Origin and characteristics of discharge at San Marcos Springs based on hydrologic and geochemical data (2008-10), Bexar, Comal, and Hays Counties, Texas


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Origin and characteristics of discharge at San Marcos Springs based on hydrologic and geochemical data (2008-10), Bexar, Comal, and Hays Counties, Texas

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Origin and characteristics of discharge at San Marcos Springs based on hydrologic and geochemical data (2008-10), Bexar, Comal, and Hays Counties, Texas
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Scientific Investigations Report
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Musgrove, Mary Lynn
Crow, Cassi L.
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The Edwards aquifer in south-central Texas is a productive and important water resource. Several large springs issuing from the aquifer are major discharge points, popular locations for recreational activities, and habitat for threatened and endangered species. Discharges from Comal and San Marcos Springs, the first and second largest spring complexes in Texas, are used as thresholds in groundwater management strategies for the Edwards aquifer. Comal Springs is generally understood to be supplied by predominantly regional groundwater flow paths; the hydrologic connection of San Marcos Springs with the regional flow system, however, is less understood. During November 2008–December 2010, a hydrologic and geochemical investigation of San Marcos Springs was conducted by the U.S. Geological Survey (USGS) in cooperation with the San Antonio Water System. The primary objective of this study was to define and characterize sources of discharge from San Marcos Springs. During this study, hydrologic conditions transitioned from exceptional drought (the dry period, November 1, 2008 to September 8, 2009) to wetter than normal (the wet period, September 9, 2009 to December 31, 2010), which provided the opportunity to investigate the hydrogeology of San Marcos Springs under a wide range of hydrologic conditions. Water samples were collected from streams, groundwater wells, and springs at and in the vicinity of San Marcos Springs, including periodic (routine) sampling (every 3–7 weeks) and sampling in response to storms. Samples were analyzed for major ions, trace elements, nutrients, and selected stable and radiogenic isotopes (deuterium, oxygen, carbon, strontium). Additionally, selected physicochemical properties were measured continuously at several sites, and hydrologic data were compiled from other USGS efforts (stream and spring discharge). Potential aquifer recharge was evaluated from local streams, and daily recharge or gain/loss estimates were computed for several local streams. Local rainfall and recharge events were
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Scientific Investigations Report, Vol. 2012-5126 (2012).

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U.S. Department of the Interior U.S. Geological Survey

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Front cover: þ U.S. Geological Survey personnel and a diver from T exas State University near Weissmuller Spring, Spring Lake, San Marcos Springs, Hays County, Texas. Back cover: Left, þ Spring Lake, San Marcos Springs, near Deep Spring, Hays County , Texas. Right, þ The Blanco River , downstream from Halifax Ranch, Hays County, Texas.

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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 U.S. Department of the Interior U.S. Geological Survey

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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. U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 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), Bexar, Comal, and Hays Counties, Texas: U.S. Geological Survey Scientific Investigations Report 2012, 94 p.

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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

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iv Figures 1. Study area and locations of water-quality sampling and datacollection 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 ................................... 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) .................................................................................................. 28 1 0. Estimated daily recharge to the Edwards aquifer from the Blanco River, south-central Texas, computed from two pairs of U.S. Geological Survey streamflow-gaging stations ...................................................................................................... 36 11. Times series (November 2008–December 2010) of stream discharge, water temperature, specific conductance, turbidity, and dissolved oxygen (daily means) for U.S. Geological Survey station 08171290, and daily average rainfall in the vicinity of San Marcos Springs, south-central Texas .................................................................................................................... 38 1 2. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for surfacewater sites sampled for the characterization of San Marcos Springs, south-central Texas .................................................................................................................... 39 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spring sites sampled preceding and in response to storm 1 (September 2009), storm 2 (October 2009), and storm 3 (September 2010), south-central Texas .................................................................................................................... 41 14. Times series (November 2008–December 2010) of hydrologic and physicochemical data for two groundwater wells near San Marcos Springs, south-central Texas .................................................................................................... 46 15. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer groundwater wells sampled for the characterization of San Marcos Springs, south-central Texas ........................................................................ 48

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v 16. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Trinity aquifer groundwater wells sampled for the characterization of San Marcos Springs, south-central Texas .................................................................................................... 50 17. Times series (November 2008–December 2010) of discharge at San Marcos Springs, selected physicochemical properties at San Marcos Springs orifices, and daily average rainfall in the vicinity of San Marcos Springs, south-central Texas .................................................................................................................... 52 18. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer springs sampled for the characterization of San Marcos Springs, south-central Texas ............. 54 19. Times series (November 2008–December 2010) of specific conductance and turbidity values at San Marcos Springs orifices, south-central Texas, preceding and in response to unsampled storms ................................................................. 59 20. Specific conductance at San Marcos Springs, south-central Texas ................................ 61 21. Relations among selected geochemical constituents for samples collected from Comal Spring 1, Hueco Spring A, and San Marcos Springs orifices, south-central Texas (November 2008–December 2010) ....................................... 63 22. Time series (November 2008–December 2010) of stream discharge for the Blanco River, spring discharge at San Marcos Springs, and timing of sample collection used for PHREEQC geochemical modeling. ......................................................... 64 23. Relation between discharge at San Marcos Springs and the modeled proportion of discharge at Deep, Diversion, and Weissmuller Springs that is composed of stream recharge from the Blanco River based on PHREEQC inverse modeling results ........................................................................................................... 77 24. Relation between chloride concentration and deuterium isotopes for two-component mixing models showing proportional mixing between surface-water (stream recharge) and springwater endmembers and for samples collected in response to and subsequent to storm 3 (September 2010) ........... 79 25. Time series (November 2008–December 2010) of San Marcos Springs daily mean discharge and estimated daily mean recharge to the Edwards aquifer from the Blanco River, south-central Texas ............................................................. 81 26. Relation between deuterium and oxygen isotopes for surface-water and spring samples, south-central Texas (November 2008–December 2010) ................. 83 27. Relation between spring discharge (daily mean) for Comal, Hueco, and San Marcos Springs and selected physicochemical and geochemical constituents for samples collected from Comal Spring 1, Hueco Spring A, and San Marcos Springs orifices, south-central Texas (November 2008–December 2010) ........................................................................................... 85 Tables 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Texas (November 2008–December 2010) ................................................................................ 10 2. Number of samples analyzed for water quality for characterization of San Marcos Springs, south-central Texas (November 2008– December 2010) .......................................................................................................................... 14

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vi 3. Characteristics of sampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010) ...................................................... 20 4. Characteristics of unsampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010) ...................................................... 24 5. Summary statistics for discharge, physicochemical properties, and selected geochemical constituents for surface-water, groundwater, and spring sites sampled during dry and wet hydrologic conditions in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010) ........................................................................................... 30 6. Summary of PHREEQC inverse geochemical modeling results for San Marcos Springs, south-central Texas (2008) ........................................................... 66 7. Statistical relations for selected geochemical constituents with spring discharge for San Marcos Springs, Comal Springs, and Hueco Springs, south-central Texas (November 2008–December 2010) ...................................................... 84 Conversion Factors and Abbreviations Inch/Pound to SI Multiply By To obtain Length inch (in.) 2.54 centimeter (cm) inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) mile (mi) 1.609 kilometer (km) yard (yd) 0.9144 meter (m) Area acre 4,047 square meter (m 2 ) acre 0.004047 square kilometer (km 2 ) square foot (ft 2 ) 929.0 square centimeter (cm 2 ) square foot (ft 2 ) 0.09290 square meter (m 2 ) square inch (in 2 ) 6.452 square centimeter (cm 2 ) square mile (mi 2 ) 2.590 square kilometer (km 2 ) Volume gallon (gal) 3.785 liter (L) gallon (gal) 0.003785 cubic meter (m 3 ) million gallons (Mgal) 3,785 cubic meter (m 3 ) acre-foot (acre-ft) 1,233 cubic meter (m 3 ) Flow rate acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m 3 /yr) Hydraulic conductivity foot per day (ft/d) 0.3048 meter per day (m/d) Hydraulic gradient foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

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vii SI to Inch/Pound Multiply By To obtain Length centimeter (cm) 0.3937 inch (in.) millimeter (mm) 0.03937 inch (in.) meter (m) 3.281 foot (ft) Volume cubic meter (m 3 ) 264.2 gallon (gal) cubic centimeter (cm 3 ) 0.06102 cubic inch (in 3 ) milliliter (mL) 0.035 ounce, fluid (fl. oz) liter (L) 33.82 ounce, fluid (fl. oz) liter (L) 2.113 pint (pt) liter (L) 1.057 quart (qt) liter (L) 0.2642 gallon (gal) Flow rate cubic meter per second (m 3 /s) 70.07 acre-foot per day (acre-ft/d) Mass gram (g) 0.3527 ounce, avoirdupois (oz) kilogram (kg) 2.205 pound (lb) Temperature in degrees Celsius (C) may be converted to degrees Fahrenheit (F) as follows: F=(1.8C)+32 Temperature in degrees Fahrenheit (F) may be converted to degrees Celsius (C) as follows: C=(F-32)/1.8 Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum. Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (S/cm at 25C). Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms per liter (g/L).

