Prepared in cooperation with the San Antonio Water System Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10), Bexar, Comal, and Hays Counties, Texas Scientific Investigations Report 2012–5126 U.S. Department of the Interior U.S. Geological Survey Front cover: U.S. Geological Survey personnel and a diver from Texas State University near Weissmuller Spring, Spring Lake, San Marcos Springs, Hays County, Texas. Back cover: Left, Spring Lake, San Marcos Springs, near Deep Spring, Hays County, Texas. Right, The Blanco River, downstream from Halifax Ranch, Hays County, Texas. Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10), Bexar, Comal, and Hays Counties, Texas By MaryLynn Musgrove and Cassi L. Crow Prepared in cooperation with the San Antonio Water System Scientific Investigations Report 2012–5126 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 This and other USGS information products are available at http://store.usgs.gov/ U.S. Geological Survey Box 25286, Denver Federal Center Denver, CO 80225 To learn about the USGS and its information products visit http://www.usgs.gov/ 1-888-ASK-USGS Any use of trade, product, or firm names 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: Musgrove, M., and Crow, C.L., 2012, Origin and characteristics of discharge at San Marcos Springs based on hydrologic and geochemical data (2008–10), Bexar, Comal, and Hays Counties, Texas: U.S. Geological Survey Scientific Investigations Report 2012–5126, 94 p. iii Contents Abstract ..........................................................................................................................................................1 Introduction.....................................................................................................................................................2 Purpose and Scope ..............................................................................................................................4 Hydrogeologic Setting .........................................................................................................................4 Summary of Previous Studies .............................................................................................................6 Methods of Investigation ..............................................................................................................................9 Study Design ..........................................................................................................................................9 Sample Collection, Analytical Methods, and Quality Control ......................................................16 Rainfall Estimation ..............................................................................................................................17 Streamflow Measurements and Recharge Estimation ................................................................17 Numerical and Statistical Methods .................................................................................................18 Climatic and Hydrologic Conditions .........................................................................................................19 Storm Characteristics ........................................................................................................................19 Rainfall Characteristics ....................................................................................................................19 Stream Recharge ................................................................................................................................29 Geochemistry of San Marcos Springs and Nearby Hydrologic Features ..........................................37 Hydrologic and Physicochemical Data ...........................................................................................37 Surface Water ............................................................................................................................37 Groundwater ...............................................................................................................................37 Springwater ................................................................................................................................47 Geochemical Variability Associated with Routine Sampling .....................................................53 Surface Water ............................................................................................................................53 Groundwater ...............................................................................................................................53 Edwards Aquifer ...............................................................................................................53 Trinity Aquifer ....................................................................................................................56 Springwater ................................................................................................................................56 Geochemical Variability in Response to Storms ............................................................................57 Surface Water ............................................................................................................................57 Springwater ...............................................................................................................................58 Interaction Between Surface Water and Groundwater ........................................................................58 Specific Conductance and Spring Discharge ...............................................................................58 Tracers of Geochemical Evolution Processes ...............................................................................60 Endmember Mixing Using PHREEQC ...............................................................................................62 Modeling Associated with Routine Sampling .......................................................................65 Modeling Based on Storm Sampling ......................................................................................76 Endmember Mixing Using Conservative Tracers ..........................................................................77 Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs .........................80 Factors Affecting Local Recharge Sources ...................................................................................80 Relation of Spring Geochemistry to Hydrologic Conditions ........................................................81 Sources of Water to San Marcos Springs ......................................................................................86 Summary........................................................................................................................................................87 References ...................................................................................................................................................89 iv Figures 1. Study area and locations of water-quality sampling and data- collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Texas ...............................................................................3 2. Idealized block diagram of the Edwards aquifer between Comal Springs Fault and Bear Creek Fault in the vicinity of Comal, Hueco, and San Marcos Springs, south-central Texas ...............................................................................5 3. San Marcos Springs, Hays County, south-central Texas.......................................................7 4. San Marcos Springs complex, Hays County, south-central Texas ......................................8 5. Time series (November 2008–December 2010) of A, stream discharge and spring discharge for multiple sites sampled for the characterization of San Marcos Springs, south-central Texas, and timing of collection of samples; and B, Rainfall hyetograph in the vicinity of San Marcos Springs, Hays County, south-central Texas ............................................................................15 6. Rainfall hyetograph, hydrographs for streams and springs, estimated stream recharge, and timing of collection of stream and spring samples for storms in the vicinity of San Marcos Springs, south-central Texas ............................21 7. Estimated recharge to the Edwards aquifer from the Blanco River, Cibolo Creek, and Dry Comal Creek, south-central Texas, 2008–10 ...................................26 8. Time series (November 2008–December 2010) of gain (positive values) and loss (negative values) for the Guadalupe River, south-central Texas ........................27 9. Relation between deuterium and oxygen isotopes for rainfall samples collected at U.S. Geological Survey station 293146982941, Bexar County, south-central Texas (2008–10) ..................................................................................................28