Aquifer Characteristics of the Pediment South of the Tortolita Mountains, Pima County, Arizona

Aquifer characteristics of the pediment south of the Tortolita Mountains, Pima County, Arizona

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Authors Raymondi, Richard Robert.

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/191713 AQUIFER CHARACTERISTICS OF THE PEDIMENT SOUTH OF THE

TORTOLITA MOUNTAINS, PIMA COUNTY, ARIZONA

by Richard Robert Raymondi

A Thesis Submitted to the Faculty of the DEPARTMENT OF HYDROLOGY AND WATER RESOURCES In Partial Fulfillment of the Requirements For the degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA

1980 STATEMENT BY AUTHOR

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

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

-7 / 2 SIGNED:

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

// $'/'& JOHN W. HARSHBARGE 4-/Daté Professor of Hydrology and Water Resources ACKNOWLEDGMENTS

The author extends his gratitude, appreciation, and admiration to Dr. John W. Harshbarger. His knowledge and direction were invaluable to the preparation of this paper. His instruction and friendship throughout my graduate career will always be remembered.

I also express special thanks to Dr. Eugene S. Simpson, who has helped plan my academic program at The University of Arizona. His instruction and concern were responsible for my steady progress toward my degree. Thanks are also extended to Dr. Stanley N. Davis, who has supported me financially through assistantships and has reviewed and advised my work.

Also recognized are James E. Posedly of the Depart- ment of Soils, Water, and Engineering of The University of

Arizona, Dr. Harold W. Bentley of the Hydrology Department of The University of Arizona, and R. Bruce Johnson of Tucson Water. Cooperation and contributions of data were also made by the State of Arizona Land Department and the

U.S. Geological Survey. I am especially indebted to my mother and father, brothers and sister, and my grandparents. They have supported and encouraged me to pursue a career concerned with the natural environment. My studies in the Hydrology

iii iv

Department at The University of Arizona have been a great step in accomplishing my personal goals. TABLE OF CONTENTS

Page LIST OF ILLUSTRATIONS vii

LIST OF TABLES

ABSTRACT xi 1. INTRODUCTION 1 Purpose and Scope 1 Location 1 Topography 2 Climate 7 Natural Resources and Cultural Features . . 10 Mineral Deposits 10 Soils and Vegetation 10 Suburban Development 12 Previous Investigations 14 Well-numbering System 19 2. GEOLOGY 21 Rock Units of the Totolita Mountains 21 Older Precambrian Rocks 21 Younger Precambrian Rocks 22 Metamorphosed Paleozoic Rocks 22 Late Cretaceous and/or Early Tertiary Rocks 23 Tertiary Rocks 23 Faulting and the Intensity of Fracturing . 24 Determination of the Southern Extent of the Buried Pediment 26 Sedimentary Units South of the Tortolita Mountains 34 Pantano Formation 35 Tinaja Beds 36 Fort Lowell Formation 37 Surficial Deposits 37 Geologic Sections 38

3. HYDROLOGIC SYSTEMS 42 Surface Water 42 Surface Drainage Pattern 42 vi

TABLE OF CONTENTS--Continued

Page

Ground Water 48 Quantitative Aquifer Characteristics . . 48 Water Budget 53 Water Quality 71 4. CORRELATION AND INTERPRETIVE ANALYSIS OF SALIENT FEATURES 77 5. CONCLUSIONS 83 APPENDIX A: ANION-CATION BALANCE OF GROUND WATER FROM FRACTURED QUARTZ MONZONITE AQUIFER OF PEDIMENT AREA. . 87 REFERENCES 93 LIST OF ILLUSTRATIONS

Figure Page

1. Location of the study area 3

2. Physiographic Provinces of Arizona 4

3. Geographic Provinces of Arizona 5 4. Topography south of the Tortolita Mountains in pocket 5. Histogram of the number of water wells drilled between 1915 and 1980 13 6. Location of water wells south of the Tortolita Mountains in pocket 7. Residual gravity anomaly map south of the Tortolita Mountains in pocket 8. Geologic map of the area south of the Tortolita Mountains in pocket

9. Fractured bedrock in sec. 21, T. 11 S., R. 13 E 27 10. The angularity of rock units in sec. 21, T. 11 S., R. 13 E. is caused by intensity of fracturing of the Tortolita quartz monzonite 28

11. Spheroidal weathering of rocks that have not been intensely fractured in sec. 24, T. 11 S., R. 13 E. 29

12. Exfoliation of rock surfaces in sec. 24, T. 11 S., R. 13 E 30

13. Depth to bedrock or isopach of alluvial sediments south of the Tortolita Mountains in pocket 14. Basement profiles south of the Tortolita Mountains in pocket

vii v iii

LIST OF ILLUSTRATIONS--Continued

Figure Page 15. 1970 water-table elevation south of the Tortolita Mountains in pocket 16. 1973 water-table elevation south of the Tortolita Mountains in pocket 17. 1979 water-table elevation south of the Tortolita Mountains in pocket 18. Scarp east of the study area in sec. 25, T. 11 S., R. 13 E 33 19. Fence diagram of alluvium and basement rock south of the Tortolita Mountains . . . .in pocket 20. Profiles of ephemeral streams south of the Tortolita Mountains in pocket 21. Surface drainage pattern south of the Tortolita Mountains in pocket 22. Angular nature of bedrock-controlled stream pattern in sec. 21, T. 11 S., R. 13 E. 44 23. Fault-controlled segment of stream channel in sec. 21, T. 11 S., R. 13 E. 45 24. Rectilinear stream pattern in sec. 15, T. 11 S., R. 13 E. 46 25. Linear segment of stream channel in sec. 21, T. 11 S., R. 13 E. trends in the same direction as fracturing in bedrock 47 26. Distribution of transmissivity in alluvial sediments south of the Tortolita Mountains in pocket 27. Flow-net analysis of alluvial sediments south of the Tortolita Mountains in pocket 28. Depth-to-water table in 1973 south of the Tortolita Mountains in pocket ix

LIST OF ILLUSTRATIONS--Continued

Figure Page 29. Depth-to-water table in 1979 south of the Tortolita Mountains in pocket

30. Water levels in alluvial sediments 69 LIST OF TABLES

Table Page 1. Climatological data recorded at Tucson International Airport and Catalina, Arizona 8 2. Records of wells south of the Tortolita Mountains 54

3. Chemical characteristics of water from wells south of the Tortolita Mountains 72 4. Compilation of water-well penetrations in the exposed pediment area 79

X ABSTRACT

The pediment area south of the Tortolita Mountains was studied by conventional field methods aided by remote- sensing images. Investigations of the direction and intensity of fracturing of the quartz monzonite bedrock as well as the structural control of the surface drainage pattern provided the basis for analysis of the hydrologic setting. Water-level measurements, reconnaissance ground-water investigations, and a flow-net analysis were used to determine quantitative aquifer characteristics. Analyses with transparent overlays suggest the quartz monzonite constitutes a potential aquifer where a high degree of correlation exists between the locations of tectonic features, bedrock-controlled stream channels, and productive water wells. The fractured aquifer of the pediment area is recharged by infiltration of surface water through the course sediments of ephemeral stream channels. The Cafiada Agua is the major ephemeral stream draining the southeastern section of the Tortolita Mountains. Numerous water wells in a major structural depression traversed and recharged by the Canada Agua serve most of the residents of the pediment area.

xi CHAPTER 1

INTRODUCTION

The Tucson Basin in southeastern Arizona is surrounded by mountain ranges, and the Tortolita Mountains form the northwestern boundary of this basin. The southernmost portion of the Tortolita Mountains is an intrusive mass of fine-grained igneous rock. Extensive pedimentation and fracturing have occurred on the southern flank of this pluton.

Purpose and Scope

The purpose of this thesis is to analyze and describe the hydrogeological characteristics of both the exposed and buried portions of the pediment south of the Tortolita Mountains. The hydrologic relationships between this aquifer and both the rock structures within the Torto- lita Mountains and the alluvial sediments of the Tucson

Basin are also discussed. Finally, the methods for locating water wells that satisfy domestic requirements in this fractured aquifer are presented.

Location

The area in this report includes approximately 75 square miles in northern Pima County, Arizona within sections from Tps. 11 and 12 S., R. 12 E.; and Tps. 11 and

1 2

12 S., R. 13 E. Figure 1 shows the location of the study area with respect to nearby geographic features. Physiographically, the Tortolita Mountains and the area of investigation are in the Desert Region of the Basin and Range province of Arizona. The physiographic provinces are consequences of structural processes and are thought to have been established during Miocene to early Pliocene time (Wilson, 1962). The division of the Desert Region and Mountain Region is between the Tortolita Mountains and the Santa Catalina Mountains in the Canada del Oro Valley (Fig- ure 2). Altitudes in the Desert Region are generally lower, mountains are more sharply carved, and valleys or plains are more extensive than in the Mountain Region (Wilson, 1962). Figure 3 illustrates the three water resource regions of Arizona. Climatic influence, topography, and geology are the criteria used to establish these regions (Harshbarger and others, 1966). The study area is in the

Desert Lowlands, which is included within the Basin and Range physiographic province.

Topography Topographic maps prepared by the U.S. Geological

Survey include the Tortolita Mountains and basin areas north of Tucson. The quadrangles used for topographic control in this study are the Mount Lemmon, 1957; Marana, 1967; and the Ruelas Canyon, 1968 quadrangles. The scale 3

Figure 1. Location of the study area 4

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Figure 2. Physiographic provinces of Arizona. -- After Wilson (1962). 5

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Figure 3. Major water resource regions. -- After Harshbarger and others (1966). 6 of the Ruelas Canyon and Marana quadrangles is 1:24,000, and the scale of the Mount Lemmon quadrangle is 1:62,500. Hydro- logical and geologic information given in this report was plotted on these maps. The surface topography of the pediment and alluvial sediments south of the Tortolita Mountains is shown in

Figure 4 (in pocket). Elevations range from over 3,140 ft on the exposed pediment surface to 2,060 ft in the Santa Cruz Valley. Topographically, the Tortolita Mountains are relatively low compared to the Santa Catalina Mountains. The highest peak in the study area is 4,651 ft above mean sea level.

Figure 4 indicates that in most of the area of investigation the land surface has an undulating, 1 to 3 percent slope dipping to the southwest. This shallowly dissected topography has developed on the large alluvial fans deposited by erosional processes acting upon the

Tortolita Mountain block (Richardson and Miller, 1974). Rolling to steeply sloping areas along the fringes of the mountain front are due to the numerous bedrock outcrops of the pediment area. The erosional effects of the major washes can be noted at the entrances of Cochie, Wild Burro, and Ruelas Canyons. Fluvial incision by the major ephemeral streams has produced steeply sloping ridges and long, narrow drainageways. The alluvial surface in the eastern part of the 7 study area is even more deeply dissected. Depositional

processes, such as those associated with the broad alluvial

fans to the west, have not taken place in this area. The result is a highly eroded surface southeast of the Tortolita Mountains.

