur in an alkaline environment and although weathering of primary silicates continues, the relative amounts of 

primary silicates change and precipitation of calcium 

carbonate and probably of silica become dominant processes. In the recharge area, reactions of meteoric water, 

gases, and minerals may occur to a large degree in the 

soil and unsaturated zones. Downgradient, reactions 

occur only within the saturated zone.

The compositions of recharge and downgradient 

waters used in the modeling do not balance perfectly in 

charge. Electrical neutrality was not attempted by 

adjusting any of the elements in solution. In development 

of the recharge and subsequent flow models, any imbalance was carried through the calculations, which in 

essence is distributing the imbalance equally among the 

dissolved species (Plummer, 1984).

EVOLUTION OF THE GROUND WATER IN 

RECHARGE AREAS

The composition of the ground water in recharge areas 

is defined by an average of chemical analyses of water 

from wells in seven basins (table 2). Well selection was 

based on proximity to the mountains. Unfortunately, few 

wells were available for sampling in these areas, as most 

development occurs in the central parts of basins, where 

the land surface is flatter and the soil is better for 

agriculture. A greater number of wells in recharge areas 

is desirable because of the differences in the evolution of 

these waters and waters downgradient. The problem of 

poor well control may be minimized somewhat by the 

general uniformity of the water chemistry in recharge 

areas (as indicated by the relatively small standard 

deviations, especially of calcium, silica, and bicarbonate, 

the major constituents) relative to the variability of all 

ground water sampled. The large variability of all water 

sampled is reflected in the large standard deviations, 

which indicate that extreme values are influencing the 

mean values.

Feth and others (1964) showed that the chemistry of 

shallow and deep circulating springs could be related to 

the mineralogy and weathering of surrounding source 

rocks and to the equilibrium with secondary minerals. 

Garrels (1967) also used solute ratios to predict the 

compositions of the igneous rocks. However, the 

observed ground-water chemistry of the recharge area 

does not seem to reflect in all respects the primary rock 

composition. Magnesium concentrations in the ground 

water are variable and are distinctly greater than those 

expected from the weathering of primary rocks of the 

area, assuming that primary silicates are the source of 

magnesium. However, large silica concentrations suggest that hydrolysis of primary silicates is a major 

process, and, in a slowly fluxing system, the formation of 

secondary clay minerals would be expected. Sulfate and 

chloride concentrations, although generally small, 

become so significant that they require gypsum and 

halite as reactants.

Reactions occurring in the recharge areas that can 

account for the observed water compositions are the 

weathering of primary silicates, chiefly feldspar and 

ferromagnesian minerals, and the dissolution of a calcium 

magnesium carbonate phase. The carbonate phase is 

selected to account for the large observed magnesium 

concentrations. These reactions, as well as subsequent 

reactions downgradient, can be described by a 13- 

component system CaO, MgO, NagO, K20, A1203 , 

FeO, Si02, C02, 02, HC1, H2S04, HF, H20-and can be 

written in the general form

Primary rock+calcium magnesium carbonates

+soil C02+02 +H20 n:lay minerals+oxide phases

+dissolved species in solution. (13)
C30 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

Assuming that the observed concentrations in the 

recharge area do indeed represent reactions of meteoric 

water with the source rock, two models that are compatible with the water composition were developed: (1) a 

mafic model for basalt-andesite rocks and (2) a felsic 

model for granite rocks. The models represent the two 

major rock associations of the mountains within the 

study area, as indicated by the bimodal distribution of 

plagioclase (fig. 9), the most obvious silicate source of 

calcium and sodium.

According to equation 13, the chemical evolution of 

ground water in recharge areas of basins having mafic 

terranes can be written to the following balanced hydrolysis and dissolution reactions:

Hydrolysis of calcite:

CaC03 +H2O^Ca+2 +HC03-+OH- (14) 

C02+H2O^HC03~+H+ , (15)

Hydrolysis of dolomite:

CaMg(C03)2+2H2O^Ca+2+Mg+2 +2HC03~+20H~ (16) 

2C02+2H20^2HC03"+2H+ , (17)

Hydrolysis of andesine:

Cao.45Nao.55AlL45Si2. 5508+8H20-*0.45Ca+2

+0.55Na+ + 1.45Ar3 +2.55H4Si04 +5.80H~, (18) 

Hydrolysis of augite:

Ca0. 4Mg0. 8Fe0. 7Al02Si1 . 906+6H20-*0.4Ca+2 

+0.8Mg+2+0.7Fe+2+0.2Ar3 +1.9H4Si04+4.40H-,(19)

Hydrolysis of potassium feldspar (which for modeling 

purposes is specified as the sole source of potassium):

KAlSi308+8H2O^K+ +Al+3 +3H4Si04 +40H-. (20)

Other reactions occurring are the dissolution of gypsum 

(or anhydrite) and halite, respectively,

and

(21)

(22)

In determining the mass transfer of the silicate phases, 

it is assumed through the selection of the plausible 

phases that aluminum is conserved (that is, in eq. 8, 

AmrA.=0) among the primary and secondary silicate 

phases and that montmorillonite is the controlling phase 

for magnesium and aluminum. Assuming conservation

of mass, a summary equation for the recharge area can 

be written to the predominant dissolved species 

using values of ap for each phase and balanced dissolution 

reaction under observed pH and redox conditions:

Sii. 906+0.79CaC03 +0.55CaMg(C03)2+0.10KAlSi308 

+4.68H20+3.52C02(g)+0.0602+0.13Si02+0.57NaCl

+0.22CaS04^

0.62Cao. 13K0. 08Al199Mgo.4oFeo. 1oSi3. 6501o(OH)2 

+0.19FeOOH+0.60C02(aq)+1.95Ca+2+0.59Mg+2 

+0.052K+ +0.97Na+ +0.57Cr+0.22S04~ 2+4.81HC03~

+0.72H4Si04 +0.240H~. (23)

Just as conservation of aluminum leads to no dissolved 

aluminum species appearing in the final summary equation, the oxidation of iron and its conservation in the solid 

phase also leads to no dissolved Fe+2 or Fe+3 appearing 

in the summary equation. The only aqueous species 

involving redox that explicitly appears in the equation is 

dissolved oxygen. Note that the iron-oxygen component 

of the redox reactions could be written in terms of oxide 

components, for example,

4FeO (augite)+02(aq)^2Fe203 (smectite) (24) 

or 

4FeO (augite) 02(aq)+2H20^4FeOOH (goethite). (25)

Analogously, similar reactions and mass transfer can 

be derived for the evolution of recharge waters in a felsic 

terrane using the corresponding mineralogy. The reactant and product phases used in the mafic and felsic 

models and the mass transfer derived by the models are 

listed in tables 3 and 4, respectively. Biotite and hornblende were used in the felsic model because they are 

common ferromagnesian minerals in granites in the 

study area. Table 4 shows that, in the evolution of the 

felsic water, smaller amounts of plagioclase and ferromagnesian minerals are weathered and slightly greater 

amounts of carbonate minerals are dissolved. This difference in weathering would be in agreement with weathering rates of the respective silicate minerals; sodic 

plagioclase and biotite show greater stability in the 

weathering environment than do the more calcic plagioclase and augite.

The chemical compositions of the reactant silicate 

minerals are consistent with the compositions of their 

respective rock types. The mass transfer associated with 

the primary minerals is expressed in oxide compositions
CHEMICAL EVOLUTION OF THE GROUND WATER C31

TABLE 3. Phases used for derivation of mass transfer

Reactant phases Stoichiometry

Calcite....................... CaC03

Dolomite. .................... CaMg(C03)2

Oligoclase .................... Ca0 25Nao ^Al^ 25Si2 7508

V-^ciQ 30 0 70"^^ 1 30 2 TO ^*^8

A ViriOCmO 1*Q "Wd A 1 Ql ll

Ca0 ^Nao.ssAlj^ 45bi2_5508 

Potassium feldspar. .......... KAlSi308

An o*ii"o I^Q TV/Trr tVd A 1 Si 11

Hornblende .................. CaMgFeAlSi3 5OnOH

Biotite ....................... KMg! 5Fe: 5AlSi3010(OH)2

Silica (volcanic glass, quartz). Si02 

Halite. ....................... NaCl

Gypsum (anhydrite). ......... CaS04

Fluorite. ..................... CaF2

Aqueous phase. .............. H20, C02, 02

Product phases Stoichiometry

Calcite....................... CaC03o

TVTnT"ii"TYinTrillnYTii"£i (Yf\£if\f*\ C*m T^ TV/To* TH^O A 1

Si3.65Oio(OH)2 

Montmorillonite (felsic) ...... Ca0 ioKo i3Mg0 40Fe0 loA^ 99

Si3.650 10(OH)2 

Kaolinite. .................... Al4Si4010(OH)8

Goethite ..................... FeOOH

Silica (chalcedony) ........... SiOo

Aqueous phase. .............. Ca , Mg+2, Na+ , K+ , Al+3 ,

Fe +3, OP, S04 ~2, F-, 

HC(V, H4Si04

in table 5. Chemical analyses of the bulk-rock fraction of 

the basin fill from Vekol Valley and the Bosque basin in 

Gila Bend Plain are shown in table 6. The chemical 

composition of the derived phases in table 5 are in 

general agreement with the composition of the basin fill 

from Vekol Valley, a basin bounded by basaltic and 

andesitic rocks, and from the Bosque basin, a basin 

surrounded by granitic intrusive rocks. Published average compositions for three rock types also are given in 

table 6. The minerals and the rock types derived from the 

modeling indicate reactions of water with an andesite and 

with a granodiorite or a monzonite. The computed oxide 

compositions for the felsic model shown in table 6 are 

normalized to 20 percent additional quartz because granites contain 25 percent free quartz and granodiorites and 

quartz diorites contain about 20 percent free quartz 

(Earth, 1967, p. 56). The normalized compositions agree 

better with published data, as the quartz component of 

the rock is not represented in the water composition 

owing to its low solubility. 

The chemical compositions of the model-derived mineral phases indicate that aluminum is about 3 to 5 percent 

higher and that potassium is slightly lower than in the 

field-measured and reported rock types. The recharge 

water does not appear to contain the potassium concentrations that would be expected from the weathering of

TABLE 4. Summary of mass transfer for recharge waters 

[Values* in millimoles per kilogram H20. Dashes indicate phase not used]

pl Basalt- Granite

andesite Hornblende Biotite

02 .............................. 0.06 0.05 0.05

C02 ............................. 3.52 3.36 3.19

Calcite**....,................... .79 .86 1.06

Dolomite** ..................... .55 .59 .58

Avirlr\ci-ivir\/'Avi ^ 7Q

Oliorni^lijcof An ^ &A. &A.

Potassium feldspar. ............. .10 .12 .11

Augite.......................... .36 

Hornblende ..................... .19 

Biotite.......................... .13

Silica ........................... .13 .01 .19

Halite........................... .57 .57 .57

Gypsum......................... .22 .22 .22

Montmorillonite (mafic) ......... -.62 

Montmorillonite (felsic) ......... -.49 .46

Goethite ........................ -.19 -.14 -.15

*Reactants are positive and products are negative. 

**Computed as a single calcium magnesium phase.

most primary rocks. The smaller potassium concentrations may be explained by little breakdown of the potassium feldspars relative to the plagioclase feldspars as 

suggested by Garrels and MacKenzie (1967) for the 

genesis of spring waters. More likely, however, from 

evidence presented later, the smaller concentrations 

probably reflect adsorption of solute potassium by smectite clays rather than differential weathering. Reasons 

for the apparently higher aluminum content are discussed later. 

The maximum, minimum, and average saturation 

indices of the ground water with respect to a number of 

minerals observed or postulated within the aquifer or 

unsaturated zone are given in table 7. The ground water 

is undersaturated with respect to albite and anorthite, 

which are the end members of the plagioclase series, and

TABLE 5. Major oxides calculated from derived mineral phases 

[In weight percent]

Basalt- Granite

andesite Hornblende Biotite

CaO ............. 8.42 7.04 *5.63 3.10 *2.48

Na20 ............ 3.95 4.85 3.88 5.15 4.12

MgO............. 3.69 2.96 2.36 3.22 2.58

FeO**........... 5.74 5.28 4.22 5.75 4.60

A1203 ............ 19.92 19.40 15.52 19.13 15.30

Si02 ............. 56.77 58.29 66.64 59.02 67.22

K20.............. 1.50 2.18 1.75 4.63 3.71

*Normalized to additional 20 percent quartz. 

**A11 iron reported as FeO.
C32 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

TABLE 6. Chemical analyses of basin fill from Vekol Valley and the Basque basin and average

compositions of selected rock types 

[In weight percent. Analyses by Center for Meteorite Studies, Arizona State University]

CaO ..............

N^O. ............

MgO ..............

Fe203tt ..........

Al O

Si02 . .............

K20. ..............

Vekol Valley* 

(D-9-l)13bbd

6 84

. . . . . 2.62

. . . . . 3.97

. . . . . 4.95

. . . . . 14.27

. . . . . 63.80

. . . . . 3.55

Bosque Basin** 

(C-6-3)2acd

3.38

2.76

1.82

5.70

13.85

68.66

3.83

Effusive 

andesitet

8.13

3.77

4.47

9.21

17.63

55.65

1.14

Plutonic 

granodioritet

3.61

3.90

1.59

3.98

15.90

67.90

3.12

Plutonic 

monzonitef

6.92

3.59

3.76

7.32

16.97

56.65

4.79

*Average values of four stratigraphic intervals.

**Average values of seven stratigraphic intervals. 

tFrom Earth, 1967, p. 58. 

ttAll iron reported as Fe203.

with potassium feldspar (adularia). Plagioclase and probably potassium feldspar occur in the basin fill only as 

primary minerals. The ground water is highly undersaturated with respect to the ferromagnesian minerals 

diopside and clinoenstatite (pyroxenes), tremolite 

(amphibole), and forsterite and the minerals gypsum and 

halite. As expected, these minerals generally are not 

observed in the basin fill. The water is supersaturated 

with respect to the clay minerals montmorillonite, kaoTABLE 7. Maximum, minimum, and average saturation indices of

the recharge waters with respect to various mineral phases

[Phases and thermodynamic data from Ball and others (1980)]

Phases

Number

of Maximum Minimum Average 

samples

Adularia. 

Albite ...

Anorthite ...........

Beidellite. ...........

Boehmite. ..........

Calcite ..............

Chalcedony. .........

Clinoenstatite .......

Diopside. ............

Dolomite ............

Fluorite .............

Forsterite ..........

Goethite. ............

Gypsum .............

Halite ..............

Kaolinite ............

K-mica .............

Laumontite. .........

Leonhardite .........

Montmorillonite CA , Montmorillonite BF . Montmorillonite AB . Phillipsite ...........

Sepiolite(c) . ..........

Tremolite ..........

0.033

-1.247

-1.097

-2.604

3.299

.721

.455

.434

-2.495

-2.008 

.769

-1.460

-7.523 

5.494

-1.516

-7.691 

3.055 

1.005 

2.012

11.755

3.175

4.859

3.768

.087

-1.316 

2.649

-0.418

-1.816

-1.382

-3.094

2.174

.433

-.067 

.190

-3.908

-4.469

-.724

-2.619

-10.210 

5.112

-3.786

-8.688

2.194

.204

1.304

10.339 

2.054 

4.267 

3.294

-.316

-4.337

-6.885

-0.187

-1.544

-1.251

-2.850

2.866

.562

.129

.326

-3.413

-3.543

-.225

-2.025

-9.256 

5.324

-2.408

-8.001

2.681

.544

1.805

11.367 

2.726 

4.615 

3.638

-.073

-3.310

-3.400

linite, and K-mica (muscovite), the zeolites laumontite 

and leonhardite, and the aluminum and iron oxide minerals boehmite and goethite. The solution is at equilibrium with respect to calcite and dolomite. Zeolites are 

rarely present in the basin fill; clinoptilolite was detected 

in two samples. Muscovite, a low-temperature primary 

mineral common to igneous and metamorphic rocks, is 

ubiquitous in the clay and bulk-rock fraction and appears 

to weather very little or not at all. The most stable phase 

is montmorillonite, as suggested by its abundance in the 

basin fill as well as its saturation state.

In considering the thermodynamics of the saturation 

state of the ground water with respect to the clay 

minerals, a problem arises with the dissolved-aluminum 

data. To define the saturation state accurately, the 

concentrations in solution of all elements present in the 

minerals must be known. In examining aluminum concentrations in stream waters, Jones and others (1974) 

determined that dissolved-aluminum concentrations 

would have to be less than 0.5 jxg/L for the waters to be 

in equilibrium with kaolinite or montmorillonite. The 

average concentration of dissolved aluminum determined 

for the study area was considerably higher, about 10 

jxg/L. These aluminum concentrations may be the result 

of colloidal or particulate clay that passes through the 

0.1-jxm pore-size filter and may not represent dissolved 

molecular aluminum. Acidification of samples by standard procedures prior to analysis would transfer all the 

particulates to the aqueous phase. The supersaturation 

of the clay minerals may be caused by the large aluminum concentrations as well as a supersaturated solution.

Inclusive in the derivation of the primary mineral 

phases and the mass balance of magnesium are considerations of the isotopic composition of the solution. The 

reactions and mass transfer shown in equation 23 and in 

table 4 satisfy both stoichiometric and isotopic requirements. In determining the reactions in the recharge 

area, it is assumed that the dissolution of the carbonate
CHEMICAL EVOLUTION OF THE GROUND WATER C33

minerals is congruent and that isotopic equilibrium does 

not occur between the C02(g) in the soil zone and the 

bicarbonate in solution at depth. The isotopic composition of the total inorganic carbon in solution is estimated 

to be 12.5%o on the basis of 813C values of samples 

taken near recharge areas. Using the isotope massbalance equation (eq. 11) and molality values determined 

by the mass transfer and a 813C value of -4.0%o for the 

solid carbon species, a 813C value of \l%o for the C02 

gas [813C02(g)~813C02(aq)] was determined for the 

mafic model and a 813C value of -18%o for the felsic 

model. A predicted 813C value of -17 to -18%c for the 

C02 gas is in agreement with reported values. The 

general agreement between the selected mineral phases, 

the derived 813C isotopic composition of the solution, and 

the measured or reported 813C values suggest that the 

reactions are indeed congruent and that the reactions of 

carbon during recharge involve little fractionation.

The reactions that describe the carbon chemistry 

during recharge apply to mountain-front recharge and 

may not apply to recharge along streams. In the alluvial 

aquifers of basins along the Colorado River, for example, 

where water levels are shallow, approximately 10 ft 

below land surface, the aquifer may be open to soil C02 . 

The higher and more variable bicarbonate values 

observed in these shallow systems would be consistent 

with a large reservoir of C02 in the soil zone. The 

bicarbonate concentrations thus would be a function of a 

constant Pco2 and the availability of calcite for dissolution:

CaCOo+H20+C02^Ca+2+2HCO (26)

Under these open conditions, the soil C02(g) may be in 

isotopic equilibrium with the dissolved bicarbonate in the 

aquifer and lead to an enrichment of 813C of the bicarbonate in solution to a similar value of about -12%o 

(Deines and others, 1974).

The derived chemical and isotope mass balance for the 

recharge waters is based on an assumed 813C value of 

-4%o for the solid carbonate phase and implies that the 

isotopic composition of dolomite is the same as that of 

calcite. Although the carbon mass transfer would not be 

affected by the choice of either calcite or dolomite as 

reactants, the derived mass transfer of the other phases 

is altered substantially if the isotope content of dolomite 

or calcite differs significantly from -4.0%o. Most of the 

available carbonate isotope analyses are of caliche, a 

layered, sparingly soluble calcium carbonate deposit 

commonly found in desert soils. Until more is known 

about the 813C of the carbonates as well as all carbon 

phases, little more can be said regarding the carbonate 

evolution of the ground water. Because phases other 

than dolomite could be considered sources of magnesium 

(for example, palygorskite was strongly indicated in the

unsaturated zone in Vekol Valley), evidence for the 

selection of dolomite will be presented.

The XRD analyses of the basin fill provide conclusive 

evidence that dolomite as well as calcite is an abundant 

mineral. Dolomite was confirmed at varying depths in 

the fine-sand (200-|xm) fraction of the unsaturated zone 

but was not confirmed in the saturated zone (table 8). 

XRD analyses of soil samples show only calcite present in 

the upper few inches but dolomite present in small to 

moderate amounts at 60 in (fig. 13). Dolomite was 

detected at depths between 70 and 215 ft below land 

surface in the unsaturated zone of West Salt River 

Valley near the White Tank Mountains and, surprisingly, 

was detected at a depth of 101 ft near the center of the 

basin at about a 10:1 ratio over calcite. The XRD 

patterns of the basin fill are shown in figure 13. The 

analyses indicate that dolomite may be ubiquitous in the 

unsaturated zone; dolomite appears to be partially 

leached in the unsaturated zone but totally leached in the 

saturated zone.

Dolomite has been reported by other researchers in 

similar geologic environments. Barnes and O'Neil (1971) 

found magnesian calcites and dolomites in Holocene 

conglomerate cements and travertines of the Coast 

Range, with solid solutions ranging from Ca0 51Mg0 49 to 

CaC03 for calcite and Ca0 58Mg0 42C03 to Ca0 51Mg0 49 

C03 for dolomites. On the basis of 180 and MgC03 

content, they believed that the carbonates were direct 

precipitates and not magnesium substitutions of calcite. 

They also observed that such mechanisms as capillary 

pumping as well as complete evaporation may result in a 

constant chemical environment suitable for the formation 

of calcium magnesium carbonates.

TABLE 8. Distribution of calcite and dolomite in the unsaturated

zone 

[X-ray diffraction analyses by David M. Hendricks, University of Arizona]

Well location Depth below 

land surface

Amount of 

calcite

Amount of 

dolomite

(D-19-l)8ddd ................

Great Plain near San Simon 

Wash basin 

(A-6-5)31bcc. ................

Paradise Valley adjacent to 

McDowell Mountains 

(C-15-2)19bab. ...............

San Simon Wash area 

(B-2-l)5abc2. ................

Center of West Salt River 

Valley 

(B-2-2)34bcb. ................

West Salt River Valley 

near White Tank 

Mountains 

(D-16-25)4cdd. ...............

Willcox basin near playa

170 feet

20 feet

110 feet

190 feet 

101 feet

70 feet

50 feet 

175 feet 

215 feet

60 inches

Moderate 

Small

Moderate 

Moderate 

Small

Large

Small

Small

Large 

Moderate 

Trace

Moderate
C34 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

Calcite 3.03 

Dolomite 2.90

Calcite 2.28

Dolomite 2.1 

Calcite 2.09A

D

Calcite 

2.28

Dolomite

Calcite \ 2.90 

2.09A

70 FEET 215 FEET

Calcite 3.03 

Dolomite 2.89\

Calcite 

2.09A

150 FEET

Calcite 3.03 

Dolomite? 2.90

Calcite 2.09A

Calcite 3.86

60 50 40 30

175 FEET

20 10 0 60

26, IN DEGREES

50 40 30 20

60 INCHES

FIGURE 13. X-ray diffraction patterns of 200-micrometer fractions of basin fill from the unsaturated zone. Samples A-D are from 

well (B-2-2)34bcb, sample E is from well (C-15-2)19bab, and sample F is a soil sample from well (D-16-25)4cdd. Depth of 

sample, in feet or inches below land surface, given beneath figure.

Analyses of springs also provide indirect evidence of 

the existence of a magnesium carbonate phase. Springs 

that emerge near recharge areas commonly have compositions similar to recharge waters. Concentrations of 

major ions in spring water fall well within the standard 

deviation for the recharge waters, with the exception of

silica, whose concentration in spring water is only about 

30 percent as large as in the recharge waters. The small 

silica concentrations indicate that the weathering of 

silicates is less important, even though similar reactions 

may be occurring in recharge and spring waters, 

accounting for the calcium, magnesium, and bicarbonate
CHEMICAL EVOLUTION OF THE GROUND WATER C35

concentrations. Spring discharge waters are often controlled by fault or fracture systems, which may provide a 

direct route from recharge to discharge, diminishing 

residence and reaction times. The walls of fracture 

systems may become coated with precipitates, which also 

can inhibit the reactions of the silicates.

MOVEMENT OF WATER AND REACTIONS IN THE 

UNSATURATED ZONE

The compositions of runoff and precipitation entering 

the soil and unsaturated zones in recharge areas are 

unknown, but the compositions of streams that reflect 

surface runoff provide some information. Analyses of 

four streams sampled near their sources are shown in 

table 9. The samples taken during high discharge represent runoff, and those taken during low discharge probably include a large base-flow component. Important 

observations are that calcium and magnesium are the 

major cations, that the calcium to magnesium molar 

ratios range from about 60:40 to 67:33, and that calcium 

and magnesium are in solution as carbonates, as indicated by the small sulfate and chloride concentrations. 

Calcium and magnesium carbonates seem readily available in runoff waters. The high magnesium to calcium 

ratio of the water relative to the magnesium to calcium 

ratio of the source rocks may be due to greater alteration 

of the less stable ferromagnesian minerals relative to 

plagioclase. A slight alteration of ferromagnesian minerals could account for the observed compositions when 

considering the large volumes of exposed rocks within 

the drainage areas. Miller and Drever (1977) found 

similar calcium to magnesium ratios in streams draining 

the Absoroka Mountains in Wyoming, which they attributed to slight alteration of large volumes of rocks.

The streams listed in table 9 drain large areas and thus 

may contain larger solute concentration than does runoff 

that recharges the basin aquifers. The runoff presumably

TABLE 9. Chemical analyses of selected streams in Arizona

[In millimoles per liter except as indicated; ft3/s, cubic feet per second; °C, degrees Celsius; mg/L, milligrams per liter. Dashes indicate no data. Data from

U.S. Geological Survey, 1980]

would drain smaller areas and would not experience deep 

percolation or long residence times. The runoff shown in 

table 9 contains an average bicarbonate concentration of 

about 1 mmol/L, which suggests that most of the bicarbonate concentrations are acquired through dissolution 

of carbonates after infiltration.

The mechanism by which C02(g) enters and is transported through the unsaturated zone has not been 

defined. C02 is undoubtedly transported in the aqueous 

phase through the unsaturated zone via deep percolation, but other mechanisms seem possible. Thorstenson 

and others (1983), in work in the western Great Plains, 

determined that diffusion of gaseous species of C02 is an 

important transport mechanism in the unsaturated zone. 

Movement by diffusion of C02 of more than 50 ft to the 

water table was observed. Because little is known about 

movement of gas or liquid in arid climates where depths 

to water are on the order of 600 to 800 ft, CO2 movement 

is assumed for modeling purposes to occur in the aqueous 

phase via deep percolation during recharge by a series 

of pulses or slugs, probably along permeable preferential vertical flow paths. At some point during recharge, 

the system may become closed to C02 and other gases; 

the exceptionally great depths to water may preclude 

direct interaction of soil C02 with carbonate minerals in 

the aquifer.

At equilibrium, the high partial pressure of C02 in the 

soil zone provides dissolved C02 (carbonic acid) for the 

weathering process. The average C02 pressure of the 

recharge water is about 10~ 1-7, which is considerably 

higher than atmospheric (~10~3'5). The relation between 

PC02 and dissolved C02 is shown by the equation

C02(g)+H2O^H2C03(aq), (27)

and because H2C03(aq) is an acid, it disassociates and 

forms hydrogen and bicarbonate ions:

(28)

Station name

Verde River near ....

Clarksdale

Oak Creek near ......

Sedona

Burro Creek above . .

Boulder Creek

near Bagdad

Burro Creek at ......

