Geochemistry of Ground Water in Avra Valley, Pima County, Arizona

Geochemistry of ground water in Avra Valley, Pima County, Arizona Item Type Thesis-Reproduction (electronic); text Authors Conner, Leslee Lynn,1957- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 01/10/2021 12:48:22 Link to Item http://hdl.handle.net/10150/191892 GEOCHEMISTRY OF GROUND WATER IN AVRA VALLEY, PIMA COUNTY, ARIZONA by Leslee Lynn Conner 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 WITH A MAJOR IN HYDROLOGY In the Graduate College THE UNIVERSITY OF ARIZONA 1986 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment 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 or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained by the author. SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: gCnrn, Wtt E. S. SIMPSO Date Professor of Hydrology and Water Resources ACKNOWLEDGMENTS I am grateful to my advisor, Dr. Eugene S. Simpson for his guidance and laboratory support through the course of this research. I am also indebted to Dr. Austin Long for isotope analyses, and to Drs. Long and Stanley N. Davis for their constructive comments as members of my graduate committee. I gratefully acknowledge the information provided by the City of Tucson Water Department, Arizona Department of Water Resources, U.S. Geological Survey and Jim Posedly. In addition, I extend my gratitude to the City of Tucson and the residents of Avra Valley, who readily and generously allowed me to sample their wells. Heartfelt thanks are due Darcy Campbell, June Fabryka-Martin and Daniel Matthews, who each so generously offered support and guidance, as only dear friends can. I am also grateful to Sally Adams and Harding Lawson Associates for their technical support. TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS vi LIST OF TABLES vii ABSTRACT viii 1. INTRODUCTION 1 2. AVRA VALLEY 4 Geology 6 Mountains 6 Structure 8 Alluvium 10 Hydrology 12 Climate 12 Surface Water 12 Ground Water 13 Ground-water development 18 Ground-water chemistry 18 Past studies 21 3. METHODS 22 Sampling Program 22 Laboratory Analyses 25 4. RESULTS AND DISCUSSION 27 General Trends in Ground-water Chemistry 27 Ground Water in Southern and Mid Avra Valley 32 Ground Water in Mountain-front Samples 40 Ground Water in Northern Avra Valley 42 Minor Ion Chemistry 43 Potassium 43 Nitrate 44 Fluoride 47 Iodide 50 iv V TABLE OF CONTENTS--Continued Page Stable Isotopes 51 Deuterium and oxygen-18 51 Fractionation 52 Units 53 Isotopes in southern Arizona 53 Isotopes in Avra Valley 54 Equilibrium Approach 56 Saturation indices 57 Mineral stability diagrams 60 5. SUMMARY AND CONCLUSIONS 65 Recommendations 66 REFERENCES 68 LIST OF ILLUSTRATIONS Figure Page 1. Location of Avra Valley, Arizona 2 2. Avra Valley 5 3. Geology of the Avra Valley area 7 4. Depths to bedrock, Avra Valley 9 5. Depths to water, Avra Valley 15 6. Elevation of the water table and approximate flow lines, Avra Valley 16 7. Transmissivities of the ground water aquifer, Avra Valley 19 8. Location of sampled wells, Avra Valley 23 9. Piper trilinear diagram 31 10. Stiff diagrams 33 11. Bicarbonate - TDS scattergram 36 12. Sodium distribution in Avra Valley 39 13. Nitrate - chloride scattergram 46 14. Fluoride and iodide distribution in Avra Valley 48 15. Isotope ratios of samples from Avra Valley 55 16. Mineral stability diagram for the H20-Al203-K2 0-Si02 system 62 17. Mineral stability diagram for the H20-A103-Na20-Si02 system 63 v i LIST OF TABLES Table Page 1. Estimated volumes of flow into Avra Valley 17 2. Sampled wells in Avra Valley 24 3. Results of chemical analyses 28 4. Recommended limits for drinking water 29 5. Ca:Na ratios in Avra Valley 38 6. Saturation indices 59 v ii ABSTRACT A geochemical evaluation of the ground water in Avra Valley, Arizona using major ions and the stable isotopes deuterium and oxygen-18 verifies that ground-water recharge from the surrounding mountains is negligible and that most ground water enters the valley as underflow from Altar Valley. Primarily granitic sediments result in soft waters with elevated pH in southern Avra Valley. Increasing sedimentary mineralization in northern reaches produces slightly harder waters. Elevated nitrate in the ground water results from intense local agriculture; uneven distribution of nitrate and corresponding ground-water mounding suggest leakage around irrigation well casings as the