Climate

The thesis area lies in a semi-desert climatic zone characterized by long, hot summers, which last from May to September. Maximum daily temperatures during this period

average more than 90°F (32°C). The winter climate is mild

and dry. Early-morning temperatures are usually above the freezing point, and afternoon temperatures range between 65°F and 70°F (18°C and 21°C) (Sellers and Hill, 1974). The area has two major periods of precipitation. The rainfall in winter is derived from Pacific storms.

These disturbances produce gentle, widespread rain showers, which may continue intermittently for several days. Summer showers and thunderstorms originate in moist air that flows into Arizona from the Gulf of Mexico. Normally, rainfall is most intense in the late afternoon or early evening (Sellers and Hill, 1974).

The average monthly temperatures and total monthly precipitation recorded at the Tucson International Airport for the period from January 1978 to February 1980 appear in Table 1. Also included, for the same time period, are data

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Two important facts are demonstrated in Table 1. First, precipitation increases with elevation in the Tucson

Basin. The annual totals recorded at the Tucson Interna- tional Airport for 1978 and 1979 are 15.73 and 10.39 inches, respectively. However, the total precipitation recorded at Catalina is 31.02 and 20.10 inches for the same years. The average annual precipitation for Tucson is 10.4 inches based on over 40 years of record. Fifteen years of record at Catalina show an average of 15.6 inches of precipitation (U.S. National Oceanic and Atmospheric Administration, 1978,

1979, 1980). The second important fact concerning precipitation is the above-average rainfall in the months of December of

1978 and January of 1979. Runoff from storms caused the various ephemeral streams draining the southern portion of the Tortolita Mountains to flow for nearly 3 months, according to local residents. The significance and impact of this ground-water recharge upon the fractured pediment aquifer and alluvial sediments of the Tucson Basin will be 1 0 discussed in greater detail in later chapters of this report.

Natural Resources and Cultural Features

Mineral Deposits

Presently, no mineral production occurs in the sections of the Tortolita Mountains included in the study area.

Several copper prospects are in sec. 29, T. 11 S., R. 13 E. Numerous other copper prospects and copper and marble mining

are found to the north of the study area in the Tortolita

Mountains (Wilson, 1962).

Noteworthy sources of nonmetallics occur in the coarse-grained alluvial deposits along the stream channels

of the Cafiada del Oro and the Santa Cruz River. Sand and gravel are currently being mined in these areas. Much of

the sand and gravel mined in sec. 17, T. 12 S., R. 12 E., is manufactured into concrete at Arizona Portland Cement

Company. Four large industrial water wells have been drilled

in secs. 5 and 8, T. 12 S., R. 12 E. by this company for the plant supply.

Soils and Vegetation Five different soil associations in the area of investigation are delineated in the soil survey prepared for the U.S. Soil Conservation Service by Richardson and Miller

(1974). Minor soil series within the associations may be 11 locally extensive but will not be considered in this report.

The Rock Outcrop-Lampshire-Cellar Association consists of semiarid soils of the Tortolita Mountains and foothills. Slopes range from 5 to 25 percent on the lower foothills and old granitic pediment surfaces, to 60 percent or more on the higher mountains. Granitic and gneissic rocks are the dominant parent materials of these highly permeable soils. Natural vegetation includes palo verde, jojoba, mesquite, and other desert shrubs, grasses, and cacti

(Richardson and Miller, 1974). The White House-Bernardino-Caralampi Association consists of deep, semiarid soils on uplands in the northeast portion of the study area. According to Richardson and Miller (1974), these soils have low permeability and percolation rates and are found in old alluvium derived from granite, rhyolite, and andesite tuffs. Slopes are dominantly between 2 and 15 percent, and the natural vegetation associated with this soil group consists of perennial grasses and scattered brush. The Pinaleno-Nickel-Palos Verdes Association is deep, arid, gravelly soils on deeply dissected uplands and is in eastern sections of the thesis area. Slopes are dominantly 15 to 45 percent and restrict infiltration of precipitation.

The soils have formed from parent material derived from granite, gneissic, and rhyolitic rocks. Vegetation consists 12 of palo verde, white thorn, mesquite, bursage, creosote bush, ironwood, saguaro, and other cacti and sparse grasses (Richardson and Miller, 1974).

Most of the central and western portions of the

study area contain deep, arid soils formed on alluvial fans and valley slopes belonging to the Anthony-Sonoita Associ- ation. Soils generally have slopes ranging from 1 to 3 percent and low permeability in the upper part of the soil profile. According to Richardson and Miller (1974), the parent material consists of recent alluvium from rocks that are mainly granitic. Natural vegetation is mostly desert shrubs, cacti, and annual grasses. The final association in the study area is the Grabe- Gila-Pima Association found on the floodplains of the Santa Cruz River and the Canada del Oro. Soils are deep and well- drained, and slopes are less than 1 percent. Soils have developed in recent alluvium and are presently used for irrigated cropland and rangeland (Richardson and Miller,

1974).

Suburban Development With the exception of the floodplains of the Santa Cruz River and the Canada del Oro, most of the area considered in this study had been used in the past for grazing by a small number of ranchers. The floodplains have been utilized for the raising of irrigated crops. However, in 13

40-

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7- , 5 - - 3 2 I -

19 S 92.0 1925 1930 1935 1940 1945 1950 1955 19Go 1965 1970 1975 1980

VEP-12-.

Figure 5. Histogram of the number of water wells drilled between 1915 and 1980 14

recent years, the urban expansion of the City of Tucson has had an impact on the land use in the area south of the Tor- tolita Mountains. The amount of land being devoted to residential home sites is increasing each year.

Figure 5 is a histogram illustrating the number of water wells drilled in the study area per 5-year interval since 1915. Most of the wells drilled before and immediately after World War II were for irrigation and livestock. However, since that time nearly all wells have been drilled for domestic supply. The sharp increase on the graph beginning in 1970 reflects suburban expansion in this rural area.

The areas that have undergone suburban development can be noted in Figure 6 (in pocket). This map reveals the locations of water wells south of the Tortolita Mountains.

The density of water wells is an indication of the number of residential homes that have been constructed. An exception to this trend is the large number of wells in the southwest portion of the study area. Numerous high-capacity water wells in the floodplain of the Santa Cruz River are for industrial, municipal, and agricultural purposes.

Previous Investigations

Recent geologic studies by Davis and others (n.d.) revealed a diversity of rock ages, types, and structure within the Tortolita Mountains that had previously gone unrecognized. Davis and others (1975) have studied the 15 origin of mineral lineation in the Catalina-Rincon-Tortolita gneiss complex. Results of structural analyses in the Tor- tolita Mountains indicate that the lineation resulted from a profound N. 60 0 E., S. 60° W. extension accompanying differential vertical uplift. The similarity of the discriptions, ages, and structure of rock units in the Santa Catalina and Tortolita Moun- tains were studied by Budden (1975). Detailed geologic mapping, structural and geomorphological interpretation, and mineralogical analyses of both mountain ranges were completed. Budden concluded that the Santa Catalina and Tortolita Mountains are remnant blocks of a once continuous plutonic and metamorphic complex that was created during mid- to late-Mesozoic time. The differences in the complex are physiographic and were brought about by the Basin and Range orogeny in late-Tertiary time. Recent reconnaissance geologic mapping in the Santa

Catalina and Tortolita Mountains has been completed by

Creasey and Theodore (1975) and Banks (1976). Information obtained corresponds with new and published potassium-argon and fission-track ages (Creasey and others, 1977), which indicate that a large composite batholith of middle-Tertiary age crops out extensively in both mountains. According to

Creasey and others (1977), the quartz monzonite of the Tortolita Mountains is one of at least two intrusive phases that make up the batholith. 16 The most detailed and recent geologic information concerning the Tortolita Mountains is contained in a paper by Keith and others (n.d.). Published and unpublished geologic and geochronologic studies are summarized and discussed with reference to the ages and correlations of major rock units within the Santa Catalina-Rincon-Tortolita metamorphic core complex. Particular attention is given to the post-Paleozoic intrusions that together constitute a batholith dominating the geology of this mountain complex. Goodoff (1975) used an integrated geophysical approach, consisting of subsurface geophysical well logging and surface gravity and aeromagnetic data to determine the amount of basin fill and the alluvium-basement contact in the Cortaro Basin. The Cortaro Basin is a sub-basin of the Tucson Basin in the extreme northwest section of this larger alluvial valley. It also coincides with the southwest corner of the area of investigation. Gravimetric data were measured at various stations south of the Tortolita Moun- tains and reduced to complete Bouguer anomalies. After the complete Bouguer anomalies were corrected to remove the regional gravity field, residual gravity anomalies remained and were used to form the basis of interpretation of the major subsurface structural features in the Cortaro area. Gravity studies of the Canada del Oro Valley were undertaken by Budden (1975) to determine the depth to crystalline bedrock and approximate the displacement of 17 normal faults beneath the alluvial cover. Data were col- lected and analyzed in the same manner as in the study by Goodoff (1975). Data from both studies are compiled in the Arizona Gravity Data Base of the Laboratory of Geophysics of The University of Arizona. The combination of the two studies provided the residual gravity information for the area of investigation that appears in Figure 7 (in pocket). West (1971) developed an iterative two-dimensional gravity model program for use on the DEC-10 computer system.

The DEC-10 system has capabilities of interactive processing, that is, the programmer operates a remote terminal from which he issues commands directly to the computer. The program, known as ITMOD, generates depth-to-bedrock values below an alluvial valley from gravity data (residual or complete). ITMOD was revised by Oppenheimer (1980), and the program was used to construct the basement profiles for this thesis. Although no previous hydrogeological investigations have been made of the pediment south of the Tortolita Moun- tains, several papers have been written concerning the hydrologic and geologic nature of the alluvial sediments in the northern Tucson Basin. The most comprehensive study completed to date is contained in the U.S. Geological Survey Water-Supply Paper 1939-E by Davidson (1973). The report de- scribes the hydrologic system with emphasis on geology and its control on water-level declines, aquifer transmissivity 18 and well yields, chemical quality of the water, and ground-

water recharge and storage. The distribution of the sedimentary facies, including their description and percentages of various grain sizes ,was also analyzed.