U.S. 93 near

Bagdad

Date 

sampled

3-13-79

11-14-79

5-9-78

1-17-79

5-10-79

3-14-79

.. 4^-79

6-12-79

7-12-79

1-17-79

5-9-79

11-15-78

Discharge

(fts/s)

1,130

149

80

*2,500

43

*500

76

5.0

1.9

14,400

11

225

TT Temperature pH £C)

7.6

7.5

8.3

_

8.5

8.1

8.2

8.3

_

7.6

8.7

10.0

9.0

17.0

4.5

11.0

5.5

15.5

20.5

28.0

7.0

18.0

11.0

Dissolved 

oxygen 

(mg/L)

9.7

11.6

10.4

11.7

9.3

11.2

8.7

7.6

8.3

12.0

7.9

11.5

Calcium (Ca+2)

0.37

.75

1.40

.22

1.15

.27

.65

.77

.60

.30

1.50

.57

Magnesium Sodium 

(Mg+2) (Na+)

0.23

.38

.95

.19

.66

.19

.45

.74

.70

.15

1.48

.40

0.22

.43

1.00

.08

.20

.09

1.04

2.47

2.95

.24

2.04

.78

Potassium (K+)

0.03

.07

.06

.02

.03

.01

.11

.19

.22

.05

.18

.09

Bicarbonate Sulfate 

(HCOg) (SOJ2)

1.13

2.13

4.91

.80

2.79

.80

2.86

1.01

1.56

0.05

.09

.13

.05

.05

.06

.15

.32

.45

.06

1.24

.35

Chloride (cr)

0.021

.42

.28

.03

.03

.04

.18

.76

1.02

.05

.87

.48

Fluoride (F-)

0.01

.01

.01

.00

.01

.00

.01

.03

.04

.01

.05

.03

Silica 

(Si02)

0.23

.23

.25

.16

.22

.20

.60

.47

.56

.04

.47

.60

*Estimated.
C36 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

The hydrogen ion, which provides the impetus for the 

weathering process, dissolves carbonate minerals and 

attacks the lattice structure of the silicates, releasing 

cations and silica. The aggressiveness of dissolved C02 is 

indicated by the noticeable absence of caliche in some of 

the arroyos and washes near the mountains.

EVOLUTION OF THE GROUND WATER IN 

SELECTED BASINS

Chemical models were developed along general flow 

paths for selected basins using compositions observed in 

the recharge areas as initial water compositions and 

compositions observed downgradient as final water compositions. The final water composition is that of the 

farthest downgradient point, normally the center or 

downvalley part of the basin. Reactant mineral phases 

used in the models were chosen to be consistent with the 

basin geology. Compositions of reactant and product 

phases were idealized and were used consistently within 

and among basins to confirm the presence or absence of 

similar reactions. Reactions also were used consistently 

among basins, although a particular reaction, for example, ion exchange, may not pertain to all basins. The 

chemical models are described by basin, rather than by 

individual reaction simultaneously for all basins, to provide a clear picture of evolution in each basin.

The basins were selected to illustrate climatic and 

physiographic differences or distinctive geohydrologic 

and geochemical characteristics (fig. 14). Vekol Valley 

(southern part) was selected as typical of the mediumsized oval basins in arid south-central Arizona where 

sufficient well data and test holes were available for 

analysis. Lower San Pedro Valley was selected as an 

excellent example of the long, narrow basins in the 

southeast where the alluvial aquifer is confined and 

flowing wells exist and where distinct flow paths can be 

delineated. Ranegras Plain in the western arid region 

has a markedly different geochemistry exemplified by 

large concentrations of dissolved solids and occurrences 

of trace elements in the ground water. McMullen Valley 

is one of the few basins in the study area bounded by 

intrusive felsic rocks. The Willcox basin is typical of the 

large oval basins in southeastern Arizona where precipitation and ground-water recharge are relatively high 

and much of the annual recharge may occur along the 

valley floor. A generic model for basins along the Colorado River (Yuma area in fig. 14) is presented as an 

alternative to previously published explanations for the 

genesis of the ground water underlying the flood plain.

Some basins for which a large amount of well and 

chemical data have been collected, such as basins in the 

Phoenix, Tucson, and Yuma areas, were not selected for 

modeling. These basins have complex hydrologic systems and have been extensively developed, which substantially altered the hydrologic and chemical systems, 

thus making the modeling interpretations difficult 

or impossible.

The recharge areas of the basins were defined as those 

areas in basins adjacent to mountains receiving the 

highest annual precipitation. Hydrologic information 

from previously published studies was also considered. 

Analytical data for selected wells in the modeled basins 

are summarized in table 10.

The chemical models are divided into two fundamental 

types closed systems and open systems. Closed systems are those that do not receive additional recharge 

along the flow path after the initial recharge or are not 

influenced by liquid or gas phases other than those 

introduced during initial recharge. In closed systems the 

aqueous chemistry is determined solely by the reactions 

of the initial recharge water with the various minerals as 

it moves downgradient. Conversely, open systems are 

those whose aquifer chemistry is in part controlled or 

influenced by atmospheric gases or liquids that enter the 

system along flow paths subsequent to initial recharge. 

In the development of basin models it will be shown that 

the chemistry of most of the basins evolves under closed 

conditions.

VEKOL VALLEY

The southern part of Vekol Valley is an oval basin 

about 14 mi long and 13 mi wide (measured at the 

mountain front) (fig. 15); it is typical in shape and geology 

of many basins in arid south-central Arizona. The ground 

water contains low dissolved solids, about 400 mg/L, 

which also is typical of many basins in the area. The basin 

is bounded on the east by schist, granite, and andesite of 

the Table Top and Vekol Mountains and on the west by 

andesite, basalt, and schist of the Sand Tank Mountains 

(Wilson and others, 1969).

Lithologic and geophysical logs of several wells 

throughout the basin show that the alluvial aquifer 

consists of several hundred feet of sand and gravel 

overlain throughout much of the basin by a thick silt-clay 

unit. In ascending order, the basin fill in the central part 

of the basin consists of about 500 ft of silty sand, gravel, 

and conglomerate; 950 ft of sandy gravel; 500-550 ft of 

silt and clay; and 50-100 ft of sand and gravel. The depth 

to water in the central part is between 400 and 450 ft 

below land surface (Wilson, 1979). The major recharge 

area in the basin is adjacent to the Sand Tank Mountains, 

which are the dominant mountain range in the area and 

receive the highest precipitation. Ground-water flow is 

eastward toward the center of the basin and eventually 

southward out of the basin. A buried ridge to the north 

forms a ground-water divide between this south basin
115°

CHEMICAL EVOLUTION OF THE GROUND WATER 

113° 111°

C37

109°

NEVADA

Las Vegas

UTAH

ARIZONA

0\o^

! Durangoo

I COLORADO

* O*. '<*<*,

35°

(, \ ( ,^M %_ 7

 ^* Wl Ivr^-1

II Ji" _ \/ /-^V*1

-Flagstaff

^

>jn^y * £ X^. ,-f« v m/¥ f u^^K, "Jr«- A_..-^^ / I . \ >*^ .T 

.,.KJ\ ) : .-^ X

/ /Z--^J -: - *^M\\\ \ Ki,-^ ) '' v ^ /

X

-GGallup

NEW MEXICO

Silver 

'oCtty

31

GULF OF 

CAL/FORMA

4-V4-

100 MILES

_I

0 50 100 KILOMETERS 

EXPLANATION

BASIN FOR WHICH DATA WERE COLLECTED

BASIN FOR WHICH DATA WERE COLLECTED AND 

-£ MODELED-1, Vekol Valley; 2, lower San Pedro 

Valley; 3, Ranegras Plain; 4, McMullen Valley; 

5, Willcox basin; 6, Yuma area (Colorado River)

 " BASIN BOUNDARY 

FIGURE 14. Basins for which data were collected and for which models were developed.
C38 EEGIONAL AQUIFEE-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

TABLE 10. Analytical data for selected wells in Vekol Valley, lower San Pedro Valley, Ranegras Plain, McMutten Valley, and the Willcox

basin

[In millimoles per liter except as indicated; °C, degrees Celsius; meq/L, milliequivalents per liter; mg/L, milligrams per liter. Vek, Vekol Valley; LSP, lower San Pedro 

Valley; Ran, Ranegras Plain; McM, McMullen Valley; Wil, Willcox basin; well names keyed to figures. Dashes indicate no data]

Well 

location

(D-S-l)31cbd ....

(D-9-l)lldbdl . . .

(D-9-l)13bbd ....

(D-ll-18)3cdc. . . .

(D-S-17)32daa . . .

(B-4-15)14caa. . . .

(B-4-15)18bbc . . .

(B-4-15)18bbb . . .

(B-6-16)23ddc . . .

(B-6-13)25bbb . . .

(B-5-13)lcba

(B-7-8)17bbc ....

(B^8-9)32baa ....

(D-17-27)31ddd . .

(D-17-26)25bad . .

(D-16-26)9daa . . .

(D-16-25)4cdd . . .

(D-15-25)27cdd . .

- Sft >" '

Vekl 715 7.4

Vek 2 1,738 8.0

Vek 3 1,975 8.0

LSP1 387 8.3

LSP 2 1,485 8.4

Rani 900 8.4

Ran 2 870 8.1

Ran 3 1,005 7.7

Ran 4 1,100 7.7

McMl 930 7.8

McM 2 730 8.2

McM 3 1,896 7.9

McM 4 1,003 9.4

Wil 1 555 8.0

Wil 2 1,020 8.4

Wil 3 460 8.0

Wil 4 550 7.9

Wil 5 1,100 7.8

"emperature (°C)

36.5

37.0

36.5

26.5

42.0

34.2

27.0

29.0

26.0

33.0

31.0

30.0

33.5

22.0

22.0

24.5

22.5

26.5

Field 

alkalinity 

(meq/L)

3.560

2.083

1.917

2.373

1.822

.359

1.442

.951

1.978

3.048

2.491

2.398

1.645

2.065

1.999

2.032

1.606

2.491

Ca+2 Mg+2

0.748 0.576

.325 .066

.324 .091

.232 .045

.245 .002

6.736 .033

.674 .128

6.487 .202

8.483 3.292

.574 .300

.185 .074

.325 .247

.035 .000

.424 .074

.375 .066

.824 .095

1.248 .103

.724 .050

K+ Na+

0.043 2.436

.079 5.220

.168 5.655

.046 2.784

.064 5.655

.069 13.050

.094 7.395

.113 8.700

.306 41.304

.082 3.828

.046 4.785

.066 2.306

.013 4.785

.026 1.392

.028 1.871

.064 1.740

.066 1.958

.082 2.219

cr

0.592

1.918

2.200

.310

1.241

3.667

3.384

3.949

22.568

1.185

1.326

.395

1.382

.150

.223

.279

1.016

.451

SOJ2

0.271

.750

.988

.198

1.457

11.451

1.770

8.120

19.779

.468

.541

.219

.469

.035

.136

.635

1.145

.500

F~

0.037

.032

.026

.105

.417

.526

.279

.190

.234

.074

.111

.253

.200

.126

.232

.116

.153

.068

Si02

0.799

.699

.732

.583

.566

.266

.649

.533

.433

.399

.349

.399

.399

.666

.570

.466

.516

.665

Log?CO2*

 1.98

-2.80

-2.88

-3.09

-3.26

-4.69

-3.20

-2.97

-2.82

-2.50

-3.00

-2.68

-4.12

-2.92

-3.48

-2.43

-2.99

-2.61

NOg-N

0.286

.543

.593

.114

.023

.357

.328

1.356

.228

.257

.114

.221

.034

.079

.001

.058

.043

Dissolved 

solids 

(mg/L)

324

421

524

223

417

1,830

649

1,530

4,325

338

343

218

156

162

183

239

327

259

*Calculated.

and a north basin. An estimated 4.6 million acre-ft of 

recoverable ground water is in storage in the upper 1,500 

ft of the aquifer. Annual recharge of an estimated 370 

acre-ft indicates long residence time of the water in 

storage (Matlock, 1981).

Reactions were modeled along a general flow path 

from the recharge area about 2 mi downgradient to Vek 

1 (Vekol 1), 7 mi downgradient to Vek 2, and 8 mi 

downgradient to Vek 3 near the center of the basin (fig. 

15). XRD analyses of the bulk-rock fraction of the basin 

fill at Vek 3 indicated that labradorite, bytownite, and 

andesine are present throughout the column, with substantial albite and oligoclase between 1,400 and 1,900 ft. 

Andesine, augite, and potassium feldspar were selected 

as the primary reactant minerals. The changes in 

ground-water chemistry as a function of distance along 

the flow path are illustrated in figure 16. The phases and 

mass transfer associated with these changes are shown in 

table 11.

On the basis of the water composition and selected 

phases, hydrolysis of augite and andesine and precipitation of calcite are the dominant reactions occurring 

between recharge and Vek 1 (reaction 1). In contrast to 

the recharge area, calcite precipitates downgradient. 

Calcium decreases early along the flow path, primarily 

because of calcite precipitation but partly because of 

montmorillonite formation. A significant amount of magnesium, 0.42 mmol, is removed from solution by montmorillonite formation, but presumably it is replenished 

by weathering of ferromagnesian minerals, whose presence is suggested by an adjacent basalt outcrop. Sodium, 

sulfate, chloride, and pH increase downgradient between

Vek 1, 2, and 3 (reactions 2 and 3). Alkaline earths are 

almost wholly depleted downgradient, and bicarbonate 

decreases to nearly one-third that of the recharge water 

(reaction 4). Potassium and fluoride show little change 

downgradient, but fluoride concentrations increase from 

recharge to require an additional phase, fluorite. Ion 

exchange of calcium replacing sodium on the substrate, 

the most likely exchange reaction, accounts for a portion 

of the calcium and sodium concentrations. Ion exchange 

and its implication are discussed in detail in the section 

on "Ion Exchange," where it is postulated that ion 

exchange does not occur in dilute waters.

The large mass transfer of the ferromagnesian mineral 

and the large concentration of magnesium in the ground 

water at Vek 1 point out two significant weathering 

traits: (1) the high weathering rates of the ferromagnesian minerals in the upgradient portion of the aquifer and 

(2) the overall high chemical aggressiveness occurring in 

the aquifer upgradient. C02(aq) decreases from an average concentration of about 0.60 mmol in the recharge 

area to 0.27 mmol upgradient at Vek 1 to 0.00 mmol 

downgradient at Vek 3. The decrease in C02(aq) and the 

high but decreasing weathering rates of the silicates 

indicate that carbonic acid plays the predominant role in 

the weathering process and that the residence time of the 

ground water is less critical. C02(aq) decreases downgradient as hydrogen ions are consumed, and carbonic 

acid disassociates to form bicarbonate ions that are 

subsequently precipitated as calcite in response to 

increases in pH and temperature. The high chemical 

reactivity of the water and the instability of the ferromagnesian minerals are indicated by the mass transfer of
CHEMICAL EVOLUTION OF THE GROUND WATER C39

112°30'

saw

15'

32°30'

Base from U.S. Geological Survey 

State base map,1974,1: 500,000

10 MILES

_J

-70-

0 10 KILOMETERS 

EXPLANATION

ALLUVIUM 

BASALTIC ROCKS 

INTERMEDIATE VOLCANIC ROCKS

GRANITIC AND METAMORPHIC ROCKS, LOCALLY 

SEDIMENTARY ROCKS

LINE OF EQUAL AVERAGE ANNUAL PRECIPITATION, 

1930-60-Interval, in inches, is variable

DIRECTION OF GROUND-WATER MOVEMENT

»Vek 1 WELL- Vekl is well identifier 

....... GROUND-WATER DIVIDE

FIGURE 15. Generalized geology, wells, and ground-water movement 

in Vekol Valley.

the silicates from recharge to Vek 1; approximately 2.1 

mmol of andesine and 0.9 mmol of augite are weathered 

over the initial distance of 1.8 mi, whereas 4.5 mmol of 

andesine and 1.2 mmol of augite are weathered over the 

total distance of about 8 mi.

Silicate hydrolysis increases the pH of the water from 

about 7 in the recharge area to 8 downgradient in spite of 

calcite precipitation of several millimoles. The precipitation of calcite is an acid-producing reaction and in a sense

can be considered a reverse weathering reaction as C02 

and H+ are produced (Drever, 1982); it can be written in 

the reverse of reactions shown in equations 14 and 15. 

The acid reactions are neutralized by hydroxyl ions 

produced by silicate hydrolysis per equations 18, 19, 

and 20.

The summary reaction written to the predominant 

dissolved species from the recharge area along the flow 

path to Vek 3 is

3.67Ca0.45Na0. 55Al1 .45Si2. 5508 

+0.84Ca0.4Mg0.8Fe0. 7AV2Sii.906 +0.35KAlSi308

+ 1.81H20+0.60C02(aq)+0.1502

+ 1.63NaCl+0.77CaS04+0.01CaF2+1.02Naex

+ 1.95Ca+2+0.97Na+ +0.59Mg+2 +0.52K+ +4.81HC03

+0.57CP +0.22S04 +0.01F-+0.72H4Si04-»3.49CaC03

+2.93Ca0. 13K0. 08Mg0.40Fe0. 10Al1. 99Si3. 65010(OH)2 

+0.30FeOOH+1.29Si02+0.51Caex+0.32Ca+2+5.65Na+ 

+0.09Mg+2+0.17K+ + 1.92HC03~ +2.20CT +0.99S04 2 

+0.03F" +0.73H4Si04+0.310H~. (29)

Conservation of electrons as well as of mass is maintained along the flow path from recharge to Vek 3. 

Assuming that iron is Fe+2 in augite and Fe+B in 

montmorillonite and that dissolved oxygen is the oxidant, the oxidation of iron and the hydrolysis of about 

0.80 mmol of augite along the flow path can be written to 

oxygen and water:

0.80Ca0.4Mg0.8Fe0.7Al0.2Si1 . 906+0.1402+3.73H20 

-^0.31Ca+2+0.61Mg+2+1.23H4Si04

+0.08montmorillonite(mafic)+0.55FeOOH 

+ 1.830R-. (30)

The electron-balanced equation shows that 0.14 mmol of 

oxygen is being consumed, which, with an initial concentration of about 9 mg/L (0.28 mmol/L) extrapolated from 

data in figure 7, would deplete dissolved oxygen to about 

4.5 mg/L at Vek 3 (well E in fig. 7), close to the measured 

concentration of 4.3 mg/L.

The higher weathering rate of the ferromagnesian 

minerals relative to the plagioclase is supported by the 

XRD analysis of the basin fill at Vek 3. XRD analyses 

show that plagioclase is present in trace to small amounts 

in the less than 2-(xm fraction and, along with quartz, 

calcite, and potassium feldspar, is dominant in the less 

than 62-(xm fraction. Amphiboles and pyroxenes were 

not detected in the less than 2-(xm fraction and only 

rarely in the less than 62-(xm fraction. The decrease in
C40 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

0.2

LU

^JCC 

O LU i*

_l CC

^ LU 

^ D0 12345678

MILES DOWNGRADIENT

FIGURE 16. Changes in concentration of dissolved chemical constituents along the flow path in Vekol Valley. (Well locations

shown in fig. 15.)

the weathering of mafic minerals relative to that of 

plagioclase may explain why the existing ground water 

tends to maintain its oxidizing character for an indefinite 

period.

The saturation indices of the ground water with 

respect to selected mineral phases are shown in table 12. 

The saturation state of the ground water is consistent 

with the weathering and precipitation reactions and with 

the minerals observed and postulated in the basin fill. 

Along the flow path, the ground water maintains equilibrium with respect to calcite but becomes increasingly

undersaturated with respect to dolomite. The water 

becomes slightly less undersaturated but is still highly 

undersaturated with respect to the ferromagnesian minerals clinoenstatite and diopside. The water is saturated 

with respect to potassium feldspar and, as determined in 

the recharge area, is supersaturated with respect to clay 

minerals, iron oxides, zeolites, and quartz minerals. Poor 

aluminum data again may be the cause of the apparent 

supersaturation of the clay minerals. The water appears 

to be near equilibrium with respect to muscovite. Muscovite is present in the less than 62-jxm and less than
CHEMICAL EVOLUTION OF THE GROUND WATER C41

TABLE 11. Summary of mass transfer for Vekol Valley

[Values* in millimoles per kilogram H20; well names keyed to fig. 15; Vek, 

Vekol Valley]

Keaction number and flow path

Phases

Kecharge 

to Vek 1

2

Vek 1 to 

Vek 2

3

Vek 2 to 

Vek 3

4

Kecharge 

to Vek 3

02 .....................

C02 ....................

Calcite.................

Andesine (An45) .......

Potassium feldspar.....

Augite.................

Silica ..................

Halite..................

Gypsum................

Fluorite................

Montmorillonite (mafic) 

Goethite ...............

Ion exchange**........

0.09 

.33

-1.58

1.33

.06

.51

-.63 

.02 

.05 

.01

-1.06

-.25 

.35

0.05 

.25

-1.73 

2.33

.17 

.28

-.44

1.33

.48

.00

-1.82

-.02 

.09

0.01 

.02

-.18 

.01 

.09 

.05

-.17 

.28 

.24 

.00

-.04

-.02 

.08

0.15 

.60

-3.49

3.67

.32

.84

-1.24

1.63

.77

.01

-2.92

-.29 

.52

*Keactants are positive and products are negative.

**Calcium replacing sodium on the substrate.

2-fjim fractions of the basin fill and appears to be an inert 

mineral downgradient as well as in the recharge area. 

Apparently, owing to the stability of muscovite, any 

formation of a new mineral may not involve breakdown of 

the lattice structure but may involve only ionic substitution in the original sheet structure.

The trilinear plot in figure 17 illustrates the evolution 

of the recharge water from the calcium and bicarbonate 

apexes toward the sodium and chloride sulfate apexes. 

The plot indicates that early along the flow path, as

interpreted from the small sulfate and chloride concentrations, the ground water evolves into an intermediate 

sodium magnesium calcium bicarbonate type or a sodium 

calcium magnesium bicarbonate type owing to enrichment of magnesium relative to calcium. The trend is an 

apparent result of the concurrent reactions of calcite 

precipitation and the weathering rates of the ferromagnesian minerals relative to plagioclase as determined by 

the modeling.

The trends in chemistry in Vekol Valley depicted in 

figures 16 and 17 are observed in all dilute ground waters 

that evolve under closed conditions throughout the Basin 

and Range lowlands province. The similarities are immediately apparent in figure 18, which shows chemical 

trends for Butler, McMullen, lower San Pedro, and San 

Simon Valleys and the Big Sandy and Safford basins. 

These chemical trends need not be invariably similar and 

may differ markedly in some basins. Modeling of lower 

San Pedro Valley, McMullen Valley, and Ranegras Plain 

will demonstrate that regardless of differences in water 

chemistry, the reactions and mass transfer of many 

phases are indeed virtually invariant.

LOWER SAN PEDRO VALLEY

Lower San Pedro Valley is a long, narrow, northwesttrending basin in semiarid southeastern Arizona where 

ground water occurs under confined conditions. The 

basin is structurally and hydrologically typical of several 

elongated basins in the southeastern and northwestern 

parts of the study area, some of which contain major

TABLE 12. Saturation indices of the ground water in Vekol Valley with respect to various minerals

[Vek, Vekol Valley; well locations shown in fig. 15. Phases and thermodynamic data from Ball and others (1980). Dashes

indicate values not calculated]

Saturation indices for basin

Phases

Adularia ...............

Albite. .................

Beidellite ..............

Calcite .................

Chalcedony ............

Clinoenstatite..........

Diopside ...............

Dolomite ...............

Fluorite. ...............

Goethite ...............

Gypsum. ...............

Halite. .................

K-mica. ................

Montmorillonite CA. . . . 

Montmorillonite BF. . . . Montmorillonite AB. . . . 

Tremolite ..............

Vekl

-0.057 

.298 

-2.502 

-2.316 

-.121 

-1.451

-2.237 

-7.525

Vek 2

-.088 

.201 

-2.856 

-1.414 

-1.838 

-2.185

-2.477 

-6.566

Vek 3

0.489 

-.422 

1.49 

-.158 

.254 

-2.183 

-1.243 

-.759 

-2.122 

6.868 

-2.054 

-6.596 

0.015 

1.308 

6.044 

5.352 

4.298

Number 

of 

samples

3 

3 

3 

11 

11 

2 

8 

11 

11

8

3 

3 

2

4 

1

Maximum

0.489 

-.422 

.27 

.238 

.341 

-1.721 

-1.243 

.053 

-.402

-1.986

.110 

1.904 

6.044 

5.352 

4.298

Minimum

-0.027 

-.929 

-.57 

-.158 

.133 

-3.334 

-3.352 

-1.838 

-2.185

-3.792

-.468 

.888 

5.161 

4.573 

4.298

Average

0.160 

-.693 

.397 

.039 

.224 

-2.615 

-2.217 

-.345 

-1.506

-2.411

-.114 

1.367 

5.602 

4.962 

4.298
C42 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

°o

CHLORIDE

.\\

V

PERCENT 

EXPLANATION 

COMPOSITION OF RECHARGE WATER

COMPOSITION ALONG FLOW PATH

 1 COMPOSITION OF Vek 1 

FIGURE 17. Relative compositions of ground water in Vekol Valley, in equivalent percents. (Well locations shown in fig. 15.)

stream systems. The valley is 65 mi long and 15 to 20 mi 

wide and extends from the Narrows to Winkelman, 

where the San Pedro River joins the Gila River (fig. 19). 

The basin is bounded on the west by granite of the 

Tortilla Mountains and by gneiss and granite of the Santa 

Catalina Mountains, and on the east wholly by intermediate and silicic volcanic rocks of the Galiuro Mountains

(Wilson and others, 1969). The basin fill consists mainly 

of lower basin-fill deposits; most of the upper basin fill 

has been removed by late Cenozoic uplift and valley 

erosion. The aquifer consists of coarse-grained deposits 

of the lower basin fill overlain by a fine-grained confining 

unit. In a large area extending about 20 mi from Redington to Mammoth, the ground water occurs under
CHEMICAL EVOLUTION OF THE GROUND WATER C43

confined conditions at depths of 300 ft near Redington to 

1,400 ft near Mammoth. Artesian pressure is sufficient 

for wells to flow at rates of about 10 to 500 gal/min in this 

area. Confined conditions are known to exist an additional 10 mi north of Mammoth, but no flowing wells are 

reported. The flood-plain deposits along the San Pedro 

River and the stream alluvium along its tributaries 

consist of mixtures of sand, gravel, and silt and also 

constitute an important local aquifer (Roeske and 

Werrell, 1973).

Ground water in lower San Pedro Valley, as indicated 

by water-level data, is recharged by precipitation and 

runoff along the Galiuro, Tortilla, and Santa Catalina 

Mountains and moves toward the basin axis (Roeske and 

Werrell, 1973). The water chemistry clearly indicates 

that the ground water of the confined aquifer originates 

near the Galiuro Mountains and not in the Santa Catalina 

Mountains on the west side of the valley. The similarity 

in concentrations of major elements and the large fluoride concentrations of the geochemically closed confined 

aquifer and wells sampled between the San Pedro River 

and the Galiuro Mountains are cited as evidence. These 

waters stand out in contrast to the different major 

element chemistry, the lower ground-water temperatures, and the small fluoride concentrations of the open 

system draining the Santa Catalina Mountains. The 

confined aquifer offers an excellent opportunity to define 

the mass transfer along a distinct flow path of known 

origin.

Reactions were modeled along two flow paths from the 

Galiuro Mountains to the axis of the basin. One flow path 

is from the mountains to well LSP 1 (lower San Pedro 1) 

upvalley at the beginning and shallowest part of the 

confined aquifer. The other flow path is from the mountains to well LSP 2 (well M in fig. 7) downvalley at the 

deepest point. LSP 1, near Redington, is 387 ft deep and 

flows at about 15 gal/min. The top of the aquifer is at 370 

ft and is overlain by about 360 ft of fine-grained sediments. The ground water is highly dilute, indicating an 

absence of the gypsiferous deposits within the aquifer 

found elsewhere in the basin. The recharge area is about 

7 to 8 mi eastward, normal to the axis of the valley. LSP 

2, near Mammoth, is 1,485 ft deep and flows at about 50 

gal/min. The flow path of 7 to 8 mi is similar in distance 

to the flow path to LSP 1, but the aquifer is situated 

considerably deeper, and the ground water is higher in 

dissolved solids. The lithologic log of LSP 2 indicates that 

the top of the aquifer is at about 1,370 ft below land 

surface, underlying fine-grained gypsiferous and red clay 

deposits. Examination of the red clay reported by drillers in the area shows that the clay is entirely of "water 

worn particles of volcanic rocks" (Halpenny and others, 

1952, p. 89). The chemistries along both flow paths are 

shown in figure 20.