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viii Explanation of Isotope Units Per mil: A unit expressing the ratio of stable-isotope abundances of an element in a sample to those of a standard material. Per mil units are equivalent to parts per thousand. Stable-isotope ratios are computed as follows (Coplen and others, 2002): X = (R sample /R standard -1) x 1,000, where is the “delta” notation, X is the heavier stable isotope, and R is the ratio of the heavier, less abundant isotope to the lighter, stable isotope in a sample or standard. The values for stable-isotope ratios discussed in this report are referenced to the following standard materials: Element R Standard identity and reference Carbon Carbon-13/carbon-12 Vienna PeeDee Belemnite (Fritz and Fontes, 1980) Hydrogen Hydrogen-2/hydrogen-1 Vienna Standard Mean Ocean Water (Fritz and Fontes, 1980) Oxygen Oxygen-18/oxygen-16 Vienna Standard Mean Ocean Water (Fritz and Fontes, 1980) Abbreviations HCO 3 bicarbonate B boron Br bromide Ca calcium Cl chloride 13 C delta carbon-13 D delta deuterium 18 O delta oxygen-18 F fluoride FNU formazin nephelometric units Mg magnesium Mg/Ca magnesium to calcium, molar ratio g/L micrograms per liter S/cm microsiemens per centimeter at 25 degrees Celsius mg/L milligrams per liter NCDC National Climatic Data Center NWIS National Water Information System NWS National Weather Service NO 3 nitrate NO 2 nitrite NO 2 +NO 3 nitrite plus nitrate N nitrogen K potassium Na sodium Sr strontium 87 Sr/ 86 Sr strontium isotope ratio Sr/Ca strontium to calcium, molar ratio SO 4 sulfate

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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 Abstract The Edwards aquifer in south-central Texas is a productive and important water resource. Several large springs issuing from the aquifer are major discharge points, popular locations for recreational activities, and habitat for threatened and endangered species. Discharges from Comal and San in Texas, are used as thresholds in groundwater management strategies for the Edwards aquifer. Comal Springs is generally understood to be supplied by predominantly regional less understood. During November 2008–December 2010, a hydrologic and geochemical investigation of San Marcos Springs was conducted by the U.S. Geological Survey (USGS) in cooperation with the San Antonio Water System. characterize sources of discharge from San Marcos Springs. During this study, hydrologic conditions transitioned from exceptional drought (the dry period, November 1, 2008 to September 8, 2009) to wetter than normal (the wet period, September 9, 2009 to December 31, 2010), which provided the opportunity to investigate the hydrogeology of San Marcos Springs under a wide range of hydrologic conditions. Water samples were collected from streams, groundwater wells, and springs at and in the vicinity of San Marcos Springs, including periodic (routine) sampling (every 3 weeks) and sampling in response to storms. Samples were analyzed for major ions, trace elements, nutrients, and selected stable and radiogenic isotopes (deuterium, oxygen, carbon, strontium). Additionally, selected physicochemical properties were measured continuously at several sites, and hydrologic data were compiled from other USGS efforts (stream and spring discharge). Potential aquifer recharge was evaluated from local streams, and daily recharge or gain/loss estimates were computed for several local streams. Local rainfall and recharge events were compared with physicochemical properties and geochemical variability at San Marcos Springs, with little evidence for dilution by local recharge. Hydrologic and geochemical variability at San Marcos Springs was compared with that at Comal Springs and Hueco Springs. A small range of variability was observed at Comal Springs, and a large range was observed at Hueco Comal Springs and Hueco Springs are representative of two endmember Edwards aquifer spring types, with Hueco sourced recharge and Comal Springs predominantly affected geochemistry of discharge at San Marcos Springs from three recharge and groundwater supplying the springs. During the dry period, the geochemistry of Deep Spring indicates that it was affected by a small component of saline groundwater. The geochemistry of Deep Spring was not responsive to changes in hydrologic conditions from the dry period to the wet period, indicating that Deep Spring is likely dominated was more responsive to changes in hydrologic conditions, indicating that Diversion Spring was affected by some changes in discharge sources. From the dry period to the wet period, the geochemistry of Diversion Spring became more like that at that increased discharge included an increased component of saline groundwater. Weissmuller Spring was sampled only Diversion Spring, indicating that Weissmuller and Diversion Geochemical models (using PHREEQC) indicate that a small amount of saline groundwater (generally less than 1 percent), in addition to a dominant component of regional of water from Deep, Diversion, and Weissmuller Springs. Potential sources of saline groundwater are the downdip both sources are hydrologically and geochemically plausible, model results indicate that mixing with groundwater from the

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2 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Edwards aquifer saline zone is likely a better geochemical Geochemical model results for the wet period yielded different proportions of local recharge sources (the Blanco River) to discharge from San Marcos Springs ranging from Deep Spring than for Diversion Spring, which is consistent with the nominal response of Deep Spring to changes in hydrologic conditions. In response to storm events, when focused recharge of dilute surface water is likely to occur, for example, the modeled proportion of local surface-water recharge contributing to Diversion Spring was mostly less than 10 percent directly following storms and for several subsequent months. These results place further constraints on estimates using PHREEQC and suggest that the proportion of local recharge contributing to discharge at San Marcos Springs is likely on the order of no more than 10 percent. The results of this study indicate that discharge at San Marcos Springs is dominated by regional recharge conditions when aquifer recharge is likely occurring from local streams. Local surface-water recharge sources do not discharge. Knowledge of recharge sources to San Marcos Springs and how they vary spatially and temporally is useful for water-resource management strategies and for understanding geochemical and hydrologic processes that affect discharge at San Marcos Springs. Introduction The San Antonio segment of the Edwards aquifer (hereinafter, Edwards aquifer) in south-central Texas is one of the most productive aquifers in the world and is a designated sole-source aquifer that is the largest water supply for more than two million people in a rapidly urbanizing region Agency, 2006). Springs issuing from the Edwards aquifer provide habitat for several threatened and endangered species (Edwards Aquifer Research and Data Center, 2010), serve as locations for recreational activities, and supply downstream second largest spring complexes in Texas, are major discharge points for the Edwards aquifer (Brune, 1975) Springs complexes, along with water-table altitudes in the thresholds for enacting various water management strategies in the San Antonio area (Texas Legislature Online, 2007). A comprehensive understanding of the hydrogeology and sources of water to Comal Springs and San Marcos Springs is needed for effective aquifer management. Discharge at Comal Springs is generally understood to be predominantly Although San Marcos Springs has been hypothesized to be supplied by both regional and local recharge sources (Guyton interconnection of San Marcos Springs with the regional During November 2008–December 2010, the U.S. Geological Survey (USGS), in cooperation with the San Antonio Water System, collected and analyzed hydrologic and geochemical data in Bexar, Comal, and Hays Counties, Tex. characterize sources of recharge and groundwater supplying San Marcos Springs. During the study, climatic and hydrologic conditions underwent a transition from exceptional drought (U.S. Drought Monitor, 2011) to wetter than normal. Between November 1, 2008 and September 8, 2009 is referred to as the “dry period,” and the period between September 9, 2009 and December 31, 2010 is referred to as the “wet period.” Rainfall and hydrologic conditions, including surface-water recharge and spring discharge during the period of the study, are described in detail in the Climatic and Hydrologic Conditions section of this report. Collection of routine and storm-associated water samples from streams, groundwater wells, and springs over the 25 months of the study provided an opportunity to investigate the hydrogeology of San Marcos Springs under a wide range of climatic and hydrologic conditions and the response of the karst system to drought and post-drought conditions. The Edwards aquifer is karst, in which soluble host rocks have dissolved preferentially to form large interconnected voids and conduits (White, 1988). Most groundwater storage in karst aquifers occurs within the bedrock matrix (primary pores and bedding planes), but most transport occurs within the secondary conduits, which often dominate groundwater through the primary aquifer matrix is typically diffuse and Desmarais and Rojstaczer, 2002). As a result, karst aquifers tend to be heterogeneous with large variability and rapid springs are integrators of input from various sources and such, are ideal sites for studying aquifer processes and surfacewater/groundwater interaction (Quinlan, 1989). Karst springs moving through a well-developed conduit system can be highly variable in discharge quantity and quality. Karst springs to exhibit less temporal variability in spring discharge and paths than it does in conduits, and has more opportunity to interact with the rock matrix and mix with matrix groundwater changes in climatic and hydrologic conditions directly affect