Matlock and Davis (1972) reported the results of a hydrologic investigation in the Santa Cruz Valley. Water-

level data and well capacities, depth-to-water contour maps, ground-water level contour maps, and water-level changes for the 1965-70 and 1947-70 periods were included in the paper. Water use and the hydrologic impacts of ground-water pumpage in the Tortolita area were also discussed.

Ground-water modeling studies of the hydrologic system in the area of investigation were conducted by

Anderson (1972) and Fogg (1978). Anderson (1972) completed the construction and calibration of an electric analog model for the Tucson Basin. The data used in formulating the model included measured and estimated coefficients of transmissivity and storage, measured and estimated rates of ground-water pumpage from 1940 through 1965, estimated rates of underflow entering and leaving the basin, and estimated recharge through stream channels. The model was then used to predict future water-level conditions in 1985. This prediction was based on the assumption that pumpage would continue at its present rate and areal distribution. Fogg (1978) used a finite-element model to gain an understanding of the ground-water flow regimen and aquifer 19 characteristics of the Cortaro Basin. The basic information used in calibrating the model was similar to the data used by Anderson (1972), but Fogg (1978) used constant-head boundaries whereas Anderson used constant-flux boundaries. The results of the investigation by Fogg (1978) included the determination of a distribution of transmissivity values, recharge rates, and subsurface flow rates for the Cortaro

Basin.

Well Numbering System

The well numbers used in this report follow the system used by the U.S. Geological Survey in Arizona and are in accordance with the Bureau of Land Management's system of land subdivision. The land survey in most of Arizona is based on the Gila and Salt River Meridian and Base Line, which intersect to form a point of origin and divide the State into four quadrants. These quadrants are designated counter-clockwise by the capital letters A, B,

C, and D. All land northeast of the point of origin is in A quadrant, that northwest in B quadrant, that southwest in C quadrant, and that southeast 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 lowercase 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, 20 the second a 40-acre tract, and the third a 10-acre tract.

These letters are assigned in a counterclockwise direction, beginning in the northeast quarter. If the location is known to within the 10-acre tract, the lowercase letters are shown in the well number. CHAPTER 2

GEOLOGY

Rock Units of the Tortolita Mountains

The southern Tortolita Mountains contain a large diversity of rock types, ages, and structure (Figure 8, in

pocket). Plutonic rocks of Tertiary age underlie most of the area and are part of a large batholith emplaced during this time period (Keith and others, n.d.). Extensive faulting and fracturing have taken place in the south- central sections of the Tortolita Mountains as a result of the tension produced by uparching of the plutonic complex (Davis and others, 1975).

The southern flank of the Tortolita Mountains has undergone extensive pedimentation. A large, gently inclined, erosional surface carved in bedrock extends nearly a mile from the mountain front (Figure 8). Beyond this point, the pediment underlies erosional products from the retreating front range of the Tortolita Mountains.

The surface of the pediment has been covered with more than 500 ft of alluvial materials. This part of the erosional surface will be referred to as the buried Pediment.

Older Precambrian Rocks

The oldest rocks in the area of investigation are exposures of Pinal Schist (Budden, 1975). This rock unit 21 22 occurs along the margins of the Coche Canyon and consists of thinly laminated series of quartzites, biotite and muscovite schists, and phyllites. Foliation tends to parallel the compositional layering of these metamorphic rocks, which may well be the original sedimentary layering (Budden, 1975).

Younger Precambrian Rocks

An east-northeast- to east-west-trending arcuate screen of metasedimentary rocks crop out in the northwest sections of the study area. Quartz-sericite schists, quartzites, amphibolite schists, metaconglomerates, marbles, epidote skarns, gnarly phyllites, and thinly laminated argillites comprise the Apache Group of the Tortolita Mountains (Davis and others, 1975). Budden (1975) indicated that the inselberg in sec. 29, T. 11 S., R. 12 E. is a quartzite of the Apache Group. Foliation within the Apache Group tends to parallel the compositional layering, strikes east-northeast, and dips moder-

ately southeast (Budden, 1975).

Metamorphosed Paleozoic Rocks Marbles, epidote skarns, gnarly phyllites, and

thinly laminated argillates are exposed in a faulted but

continuous band directly east and northeast of the Apache Group (Budden, 1975) (Figure 8). Both the Apache Group

and the metamorphosed Paleozoic rocks are surrounded by the intrusive Catalina quartz monzonite (Keith and others, 23 n.d.). Thickness of the Paleozoic section range from 320 to 640 ft. According to Budden (1975), foliation is generally parallel to the compositional layering and consistently strikes east-northeast to east.

Late Cretaceous and/or Early Tertiary Rocks The Chirreon Wash grandodiorite (Keith and others, n.d.) is an east-west elongate intrusion in the north- central Tortolita Mountains. This pluton is in the extreme northwest corner of the study area and is generally separated from the Catalina quartz monzonite to the south by a screen of schistose rocks interpreted by Keith and others (n.d.) as Pinal Schist. The Chirreon Wash granodiorite displays foliation which dips vertically or steeply southeast (Budden, 1975) and is cut by a low- angle cataclastic foliation (Keith and others, n.d.).

Tertiary Rocks A large east-northeast-trending, dikelike, quartz monzonite mass in the central Tortolita Mountains is correlated with the Catalina quartz monzonite by Sudden (1975) and Keith and others (n.d.). This pluton surrounds inclusions of the Apache Group and metamorphosed Paleozoic rocks. Also, large inclusions of biotite quartz diorite are common within the Catalina quartz monzonite. The biotite quartz diorite is thought to correlate with the dioritic phase of the Chirreon Wash granodiorite (Keith and others, n.d.). 24 Contacts between the various inclusions are sharp and essen- tially undeformed. Mineral foliation in the Catalina quartz monzonite strikes east-northeast, dips moderately southeast, and is cut by a lower-angel cataclastic foliation (Keith and others, n.d.).

The southern third of the Tortolita Mountains is comprised of a large pluton which occupies about 27 square miles of the mountain range. This igneous intrusion is a fine-grained quartz monzonite that is undeformed. The Tortolita quartz monzonite (Keith and others, n.d.) contains more quartz, less calcic plagioclase, the same amount of K-feldspar, and less mafic minerals than the Catalina quartz monzonite. According to Keith and others (n.d.), the western portion of the Tortolita quartz monzonite pluton is highly fractured and displays myloni- tic cataclastic foliation. The cataclastic foliation and intensity of fracturing decrease rather abruptly east of the gradational contact indicated in Figure 8 (Keith, n.d.).

Faulting and the Intensity of Fracturing

The faulting and the intensity of fracturing of bedrock in the southern Tortolita Mountains have important hydrogeological significance. The igneous, metamorphic, and metasedimentary rocks have extremely low primary permeability. Primary permeability is defined as the 25 ability of a rock unit to transmit water through its inter- granular or intercrystalline pore spaces. As a result of deformation and weathering, the rocks in the study area contain secondary permeability. Fracturing and faulting have physically modified the grain-pore matrix and provided the rock units the potential for ground-water resource development for domestic purposes.

An investigation of the tectonics of both the exposed pediment surface and the mountain front was undertaken. Information obtained by surface observation and mapping techniques was correlated with images produced by several remote sensing systems. The results of this exercise are illustrated in Figure 8. A number of high- angle normal faults strike N. 23° W. (Budden, 1975) per- pendicular to the trend of cataclastic mineral lineation.

The faults are northeast dipping and show consistently the northeast side down. Contributions to the mapping of

structural features were made by Budden (1975), Keith

(n.d.), Davis and others (n.d.), and the author. The structural features in the Tortolita quartz monzonite are similar to those displayed by other crystal-

line rocks of plutonic origin. This similarity aided the geologic interpretation of field observations and images for of remote sensors. Radar imagery proved to be useful moisture differ- the study of faults because relief and ences associated with these linear features cause them to 26 be conspicuous (American Society of Photogrammetry, 1975).

Available black-and-white aerial photographs with a scale of 1:400 were also used in the reconnaissance of the fault system. The black-and-white images aided in the interpretation of linear features noted in the field. Zones of intensely fractured Tortolita quartz monzonite are shown in Figure 8. The principal direction of fracturing in the rocks of the exposed erosional surface coincides with the strike of the fault system (N. 23° W.) (see Figure 9). Some fracturing was noted normal to the primary direction. Rock units of secs. 16, 20, 21, and the W1/2 of sec. 22, T. 11 S., R. 13 E., were most intensely fractured. Tectonism has also produced numerous slickensides and highly angular rock units in these areas

(see Figure 10). A gradational contact mapped by Keith (n.d.) between the highly fractured and nonfractured rocks of the Tortolita quartz monzonite is shown in Figure 8. Rocks to the east of this boundary are rounded due to spheroidal weathering processes and the exfoliation of rock surfaces (Figures 11 and 12).

Determination of the Southern Extent of the Buried Pediment Well logs, geophysical data, water-table elevation maps, the depth-to-bedrock map, and field observations all indicate a fault south of the Tortolita Mountains, which 27

Figure 9. Fractured bedrock in sec. 21, T. 11 S., R. 13 E.--Note that the fracturing trends in a similar direction as linear features in background. 28

Figure 10. The angularity of rock units in sec. 21, T. 11 S., R. 13 E. is caused by the intensity of fracturing of the Tortolita quartz monzonite 29

Figure 11. Spheoridal weathering of rocks which have not been intensely fractured in sec. 24, T. 11 S., R. 13 E. 30

Figure 12. Exfoliation of rock surfaces in sec. 24, T. 11 S., R. 13 E. 31 limits the southern edge of the erosional surface. This structural feature was mapped by using the above information.

Geologic and geophysical information was compiled to construct the depth-to-bedrock map. The depth below land surface where crystalline rock was encountered by drilling operations was plotted on Figure 13 (in pocket). Contours connect points of equal bedrock depth and coincide with the thickness of alluvial material. A steeply sloping ridge cuts across the northern parts of sections

32 and 33, the southern part of section 27, and the central part of section 26, T. 11 S., E. 13 E. Some water wells drilled into the alluvium encountered basement rock

south of this ridge (see Figure 13) approximately 150 ft below the level of the pediment surface. Residual gravity data from surface exploration by

Budden (1975) and Goodoff (1975) are illustrated in Figure

7. The residual gravity is the portion of a gravity effect remaining after the removal of the regional gravity gradient. If correct values of local rock densities are used

to reduce gravity data, the residual gravity anomaly will

reflect the topography of the buried bedrock surface. The density of the quartz monzonite was assumed to be 2.67 3 g/cm , a value used for granitic material. Figure 7 displays a subsurface anomaly that is in the same position and trends in the same direction as the ridge in Figure 13. 32

The values of residual gravity from Figure 7 were also used to construct the four basement profiles

that appear in Figure 14 (in pocket). The profiles were generated from an iterative two-dimensional gravity model

program developed by West (1971) and revised by

Oppenheimer (1980). This program, known as ITMOD, provided estimates of bedrock depth below the surface of the alluvium in areas where basement rock was not encountered in drilling operations. The estimates are entered into Figure 13, the map showing depth to bedrock in the area south of the Tortolita Mountains.