The reactant phases were selected through a review of 

the published literature (Creasey, 1965; Krieger, 1979, 

1980) on the mineralogy of the Galiuro Mountains. Biotite 

and hornblende are both found in the rhyolitic tuffs. 

Krieger (1979) did not detect biotite in 22 of 62 samples 

of the tuffs but did detect trace amounts in the remaining 

samples. Hornblende was detected in 15 of the samples; 

some was visible in hand specimens. Hornblende was 

selected because it is the more common ferromagnesian 

mineral in andesite, the other dominant rock type of 

the Galiuro Mountains, and in granodiorite and diabase 

found throughout the valley (Creasey, 1965). The phases 

and the mass transfer along the flow paths are shown in 

table 13.

Reaction 1 in table 13 shows that the mass transfer of 

most phases along the flow path from the recharge area 

to LSP 1 is similar to the mass transfer derived for Vekol 

Valley. The mass transfer of the silicates, in particular, is 

markedly similar. The major differences in water chemistries between the two basins can be attributed to the 

dissolution of the soluble components of halite and gypsum. Ion exchange of calcium for sodium again the most 

likely exchange reaction does not appear to be occurring and is interpreted to be a result of the dilute waters.

Reaction 2 describes the mass transfer along a similar 

flow path from the Galiuro Mountains to LSP 2, located 

approximately 17 mi downvalley. Reactant phases used 

in the modeling of LSP 1 were selected for LSP 2 on an 

assumed homogeneity of the geology of the Galiuro 

Mountains. The mass transfer of most phases along the 

flow path from the recharge area to LSP 2 (reaction 2) is 

similar to that determined near the origin of the aquifer 

system near Redington (reaction 1). Significant differences lie only in the greater mass transfer of gypsum 

and, consequently, the presumed presence of ionexchange reactions. The similarity in the mass transfer 

of the silicates supports the assumed homogeneity of the 

source rock and the transferability of the reactions. A 

slightly greater mass transfer of andesine (0.18 mmol) 

and hornblende (0.01 mmol) may be due to greater 

hydrolysis as a result of deep circulation and higher 

temperature (42 °C). Similarly, the greater amount of 

calcite precipitated (0.57 mmol) also may be related to 

these variables, particularly temperature.

Reactions 2 and 3 suggest that ion exchange of calcium 

for sodium is occurring. The analytical data in table 10 

show that the ground water in the downvalley well (not 

downgradient), well LSP 2, contains appreciably more 

sodium than that in the upvalley well, well LSP 1 (5.65 

and 2.78 mmol, respectively). The increase in sodium 

could be interpreted to be a result of additional weathering reactions because the difference in mass transfer of 

halite between the two waters is smaller than the 

difference in molar sodium concentrations and because
C44 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

6.5

CO

LU

IS o 1 

I

K+ ____________ 

0 1

F"

1 1 1

K+

, F2345678012345678

(

-» 

^ D

MILES DOWNGRADIENT MILES DOWNGRADIENT

FIGURE 18. 

A

Changes in concentration of dissolved chemical constituents along flow paths in selected basins in the Basin and Range lowlands. 

Butler Valley; B, McMullen Valley; C, lower San Pedro Valley; and D, San Simon Valley. (Continued on next page.)

the downvalley well has the larger concentrations. The 

general agreement of the silicate mass transfer accompanied by the mass transfer of halite and gypsum derived 

between the final water compositions in the upvalley and 

downvalley wells implies that differences in sodium 

concentrations are best explained by ion exchange and 

not by continued hydrolysis.

Reaction 4 shows the mass transfer derived to LSP 1 

using biotite, the other pertinent ferromagnesian mineral in the Galiuro volcanic complex, as a reactant. The 

use of biotite as an alternate source of magnesium results 

in significant ion exchange of sodium exchanging for 

calcium. Ion exchange of monovalent sodium for divalent 

calcium in the dilute waters is unlikely, and reaction 4 is 

not a realistic alternative.

The hydrolysis of hornblende and the oxidation of iron 

are written analogously to the hydrolysis and oxidation 

of augite. The hydrolysis of hornblende along both flow
CHEMICAL EVOLUTION OF THE GROUND WATER C45

O

<J -1 

X DC 4

(JO

CD 

<J

E 0.2 

-I DC

__ O LU

L. | 5 o.i

^ -I DC * W

S QZ 0

2 3 4 5 e 

£

MILES DOWNGRADIENT

8 0 2 3 4 5 e

F 

MILES DOWNGRADIENT

FIGURE 18. Changes in concentration of dissolved chemical constituents along flow paths in selected basins in the Basin and Range lowlands.

E, the Big Sandy basin; and F, the Safford basin.

paths is approximately 0.48 mmol, which results in 

consumption of 0.12 mmol of dissolved oxygen:

0.48CaMgFeSis. 5011OH+0.1202+2.66H20

-^0.46Ca+2 +0.38Mg+2 +0,80H4Si04 

+0.24 montmorillonite(felsic)+0.46FeOOH

+ 1.660H". (31)

The saturation state of the ground water with respect 

to most minerals (table 14) is in general agreement with 

the derived mass transfer. The ground water is undersaturated with respect to dolomite, plagioclase, the 

ferromagnesian minerals, and, in contrast to the ground 

water in Vekol Valley, to potassium feldspar. The 

ground water is slightly supersaturated with respect to 

chalcedony and is highly supersaturated with respect to 

goethite and most of the clay minerals. The water is 

undersaturated at LSP 1 but saturated at LSP 2 with 

respect to fluorite. Fluoride concentrations in the artesian aquifer of lower San Pedro Valley are some of the 

largest in the study area.

Surprisingly absent from the ground water are the 

large silica concentrations normally associated with the 

weathering of welded tuffs and silicic volcanic rocks. 

Analyses of ground water from volcanic and welded tuff 

terranes in Nevada show that the ground water contains 

considerably larger silica concentrations (Winograd,

1971; White, 1979). Concentrations in wells sampled in 

the artesian aquifer along the San Pedro River and 

between the river and the Galiuro Mountains do not 

indicate large silica contents but are comparable to 

concentrations found in other terranes. The reasons for 

these differences are unclear.

The nearly identical mass transfer of the silicates 

determined along the two flow paths 17 mi apart and the 

negligible mass transfer for the flow downvalley clearly 

show that no reactions are occurring and suggest that 

little or no water is moving downvalley. Precipitation 

apparently recharges the aquifer near the Galiuro Mountains, moves toward the center of the basin, and discharges near the San Pedro River. The presence of a 

dense mesquite forest along the axis of the basin is 

indicative of large evapotranspiration losses. Measurements of 14C specific activity and of tritium strongly 

support the modeling conclusions of little downvalley 

movement of water. Specific activity is a measure of the 

extent of the radioactive decay of 14C, the radiogenic 

isotope of carbon, and of any addition of nonradiogenic 

carbon; it is expressed as a percent of the 14C that has not 

undergone any radioactive decay, usually considered to 

be 100 PMC (percent modern carbon) (Fontes and Garnier, 1979). The 14C activity of about 20 PMC for the 

tritium-free waters and similar 13C content at LSP 1 and 

LSP 2 would give nearly identical corrected ages under
C46 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

451 301 15' 110W llfOO1 

331001

32°00'L

Base from U.S. Geological Survey 

State base map,1974,1:500,000

10 MILES

0 10 KILOMETERS 

EXPLANATION

ALLUVIUM

INTERMEDIATE AND SILICIC 

VOLCANIC ROCKS

SEDIMENTARY ROCKS

GRANITIC AND METAMORPHIC 

ROCKS

-20 LINE OF EQUAL AVERAGE ANNUAL 

PRECIPITATION, 1930-60-Interval 

in inches, is variable

-»- DIRECTION OF GROUND-WATER 

MOVEMENT

LSP1, WELL-LSP1 is well identifier

FIGURE 19. Generalized geology, wells, and ground-water movement in lower San Pedro Valley.
CHEMICAL EVOLUTION OF THE GROUND WATER C47

11

<0

t- CM 

Q. Q. 

CO CO

cc

UJ

UJ 

Q_

UJ

O

3

O

(75

 « 

O O

O

co 

O

LSP1 

 LSP2

246

MILES DOWNGRADIENT

246

MILES DOWNGRADIENT

FIGURE 20. Comparison of ground-water compositions along the flow paths in lower San Pedro Valley. (Well locations shown

in fig. 19.)

presumed identical correction factors indicated by the 

mass transfer. Ground-water movement in the lower San 

Pedro Valley formerly was characterized as moving from 

the mountains toward the basin axis and downvalley 

(Halpenny and others, 1952; Roeske and Werrell, 1973), 

a direction that is not supported by the geochemical 

model.

The similarity in the mass transfer of the silicate 

phases in the water-table aquifer of Vekol Valley and the 

confined aquifer of lower San Pedro Valley (mass transfer of plagioclase for the four downgradient wells is 3.52 

±0.19 mmol) suggests that the aquifers are similar 

closed systems. The agreement clearly indicates that in 

Vekol Valley ground water is being recharged along the 

mountain front and virtually no precipitation or runoff 

recharges the aquifer along the valley floor. As demonstrated later in modeling the Willcox basin, satisfactory 

mass transfer cannot be derived for the evolution of the

ground water if the system is open to C02 and additional 

recharge.

RANEGRAS PLAIN

Ranegras Plain is a moderately large basin about 50 mi 

long and 20 mi wide in arid western Arizona that contains 

ground water of relatively high salinity (fig. 21). The 

rock types of the mountains surrounding the basin are 

similar to those in Vekol Valley; unlike Vekol Valley, 

however, the basin fill contains evaporites near the 

surface that are responsible for the high salinity (table 

10). The basin is bounded on the east by basalt and 

granite of the Little Harquahala Mountains and granite 

of the Granite Wash Mountains; on the south by basalt 

and andesite of the Little Horn, Eagle Tail, and Kofa 

Mountains; and on the west and north by andesite of the 

Plomosa and New Water Mountains and andesite and 

granite of the Bouse Hills (Wilson and others, 1969).
C48 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

TABLE 13. Summary of mass transfer for lower San Pedro Valley

[Values* in millimoles per kilogram H20; LSP, lower San Pedro Valley; well 

names keyed to fig. 19. Dashes indicate phase not used]

Reaction number and flow path

Phases

Recharge 

to LSP 1

Recharge 

to LSP 2

3

LSP1 

to LSP 2

4

Recharge 

to LSP 1

C02 ....................

Calcite .................

Andesine (An33) .......

Potassium feldspar.....

Hornblende ............

Biotite .................

Silica...................

Halite..................

Gypsum................

Fluorite................

Montmorillonite (felsic). 

Goethite................

Ion exchange**.........

0.12 

.57

-3.01

3.16

.32

.46

-1.99 

.00 

.00 

.05

-2.50

-.20 

.00

0.12 

.59

-3.58

3.34

.36

.47

-2.13

.67

1.24

.20

-2.65

-.21

0.00 

.02

-.57 

.18 

.04 

.01

-.14 

.67

1.24 

.15

-.15

-.01

0.17 

.57

-3.01

3.92

.39

.45

-2.01 

.00 

.00 

.05

-3.04

-.37

-.28

*Reactants are positive and products are negative.

**Calcium replacing sodium on the substrate.

The aquifer throughout most of the basin consists of 

sand and gravel of the lower basin fill that are partially 

overlain by an areally extensive fine-grained unit. The 

fine-grained unit extends to the west from the basin 

center midway to the Bear Hills, to the north nearly to 

the Bouse Hills, to the east to the basalt flows adjacent 

to the Little Harquahala Mountains, and to the south for 

an undetermined distance probably covering much of the 

southern part of the basin. The fine-grained unit is about

500 to 600 ft thick in the central part of the basin near 

Bear Hills and contains evaporites in the lower part. 

Predevelopment and present (1981) depths to water in 

the center of the basin were about 120 ft.

The major recharge area for the basin is adjacent to 

the Little Harquahala and Granite Wash Mountains 

owing to the greater precipitation relative to that in 

adjacent mountains. The Plomosa and New Water Mountains to the west, although smaller in area and lower in 

altitude, receive significant precipitation and probably 

also contribute recharge. The Kofa Mountains in the 

southwest also may contribute significant recharge to the 

basin, although much of the area drains to the south. 

Briggs (1969) indicated that water movement is from 

southeast to northwest throughout the basin, but owing 

to the scarcity of water-level data for the southern part 

of the basin and the flatness of the potentiometric surface 

determined from available water-level data, areas of 

recharge and the direction of ground-water movement 

are poorly defined. Indeed, the 8D and 8180 data suggest 

that some ground water may be moving to the south. An 

estimated 10 million acre-ft of water is in storage, and 

average annual recharge to the basin is estimated to be 

500 to 1,500 acre-ft (Briggs, 1969).

The ground water throughout much of the basin is not 

suitable for all purposes. The water contains dissolvedsolids concentrations greater than 3,000 mg/L and large 

concentrations of trace elements. Arsenic, boron, chromium, fluoride, and selenium occur naturally and commonly exceed the maximum contaminant level for these 

constituents. The ground water evolves from a sodium

TABLE 14. Saturation indices of the ground water in lower San Pedro Valley with respect to various

minerals

[LSP, lower San Pedro Valley; well locations shown in fig. 19. Phases and thermodynamic data from Ball and others (1980).

Dashes indicate values not calculated]

Saturation indices for basin

Phases

Adularia .....................

Albite. .......................

Beidellite ....................

Calcite.......................

Chalcedony ..................

Clinoenstatite. ...............

Diopside .....................

Dolomite. ....................

Fluorite. .....................

Goethite .....................

Gypsum. .....................

Halite. .......................

K-mica. ......................

Montmorillonite CA. .........

Montmorillonite BF. .........

Montmorillonite AB. .........

Tremolite ....................

LSPl

...... 0.185

...... -.586

...... 1.572

...... -.022

...... .262

...... -2.490

...... -1.511

...... -.691

...... -.860

...... 6.464

...... -2.765

...... -7.708

...... -.270

...... 1.389

4 864

...... 4.265

...... 3.402

LSP 2

-1.186 

-1.614

.154 

.064 

-2.849 

-1.243 

-1.564 

-.132

-2.036 

-6.857 

-3.866

-1.828

1.823

Number 

of 

samples

4 

4 

4 

8 

8 

2

8 

8 

3 

8

4 

3 

2 

1 

2

Maximum

0.487 

.295 

1.572 

.321 

.315 

-2.490

.532 

.459 

6.801 

-1.689

-.043 

1.512 

4.864 

3.958 

3.402

Minimum

-1.186 

-1.614 

-.574 

-.022 

.064 

-2.849

-1.564 

-.860 

5.623 

-2.765

-3.866 

-1.828 

4.517 

3.958 

1.823

Average

-0.449 

-1.101 

.635 

.126 

.213 

-2.670

-.889 

-.290 

6.296 

-2.226

-2.447 

-.358 

4.691 

3.958 

2.613
CHEMICAL EVOLUTION OF THE GROUND WATER 

114°00' 50' 40' 30'

C49

113020'

33°20'

Base from U.S. Geological Survey 

State base map,1974,1:500,000

10 MILES

I ' ' '' I

0 10 KILOMETERS 

EXPLANATION

ALLUVIUM

BASALTIC ROCKS 

INTERMEDIATE VOLCANIC ROCKS

GRANITIC AND METAMORPHIC ROCKS, 

LOCALLY SEDIMENTARY ROCKS

-10 LINE OF EQUAL AVERAGE ANNUAL PRECIPITATION, 

1930-60-Interval, in inches, is variable

-*-. DIRECTION OF GROUND-WATER MOVEMENT

Ran 1 WELL-Ran 1 is well identifier

FIGURE 21. Generalized geology, wells, and ground-water movement in Ranegras Plain.
C50 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

°o * jS>

CHLORIDE

PERCENT

EXPLANATION

COMPOSITION OF RECHARGE WATER

i\

\

"^ __ > COMPOSITION ALONG FLOW PATH -" ~V

 1 COMPOSITION OF Ran 1 

FIGURE 22. Relative compositions of ground water in Ranegras Plain, in equivalent percents. (Well locations shown in fig. 21.)

mixed-anion type near the basin margins, where 

dissolved-solids concentrations range between 500 and 

1,000 mg/L, to a sodium chloride sulfate or sodium 

sulfate chloride type toward the basin interior, where 

concentrations range between 1,000 and 3,000 mg/L. The 

trilinear plot in figure 22 shows the evolutionary path of 

the ground water from a bicarbonate type to chloride and

sulfate types. Mesozoic sedimentary rocks that crop out 

near the Harcuvar and Granite Wash Mountains and 

Mesozoic and Paleozoic sedimentary rocks in the Little 

Harquahala Mountains may be sources of the large 

sulfate and chloride concentrations.

Reactions were modeled along flow paths between the 

Little Harquahala and Granite Wash Mountains to Ran 1
CHEMICAL EVOLUTION OF THE GROUND WATER C51

(Ranegras 1), a well near the depositional center of the 

basin, and between the Plomosa and New Water Mountains to Ran 2 and 3 near the center of the basin. The 

selected paths probably are representative of the direction of flow; waters originating as recharge on both 

flanks move toward the center of the basin. Recent 

water-level and stable-isotope data obtained during 

this study support this direction of flow in the east 

recharge area.

The changes in ground-water chemistry downgradient 

from the east and west recharge areas to the center of 

the basin are shown in figure 23. The changes in composition along both flow paths are markedly different from 

the changes observed along the flow paths in Vekol 

Valley and lower San Pedro Valley. The compositions 

generally are higher in calcium, sodium, sulfate, and 

chloride and are lower in bicarbonate. In view of the 

large areal extent and diversity of physiography of the 

study area, differences in chemistry may be caused by 

several factors, such as (1) occurrence of additional 

reactions, (2) occurrence of similar reactions but with 

different minerals or mineral compositions, or (3) differences in the mass transfer of the silicates, halite, or 

gypsum.

The summary of mass transfer for Ranegras Plain is 

shown in table 15. The silicate phases used for Vekol 

Valley were adopted for Ranegras Plain because of the 

similarity in geology between the two basins, as the 

south-central areas of both basins are bounded by andesite and basalt. Reaction 1 shows that along the flow path 

from the east recharge area to Ran 1, identical reactions 

and virtually identical mass transfer of the silicate phases 

are occurring in Ranegras Plain and Vekol Valley, in 

spite of the considerable differences in water chemistry. 

Differences in chemistry to Ran 1 can be explained by 

simple dissolution of gypsum and halite and modification 

by an additional reaction, ion exchange. Calcium 

exchanging for sodium apparently is occurring in Ranegras Plain and thus is a highly significant reaction in 

controlling the sodium and calcium concentrations in the 

ground water. The calcium and sulfate concentrations 

and the additional ion-exchange reaction and its mediating effect on the chemistry are reflected in the trilinear 

plot (fig. 22). The orderly sequence of the evolution of 

dilute waters downgradient toward the sodium and sulfate apexes are not as apparent.

Figure 24 and reaction 2 in table 15 show the changes 

in chemistry and mass transfer along the flow path from 

the west recharge area to Ran 2. In spite of differences 

in chemistry between Ran 1 and Ran 2, the mass transfer 

of most phases at Ran 2 are again markedly similar to 

those determined along the flow path to Ran 1. The 

differences are attributed largely to a greater mass 

transfer of gypsum and calcite. The similarity of the mass

transfer of the silicates provides a decisive factor for the 

validity of transferability of reactions. The validity is 

based on the assumption that the kinetics of the silicates 

are invariant in ground water under identical hydrologic 

and temperature conditions and mineral environments. 

The total mass transfer of the aluminosilicates of 5.09 and 

5.01 mmol and the mass transfer of augite of 0.86 and 

0.93 mmol for Ran 1 and Ran 2, respectively, attest to 

the similarity and transferability of reactions within the 

basin.

The mass transfer derived along the flow path from the 

west recharge area to Ran 3 (reaction 3), however, is 

unlike that determined for Ran 2 and in the initial 

analysis does not support the transferability of reactions. 

Reaction 3 shows a distinct difference in the mass 

transfer of the silicates relative to Ran 1 or Ran 2 or to 

the mass transfer determined in Vekol and San Pedro 

Valleys. Reactions 2 and 3 show that, although Ran 2 and 

Ran 3 are close together about 500 ft apart and the 

same distance down the flow path, the mass transfer of 

the silicates is significantly greater at Ran 3 than at Ran 

2. The differences in water chemistry between the two 

wells are shown in figure 24. The differences between the 

two waters are the result of differences in well depths; 

Ran 2 is reported to be 870 ft deep and thus does not 

intercept the evaporites, as does Ran 3, which is 1,005 ft 

deep. Ran 3 has higher sodium, chloride, and sulfate, and 

significantly higher calcium, than the shallower well 

(table 10). Whereas the differences in these four dissolved constituents may be explained by dissolution of 

halite and gypsum, the differences in the silicate mass 

transfer cannot.

The poor agreement of the silicates between Ran 2 and 

3 may not lie in the difference in the mass transfer of 

halite and gypsum, or in ion-exchange reactions as 

determined earlier, but in the mass transfer of total 

carbon. The ground water is saturated with respect to 

calcite (SIcai=0.192) at Ran 3 but is slightly undersaturated with respect to calcite (SIcal=-0.537) at Ran 2 

(table 16). The difference in the saturation state of the 

water may affect the mass transfer of carbon by increasing precipitation of calcite. Reaction 4 shows the mass 

transfer derived for Ran 3, assuming equal precipitation 

of calcite for both wells. Values of 3.63 and 0.95 mmol for 

andesine and augite, respectively, at Ran 3 are in good 

agreement with 3.83 and 0.93 mmol at Ran 2. The 

difference between the two wells in total mass transfer 

for all silicate phases is 0.17 mmol; the small difference in 

the mass transfer of the individual silicate phases for 

reactions 2 and 4 may represent the uncertainty of the 

field and laboratory analytical data. Thus, the difference 

in chemistry and silicate mass transfer for the two wells 

may be explained if the differences in bicarbonate concentrations of the two wells are interpreted as additional
C52 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

DC CO

MILES DOWNGRADIENT

FIGURE 23. Changes in concentration of dissolved chemical constituents along the flow paths in Ranegras Plain. (Well

locations shown in fig. 21.)
CHEMICAL EVOLUTION OF THE GROUND WATER C53

TABLE 15. Summary of mass transfer for Ranegras Plain

[Values* in millimoles per kilogram H20; Ran, Ranegras Plain; well names 

keyed to fig. 21]

Reaction number and flow path

Phases

02 . .............

C02 . ............

Calcite..........

Andesine (An45) 

Potassium ......

feldspar 

Augite .......... o

Silica ...........

Halite. ..........

Gypsum. ........

Fluorite. ........

Montmorillonite . 

(mafic) 

Goethite ........

Ion exchange! . .

i

Recharge 

to Rani

. 0.15

. .60

. -5.05

. 3.99 

.24

. .86

. -1,61

. 3.10

. 11.24

.26

. -3.12 

. -.29

. 3.39

2 

Recharge 

to Ran 2

0.16 

.55 

-3.92 

3.83 

.25

.93 

-1.37 

2.82 

1.55 

.13 

-3.02

-.35

.75

3 

Recharge 

to Ran 3

0.21 

.57 

-4.43 

4.28 

.30

1.22 

-1.92 

3.38 

7.90 

.09 

-3.39

-.51 

.99

4 

Recharge 

to Ran 3**

0.17 

.57 

-3.92 

3.63 

.26

.95 

-1.55 

3.38 

7.90 

.09 

-2.87

-.38 

1.18

5 

Vek3 

to Ran 1

0.01 

.00 

-1.56 

.32

-.08

.02 

-.37 

1.47 

10.46 

.25 

-.19

.00

2.87

*Reactants are positive and products are negative.

**Calcite precipitation corrected for gypsum. 

tCalcium replacing sodium on the substrate.

calcite precipitation caused by the presence of gypsum 

and larger calcium concentrations in the lower interval.

In addition to the saturation data, the differing pH 

values and dissolved-oxygen content of the ground water 

strongly argue against the greater mass transfer. The 

pH of 7.7 of the water at Ran 3 is significantly lower than 

the pH of 8.1 at Ran 2 and is anomalously low for a 

downgradient well. A lower pH would be an expected 

result of greater calcite precipitation in a closed system. 

The increase in hydrolysis suggested by reaction 3 would 

increase the pH of the water at Ran 3, not lower it, and 

any additional weathering of the silicates would severely 

deplete the ground water of dissolved oxygen. The 

oxygen required for weathering of augite per reaction 3 

specified by similar reactions shown for Vekol Valley is 

7.0 mg/L, a depletion that was not observed at Ran 3, 

assuming an initial concentration of about 9 mg/L for the 

recharge waters. Using the mass transfer of augite of 

0.95 mmol derived by reaction 4, 5.4 mg/L, or 0.17 mmol, 

of dissolved oxygen would be removed from solution, 

which is in accord with measured values. Moreover, as 

discussed later, the larger increase in ion exchange of 

calcium replacing sodium is in accord with the larger 

sulfate concentrations. On the basis of the geology and 

the overall water chemistry within the basin and other 

basins, reaction 4, not reaction 3, is more suitable to 

explain the water chemistry.

The final water composition of Vekol Valley (Vek 3) 

and the higher salinity water of Ranegras Plain (Ran 1) 

are compared in figure 25 and illustrate the large difference in composition. The final waters of both basins are 

old, greater than 10,000 years, judged by flow distances, 

hydraulic gradients, and 14C ages. Reacting the final 

water composition in Vekol Valley with the final water 

composition in Ranegras Plain (reaction 5) gives only a 

slight difference in the mass transfer of the silicates. 

These differences become even smaller when comparing 

water from Vek 3 with that from Ran 2 or 3.

The close similarity of the mass transfer of the silicates 

among the three basins, supported by correspondingly 

similar saturation states of the ground water, is highly 

significant and suggests that (1) the geochemical models 

developed have identified the major reactions that are 

occurring in the basins and (2) the reactions are indeed 

transferable among basins. In spite of relatively large 

differences in the composition of all major dissolved 

species, the compositions can be related by similar 

reactions and mass transfer for most phases. The models 

have defined the nearly identical mass transfer for the 

silicate phases, the differing mass transfer for gypsum 

and halite, and an ion exchange of calcium for sodium.

McMULLEN VALLEY

McMullen Valley, a long, narrow basin in arid westcentral Arizona about 50 mi long and 15 mi wide, is one 

of the few basins in Arizona bounded chiefly by granitic 

rocks (fig. 26). The Harcuvar Mountains on the north 

consist of granite and gneiss, with granite outcropping at 

the southern and central parts. The Granite Wash Mountains south of the Harcuvar Mountains also consist of 

granite. The Harquahala Mountains to the south consist 

chiefly of gneiss, with granite outcroppings in the northcentral part. Locally, older metasediments, schist, and 

volcanic rocks are present (Wilson and others, 1969).

The valley fill has been divided into four units, in 

ascending order: (1) conglomerate, (2) sand and gravel 

deposits, (3) playa deposits, and (4) alluvium (Kam, 

1964). The conglomerate, a unit of low permeability, has 

not been encountered extensively by drilling; near 

Aguila, the unit has been penetrated at 850 ft. The 

second unit, probably lower basin fill, consists of 

unsorted clay, sand, and gravel but may contain beds of 

sorted sand and gravel that yield large amounts of water 

to wells. The coarse-grained facies of this unit forms the 

major aquifer of the basin. Overlying the sand and gravel 

deposits are playa deposits that consist of clay, silt, fine 

sand, and, locally, gypsum and halite. These fine-grained 

deposits extend from approximately midway between 

Wenden and Aguila to the south end of the basin. Near 

Wenden, the unit reaches its maximum thickness of 1,000 

ft and is described as a reddish-orange to pale-red 

clay-silt unit containing gypsum and halite. The unit 

decreases in thickness to the southwest. Overlying the
C54 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

If

^ o

UJ

o

8 4

'cT

c/) 

o

+ * 3

03

03

O

 Ran 2 

Ran 3

\

246

MILES DOWNGRADIENT

246

MILES DOWNGRADIENT

10.5

 10.0

C/)

CC 

< 

Q

1

FIGURE 24. Changes in concentration of dissolved chemical constituents along the flow path to wells Ranegras 2 and Ranegras 3. (Well

locations shown in fig. 21.)

playa deposits are clay, sand, and gravel of the alluvium, 

which for the most part lies above the water table.