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Introduction 3 1,000 mg/L lllllllllllllll llll llllllll llllllllllllllllll llll l llll l Kyl e A u s tin S an Ant o nio N e w B r a unfels S an M a r c o s S egu i nBoerne Wimberley Bulverde Selma Sattler Base modified from U.S. Geological Survey 1:24,000 digital data Geographic Coordinate System North American Datum of 1983 H A YS C OUN T YC OM AL C O UNT YK E NDA L L C OUN T Y BL AN C O C OUN T YBE XA R C OUN T YTR A VIS C OUN T YGUADA L UP E C OUN T Y !!# # # # # # # # # # # # #Q13 Q12 Q11 Q10Q9 Q8 Q7 Q6 Q5 Q4 Q3 Q2 Q1 S3 S2 S1 S4S5, S6, S7 S5, S6, S7W21 W20 W19 W18 W17 W16 W15 W14 W13 W11 W10 W6 W12 P1 W9 W8 W5 W4 W3 W2 W1 W22" " " " " " " " " " " " " " " " " " " " " " W7 CiboloC r ee kDryComalC r ee kAlligatorC r ee kBl an c oRive rOn io nC r ee kS a nM a r c o sRive rPl u mC r ee kY or k C r ee kCAN YO N L AK EPu r g ato r yC r ee kS in kC r ee kComal RiverComal Springs San Marcos Springs San Marcos Springs G u a d a lu p eRiverG u a d a lu p eRive rHueco Springs0 5 7.5 10 MILES 2.5 0 5 7.5 10 KILOMETERS 2.5 Gulf of MexicoEDWARDS PLATEAU B e xar C o u nty i n dex well (J) TEX A S Study area EXPL A N A TIO NEdwards aquifer (Ashworth and Hopkins, 1995) Contributing zone Recharge zone Conned zoneBalcones Escarpment (modied from Abbott and Woodruff, 1986) Municipal boundary Groundwater divide (modied from Maclay, 1995, plate 1; Mahler and others, 2006, g. 1) Freshwater/saline-water interface (1,000 milligrams per liter [mg/L] dissolved solids concentration) (Schultz, 1994) Streamflow-gaging station or surface-water site and map identifier— Table 1 Groundwater well, Edwards aquifer and map identifier—Table 1 Groundwater well, Edwards and Trinity aquifers and map identifier—Table 1 Groundwater well, Trinity aquifer and map identifier—Table 1 Spring and map identifier—Table 1 Rain sample collection site—Table 1 National Weather Service cooperative station and identifierQ3#"W5 S5"W10 W21 P1 l l 417983 98' 98' 98' 30' 30' 29' 418544 411429 419815 419815 412585 417983 417983 416276 416276 Figure 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.

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4 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) the aquifer through processes including dilution of recharge, surface-water/groundwater mixing, the extent of water-rock interaction, and corresponding aquifer geochemistry. This study evaluates hydrologic and geochemical processes that affect discharge at San Marcos Springs and provides insight into the origin and characteristics of discharge at San Marcos Springs and how they vary spatially and temporally. Purpose and Scope The purpose of this report is to (1) describe hydrologic and geochemical data for November 2008–December 2010 from streams, groundwater wells, and springs in parts of in hydrologic and geochemical data for San Marcos Springs and other hydrologic features in parts of Bexar, Comal, and Hays Counties in response to changes in hydrologic conditions groundwater supplying San Marcos Springs. Hydrogeologic Setting The karstic Edwards aquifer developed in Lower Cretaceous limestone and dolomite rocks of the Edwards Group (Person and Kainer Formations) (Rose, 1972) and the Georgetown Formation. The aquifer is present in a narrow band along the Balcones fault zone, which is characterized by a series of high-angle en echelon down-to-the-coast normal faults within a series of fault blocks that trend southwest to northeast (Barker and Ardis, 1996). The fault blocks, and their subsequent erosion and dissolution, are major factors affecting to the Edwards aquifer occurs in the recharge zone, where the porous and permeable Edwards Group outcrops. Streams faulted and fractured limestone as they cross the recharge zone, supplying from 60 to 80 percent of aquifer recharge recharge occurs west of Bexar County with additional recharge points in Bexar County, mainly municipal water-supply wells. Water not discharged to wells continues generally toward the northeast along and parallel to the Balcones Escarpment, the surface expression of the Balcones fault zone (McKinney and Sharp, 1995), to discharge points in Comal and Hays Counties, primarily Comal Springs and San Marcos Springs (Maclay paths that supply discharge to Comal Springs also supply discharge to San Marcos Springs (Johnson and Schindel, 2008). Depending on the magnitude of displacement, faults adjacent fault blocks. Faults that juxtapose rocks of the Edwards aquifer against those of the underlying Trinity aquifer could allow groundwater from the Trinity aquifer to enter the (Lindgren and others, 2004). The downdip limit of potable water in the Edwards aquifer, the freshwater/saline-water Comal, San Marcos, and Hueco Springs are karst Comal Springs is a complex of springs that issue from numerous solution cavities along a 1,500-foot (ft) section of the Comal Springs Fault (Puente, 1976), a normal fault with as much as 500 ft of offset (Maclay and Land, 1988). Longterm discharge from the Comal Springs complex (USGS station (08168710) (1927) has ranged from 0 to 534 cubic feet per second (ft 3 /s) with an average value of 292 ft 3 /s period (November 2008–December 2010) ranged from 158 to 391 ft 3 /s with an average value of 293 ft 3 /s (median of 306 ft 3 /s) (U.S. Geological Survey, 2011). Water discharging from Comal Springs is impounded by a low-head dam that forms a small lake in Landa Park in New Braunfels, Tex. About 25 percent of the discharge from the Comal Springs beneath the lake in the downthrown block of the fault (Ogden at an average altitude of about 623 ft (Guyton and Associates, of a 2-mile(mi-) long tributary to the Guadalupe River known as the Comal River. Hueco Springs is located about 5 mi north-northwest of Comal Springs, in the outcrop area of the Edwards aquifer Hueco Springs Fault, which has 380 ft of offset in the vicinity of the springs. Long-term discharge from the Hueco Springs complex (USGS station 08168000) (2002) has ranged from 1.3 to 148 ft 3 /s with an average value of 53 ft 3 discharge during the study period (November 2008–December 2010) ranged from 1.3 to 121 ft 3 /s with an average discharge of 44 ft 3 /s (median of 40 ft 3 /s) (U.S. Geological Survey, 2011). the alluvium along the west side of the Guadalupe River and 652 ft, Hueco Springs is 29 ft higher than the average altitude of Comal Springs and about 78 ft higher than the average altitude of San Marcos Springs (Guyton and Associates, an approximately 0.3-mi-long channel that empties into the Guadalupe River.