Water-table elevation maps constructed for the years 1970, 1973, and 1979 were used as a hydrogeologic aid in determining the location of the buried fault. Water-table elevations in the alluvial sediments and fractured pediment were plotted and contoured and appear in Figures 15, 16, and 17 (in pocket). The elevation of the water table in the fractured rocks is not as precise as indicated by the contours in the illustrations. The water levels are controlled by the elevation of permeable zones in the bedrock. However, plotting and contouring water- table elevations in the fractured aquifer show the location of this partially buried unit.

A scarp to the east of the study area may be part of the fault system that separates the pediment from basin fill (see Figure 18). This scarp is the 33

Figure 18. Scarp east of the study area in sec. 25, T. 11 S., R. 13 E. 34 southernmost bedrock outcrop of the Tortolita Mountains and is at the confluence of Big Jim Wash and Honey Bee Canyon. Extensive downcutting by the ephemeral streams has exposed the quartz monzonite. The trend of the escarp- ment is in the same direction as the strike of the buried fault. Correlation of the previous data with the locations of water wells south of the Tortolita Mountains (Figure 6), was used to locate the buried fault. Wells which do not yield domestic requirements occur only on the upthrown side of the fault. This major structural feature of the area of investigation is shown in Figures 6 and 8 (in pocket).

Sedimentary Units South of the Tortolita Mountains

Three principal sedimentary units occur in the subsurface of the Tucson Basin. They include: (1) indu- rated sediments, (2) basin-fill deposits, and (3) floodplain alluvium. These units occur within the area of investigation and are hydraulically connected and form a single aquifer. The sedimentary deposits have also been distinguished according to hydrologic characteristics.

This differentiation will serve as the basis for the various sedimentary units described in this report. According to Davidson (1973), the Tucson Basin contains four major sedimentary units, which unconformably 35 overlie one another. The units are the Pantano Formation,

Tinaja beds, Fort Lowell Formation, and surficial deposits. They have been differentiated on the basis of color, rock- fragment content, degree of cementation, and spatial position. Particle size analyses of materials penetrated by wells are an effective supplementary aid in correlating information.

Pantano Formation

The Pantano Formation (Finnell, 1970) typically is

a reddish-brown silty sandstone to gravel, which is weakly cemented by calcium carbonate. The formation contains a few interbedded volcanic flows and tuffs and crops out in

the extreme southwest corner of the study area in the foothills of the Tucson Mountains. No traces of the Pantano Formation are mapped along the margins of the Tortolita Mountains. The Pantano Formation is believed to have been

deposited in early-Tertiary time and is the least known unit in the Tucson Basin (Davidson, 1973). It occurs at

depths of over 1500 ft in the center of the basin and is known to be over 6400 ft thick in some areas (Finnell,

1970). Most of the Pantano Formation penetrated by wells

has no less than 30 percent sand and gravel (detritus greater than 0.061 mm in diameter); the average content is

about 50 percent sand and gravel; where 50 percent of the 36

sample is sand and gravel, about 10 percent, or slightly more, is gravel (detritus greater than 2.0 mm) (Davidson,

1973). According to Davidson (1973), the Pantano Formation

includes a sandstone aquifer tapped by wells in the central and eastern parts of the basin.

Tinaja Beds

The Tinaja beds unconformably overlie the Pantano

Formation and consist of sandy gravel at the basin margins

grading into gypsiferous clayey silt and mudstone along

the central axis of the basin (Davidson, 1973). Davidson

assigns the age of the beds as late Tertiary. No outcrops

of the Tinaja beds appear in the area of investigation but

are found to the east along the western edge of the Santa Catalina Mountains.

The Tinaja beds are a major part of the aquifer in

the Tucson Basin and range in thickness from several to

2000 ft. Some reports indicate the Tinaja beds may be

over 5000 ft thick (Davidson, 1973). In the northern and

eastern parts of the basin, the beds contain abundant

granitic fragments in a feldspathic to arkosic sand matrix;

some of the beds may be mudflows or landslide breccias, but none have yet been positively identified. According

to Davidson (1973), volcanic and sedimentary rock fragments are present in varying amounts but do not constitute much more than 20 percent of the coarse, megascopically identifiable clasts. 37 Fort Lowell Formation

The Fort Lowell Formation (Davidson, 1973) is a

locally derived sedimentary deposit that underlies most of the basin surface and unconformably overlies the Tinaja beds. The formation is 300 to 400 ft thick in most of the basin and thins toward the mountain fronts. The lower contact between the Fort Lowell Formation and the Tinaja beds has been determined on the basis of (1) a color-hue change

from dark-reddish-brown Fort Lowell to less-dark-brown or gray Tinaja beds, and (2) a marked increase in cementation from the Fort Lowell to the Tinaja beds (Davidson, 1973). The sediment of the Fort Lowell Formation is loosely packed to weakly cemented and grades from a silty gravel near the margin of the basin to a silty sand and clayey silt in the central part of the Tucson Basin (Davidson, 1973). According to Davidson (1973), the grain-size distribution is similar to that in the upper part of the Tinaja beds, but the Fort Lowell consistently contains more coarse material than does the Tinaja. In most of the basin, the Fort Lowell is 50 to 90 percent coarser than silt (more than

0.061 mm in diameter) and 25 to 60 percent coarser than sand (more than 2.0 mm in diameter).

Surficial Deposits The surficial deposits are mainly gravel and gravelly sand to sandy silt of fluvial origin. They include 38 alluvial-fan, sheet-flow, and stream-channel deposits. The surficial deposits overlie and partially conceal all older sedimentary units and vary from a thin veneer to a cover of tens of feet thick (Davidson, 1973). The surficial deposits are confined mainly to the basin, but mappable deposits extend into the larger canyons of the Tortolita Mountains and are shown in Figure 8. In the study area, the surficial deposits consist of stream and flood-plain alluvium and undifferentiated deposits of the alluvial fans southwest of the Tortolita Moun- tains and the terraces along the Santa Cruz River. Thick deposits of stream and flood-plain alluvium are found along the channels of the Santa Cruz River and the Caiiada del Oro. This material is loosely packed and generally not cemented. According to Davidson (1973), the thickness ranges between

40 and 100 ft and averages about 50 ft. Undifferentiated alluvial deposits extend over most of the surface of the study area. They range from 5 ft to probably no more than 100 ft in thickness. The rock fragments in both the undifferentiated alluvial deposits and the stream and flood-plain deposits reflect the composition of the nearby mountains and foothills.

Geologic Sections

A site-specific analysis of the perisediment (deposits underlain by the buried pediment surface) and the basin 39

fill immediately beyond the pediment appears in Figure 19

(in pocket). This fence diagram was constructed with data obtained from drilling logs and results of particle-size analyses. The sediments are composed of high percentages of coarse sands and gravels. A comparison of the grain-size distributions of samples from the study area with those in the report by Davidson (1973) reveals that the sediments near the mountain front are coarser than those located in the central parts of the Tucson Basin.

An analysis of the depositional environment near the mountain front explains the abundance of coarse materials. Precipitation in the area of investigation is often very intense, localized, and brief. The erosional potential of the runoff is high due to the lack of vegetal cover on the rock surfaces. Runoff eventually collects in arroyos incised into the mountain blocks. Flows may last only a few minutes and seldom continue for more than several hours. Outwash consists of bedload material, which are sands and gravels rolled along the bottom of the channel by moving water, and the suspended load consisting of the silt-and- clay fraction. The volume of ephemeral stream discharge decreases with the length of flow (Garner, 1974) through infiltration into the dry stream channels. Losses in the volume of runoff result in a reduced sediment-carrying capacity and deposition of the bed load. Fine materials are 40 carried further from the mountain front until the flow volume totally dissipates.

Sediment-carrying capacity also decreases with

channel slope as ephemeral streams approach a local base level. The erosional energy of streams in mountain areas

is greater in basin environments because of the steep

gradient of the land surface. As the gradient decreases away from the mountain front, the erosional capacity is

reduced, and deposition begins. The percentage of coarse material carried by streams decreases as the flow regimen weakens. Figure 20 (in pocket) contains profiles of two

ephemeral streams which drain the southern Tortolita Mountains. The profiles demonstrate the topography along the arroyos.

In addition to the coarse nature of the perisediment and basin fill, Figure 19, the fence diagram, illustrates a continuous layer of fine sand immediately above the downthrown block of the basement rock. The fine sand encountered by drillers is distinctive in content, color, and size distribution. According to particle-size analyses of samples from drill holes, the sand is much finer than the deposits above this layer. A small percentage of the samples comprises gravel (detritus greater than 2.0 mm) and is 80 to 90 percent granitic. The fine sand is less than 100 ft thick, does not occur on the upthrown block of the 41 buried pediment, and does not fit the description of the other sedimentary units of the Tucson Basin. The color, composition, and occurrence of the fine sand immediately above bedrock indicate that it was derived from the weathering and decay of the crystalline quartz monzonite in situ. The development of this layer of sapro- lith indicates that the downthrown block of the quartz monzonite has not undergone the recent degradation that has created the erosional surface on the upthrown block. CHAPTER 3

HYDROLOGIC SYSTEMS

Surface Water

Surface Drainage Pattern

The ephemeral streams in the area of investigation drain into the Santa Cruz River or the Cafiada del Oro.

The surface drainage pattern of the streams south of the Tortolita Mountains is illustrated in Figure 21 (in pocket). An investigation and analysis of the drainage pattern were undertaken to aid in the structural interpretation of the geologic setting. Black-and-white aerial photographs, orthophotographs, and surface mapping procedures were used for this purpose.

The drainage pattern is the configuration formed by the aggregate of drainageways. The configuration formed by a single drainageway is defined as a stream pattern (Howard, 1967). Two basic drainage patterns are shown in Figure 21. Most of the study area exemplifies the parallel drainage pattern which indicates moderate to steep slopes or a topographic control of the system (Howard, 1967).

Very little structural control or resistance to erosion is exerted by the thick alluvial sediments south of the Tortolita Mountains.