Predevelopment depth to the water table at midbasin 

ranges from about 125 ft near Salome to 425 ft near 

Wenden. Recharge occurs primarily from runoff from 

the Harcuvar and Harquahala Mountains. Ground water

occurs chiefly under unconfined conditions (Kam, 1964). 

The ground water is characterized as a sodium bicarbonate or sodium mixed-anion type (fig. 27). In general, the 

quality is good and the water is suitable for most 

purposes; dissolved-solids concentrations generally 

range between 250 and 500 mg/L. As in many basins in
CHEMICAL EVOLUTION OF THE GROUND WATER C55

TABLE 16. Saturation indices of the ground water in Ranegras Plain with respect to various minerals

[Ran, Ranegras Plain; well locations shown in fig. 21. Phases and thermodynamic data from Ball and others (1980). Dashes

indicate values not calculated]

Saturation indices for basin

Phases Ran 1 Ran 2 Ran 3 Number

of Maximum Minimum Average 

samples

Adularia. ...............

Albite ..................

Calcite .................

Chalcedony. ............

Clinoenstatite ..........

Diopside. ...............

Dolomite ...............

Fluorite ................

Goethite. ...............

Gypsum ................

Halite ..................

K-mica .................

Montmorillonite CA

Montmorillonite BF . . . .

Montmorillonite AB

Tremolite ..............

. -0.832

. -1.010

.875

-.219

. -1.665

1.576

-.440

-.149

. -6.086

. -2.021

-.793

11.143

_

-.537

.659

-4.042

-4.032

-2.161

.724

-1.491

-6.212

-0.107

-.735

.192

.205

-3.101

-1.997

-1.030

.698

5.639

-.235

-6.204

-.396

1.401

4.845

4.016

.398

14

14

44

44

43

43

44

44

43

14

14

14

14

0.582

-.111

.368

.659

-1.701

-.448

.559

1.252

.070

1.654

2.869

5.249

4.495

-1.665

-2.075

-.537

-.196

-4.370

-4.743

-2.160

-1.660

-2.289

-2.860

-.802

3.142

2.471

-0.471

-1.314

-.142

.131

-2.890

-2.360

-.921

.044

-1.507

-.872

.732

4.434

3.633

the western part of the study area, however, fluoride or 

chromium locally may present a water-quality problem.

Nine reactions were derived to define the groundwater evolution along two flow paths within the basin 

(table 17). Six of the reactions define the mass transfer 

along a flow path in the lower, or southern, part of the 

valley between the Harcuvar Mountains to McM 1 

(McMullen 1) and to McM 2 in the center of the valley 

near Salome. Three of the reactions define the mass 

transfer in the upper, or northern, part between the 

Vulture Mountains to McM 3 and to McM 4 in the center 

of the valley west of Aguila. The water chemistry of the 

four wells is summarized in table 10, and the changes in 

chemistry along the flow paths in the southern and 

northern parts are shown in figures 28 and 29, respectively.

Multiple reactions were developed because of two 

possible sources of iron and magnesium (biotite and 

hornblende) and a suspected range in composition of 

oligoclase between An20 and An30 . Biotite and hornblende are associated with the calcic granites. Thinsection petrography shows that biotite is the dominant 

ferromagnesian mineral in the Harcuvar and Harquahala 

Mountains and that hornblende is dominant in the Granite Wash Mountains. Numerous diorite dikes containing 

hornblende are present in the Harcuvar and Harquahala 

Mountains.

In the southern part of the basin, four reactions were 

modeled using oligoclase An30 and biotite (reactions 1,2, 

3, and 6), one using oligoclase An25 and hornblende 

(reaction 4), and one using oligoclase An25 and biotite 

(reaction 5). Using ion-exchange constraints as a guide

for probable reactions, the reactions using An30 and 

biotite appear suitable with little exchange of calcium for 

sodium, about 0 to 0.1 mmol of calcium. This value is 

consistent with the concept of ion exchange not occurring 

in dilute waters; therefore, reactions 1, 2, 3, 5, and 6 

generally are more suitable to explain the water chemistry. Reaction 4 using hornblende shows that ion 

exchange increases to 0.3 mmol of calcium. On the basis 

of data presented later, reaction 4 is less favored because 

the increase in gypsum is less than 0.3 mmol between the 

initial and final waters and exchange should not occur. A 

reaction using oligoclase An30 and hornblende (not 

shown) was modeled but, like reaction 4, was not considered because of 0.41 mmol of calcium replacing sodium. A 

reaction using oligoclase An20 and biotite, a common 

assemblage for granite (not shown), was considered but 

was not realistic because of an improbable exchange of 

sodium replacing 0.56 mmol of calcium. Reactions 4 and 

5 indicate that the granitic rocks are slightly more calcic 

than true plutonic alkali granites. These ion-exchange 

constraints are admittedly somewhat arbitrary, particularly in view of the uncertainty of the compositions and 

presence of the minerals involved. However, reactions 1, 

2, 3, 5, and 6 are consistent with the observed geology 

and with the hypothesis regarding ion exchange developed throughout this report.

The mass transfer of hornblende and biotite for all 

reactions to McM 2 is between 0.39 and 0.47 mmol, with 

apparently little weathering of these minerals downgradient between McM 1 and McM 2. Like augite, the 

weathering of biotite and hornblende appears to be 

related to the regional flow system. For the oxidation of
C56 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

10.0

- 9.5

468

MILES DOWNGRADIENT

..->-- \\\v

468

MILES DOWNGRADIENT

6.0

FIGURE 25. Comparison of ground-water compositions in Vekol Valley and Ranegras Plain. (Well locations shown in figs. 15, 21.)

iron in 0.46 mmol of biotite, 0.17 mmol, or 5.5 mg/L, 

of oxygen is consumed, which would be in good agreement with measured values from the southern part of 

the valley. The total mass transfer of the oligoclase 

ranges from 3.29 to 4.13 mmol depending on the source 

mineral(s) for calcium. The total mass transfer of oligoclase An30 ranges between 3.93 and 3.97 mmol. For 

comparison purposes, reaction 6 shows the mass transfer 

from recharge to McM 2 with potassium included in 

biotite. With the exception of decreasing slightly the 

already small mass transfer of potassium feldspar, the 

other phases are not greatly affected, for example, the

small difference in the mass transfer cited for oligoclase 

An30.

Some differences in chemistries are seen between the 

ground water in McMullen Valley and that in Vekol 

Valley and Ranegras Plain. The concentrations of bicarbonate, calcium, and magnesium, which are typical of 

other wells sampled in the lower part of the valley, 

appear to decrease less downgradient than those in mafic 

basins, assuming identical composition of the recharge 

waters for both basin types. In accordance with the 

differences in chemistries, smaller mass transfer of the 

silicate minerals is derived for the felsic basin than for
CHEMICAL EVOLUTION OF THE GROUND WATER C57

112°45'

33*30'

Base from U.S. Geological Survey 

State base map,1974,1:500,000

10 MILES

McMI

0 10 KILOMETERS 

EXPLANATION

ALLUVIUM

INTERMEDIATE VOLCANIC ROCKS

SEDIMENTARY ROCKS

GRANITIC AND METAMORPHIC ROCKS

LINE OF EQUAL AVERAGE ANNUAL PRECIPITATION, 

1930-60-Interval, in inches, is variable

DIRECTION OF GROUND-WATER MOVEMENT 

WELL-McM 1 is well identifier

FIGURE 26. Generalized geology, wells, and ground-water movement in McMullen Valley.
C58 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

/ V \7 V V V

PERCENT 

EXPLANATION

fj COMPOSITION OF RECHARGE WATER

^ COMPOSITION ALONG FLOW PATH 

' V 

 1 COMPOSITION OF McM 1 

FIGURE 27. Relative compositions of ground water in McMullen Valley, in equivalent percents. (Well locations shown in fig. 26.)

the mafic basins. Total silicate mass transfer of about 4.1 

to 5.0 mmol for McMullen Valley is compared with 4.9 to 

5.8 mmol for the mafic basins. The difference may be 

due to the weathering rates of the lower temperature 

minerals.

On the basis of minerals selected and initial and final 

water compositions, about 4 mmol of oligoclase An25 to 

An30 and about 0.5 mmol of biotite with minor hornblende are weathered. The derived phases and weathering sequences agree with the subsurface geology of the 

area and, except for the Aberdeen and Belle Fourche 

montmorillonites, with the saturation state of the ground 

water (table 18). The lithologic log of well (B-6-12)15bbb 

near Wenden reveals that the coarse-grained facies 

underlying the reddish playa deposits consist of granite, 

gneiss, schist, and quartz fragments (Kam, 1964).
CHEMICAL EVOLUTION OF THE GROUND WATER C59

TABLE 17. Summary of mass transfer for McMullen Valley 

[Values* in millimoles per kilogram H20; McM, McMullen Valley; well names keyed to fig. 26. Dashes indicate phase not used]

Reaction number and flow path**

Phases

02 .. .........................

C02 . .........................

Calcite .......................

Oligoclase ( An25) .............

Oligoclase (An30) .............

Andesine (An45) .............

Potassium feldspar. ..........

Augite .......................

Hornblende ..................

Biotite .......................

Silica ........................

Halite. .......................

Gypsum. .....................

Fluorite. .....................

Montmorillonite (felsic) ......

Montmorillonite (mafic) ......

Goethite .....................

Ion exchangeft ..............

i

Recharge 

to McM 1

0.18

.51

-2.27

3.03

.34

.44

-2.19

.62

.25

03

-2.37

-.42

.05

2

McM 1 to 

McM 2

0.00 

.06 

-.62

.94 

.05

.02 

-.42 

.14 

.07 

.02 

-.65

.03

.08

3 

Recharge 

to McM 2

0.17 

.57 

-2.89

3.97 

.39

.46 

-2.61 

.76 

.32 

.05 

-3.02

-.39 

.13

4 

Recharge 

to McM 2

0.12 

.57 

-2.89 

3.29

.31

.47

-3.02 

.76 

.32 

.05 

-2.46

-.22 

.30

5 

Recharge 

to McM 2

0.17 

.57 

-2.89 

4.13

.39

.46 

-3.25 

.76 

.32 

.05 

-3.02

-.39 

-.02

6 

Recharge 

to McM 2

0.15 

.57 

-2.89

3.93 

.04

t.39 

-2.02 

.76 

.32 

.05 

-2.74

-.31 

.15

7 

Recharge 

to McM 3

0.18 

.57 

-2.98

3.48 

.37

.49 

-2.42 

.00 

.01 

.12 

-2.70

-.47 

-.46

8 

Recharge 

to McM 3

0.12

.57 

-2.98

2.77 

.17 

.66

-1.22 

.00 

.01 

.12

-2.17 

-.24 

.00

9 

Recharge 

to McM 4

0.17 

.60 

-3.76

4.24 

.19 

.90

-1.46 

.82 

.25 

.09

-3.27 

-.30 

.33

*Reactants are positive and products are negative.

**Reactions 1 through 6 refer to lower basin, 7 through 9 to upper basin.

tPotassium included in biotite.

ttCalcium replacing sodium on the substrate.

In modeling reactions in the upper part of the basin, 

phases used in the lower part were considered probable 

phases on an assumed homogeneous geology of the 

Harquahala and Harcuvar Mountains throughout the 

basin. Reaction 7, which defines the mass transfer along 

the flow path from recharge areas to McM 3 using 

oligoclase An30 and biotite, is considered unlikely, however, because of an unrealistically large exchange of 0.92 

mmol of sodium for calcium. A reaction with substitution 

of hornblende for biotite (not shown) also gives an 

exchange of 0.42 mmol of sodium for calcium. Reactions 

8 and 9, using andesine An45 and augite, define mass 

transfer that is consistent with the water chemistry and 

with that determined in other basins. Compatible mass 

transfer is derived by compensating for solute calcium 

through higher calcic phases of the mafic model.

The surficial geology appears similar in upbasin and 

downbasin areas; however, examination of geologic logs 

from the upper part show that the subsurface geology in 

the west half of T. 7 N., R. 8 E., contains abundant basalt 

flows and fragments. Basalt flows are confirmed from 

340 to 1,704 ft below the land surface. The geologic log of 

Department of Energy test well (B-8-9)29dc just north of 

the flow path between the Harcuvar Mountains and McM 

4 also shows abundant basalt between the land surface 

and 740 ft and traces of basalt between 740 and 3,910 ft

below the land surface (U.S. Geological Survey, unpub. 

data). Using the reactant and product phases of the mafic 

model, the mass transfer determined is consistent with 

the observed subsurface geology and, although not so 

obviously, with the surficial geology. The higher ionexchange value derived by reaction 9 suggests that McM 

4 may be located in a transitional zone between the mafic 

and felsic terranes.

Reaction 9 also shows that, unlike the trend in other 

basins, the ferromagnesian mineral (augite) is weathered 

downgradient. Assuming the changes in chemistry do 

indeed represent changes along the true flow path, the 

weathering of additional augite and andesine may be 

explained through the presence of unweathered basalt 

within the alluvium, as high-temperature ferromagnesian minerals are not stable in the low-temperature 

weathering environment. (The question of the flow path 

is raised because, of all the paths modeled, the flow path 

to McM 4 is the least defined; some underflow may be 

recharged near the northernmost parts of the Harcuvar 

Mountains.) The pH of 9.4 of the ground water at McM 4 

is indicative of the extent of the hydrolysis reactions. 

This pH measurement was the highest in the entire 

study area. The derived mass transfer of the silicates 

predicts a highly alkaline solution, which is in agreement 

with this observed high pH value.
C60 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

01234567

MILES DOWNGRADIENT

FIGURE 28. Changes in concentration of dissolved chemical constituents along the flow path in lower McMullen Valley. (Well locations

shown in fig. 26.)

WILLCOX BASIN

The Willcox basin is typical of several large, broad 

basins in semiarid southeastern Arizona. These broad 

basins contrast with narrow basins like the San Pedro 

Valley. The Willcox basin is the highest in altitude of the 

basin lowlands, about 4,100 ft above sea level, and is 

surrounded by mountains with altitudes greater than 

10,000 ft. The mountains that bound the basin receive 

relatively high precipitation; for example, annual precipitation in the Chiricahua Mountains exceeds 35 in. The 

Willcox basin is the only topographically closed basin in

the study area with centripetal drainage that discharges 

ground water toward its center at the Willcox Playa (fig. 

30). The basin also is unique geochemically and hydrologically among those modeled because a major portion of 

the recharge apparently is a result of infiltration of 

precipitation and runoff along the valley floor throughout 

much of the basin.

The Chiricahua and the Dos Cabezas Mountains bound 

the basin on the east, the Dragoon and Winchester 

Mountains on the west, and the Pinaleno Mountains on 

the north. The Chiricahua and Winchester Mountains are
CHEMICAL EVOLUTION OF THE GROUND WATER C61

6 8 10 

MILES DOWNGRADIENT

12 14 16

FIGURE 29. Changes in concentration of dissolved chemical constituents along the flow path in upper McMullen Valley. (Well

locations shown in fig. 26.)

predominantly silicic volcanic rocks, and the Dos Cabezas 

and Dragoon Mountains are predominantly granitic and 

volcanic rocks. The Pinaleno Mountains consist chiefly of 

granite and granitic gneiss. The basin fill consists of 

alluvial and stream deposits that contain clay, silt, sand,

gravel, cobbles, and boulders. The permeable stream 

deposits of the upper basin fill, which range from a few 

hundred feet to more than 1,000 ft thick, constitute the 

most important aquifer of the Willcox basin (Brown and 

Schumann, 1969). The confining clayey layer overlying
C62 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

TABLE IS. Saturation indices of the ground water in McMullen 

Valley with respect to various minerals

[McM, McMullen Valley; well locations shown in fig. 26. Phases and thermodynamic data from Ball and others (1980)]

Phases McM 1 McM 2 McM 3 McM 4

Adularia. ..............

Albite.................

Calcite ................

Clinoenstatite ..........

Diopside. ..............

Dolomite ..............

Fluorite ...............

Goethite. ..............

Gypsum ...............

Halite.................

K-mica ................

Montmorillonite CA

Montmorillonite BF

Montmorillonite AB ....

Tremolite. .............

-1.130

-1.937

.079

.Uo£

-2.456

-1.994

-.025

-.919

5.827

-2.100

-7.020

-2.468

-.545

3.736

3.058

1.945

-1.422

-1.905

-.134

01 1

-2.421

-1.773

-.572

-1.021

6.043

-2.498

-6.866

-3.376

-1.378

3.171

2.616

2.559

-1.395

-2.366

-.174

-2.452

-2.090

-.385

-.012

6.051

2.851

-7.696

-3.670

-1.297

2.921

2.357

1.961

-1.022

-.920

-.178

-.063

-3.198

-1.452

-2.572

-1.273

5.179

-3.296

-6.851

-2.571

-.916

3.451

2.644

.682

the aquifer in other basins is not present in the Willcox 

basin.

Depths to water are considerably shallower relative to 

most basins and range from about 200 to 400 ft at the 

mountain front, in the case of the Chiricahua Mountains, 

to less than 20 ft as the playa is approached. The major 

recharge area of the basin is adjacent to the Chiricahua 

Mountains that bound the southeastern part of the basin. 

Estimated annual recharge to the basin is 54,000 to 

75,000 acre-ft. Nearly one-half of this amount originates 

near the Chiricahua Mountains and flows through the 

Kansas Settlement area; some recharge originates near 

the Dos Cabezas Mountains (Brown and Schumann, 

1969). Recharge from the Pinaleno Mountains, which 

also receive annual precipitation in excess of 35 in, drains 

partly into Aravaipa Valley to the north as well as into 

the Willcox basin. The waters are low in dissolved solids 

throughout much of the basin. Chloride and sulfate 

concentrations commonly are less than 10 mg/L. The 

ground water is a calcium sodium bicarbonate or sodium 

calcium bicarbonate type.

Reactions were modeled along a flow path of about 30 

mi, from the origin of Turkey Creek, the major drainage 

area of the Chiricahua Mountains, along the area near 

Turkey Creek to the playa (fig. 30). Oligoclase An25 and 

biotite were selected as the probable silicate reactant 

phases on the basis of the geology of the Chiricahua 

Mountains. The granitic and syenitic gneisses and the 

extensively developed acidic lavas found in the Dos 

Cabezas and Chiricahua Mountains contain chiefly 

quartz, feldspar, and mica (Meinzer and Kelton, 1913).

In contrast to the definite water-chemistry patterns 

observed along the flow path of dilute waters in other 

basins, trends in the Willcox basin are less clear (fig. 31). 

Bicarbonate, calcium, and silica increase and sodium,

sulfate, chloride, and pH decrease at points along the 

flow path. The difference in water chemistry is accompanied by differing mass transfer that is inconsistent 

with that of other basins or is not thermodynamically 

valid.

Reaction 1 in table 19 shows the mass transfer derived 

from recharge to Wil 1 (Willcox 1) using biotite and 

oligoclase. The reaction is thermodynamically valid but 

shows an improbable ion exchange of 2.32 mmol of 

sodium replacing calcium. Reaction 2 derives the mass 

transfer to Wil 1 using minerals with larger calcium 

content to affect lower solute sodium. The exchange of 

sodium for calcium is reduced only slightly; thus reaction 

2 or other reactions with reactants higher in calcium 

would not be valid in terms of the mass balance of sodium 

and calcium. Reaction 3, showing the mass transfer from 

recharge to Wil 2, is nearly identical to reaction 1. For 

subsequent reactions, reactions 4, 5, and 6 between Wil 

2 and the playa, phases are derived that do not agree 

with the speciation data and the saturation state of the 

ground water (table 20). The mass transfer of the silicate 

phases is reversed; potassium feldspar, plagioclase, and 

biotite are precipitating out of solution and montmorillonite, goethite, and silica are dissolving.

The difference between the mass transfer determined 

for the Willcox basin and for basins previously discussed 

may be explained by the mixing of additional recharge 

water with underflow owing to the shallow water table 

and permeable deposits. In the basins previously discussed, a geochemically closed system was postulated 

and was subsequently supported by the mass-transfer 

calculations, the aqueous speciation and saturation data, 

and, in most basins, the isotope data. Depths to water in 

these basins generally ranged from 125 to 500 ft in the 

shallow parts to more than 800 ft near the mountains. 

Predevelopment depth to water was about 120 ft at Wil 

1 and about 80 ft at Wil 2 (Meinzer and Kelton, 1913). The 

overlying coarse-grained deposits and relatively large 

amounts of precipitation and runoff apparently allow 

significant recharge through the overlying deposits to 

depths as great as 120 ft. As depths to water decrease 

downgradient, leakage and mixing increase, which 

results in the reversed trends in chemistry and mass 

transfer.

Recharge through the overlying sediments throughout 

the basin can be interpreted from trends in the trilinear 

diagram (fig. 32) and from the isotope data. The trilinear 

diagram indicates that calcium, magnesium, and bicarbonate ions are predominant or occur in relatively larger 

concentrations than in similar dilute waters of Vekol, San 

Pedro, or McMullen Valleys. The ground water throughout the Willcox basin is generally characterized by high 

calcium, magnesium, and bicarbonate typical of 

recharge waters and does not evolve into a sodium
CHEMICAL EVOLUTION OF THE GROUND WATER 

11 (TOO1 45' 30'

C63

109°15'

32° 00' **

31° 45'

Base from U.S. Geological Survey 

State base map,1974,1:500,000

10 MILES

^ I

10 KILOMETERS

EXPLANATION

ALLUVIUM

SILICIC VOLCANIC ROCKS

SEDIMENTARY ROCKS

GRANITIC AND METAMORPHIC ROCKS

-20 LINE OF EQUAL AVERAGE ANNUAL 

PRECIPITATION. 1930-60-Interval, 

in inches, is variable

-*^ DIRECTION OF GROUND-WATER 

MOVEMENT

Wil 1

WELL-Wil 1 is well identifier

.............. GROUND-WATER DIVIDE

FIGURE 30. Generalized geology, wells, and ground-water movement in the Willcox basin.
C64 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

< w

10 15

DISTANCE, IN MILES

20 25

FIGURE 31. Changes in concentration of dissolved chemical constituents along the flow path in the Willcox basin. (Well locations shown

in fig. 30.)

mixed-anion or sodium chloride sulfate water, as does 

water in basins that evolve under closed conditions. 

Isotope data also lend some support to an open system. 

Wil 4, one of the two farthest downgradient wells, is in a 

sandy area that has no significant irrigation history 

onsite or upgradient for several miles. The well was 

recently deepened from 350 to 500 ft, and most of the

800-gal/min yield is contributed from a 400- to 500-ft 

sandy zone. Depth to water, which is near predevelopment levels, is 50 ft. A 14C measurement of the tritiumfree water (<0.2 TU) was 26 PMC, which is unusually 

high. Unless a process such as gaseous diffusion 

(Thorstenson and others, 1983) or exceptionally high 

ground-water velocities are considered, the 14C activity
CHEMICAL EVOLUTION OF THE GROUND WATER C65

TABLE 19. Summary of mass transfer for the Willcox basin 

[Values* in millimoles per kilogram H20; Wil, Willcox basin; well names keyed to fig. 30. Dashes indicate phase not used]

Reaction number and flow path

Phases

02 ........................

C02 . ......................

Calcite....................

Oligoclase (Arw) ..........

Oligoclase (An30). .........

Potassium feldspar. .......

Augite ....................

Biotite ....................

Silica .....................

Halite. ....................

Gypsum. ..................

Fluorite. ..................

Montmorillonite (felsic) . . . 

Goethite ..................

Ion exchange** ...........

i 

Recharge 

to Will

0.18

.56 

-3.31 

4.21

.37

.47 

-2.97 

.00 

.00 

.06 

-3.07 

-.41 

-1.16

2 

Recharge 

to Will

0.11 

.56 

-3.31

3.32 

.15 

.63

-.84 

.00 

.00 

.06 

-2.56 

-.19 

-.49

3 

Recharge 

to Wil 2

0.18

.58 

-3.39 

4.37

.39

.50 

-3.19 

.00 

.00 

.11 

-3.19 

-.43 

-1.02

4 

Wil 2 

to Wil 3

-0.02 

.05 

-.25

.02

-.02 

.02 

.06 

.50 

-.06 

.16 

.02 

.00

5 

Wil 2 

to Wil 4

-0.02 

-.37 

-.13

.03

-.01 

-.04 

.79 

1.01 

-.04 

.07 

.02 

-.30

6 

Wil 2 

to Wil 5

-0.06 

.55

-.78

-.02

-.17 

.67 

.23 

.36 

-.08 

.58 

.19 

.35

*Reactants are positive and products are negative.

**Calcium replacing sodium on the substrate.

shows additional recharge along the 30-mi flow path 

between the mountain front and Wil 4.

In addition to leakage, another factor to be considered 

in the Willcox basin is the uncertainty of the composition 

of the recharge water. Chemical analyses of the ground 

water near the Chiricahua Mountains are not available to 

define the possible silicic-sodic composition expected 

from the weathering of a felsic terrane. The composition 

of the ground water throughout the eastern part of the 

basin, however, does not reflect dissolution of a rhyolitic

tuff or weathering of silicic volcanic rocks present in the 

Chiricahua Mountains; silica and sodium concentrations 

are smaller, and calcium and magnesium larger, than 

expected and are consistent with other basins in the 

study area. In any event, increasing the sodium content 

of the recharge water, as would be expected with silicic 

reactions, would not improve the modeling results 

because the average recharge water already appears too 

high in sodium. The ground-water compositions in the 

Willcox basin suggest that reactions involving an open

TABLE 20. Saturation indices of the ground water in the Willcox basin with respect to various minerals

[Wil, Willcox basin; well locations shown in fig. 30. Phases and thermodynamic data from Ball and others (1980). Dashes indicate values not

calculated]

Saturation indices for basin

Phases Will Wil 2 Wil 3 Wil 4 Wil 5 Number

of Maximum Minimum Average 

samples

Adularia ................

Albite. ..................

Calcite ..................

Chalcedony .............

Clinoenstatite. ..........

Diopside ................

Dolomite....... .........

Fluorite. ................

Goethite ................

Gypsum. ................

Halite. ..................

K-mica. .................

Montmorillonite CA. ....

Montmorillonite BF.

Montmorillonite AB.

Tremolite ...............

.. -0.

.. -2.

.. -2.

 ,

-

5.

.. -3.

.. -8,

1,

,106

.377

,911

,215

.947

.354

,472

.215

.309

.104

_

0.198

.299

-2.254

-.905

-.336

.100

5.338

-2.705

-8.010

5.616

_

0.151

.192

-2.883

-2.030

-.588

-.240

5.858

-1.769

-7.961

1.228

_

0.122

.261

-3.023

-2.127

-.807

.172

6.328

-1.460

-7.354

-.802

__

0.020

.326

-3.308

-2.692

-1.062

-.722

5.907

-1.925

-7.652

-1.351

9

9

19

19

19

19

19

19

19

9

9

9

9

0.942

-.036

0.231

.571

-2.090

-.462

-.034

.172

-1.192

1.536

3.197

4.803

4.078

-0.802

-1.

-1.

643

565

.089

-4.495

-5.

-3.

-2.

-4.

-1.

3.

2.

,637

,587

,640

,385

.882

,272

,275

.726

0.200

-1.063

-.029

.285

-2.929

-2.268

-.788

-.988

-2.574

-.256

1.652

4.163

3.474
C66 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

CALCIUM

PERCENT 

EXPLANATION 

COMPOSITION OF RECHARGE WATER

\

__ COMPOSITION ALONG FLOW PATH ^"~ ~V

 1 COMPOSITION OF Wil 1

FIGURE 32. Relative compositions of ground water in the Willcox basin, in equivalent percents. (Well locations shown in fig. 30.)

system or reactions in the unsaturated zone may be as 

influential as the mineralogy of the surrounding mountains in determining solute chemistry.