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Introduction 5 FAULTFAULTMOUNTAIN CITY SinkCreekBlancoRiver Comal SpringsCanyon Lake San Marcos Springs Hueco Springs Infiltration and recharge SOUTHWEST NORTHEASTOnionCreek Y orkCreekBase modified from U.S. Geological Survey digital data, 1:24,000 Universal T ransverse Mercator projection, zone 14 Datum, North American Datum of 1983 Not to scaleInteraquifer flow from T rinity into Edwards Regional Edwards aquifer flow S a n M a r c o s R i v e r San Marcos RiverGuadalupeRiverPurgatory Creek SAN MARCOS SPRINGS FAULTCOMAL SPRINGS FAULTHUEC O SPRINGS FAUL TBEAR CREEK FA ULTBAT CAVE FAULTMUSTANG BRANCHACADEMY FAULT Upper confining units (Ashworth and Hopkins, 1995)Edwards aquifer (Ashworth and Hopkins, 1995)T rinity aquifer (Ashworth and Hopkins, 1995)Fault—Dashed where approximately located. Arrow indicates relative direction of movementStream—Dashed where ephemeral SpringEXPLANA TION Figure 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.

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6 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) San Marcos Springs is a complex of springs that normal fault with as much as 470 ft of offset (LBG-Guyton Associates, 2004) that juxtaposes the Edwards Group against the San Marcos Springs complex (USGS station 08170000) (1955) has ranged from 46 to 451 ft 3 /s with an average value of 174 ft 3 during the study period (November 2008–December 2010) ranged from 83 to 270 ft 3 /s with an average discharge of 160 ft 3 /s (median of 176 ft 3 /s) (U.S. Geological Survey, 2011). LBG-Guyton Associates (2004) estimated that approximately 25 percent of the total discharge from San Marcos Springs 75 percent from sand boils. Water discharging from San Marcos Springs is impounded by a concrete dam to form structures. The two channels converge downstream from the Because San Marcos Springs is 49 ft lower in altitude than Comal Springs to San Marcos Springs likely occurs (Guyton Summary of Previous Studies Previous studies provide insight into the hydrology and geochemistry of the regional Edwards aquifer and Comal, San Marcos, and Hueco Springs. Most notably, studies focused on the hydrogeology of San Marcos Springs have described a combination of regional and local sources supplying spring it has been previously noted that the correlation between water-table altitudes for well J and San Marcos Springs discharge is not as well correlated as that between J and Comal Springs and at San Marcos Springs indicates that they might have differences in discharge sources (LBG-Guyton sources for spring discharge. Puente (1976) used relations between water levels, spring discharge, and stream discharge to develop statistical correlations and concluded that Comal aquifer to the west and southwest and secondarily by local recharge. Rothermel and Ogden (1986) considered several lines of evidence that implied a lack of local recharge to Comal Springs including negligible turbidity during and after storms, inability to trace dye from local sinks to the constituents, and a mean annual water temperature that was warmer than the mean annual air temperature. McKinney and Sharp (1995), in an investigation of the feasibility of studies to conclude that Comal Springs is supplied by regional aquifer, with little to no evidence for local recharge sources. Otero (2007) used geologic, hydrologic, and geochemical conveying water to Comal Springs. recharge source for spring discharge (Guyton and Associates, some unknown percentage of water derived from the Trinity 2004). Hueco Springs is the only large spring in the Edwards aquifer that is located in the outcrop area and has been hypothesized to have a much smaller contributing area than a smaller mean discharge than do other major springs in the droughts, which is consistent with the hypothesis of a smaller contributing area and a local recharge source possibly comprising the upper part of the Dry Comal Creek Basin and the Guadalupe River Basin recharge areas west of the river. Some water from Hueco Springs might also be sourced from the main part of the aquifer between San Antonio and Comal Springs under wet hydrologic conditions (Guyton and Associates, 1979). Studies of San Marcos Springs have described a combination of regional and local sources for spring discharge. Pearson and others (1975) concluded, on the basis of tritium results, that approximately 35 percent of the discharge from San Marcos Springs might come from local recharge in areas east of Bexar County, in Comal and Hays Counties. Puente (1976), using statistical correlations between water levels, spring discharge, and stream discharge, concluded that San Marcos Springs is supplied by a combination of regional northern Comal and Hays Counties. Using historical tritium concentrations and spring discharge correlations, Guyton and Associates (1979) concluded that roughly 55 percent of discharge from San Marcos Springs originates from regional Springs from the southwest. Ogden and others (1985a, 1985b, 1986) concluded from an integrated study of chemistry and hydrologic data that the springs in the southern part of Spring Fault Block and that those in the northern part of the lake are supplied by local recharge from the Blanco River and Sink of the lake are separated by faults or a “pressure boundary” Wanakule (1988) accounted for a weaker correlation between the water-table altitude in well J and discharge from San Marcos Springs, relative to discharge from Comal Springs, on recharge from the Blanco River Basin. McKinney and Sharp (1995) concluded that, although supplied mainly by regional

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Introduction 7 #! ! !" " "R W8 W7 S7 S6 S5 COMAL SPRINGS FAULTSAN MARCOS SPRINGS FAULT SanSpring LakeMarcos RiverSink Creek U DU DD UU DU D U D A' A 0 0.4 0.8 MILES 0.2 0 0.4 0.6 0.8 KILOMETERS 0.2 0.6 97' 97' 29' 29' Kp Kp ucu ucu Qal Qal Qal Qal A' AEXPLANATIONSystem Quaternary Cretaceous Fault—U, Upthrown side; D, downthrown side. Dashed where inferred or approximately located; dotted where concealed Line of section U D Geologic unit Alluvium Upper confining units Person Formation Kainer Formation (does not outcrop in area shown) Glen Rose Formation (does not outcrop in area shown)Qal ucu Kp Kk Kgr 0 0.4 0.8 MILES 0.2 0 0.4 0.6 0.8 KILOMETERS 0.2 0.6 COMAL SPRINGS FAULT SAN MARCOS SPRINGS FAULT San Marcos Springs Spring Lake A' A Kp Kp Kp ucu ucu Kp ucu Qal Qal Qal Kk Kk Kk Kk Kgr Kgr Kgr Kgr Qal NORTHWEST SOUTHEAST -600 -700 -500 -400 -300 -200 -100 100 NAVD 88 200 300 400 500 600 700 800 900 FEET -600 -700 -500 -400 -300 -200 -100 100 NAVD 88 200 300 400 500 600 700 800 900 FEET Interstate Highway 35 Surface geology modified from DeCook (1956), DeCook (1960), and Guyton and Associates (1979) Base modified from Guyton and Associates (1979) Universal Transverse Mercator projection, Zone 14 North American Datum of 1983 Modified from Guyton and Associates (1979) NORTH AMERICAN VERTICAL DATUM OF 1988 VERTICAL EXAGGERATIONEXPLANATION Geologic unit Alluvium Upper confining units Person Formation Kainer Formation Glen Rose Formation Contact—Dashed where approximately located Fault—Dashed where approximately located. Arrow indicates relative direction of movementQal ucu Kp Kk Kgr A B Figure 3. San Marcos Springs, Hays County, south-central Texas. A , Surface geology. B , Geologic section.

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8 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) SPR ING LAKE DAM U DU D S AN M AR C OS SPR INGS F AULTSp r ing Lake Spr ing LakeDiv ersion Spr ing (S7) W eissmuller Spr ing (S6) D eep Spr ing (S5) 0 0.05 0.1 MILE 0 0.05 0.1 KIL OME TER Fault—U, upthrown; D, downthrown ( Guyton and Associates, 1979) S p r i n g sample collection location— See table 1E X PLA N A T ION U D U D97'50" 97' 29'40" 29'30" 29'20" Base modified from U.S. Geological Survey digital data, 1:24,000 U.S. Department of Agriculture, 2010 Natural Agriculture Imagery Program (NAIP) 1 meter Geographic Coordinate System, North American Datum of 1983 East outflow structure West outflow structure include local recharge sources 10 mi west of the springs. LBG-Guyton Associates (2004) concluded that San Marcos path in Edwards aquifer fault blocks on the upthrown side of the Comal Springs Fault that is bypassing discharge to Comal be supplied by local recharge sources from the Blanco River and associated drainage basins in Hays County. Johnson and Schindel (2008) concluded that discharge from San Marcos the western part of the aquifer (more than 90 percent of the sources such as the Blanco River, Sink Creek, Cibolo Creek, the Guadalupe River, and Dry Comal Creek contribute a small percentage of water that most likely discharges from springs in the northern part of Spring Lake. Dye tracing has been used in previous studies to assess efforts by Ogden and others (1986) indicate that, during below-average discharge conditions at San Marcos Springs (approximately 140 ft 3 /s), groundwater moved from Ezell’s Cave, located approximately 2 mi southwest of Spring Lake, Figure 4. San Marcos Springs complex, Hays County, south-central Texas.