42 43

A rectangular drainage pattern is exhibited in

Figure 21. This pattern is often found in igneous terrain wherever bedrock is exposed or under a thin veneer of sediment (Melton, 1959) and is typified in the Tortolita

Mountains on the erosional surface to the south. The bends of the ephemeral streams are abrupt and angular (see Figure 22). Tectonic movements and the associated fracturing of bedrock surfaces have shaped the drainage pattern and caused directional changes, adjustments, and anomalies (von Bandat, 1962). The features of the drainage pattern have formed the basis of analysis and structural interpretation of the pediment.

A number of streams have long, rectilinear channels (see Figures 23 and 24) and are found in the area of investigation. This stream pattern indicates fractures or easily erodible veins or dikes (Howard, 1967) in the Tortolita quartz monzonite pediment. The channels are aligned in nearly the same direction as the strike of the faults (N. 23° W.) in the Tortolita Mountains. The direction of fracturing in the rocks of the exposed pediment surface is also parallel to the primary direction of the stream channels (see Figure 25). The angular nature of the drainage pattern extends a short distance into the perisediment. Higgins (1961) suggested that bedrock fracture sets can be expressed upward through a cover mass and thereby influence drainage patterns. 44

Figure 22. Angular nature of bedrock-controlled stream pattern in sec. 21, T. 11 S., R. 13 E. 45

Figure 23. Fault-controlled segment of stream channel in sec. 21, T. 11 S., R. 13 E. 46

Figure 24. Rectilinear stream pattern in sec. 15, T. 11 S., R. 13 E. 47

Figure 25. Linear segment of a stream channel in sec. 21, T. 11 S., R. 13 E. trends in the same direction as fracturing in bedrock 48

Ground Water

Quantitative Aquifer Characteristics

The hydraulic characteristics of the unconfined, fractured aquifer have not been determined by aquifer tests. Very few long-term aquifer tests have been undertaken in the northwestern Tucson Basin. The lack of available data necessitated reconnaissance ground-water investigations to estimate hydrologic properties of the aquifer system.

Transmissivity and Hydraulic Conductivity. Trans- missivity is an important parameter for the determination of ground-water flow through an aquifer. In an unconfined aquifer, it is defined as the product of the saturated thickness and the average hydraulic conductivity of the medium. Transmissivity is the rate of flow through a vertical strip of an aquifer 1 foot wide under a hydraulic gradient of 1 foot per foot, at the prevailing water temperature. It is usually expressed in consistent units such as ft 2 /dav or m 2 /day or may be expressed in U.S. practical units of gpd/ft. A medium has a hydraulic conductivity of unit length per unit time if it will transmit in unit time a unit volume of ground water at the prevailing viscosity through a cross section of unit area, measured at right angles to the direction of flow, under a hydraulic gradient of unit change in head through unit length in flow (Lohman, 1972). 49

Hydraulic conductivity is a function of the fluid and the porous medium and is usually expressed in consistent units such as ft/day or m/day. English units of hydraulic conductivity include inches per hour or per day and the U.S. gallons per day through a cross-sectional area of 1 ft 2 and per unit hydraulic gradient (gpd/ft 2 ). The distribution of transmissivity values in the

alluvial sediments south of the Tortolita Mountains is illustrated in Figure 26 (in pocket). The general pattern was established from aquifer tests and specific capacity

data in order that ground-water flow through the alluvium could be determined. A number of short-term aquifer tests were conducted in the flood plains of the Santa Cruz River

and the Cariada del Oro. The values of transmissivity determined are questionable owing to the short duration of the aquifer tests.

The specific capacity of a well is functionally related to the transmissivity of the materials from which water is obtained. Specific capacity is the ratio between the pumping rate and the amount of drawdown in a well. It is usually reported in gallons per minute per foot of drawdown after some specified period of pumping, generally one day. Well construction and development affect specific capacity, and as a result, this parameter cannot be an exact criterion of transmissivity. It is used to estimate 50

transmissivity in reconnaissance ground-water investiga-

tions, and several methods concerning this approach have been published.

Studies of aquifer tests in Solano County, Califor- nia, and other sites throughout that state by Thommasson,

Olmsten, and LeRoux (1960) indicate a consistent empirical

relationship between specific capacity and transmissivity. An average factor of 1700 was applied to pump-efficiency tests to approximate transmissivity and to estimate ground- water flow through the unconfined aquifers. Values of transmissivity were obtained by Thommasson, Olmsten, and LeRoux (1960) from the following equation,

Transmissivity = 1700 X Specific Capacity in gpd/ft in gpm/ft of drawdown

This empirical relationship was used to obtain the approximate distribution in Figure 26 (in pocket). The values of transmissivity are highest in the coarse alluvial deposits of the floodplain of the Santa Cruz River and

relatively high in areas along the Cariada del Oro. Aquifer

tests may indicate values that locally do not fit the distribution. However, Figure 26 is representative of the general distribution south of the Tortolita Mountains and is similar to the patterns that appeared in the studies by Anderson (1972) and Davidson (1973). 51

Permeability and Porosity. The primary permeability

of the Tortolita quartz monzonite is negligible because the constituent grains of the rock are closely interlocked.

Permeability exists where the grain-pore matrices have been physically modified by fracturing and subsequent weathering

of the rock. The most permeable zones of the aquifer have been determined from surface mapping of the intensity of fracturing and are indicated in Figure 8.

The direction of fracturing and the physical location of the most intensely fractured rocks suggest that the fractures are associated with the major fault system in the Tortolita Mountains. The faults are generally northeast dipping (Buddin, 1975). Fractures on the erosional surface are approximately vertical or dip slightly northeast, and openings are generally 1 to 2 inches in width. The removal of dissolved mineral matter from the rocks along the bottoms of bedrock-controlled stream channels has resulted in the enlargement of fractures to approximately 6 inches in width. Fractures are generally not evenly distributed and are 1 to 2 inches apart in intensely fractured zones and 10 to 20 feet apart where fracturing is minimal. A characteristic feature of crystalline rocks is the general trend that permeability decreases with depth (LeGrand, 1962). Fractures tend to close at depth because of vertical and lateral stresses imposed by overburden loads (Ellis, 1909). In the fractured aquifer of the 52 pediment area, nearly all wells have perforated intervals between 200 and 400 ft below land surface. Only three wells have been drilled deeper than 480 ft. Yields from the deep wells were not more than 6 gpm, which is an indication that large permeable zones were not encountered at depth.

Logs of the most productive wells indicate that

the Tortolita quartz monzonite is fractured nearly the entire depth of penetration at these sites. In most locations, drilling was continued until yields satisfied domes-

tic requirements. If sufficient water-bearing zones were not encountered in 500 ft of penetration, the borehole was abandoned.

In three of 18 reported drilling efforts, water- bearing zones were encountered below thick layers of solid quartz monzonite. LeGrand (1949) attributes near-horizontal fracturing or sheet structure in granitic rocks to unloading or the removal of overlying material by erosion. According to Jahns (1943), extensive sheet fracture development would not be expected in rocks that are moderately to heavily jointed such as the quartz monzonite of the pediment area. Chemical weathering also increases the permeability and porosity of rock (Clark, 1924). The total amount of mineral matter dissolved by water depends on the length of time water is in contact with the soluble rock, the solu- bility of the mineral constituents, and the aggressiveness of the water with respect to its ability to react with 53

rock-forming minerals (Feth, Roberson, and Polzer, 1964). Drilling reports indicate clay and sand were encountered at

depth in fractured zones of the aquifer in the area of investigation. This material is partially decomposed

quartz monzonite created by dissolution of the rock forming minerals and is found as deep as 500 ft below land surface in the area of investigation.

Compilation of Water-well Data. A compilation of available data obtained from reports of water-well drillers is presented in Table 2. Data were obtained from the Department of Soils, Water, and Engineering of The Univer- sity of Arizona, the U.S. Geological Survey, the Arizona Water Commission, the State of Arizona Land Department, and Tucson Water.

Information concerning bedrock depth, well construction, well ownership, and pumping data were included in Table 2. Well yields available for 9 wells in the fractured aquifer range from 2 to 50 gpm. The average yield, including those obtained from personal communications with owners of wells not officially registered with the State of Arizona, is approximately 10 gpm.

Water Budget

Ground-water Recharge. Several different processes contribute natural recharge to the fractured aquifer south

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44 U1 I 1 II I I I I I I I I I I 0 Q) Q) ; Cd 41W 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4- NO 4-.1- 58 of the Tortolita Mountains. The ground-water system receives some recharge directly from precipitation, which

percolates into exposed, fractured bedrock. Most recharge occurs by infiltration of surface water through the permeable channels of ephemeral streams and into fracture zones that control the drainage system or are crossed by Natural recharge to the fractured aquifer in the pediment area may occur locally or at points within the Tortolita Mountains. In the latter process, ground water moves from

the mountain area toward the pediment through the fracture system where the breaks in the rocks are interconnected. Figure 21 indicates areas where significant ground- water recharge occurs from the infiltration of surface water. Recharge zones occur along thick, areally extensive deposits of stream-channel alluvium at or near the Tortolita Mountain front. The infiltration process involves vertical leakage of water through surficial deposits. The quantity of vertical leakage varies from place to place and is controlled by the hydraulic conductivity and thickness of deposits through which leakage occurs, the head differential between the sources of water and the aquifer, and the area through which leakage occurs (Walton, 1970). Effective precipitation flows from the mountain areas to the gently sloping pediment surface in ephemeral stream channels. Water infiltrates through the coarse sediments along the bottom of the major channels until this 59 material becomes saturated. Total saturation occurs when the percolating waters encounter the water table of the fractured bedrock aquifer or relatively impermeable zones of unfractured bedrock. At this point, the difference in pressure head between the water at the infiltration front and the water in the stream channel becomes negligible.

Any additional recharge in the form of surface flow is rejected and continues downstream until other unsaturated zones are encountered.

According to local residents, heavy precipitation during the months of December 1978 and January 1979 caused the ephemeral streams in the area of investigation to flow for nearly 3 months. During this period, additional recharge to the fractured aquifer was rejected. Infiltra- tion occurred through stream channels into the perisediment, basin fill, or floodplain alluvium of the northwestern Tucson Basin. Water levels in wells in the pediment area and in the perisediment were higher in 1979 than levels in the same wells for 1973 or 1970 (Figures 15, 16, and 17). Thus, the aquifer is rapidly recharged by major precipitation events.

Ground-water Flow System. The water-table elevation maps (Figures, 15, 16, and 17) illustrate the ground-water flow system in the area south of the Tortolita Mountains. Underf low from the Canada del Oro Valley travels from the 60

northeast to southwest until it joins the underflow in the

valley of the Santa Cruz River. The entire ground-water

system then flows northwest and leaves the Tucson Basin at Rillito, Arizona. At this point, the Tucson Basin is narrow and relative shallow (Figure 13). The water-level contours indicate that nearly all the ground water flows between the inselberg in sec. 29, T. 11 S., E. 12 E. and volcanic rocks of the Tucson Mountains. Little ground water leaves the basin between the Tortolita Mountains and the

inselberg. Basement profile D-D' in Figure 14 indicates the depth to bedrock and configuration of the water table in this area.