Although the mass transfer along the flow path has not 

been defined, several distinct trends are observed. The 

system is apparently open to C02 , as observed by a slight 

decrease in pH downgradient. The ground water downgradient is dissolving carbonate minerals and shows

increases in silica, calcium, and magnesium, all of which 

are indicative of open-system reactions. The final water 

compositions near the playa at Wil 4 and Wil 5 show an 

inverse relation between calcium and bicarbonate. Wil 5 

shows larger bicarbonate and smaller calcium concentrations, a result of equilibrium with respect to calcite, 

which is maintained along the entire flow path. Wil 4 is 

shallower, and the larger concentrations of calcium are a
CHEMICAL EVOLUTION OF THE GROUND WATER C67

result of continuous input of additional calcium through 

percolating waters and dissolution reactions along the 

valley floor. In a geochemically closed system, calcium 

and bicarbonate ions are effectively removed by secondary minerals. The reactions and mass transfer determined for the chemically closed basins and the reactions 

described above show the importance of open versus 

closed systems in determining the evolution of the 

ground-water chemistry.

BASINS ALONG THE COLORADO RIVER

The basins along the Colorado River in Arizona, 

California, and Nevada are unique in the study area as 

they were formed and are hydrologically dominated by a 

major river. Mohave Valley between Davis Dam and 

Topock, the Parker, Palo Verde, and Cibola Valleys 

between Parker Dam and the Narrows below Cibola, and 

the Yuma area between Imperial Dam and the international boundary were formed during several periods of 

degradation and aggradation of the Colorado River (fig. 

33). The flood plains, which are as much as 10 mi wide, 

were formed by the last aggradation of the river. The 

boundary between the flood plain and the terraces is the 

contact between the older and younger alluviums 

(Metzger and Loeltz, 1973; Metzger and others, 1973; 

Olmsted and others, 1973).

Ground-water development occurs primarily within 

the flood plain. Virtually all the ground water underlying 

the flood plain originated either as seepage from the 

river channel or as overbank flooding of the Colorado 

River. Ground-water discharge occurs primarily through 

evapotranspiration in the flood plain. In Parker, Palo 

Verde, and Cibola Valleys and the Yuma area, ground 

water is also discharged through artificial drains and as 

subsurface return flow.

The source of the ground water the Colorado River, 

ultimately from central Colorado is seen in the relation 

between 8D and 8180 for selected wells in Mohave, 

Parker, Palo Verde, and Cibola Valleys and the Yuma 

area (fig. 34). The deuterium content of -100 to -120%o 

is approximately 60%c depleted in deuterium relative to 

precipitation that recharges the ground water along the 

flanks of the basins. The plot of 8D and 8180 in figure 34 

shows a departure from the meteoric water line of 

8D=88180+10 (Craig, 1961); a slope of evaporation of 

about 5 indicates that extensive evaporation of the 

Colorado River has occurred. An estimated initial deuterium content of approximately -120%c is indicative of 

the precipitation and runoff from central Colorado (Doe, 

1983).

Deuterium, sulfate, and chloride contents of the 

ground water (fig. 33) suggest that recharge from the 

river occurs by seepage beneath the flood plain and

overbank flow. In the Mohave Valley flood plain the 

ground water in wells G, H, I, and J seems to have been 

recharged by seepage. The similarity of deuterium content of the samples (-104 to -109%c) to that of the river 

water (-106%c) at Hoover Dam in 1980, the larger 

sulfate and chloride concentrations, and the lower sulfate 

to chloride ratios of the samples relative to the water at 

Hoover Dam are evidence. The ground water in well F, 

about 2.7 mi from the river at the contact of the flood 

plain and the piedmont slope, may have been recharged 

by overbank flow. The depleted deuterium content of 

 117%c and the small sulfate concentration of 150 mg/L 

suggest that recharge may have occurred during spring 

runoff before construction of the dam. Beginning in the 

early 1940's with the completion of Hoover Dam, the 

river has been controlled by a series of dams, and, as a 

result, overbank flow generally no longer occurs.

MOVEMENT OF WATER INTO THE PIEDMONT AREAS

In Mohave, Palo Verde, and Parker Valleys, the 8D 

and 8180 of several samples indicate that water movement from the Colorado River transgresses the floodplain deposits and extends a considerable distance into 

the piedmont areas. Wells B, L, and M on the piedmont 

slopes show mixing of flood-plain waters and ground 

water recharged from the basin flanks (fig. 34). The 

proportions of water from the Colorado River are calculated by the following equation:

Percentage of river water = 8-8 x 100 (32)

where

n = ground water from basin flanks

(8D=-61.25%o; 8180=-8.25%o), 

x =sample (8D and 8180 of wells B, L, and M), and 

c =river water (8D=-117%C; 8180=-15.3%c).

For ground water recharged from the basin flank, a 8D 

of -61.25±0.9%o and a 8180 of -8.25±0.3%c were used; 

these are average values of four samples from Palo 

Verde and Mohave Valleys and the Yuma area. For the 

Colorado River, a 8D of -H7%c and a 8180 of -15.3%c 

(well F) were used for samples from wells L and B. These 

values were used for the river water because the isotopic 

compositions of the samples lie on a mixing line between 

the native ground water and these values. The isotope 

content of the Colorado River at Hoover Dam, -106%c, 

was used for the mixing calculation at well M for similar 

reasons. For comparison purposes, both sets of isotope 

values of the river water were used for the mixing 

calculations at well M.

The percentages of water from the Colorado River 

derived from both 8D and 8180 for the calculations are 

shown in table 21. The 8D and 8180 indicate that about 38
C68 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

ii5°oo 34015'

451 301

34° 301

Base from U.S. Geological Survey 

State base map,1974, 1:500,000

EXPLANATION

ALLUVIUM

-74 

360 

270 

90 

60E

CONSOLIDATED ROCKS 

FLOOD PLAIN

WELL-Lerter, R, is well identifier

WELL DATA-Numbers indicate, from top to bottom, 

deuterium, in per mil; chloride and sulfate, 

in milligrams per liter: and depth of well 

and depth to water, in feet. 

E = estimated

 BASIN BOUNDARY

 BOUNDARY OF STUDY AREA

33° 00' -

Parker 

15' /Dam 114° 00'

10

1

20

I

30 MILES

10 20 30 KILOMETERS

32° 151

301 -

Base from U.S. Geological Survey 

State base map,1974,1:500,000

FIGURE 33. Basins and wells sampled along the Colorado River.

percent of the water in well B and 43 percent of the water 

in well L originated from the river. Well M, which is less 

than a mile from the base of the Big Maria Mountains, 

receives roughly 25 percent of its water from the river. 

The contribution is noteworthy, as the Big Maria Mountains are one of the largest ranges along the river and 

thus probably receive more precipitation than other 

nearby ranges. It is hydrologically interesting that river 

water is present even near the mountain-front recharge 

area of the basin. The interpretation of the isotope data,
CHEMICAL EVOLUTION OF THE GROUND WATER C69

-50

-60

-70

-80

o -90

O

eo

-100

-110

-120

-130

-17 -16 -15 -14 -13 -12 -11 

518 0°/oo

-10 -9 -8 -7

EXPLANATION

-|- COLORADO RIVER WATER AT HOOVER DAM 

X COLORADO RIVER WATER AT IMPERIAL DAM

GROUND WATER ORIGINATING FROM THE COLORADO RIVER 

§B Mohave Valley

| Parker-Cibola-Palo Verde Valleys 

(S Yuma area

GROUND WATER ORIGINATING FROM LOCAL RECHARGE 

( ) Mohave Valley

( ) Parker-Cibola-Palo Verde Valleys 

(A) Yuma area

PROBABLE MIXTURES

-^- Mohave Valley

-^h Parker-Cibola-Palo Verde Valleys 

J WELL IDENTIFIER 

FIGURE 34. 8D and 8180 content of Colorado River water and of ground water in basins along the Colorado River.

if valid throughout the area, indicates that significant 

amounts of river water are moving beyond the flood plain 

into the piedmont areas. Interpretations based on hydrologic data have indicated that significant amounts of 

water probably are not moving beyond the flood plain 

(Metzger and others, 1973, p. 55).

EVOLUTION OF THE GROUND WATER

Because infiltration of Colorado River water recharges 

the aquifers along the river, chemical evolution of the

ground water must be related to the river water. As will 

be seen, there is more than one way the ground-water 

composition can be derived from that of the river water. 

Olmsted and others (1973) concluded that the changes in 

the chemical characteristics of the ground water in the 

Yuma area were related primarily to five reactions: (1) 

concentration by evapotranspiration, (2) ion exchange 

(calcium replacing sodium on the substrate), (3) precipitation of calcium and magnesium carbonates, (4) sulfate 

reduction, and (5) hardening (sodium replacing calcium
C70 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

TABLE 21. Percentage of Colorado River water in wells 

[Well locations shown in fig. 33]

Well

L ....................

B ....................

M.... ................

M* ...................

8D data

41.6

38.1

22.9

28.5

8180 data

45.4

38.3

19.2

29.0

Average

43.5

38.2

21.1

28.8

*Using isotope data of Colorado River at Hoover Dam.

on the substrate). Reaction 1 was required to account for 

the higher salinity, and reactions 3 and 4 were required 

to reduce the subsequent elevated sulfate and bicarbonate concentrations to observed levels. Reactions 2 and 5 

were used to adjust any calcium or sodium imbalance. 

Metzger and Loeltz (1973) and Metzger and others (1973) 

also used these processes to account for the evolution of 

the ground water in the Needles area and the ParkerBlythe-Cibola area, respectively.

Data collected and chemical modeling done during this 

study were used to develop alternative processes. Two 

fundamental differences between Colorado River water 

and the ground water are the increased salinity and the 

lower sulfate to chloride ratios of the ground water. The 

sulfate to chloride ratio of the Colorado River water from 

1941 to 1965 ranged from 3:1 to 5:1, the weighted-average 

value being about 3:1 (Olmsted and others, 1973), 

whereas the ground water contains much lower ratios, 

commonly less than 1:1. In this study, an alternate 

process consisting of dissolution reactions, primarily 

halite, is offered to account for the observed changes.

The water composition of well Q in the Yuma area 

(table 22) was derived from the river water using reactions and phases determined during this study. Equation 

33 shows how the composition of a saline ground water in 

the Yuma area can be derived from the river water 

starting with the weighted-average analysis of the 

Colorado River water:

3.38C02 +3.83H20(Colorado River water)+0.0502

+0.62CaC03 +0.21 CaMg(C03)2 +0.89andesine( An45)

+0.08Kspar+0.20augite+3.87NaCl+2.51CaS04

+(0.86Mg 0.44Na)ex^O. 70montmorillonite(mafic)

+0.07FeOOH+0.10Si02 +1.08Caex +4.99Ca+2 

+2.10Mg+2 +9.57Na+ +0.16K+ +6.49Cr+5.73S04~2

+7.11HC03 +0.37H4Si04+0.98H+ . (33)

The major reactions suggested are the dissolution of 

halite, gypsum, calcite, and dolomite; the weathering of 

silicates; a decrease in pH, from soil C02 ; and the 

exchange of calcium for magnesium and sodium.

Dissolution of halite, gypsum, and carbonates and 

weathering reactions are favored over the previously

TABLE 22. Weighted-average analysis of Colorado River water and 

analysis of well Q in the Yuma area

[In millimoles per liter; values in parentheses are in milligrams per liter. Well 

location shown in fig. 33]

Weighted average

river water,

1941-65*

WellQ

(C-9-24)19bdd, 5-20-80**

Ca+2

Na

cr .. so4-2 .

Si02

2.35

1.15

4.78

.13

2.62

3.22

2.70

.17

(94)

(28) 

(110) 

(5.0)

(93) 

(309) 

(165)

(10)

4.99

2.10

9.57

.16

6.49

5.73

7.11

.37

(200)

(51)

(220)

(6.4)

(230)

(550)

(434)

(22)

*01msted and others (1973, p. 125).

**Date sampled.

cited processes because of the known geology of the area, 

isotopic content, high PCO!), and dissolved-oxygen content of the ground water. Equation 33 shows that the 

dissolution of halite alone can account for the decrease in 

sulfate to chloride ratios and for a major part of the 

increase in salinity. The low sulfate to chloride ratios of 

water from wells in the piedmont areas as well as on the 

flood plains indicate that dissolution of halite is an 

important reaction throughout the area. Sodium chloride 

or residual pore fluids of the marine Bouse Formation 

could readily cause an increase of chloride concentrations 

through upward leakage or dissolution. The Bouse Formation is observed as far north as Lake Mohave (Bentley, 1979). The lack of correlation between 8D and sulfate 

and chloride supports a geologic control for the increase 

in chloride and sulfate concentrations. The concentrations of sulfate and the ion ratios of sulfate and chloride 

also do not represent a simple reduction in volume of the 

river water; the sulfate to chloride ratio of about 3:1 of 

the river water is not represented in the ground water, 

and at well E the ratios are reversed. The commonly 

observed threefold increase in bicarbonate concentrations from about 150 mg/L in the river water to 450 mg/L 

in the ground water can result from dissolution of 

carbonate minerals. Shallow depths to water of 5 to 15 ft 

and the abundance of vegetation may favor dissolution 

rather than precipitation of calcite. A constant PCOw 

associated with plant respiration could supply the hydrogen ions for dissolution reactions (eq. 26). Lastly, sulfate 

reduction probably is not occurring in the oxygenated 

ground water. Aquifer tests indicate that transmissivities in the Yuma area and in the Parker-Blythe-Cibola 

area are on the order of several hundred thousand to 

several million gallons per day per foot (Metzger and 

others, 1973, p. 68; Olmsted and others, 1973, p. 70). The 

water of the Colorado River is saturated with dissolved 

oxygen ( 10 mg/L), and it is unlikely the sand and gravel 

would deplete the ground water of oxygen. The assumed 

oxygenated waters are supported by field data, as all
SENSITIVITY ANALYSIS C71

wells sampled contained dissolved oxygen. The overall 

data suggest that evaporation occurs before infiltration 

and that dissolution of halite and gypsum within the 

alluviums are the major reactions controlling the groundwater chemistry.

Equation 33 shows that different types of reactions can 

account for the observed chemistry. The reactions shown 

in the equation are more realistic than those cited from 

previous studies, but they may not fully describe the 

evolution of the water. For example, magnesium may 

have major sources other than ion exchange. Because of 

the differences in chemistry of the recharge waters and 

the differences in geology and hydrology of the Colorado 

River area, the reactions derived for the other basins 

may not be fully applicable. Because of limitations of the 

mineralogic and isotope data of the aquifer and the 

isotope data of the ground water and soil gas, all reactions could not be defined unequivocally for the basins 

along the Colorado River.

SENSITIVITY ANALYSIS

The selection and composition of the phases are the 

most critical considerations in the development of chemical models. To provide a consistent framework for the 

development and transferability, several fundamental 

assumptions were made regarding the selection: (1) the 

minerals in the rocks surrounding the basins also are 

spatially distributed in the basin fill, (2) the compositions 

of individual minerals are invariant throughout and 

among basins, (3) the variation in compositions of individual minerals is not large, (4) relatively few phases are 

primarily responsible for controlling the ground-water 

chemistry, and (5) the phases selected for the modeling 

are the correct phases. During the course of the modeling, several questions arose concerning the phases:

1. Were the minerals selected or observed primary or 

secondary?

2. Were any minerals related to the regional flow system?

3. Are there any differences between the mineralogy or

chemistry of the coarse-grained fraction versus the

fine-grained fraction of the alluvium?

The dissolved constituents in the ground water that

are considered in the mass-transfer calculations are Na+ ,

K+ , Ca+2, Mg+2 , Cr, S04"2 , F~, Al+3, Fe+3, Si02 , C02 ,

and 02 . Although iron and aluminum are in solution only

in trace amounts, they must be considered because they

remain conservative in the solid phases. As the number

of common minerals exceeds the number defined by the

constituents, each probable phase must be tested by field

and laboratory criteria and, if found in valid, eliminated. A

sensitivity analysis was developed to test primary minerals, clay minerals, and initial water compositions systematically. Reactions were eliminated on the basis of 

criteria in three major categories: (1) mineralogic, (2) 

thermodynamic, and (3) isotopic. The reader is again 

referred to Plummer and others (1983) for pitfalls in 

selecting unique phases for models, as for most systems 

more than one set of phases may satisfy these or other 

criteria.

The selection of reactant silicate phases was based on 

the known mineralogy of the mountains and on the 

mineralogy of the sedimentary rocks in the basin fill 

determined through XRD data or thin-section petrography. The mineralogic information on the basin fill generated through the reconnaissance solid-species program 

proved invaluable in the chemical modeling. Where such 

data are not available (most basins), the mineralogy of 

the basin fill was assumed to be identical to the mineralogy of the surrounding mountains, which was estimated from information from previous studies.

Idealized formulas were adopted for the primary minerals. Fortunately, compositional variation of these minerals, particularly feldspars, is small relative to the clay 

minerals (Deer and others, 1976, 1980) and therefore a 

minimal error is introduced into the reactant side of the 

equations. However, clay minerals, smectites in particular, vary widely in their composition and could not be 

idealized from the XRD analysis or from the literature. 

The smectite composition was defined by establishing 

fixed compositions for the primary minerals and modeling reactions using end members of the smectite group 

and initial water compositions. The end members established brackets, or confidence intervals, of suitable smectite compositions as determined by the mass transfer.

The smectites have an enormous compositional variation through substitution of elements in the octahedral 

layer and the two tetrahedral layers of the 2:1 structure. 

Substitution of Al+3 ions for Si+4 in the tetrahedral layer 

causes a net deficiency of charge in the 2:1 layer, which 

is balanced by a cation in an interlayer position between 

the two tetrahedral layers. Substitution of Mg+2 and 

Fe+2 for Al+3 in the octahedral layer will also introduce 

cations into the interlayer position. Substitution of Fe +2, 

Fe+3 , and Mg+2 for Al+3 or of Fe+2 , Fe+3 , and Al+3 for 

Mg+2 in the octahedral layers may be balanced by 

interlayer cations or by introducing vacancies in the 

octahedral layer (Deer and others, 1976).

The end members selected for the analysis were 

beidellite, nontronite, saponite, montmorillonite, and the 

clay mineral illite. End members were derived by substitution in the dioctahedral pyrophyllite (Al2Si4010 

(OH)2) and trioctahedral talc (Mg4Si4010(OH)2), which 

may be considered the simplest structures of the smectite group. Substitutions were Al+3 for Si +4 in the 

tetrahedral layer of pyrophyllite (beidellite), Fe +3 for
C72 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

TABLE 23. Clay phases considered in the mass transfer

Phases Stoichiometry

Beidellite

Nontronite.......

Saponite.........

Illite (sodium)....

Illite (potassium). 

Montmorillonite..

Nao. 5Al2. 5Si3.5010(OH)2 

Ca0 165A12 33Si3 67O10(OH)2 

Ca0. 25Al2. 5Si3. 5010(OH)2 

Ca0.i65lre2.oAl0 .ssSig 67010(OH)2

Ca0. 165Mg3.oAlo.33Si3.670lo(OH)2

Na0. 6Mg0. 25Al2. 3Si3. 501 o(OH)2

Cao. 165Mg0.5Al1 .56Si4.0010(OH)2 

Ca0. 25Mg0. 5Al1 . 50Si4. 0010(OH)2

Al in the tetrahedral layer of pyrophyllite (nontronite), 

Al+3 for Si+4 in the tetrahedral layer of talc (saponite), 

and Mg+2 for Al+3 in the octahedral layer of pyrophyllite 

(montmorillonite). Two formulas for the mineral illite 

were used to represent the clay-sized minerals belonging 

to the mica group. Small compositional variations were 

made within these end members to affect a range of the 

interlayer charge between 0.33 and 0.50. Calcium, 

sodium, and potassium were used as interlayer cations. 

The phases and stoichiometry are given in table 23.

The primary minerals of the mafic model were reacted 

with the recharge water and variations of the recharge 

water using the selected clay end members. The composition of the recharge water was varied, maintaining 

electrical neutrality, within the standard deviation of 

sodium and bicarbonate, calcium and bicarbonate, and 

sodium and calcium. The magnesium and calcium contents of the carbonate phase were varied from a nearly 

pure dolomite to pure calcite (60-, 80-, and 100-mol 

percent calcium). Thus, each model with a clay end 

member represents at least 21 reactions. The results are 

summarized in table 24.

Each reaction was examined for validity against the 

three categories of criteria cited earlier. In table 24, a 

checkmark is entered for a reactant or product phase if 

the reaction is consistent. "Diss" or "ppt" signifies that a 

phase is dissolving or precipitating in violation of thermodynamic constraints. "L" signifies that a phase is 

dissolving or precipitating in compliance with thermodynamic constraints but that the mass transfer is exceedingly large, thereby violating mineralogic and chemical 

constraints. "1" signifies that the derived isotopic composition of the C02 gas is too light, and "h" too heavy, as 

dictated by isotopic constraints. A derived 8 13C of 

greater than 14%o and less than 22%o was considered 

unacceptable. On the basis of the agreement of the 

derived mass transfer with these constraints, a reaction 

is designated either unacceptable or acceptable by "U" or 

"A." In all cases, reactions considered unacceptable had 

violated more than one constraint.

The sensitivity analysis shows that three compositions 

or their variations are suitable phases: a dioctahedral

beidellite, a dioctahedral montmorillonite, and a 

potassium-rich illite. Although beidellite does not contain 

magnesium, an acceptable mass transfer was derived 

because its composition is similar to that of montmorillonite. The absence of magnesium in beidellite can be 

compensated for by dissolution of dolomite. However, 

beidellite becomes an unacceptable phase in computing 

mass transfer in basins because dolomite is not present 

as a reactant or product.

The analysis revealed several general trends. Calcium 

is the major interlayer cation; potassium also probably is 

in the interlayer position. The variation of the interlayer 

charge from 0.33 to 0.50 does not affect the model, as the 

mass transfer derived is virtually identical in this range. 

The model is most sensitive to variations in magnesium, 

but, surprisingly, the variation in aluminum between 

1.56 and 2.33 atom has no dramatic effect that is not 

compensated for by compositional variations of the smectite. On the basis of the results of the analysis, a smectite 

composition that fell within the limits of the three 

suitable phases was selected for the modeling. Additional 

reactions were subsequently run for final adjustment of 

the magnesium content of the carbonate phase for compliance with the three constraints, particularly the 813C 

mass balance. More than 200 mineral assemblages among 

27 phases that were deemed realistic were considered in 

development of the recharge models. Under assumptions 

set forth at the beginning of this section, the stoichiometries derived for minerals in the recharge model thus 

also became those used in the modeling of the basins.

CLAY MINERALOGY

Near the end of the study, four clay samples (less than 

2-(Jim fraction) from three basins were selected for structural formula determination, as some detailed information on the smectite was desirable for testing the conclusions and assumptions of the chemical models. The 

samples selected were those showing the least content of 

mica, kaolinite, and mixed layering as determined by the 

XRD analyses. The chemical analyses of the four samples 

are shown in table 25, and the formulas determined from 

the analyses are shown in table 26. Because sodium 

phosphate and sodium chloride were used in the separation procedures and are abnormally high, the analyses 

are shown recalculated without sodium and phosphorus. 

The structural formulas of the field-determined clay 

samples are those of an end member montmorillonite and 

are similar to the formulas determined by the sensitivity 

analysis. The major differences between the field- and 

model-derived montmorillonites are lower Al203/Si02 

ratios and higher Fe203/MgO ratios.

Any conclusions based on four samples are tenuous, 

particularly in the absence of sodium data and possible
CLAY MINERALOGY C73

TABLE 24. Sensitivity analysis and summary of mass-transfer calculations

[In millimoles per liter; * *, reactant is consistent; ppt, precipitating; diss, dissolving; L, abnormally large; 1, derived 813C too light; h, derived 8 13C too heavy; U,

unacceptable; A, acceptable. Dashes indicate element not used]

Recharge water*

Clay mineral

Beidellite ........

Saponite .........

Nontronite.......

Illite (sodium). . . .

Illite (K) .........

Montmorillonite. .

Mineral phase

Plagioclase

K-spar

Augite

FeOOH

Smectite

Si02

513C

Summary

Plagioclase

K-spar

Augite

FeOOH

Smectite

Si02

513C

Summary

Plaonnfljmp A l<*^lL/V*ldoC

K-spar

Augite

FeOOH

Smectite

Si02

513C

Summary

Plagioclase

K-spar

Augite

FeOOH

Mica

Si02

513C

Summary

Plagioclase

K-spar

Augite

FeOOH

Mica

Si02

513C

Summary

Plagioclase

K-spar

Augite

FeOOH

Smectite

Si02

513C

Summary

Variation of interlayer charge

(Na) 

0.5

L

|X

|X

|X

L

|X

h

U

(Ca) 

0.165

|X

IX

|X

IX

JX

|X

IX

A

JX

IX

ppt

diss

diss

L

h

U

IX

JX

IX

diss

L

L

JX

U

L

L

L

IX

L

L

h

U

IX

IX

IX

IX

IX

IX

|X

A

IX

IX

IX

IX

IX

IX

IX

A

(Ca) 

0.25

JX

IX

IX

|X

IX

IX

|X

A

»x

|X

|X

(X

|X

|X

»x

A

Variation in composition of recharge water*

Variation of Na+ and HC03

Na+ 0.30 

HCOg 4.14

ppt

|X

|X

|X

diss

|X

|X

U

ppt

|X

L

L

|X

L

h

U

ppt

|X

IX

L

diss

L

L

U

ppt

ppt

ppt

diss

diss

L

1

U

ppt

ppt

»x

|X

diss

»x

1

U

ppt

JX

|X

IX

diss

IX

1

U

1.64 

5.48

L

|X

|X

|X

L

|X

h

U

L

|X

ppt

diss

diss

L

h

U

L

|X

JX

diss

L

L

1

U

L

L

L

IX

L

L

h

U

L

L

ppt

L

L

L

h

U

L

IX

L

L

L

»x

h

U

Variation of Ca+2 and HCOg

Ca+2 1.61 

HCOg 4.14

IX

IX

|X

IX

IX

|X

IX

A

IX

IX

ppt

diss

diss

L

h

U

IX

IX

IX

diss

L

L

IX

U

L

L

L

JX

L

L

h

U

|X

»x

L

|X

»x

L

h

U

IX

|X

IX

|X

IX

|X

|X

A

2.28 

5.48

|X

|X

|X

|X

|X

|X

|X

A

|X

|X

ppt

diss

diss

L

h

U

|X

IX

|X

diss

L

L

|X

U

L

L

L

|X

L

L

h

U

|X

|X

|X

|X

|X

|X

|X

A

IX

|X

|X

»x

|X

|X

IX

A

Variation of Ca+2 and Na+

Ca+2 1.60 

Na+ 1.66

L

|X

|X

IX

L

|X

h

U

L

|X

L

diss

diss

L

h

U

L

|X

|X

diss

L

L

|X

U

L

L

L

|X

L

L

h

U

L

L

L

|X

L

L

h

U

L

IX

L

|X

L

IX

h

U

2.29 

0.28

ppt

ppt

|X

diss

diss

|X

|X

U

ppt

|X

L

|X

L

L

h

U

ppt

|X

|X

L

diss

L

|X

U

ppt

ppt

ppt

diss

diss

L

1

U

ppt

ppt

IX

JX

diss

JX

JX

U

IX

|X

IX

IX

IX

|X

IX

A

*See table 2 for composition of recharge waters.
C74 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

TABLE 25. Chemical analyses of four clay samples from selected

basins

[In weight percent. Analyses by Center for Meteorite Studies, Arizona State

University]

Well number and location and depth of sample below land surface

MgO . . . AI n

Si02 ....

K20 . . . .

CaO....

Ti02 ... 

MnO . . . 

Fe203 ..

