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Simulation of Flow in the Edwards Aquifer, San Antonio Region, Texas, and Refinement of Storage and Flow Concepts

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Simulation of Flow in the Edwards Aquifer, San Antonio Region, Texas, and Refinement of Storage and Flow Concepts THE EDWARDS-TRINITY AQUIFER SYSTEM, SAN ANTONIO REGION, TEXAS United States Geological Survey Water-Supply Paper 2336-A _ EDWA Prepared in cooperation with the San Antonio City Water Board AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the cur­ rent-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Sur­ vey publications released prior to the current year are listed in the most recent annual "Price and Availability List" Publications that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available. Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated month­ ly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 25425, Denver, CO 80225. Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. Order U.S. Geological Survey publications by mail or over the counter from the offices given below. BY MAIL OVER THE COUNTER Books Books Professional Papers, Bulletins, Water-Supply Papers, Techniques of Water-Resources Investigations, Circulars, publications of general in­ Books of the U.S. Geological Survey are available over the terest (such as leaflets, pamphlets, booklets), single copies of Earthquakes counter at the following Geological Survey Public Inquiries Offices, all & Volcanoes, Preliminary Determination of Epicenters, and some mis­ of which are authorized agents of the Superintendent of Documents: cellaneous reports, including some of the foregoing series that have gone out of print at the Superintendent of Documents, are obtainable by mail from WASHINGTON, D.C.-Main Interior Bldg., 2600 corridor, 18th and CSts.,NW. U.S. Geological Survey, Books and Open-File Reports DENVER, Colorado-Federal Bldg., Rm. 169,1961 Stout St. Federal Center, Box 25425 LOS ANGELES, California-Federal Bldg., Rm. 7638,300 N. 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E-146,701 C St. Maps For maps, address mail orders to U.S. Geological Survey, Map Distribution Maps Federal Center, Box 25286 Maps may be purchased over the counter at the U.S. Geologi­ Denver, CO 80225 cal Survey offices where books are sold (all addresses in above list) and at the following Geological Survey offices: Residents of Alaska may order maps from ROLLA, Missouri-1400 Independence Rd. Alaska Distribution Section, U.S. Geological Survey, DENVER, Colorado-Map Distribution, Bldg. 810, Federal New Federal Building - Box 12 Center 101 Twelfth Ave., Fairbanks, AK 99701 FAIRBANKS, Alaska-New Federal Bldg., 101 Twelfth Ave. Chapter A Simulation of Flow in the Edwards Aquifer, San Antonio Region, Texas, and Refinement of Storage and Flow Concepts By R.W. MACLAY and L.F. LAND Prepared in cooperation with the San Antonio City Water Board U.S. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 2336 THE EDWARDS-TRINITY AQUIFER SYSTEM, SAN ANTONIO REGION, TEXAS DEPARTMENT OF THE INTERIOR DONALD PAUL MODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade names and trademarks in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 Library of Congress Cataloging-in-Publication Data Maclay, Robert W. Simulation of flow in the Edwards Aquifer, San Antonio region, Texas, and refinement of storage and flow concepts. (United States Geological Survey water-supply paper; 2336) "Prepared in cooperation with the San Antonio City Water Board." Bibliography: p. Supt. of Docs, no.: I 19.13:2336 I. Groundwater flow Texas Edwards Aquifer Simulation methods. 2. Groundwater flow Texas San Antonio Region Simulation methods. I. Land, Larry F. II. San Antonio City Water Board. III. Title. IV. Series: U.S. Geological Survey water-supply paper; 2336. GB1197.7.M33 1988 551.4'9'097643 88-600133 CONTENTS Abstract Al Introduction 1 Investigative background 1 Purpose and scope 3 Approach 3 Physiographic and hydrologic setting 4 Acknowledgments 5 Geologic framework 5 Rock properties 5 Structural properties 9 Hydrogeologic framework 12 Hydrology 12 Hydraulic characteristics 17 Initial storage and flow concepts 17 Mathematical model 20 Representation of the aquifer 21 Hydrologic data 23 Hydrogeologic data 23 Simulation of flow 26 General description of simulations 26 Selected simulation 27 Refinement of storage and flow concepts 31 Delineation of storage units 31 Delineation of flow units 36 Identification of possible flow across external boundaries 36 Description of geologic structures that restrict and convey ground water 38 Topics for future investigation 42 Summary and conclusions 44 Selected references 45 Glossary 48 FIGURES 1. Map showing location of San Antonio region, physiographic regions, and drainage basins that contribute recharge to Edwards aquifer A2 2. Maps showing depositional provinces and geologic structure of south Texas 6 3. Chart showing correlation of Cretaceous stratigraphic units in south Texas 9 4-7. Maps showing: 4. Surface geology of study area 10 5. Major faults in study area 11 6. Configuration of base of Del Rio Clay 13 7. Regional water levels during winter of 1973, regional ground- water-flow pattern, and location of selected observation wells 14 8. Two typical hydrogeologic sections along geologic dip 15 9. Graphs showing annual recharge, discharge, and accumulated recharge and discharge, 1934-82 16 10. Map showing relative transmissivity values by subareas 18 11. Schematic diagrams showing possible hydraulic restrictions and connections across faults 19 12. Map showing model grid and recharge and discharge areas 22 13. Map showing delineation of transmissivity subareas 24 Contents III 14. Graphs showing distribution of recharge by drainage basin, 1972-76, and of combined ground-water withdrawals and spring discharge by county, 1972-76 A25 15. Graph showing temporal distribution of total recharge and ground-water withdrawals assigned to model 26 16. Hydrographs showing comparison of simulated and measured water levels in selected observational wells 31 17. Hydrographs showing comparison of simulated and measured discharge at Comal and San Marcos Springs 31 18-23. Maps showing: 18. Simulated regional water levels, end of 1972 32 19. Transmissivities used in selected simulation 33 20. Anisotropy factors used in selected simulation 34 21. Simulated flow vectors at each cell in model 35 22. Storage and flow units and areas of flow across external boundaries 37 23. Locations of selected geologic structures restricting and conveying ground water 40 TABLES 1. Major structural and depositional features A7 2. Summary of series of simulations 28 3. Major storage units 36 4. Major flow units 38 5. Ground-water flow across external boundaries 39 6. Major geologic barriers that restrict ground-water flow 41 7. Major geologic gaps and channels that convey ground water 43 Metric Conversions The inch-pound units of measurement used in this report may be converted to metric units (International System) by the following factors: Multiply inch-pound unit By To obtain metric units cubic foot per second (ft3/s) 0.02832 cubic meter per second foot (ft) 0.3048 meter gallon per minute (gal/min) 0.06308 liter per second gallon per minute per foot [(gal/min)/ft] 0.207 liter per second per meter inch (in.) 25.40 millimeter mile (mi) 1.609 kilometer square foot per day (ft2/d) 0.0929 square meter per day square foot per second (ft2/s) 0.0929 square meter per second square mile (mi2) 2.590 square kilometer IV Contents Simulation of Flow in the Edwards Aquifer, San Antonio Region, Texas, and Refinement of Storage and Flow Concepts By R.W. Maclay and L.F. Land Abstract INTRODUCTION The Edwards aquifer is a complexly faulted, carbonate aquifer lying within the Balcones fault zone of south-central The Edwards Limestone of Early Cretaceous age in Texas. The aquifer consists of thin- to massive-bedded limestone south Texas contains one of the most permeable and pro? and dolomite, most of which is in the form of mudstones and ductive carbonate aquifers in the United States. The Edwards wackestones. Well-developed secondary porosity has formed in association with former erosional
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