Water-level contours parallel the mountain front south of the Tortolita Mountains which indicates ground- water recharge to the basin fill from the mountain areas. Water-level measurements in the fractured aquifer of the pediment area show ground water generally flows south.

Ground water moves southeast along the primary direction of the fracture system and southwest along the fractures that are normal to the major fracture pattern. The net direction of flow in the bedrock-controlled aquifer is south as

indicated in Figures 15, 16, and 17. Water-level measurements and well logs indicate a relatively thin saturated layer of perisediment. Ground- water flow in the perisediment is generally south but follows the most permeable zones and sloping channels in the 61 buried erosional surface. The saturated thickness of the perisediment ranges from a few feet to as much as 40 ft in small pockets. Figure 20, the stream profiles of Canada

Agua and the next major ephemeral stream immediately to the east, shows the saturated zone above the buried pediment.

Ground-water Discharge. Ground water is discharged

from the fractured aquifer by springs, through plant transpiration, from joints and other openings in the rocks below the land surface, and from wells in the pediment area. Springs in the higher parts of the Tortolita Mountains are fed by water moving through fractures. The permanence and amount of their flow depends on the extent of the fracture system and the frequency and amount of precipitation (Davidson, 1973).

Some ground water from the fractured aquifer of the pediment area is discharged by natural plant transpiration. A moderately dense cover of mesquite and other phreatophytes, plants with deeply penetrating roots that habitually

reach the water table (Davis and DeWiest, 1966), grow along the ephemeral streams in the area of investigation. These plants are also found in areas mapped as recharge zones as shown in Figure 21. The depth to water beneath the pediment surface is less than 25 ft in many places, especially after recharge events. In this situation, the plants probably receive their water from rainfall, streamflow 62

infiltration, and ground water in storage. A considerable growth of phreatophytes occurs along the channel of the Cariada Agua.

Ground water is also discharged from joints and

other breaks in the rocks into the basin fill near the southern edge of the pediment. This discharge and infiltrated water from the many ephemeral stream channels that

drain the mountains together constitute recharge along the mountain front to the alluvial basin aquifer (Davidson,

1973). An estimate of the volume of annual recharge to the ground-water system from the southern perimeter of the Tortolita Mountains was made by the construction and analysis of a flow net (Figure 27, in pocket). A flow net is composed of two families of lines (or curves); one family represents flow lines, paths followed by particles of water as they move through an aquifer in the direction of decreasing head. Intersecting the flow lines at right angles is a family of lines (or curves) termed "equipotential" lines (Walton, 1970), which, in this case, represent water-table contours. The flow net was constructed using only a few flow lines and equipotential lines. Analysis of the flow net was made with modified forms of the Darcy equation. Previous to analysis, it was assumed that no significant additional ground-water recharge is added to the basin fill south of the Tortolita Mountains other than 63 mountain-front recharge. Studies by Turner and others

(1973) and Smith (1940) indicate very little recharge occurs from rainfall upon the desert floor. Most of the recharge

takes place at the mountain front in the areas indicated in Figure 21. Therefore, ground-water recharge along the perimeter equals underflow in this part of the Tucson Basin.

The flow-net analysis proceeded first by the determination of the boundary between underf low of the Cahada del Oro Valley and underflow derived from recharge along the southern perimeter of the Tortolita Mountains. This is represented by the southernmost flow line in Figure 27 and was determined by analyzing the configuration of both the Tortolita Mountain block and the northern Tucson Basin, as well as the water-table contours. A second flow line was drawn parallel to and north of the first line so that the intersections of flow lines and equipotential lines formed a system of squares.

A representative value of transmissivity determined by aquifer tests was assigned to an elemental square. The quantity of ground water moving through each flow tube was estimated from the following form of the Darcy Equation, Q = TIL where Q = rate of flow of water through cross section of aquifer, in gallons per day (gpd) T = coefficient of transmissivity in gallons per day per foot (gpd/ft) 64 I = hydraulic gradient, in feet per foot (dimensionless)

L = average width of cross section of aquifer, in feet

Calculations show the volume in tube 5 (Figure 27) equals approximately 560,000 gpd or 75,000 ft 3 /day.

Analysis of the Darcy equation reveals that the gradient of the water table in the steady flow condition is inversely proportional to and closely controlled by aquifer transmissivity. The steepening and flattening of the gradient indicate changes in this aquifer parameter. Values of transmissivity were calculated along the flow tube using the Darcy equation and coincide with the distribution in Figure 26. Flow-net analyses were used to determine approximate coefficients in areas where aquifer test data were not available. Upon construction of the second flow tube (tube

4), the lack of transmissivity data required analysis of the gradient of the water table. It was assumed that if the gradient is the same in the area above a flow element as it is within the square, the values of transmissivity are similar. Values obtained by use of the Darcy equation could be extrapolated to obtain the width of the second flow tube by solving for the term representing the average width of the cross section of the aquifer.

Five flow tubes of equal discharge were constructed from calculations using modified forms of the Darcy 65

equation. The annual recharge to the ground-water flow system from nearly 10 miles of the perimeter along the southern Tortolita Mountains is approximately 3,150 acre-feet per year. This figure is comparable to the 4,000 acre-feet per year computed by Anderson (1972). However, Anderson (1972) included approximately 5 more miles of the eastern and western Tortolita Mountain front in his estimate.

Ground-water Pumpage and Water-level Declines. Water- level declines caused by ground-water pumpage over the past decade are evident from comparison of the water-table elevations during 1970, 1973, and 1979. The same trends are expressed by the increases in depth to the water table from 1973 to 1979 shown on Figures 28 and 29 (in pocket). Figure 30 presents hydrographs of representative wells in the alluvium of the northwestern Tucson Basin. These hydrographs show the trends in water levels from 1955 to 1979. Water levels in the fractured aquifer of the pediment area are influenced by the availability of recharge as well as ground-water pumpage and tend to fluctuate markedly.

Water-table elevation maps (Figures 15, 16, and 17) show that the location of the buried fault which separates the pediment from the basin fill has become more evident during the past decade. The outline of the hydrologic boundary is most conspicuous in Figure 17. Ground-water withdrawals from the lower Tucson Basin, especially immediately south 66 of the pediment, have caused a relatively rapid rate of

decline and migration of water table contours in secs. 32 33, 34, and 35, T. 11 S., R. 13 E.

The rate of decline of wells close to the pediment,

(D - 11-13)34bdc, (D-11-13)33dcc, and (D-11-13)34cac, is greater than in the wells farther from the pediment, as shown in Figure 30. This difference in rates of decline indicates that dewatering of the alluvium is greater near the buried erosional surface. Numerous water wells (see

Figure 6) have been pumping and creating a cone of depression in the water table in this area. Total withdrawals from the basin fill are greater than the recharge to the mountain front. In addition, the less permeable uplifted

block of the pediment has become a barrier to the spread of the cone of depression and constitutes a negative hydrologic boundary responsible for the significant water-level declines in this area.

The various maps and graphical representations of the water table in the alluvial sediments south of the

Tortolita Mountains indicate it has been lowered between 10 and 15 ft in the past decade. The depth of the level of saturation below land surface presently ranges between 100 and 620 ft. The greatest depths to the water table are immediately south of the pediment where the static water level in a domestic well measured 620 ft below land surface (Figure 29). 67

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1980 1955 1 •91 19G' 5 1970 1975 YEJP Figure 30. Water levels in alluvial sediments ▪

370

*DATA, tDE57-Aat,IED FROM TNE DEPARTMENT OF SO1I_S, wAzr E , ENG11,4Eata InJG

UNIV Es 'T'? OF Plat -Lot-SA..

• IzacolaDED v44.-TEEz LEVEL MEP•suIZEME1.4 -T

'JELL tsx0.-1- 10N (D l2)I CC CI

ovsi1JE12_: c.EREEIR.A.L. PA. Lsy Fou DA,--riotsi USE: 1=> CMEST

Me rs,,sUR-1,--1C2 POIr•IT ELEVATION • 22-I -11,9 FEET ABOVE MSL_

• IzecoR.DED WAWEtz LEVEL ME A.SU EmENT WEu_ Lock-not., t (-- a) cidc

ow NE12. : JON/1.3 -7 USE

ME NSUR.11.4G PO IT ELEvA-1- toK 2392,05 FEET ABOVE MS1_

I 9SS I 5G0 1965 1970 17S 19E30 V EAR Figure 30. -- Continued 6 9

460 -- • RE GOP-0ED vvATER. LEN/EL ts.f(E.P,SIJR.EmENT *

WELL LOCATION : (D-12 - 13) 5 C.I:i b owNIER...: GEORGE Pusc.F-1 USE : ST0c.x._

MEA,SUR.11.46 PoINT ELEVATION .. 2-Fzi-4 0.14 FEET As12,0n./E. MS_

+0IszTP. 05TAhat•IED 1Z.0N.A T1-1E IDEPAZTME-R-T OF SOILS, WATER_ , A.1.1D Et.IGW4EET-21k4G, n u 1 4 Iv E.R.51-7-1' oF r:-.R.17.0(--4 A..

• RECOIZIDED WgrER LEVEL MEASUREMEWT )14.

WELL LOc.AMC1.1 ". (0- 1 1 - 12..) 36 Cd 6

oWNJER:. 1,11GoL_A...1 505 —

UsE : DOMES-Tic MEASURNG PoiNT ELEVATION : 2591.97 FEET ABOVE MS1-

i I I i 1955 (960 i 95 197o I 975 i 9.So YEAR. Figure 30. -- Continued 70

1 25 ‘,1‘' DATA 0WTA.11.-.1EiD FIZOM -rw DE.P A.A2,7Ewr C: 501LS, v.tp.;TE12k,t.ID EINIGNEERANG UNIVER_SVIN OF A.R...12.01-114,

• mecozoer) Wis..-reQ. LEVEL ME P•SuIZE ME. Ki 7 *

\A/ELL - Lact,.... x-lot.i : (o- iz-‘2) s bbc

owNerz, : J O'Y t--3 -1- USE. UNUSED AG R...1C U....7\312.e

MEASU RING PO t t•31- ELEVAZT1ON ; 2055.5 FEET A 13 ONI E. MS L.