(B-2-2)34bcb 

West Salt 

River Valley 

Depth, 

330 feet

5.51

IS 04

. . 62.43

1.19

3.02

.39 

.10 

9.32

(D-16-25)4cdd 

Willcox basin 

near playa 

Depth, 

400 feet

4.06 

19.10 

62.52 

2.44

2.88 

.54 

.08 

8.38

(D-2-8)26caa 

East Salt 

River Valley 

Depth, 

802 feet

5.67 

16.90 

65.15 

1.38 

2.59 

.39 

.16 

7.76

(D-2-8)26caa 

East Salt 

River Valley 

Depth, 

903 feet

4.97 

18.08 

63.77 

2.08 

3.01 

.50 

.07 

7.52

inclusion of noncrystalline iron and silica, but support is 

provided for the major assumptions set forth for the 

modeling. The composition of the clays suggests that the 

clay mineralogy indeed may be spatially invariant within 

and among basins throughout the area. The samples 

represent a range of basin types, from a basin having the 

highest altitude and precipitation (Willcox) to one having

low altitude and precipitation (West Salt River Valley), 

each showing distinct differences in vegetation, climate, 

geology, and hydrology. The chemical analyses of the 

bulk rock support the assumptions that the clays are a 

sink for magnesium and a partial sink for iron and that 

aluminum generally is conserved in the silicate phase.

Chemical analyses of the bulk-rock fraction of core 

from well (D-2-8)26caa in East Salt River Valley are 

shown in table 27 with and without sodium and phosphorus. The clay analyses of the same well (table 25) show an 

enrichment of magnesium of about a factor of two 

relative to the bulk-rock samples. The enrichment of iron 

appears to be somewhat less than two. Aluminum 

appears to be generally conserved but is slightly 

depleted relative to the initial primary phases and 

slightly enriched relative to the bulk-rock fraction. 

Chemical analyses of the bulk-rock fraction of core from 

two additional wells, (A-l-7)36daa2 and (D-6-9)29bba, 

in East Salt River Valley are shown in table 27 to 

illustrate the similarity and spatial variation in sediment chemistry. Calcium in particular varies vertically 

owing to the presence of calcite, which is indicated by the 

large losses on ignition of samples from the lowest 

interval of well (D-2-3)26caa. Wells (D-2-8)26caa and

TABLE ^. Structural formulas and location of four clay samples

Atoms per 2:1 phyllosilicate half cell

Location

(B-2-2)34bcb West Salt River Valley ..................

(D-16-25)4cdd Willcox basin near playa. ...............

(D-2-8)26caa East Salt River Valley, Depth 802 feet . . 

(D-2-8)26caa East Salt River Valley, Depth 903 feet . .

Tetrahedral

Si

. 3.82

. 3.82

. 3.94

. 3.88

Al

0.18 

.18 

.06 

.12

Total 

tetrahedral

4.00 

4.00 

4.00 

4.00

Mg

0.50 

.37 

.51 

.45

Al

1.12 

1.20 

1.15 

1.18

Octahedral

Fe

0.43 

.39 

.35 

.34

Mn

0.01 

.01 

.01 

.01

Ti

0.01 

.03 

.01 

.02

Total 

octahedral

2.07 

2.00 

2.04 

2.02

Interlayer

Ca

0.19 

.18 

.17 

.19

K

0.09 

.19 

.11 

.16

TABLE 27 . Chemical analyses of the bulk-rock fraction of core from selected wells in East Salt River Valley 

[In weight percent. Analyses by Center for Meteorite Studies, Arizona State University. Dashes indicate element not included]

(D-2-8)26caa

Depth,* 

375 feet

Na20 . .

MgO. . .

A1203

Si02 . . .

P20S . . .

K20 . . .

CaO ...

Ti02 ...

MnO. . .

Fe203 . .

LOP* .

. 1.82

. 2.28

10.95

. 61.84

.24

. 2.63

. 5.10

.66

.04

. 3.87

. 6.63

2.60

12.54

70.78

3.00

5.84

.76

.05

4.43

Depth,* 

712 feet

1.43

3.31

13.88

59.46

.28

3.08

3.89

.71

.08

5.47

7.16

3.68

15.44

66.16

3.43

4.33

.79

.08

6.09

Depth,* 

1,102 feet

2.06

2.41

12.31

65.31

.23

2.99

4.13

.63

.06

4.27

6.04

2.62

13.36

70.90

3.43

4.48

.69

.07

4.63

Depth,* 

1,600 feet

1.73

1.79

9.24

47.82

.11

2.14

18.84

.55

.07

4.07

17.22

2.11

10.94

56.60

2.53

22.28

.65

.08

4.82

Depth,* 

100 feet

2.13

2.49

11.72

63.08

.26

2.31

4.94

.73

.06

4.45

6.51

2.78

13.05

70.26

2.57

5.50

.82

.07

4.96

(A-l-7)36daa2

Depth,* 

595 feet

2.39

3.22

14.42

60.59

.30

2.27

4.54

.73

.07

4.87

7.20

3.55

15.89

66.80

2.54

5.00

.80

.08

5.35

Depth,* 

1,021 feet

1.99

2.40

12.60

50.96

.22

2.16

12.60

.52

.06

3.72

13.91

2.82

14.82

59.93

2.55

14.82

.60

.07

4.38

Depth,* 

306 feet

1.72

2.24

11.83

50.34

.25

2.67

12.06

.60

.09

4.40

12.48

2.66

14.05

59.77

3.16

14.32

.71

.10

5.22

(D-6-9)29bba

Depth,* 

708 feet

1.68

4.76

12.47

54.81

.33

3.28

6.62

.68

.11

5.61

9.18

5.39

14.11

62.05

3.72

7.50

.76

.13

6.34

Depth,* 

1,203 feet

3.08

1.49

12.88

65.49

.21

3.50

4.46

.45

.06

2.86

4.05

1.63

14.12

71.81

3.85

4.89

.50

.07

3.14

Depth,* 

1,799 feet

2.79

2.66

13.84

60.73

.32

3.27

5.46

.79

.11

5.60

4.49

2.87

14.97

65.70

3.53

5.90

.84

.11

6.06

*Depth of sample below land surface.

**LOI = Percent loss on ignition.
CLAY MINERALOGY C75

analyses of samples from depth intervals shown in table 

27 suggest that if differences in sediment chemistry 

exist, they may be greater in the vertical dimension than 

in the horizontal.

MASS TRANSFER OF OXYGEN AND SILICA

The modeling and the field-determined montmorillonites suggest that a typical montmorillonite composition 

observed in Arizona may have lower Al203/Si02 ratios 

than those derived by the modeling but that the amount 

formed will be more in accord with the higher Al203/Si02 

ratio. In other words, less montmorillonite is formed 

from weathering a given amount of rock than is indicated 

by the observed smectite composition, assuming conservation of aluminum. The mass transfer determined by 

using the model-derived montmorillonite with the higher 

ratio appears more compatible with the chemical and 

mineralogic data when stoichiometry constraints of the 

ferromagnesian minerals, dissolved-oxygen content in 

the ground water, and possibly silica content of the 

ground water are considered.

Reactions 1, 2, 3, and 4 in table 28 show the mass 

transfer in Vekol Valley using the formulas of the four 

montmorillonite samples from table 26. The oxidation of 

iron in augite using all formulas depletes the ground 

water of dissolved oxygen, total consumption ranging 

between 12 and 17 mg/L. The graph of dissolved oxygen

TABLE 28. Mass transfer derived using structural formulas of jour 

clay samples 

[Values* in millimoles per kilogram H20; Vek, Vekol Valley; well names keyed to 

fig. 15]

Flow path and reaction number**

Phases 

1234

Recharge to Vek 3 

02 ..................... 0.41 0.45 0.40 0.31

C02 .................... -.60 -.60 -.60 -.60

Calcite................. -3.49 -3.49 -3.49 -3.49

Andesine (An45) ....... 3.56 3.36 3.70 3.92

Potassium feldspar. .... .54 .67 .95 1.10 

Augite................. 2.34 2.57 2.31 1.77

Silica .................. 2.99 4.28 3.57 3.11

Halite.................. 1.63 1.63 1.63 1.63

Gypsum. ............... .77 .77 .77 .77

Fluorite................ .01 .01 .01 .01

Montmorillonite........ -4.75 -5.01 -5.21 -5.17

Goethite............... .40 -.05 .16 .78

Ion exchanget ......... .55 .60 .51 .45

*Reactants are positive and products are negative. 

** Indicates location of clay sample: 1, (B-2-2)34bcb; 2, (D-2-8)26caa (802); 3, 

(D-2-8)26caa (903); 4, (D-16-25)4cdd. 

tCalcium replacing sodium on the substrate.

(A-l-7)36daa2 are near the recharge area of the Superstition Mountains, and well (D-6-9)29bba is in the basin 

center between Casa Grande and Florence. The chemical

9 

8

oc

LLJ

5 7 

OC

HI 

Q_

C/D e

^ 6 

<

oc

CJ 

=1 5

^

"Z.

-Z. 4

HI

C3

Z

0 3

Q

DISSOLVE

103 _i 

1 1

A MEASURED DISSOLVED-OXYGEN 

CONCENTRATIONS FROM WELLS 

_ IN SELECTED BASINS _ 

 Vekol Valley

A Butler Valley

_ A San Pedro Valley _ 

A E San Simon Wash 

0A' MODEL-DERIVED DISSOLVED-OXYGEN

B CONCENTRATIONS FOR SELECTED

_ 8 B. CORRESPONDING WELLS _

C

0 A' *

M'o>L' 

 »D 

P , _ A L OE ^

G «E.H ' .J D N

r. «F °J'

°G AM

A*

WELL NUMBER 

 A (D-7-2)18aba H (B-6-16)33aaa 

B (D-8-1)31cbc I (B-6-16)26aad 

C <D-7-1)10cbc2 J (B-3-15)2dab 

D (D-8-1)14baa K (B-8-14)29cdd 

~~ E (D-9-1)13bbd L (D-9-17)24dcc ~ 

F (D-8-1)35abd M (D-8-17)32daa 

G (B-4-15)18bbb N (C-16-1)10cca

0 5 10

DISTANCE FROM MOUNTAIN-FRONT 

RECHARGE AREA, IN MILES

FIGURE 35. Measured and model-derived concentrations of dissolved 

oxygen from wells in selected basins.

and distance along flow paths shown in figure 7 is 

illustrated again in figure 35, showing the agreement of 

measured dissolved-oxygen concentrations with modelderived concentrations. The model-derived concentrations are computed from the total consumption of dissolved oxygen along the flow path. The total 

consumption is defined as the amount of oxygen required 

to oxidize Fe+2, presumably the only reductant in the 

hydrochemical system, from the time of infiltration of 

water to the final water. The initial dissolved-oxygen 

content of the ground water at the time of infiltration is 

placed at 11 mg/L, which would be the amount in the 

ground water at saturation at 10 °C and 3,000-ft altitude, 

an estimated average temperature and altitude for winter recharge of the basin lowlands. This value is also in 

good agreement with field measurements of dissolved 

oxygen in streams, as listed in table 9.

The mass transfer of silica also places a limit on the 

amount of montmorillonite formed. The modeling of the 

basins using all montmorillonite compositions indicates 

that silica is controlled primarily by the clay phase. Silica
C76 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

(1000/TEMPERATURE, IN DEGREES KELVIN)

3.5 3.4 3.3 3.2

CC 

UJ 

Q_

CC

GO:D

s

10

20 30 40

TEMPERATURE, IN DEGREES CELSIUS

50

EXPLANATION 

BASIN

SYMBOL BASIN 

NUMBER

1 Ranegras Plain

2 Verde Valley

3 West Salt River Valley

4 East Salt River Valley

5 Harquahala Plain

6 Rainbow Valley

7 McMullen Valley

8 Hualapai Valley

9 Big Sandy basin

10 Sacramento Valley

11 La Posa Plain

12 Butler Valley

13 Altar Valley

14 Avra Valley

15 Upper Santa Cruz

16 Lower San Pedro Valley

17 Papago Indian Reservation

(includes basins 25, 27, 28, 43, 

44, 45)

18 Vekol Valley

19 Willcox basin

20 Colorado River (includes basins 

3, 5, 6, 7)

21 Safford basin

FIGURE 36. Relation of average silica concentrations in ground 

water from selected basins to silica minerals and temperature.

released to the aqueous phase through silicate weathering in Vekol Valley, for example, is about 12.0 mmol, of 

which montmorillonite removes about 90 percent. A 

small change in the silica composition of the montmorillonite may require control by an additional phase or a 

source of additional silica. For example, an increase in 

silica in montmorillonite from 3.65 to 3.82 atoms would 

require dissolving at least 3 mmol (180 mg/L) of additional silica. Dissolving this amount of silica is unlikely 

because of the low solubility of quartz, the most abundant silica source.

Silica may not be dissolving but, as indicated by the 

modeling, may be precipitating along the flow path. 

Average silica concentrations in ground water from 

several basins and the solubilities of amorphous silica, 

chalcedony, and quartz are shown in figure 36. Wells 

sampled in the Big Sandy and Willcox basins and in 

Hualapai, Vekol, and Sacramento Valleys are near 

recharge areas. Wells sampled in Ranegras and Harquahala Plains and in lower San Pedro, Butler, McMullen, 

and Rainbow Valleys are near the center or along the 

axis of the basin and represent downgradient waters. 

The trend in silica concentrations along the chalcedony 

phase boundary suggests that chalcedony may be an 

additional minor control for removal of silica.

The interpretations using the depletion of dissolved 

oxygen in support of the models are valid under the 

following assumptions: (1) most of the iron in the ferromagnesian minerals is Fe+2, (2) the unoxidized iron and 

magnesium mole ratios of the minerals are near unity, (3) 

the less than 62-jjim fraction adequately reflects the 

bulk-rock chemistry of the basin fill, (4) no additional 

reductants are present in the aquifer, and (5) the initial 

dissolved-oxygen concentration of the ground water at 

the time of recharge is about 10 to 12 mg/L. Assumptions 

1 and 2 regarding the oxidation state and content of the 

iron are the weakest. Primary minerals are known to 

contain some Fe+3 and, as was discussed earlier in the 

section on reactions in the unsaturated zone, the ferromagnesian minerals are preferentially leached relative to 

other minerals. The basic assumptions that the proper 

minerals have been identified and that the derived mass 

transfer is correct also apply to the above interpretation.

MASS TRANSFER OF KAOLINITE

The apparent smaller mass transfer of montmorillonite 

may be related to the geochemical behavior of aluminum 

and its distribution in the basin. One explanation may be 

that the rocks of the mountains contain less aluminum 

than the idealized mineral phases. The aluminum content 

of the selected silicate phases is greater than that 

indicated by chemical analyses of the bulk rock (less than 

62-jxm fraction) of the basin fill, supporting the depletion 

of aluminum (tables 5, 6, 27). Alternately, aluminum may 

be conserved in aluminum oxyhydroxides or, more 

likely, in additional secondary aluminosilicates. Supporting the oxyhydroxides, the saturation state of the ground
ION EXCHANGE C77

water is at equilibrium with respect to boehmite and is at 

supersaturation with respect to diaspore and gibbsite. 

The amount of any aluminum oxyhydroxides forming 

should be small, however, as these minerals are not 

stable under equilibrium conditions with kaolinite and 

montmorillonite in the quartz-rich environment. Kaolinite (Al4Si4010(OH)8), another aluminosilicate, however, 

can be in equilibrium with montmorillonite. The mass 

transfer of the silicates and oxides along the flow path in 

Vekol Valley using kaolinite as an additional secondary 

clay mineral and a lower Al203/Si02-higher Fe203/MgO 

montmorillonite is shown in the following equation:

0.1402 +3.67Ca0.45Na0. 55Al 1 .45Si2. 5508+0.35KAlSi308

+0.84Cao.4Mgo.gFeo.7Alo.2Sii.906+0.73Si02 +0.32FeOOH

-+2.93Cao. 13K0.o7Mg0. 4Feo.3iAl1 . 5Si3.8601o(OH)2

+0.36Al4Si4010(OH)8. (34)

The reactions in equation 34 not only satisfy the 

aluminum balance but also alleviate the problem of silica. 

A reduction in the amount of precipitated or dissolved 

silica is more compatible with the kinetic behavior of 

silica. The subsequent conservation of aluminum among 

all primary and secondary silicates supports the major 

modeling assumptions. The inclusion of about 10 percent 

of kaolinite as an additional phase is compatible with the 

low Al203/Si02 ratio of the field montmorillonite and, 

with the exception of the amount of precipitated silica, is 

consistent with the derived mass transfer presented in 

this paper.

The questions concerning the small discrepancies in 

iron and silica in equation 34 will require more precise 

analysis of the crystalline iron and silica content of the 

montmorillonite. For example, a reduction in the iron 

content of montmorillonite from 0.32 to 0.20 atom would 

affect the formation and not the dissolution of goethite. 

Similarly, a small decrease in the mass transfer of 

montmorillonite or in the silica content of montmorillonite would affect silica precipitation. An excess of aluminum in kaolinite over that required for the theoretical 

lattice or the presence of aluminum oxyhydroxides would 

require even smaller amounts of kaolinite and also would 

minimize any problem of both silica and goethite. Resolving any differences in aluminum content among the 

primary rock, basin fill, and derived phases will require 

more knowledge of the mineralogy and chemistry of 

different fractions of the basin fill.

ION EXCHANGE

The imbalance of charge in smectite structures 

through substitution in the octahedral and tetrahedral 

sites causes the smectite substrate to attract other ions

on its surface or to incorporate cations within the interlayer position. Among the cations, a preferential concentration of the multivalent cations occurs in these interlayer exchange positions because of their higher charge. 

This preferential exchange (selectivity) increases markedly with increased dilution of the water. The laws of 

mass action and thermodynamic relations have been 

applied in approaches regarding treatment of ionexchange equilibrium (Thorstenson and others, 1979; 

Stumm and Morgan, 1981). During this study, ion 

exchange was treated in a concept of mass transfer, 

mineral phases, and water compositions.

The reaction modeling has shown that ion exchange is 

not a significant reaction in the ground water in most 

basins. In dilute waters, exchange accounts for little, if 

any, solute sodium, the major ion replaced on the substrate, although these dilute waters are sodic in composition. In basins that contain large concentrations of 

dissolved solids and sulfate, however, ion-exchange reactions have a significant impact on the water chemistry.

The absence of ion-exchange reactions in most basins 

can be explained by the composition of the soils, smectite, and ground water. Owing to the ubiquitous presence 

of calcite in soils and dissolved calcium in recharge 

waters, the divalent calcium ion is preferentially concentrated over sodium and potassium in the interlayer 

position of the smectite. Although ground water initially 

high in calcium evolves into a sodic water, the water 

remains too low in ionic strength to reduce the selectivity 

of the divalent cation in either the authigenic or the 

detrital clay. Calcium that enters the aqueous phase 

through hydrolysis is dissipated at the weathering rate 

through calcite precipitation or uptake of newly formed 

authigenic clay. Concentrations of calcium do not reach 

levels at which they may effect an exchange for sodium, 

potassium, or magnesium ions, which probably also are 

present in small amounts in the interlayer position. If 

large concentrations of calcium suddenly enter the aqueous phase, such as by dissolution of gypsum, all three 

cations may be exchanged and transferred to the dissolved phase.

SODIUM

As determined in the modeling of Ranegras Plain, 

ion-exchange reactions were influential in the evolution 

of the high-sodic ground water. Examining the analytical 

data of Ran 2 and Ran 3 in Ranegras Plain again, the 

increase of 1.31 mmol of sodium between Ran 2 and 3 

accompanied by an increase of only 0.56 mmol of chloride 

is difficult to explain without an ion-exchange mechanism 

or an unrealistic mass transfer of the silicates. At Ran 4, 

which is in an area of larger dissolved-solids concentrations in the northern part of the basin, the ground water
C78 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

contains 41.3 mmol of sodium, 22.6 mmol of chloride, and 

19.8 mmol of sulfate (table 10). Well (B-6-16)25bbc about 

1 mi upgradient from Ran 4 contains 14.8 mmol of 

sodium, 7.9 mmol of chloride, and 4.5 mmol of sulfate. 

Well (B-6-16)25bbc is 1,000 ft deep; Ran 4 is 1,100 ft 

deep and apparently penetrates more of a buried playa 

deposit. An increase of 11.8 mmol of sodium in the 

ground water over that of the chloride increase between 

the two wells indicates that ion exchange is an extremely 

important reaction in the evolution of the water. The 

exchange of more than 10 mmol of sodium indicates an 

availability of exchangeable sodium.

The basin-fill sediments in Vekol Valley and East Salt 

River Valley contain an abundance of sodium (tables 6, 

27). Presumably, a small amount of this sodium is 

situated in the interlayer position of the montmorillonite, 

although most of the sodium present in the basin fill 

probably is incorporated in minerals other than clays. 

Sodium in primary minerals is suggested by the vertical 

distribution of sodium in Vekol Valley, where the highest 

content is found in the sandy gravels of the lower unit, 

which is predominantly quartz and feldspar. The upper 

silt-clay unit, however, also contains appreciable sodium. 

Leachate analyses of drill cuttings and core from the 

wells indicate that the basin fill is low in chloride, and 

thus halite is not a major source of sodium.

POTASSIUM

Potassium presented difficulties in relating water compositions to specific rock-water reactions because of 

lower concentrations than would be expected from 

weathering of the source rocks. The geochemistry of 

potassium requires two fundamental considerations: (1) 

silicate minerals weather at different rates and (2) potassium may exchange for other cations on clay minerals. 

Minerals that form first in the crystallization series are 

unstable in the weathering environment (Goldich, 1938). 

In the case of feldspars, stability increases with increasing sodium and potassium content; potassium feldspar 

and muscovite are relatively stable minerals. Colman 

(1982), in a study of chemical weathering of rinds of 

basalts and andesites, determined that volcanic glass was 

the least stable phase and that pyroxene and plagioclase 

have intermediate stabilities. Potassium feldspar and the 

opaque minerals were resistant to weathering but did 

eventually weather completely. The relative elemental 

mobilities determined by Colman (1982) were 

Ca>Na>Mg>Si>Al>K>Fe>Ti, showing that calcium 

is the most mobile and titanium the least.

The average potassium concentration determined for 

recharge areas is small, 0.05 mmol (2.0 mg/L), but it 

varies significantly and may increase downgradient severalfold or may remain the same. Potassium correlates

positively (p<0.0001) with calcium, sodium, magnesium, 

chloride, sulfate, and dissolved solids in several basins. 

Average sulfate and potassium concentrations in basins 

that contain sulfate concentrations greater than 200 

mg/L are plotted in figure 37. The general trend in these 

basins is that potassium concentrations increase downgradient with increasing sulfate concentrations. In La 

Posa Plain and McMullen, Vekol, and lower San Pedro 

Valleys, where average sulfate concentrations are small 

(less than 50 mg/L), the potassium concentrations also 

are small (2-4 mg/L) and do not increase downgradient or 

show significant correlations with sulfate or other major 

elements.

The increase in the concentrations of potassium downgradient, if caused by a mineral source, would require 

weathering of potassium feldspar or possibly biotite, the 

other source of potassium. The long residence time, the 

variability of potassium concentrations, and the differing 

saturation state of the ground water with respect to 

potassium feldspar tend to diminish the importance of 

potassium feldspar as a source. The difficulties in nucleation of potassium feldspar and its instability with 

respect to montmorillonite would discount the precipitation of potassium feldspar (White, 1979). The diminishing 

weathering rate of the ferromagnesian minerals downgradient tends to discount biotite. Removal of potassium 

by the clay minerals, as suggested by the statistical 

trends, is the likely control rather than the weathering or 

forming of primary or secondary minerals.

MAGNESIUM

In addition to weathering and precipitation reactions, 

magnesium may be controlled by ion-exchange reactions. 

In the study area, magnesium correlates positively 

(p<0.001) with calcium, dissolved solids, potassium, sulfate, sodium, and chloride. Except for the correlation 

with calcium, the positive correlations are the reversal of 

the negative correlations determined in Vekol Valley and 

other basins containing dilute, low-sulfate water. In 

Vekol Valley, for example, magnesium correlated positively with calcium and alkalinity but negatively with 

sodium, sulfate, chloride, dissolved solids, and pH in 

accordance with the aforementioned reactions. The 

ground water contains small sulfate concentrations and, 

accordingly, small magnesium concentrations (1-4 

mg/L).

Magnesium correlates positively with sodium, sulfate, 

chloride, and dissolved solids (p<0.0001) in the ground 

waters of Ranegras Plain, Verde Valley, the Safford 

basin, East and West Salt River Valleys, and basins 

along the Colorado River. As these basins contain the 

largest concentrations of sulfate and magnesium, the 

strong correlations with salinity and sulfate suggest that
ION EXCHANGE C79

25

20

15

10

DC 

UJ

DC 

UJ 

Q_

CO

DC 

(D

500 1,000 1,500 2,000 2,500

S042 , IN MILLIGRAMS PER LITER

FIGURE 37. Relation of potassium to sulfate for selected basins.

ion exchange is a major control of magnesium. As 

demonstrated later, ion-exchange reactions dominate the 

chemistry in these basins to the extent that they totally 

mask the fundamental reactions controlling the mass 

transfer of the primary and secondary silicate minerals of 

magnesium. The critical aspects of the basin selection for 

modeling are clearly revealed by the ion-exchange constraints.

Magnesium normally resides in the octahedral site of 

the montmorillonite; however, some magnesium seems 

to be available for exchange in the interlayer position. 

Exchangeable magnesium is inferred by the formula 

derived by the modeling and the formulas determined for 

the dioctahedral montmorillonites in table 26. The number of octahedral atoms exceeds two and suggests that 

some magnesium may be present in the interlayer site 

(Jones and Weir, 1983). In areas that contain large 

dissolved-solids and sulfate concentrations, the magnesium may be replaced by calcium, causing the observed 

large concentrations of dissolved magnesium.

ION EXCHANGE IN RANEGRAS PLAIN

The ion-exchange reactions of calcium, sodium, potassium, and magnesium are quantified through the derivation of the mass transfer using the composition of a 

high-sulfate, high-chloride ground water in Ranegras 

Plain. Along the flow path from the Granite Wash 

Mountains to Ran 4 (fig. 21), sodium concentrations

increase from 0.97 to 41.30 mmol (22 to 950 mg/L), 

magnesium concentrations increase from 0.6 to 3.3 mmol 

(15 to 80 mg/L), and potassium concentrations increase 

from 0.05 to 0.31 mmol (2 to 12 mg/L). The ground water 

at Ran 4 contains the largest sulfate concentration, 19.8 

mmol (1,900 mg/L), and the largest magnesium and 

potassium concentrations in the basin. The reactions 

along the flow path can be written to the predominant 

species as

Recharge water+0.1502 +0.93H20

+3.68oligoclase (An30)+0.58hornblende+0.41Kspar

+22.00halite+19.57gypsum+0.11fluorite-^3.30calcite

+2.90montmorillonite+0.59goethite+2.92silica

+8.48Ca+2 +3.29Mg+2 +41.30Na+ +22.57Cr

+ 19.78S04 2 +1.98HCO^+0.43H4Si04+0.22F 

+0.31K+ +0.040H~. (35)

Exchange reactions that influence calcium, sodium, 

potassium, and magnesium concentrations in the ground 

water can be expressed by the following general equations:

Ca+2 +2Naex-^Caex +2Na"

Ca-t-2+2Kex-*Caex +2K

and

Ca+2+Mgex-^Caex +Mg.+2

(36)

(37)

(38)
C80 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

and by mass balance

11.26Ca+2+(15.76Na, 3.26Mg, 0.23K)ex^11.26Caex

+ 15.76Na+ +3.26Mg+2+0.23K+ . (39)

Thus, an exchange of 11.26 mmol, or 451 mg/L, of 

calcium can account for 362 of the 950 mg/L of sodium, 79 

of the 80 mg/L of magnesium, and 8.9 of the 12 mg/L of 

potassium concentrations in solution. It is clear that the 

cation concentrations, when adjusted for ion exchange, 

and in the case of sodium, also for halite dissolution, are 

representative of concentrations found in dilute downgradient waters. Also apparent is the nearly identical 

mass transfer of the aluminosilicates determined for 

other basins containing dilute waters.

An alternate reaction that may account for the large 

magnesium concentrations at Ran 4 is the dissolution of 

dolomite. The ground water is at equilibrium with 

respect to calcite (SIcal=0.040) and gypsum 

(SIffyp=Q.Q7Q) and is undersaturated with respect to 

dolomite (<S/doZ =-0.674). Incongruent reactions as 

described by Plummer (1977) could occur where gypsum 

dissolution and calcite precipitation may cause dissolution of dolomite (dedolomitization). The general state of 

dolomite undersaturation of the ground water and the 

fact that dolomite was not detected during XRD analysis 

within the saturated zone favors the ion-exchange model. 

