HYDROGEOLOGIC FRAMEWORK AND GEOCHEMISTRY OF THE EDWARDS AQUIFER SALINE-WATER ZONE, SOUTH-CENTRAL TEXAS U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 97–4133 FRESHWATER ZONE SALINE-WATER ZONE Prepared in cooperation with the EDWARDS AQUIFER AUTHORITY and SAN ANTONIO WATER SYSTEM HYDROGEOLOGIC FRAMEWORK AND GEOCHEMISTRY OF THE EDWARDS AQUIFER SALINE-WATER ZONE, SOUTH-CENTRAL TEXAS By George E. Groschen and Paul M. Buszka U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 97–4133 Prepared in cooperation with the EDWARDS AQUIFER AUTHORITY and SAN ANTONIO WATER SYSTEM Austin, Texas 1997 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Acting 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: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 8011 Cameron Rd. Box 25286 Austin, TX 78754–3898 Denver, CO 80225–0286 ii CONTENTS Abstract ................................................................................................................................................................................ 1 Introduction .......................................................................................................................................................................... 1 Purpose and Scope .................................................................................................................................................... 3 Methods of Data Collection and Analysis ................................................................................................................ 3 Sampling Methods ........................................................................................................................................ 3 Sample Preparation and Laboratory Analyses .............................................................................................. 4 Well-Numbering System .......................................................................................................................................... 6 Acknowledgments .................................................................................................................................................... 6 Hydrogeologic Framework .................................................................................................................................................. 6 Geology and Stratigraphy of Aquifer Units ............................................................................................................. 6 Kainer, Person, and Georgetown Formations ................................................................................................ 6 Devils River Formation ................................................................................................................................. 10 West Nueces, McKnight, and Salmon Peak Formations ............................................................................... 10 Stuart City Formation .................................................................................................................................... 10 Stratigraphy of Underlying and Overlying Units ..................................................................................................... 10 Glen Rose Limestone .................................................................................................................................... 10 Del Rio Clay, Buda Limestone, and Eagle Ford Group ................................................................................ 10 Geologic Structure and Faults .................................................................................................................................. 11 Hydrologic Framework ............................................................................................................................................. 11 Geochemistry of the Saline-Water Zone .............................................................................................................................. 13 Equilibrium Species Distribution ............................................................................................................................. 13 Application of Equilibrium-Computation Program ...................................................................................... 13 Uncertainties in Equilibrium Species Distribution ....................................................................................... 19 Temperature and Hydrogen Activity ........................................................................................................................ 19 Dissolved Constituents ............................................................................................................................................. 21 Dissolved Solids ............................................................................................................................................ 21 Chloride ......................................................................................................................................................... 24 Calcium and Magnesium .............................................................................................................................. 25 Sulfur ............................................................................................................................................................. 27 Dissolved, Gas Phase, and Nonaqueous Liquid Phase ...................................................................... 27 Stable Isotopes ................................................................................................................................... 28 Carbon ........................................................................................................................................................... 31 Dissolved ........................................................................................................................................... 31 Stable Isotopes ................................................................................................................................... 33 Other Dissolved Species ............................................................................................................................... 34 Other Isotopes ............................................................................................................................................... 36 Stable Isotopes of Hydrogen and Oxygen ......................................................................................... 36 Stable Isotopes of Strontium and Boron ............................................................................................ 37 Unstable Isotopes of Carbon and Chloride ........................................................................................ 41 Summary and Conclusions ................................................................................................................................................... 43 References ............................................................................................................................................................................ 44 FIGURES 1. Map showing location of the study area ............................................................................................................... 2 2. Diagram showing well-numbering system ........................................................................................................... 7 3. Chart showing correlation of Cretaceous stratigraphic units in south Texas and Edwards aquifer hydrostratigraphic units in the San Marcos platform area, south-central Texas ................................................... 8 CONTENTS iii 4. Map showing location of selected faults, oil and gas fields, and depositional provinces near the Edwards aquifer saline-water zone, south-central Texas ...................................................................................... 9 5. Generalized hydrogeologic section showing the relation between the Edwards aquifer and the geopressured zone in the subsurface of the Texas Gulf Coast ............................................................................. 12 6–7. Maps showing: 6. Location of wells in the Edwards aquifer saline-water zone, south-central Texas, for which water-sample analyses are available .......................................................................................................... 14 7. Location of sampled wells and monitor-well transects in San Antonio, New Braunfels, and San Marcos, Texas ............................................................................................................................................ 15 8–10. Graphs showing: 8. Relation between estimated in-place temperature of ground-water samples and distance of well from the downdip limit of freshwater in the Edwards aquifer, south-central Texas .......................... 20 9. Relation between corrected pH and dissolved solids concentration in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas ............................................................ 20 10. Relation between dissolved solids concentration in ground-water samples