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Methods of Investigation 9 northern part of Spring Lake. The Edwards Aquifer Authority conducted numerous tracer tests near San Marcos Springs in sites to the south, southwest, and west of San Marcos Springs with groundwater velocities of approximately 1,400 ft per day exists parallel to the Hueco Springs Fault (Johnson and the southwest and west of San Marcos Springs during belowaverage to average discharge conditions (approximately 168– 196 ft 3 /s) in 2005 that had relatively long traveltimes and were parts of Spring Lake (Johnson and Schindel, 2008). Ogden and others (1986) introduced dye into Rattlesnake Cave, located approximately 4,000 ft to the northeast of Spring Lake, during below-average discharge conditions at San Marcos Springs (approximately 60 ft 3 /s), and the dye was detected Spring Lake 40 days after injection. Johnson and Schindel (2008) replicated the Rattlesnake Cave dye-tracing test during average discharge conditions (approximately 150 ft 3 /s), and and southern parts of Spring Lake approximately 3 days after injection. Dye introduced by Ogden and others (1986) into Tarbuttons Showerbath Cave, located approximately 20 ft from the Blanco River and approximately 6 mi north of Spring Lake, was continually detected for a 6-week period following Marcos Springs ranged from 100 to 218 ft 3 /s during the year between dye injection and detection with an average discharge of 141 ft 3 /s. The year-long traveltime was viewed skeptically paths between the Blanco River and San Marcos Springs might not be particularly rapid or direct. Based on these results, the role of the Blanco River in supplying recharge to San Marcos Springs was unclear, although the potentiometric surface is consistent with groundwater movement from the Blanco River towards San Marcos Springs. On the basis of a series of more recent dye-tracing tests conducted during a paths to San Marcos Springs from the Blanco River but noted that dye-tracing tests indicated that groundwater gradients are relatively steep directly south of the Blanco River and become more shallow closer to San Marcos Springs and, as a result, also indicate that Sink Creek might be a minor local recharge source to San Marcos Springs (Johnson and others, 2012). Their groundwater velocity results indicate that most San Marcos Springs discharge originates from the southwest along Methods of Investigation Study Design A multiphase approach was used to evaluate the water chemistry from San Marcos Springs, the streams that potentially contribute to its recharge, nearby groundwater, and other nearby springs (Comal and Hueco Springs) and to characterize sources of discharge at San Marcos Springs. Hydrologic and geochemical data were collected from streams (surface water), groundwater wells (groundwater), and springs (springwater) in the study area during November 2008 through December 2010. Samples were collected from 5 streams, samples (from one site in northern San Antonio) also were P1). Continuous water-quality monitors were installed at one at San Marcos Springs. Water-table altitude was continuously stage-discharge ratings were developed for two of the stations at San Marcos Springs were selected to be representative of larger springs at the spring complex on the basis of comparison with historical data (Ogden and others, 1985a, 1985b, 1986) Synoptic and periodic sampling of surface water and groundwater was conducted to evaluate spatial and temporal hydrologic variability in the study area. A broad synoptic sampling effort in December 2008 characterized surface water and groundwater at 5 stream sites, 19 groundwater sites, and (table 1). A subset of these sites was selected for periodic (routine) sampling to characterize changes in water quality and in response to hydrologic conditions, with samples collected approximately every 3 weeks throughout the study period (no samples were collected between March and May 2010) (table 2). Routine samples were collected from two stream sites, eight groundwater wells (three with relatively short sampling periods that ended in February 2010), one spring were analyzed for major ions, trace elements, nutrients, and selected stable and radiogenic isotopes (deuterium, oxygen, carbon, and strontium) as described by Crow (2012). Because discharge at the stream and spring sites varied in response to rainfall, routine samples should not be assumed to represent discharge values for Comal, Hueco, and San Marcos Springs represent discharge for the spring complexes, whereas physicochemical and geochemical data were collected from

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10 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Texas (November 2008–December 2010). Map iden tifier (fig. 1) U.S. Geological Survey station number U.S. Geological Survey station name (surfacewater sites) or State well number (groundwater wells and springs) Short name Latitude, in dd mm ss (NAD 83) Longitude, in dd mm ss (NAD 83) Altitude of land surface (NAVD 88) (feet) Contributing aquifer(s) Well depth (feet) Well open interval (feet) Well completion type Data type(s) Surface-water sites Q1 08167990 Guadalupe River at River Road near Sattler, Tex. Guadalupe at River Road 29'55" 98'31" 640 N/A N/A N/A N/A QW Q2 08168500 Guadalupe River above Comal River at New Braunfels, Tex. Guadalupe above Comal 29'54" 98'36" 587 N/A N/A N/A N/A Q, QW Q3 08169500 Guadalupe River at New Braunfels, Tex. Guadalupe at New Braunfels 29'53" 98'24" 572.9 N/A N/A N/A N/A QW Q4 08169932 Sink Creek near San Marcos, Tex. Sink Creek 29'45.57" 97'39.33" 742 N/A N/A N/A N/A Q, QW Q5 08169958 Purgatory Creek at Mountain High Drive near San Marcos, Tex. Purgatory Creek 29'21.12" 98'14.1" 690 N/A N/A N/A N/A G, QW Q6 08171000 Blanco River at Wimberley, Tex. Blanco at Wimberley 29'40" 98'20" 797.6 N/A N/A N/A N/A Q, QW Q7 08171290 Blanco River at Halifax Ranch near Kyle, Tex. Blanco at Halifax 30'20" 97'09" 675 N/A N/A N/A N/A M, Q, QW Q8 08171300 Blanco River near Kyle, Tex. Blanco near Kyle 29'46" 97'36" 620.5 N/A N/A N/A N/A Q, QW Q9 08184300 Cibolo Creek at Farm Road 1863 below Bulverde, Tex. Cibolo at 1863 29'57.6" 98'22.98" 941 N/A N/A N/A N/A QW

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Introduction 11 Q10 08167800 Guadalupe River at Sattler, Tex. Guadalupe at Sattler 29'33" 98'48" 742 N/A N/A N/A N/A Q Q11 08168797 Dry Comal Creek at Loop 337 near New Braunfels, Tex. Dry Comal 29'16.8" 98'17.4" 610 N/A N/A N/A N/A Q Q12 08183890 Cibolo Creek at Cibolo Nature Center near Boerne, Tex. Cibolo Nature Center 29'52.07" 98'46.19" 1358 N/A N/A N/A N/A Q Q13 08185000 Cibolo Creek at Selma, Tex. Cibolo at Selma 29'39" 98'40" 728 N/A N/A N/A N/A Q Groundwater wells W1 294604098060701 4D well --802 Edwards 400 195 QW W2 294739098075301 Bonem well 29'39" 98'53" 860 Edwards 375 220 W QW W3 295019097592701 LR13 Fish Hatchery 1 well 29'19.46" 97'26.65" 714 Edwards 280 216 QW W4 295033098041201 Mendez well 29'33" 98'12" 991 Trinity 640 580 S QW W5 295052098070801 Sac-N-Pac well 29'52" 98'08" 1,020 Trinity 408 256 QW W6 295314097565701 LR TSU-West Campus well 29'14.04" 97'57.47" 751 Edwards 210 103 M, QW, WL W7 295323097561101 LR TSU-Artesian well 29'22.5" 97'11.2" 577 Edwards 600 0 QW W8 295325097564301 LR TSU-Jackson 1 well 29'25" 97'42.9" 740 Edwards 191 135 QW W9 295345098001001 LR SMBA 1 well 29'45" 98'10" 770 Edwards 335 200 QW Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued Map identifier (fig. 1) U.S. Geological Survey station number U.S. Geological Survey station name (surfacewater sites) or State well number (groundwater wells and springs) Short name Latitude, in dd mm ss (NAD 83) Longitude, in dd mm ss (NAD 83) Altitude of land surface (NAVD 88) (feet) Contributing aquifer(s) Well depth (feet) Well open interval (feet) Well completion type Data type(s) Surface-water sites—Continued

