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Hydrogeology and Ground-Water Flow in the Edwards-Trinity Aquifer System, West-Central Texas

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Hydrogeology and Ground-Water Flow in the Edwards-Trinity Aquifer System, West-Central Texas Hydrogeology and Ground-Water Flow in the Edwards-Trinity Aquifer System, West-Central Texas Regional Aquifer-System Analysis—Edwards-Trinity By Eve L. Kuniansky and Ann F. Ardis Professional Paper 1421-C U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director U.S. Geological Survey, Reston, Virginia: 2004 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225 For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to repro- duce any copyrighted materials contained within this report. Suggested citation: Kuniansky, Eve L., and Ardis, Ann F., 2004, Hydrogeology and ground-water flow in the Edwards-Trinity aquifer system, west-central Texas: Regional aquifer-system analysis—Edwards-Trinity: U.S. Geological Survey Professional Paper 1421-C, p. Library of Congress Cataloging-in-Publication Data Kuniansky, Eve L. Hydrogeology and ground-water flow in the Edwards-Trinity aquifer system, west-central Texas by Eve L Kuniansky and Ann F. Ardis p. cm. —(Regional aquifer-system analysis—Edwards-Trinity aquifer system) (U.S. Geological Survey professional paper; 1421-C) Includes bibliographical references. Supt. of Docs. no.: 1. Edwards-Trinity Aquifer (Tex.) 2. Edwards Aquifer (Tex.) 3. Trinity Aquifer (Tex.) 4. Water, Underground-Texas I. Title II. Series. III. Series: U.S. Geological Survey professional paper 1421-C CIP ISBN iii FOREWORD THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM The RASA Program represents a systematic effort to study a number of the Nation’s most important aquifer systems, which, in aggregate, underlie much of the country and which represent an important component of the Nation’s total water supply. In general, the boundaries of these studies are identified by the hydrologic extent of each system and, accordingly, transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capability that will contribute to the effective management of the system. The use of computer simulation is an important element of the RASA studies to develop an understanding of the natural, undisturbed hydrologic system and the changes brought about in it by human activities and to provide a means of predicting the regional effects of future pumping or other stresses. The final interpretive results of the RASA Program are presented in a series of U.S. Geological Survey Professional Papers that describe the geology, hydrology, and geochemistry of each regional aquifer. Each study within the RASA Program is assigned a single Professional Paper number beginning with Professional Paper 1400. Charles G. Groat Director v Contents Abstract . .1 Introduction . .3 Purpose and Scope . 3 Previous Studies . 5 Physiography and Hydrologic Setting. 6 Hydrogeology. .10 Edwards–Trinity Aquifer System and Contiguous Hydraulically Connected Units . 10 Structural Controls on Ground-Water Flow . 15 Hydraulic Characteristics. 16 Transmissivity. 16 Anisotropy. 19 Vertical Hydraulic Conductivity. 19 Storage Coefficient . 19 Ground-Water Use. 20 Long-Term Water-Level Variations. 28 Potentiometric Surface. 29 Natural Recharge and Discharge. 34 Ground-Water Flow . .38 Regional Steady-State Simulations of Ground-Water Flow. 40 Regional Model Development . 40 Finite-Element Method . 41 Regional Finite-Element Mesh and Lateral Boundaries . 41 Internal Boundaries. 41 Water Budgets from Steady-State Regional Simulations. 41 Direction of Ground-Water Movement . 46 Subregional Transient Simulations of Ground-Water Flow in the Edwards and Trinity Aquifers . 46 Subregional Model Development . 46 Subregional Finite-Element Mesh, Lateral and Internal Boundaries. 47 Model Layering . 47 Transient Simulations of Recent Conditions (1978–89) . 48 Water Budget from Transient Simulation . 53 Direction of Ground-Water Movement and Description of Flow Paths . 56 Limitations of the Subregional Model and Flow Path Analysis. 60 Summary and Conclusions . .61 Selected References . .62 Initial Conditions . .72 Time-Step Size. .72 Calibration . .72 Sensitivity Analysis . .74 vi Edwards–Trinity Aquifer System, West-Central Texas Plates [In pocket] 1. Regional model and finite-element mesh 2. Subregional model and finite-element mesh and layering 3. Regional potentiometric surface, magnitude and direction of flow 4. Subregional potentiometric surface, magnitude and direction of flow Figures 1–2. Maps showing— 1. Study area and major and minor aquifers, west-central Texas . 4 2. Faults associated with the Balcones fault zone, central Texas. 8 3. Graphs showing mean monthly precipitation and pan evaporation at selected locations, west-central Texas. .9 4. Correlation chart showing hydrogeologic units, major aquifers, and their stratigraphic equivalents, west-central Texas . .11 5. Diagram showing generalized section showing the geologic or hydrogeologic units simulated as one layer in the regional model, west-central Texas. .13 6–10. Maps showing— 6. Estimated transmissivity of the Edwards–Trinity aquifer system and contiguous, hydraulically connected units from the regional model, west-central Texas . 17 7. Estimated transmissivity of the Edwards and Trinity aquifers from the subregional model, central Texas. 18 8. Estimated distribution of anisotropy in the Edwards and Trinity aquifers from the subregional model, central Texas . 20 9. Estimated vertical leakage coefficients of confining units from the subregional model, central Texas . ..
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