primary pathway for nitrates to the ground water. viii CHAPTER 1 INTRODUCTION This report evaluates ground-water geochemistry in Avra Valley, west of Tucson, Arizona (Figure 1). The objectives are to characterize the chemistry of the ground water and to understand the processes which determine the chemistry. In particular, the influence of mountain-front recharge, sediment lithology, and agricultural activity on the chemistry of Avra Valley ground water are examined. In addition, the extent to which ground-water chemistry supports or changes the present understand- ing of ground-water movement in Avra Valley is reviewed. For the study, ground-water samples were collected from Avra Valley and analyzed for major ions, selected minor ions, and stable isotope ratios of hydrogen and oxygen. The chemical results were evaluated with reference to principles of water chemistry and geochemical processes. The value of geochemical investigations of ground water lies in the contributions of such studies to our knowledge of the occurrence and movement of the water in the subsurface environment (see Garrels, 1967; GarrelS and MacKenzie, 1967; Helgeson, 1968). Similar research in southern Arizona has included the Cortaro (Martin, 1980) and Oracle (Winstanley, 1984) areas, San Pedro Valley (Usunoff, 1984; DeWald, 1984), the Santa Rita Mountains (Merz, 1984) and the Tucson Basin (Bostick, 1978; Gallaher, 1979; Thorne, 1982; Olson, 1982; Koglin, 1984; 1 2 0 50 100 MILES Figure 1. Location of Avra Valley, Arizona. 3 and Mohrbacher, 1984). The approach in these studies was similar to that in the present evaluation: characterization of the major ion chemistry of ground water and identification of geochemical trends, often supplemented by use of stable isotopes. Knowledge of the local geology in such studies is helpful, because the composition of the aquifer materials through which the ground water passes plays a major role in the chemistry of the ground water. CHAPTER 2 AVRA VALLEY Avra Valley is a north-trending alluvial basin in south-central Arizona, 12 miles west of the city of Tucson. Covering roughly 520 square miles, the valley extends 40 miles from south of Three Points, Arizona to the Pima-Pinal County line north of Marana (Figure 2). East to west, Avra Valley ranges from 7 to 20 miles (White, Matlock and Schwalen, 1966). The Tucson Mountains form the eastern boundary of Avra Valley, separating it from the Tucson Basin. The Silverbell, Waterman, and Roskruge Mountains bound the valley along the west, and the Sierrita Mountains lie to the south. These mountains are of moderate height, rising up to 2000 feet above the valley floor. The sediments which fill Avra Valley derive from mountains bounding the valley on the east, west and south. The floor of Avra Valley slopes gently to the north with an average gradient of 26 feet per mile, from an elevation of 2550 feet near Three Points to 1950 feet outside Marana. The study area is defined as that portion of Avra Valley lying between the southern line of Township 11 South and State Highway 86 (Figure 2). This area is 23 miles in length and covers approximately 345 square miles. 4 5 R 9 E / \fl io E R 11 E R 12 E I r z -"*/- f-J-r .-"s•• Marana ,,,4 . I-10 1,12. 091.,;6•>._ o te-)7'S ,r 1r TWIN PEAKS o. TERM\ °MOUNTAINS ..-/ / \\ 1... • n n"'"-'".:-.:-. -7-- 7 '‘v\. ( • '• / ' ,, ...-K... / • z. – ;:- f //.„1 k t 13 . ,../. • f 1'..—...( /(•• S. 404, r A-- 4t ( / ( r ( \ 4.1e IT%.4 ) ... +4, \\ ., 4-, ff 1..---. I 1•.- qt • Jf(7,9 .). : : •••Y • ./ / "N stei>. co .."......... 1.7 ,6tae-e.. Three Points 4Z. V- 4 I (c, - \.n • ,/ t• • 4 ERR I TA 0 MILES 5 MC/017AM S SCALE: 1:250,000 Figure 2. Avra Valley. 6 Geology Avra Valley lies in the Basin and Range Lowlands physiographic province which extends across southern Arizona. This province is characterized by predominantly northwest-trending alluvial basins flanked and separated from each other by steeply rising mountains of volcanic origin. The topography of the area is believed to be the result of an extensive period of block-faulting which occurred in Tertiary time, during which down-thrust blocks created grabens roughly parallel to adjacent uplifted mountains. Subsequently, these grabens filled with sediments eroded from the mountain slopes, creating the deep alluvial basins present today (Eberly and Stanley, 1978).

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