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12 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued Map identifier (fig. 1) U.S. Geological Survey station number U.S. Geological Survey station name (surfacewater sites) or State well number (groundwater wells and springs) Short name Latitude, in dd mm ss (NAD 83) Longitude, in dd mm ss (NAD 83) Altitude of land surface (NAVD 88) (feet) Contributing aquifer(s) Well depth (feet) Well open interval (feet) Well completion type Data type(s) Groundwater wells—Continued W10 295352098071201 Riedel well 29'52" 98'12" 1,150 Edwards 240 239 S QW W11 295406097551201 LR Horton well 29'06" 97'12" 610 Edwards 80 10 QW W12 295443097554201 LR Tipps well 29'43" 97'42" 601.3 Edwards 32.5 0.5 W M, WL W13 295515097581801 LR Solar well 29'15.1" 97'17.9" 688 Edwards --QW W14 295524098114401 Eagle Peak well --1,177 Trinity 732 200 QW W15 295530097563201 LR Neff well 29'30" 97'32" 733 Edwards 280 180 S QW do do do do do do do do do 240 -W16 295538098042101 LR Burns well 29'38" 98'21" 1,041 Trinity 700 400 S QW W17 295709098000301 LR Laguna well 29'08.5" 98'03" 906 Edwards 600 -QW W18 295806097540901 LR Aqua well --683 Edwards 520 300 QW W19 295915097525501 LR City of Kyle 2 well --753 Edwards 658 328 QW W20 300041097563901 LR Halifax well 30'41" 97'39" 740 Edwards 220 161 W QW W21 300331097551601 LR Ruby Ranch well --830 Edwards and Trinity 405 182 QW W22 300453097503301 LR Buda well --710 Edwards 390 168 QW Springs Hueco Springs 08168000 Hueco Springs near New Braunfels, Tex Hueco Springs 29'34" 98'24" 644.9 Edwards N/A N/A N/A Q Comal Springs 08168710 Comal Springs at New Braunfels, Tex. Comal Springs 29'22" 98'21" 582.9 Edwards N/A N/A N/A Q San Marcos Springs 08170000 San Marcos Springs at San Marcos, Tex. San Marcos Springs 29'21" 97'02" 557.8 Edwards N/A N/A N/A Q

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Introduction 13 S1 294255098080501 Comal Spring 3 29'55.38" 98'04.92" 625 Edwards N/A N/A N/A QW S2 294300098080001 Comal Spring 1 29'46" 98'15" 623.43 Edwards N/A N/A N/A QW S3 294314098074101 Comal Spring 5 29'14.22" 98'41.46" 626 Edwards N/A N/A N/A QW S4 294533098082301 Hueco Spring A 29'34" 98'24" 652.53 Edwards N/A N/A N/A QW S5 295322097561000 LR Deep Spring (San Marcos) 29'33" 97'54" 600 Edwards N/A N/A N/A M, QW S6 295322097561002 LR Weissmuller Spring (San Marcos) 29'36" 97'48" 600 Edwards N/A N/A N/A M, QW S7 295336097555201 LR Diversion Spring (San Marcos) 29'35.64" 97'51.9" 580 Edwards N/A N/A N/A M, QW Rain sample collection site P1 293146982941 San Antonio Sub district at San Antonio, Tex. USGS San Antonio 29'47" 98'42" 970 N/A N/A N/A N/A QW Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued Map identifier (fig. 1) U.S. Geological Survey station number U.S. Geological Survey station name (surfacewater sites) or State well number (groundwater wells and springs) Short name Latitude, in dd mm ss (NAD 83) Longitude, in dd mm ss (NAD 83) Altitude of land surface (NAVD 88) (feet) Contributing aquifer(s) Well depth (feet) Well open interval (feet) Well completion type Data type(s) Springs—Continued

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14 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 2. Number of samples analyzed for water quality for characterization of San Marcos Springs, south-central Texas (November 2008–December 2010). Map identifier (fig. 1) U.S. Geological Survey station number Short name 1 Number of routine samples Number of storm samples Dry period Wet period Surface-water sites Q1 08167990 Guadalupe at River Road 7 9 2 Q2 08168500 Guadalupe above Comal 1 0 0 Q3 08169500 Guadalupe at New Braunfels 1 0 0 Q4 08169932 Sink Creek 0 0 2 Q5 08169958 Purgatory Creek 0 0 1 Q6 08171000 Blanco at Wimberley 1 0 0 Q7 08171290 Blanco at Halifax 7 9 4 Q8 08171300 Blanco near Kyle 0 0 2 Q9 08184300 Cibolo at 1863 0 0 3 Groundwater wells W1 294604098060701 4D well (DX) 6 9 0 W2 294739098075301 1 0 0 W3 295019097592701 Fish Hatchery 1 well (LR13) 3 1 0 W4 295033098041201 Mendez well (DX) 7 10 0 W5 295052098070801 Sac–N–Pac well (DX) 7 5 0 W6 295314097565701 TSU-West Campus well (LR) 3 9 0 W7 295323097561101 TSU-Artesian well (LR) 1 0 0 W8 295325097564301 TSU-Jackson 1 well (LR) 1 0 0 W9 295345098001001 SMBA 1 well (LR) 1 0 0 W10 295352098071201 1 0 0 W11 295406097551201 Horton well (LR) 1 0 0 W13 295515097581801 Solar well (LR) 6 8 0 W14 295524098114401 1 0 0 W15 295530097563201 Neff well (LR) 7 10 0 W16 295538098042101 Burns well (LR) 3 0 0 W17 295709098000301 Laguna well (LR) 1 0 0 W18 295806097540901 Aqua well (LR) 7 5 0 W19 295915097525501 City of Kyle 2 well (LR) 1 0 0 W20 300041097563901 Halifax well (LR) 1 0 0 W21 300331097551601 Ruby Ranch well (LR) 6 3 0 W22 300453097503301 Buda well (LR ) 2 0 0 Springs S1 294255098080501 1 0 0 S2 294300098080001 Comal Spring 1 (DX) 7 2 10 8 S3 294314098074101 1 0 0 S4 294533098082301 Hueco Spring A (DX) 7 2 10 8 S5 295322097561000 Deep Spring (San Marcos) (LR) 7 9 17 S6 295322097561002 Weissmuller Spring (San Marcos) (LR) 0 5 6 S7 295336097555201 Diversion Spring (San Marcos) (LR) 7 9 17 1 See table 1 for complete U.S. Geological Survey station names and numbers. 2 One sample analyzed only for stable isotopes.

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Introduction 15 Nov. 2008 Oct. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010Date0 5 0 0 1 , 0 0 0 1 , 5 0 0 2 , 0 0 0 2 , 5 0 0 3 , 0 0 0 3 , 5 0 0 4 , 0 0 0 1 2 3 4 5 6 0 7 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0S tream discharge, in cubic feet per second S pring discharge, in cubic feet per secondDaily average rainfall, in inchesA B 1 2 3 1 2 3 Wet period Dry period Discharge (daily mean) at spring— Composite of individual orifices (table 1) San Marcos Springs Comal S p r i n gs H u e c o Springs Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Tex. (USGS station 08171290) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) EXPLANATION 3 Sample, by spring San Marcos Springs—Sample collection at Deep and Diversion Springs (USGS stations 295322097561000 and 295336097555201, respectively) San Marcos Springs—S a m p l e co l l e c t i o n at Deep and Diversion Springs and at Weissmuller Spring (USGS station 295322097561002) Comal S p r i n gs—S a m p l e co l l e c t i o n at Comal Spring 1 (USGS station 294300098080001) H u e c o Springs—S a m p l e co l l e c t i on at Hueco Spring A (USGS station 294533098082301) USGS, U.S. Geological Survey Time period of major storm response sampling and and storm identifier Discharge (daily mean) at surface-water site, by stream Blanco River—Halifax Ranch near Kyle, Texas (USGS station 08171290) Guadalupe River—Sattler, Tex. (USGS station 08167800) Figure 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 (mean for National Weather Service Cooperative Stations 411429, 412585, 416276, 417983, 418544, and 419815, National Oceanic and Atmospheric Administration, 2011).

