In cooperation with the Bexar-Medina-Atascosa Counties Water Control and Improvement District No. 1, Bexar Metropolitan Water District, Texas Water Development Board, and Edwards Aquifer Authority Hydrogeology, Hydrologic Budget, and Water Chemistry of the Medina Lake Area, Texas Water-Resources Investigations Report 00-4148 Cover: Medina Dam and Medina Lake, August 1994. (Photograph by Ted A. Small, U.S. Geological Survey.) U.S. Department of the Interior U.S. Geological Survey Hydrogeology, Hydrologic Budget, and Water Chemistry of the Medina Lake Area, Texas By Rebecca B. Lambert, Kenneth C. Grimm, and Roger W. Lee U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 00–4148 In cooperation with the Bexar-Medina-Atascosa Counties Water Control and Improvement District No. 1, Bexar Metropolitan Water District, Texas Water Development Board, and Edwards Aquifer Authority Austin, Texas 2000 U.S. DEPARTMENT OF THE INTERIOR Bruce Babbitt, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to District Chief U.S. Geological Survey 8027 Exchange Dr. Austin, TX 78754–4733 E-mail: [email protected] Copies of this report can be purchased from U.S. Geological Survey Branch of Information Services Box 25286 Denver, CO 80225–0286 E-mail: [email protected] ii CONTENTS Abstract ................................................................................................................................................................................ 1 Introduction .......................................................................................................................................................................... 2 Purpose and Scope .................................................................................................................................................... 2 Previous Studies .............................................................................................................. .......................................... 4 Methods of Analysis ................................................................................................................................................. 5 Acknowledgments .................................................................................................................................................... 6 Hydrogeology ....................................................................................................................................................................... 6 Geologic Framework ................................................................................................................................................ 7 Flow Systems ............................................................................................................................................................ 8 Hydrologic Budget ............................................................................................................................................................... 9 Precipitation .............................................................................................................................................................. 12 Evaporation ............................................................................................................................................................... 12 Mass-Transfer Theory ................................................................................................................................... 12 Evaporation Rates ......................................................................................................................................... 13 Surface Water ............................................................................................................................................................ 15 Storage ...................................................................................................................................................................... 16 Ground Water ............................................................................................................................................................ 16 Ground-Water Inflow .................................................................................................................................... 16 Ground-Water Outflow ................................................................................................................................. 17 Estimated Ground-Water Recharge Methods ....................................................................................................................... 23 Water Chemistry ................................................................................................................................................................... 26 Surface Water ............................................................................................................................................................ 26 Ground Water ............................................................................................................................................................ 27 Environmental Isotopes ............................................................................................................................................ 28 Geochemistry ............................................................................................................................................................ 33 Summary and Conclusions ................................................................................................................................................... 34 References Cited .................................................................................................................................................................. 36 Appendices A. Hydrologic Data ................................................................................................................................................... A–1 B. Hydrologic Budgets ............................................................................................................................................. B–1 C. Water-Quality Data .............................................................................................................................................. C–1 PLATES [Plates in pocket} 1. Map showing hydrogeologic sections A–A' and B–B' and showing subdivisions of the Edwards and Trinity aquifers, Medina Lake area, Texas 2. Map showing locations of water-level and water-quality sites of the Edwards and Trinity aquifers, Medina Lake area, Texas CONTENTS iii FIGURES 1–3. Maps showing: 1. Location of the Medina Lake area ............................................................................................................. 3 2. Regional ground-water-flow units of the Edwards aquifer, Texas ............................................................. 10 3. Location of hydrologic budget data-collection sites and Quihi monitoring well (TD–69–40–102), Medina Lake area, Texas ............................................................................................................................ 11 4–9. Graphs showing: 4. Daily evaporation rates for Medina and Diversion Lakes, Medina Lake area, Texas ................................ 14 5. Relation of ground-water outflow (GWout) to Medina Lake stage for selected hydrologic budget periods, Medina Lake area, Texas .............................................................................................................. 18 6. Relation of monthly average Quihi well water-level altitude to monthly average Medina Lake stage, Medina Lake area, Texas, October 1993–February 1997 ................................................................ 19 7. Relation of ground-water outflow (GWout) to Diversion Lake stage for selected hydrologic budget periods, Medina Lake area, Texas .............................................................................................................. 21 8. Relation of Medina/Diversion Lakes combined ground-water outflow (GWout) to Medina Lake stage for selected budget periods, Medina Lake area, Texas ..................................................................... 22 9. Relation of estimated monthly average recharge from Medina/Diversion Lakes using three methods to monthly average Quihi well water-level altitude, Medina Lake area, Texas, December 1995–July 1996.......................................................................................................................... 25 10–11. Trilinear diagrams of: 10. Waters for surface-water and lake-water samples, Medina Lake area, Texas ............................................ 26 11. Waters for ground-water samples in the Edwards and Trinity aquifers, Medina Lake area, Texas ........... 27 12–13. Graphs showing: 12. Relation between deuterium (δD) and del oxygen (δ18O) in water samples from Medina Lake, Medina River, and from selected wells and springs, Medina Lake area, Texas ......................................... 30 13. Relation of 1/strontium to 87Sr/86Sr in aquifer and lake water, Medina Lake area, Texas ........................ 32 TABLES 1. Summary of lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards