In cooperation with the Texas Commission on Environmental Quality Recent (2003–05) Water Quality of Barton Springs, Austin, Texas, With Emphasis on Factors Affecting Variability Scientific Investigations Report 2006–5299 U.S. Department of the Interior U.S. Geological Survey Cover. Top left: Main Spring of Barton Springs (photograph courtesy of David Johns, City of Austin). Top right: Eliza Spring of Barton Springs (photograph courtesy of David Johns, City of Austin). Bottom right: Upper Spring of Barton Springs (photograph by Greg Stanton, U.S. Geological Survey). Bottom left: Old Mill Spring of Barton Springs (photograph by Brad Garner, U.S. Geological Survey). Recent (2003–05) Water Quality of Barton Springs, Austin, Texas, With Emphasis on Factors Affecting Variability By Barbara J. Mahler, Bradley D. Garner, MaryLynn Musgrove, Amber L. Guilfoyle, and Mohan V. Rao In cooperation with the Texas Commission on Environmental Quality Scientific Investigations Report 2006–5299 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Meyers, Director U.S. Geological Survey, Reston, Virginia: 2006 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 reproduce any copyrighted materials contained within this report. Suggested citation: Mahler, B.J., Garner, B.D., Musgrove, M., Guilfoyle, A.L., and Rao, M.V., 2006, Recent (2003–05) water quality of Barton Springs, Austin, Texas, with emphasis on factors affecting variability: U.S. Geological Survey Scientific Investigations Report 2006–5299, 83 p., 5 appendixes. iii Contents Abstract . 1 Introduction . 1 Purpose and Scope . 4 Overview of Karst Systems . 5 Study Area Description and Previous Studies . 5 Geologic Setting . 7 Sources of Recharge . 7 Discharge From the Aquifer . 7 General Ground-Water-Flow Direction . 7 Saline Zone Boundary . 9 Water Quality in the Barton Springs Segment of the Edwards Aquifer . 9 Chemical Species and Contaminants of Interest . 10 Nutrients . 10 Pesticides . 10 Volatile Organic Compounds . 11 Acknowledgments . 11 Methods . 11 Study Design . 11 Discharge Measurements . 13 Recharge Estimation . 13 Physicochemical Property Measurements . 14 Water-Quality Sample Collection . 14 Analytical Methods . 14 Water Quality of Barton Springs (2003–05) . 15 Continuous Discharge and Physicochemical Measurements . 15 Spring Discharge . 15 Specific Conductance . 15 Turbidity . 17 Temperature . 17 pH . 19 Dissolved Oxygen . 19 Water Quality Over an Annual Cycle . 20 Major Ions . 20 Differences in Geochemistry Among Spring Orifices . 20 Temporal Variability in Major Ion Concentrations and Relation to Recharge and Discharge . 22 Nutrients . 26 Differences in Nitrate Concentrations Among Spring Orifices . 26 Temporal Variability in Nitrate Concentrations and Relation to Recharge and Discharge . 27 Trace Elements . 27 Soluble Pesticides . 30 iv Detection Frequencies and Concentrations . 30 Differences in Pesticide Concentrations Among Spring Orifices . 30 Temporal Variability in Pesticide Concentrations and Relation to Recharge and Discharge . 32 Volatile Organic Compounds . ..