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16 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Stream sites selected for routine sampling were the Blanco River at Halifax Ranch near Kyle, Tex. (USGS station Guadalupe River at River Road near Sattler, Tex. (USGS Wells selected for routine sampling (detailed in table 2) included wells completed in the Edwards aquifer and in the (Comal Spring 1) in the Comal Springs complex, and large spring that emerges from the Comal Springs Fault at the Comal Springs complex. Hueco Spring A is the lower collected from Comal Spring 1 and Hueco Spring A are considered for the purposes of this report to be representative of discharge from Comal and Hueco Springs. At San Marcos submerged at a depth of about 28 ft in the southwest part of Spring Lake) was selected to be representative of springs discharging from the southern part of Spring Lake. Diversion Spring (LR, submerged at a depth of about 20 ft in the northeast part of Spring Lake) and Weissmuller Spring (LR, submerged at a depth of about 24 ft in the northeast part of Spring Lake) were selected to be representative of springs discharging from the northern part of Spring Lake. In contrast with the 2008 sampling period for the other springs, the sampling period for Weissmuller Spring was from June through December 2010. To characterize changes in water quality in response to storm events, samples were collected in response to three major storms (storms 1) from nearby streams that potentially contribute recharge to San Marcos Springs, from Comal Spring 1, from Hueco Spring A, and from the three were collected in response to storms. Storm samples from At the springs, in response to storms 1 and 2, single samples collected at closely spaced intervals (hours to days) from Deep and Diversion Springs. In response to storm 3, six samples were collected at closely spaced intervals (hours to days) from Comal Spring 1, Hueco Spring A, Deep Spring, Diversion Spring, and Weissmuller Spring. Of these storm 3 samples for each of these sites, one sample considered as a storm sample in Crow (2012) is considered as a routine sample in this report. All samples were analyzed for the same constituents as routine samples, as described by Crow (2012). In addition to routine and storm sample collection, discharge and selected physicochemical properties were measured at 15-minute intervals (“continuously”) at the Blanco at Halifax stream site from February 2009 through and Weissmuller Spring, from June 2010 through December and dissolved oxygen concentration were recorded at conductance were measured at hourly intervals in two wells near San Marcos Springs: (1) LR (hereinafter, May 2010 and (2) LR (hereinafter, January 2009–December 2010. Rain samples were collected periodically (March 2009–September 2010) and in response to storms 1 and were analyzed for stable isotope ratios of 18 O). Continuous discharge data for the San Marcos Springs complex, the Comal Springs complex, the Hueco Springs complex, and other stream sites from ongoing data-collection efforts by the USGS (table 1) also were used in data analysis. These continuous data, available from the USGS National 2011), are used as interpretative aids throughout this report. Daily mean values for all continuous discharge, water-table altitudes, and physicochemical properties are presented and discussed throughout this report. Hydrologic and geochemical variability at San Marcos Springs was compared with that at Comal Springs and Hueco Springs. Sample Collection, Analytical Methods, and Quality Control as descriptions of sample collection, sample processing, described in detail in a companion USGS Data Series report (Crow, 2012). All nitrogen (N) species are reported and discussed as N. Recorded turbidity values below the method reporting level of 0.3 formazin nephelometric units (FNU) are considered nondetections (<0.3 FNU), although measured report for completeness.

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Methods of Investigation 17 Rainfall Estimation Daily rainfall data were compiled from the National Climatic Data Center (NCDC) for six National Weather Service (NWS) meteorological stations within the study area: Canyon Dam, Dripping Springs 6 E, New Braunfels, San Marcos, Spring Branch 2se, and Wimberley 1 NW (Cooperative Stations 411429, 412585, 416276, 417983, 418544, and 419815, respectively) (National Oceanic and Atmospheric Administration, 2011). Daily rainfall data from these sites were averaged to obtain a composite rainfall record for the study. Streamflow Measurements and Recharge Estimation (discharge) records are computed from the stage, or gage height, which is measured continuously. A stage-discharge relation is developed for each site by the USGS on the basis the time of measurement (Kennedy, 1984), which is used to Survey, 2011). Monthly recharge to the San Antonio segment of the Edwards aquifer has been estimated by the USGS for each of the contributing drainage basins since 1934 (Edwards Aquifer Authority, 2010) by using a water-balance approach based Puente (1978), uses data collected from a network of USGS Marcos Springs were evaluated as potential local recharge sources. Recharge estimates were computed daily for 2008 for the Blanco River, Cibolo Creek, and Dry Comal gaging stations in each basin. Daily mean recharge estimates were disaggregated from the annual recharge estimates determined by the Puente (1978) method by multiplying the annual recharge value by the fraction of annual basin discharge occurring on that day (R.N. Slattery, U.S. Geological Survey, written commun., 2011). During periods of relatively large amounts of runoff, estimated daily recharge values exceeded measured daily discharge at some sites. As a result, a 5-day average recharge estimate also was considered to adjust for the allocation of large amounts of recharge to days with large events. For the Blanco River and Cibolo Creek, recharge estimates were based on discharge measured at paired USGS Edwards aquifer recharge zone in each basin. For the Blanco River, the upstream station was either 08171000 (Blanco downstream station was 08171300 (Blanco River near Kyle, the downstream station was 08185000 (Cibolo Creek at estimated from the discharge at station 08168797 (Dry Comal Creek at Loop 337 near New Braunfels, Tex., hereinafter, Dry estimated from the discharge occurring in the Guadalupe River Basin between station 08167800 (Guadalupe River at Geological Survey, written commun., 2011). For the Blanco River, historical recharge estimates have been based on stream discharge at the upstream Blanco at Wimberley gage and the downstream Blanco near Kyle gage (hereinafter referred to as the “Wimberley-to-Kyle method”). For 2009, recharge for the Blanco River also was estimated by using a second method where discharge from the Blanco at Halifax gage was considered as the upstream gage instead of the Blanco at Wimberley gage. The Blanco at Wimberley gage used in the Wimberley-to-Kyle method is approximately 12 mi upstream from the recharge zone. The Blanco at Halifax gage, installed for this study, is less than 1 mi upstream from the recharge zone and provided data that might yield a more accurate assessment of potential recharge to the Edwards aquifer from the Blanco River. This second method of estimating recharge on this shorter section of the Blanco River for 2009 (hereinafter referred to as the “Halifax-to-Kyle method”) required that the drainage upstream drainage area and the intervening area between the upstream and downstream stations. Historical recharge estimates for the Edwards aquifer have not included estimates for the Guadalupe River because the Guadalupe River Basin likely does not contribute appreciable recharge to the aquifer (Puente, 1978). Nonetheless, for this study, a rudimentary gain-loss summary was estimated for the Guadalupe River between the Guadalupe at Sattler station and the Guadalupe