DATE. DRILLED : lea L-i 1 sTATic WATER. LEVEL_ :155 FEET BELOW L1:440 SUR-F A.c E 1 r I t 1955 19 Go 19G9 1S70 )975 i 980 NI/ EAll

Figure 30. -- Continued 71 In some localized areas along the floodplain of the

Santa Cruz River, water levels in wells have risen or declined less than in surrounding areas. Irrigation, treated sewage effluent, and industrial return water have been artificially recharged to the floodplain aquifer. Note the rise of the water table in well (D-12-12)5bbc (Figure 30), an unused agricultural well, between 1957 and 1977. In local irrigation operations, water in excess of the moisture retention capacity of the soil is normally applied to flush out the salts concentrated by the evaporation of irrigation water (Davidson, 1973). The dramatic increase in the water level over a 20-year period in this particular well indicates pumpage has flushed through the soil, perco- lated downward, and reached the saturated zone of the floodplain aquifer.

Water Quality

The chemical characteristics of ground water from wells in the northern Tucson Basin are listed in Table 3. The samples of ground water from the five wells that pene- trate the fractured quartz monzonite aquifer of the pediment area were analyzed in the Water Resources Research Center (WRRC) and the University Analytical Center, both of

The University of Arizona. Field testing included the determination of pH and temperature of the ground water.

Cation-anion balances were compared in equivalents per

▪ •

to 44-1 If it It It It It HH ++ 4-F 4-1. It COO OCOHH 4-10, 00)0 (CH 0 NICC 0 004,0 CO3 NN NN Q) tt N NNN h 0 0000 00,0, 010,000,0/01010,00 HHHHH HHHHHHHHHHHHH

0) 0, !CON. H11/4(..) 1 Ot to I rn V141 0 4-1 IV 44 — NNNN I rz. tal

01 to INNODOCOICONNC71144•01C0 N I to I 40 V) V) h V:, I h h C 0 h I h h I h

H1f, HOO 40411.-4 MONO 0 00 0011-1 NVC0001NN tn NON -1 00, d'H h NON 0, 00,00 0.-10 r44), h:0N h H enr- 4.0NHIHNNN E-1 0U)

4-1

O 000 N01.1,14-1N00:101NVHON 0.4•41-00 FONCC COQ, 000 .41•NHNN•0.-1

1-• •41. N 01 H rl 0 '0 1 • I 4 H H 4 I I

N NH

• 01.41r4 r- N No.* coNul CO N NNNN H N H

0 h N CC CC N N CC V, cr 0 CO 0 h a) rn el 0 • N 4-10 000 0.04-4 HIHNN vINNN H

to 2, ,E 411 411 to ID V) 00000 4-I to

I 0 O 0 00 t I VV. VV I >, • G, 4-t toI-t C to 0.1 h000 0 NN Nr•-• 41,1U cf h 11-Ncr NNNIJINNHINNO‘r 't 0/ • I > 4J. I H CM CVU o E •-n

- 7.) O 0 ,-4 CO C C144r1 NO) 01 TrN.41.0 .0 /4 Q) to I HNN•41-0700.414NOIN 01.0 HN 0 4, >••• C 4.4 CHn

to 0/10 to 44 > C CC to t,r. co H H 0 0 0 I N 0 In N 0 N N W 4 0H•zr 04/1000110,0, H H DI 00 HIH 14 CIL 9 -G to to N>4 01 CO 0 N Q CD, 4-1 .0 -14 ODO t 4hr-IN01000DOONMNN h 0 O 111O .11N I h000,0,HINCOOD.-4 H H HCC 4400 0 U1 1111•-• 4, U

O r,

•tr .1.) • la O 0 10 0 0 0 0 0 0 0 00 0 00 , 0 al 0 s. E c ta • 0, 0, 3 • 7 • X 0 U W U 000 tO 1.0 .0 14 DI 7 1 .14 a) E" HH I Mr4c)..0 u1CONN0H0C0h0N 003 I NNOH1010,01-1N0NINNNN E 0 E NH I NVI•ur n1,--1,4NNHH HHH 0 34 C 11-4•HU1 1•14 u :g -2 a) Il li r'S g 2- 2- 'g 7 G rg .-`4 g :g ‘1,-) 0`,) • -1 0 ta r•INN NNININNNUICOMN.HIHHHH ta 34 D 4, NNN 4-1 N 141,, NrINNNNHINMNIn .0 'D 17 HHH HHHHHHHHHHHHHHH O 0 1 t t I I I I I t t I r 1 3.. H H H H H H H H N N N N N c.2N N H H H H H H H H H 74 H H H H H H H H 11111,4„1 I RI a) r0 0 0 0 0 0 0 0 0 0 0 0 f!, 0 0 0 0 0 013 0 * -3- -H- 73

million (epm), to help check the accuracy of the analyses and appear in the Appendix.

Concentrations of dissolved solids in the water of the fractured aquifer are considerably higher than in water of the alluvial sediments of the northern Tucson Basin. The total dissolved solids (TDS) ranges from 680 to 760 parts per million (ppm) which exceeds the recommended permissible

concentration limits established by the U.S. Environmental Protection Agency (1975) for drinking water standards. Ground water in the study area may be characterized as calcium, sodium, magnesium, and bicarbonate water. Bicarbonate is the dominant anion and ranges in concentra-

tion from 372 to 529 ppm in the five wells tested. The bicarbonate concentrations in the ground water of the pediment area are derived from the solution of carbonate minerals in bedrock such as calcite and dolomite, from the solution of relict salts during streamflow infiltration along the major drainage washes, and from carbon dioxide- charged water which infiltrates through the soil zone in recharge areas (Laney, 1972). The relatively high concentrations of calcium and bicarbonate in the ground water of the pediment area may be partially derived from the solution of the Precambrian and Paleozoic rocks in the Tortolita Mountains. Discon- tinuous fault zones extend from the pediment area into these rocks which include metaconglomerates, epidote skarns, 74 and marbles. Calcium and bicarbonate enter the ground

water directly by solution of rocks and percolation into the fault zones or by solution of relict calcite along the channels of ephemeral streams.

In the recharge zones of the pediment area, the growth of phreatophytes and accumulation of organic material

have contributed to the development of a soil profile. Bio- chemical and hydrochemical processes in the soil provide a

supply of CO 2 to the soil air, which dissolves in water to produce H 2 CO 3 . Carbonic acid dissociates into the bicarbonate ion and the hydrogen ion in a complex reaction controlled by pH, the partial pressure of CO 2' the concentration of calcium, the concentration of inorganic carbon species, and the concentration of other ions which influence the carbonate equilbrium of the ground water. The feldspars, micas, and other silicate minerals of igneous rocks are weathered by the chemically aggressive

nature of water containing dissolved CO 2 . Weathering pro- duces an aluminosilicate residue, usually clay minerals such as kaolonite, illite, and montmorillonite. Cations + + released to the water are normally Na , K , Mg++ , and Ca ++

(Freeze and Cherry, 1979). In the study area, the relative abundance of the cations in the ground water from the five

+ +I- wells sampled is Ca ++ > Na > mg > K+ .

Dissolved-solid concentrations in ground water of

the pediment area are relatively high in comparison to 75 concentrations in most wells in the alluvial deposits of the northern Tucson Basin. Laney (1972) indicated that ground water along Rillito Creek, Tanque Verde Creek, and Pantano

Wash are higher in TDS than in ground water in the surrounding areas as a result of the solution of relict salts along these stream channels. The solution of relict salts along the Cahada Agua and the other major ephemeral streams of the pediment area, the solution of the calcareous Precambrian and Paleozoic rocks of the Tortolita Mountains, and the weathering of the silicate minerals of the intensely fractured quartz monzonite contribute to the high concentrations of dissolved solids in the ground water. As ground water flows from the recharge zones of the fractured aquifer into the alluvial sediments of the northern Tucson Basin, the rate of CO 2 replenishment decreases. As reacts with H2 CO3 silicate minerals of igneous rocks CO 2 (aq) is consumed and the reaction may proceed to a condition of saturation with respect to the carbonate and calcium minerals present in the system. When the ground water enters the alluvial sediments, changes in ground-water temperature and the introduction of other + _ . ions + mineral forming such as Na , K , Cl , and SO4 will influence the carbonate mineral equilibria. CaCO 3 will accordingly precipitate in the alluvium and provides varying degrees of cementation for the sedimentary units of the Tucson Basin (Davidson, 1973). Precipitation of calcium 76 carbonate also lowers the concentration of dissolved solids in the ground water in the alluvial sediments. CHAPTER 4

CORRELATION AND INTERPRETIVE ANALYSIS OF SALIENT FEATURES

The occurrence, movement and distribution of ground water in the fractured aquifer of the pediment area are governed by the location and intensity of fracturing of bedrock and the source and amount of natural ground-water recharge. Results of analyses with transparent overlays suggest that the quartz monzonite constitutes a potential aquifer where a high degree of correlation exists between the locations of tectonic features, bedrock-controlled

stream channels, and productive water wells. Figure 6

illustrates the locations of water wells and dry boreholes in the area of investigation. Bedrock is most intensely fractured immediately west of the Cafiada Aqua in sec. 21, T. 11 S., R. 13E. Faults extend from the central Tortolita Mountains into this zone. a linear segment of the Caiiada Aqua is approximately one mile in length and indicates that fracturing controls the stream pattern (see Figs. 8 and 22). A structural depression along the Cafiada Aqua has been created by tectonic and subsequent weathering and erosion of the quartz monzonite. The intensity of fracturing and weathered nature of the rock at depth were noted in drilling logs. Caving of the

77 78 boreholes indicates the bedrock is highly weathered and fractured. Depressions in upland areas underlain by diorite and gabbro-diorite were recognized by LeGrand (1952) and attributed to the solution of rock-forming silicate minerals.

The vigorous growth of phreatophytes and the extensive deposits of stream-channel alluvium suggest that ground water is recharged to the fractured aquifer along this stretch of the Canada Agua. Surface water has infiltrated through the coarse stream channel into the fracture system. Wells in this structural depression provide water for most of the residents of the pediment area.

Less extensive structural depressions occur on the exposed erosional surface where ground-water resources have also developed. Five water wells have been drilled adjacent to a fracture-controlled stream channel immediately east of the Canada Agua. The wells are closely spaced in an area mapped as a recharge zone. Two other wells in the eastern part of sec. 22, T. 11S., R. 13 E. are also near a rectilinear segment of an ephemeral stream.