Moreover, dedolomitization would not explain the 

increase in potassium and sodium concentrations.

COMPARISON OF MASS TRANSFER 

AMONG BASINS

A comparison of mass transfer among basins indicates 

that significant mass transfer of the silicates is not 

occurring downvalley and suggests that many basins 

may not have significant downvalley movement of water. 

Mass transfer of the silicates in the confined aquifer of 

lower San Pedro Valley was observed only along flow 

paths. Ground water originates as runoff and precipitation along the Galiuro Mountains, moves normal to the 

mountains to the basin axis, and is discharged by dense 

phreatophytes along a line sink. The narrow basins in the 

southeastern part containing line sinks, and at least one 

basin in the northwestern part, Big Sandy basin (Davidson, 1973b), do not have significant downvalley movement of water.

Small ground-water flows moving downvalley may not 

be limited to basins in the southeastern and northwestern parts that contain extensive linear sinks. For example, in arid McMullen Valley, where confined conditions 

and extensive phreatophytes are not present as in lower 

San Pedro Valley, a similar lack of downvalley movement 

is suggested by the high degree of chemical reaction from 

the recharge area along the mountains to the center of

the basin and by the small mass transfer of the silicates 

between the upper and lower parts of the valley. In fact, 

larger reaction coefficients were derived for the upper 

part than for the lower part. An explanation for the 

observed mass transfer may lie in the distribution of 

precipitation and recharge and the evapotranspiration 

patterns. A major hydrologic difference between the two 

valleys is that the lower San Pedro Valley drainage area 

receives greater precipitation and recharge. The greater 

recharge and ground-water flow cannot be discharged by 

underflow rapidly enough to prevent the buildup of 

heads, resulting in high potentiometric surface and shallow water tables which, in turn, result in the extensive 

linear discharge. Underflow out of a basin is controlled 

by the hydraulic conductivity and the cross-sectional 

area of the saturated sediments at the outlet. As the 

cross-sectional area is considerably smaller at the outlet 

than at midbasin, owing to typical narrowing of the basin 

and thinning of the sediments, shallow water levels may 

be present even in downvalley areas in basins that 

presumably receive minimal or insignificant recharge. 

Although basins in arid areas like McMullen Valley do 

not discharge ground water along their entire axis, some 

discharge probably occurs near the outlet.

The amount and distribution of precipitation are crucial to the annual recharge of a basin. Metzger and Loeltz 

(1973) determined that the total annual precipitation 

within the watershed of the lower Colorado River area 

must exceed 8 in before any recharge can occur. Precipitation below this threshold value serves only to satisfy 

soil-moisture deficits and evapotranspiration losses. In 

McMullen Valley, only the precipitation within the central parts of the Harcuvar and Harquahala Mountains 

may be providing recharge to the basin aquifer, as 

precipitation in the distal areas is marginally greater 

than a presumed threshold value of 8 to 10 in. Thus, the 

ground water, which may be recharged primarily at 

midvalley, moves only a short distance downvalley and 

discharges under shallow water-table conditions near the 

basin outlet during periods of high evapotranspiration. 

Phreatophyte growth, although not as dense as the 

mesquite forest in lower San Pedro Valley, may be 

commensurate in abundance with the discharge. Significant phreatophyte growth is present in downvalley areas 

of McMullen Valley, Butler Valley, Ranegras Plain, 

Harquahala Plain, La Posa Plain, and other west-central 

basins. Analogous to lower San Pedro Valley, large 

fluxes of water may not be moving down the entire valley 

and out of the basin. Thus, the chemical cycle repeats 

itself in the adjoining basin of lower altitude, a trend 

observed thoughout most of the study area. The chemical 

data suggest that ground-water movement is not characterized by a continual increase in downvalley flow from 

basin to basin.
BASIN CATEGORIZATION AND INFORMATION TRANSFER C81

DISTRIBUTION PATTERNS OF MASS TRANSFER

The mass transfer derived for the basins may be 

illustrated by two general distribution patterns those 

shown by the narrow basins like lower San Pedro Valley 

in the southeast and those shown by the oval basins like 

Vekol Valley and Ranegras Plain in the south-central and 

western areas. The patterns are related primarily to the 

difference in precipitation and distribution of recharge. 

Two patterns of mass transfer derived through weathering of andesine and augite are shown in figure 38. The 

mass transfer in the basins of the southeast (figs. 38A, 

385) is characterized by an equipotential pattern that 

parallels the mountains. The equipotential distribution is 

indicative of significant recharge along the entire perimeter of the basin and corresponding discharge along the 

entire central drainage. The idealized pattern of the 

west-central basins is indicative of lesser recharge confined to, in this example, two general areas (figs. 38C, 

38D).

Figures 38C and 38Z) show the effect of an ephemeral 

stream on the water chemistry in the downvalley area 

under shallow water-table conditions; the small mass 

transfer of andesine and augite shown was derived along 

the extreme lower reach of Bouse Wash near Bouse in 

Ranegras Plain. Recharge is occurring along a sandy 

phreatophyte-free channel under shallow ground-water 

conditions of about 80 ft to water. Chemical and isotope 

data from well (B-6-17)12dbb2 near the wash confirm 

recharge along the wash; a 513C of - 12.7%o and the major 

element chemistry are in good agreement with 

mountain-front recharge compositions. The deuterium 

content of -54.0%e of water from this narrow band of 

local recharge is enriched about 15%o relative to ground 

water in the regional system, which originates near the 

Plomosa and Granite Wash Mountains. The enrichment 

in deuterium along the wash relative to the regional 

system is attributed to infiltration of water during summer flood events, as higher seasonal temperatures favor 

enrichment. Surface runoff from the mountain fronts 

commonly reaches the valley floors in response to highintensity summer thunderstorms.

The distribution of stable-isotope and chemical data in 

Ranegras Plain validates the two fundamental principles 

regarding the chemical models and basin hydrology: (1) 

the predominance of mountain-front recharge and (2) the 

existence of a closed hydrochemical system. The hydrochemical system of most basins in the study area is 

depicted as the regional recharge being attributed to 

winter precipitation along the mountain fronts and the 

local infiltration of water being attributed to summer 

flow events only in areas of shallow water. Most of the 

water in storage originates as winter precipitation that 

infiltrates along mountain fronts and evolves under

closed conditions through rock-water weathering reactions, as suggested by the distribution patterns of mass 

transfer shown in figure 38.

BASIN CATEGORIZATION AND 

INFORMATION TRANSFER

A primary goal of the geochemistry study was to 

develop information transfer from basins for which the 

geochemistry and the water quality are understood to 

basins for which data are inadequate and geochemical 

histories are complex. The information, which includes 

geochemical concepts, reactions, and water quality, was 

transferred or projected through categorization of the 

basins according to physiographic, geologic, and hydrologic characteristics and their effect on geochemistry. 

The basins in the study area were divided geochemically 

into two general groups on the basis of an open or closed 

chemical evolution. Geochemistry and water quality can 

be reasonably predicted for other basins by evaluating 

the geologic and hydrologic parameters of these basins 

and using the assumptions and model results pertinent to 

each group.

BASIN CATEGORIZATION

The basins were categorized as open basins, basins 

that contain open evolutionary systems, in the eastern 

part of the study area, and closed basins, basins that 

contain closed evolutionary systems, in the west-central 

part. (See section entitled "Evolution of the Ground 

Water in Selected Basins.") The eastern basins have 

bordering mountains of high altitude that provide 

greater precipitation and greater recharge potential, and 

some have a coarse-grained upper basin-fill unit that 

allows infiltration and percolation of runoff along the 

valley floor. The west-central basins receive less precipitation and thus less recharge, and many have a finegrained unit overlying the aquifer, factors that favor a 

closed system. Exceptions are basins in the western part 

that are dominated by the Colorado River and parts of 

basins in the southeast that contain the confined aquifers.

OPEN BASINS

The Willcox and Douglas basins, upper San Pedro 

Valley, and the Tucson basin in the upper Santa Cruz 

area are categorized as open basins. The Tucson basin, 

one of the largest basins in the study area, is a good 

example of an eastern basin. The open system is a result 

of the high annual precipitation in the Santa Catalina 

(greater than 35 in), Tanque Verde (greater than 25 in), 

and Rincon (greater than 30 in) Mountains bordering the
C82 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES 

DISTRIBUTION OF MASS TRANSFER IN NARROW BASINS IN THE SOUTHEASTERN PART OF THE STUDY AREA

<&

A. Mass transfer of andesine B. Mass transfer of augite

DISTRIBUTION OF MASS TRANSFER IN OVAL BASINS IN THE WEST-CENTRAL PART OF THE STUDY AREA

C. Mass transfer of andesine D. Mass transfer of augite

8 MILES

J

8 KILOMETERS

EXPLANATION

-1.0 LINE OF EQUAL MASS TRANSFER-lnterval, in millimoles per 1,000 

grams of water, is variable. Line dashed where extrapolated

FIGURE 38. Idealized distribution of mass transfer.
BASIN CATEGORIZATION AND INFORMATION TRANSFER C83

northern and eastern parts of the basin and the numerous streams that drain these mountains. Major streams 

that traverse the basin floor are the Santa Cruz River, 

Pantano and Aqua Caliente Washes, Canada del Oro, and 

Tanque Verde, Rillito, Big, Sabino, and Rincon Creeks. 

Most of the estimated annual recharge to the aquifer of 

100,000 acre-ft is supplied by infiltration along these 

streams (Davidson, 1973a). The shallow depths to water 

of 25 to 250 ft (Davidson, 1973a) throughout the basin and 

the generally permeable deposits of the upper basin fill 

(Fort Lowell Formation of Pleistocene age) favor infiltration and recharge in many other canyons and stream 

channels in the basin.

The shallow water-table aquifers overlying the confined aquifers in the eastern part also are open systems. 

These open systems include the saturated stream sediments along the San Pedro, Gila, and Salt Rivers and San 

Simon Wash. Open systems also occur in the shallow 

stream aquifers in the intermontane basins of the Central 

highlands water province. Altar and Avra Valleys also 

are open or partially open basins and are in a transition 

zone between the open and closed systems, primarily 

because of the westward trend of decreasing precipitation. Open systems may occur in large parts of Altar and 

Avra Valleys owing to the presence of a moderately 

coarse grained upper basin-fill unit. Closed conditions 

may exist in many basins near the mountains depending 

on the depth to water and, locally, on the thickness, 

vertical hydraulic conductivity, and areal extent of any 

fine-grained unit overlying the aquifer.

CLOSED BASINS

Most of the basins in the western and central parts of 

the study area are categorized as closed; the eastern 

basins that contain the confined aquifers also are considered closed basins. Through examination of water-level 

data, water chemistry, geology, or stratigraphy, closed 

systems were indicated in Vekol, McMullen, Butler, 

Rainbow, and Hualapai Valleys, Harquahala and La 

Posa Plains, the Bullard Wash area, and the Big Sandy 

basin in the west-central area. A large part of the Big 

Sandy basin may contain an extensive aquifer underlying 

the shallow water-table aquifer along the Big Sandy 

River. Because of an abundance of shallow ground water 

along the river and lack of development elsewhere, data 

to define the underlying aquifer were not available 

throughout the rest of the basin. In the eastern part of 

the area, the confined aquifers in lower San Pedro, San 

Simon, and San Bernardino Valleys, the Benson area, 

and the Safford basin are also categorized as closed 

systems.

Closed systems occur in East and West Salt River 

Valleys in areas away from the river where depths to

water exceed approximately 200 ft and are not influenced 

by irrigation return flow. A closed system is also found in 

a large part of Verde Valley. Closed conditions occur in 

parts of Little Chino Valley and of the adjacent Agua 

Fria River area that are not influenced by streams of the 

area. The analysis of available data for the MaricopaStanfield and Eloy areas and many of the smaller nearby 

basins on the Papago Indian Reservation indicates that 

most of the ground water probably evolves under closed 

conditions. On the basis of few data, open and closed 

conditions apparently occur in parts of Sacramento and 

Detrital Valleys. Virtually no data are available for the 

Castle Dome and Palomas Plains, King and Mohawk 

Valleys, the San Cristobal Wash and Growler Wash 

areas, and Lechuguilla Desert in the extreme southwestern part of the study area, but owing to the sparse 

precipitation in the area, ground water may evolve under 

predominantly closed conditions.

PROJECTION OF GROUND-WATER CHEMISTRY

OPEN SYSTEMS

The ground water evolving under open conditions is 

characterized by large and variable concentrations of 

calcium, magnesium, and bicarbonate ions. In dilute 

waters, the concentrations of these ions are the major 

differences in chemistry between the open and closed 

systems. The chemical data available for the Tucson 

basin show such variability and suggest that the ground 

water evolves under open conditions throughout much of 

the basin. Variable concentrations may be the result of 

dissolution of calcite or dolomite under shallow watertable conditions, as described by equation 26. Concentrations of these elements would be determined mainly by a 

reservoir of soil C02 and the availability of calcite or 

dolomite in the soil or basin fill.

Upward leakage from the confined aquifers may be an 

additional factor affecting the water chemistry of the 

overlying shallow unconfined aquifers. In the unconfined 

aquifers along the rivers in lower San Pedro Valley and 

in the Safford basin, for example, the water chemistry of 

all dissolved species is variable. Some of the variability, 

particularly of sulfate and chloride, could be caused by 

upward leakage through the gypsiferous deposits known 

to be present. Highly variable fluoride concentrations 

unique to the unconfined aquifers that overlie the confined aquifers may also be a result of upward leakage 

(Robertson and Garrett, 1988). The presence of halite or 

gypsum, although much less common in the upper unit or 

stream alluvium, would also cause some variability. 

Because of the inherent problem in open systems of the 

mixing of waters having different compositions, a general process was not developed for describing reactions
C84 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

quantitatively and for projecting water chemistry. However, if ground water in an open system is anticipated to 

be high in dissolved solids, techniques described in the 

next section may make it possible to project water 

chemistry. Projections are possible because the differences in the chemical compositions of the water evolved 

under the two processes decrease as the salinity 

increases and because the salinity is caused primarily by 

dissolution of halite and gypsum and their mediation of 

other ions.

CLOSED SYSTEMS

The modeling used in conjuction with the analytical 

techniques developed during this study has proved valuable in projecting water chemistry and geochemical 

trends in basins with closed systems. The concentrations 

of the major dissolved species can be estimated with 

reliability, and occurrences of some trace elements can 

be projected. Average values for major ions in downgradient waters, based on 21 analyses from the Big Sandy 

basin, Harquahala Plain, San Simon Wash area, and 

Little Chino, lower San Pedro, Butler, McMullen, West 

Salt River, Hualapai, Paradise, and Altar Valleys, where 

closed conditions exist, are shown in table 29. If ground 

water in any basin evolves in a closed system and is low 

in dissolved solids (less than 50 mg/L of sulfate and 

chloride), the concentrations of the major ions should fall 

within the range indicated by the analyses. If the ground 

water contains larger dissolved-solids concentrations, 

the water will have the composition of these dilute 

waters plus the additional sulfate and chloride concentrations, plus sodium concentrations equal to molar 

increases of chloride and any exchange of calcium for 

sodium provided by the dissolution of gypsum. Calcium 

concentrations would be equal to those in table 29 plus 

the increase in molar concentrations in sulfate, minus any 

exchange of calcium for sodium. The nature of the 

ion-exchange mechanism and the extent of exchange of 

all cations in waters of differing salinity are not clear but 

can be empirically estimated for sodium by assigning 40

TABLE 29. Average major ion concentrations in dilute 

downgradient waters

Millimoles per liter Milligrams per liter

Mean Standard 

deviation Mean Standard 

deviation

Ca+2 . Mg+2

Na+ .

c

HC03~ 

Si02 ..

0.215

.084

3.782

.047

.929

.387

2.387

.443

0.11

.07

1.15

.02

.53

.23

.47

.17

8.6

2.0

87

1.8

33

37

146

27

4.4

1.7

28

.8

19

22

29

10

percent of the calcium increase supplied by the gypsum 

increase to ion exchange. A twofold or threefold increase 

in magnesium and potassium concentrations may occur if 

the ground water contains concentrations of sulfate in 

excess of 500 mg/L. These concentrations may increase 

severalfold if sulfate concentrations increase to more 

than 1,000 mg/L. The composition of the ground water 

can be estimated after an analysis for chloride and 

sulfate, which can be done quickly and accurately in 

the field.

In an area where a water sample is not available for 

analysis, the chemical composition of the ground water 

can nonetheless be estimated from basin-fill material. 

Techniques for analyzing the solid species well cuttings 

or drill core developed during the study may also be 

used for projection of ground-water chemistry. The 

analysis consists of leaching small amounts (2-4 grams) of 

the material in distilled water and analyzing the leachate 

for chloride, sulfate, nitrate, and fluoride by ion chromatography. The concentrations of chloride, sulfate, and 

nitrate in the leachate closely approximate actual concentrations of these ions in the ground water from the 

completed wells. Analyses of well-cuttings leachate and 

of ground-water compositions from Vekol Valley and the 

Bosque basin (in Gila Bend Plain) are shown in table 30 

and illustrate the degree of confidence of the predictive 

technique. Thus, most major ion concentrations can be 

approximated without the expense of large-diameter 

boreholes and installation of casing and pumping equipment. Water compositions can be estimated by analyses 

of samples from small-diameter drill core or of material 

that is already stored in repositories. This predictive 

technique is a potentially powerful tool for evaluation of 

water quality and geochemistry in undeveloped areas in 

the southwest as well as in the study area.

Occurrences of Cr+6 and larger concentrations of fluoride can be expected in the arid west-central basins. 

Closed systems favor high pH values and the removal of 

solute calcium, factors that may produce large fluoride 

concentrations. The high pH values and the long residence time of these waters also may cause large hexavalent chromium concentrations. The few data available in 

the western part of the study area support this projection, as the ground water contains noticeably large 

fluoride and hexavalent chromium concentrations.

Hexavalent chromium, however, is conspicuously 

absent in the southwestern part of the study area near 

Yuma, which is the most arid part. The absence probably 

is due to the distribution of the sampling points. A large 

part of the area consists of bombing and gunnery ranges 

or other military installations, and little development has 

occurred. Most samples were taken from wells in the 

Colorado River flood plain or along the Gila River 

between Gila Bend and Yuma and were from shallow,
SUMMARY C85

TABLE 30. Analyses of well-cuttings leachate for Monde, sulfate, 

nitrate, and fluoride from wells in Vekol Valley and the Basque 

basin (Gila Bend Plain)

[In parts per million* except as indicated. Analyses by Center for Meteorite 

Studies, Arizona State University]

Depth of sample, 

in feet below 

land surface

cr so4-2 N03- F~

(D-&-l)13bbd (Vekol Valley)

370. .........

470. .........

730. .........

1,410. .........

1,610. .........

1,910..........

2,100..........

Average ......

Analysis** ....

103 

65 

54 

51 

69 

97 

94 

76 

78

62 

54 

44 

55 

71 

57 

104 

62 

95

<30 

<30 

<30 

<30 

<30 

<49 

36 

<42 

37.4

24 

7 

9 

6 

7 

7 

8 

9.25 

0.5

(C-6-3)2acd (Bosque basin)

320. .........

640. .........

840. .........

1,040. .........

1,350. .........

1,620. .........

1,950. .........

2,120..........

A vpva ITP

Analysis** ....

166 

278 

117 

232 

250 

188 

280 

94 

201 

170

177 

149 

45 

131 

110 

60 

136 

72 

110 

120

<30 

<30 

<30 

<30 

66 

<30 

<30 

<30 

<66 

31.6

9 

13 

4 

9 

6 

4 

7 

4 

7 

0.3

*Reported on dried sample basis.

**Analysis of ground water from completed well.

open-system aquifers. It is quite probable that if wells 

are drilled away from the rivers where depths to water 

exceed 150 ft, the presence of chromium in the ground 

water will be detected.

SUMMARY

Ground water in the alluvial basins of the Southwestern United States has been the subject of geochemical 

and water-quality investigations as part of the Southwest Alluvial Basins Regional Aquifer-System Analysis 

Program. The study area encompasses about 82,000 mi2 

in the Basin and Range physiographic province in Arizona and small parts of adjacent States. The principal 

landforms of the area are a series of north- to northwesttrending mountains and basins, which were formed by 

large-scale normal faulting during the Basin and Range 

disturbance 14 to 6 million years ago. The mountains in 

the area consist of Precambrian and Tertiary metamorphic and intrusive rocks, Paleozoic sedimentary rocks, 

Mesozoic igneous and sedimentary rocks, and Tertiary 

volcanic and sedimentary rocks. The basins contain alluvial deposits of late Cenozoic age that attain thicknesses 

of many thousands of feet. The basin fill was deposited by

local drainage processes, as indicated by the clast type, 

the basinward increase in fine-grained materials, and the 

proximity of the finest grained and thickest deposits to 

the basin center. The basin deposits are divided into 

three units the lower basin fill, the upper basin fill, and 

the stream alluvium on the basis of source, particle 

size, and composition. The lower basin fill generally is 

highly consolidated and deformed and contains the finest 

grained deposits. Evaporites, where present, occur predominantly in this unit. The upper basin fill was deposited near the end of the Basin and Range disturbance and 

is coarser grained and less cemented, deformed, tilted, 

or faulted than the lower unit. The sediments of the 

stream alluvium generally are less than 100 ft thick and 

are found along rivers and streams.

The mountains that surround the basins have little 

permeability and form ground-water divides that separate the study area into 72 basins or individual alluvial 

aquifers. These aquifers join together in a dendritic 

pattern to form the regional aquifer system. Ground 

water is recharged to the aquifers by infiltration of 

precipitation and runoff along the mountain fronts, by 

infiltration along streams, and by underflow from adjacent basins of higher altitude. The chemical and isotope 

data indicate that in most basins the major recharge 

occurs along the mountain fronts. Ground-water discharge from basins occurs through evapotranspiration, 

ground-water underflow, pumping, and discharge to 

streams. Ground water generally is under unconfined 

conditions, but confined conditions are common throughout the eastern part of the study area where areally 

extensive fine-grained facies of the basin fill and sufficient recharge occur.

X-ray diffraction analyses of the less than 62-|jim 

fraction of the basin fill show that plagioclase, potassium 

feldspar, calcite, quartz, muscovite, and smectite are the 

most common minerals. Dolomite was detected in the 

unsaturated zone of several basins but not in the saturated zone. Montmorillonite, a magnesium dioctahedral 

smectite, is the dominant clay in the less than 2-|jim 

fraction, but trace to small amounts of kaolinite and 

muscovite are present. Chlorite and vermiculite are not 

present. Montmorillonite is the most stable phase within 

the basin fill, as indicated by its abundance and the 

saturation state of the ground water. Variations in 

precipitation, altitude, and vegetation among the basins 

apparently have little influence on the type of clay 

formed. The clay mineralogy of the soil zone is similar to 

that of the basin fill; montmorillonite is the dominant 

clay, but mica and kaolinite may be more abundant.

The chemical quality of the ground water is suitable for 

most purposes but is not without problems. Most basins 

contain ground water with less than 1,000 mg/L 

dissolved-solids concentrations, but ground water in
C86 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

some basins contains concentrations in excess of 3,000 

mg/L and can be classified as hard to very hard. All 

ground water sampled contained dissolved-oxygen concentrations near 70 percent or more of saturation. Owing 

primarily to the oxidizing environment and the abundance of trace elements in the basin fill, the ground water 

in several basins contains large concentrations of trace 

elements that exceed Federal and State maximum contaminant levels or toxicity levels for plants. The constituents of fluoride, chromium, arsenic, boron, and nitrate, 

in particular, present environmental problems in several 

basins.

The occurrence of fluoride in the ground water may be 

related to mineral-equilibrium and sorption or desorption 

reactions. The mineral fluorite apparently is a control of 

fluoride in water that contains concentrations greater 

than about 5 mg/L. Smaller concentrations seem to be 

controlled by pH-dependent exchange reactions. Availability of the fluoride in the rocks may be an additional 

control; large fluoride concentrations are correlated with 

acidic and intermediate volcanic rocks and their weathering products. Hexavalent chromium is a naturally 

occurring constituent in ground water throughout much 

of central and western Arizona. The observed concentrations are a result of oxidation of Cr+3 in the basin fill, 

apparently under thermodynamic-equilibrium conditions. The larger concentrations tend to be associated 

with intermediate volcanic rocks. The weathering of 

sulfide and arsenide mineral deposits probably is the 

primary source of arsenic. Mineralogic controls appear to 

have little or no effect on arsenic solubility in the 

oxygenated environment in which arsenic occurs as the 

arsenate ion. A major control of arsenic occurrence and 

removal is the coprecipitation or adsorption of the arsenate ion on ferric oxide or ferric oxyhydroxide surfaces. 

Boron concentrations are larger in the western basins 

and may be a function of reworking and concentrating of 

boron in the soil. The largest concentrations are related 

to playa or evaporite deposits where the soluble borate 

compounds tend to be concentrated. The geochemical 

behavior of nitrogen is parallel to that of boron. Nitrate, 

the most stable species of nitrogen in the ground water, 

is highly soluble and also tends to be enriched in the basin 

centers. Large nitrate concentrations have multiple 

sources. Selenium is geochemically similar to arsenic, 

and, like arsenic, concentrations may be controlled by 

sorption-desorption reactions on ferric oxide or ferric 

oxyhydroxide surfaces. The concentrations of barium in 

the ground water are controlled solely by the solubility of 

the mineral barite and generally do not exceed the 

maximum contaminant level of 1.0 mg/L. The weathering of alkali feldspars is the major source of barium in the 

ground water.

Geochemical models were developed to identify reactions and mass transfer responsible for the evolution of

the ground water. On the basis of the mineralogy and 

chemistry of the rocks of the surrounding mountains and 

of the basin fill, two chemical models a mafic model and 

a felsic model were developed to define reactions. Two 

distinct hydrochemical processes were identified: reactions of meteoric water with minerals and gases in the 

recharge areas, and reactions of the ground water as it 

moves downgradient within the aquifer. Major reactions 

controlling the recharge chemistry are the weathering of 

feldspars and ferromagnesian minerals, dissolution of 

carbonate minerals, and formation of montmorillonite 

and iron oxyhydroxides in an acid environment. Major 

reactions controlling the chemistry downgradient are the 

weathering of plagioclase, formation of montmorillonite, 

precipitation of calcite, and possibly of silica, and, in 

some basins, ion exchange of calcium for sodium, potassium, and magnesium. Geochemical models were developed for several basins to determine geologic, climatic, 

and physiographic effects on ground-water chemistry. 

Relatively few phases are required to account for 

the chemical evolution of the ground water; for each 

basin, 14 phases 12 mineral and 2 gas accounted for 

the ground-water evolution and changes in water 

chemistry.

The high partial pressure of soil C02 and the high 

reaction rates of the silicates in the recharge waters 

suggest that carbonic acid supplies the impetus in 

the weathering process and is considerably more important than the residence time of the ground water. 

The transport of C02(g) through the unsaturated zone 

has not been defined but is postulated to occur via deep 

percolation during recharge. Reactions in the unsaturated zone appear to be highly influential in determining 

the composition of the ground water. Compositions of 

some ground water from different geologic terranes 

appear similar to or unrelated to obvious source rocks. 

Reasons for these similarities or anomalies are unclear; 

possible explanations are inconsistencies between the 

mineralogy of the basin fill and that of the mountains and 

reactions mediated by climate and precipitation in the 

unsaturated zone that may be obscuring or altering 

initial compositions.

The basins were divided geochemically into two categories, closed and open systems. In closed systems the 

aqueous chemistry is determined solely by reactions of 

the initial recharge water with minerals and gases as it 

moves downgradient. Conversely, in open systems the 

aquifer chemistry is controlled or influenced by gases or 

water that enter the system after the initial recharge.

In closed systems, the reactions and associated mass 

transfer were successfully defined along flow paths and 

were found to be consistent within and among basins. In 

open systems, probable reactions were identified but the 

mass transfer along the flow path could not be defined, as 

the chemistry is influenced by additional recharge. Reac-
SELECTED REFERENCES C87

tions in the unsaturated zone seem to play a larger role in 

open systems than in closed systems. The decisive factors in determining whether reactions occur in an open or 

closed system are the annual precipitation that falls 

within the basin drainage area and the presence or 

absence of fine-grained units. Basins in areas of higher 

precipitation in the eastern part of the study area and 

those along the Colorado River were categorized as open 

systems. Most basins in the central and western parts 

were categorized as closed systems, as were the confined 

aquifers in basins in the east. The closed systems were 

supported by stable and radiogenic isotope data and by 

the transferability and consistency of reactions from the 

confined aquifers of the eastern basins to the generally 

unconfined aquifers of the western basins. The consistency of the mass transfer of the silicates among basins 

clearly indicates that the major reactions have been 

identified.

The basins adjoining the Colorado River are unique 

among other basins in the region because they were 

formed by and are hydrologically dominated by a major 

river. Most of the ground water in basins along the river 

originated as infiltration of river water or, before construction of the dams, as overbank flow during spring 

floods. Ground water that originates from the river may 

extend several miles beyond the flood plain into the 

piedmont areas. The 8D and 8180 of the ground water in 

these basins plot along a line having a slope of 5, 

indicating enrichment of the heavier isotopes through 

evaporation. The initial deuterium content of the river 

water of about -120%e is indicative of that of the 

precipitation in central Colorado. Reactions proposed in 

the literature to account for the chemical evolution of the 

ground water along the river, namely concentration 

through evapotranspiration, sulfate reduction, and precipitation of carbonates, are not realistic. The chemical 

and isotope data indicate that increases in salinity and 

changes in sulfate to chloride ratios generally are not a 

function of sulfate reduction or evapotranspiration. 

Reactions were developed that show weathering of silicates and the dissolution of gypsum, halite, and calcite 

are considerably more realistic in accounting for chemical 

evolution and increased salinity of the ground water.

Ion exchange is not a significant reaction in basins that 

contain ground waters having low dissolved-solids content. However, ion exchange of calcium for sodium, 

potassium, and magnesium is a significant reaction in 

basins that contain large concentrations of dissolved 

solids and sulfate. The uptake and release of potassium 

through exchange reactions is the most important control 

of solute potassium. In the gypsiferous waters of several 

basins, ion exchange of calcium for magnesium is the 

major cause of large concentrations of magnesium in the 

ground water.

The information obtained from the modeling and the 

analytical techniques developed during this study can be 

used to project ground-water chemistry throughout most 

of the area. Water compositions for the major ions can be 

estimated from a simple field analysis of the water for 

sulfate and chloride or from an analysis of the basin-fill 

material for these two ions. This predictive technique is 

a potentially powerful tool for evaluation of the water 

quality and the geochemistry in undeveloped areas in the 

Southwest.

SELECTED REFERENCES

Ahrens, L.H., 1965, Distribution of the elements in our planet: New 

York, McGraw-Hill, 110 p.

Anderson, T.W., 1972, Electrical-analog analysis of the hydrologic 

system, Tucson basin, Arizona: U.S. Geological Survey WaterSupply Paper 1939-C, 34 p.

 1980, Study plan for the regional aquifer-system analysis of 

alluvial basins in south-central Arizona and adjacent States: U.S. 

Geological Survey Open-File Report 80-1197, 22 p.

Back, W., and Barnes, Ivan, 1961, Equipment for field measurement of 

electrochemical potentials: U.S. Geological Survey Professional 

Paper 424-C, p. 366-368.

Ball, J.W., Nordstrom, O.K., and Jenne, E.A., 1980, Additional and 

revised thermochemical data and computer code for WATEQ2 A 

computerized chemical model for trace and major element speciation and mineral equilibria of natural waters: U.S. Geological 

Survey Water-Resources Investigations 78-116, 109 p.

Barnes, Ivan, 1964, Field measurement of alkalinity and pH: U.S. 

Geological Survey Water-Supply Paper 1535-H, 17 p.

Barnes, Ivan, and O'Neil, J.R., 1971, Calcium-magnesium carbonate 

solid solutions from Holocene conglomerate cements and travertines in the Coast Range of California: Geochimica et Cosmochimica Acta, v. 35, p. 699-718.

Earth, T.F.W., 1967, Theoretical petrology (2d ed.): New York, John 

Wiley, 416 p.

Bentley, C.B., 1979, Geohydrologic reconnaissance of Lake Mead 

National Recreation Area Mount Davis to Davis Dam, Arizona: 

U.S. Geological Survey Open-File Report 79-691, 34 p.

Berner, R.A., 1971, Principles of chemical sedimentology: New York, 

McGraw-Hill, 328 p.

Bowen, N.L., 1956, The evolution of the igneous rocks: New York, 

Dover Publications, 332 p.

Briggs, P.C., 1969, Ground-water conditions in the Ranegras Plain, 

Yuma County, Arizona: Arizona State Land Department WaterResources Report 41, 28 p.

Brown, Eugene, Skougstad, M.W., and Fishman, M.J., 1970, Methods 

for collection and analysis of water samples for dissolved minerals 

and gases: U.S. Geological Survey Techniques of Water-Resources 

Investigations, Book 5, Chapter Al, 160 p.

Brown, S.G., Davidson, E.S., Kister, L.R., andThomsen, B.W., 1966, 

Water resources of Fort Huachuca Military Reservation, southeastern Arizona: U.S. Geological Survey Water-Supply Paper 

1819-D, 57 p.

Brown, S.G., and Schumann, H.H., 1969, Geohydrology and water 

utilization in the Willcox basin, Graham and Cochise Counties, 

Arizona: U.S. Geological Survey Water-Supply Paper 1959-F, 

32 p.

Colman, S.M., 1982, Chemical weathering of basalts and andesites 

Evidence from weathering rinds: U.S. Geological Survey 

Professional Paper 1246, 51 p.
C88 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

Craig, Harmon, 1961, Isotopic variations in meteoric waters: Science,

v. 133, no. 3465, p. 1702-1703. 

Creasey, S.C., 1965, Geology of the San Manuel area, Final County,

Arizona, with a section on Ore deposits, by J.D. Pelletier and S.C.

Creasey: U.S. Geological Survey Professional Paper 471, 64 p. 

Davidson, E.S., 1973a, Geohydrology and water resources of the

Tucson basin, Arizona: U.S. Geological Survey Water-Supply

Paper 1939-E, 81 p.

 1973b, Water-resources appraisal of the Big Sandy area,

Mohave County, Arizona: Arizona Water Commission Bulletin 6,

40 p. 

Deer, W.A., Howie, R.A., and Zussman, J., 1976, Rock forming

minerals, v. 3, Sheet silicates: London, Longman Group Limited,

270 p.

 1980, An introduction to the rock-forming minerals: London,

Longman Group Limited, 528 p. 

Deines, Peter, Langmuir, Donald, and Harmon, R.S., 1974, Stable

carbon isotope ratios and the existence of a gas phase in the

evolution of carbonate ground waters: Geochimica et Cosmochimica Acta, v. 38, p. 1147-1164. 

Dixon, J.B., and Weed, S.B., 1979, Minerals in soil environments:

Madison, Wis., Soil Science Society of America, 948 p. 

Doe, B.R., 1983, Isotopic research, climate, and the genesis of mineral

deposits, in Cronin, T.M., Cannon, W.F., and Poore, R.Z., eds.,

Paleoclimate and mineral deposits: U.S. Geological Survey Circular 822, p. 44H18. 

Drever, J.I., 1982, The geochemistry of natural waters: London,

Prentice-Hall, 388 p. 

Dunbier, Roger, 1970, The Sonoran Desert, its geography, economy,

and people: Tucson, University of Arizona Press, 426 p. 

Durfor, C.N., and Becker, Edith, 1964, Public water supplies of the 100

largest cities in the United States, 1962: U.S. Geological Survey

Water-Supply Paper 1812, 364 p. 

Eberly, L.D., and Stanley, T.B., Jr., 1978, Cenozoic stratigraphy and

geologic history of southwestern Arizona: Geological Society of

America Bulletin, v. 89, no. 6, p. 921-940. 

Emrich, K., Ehhalt, D.H., and Vogel, J.C., 1970, Carbon isotope

fractionation during the precipitation of calcium carbonate: Earth

and Planetary Science Letters 8, p. 363-371. 

Faure, Gunter, 1977, Principles of isotope geology: New York, John

Wiley, 464 p. 

Fenneman, N.M., 1931, Physiography of Western United States: New

York, McGraw-Hill, 534 p. 

Ferguson, J.F., and Davis, Jerome, 1972, A review of the arsenic cycle

in natural waters: Water Resources Research, v. 6, p. 1259-1274. 

Feth, J.H., 1966, Nitrogen compound in natural water A review:

Water Resources Research, v. 2, no. 1, p. 41-59. 

Feth, J.H., Roberson, C.E., and Polzer, W.L., 1964, Sources of

mineral constituents in water from granitic rocks, Sierra Nevada,

California and Nevada: U.S. Geological Survey Water-Supply

Paper 1535-1, 70 p. 

Fontes, Jean-Charles, and Gamier, Jean-Marie, 1979, Determination

of the initial 14C activity of the total dissolved carbon: A review of

the existing models and a new approach: Water Resources

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Garrels, R.M., 1967, Genesis of some ground waters from igneous

rocks, in Abelson, P.H., ed., Researches in geochemistry, v. 2:

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Garrels, R.M., and MacKenzie, F.T., 1967, Origin of the chemical

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Gillespie, J.B., andBentley, C.B., 1971, Geohydrology of Hualapai and

Sacramento Valleys, Mohave County, Arizona: U.S. Geological

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Goldich, S.S., 1938, A study in rock weathering: Journal of Geology, 

v. 46, p. 17-58.

Goldschmidt, V.M., 1958, Geochemistry: London, Oxford University 

Press, 730 p.

Gough, L.P., Shacklette, H.T., and Case, A.A., 1979, Element concentrations toxic to plants, animals, and man: U.S. Geological 

Survey Bulletin 1466, 80 p.

Gulens, J., Champ, D.R., and Jackson, R.E., 1979, Influence of redox 

environments on the mobility of arsenic in ground water: American 

Chemical Society Symposium Series 93, p. 81-93.

Halpenny, L.C., and others, 1952, Ground water in the Gila River 

basin and adjacent areas, Arizona A summary: U.S. Geological 

Survey open-file report, 224 p.

Heindl, L.A., 1967, Ground water in fractured volcanic rocks in 

southern Arizona: Hydrology of fractured rocks, v. 2: International Association of Scientific Hydrology Publication 74, lip.

Helgeson, H.C., Brown, T.H., and Leeper, R.H., 1969, Handbook of 

theoretical activity diagrams depicting chemical equilibria in geologic systems involving an aqueous phase at one atm and 0° to 

300°C: San Francisco, Calif., Freeman, Cooper & Co., 253 p.

Hem, J.D., 1970, Study and interpretation of the chemical characteristics of natural water (2d ed.): U.S. Geological Survey WaterSupply Paper 1473, 363 p.

 1978, Redox processes at surfaces of manganese oxide and their 

effects on aqueous metal ions, in Chemical geology, v. 21: Amsterdam, Elsevier Scientific Publishing Co., p. 199-218.

-1985, Study and interpretation of the chemical characteristics of

natural water (3d ed.): U.S. Geological Survey Water-Supply 

Paper 2254, 263 p.

Jones, B.F., 1966, Geochemical evolution of closed basin water in the 

Western Great Basin, in Rau, J.L., ed., Second Symposium on 

Salt: Cleveland, Ohio, Northern Ohio Geological Society, v. 1, 

p. 181-200.

Jones, B.F., and Bowser, C.J., 1978, The mineralogy and related 

chemistry of lake sediments, in Lerman, A., Lakes Chemistry, 

geology, physics: New York, Springer-Verlag, p. 179-234.

Jones, B.F., Kennedy, V.C., and Zellweger, G.W., 1974, Comparison 

of observed and calculated concentrations of dissolved Al and Fe in 

stream water: Water Resources Bulletin, v. 10, no. 4, p. 791-793.

Jones, B.F., and Weir, A.H., 1983, Clay minerals of Lake Albert, an 

alkaline, saline lake: Clay and Clay Minerals, v. 31, no. 3, 

p. 161-172.

Kam, William, 1964, Geology and ground-water resources of McMullen 

Valley, Maricopa, Yavapai, and Yuma Counties, Arizona: U.S. 

Geological Survey Water-Supply Paper 1665, 64 p.

Kimball, B.A., 1981, Geochemistry of spring water, southeastern 

Uinta Basin, Utah and Colorado: U.S. Geological Survey WaterSupply Paper 2074, 30 p.

Kister, L.R., 1973, Quality of ground water in the lower Colorado 

River region, Arizona, Nevada, New Mexico, and Utah: U.S. 

Geological Survey Hydrologic Investigations Atlas HA-478, 

2 sheets.

 1974a, Dissolved-solids content of ground water in the Phoenix 

area, Arizona: U.S. Geological Survey Miscellaneous Investigations Series Map I-845-G, 1 sheet.

-1974b, Dissolved-solids content of ground water in the Tucson

area, Arizona: U.S. Geological Survey Miscellaneous Investigations Series Map I-844-I, 1 sheet.

Kister, L.R., Brown, S.G., Schumann, H.H., and Johnson, P.W., 1966, 

Maps showing fluoride content and salinity of ground water in the 

Willcox basin, Graham and Cochise Counties, Arizona: U.S. Geological Survey Hydrologic Investigations Atlas HA-214, 6 p.

Kister, L.R., and Hardt, W.R., 1966, Salinity of the ground water in 

western Final County, Arizona: U.S. Geological Survey WaterSupply Paper 1819-E, 21 p.
SELECTED REFERENCES C89

Krieger, M.H., 1979, Ash-flow tuffs of the Galiuro Volcanics in the 

northern Galiuro Mountains, Final County, Arizona: U.S. Geological Survey Professional Paper 1104, 32 p.

 1980, Zeolitization of Tertiary tuffs in lacustrine and alluvial 

deposits in the Ray-San Manuel area, Final and Gila Counties, 

Arizona, in Shorter contributions to mineralogy and petrology, 

1979: U.S. Geological Survey Professional Paper 1124-D, 11 p.

Lakin, H.W., 1972, Selenium accumulation in soils and its absorption 

by plants and animals: Geological Society of America Bulletin, 

v. 83, p. 181-190.

Laney, R.L., 1970, Quality of water, in Culler, R.C., Objectives, 

methods, and environment Gila River Phreatophyte Project, 

Graham County, Arizona: U.S. Geological Survey Professional 

Paper 655-A, 25 p.

 1972, Chemical quality of the water in the Tucson basin, 

Arizona: U.S. Geological Survey Water-Supply Paper 1939-D, 

46 p.

-1977, Effects of phreatophyte removal on water quality in the

Gila River Phreatophyte Project area, Graham County, Arizona, 

with a section on Statistical analysis, by H.W. Hjalmarson: U.S. 

Geological Survey Professional Paper 655-M, 23 p.

Laney, R.L., and Hahn, M.E., 1987, Hydrogeology of the eastern part 

of the Salt River Valley area, Maricopa and Final Counties, 

Arizona: U.S. Geological Survey Water-Resources Investigations 

Report 86-4147, 1 sheet.

Laney, R.L., Ross, P.P., and Littin, G.R., 1978, Maps showing 

ground-water conditions in the eastern part of the Salt River 

Valley area, Maricopa and Final Counties, Arizona 1976: U.S. 

Geological Survey Water-Resources Investigations 78-61, 2 

sheets.

Mason, Brian, 1966, Principles of geochemistry: New York, John 

Wiley, 329 p.

Matlock, D.T., 1981, Simulative models for the analysis of groundwater flow in Vekol Valley, the Waterman Wash area, and the 

Bosque area, Maricopa and Final Counties, Arizona: U.S. Geological Survey Open-File Report 82-77, 77 p.

Meinzer, O.E., and Kelton, F.C., 1913, Geology and water resources of 

Sulphur Spring Valley, Arizona, with a section on Agriculture, by 

R.H. Forbes: U.S. Geological Survey Water-Supply Paper 320, 

231 p.

Metzger, D.G., and Loeltz, O.J., 1973, Geohydrology of the Needles 

area, Arizona, California, and Nevada: U.S. Geological Survey 

Professional Paper 486-J, 54 p.

Metzger, D.G., Loeltz, O.J., and Irelan, Burdge, 1973, Geohydrology 

of the Parker-Blythe-Cibola area, Arizona and California: U.S. 

Geological Survey Professional Paper 486-G, 130 p.

Miller, W.R., and Drever, J.I., 1977, Chemical weathering and related 

controls on surface water chemistry in the Absaroka Mountains, 

Wyoming: Geochimica et Cosmochimica Acta, v. 41, p. 1693-1702.

Moosburner, Otto, 1972, Analysis of the ground-water system by 

electrical-analog model, Avra Valley, Pima and Final Counties, 

Arizona: U.S. Geological Survey Hydrologic Investigations Atlas 

HA-215, 2 sheets.

Nesbitt, H.W., Markovies, G., and Price, R.C., 1980, Chemical processes affecting alkalis and alkaline earths during continental 

weathering: Geochimica et Cosmochimica Acta, v. 44, 

p. 1659-1666.

Nie, N.H., Hull, C.H., Jenkins, J.G., Steinbrenner, Karin, and Bent, 

D.H., 1975, Statistical package for the social sciences (SPSS) 

(2d ed.): New York, McGraw-Hill, 675 p.

Olmsted, F.H., Loeltz, O.J., and Irelan, Burdge, 1973, Geohydrology 

of the Yuma area, Arizona and California: U.S. Geological Survey 

Professional Paper 486-H, 227 p.

Osterkamp, W.R., 1974, Chemical quality of ground water for public 

supply in the Phoenix area, Arizona: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-845-F, 1 sheet.

Osterkamp, W.R., and Laney, R.L., 1974, Map showing chemical 

quality of ground water for public supply in the Tucson area, 

Arizona: U.S. Geological Survey Miscellaneous Investigations 

Series Map I-844-L, 1 sheet.

Owen-Joyce, S.J., and Bell, C.K., 1983, Appraisal of water resources in 

the upper Verde River area, Yavapai and Coconino Counties, 

Arizona: Arizona Department of Water Resources Bulletin 2, 

219 p.

Parada, C.B., 1981, Isotopic composition of soil carbon dioxide in the 

Tucson basin: M.A. thesis, University of Arizona (Tucson), 85 p.

Parkhurst, D.L., Plummer, L.N., and Thorstenson, D.C., 1982, BALANCE A computer program for calculating mass transfer for 

geochemical reactions in ground water: U.S. Geological Survey 

Water-Resources Investigations 82-14, 33 p.

Pearson, F.J., Jr., and Hanshaw, B.B., 1970, Sources of dissolved 

carbonate species in ground water and their effects on carbon-14 

dating, in Isotopes in hydrology: Symposium of the International 

Atomic Energy Agency (Vienna, Austria), STI-Pub. 255, 

p. 271-286.

Plummer, L.N., 1977, Defining reactions and mass transfer in part of 

the Floridan aquifer: Water Resources Research, v. 13, no. 5, 

p. 801-812.

 1984, Geochemical modeling A comparison of forward and 

inverse methods, in Hitchon, Brian, and Wallick, E.I., eds., 

Practical applications of ground water geochemistry: Canadian/ 

American Conference on Hydrogeology, 1st, Banff, Alberta, Canada, June 22-26, 1984, Proceedings, p. 149-177 (pub. by National 

Water Well Association, Columbus, Ohio).

Plummer, L.N., Parkhurst, D.L., and Thorstenson, D.C., 1983, Development of reaction models for ground-water systems: Geochimica 

et Cosmochimica Acta, v. 47, p. 665-686.

Pool, D.R., 1986, Aquifer geology of alluvial basins of Arizona, in 

Anderson, T.W., and Johnson, A.I., eds., Regional aquifer systems of the United States, Southwest alluvial basins of Arizona: 

American Water Resources Association Monograph Series 7, 

p. 25-36.

Pourbaix, M.J.N., 1949, Thermodynamics of dilute aqueous solutions: 

London, Edward Arnold Co., 336 p.

Rankama, Kalervo, and Sahama, Th.G., 1950, Geochemistry: Chicago, 

The University of Chicago Press, 912 p.

Robertson, F.N., 1975, Hexavalent chromium in the ground water in 

Paradise Valley, Arizona: Ground Water, v. 13, no. 6,11 p., 7 figs., 

1 table.

 1984, Solubility controls of fluorine, barium and chromium in 

ground water in alluvial basins of Arizona, in Hitchon, Brian, and 

Wallick, E.I., eds., Practical applications of ground-water geochemistry: Canadian/American Conference on Hydrogeology, 1st, 

Banff, Alberta, Canada, June 22-26, 1984, Proceedings, p. 96-102 

(pub. by National Water Well Association, Columbus, Ohio). 

-1986, Occurrence and solubility controls of trace elements in

ground water in alluvial basins of Arizona, in Anderson, T. W., and 

Johnson, A.I., eds., Regional aquifer systems of the United 

States, Southwest alluvial basins of Arizona: American Water 

Resources Association Monograph Series 7, p. 69-80.

Robertson, F.N., and Garrett, W.B., 1988, Distribution of fluoride in 

ground water in alluvial basins of Arizona and adjacent parts of 

California, Nevada, and New Mexico: U.S. Geological Survey 

Hydrologic Atlas HA-665, 3 sheets.

Roeske, R.H., and Werrell, W.L., 1973, Hydrologic conditions in the 

San Pedro River Valley, Arizona, 1971: Arizona Water Commission Bulletin 4, 76 p.
C90 REGIONAL AQUIFER-SYSTEM ANALYSIS-SW. ALLUVIAL BASINS, ARIZ. AND ADJACENT STATES

Scarborough, R.B., and Peirce, H.W., 1978, Late Cenozoic basins of 

Arizona, in Callender, J.F., Wilt, J.C., and demons, R.E., eds., 

The land of Cochise: New Mexico Geological Society field conference, 29th, Guidebook, p. 253-259.

Schmidt, K.D., 1979, Groundwater quality in the major basins of 

Maricopa County: Phoenix, Ariz., Maricopa Association of Governments, 68 p.

Shafiqullah, M., Damon, P.E., Lynch, D.J., Reynolds, S.J., Rehrig, 

W.A., and Raymond, R.H., 1980, K-Ar geochronology and geologic history of southwestern Arizona and adjacent areas: Arizona 

Geological Society Digest, v. 12, p. 201-260.

Simpson, E.S., Thorud, D.B., and Friedman, Irving, 1970, Distinguishing seasonal recharge to ground water by deuterium analysis 

in southern Arizona (with discussion): International Association of 

Scientific Hydrology Publication 94, 9 p.

Smoor, P.B., 1967, Hydrochemical facies study of ground water in the 

Tucson basin (Pima County, Arizona): Ph.D. thesis, University of 

Arizona (Tucson), 247 p.

Stumm, Werner, and Morgan, J.J., 1981, Aquatic chemistry: New 

York, Wiley-Interscience, 583 p.

Thorstenson, D.C., 1984, The concept of electron activity and its 

relation to redox potentials in aqueous geochemical systems: U.S. 

Geological Survey Open-File Report 84-072, 45 p.

Thorstenson, D.C., Fisher, D.W., and Croft, M.G., 1979, The geochemistry of the Fox Hills-Basal Hell Creek aquifer in southwestern North Dakota and northwestern South Dakota: Water 

Resources Research, v. 15, no. 6, p. 1479-1498.

Thorstenson, D.C., Weeks, E.P., Haas, Herbert, and Fisher, D.W., 

1983, Distribution of gaseous 12C02 , 13C02 , and 14C02 in the 

sub-soil unsaturated zone of the Western U.S. Great Plains: 

Radiocarbon, v. 25, no. 2, p. 315-346.

Turner, J.V., 1982, Kinetic fractionation of carbon-13 during calcium 

carbonate precipitation: Geochimica et Cosmochimica Acta, v. 46, 

p. 1183-1191.

U.S. Environmental Protection Agency, 1977, National interim primary drinking water regulations: U.S. Environmental Protection 

Agency Report EPA-570/9-76-003, 159 p.

U.S. Geological Survey, 1969, Annual report on ground water in 

Arizona, spring 1967 to spring 1968: Arizona State Land Department Water-Resources Report 38, 54 p.

 1980, Water resources data for Arizona, water year 1979: U.S. 

Geological Survey Water-Data Report AZ-79-1, 614 p.

U.S. Soil Conservation Service, 1974, Soil survey laboratory data and 

descriptions for some soils of Arizona: U.S. Department of Agriculture Soil Survey Investigations Report 28, 383 p.

University of Arizona, Undated, Normal annual precipitation, normal 

May-September precipitation, 1930-1960: State of Arizona, scale 

1:500,000.

Van Alstine, R.E., and Moore, R.T., 1969, Fluorspar, in Mineral and 

water resources of Arizona: Arizona Bureau of Mines Bulletin 180, 

p. 349-357.

Wallick, E.I., 1973, Isotopic and chemical considerations in radiocarbon 

dating of groundwater within the arid Tucson basin, Arizona: 

Ph.D. thesis, University of Arizona (Tucson), 184 p.

White, A.F., 1979, Geochemistry of ground water associated with 

tuffaceous rocks, Oasis Valley, Nevada: U.S. Geological Survey 

Professional Paper 712-E, 25 p.

Wilson, E.D., Moore, R.T., and Cooper, J.R., 1969, Geologic map of 

Arizona: Arizona Bureau of Mines map, scale 1:500,000,

Wilson, R.P., 1979, Availability of ground water on Federal land near 

the Ak-Chin Indian Reservation, Arizona A reconnaissance 

study: U.S. Geological Survey Open-File Report 79-1165, 36 p.

Winograd, I.J., 1971, Hydrogeology of ash flow tuff: A preliminary 

statement: Water Resources Research, v. 7, no. 4, p. 994-1006.

Winograd, I.J., and Robertson, F.N., 1982, Deep oxygenated ground 

water Anomaly or common occurrence?: Science, v. 216, no. 

4551, 3 p.
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists 

and engineers. Included are reports on the results of resource studies 

and of topographic, hydrologic, and geologic investigations. They also 

include collections of related papers addressing different aspects of a 

single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations, as 

well as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present 

significant interpretive results of hydrologic investigations of wide 

interest to professional geologists, hydrologists, and engineers. The 

series covers investigations in all phases of hydrology, including 

hydrogeology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for 

distribution at no cost to the public. Information is usually of short-term 

interest.

Water-Resources Investigations Reports are papers of an 

interpretive nature made available to the public outside the formal 

USGS publications series. Copies are reproduced on request unlike 

formal USGS publications, and they are also available for public 

inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, 

maps, and other material that are made available for public consultation 

at depositories. They are a nonpermanent form of publication that may 

be cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7.5- or 15-minute quadrangle formats (scales 

mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include 

structure and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales; they show results of surveys using 

geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which reflect subsurface structures that are of economic or 

geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also 

includes 7.5-minute quadrangle photogeologic maps on planimetric 

bases that show geology as interpreted from aerial photographs. Series 

also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial 

geology, stratigraphy, and structural relations in certain coal-resource 

areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum 

potential.

Miscellaneous Field Studies Maps are multicolor or blackand-white maps on topographic or planimetric bases on quadrangle or 

irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or blackand-white maps on topographic or planimetric bases presenting a wide 

range of geohydrologic data of both regular and irregular areas; 

principal scale is 1:24,000, and regional studies are at 1:250,000 scale 

or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, 

Books and Open-File Reports Section, Federal Center, Box 25425, 

Denver, CO 80225. (See latest Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be 

purchased by mail and over the counter in paperback book form and as 

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"Publications of the Geological Survey, 1962-1970" may be 

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Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by 

mail and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by 

mail and over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback 

booklet form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" are available free of charge by mail or may 

be obtained over the counter in paperback booklet form only. Those 

wishing a free subscription to the monthly catalog "New Publications of 

the U.S. Geological Survey" should write to the U.S. Geological 

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Note. Prices of Government publications listed in older catalogs, announcements, and publications may be incorrect. Therefore, 

the prices charged may differ from the prices in catalogs, announcements, and publications.