Table 4 provides information concerning the relationship between locations of water well, abandoned boreholes, structural depressions, and ephemeral streams. Most productive water wells in the pediment area are in structural depressions traversed by ephemeral streams. The depressions are areas that have been intensely fractured by faulting 79

Table 4. Compilation of water-well penetrations in the exposed pediment area

Productive wells in structural depressions traversed by

ephemeral streams 15 Productive wells distant from structural depressions and ephemeral streams 5 Abandoned boreholes (dry) distant from structural depressions and ephemeral streams 4

Abandoned boreholes (dry) adjacent to ephemeral streams 1

Total penetrations in the exposed pediment area 25 80

in the Tortolita Mountains. Development of ground-water

resources is less tenable on the exposed erosional surface in areas distant from intensely fractured zones which control the channels of ephemeral streams.

The chances of locating favorable water-bearing

zones in the perisediment are less than in the exposed pediment area. Rocks that have resisted weathering and erosion rise above the thin, saturated zone of the pen- sediment. Domestic supply cannot be obtained from wells

penetrating these buried features unless the bedrock has been tectonically altered. Several wells did encounter

fractured bedrock at depth. However, analysis of the surface drainage pattern provides little indication of the subsurface topography or structure. Correlation of the map showing depth to bedrock (Figure 13) or the isopach of alluvial sediments with well locations (Figure 6) indicates the paths of buried channels. Numerous water wells in sec. 28, T. 11 S., R. 13 E. have penetrated a deep alluvial channel in the pen- sediment. Figure 13 shows two erosional remnants on either

side of the buried channel. Eight boreholes penetrating

these shallow bedrock remnants have been dry. Stream flow in the Canada Agua provides natural ground-water recharge to the perisediment in sec. 28, T 11 S., R. 13 E. A resistant bedrock outcrop abruptly divides the channel of this ephemeral stream (Figure 8). Most 81 surface water and subsequent ground-water recharge flow along a western segment of this braided stream. Ground- water resources have not been located in the perisediment east of the Canada Aqua.

Figure 27 illustrates several salient features

concerning the quantity of ground-water recharge from the southern perimeter of the Tortolita Mountains. Flow tubes 3, 4 and 5 are narrower than tubes 1 and 2. This indicates a greater volume of water is recharged from the southeastern perimeter of the Tortolita Mountains than is recharged from the southwestern perimeter. The additional recharge is derived from the discharge of the fractured aquifer of the pediment area.

Flow tubes 1 and 2 are broad and encompass nearly 3.5 miles of the mountain front. The steep gradient sug- gests the transmissivity of the alluvial deposits is relatively less than in areas to the south. The decrease in transmissivity is attributed to a reduction in the saturated thickness rather than a reduction in the hydraulic conductivity of the alluvial materials. Much of the recharge from Wild Burro and Ruelas Canyons is lost through evapotranspiration along stream channels before it reaches the ground-water system.

The fractured Tortolita quartz monozonite controls the channels draining the southeastern sections of the

Tortolita Mountains. A relatively small volume of water 82 moving through the fracture system is lost through the process of evapotranspiration. Discharge enters the basin fill in flow tubes 3, 4, and 5 (see Figure 27). Flow tube 4 receives discharge from the perisediment and fracture system, which are primarily recharged by the Canada Agua.

Approximately 560,000 gpd or 75,000 ft 3 /day are discharged into flow tube 4, which is nearly 2,600 ft wide. Nearly 10,000 ft of mountain front discharge the same volume into flow tube 2. The relatively narrow width of flow tube 4 indicates the magnitude of ground-water recharge from this ephemeral stream. CHAPTER 5

CONCLUSIONS

The southern third of the Tortolita Mountains is comprised of a large pluton of Tortolita quartz monzonite.

Extensive pedimentation and fracturing have occurred on the southern flank of this pluton. Fracturing of the pediment rock is a result of tectonic movements that produced a number of high-angle, normal faults in the Tortolita Mountains. The fracturing has created secondary perme-

ability by physically modifying the intrusive rocks. Well logs, geophysical data, water-table elevation maps, the depth-to-bedrock map, and field observations all indicate a fault south of the Tortolita Mountains, which limits the southern edge of erosional surface. Water-table elevation maps show the location of this buried fault has become more evident during the past decade. Ground-water withdrawals from the lower Tucson Basin, especially immediately south of the pediment, have caused a relatively rapid rate of decline in water levels in the alluvium in this area. The less permeable uplifted block of the pediment has become a barrier to the spread of the cone of depression and constitutes a negative hydrologic boundary.

83 84 The various maps and graphical representations of the water table in the alluvial sediments south of the Tortolita Mountains indicate it has been lowered between 10 and 15 ft in the past decade. The greatest depths to the water table are immediately south of the pediment. In some localized areas along the floodplain of the Santa Cruz

River, water levels in wells have risen or declined less than in surrounding areas. Treated sewage effluent and irrigation and industrial return water have been artificially recharged to the floodplain aquifer.

Water-level contours parallel the mountain front south of the Tortolita Mountains which indicates ground- water recharge to the basin fill from the mountain areas. A flow net was constructed and analyzed to estimate the volume of annual recharge to the alluvial ground-water system from the southern perimeter of the Tortolita Mountains. Transmissivity values established from aquifer tests and specific capacity data were used in the flow-net analysis. Calculations indicate the annual recharge to the ground-water flow system from nearly 10 miles of the perimeter along the southern Tortolita Mountains is approximately 3150 acre-feet per year.

The fracture system controls the drainage pattern on the erosional surface south of the Tortolita Mountains. Rectilinear stream channels follow paths where tectonic deformation has weakened the bedrock. Surface flow in the 85 channels of ephemeral streams provides ground-water recharge to the fractured aquifer of the pediment area. Recharge zones were determined from the thickness and

areal extent of stream-channel deposits and the density of phreatophytes along the ephemeral streams.

Sustained streamflow from extended periods of precipitation saturate the fractured aquifer and recharge the perisediment in the area of investigation. Saturation

and subsequent rejection of available recharge during major rainfall events indicate the limited storage capacity of the fractured aquifer.

The development of ground-water resources in the aquifer is tenable where fracturing has created secondary

permeability and porosity. Numerous water wells have been located in structural depressions traversed and recharged by ephemeral streams. Favorable water-bearing zones in the perisediment are in deep, permeable channels on the buried erosional surface.

The Canada Agua is the major ephemeral stream draining the southeastern section of the Tortolita Mountains. Infiltration through the course sediments of the stream channel provides significant recharge to the pediment and perisediment water-bearing zones. Numerous water wells have been located along the structural depression traversed by the Canada Aqua. The fractured aquifer 86 of the pediment area eventually discharges a significant volume of ground water into the alluvial deposits of the Tucson Basin.

n APPENDIX

ANION-CATION BALANCE OF GROUND WATER FROM FRACTURED QUARTZ MONZONITE AQUIFER OF PEDIMENT AREA

Analyses of chloride and bicarbonate were performed by the author in 1980. Other chemical constituents were determined by the University Analytical Center, University of Arizona, in 1980.

87 88

Table A-1. Cation-anion balance of water from well (D-11-13) 21cab in fractured aquifer of pediment area

Equivalent Weight PPm epm % epm

Cations

Ca 20.04 108.0 5.39 48.2

Mg 12.16 23.0 1.89 16.9 + Na 23.00 89.0 3.87 34.6 + K 38.10 .97 .03 .3

Total 220.97 11.18 100.0

Anions -. CO 3 30.0 0 0 0 529.0 8.82 71.9 HCO -3 61.01 Cl - 35.46 75.9 2.14 17.4

48.03 61.0 1.27 10.4 SO 4

NO 3 62.00 19.00 .76 .04 .3

Total 666.66 12.27 100.0

Percent difference in balance: 4.65

Total dissolved solids: 388 porn

89 Table A-2. Cation-anion balance of water from well (D - 11-13)21dca in fractured aquifer of pediment area

Equivalent Weight PPm epm % epm

Cations

Ca 20.04 108.0 5.39 52.2 Mg 12.16 27.0 2.22 21.5 + Na 23.00 62.0 2.69 21.5 + K 39.10 1.1 .03 .3 Total 198.1 10.33 100.0

Anions

CO= 30.00 0 3 0 0 HCO -3 61.01 372.6 6.21 60.9

Cl - 35.46 103.8 2.93 28.7 48.03 51.0 1.06 10.4 SO 4 _ 62.00 NO 3 F - 19.00 .29 .02 .1 Total 572.69 10.22 100.0

Percent difference in balance: 0.53

Total dissolved solids: 726 ppm

90 Table A-3. Cation-anion balance of water from well (D- 11-13)22acd in fractured aquifer of Pediment area

Equivalent Weight PPm epm % epm

Cations ++ Ca 20.04 102.0 5.09 47.8 ++ Mg 12.16 27.0 2.22 20.9 + Na 23.00 76.0 3.30 31.0 + K 39.10 1.3 .03 .3 Total 206.3 10.64 100.0 Anions

Co 3 30.0 0 0 0 HCO -3 61.01 404.8 6.63 59.8 Cl - 35.46 82.9 2.34 21.1

SO 4 48.03 100.0 2.08 18.8 _ NO 3 62.00 F - 19.00 .48 .03 .3 Total 588.18 11.08 100.0

Percent difference in balance: 2.03

Total dissolved solids: 794 ppm 91

Table A-4. Cation-anion balance of water from well (D-11-13)22bdb in fractured aquifer of pediment area

Equivalent

Weight PPm epm % epm

Cations

Ca 20.04 102.0 5.09 45.2

Mg 12.16 27.0 2.22 20.1 + Na 23.00 85.0 3.70 33.5 + K 39.10 .64 .02 .2

Total 214.64 11.03 100.0

Anions

CO 3 30.00 0 0 0 HCO -3 61.01 414.0 6.79 57.7

Cl - 35.46 99.9 2.82 23.9 SO 4 48.03 100.0 2.08 17.7 NO

19.00 1.5 .08 .7

Total 615.4 11.77 100.0

Percent difference in balance: 3.24

Total dissolved solids: 830 ppm 92

Table A-5. Cation-anion balance of water from well (D-11-13)22bdd in fractured aquifer of pediment area.

Equivalent Weight PPm epm % epm

Cations ++ Ca 20.05 108.0 5.39 48.0 ++ Mg 12.16 27.0 2.22 19.8 + Na 23.00 83.0 3.60 32.1 + K 39.10 .47 .01 .1

Total 218.47 11.22 100.0

Anions

CO 3 30.00 0 0 0 HCO -3 61.01 395.6 6.48 58.4

Cl - 35.46 96.2 2.71 24.4

SO4 48.03 90.0 1.87 16.8 NO

F- 19.00 .56 .04 .4

Total 582.36 11.10 100.0

Percent difference in balance: 0.54

Total dissolved solids: 801 ppm REFERENCES

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