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18 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Guadalupe at Sattler station and discharge at Hueco Springs. interpreted to represent aquifer recharge. the aquifer occurs through internal drainage sinkholes or as type of recharge has been estimated to range from 20 to 40 percent of total aquifer recharge for the San Antonio segment recharge likely occurs by leakage from the underlying Trinity and has been estimated to range from 2 to 9 percent of total extent in Hays County relative to Comal, Bexar, and Medina Counties (Mace and others, 2000). Numerical and Statistical Methods The geochemical model PHREEQC (Parkhurst and Appelo, 1999) was used to calculate equilibrium concentrations of chemical species in solution, for speciation modeling, and to simulate reactions and processes occurring in the aquifer. Speciation modeling determines the potential in natural waters for mineral dissolution or precipitation to occur phases. The precipitation or dissolution of mineral phases can be an important control on water composition. In a carbonate aquifer, such as the Edwards aquifer, the dissolution or precipitation of minerals such as calcite and dolomite, which compose the aquifer rocks, will add or remove associated 3 precipitation of common trace minerals in carbonate rocks, such as celestite or gypsum, can also affect water composition. Gypsum is present in Edwards Group rocks (Maclay and Small, 1983) and might affect groundwater sulfate (SO 4 )/ chloride (Cl) ratios and SO 4 concentrations. Saturation index values were evaluated for aragonite, calcite, celestite, dolomite, gypsum, and strontianite and for carbon dioxide and dissolved oxygen gas phases. Inverse modeling with PHREEQC attempts to account for observed water compositions by (1) identifying thermodynamically feasible geochemical reactions and major dissolved constituents and (2) quantifying mixing proportions of different initial solutions. Inverse modeling was used in this study to approximate mixing proportions of selected source (endmember) compositions and plausible initial water composition or compositions (representing matrix groundwater) was mixed with other selected water compositions (representing surface-water recharge, salinewater composition was designated (representing San Marcos with selected mineral and gas phases were allowed, and results were constrained by selected ion concentrations (for example, major ions and Sr). Concentration uncertainties different model scenarios was considered in evaluating thus, more hydrologically plausible. No model results that required an uncertainty greater than 10 percent were considered. All models included phases likely to be associated aquifer (calcite, dolomite, gypsum, quartz, carbon dioxide, exchange). Dolomite and gypsum were constrained to allow for only dissolution (no precipitation). Dissolved oxygen was constrained to allow for only its loss or consumption. Resulting mixing-reaction models are valid within the constraints of available thermodynamic data and the data Nonparametric statistical tests were used for most data interpretation. Water-resources data are commonly nonnormally distributed (Helsel and Hirsch, 2002), and as a result, traditional parametric statistical methods are less suited to evaluate water-resources data than are nonparametric tests. The Kendall’s tau test was used to measure correlation. The test used to measure the strength of the relation between x and y (linear and nonlinear) and is resistant to the effects values approaching -1 or 1 indicate an increasing strength of correlation of greater than or equal to 0.9 corresponds to a tau value of greater than or equal to about 0.7 (Helsel and Hirsch, 2002). The Mann–Whitney U test, a nonparametric

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Climatic and Hydrologic Conditions 19 test for comparing two independent groups of data, was used to test for differences. For this report, statistical results with probability values (p-values) less than 0.05 (p < 0.05) are Climatic and Hydrologic Conditions Climatic and hydrologic conditions during the study varied considerably, which offered an opportunity to investigate changes in water quality at San Marcos Springs in response to drought and relatively wet conditions and the transition between the two. Climatic and hydrologic conditions during the study period ranged from exceptional drought (U.S. conditions with regularly occurring rainfall, greater surface5). When water-quality sampling began in December 2008, dry conditions had prevailed through the prior spring and summer, resulting in a decrease in discharge at San Marcos Springs from the largest value for the year of 217 ft 3 /s on January 1, 2008, to 102 ft 3 /s on December 1, 2008, when the September 2009, resulting in discharge at San Marcos Springs falling to 86 ft 3 /s. During the dry period, 12.7 inches of rain from 83 ft 3 /s to 106 ft 3 /s, with an average discharge of 95 ft 3 3 /s to 36 ft 3 /s, with an average discharge of 9.4 ft 3 /s (monitoring at Blanco at Halifax started on December 19, 2008). In contrast, San Marcos Springs discharge ranged from 88 ft 3 /s to 270 ft 3 /s, with an average discharge of 202 ft 3 at Halifax ranged from 14 ft 3 /s to an estimate of 3,620 ft 3 /s, with an average discharge of 204 ft 3 /s (approximately 20 times larger than during the dry period). Storm Characteristics The three major storms that were sampled during this study varied in size, antecedent moisture conditions, and resulting stream (discharge and recharge) and spring transition from the dry period to the wet period and followed the driest antecedent moisture conditions. Storm 1 was intermediate between storms 2 and 3 with respect to both the amount of rainfall and the discharge response at San Marcos Springs and Comal Springs. The discharge response and estimated recharge values for the Blanco River were similar for storms 1 and 2, which were smaller than storm 3. The discharge response for Hueco Springs was similar for all three storms and was characterized by a rapid rise in discharge relative to a more muted increase at San Marcos Springs antecedent moisture conditions (table 3). Storm 3, which was a named tropical cyclone storm (Hermine) ( National Aeronautics and Space Administration , 2012), was the largest storm to occur during the study period with respect to rain recharge estimates for the Blanco River and Cibolo Creek. For estimated recharge were larger for storm 2 than for storm 3. at both Sink Creek and Purgatory Creek. In addition to the three sampled storms, there were other storms that occurred during the study that generated large table 4). On June 9, 2010, a geographically isolated rain event occurred near New Braunfels, Tex. The average rainfall for the study area on that day was 2.91 inches, but 6.59 inches was recorded at New Braunfels, and 3.98 inches was recorded at Canyon Dam (NWS Cooperative stations 416276 and 411429, respectively). This rain event resulted in very large discharge values on the Guadalupe River and Dry Comal Creek, with only small changes in discharge and estimated recharge for the Blanco River and Cibolo Creek. This event was well suited to investigate potential recharge to San Marcos Springs from the from Dry Comal Creek was large in response to this rain physicochemical properties at San Marcos Springs indicates that these streams likely do not contribute notably to San Marcos Springs discharge. Rainfall Characteristics Stable isotope results for rainfall samples collected (USGS station 293146982941) covered a large range: D values ranged from -139.0 to 8.9 parts per thousand (per 18 O values ranged from -19.13 to -0.32 per mil. These data are consistent with a meteoric origin based on

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20 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 3. Characteristics of sampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010). [ft 3 Storm description Antecedent moisture conditions 1 Hydrologic response 2 Storm Date of storm onset Total (inches) 1 Temporal distribution of rainfall Rainfall in prior 3 months (inches) 1 Days since last storm 1 (>1 inch of rain in a day) Spring discharge response (prestorm value to 10-day poststorm max.): ft 3 /s (% change) Stream discharge response (prestorm value to 7-day poststorm max.): ft 3 /s Estimated stream recharge (ft 3 /s): range (5-day total) 1 9/9/2009 5.4 5.4 inches of rain fell between 9/9 and proportion on any which fell on 9/11 2.5 134 San Marcos Springs: Springs: 173 to 210 Guadalupe at Sattler: 57 to Cibolo near Boerne: 1.9 to Blanco River 4 : 0.8 to 83.9 Creek: 0 to 487 (758) 2 10/3/2009 2.5 2.5 inches of rain fell between 10/3 and additional 2.2 inches of rain fell between 10/9 and 10/14 10.2 20 San Marcos Springs: Springs: 210 to 274 Guadalupe at Sattler: 69 to to 3,500 Blanco River 4 : 6.2 to 92.3 Dry Comal Creek: 7.4 to 10,022 (13,452) 3 9/7/2010 8.1 8.1 inches of rain fell between 9/7 and was a named tropical storm (Hermine) 10.1 4 San Marcos Springs: Springs: 313 to 348 Guadalupe at Sattler: 201 to 3 Comal: 0.7 to 2,660 Blanco River 4 : 5.2 to 538 Dry Comal Creek: 38.4 to 3,607 (4,995) 1 for these sites were averaged to obtain a composite rainfall record in the vicinity of San Marcos Springs. 2 3 4

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Climatic and Hydrologic Conditions 21 Sept. 21 Aug. 28 Sept. 15 Aug. 31 Sept. 3 Sept. 6 Sept 18 Sept. 9 Sept. 1220090 1 2 3 4 5 6 7 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 2 3 0 2 4 0 0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 0Daily average rainfall, in inches Stream discharge, in cubic feet per second Estimated daily recharge, in cubic feet per second Spring discharge, in cubic feet per secondRainfall Blanco River*, Cibolo Creek, and Dry Comal Creek Hueco Springs Comal Springs San Marcos Springs Cibolo Creek Guadalupe River Blanco RiverA Onset of major storm Discharge at surface-water site, by stream Blanco River—Halifax Ranch near Kyle, Texas (USGS station 08171290) Guadalupe River—Sattler, Tex. (USGS station 08167800) Cibolo Creek—Cibolo Nature Center near Boerne, Tex. (USGS station 08183890) Discharge (daily mean) at spring—Composite of individual orifices (table 1) San Marcos Springs Comal S p r i n gs H u e c o Springs Sample, by stream or spring Blanco River—Sample collection at Halifax Ranch near Kyle, Tex. (USGS station 08171290) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) San Marcos Springs—S a m p l e co l l e c t i o n at Deep and Diversion Springs (USGS stations 295322097561000 and 295336097555201, respectively) Comal S p r i n gs—S a m p l e co l l e c t i o n at Comal Spring 1 (USGS station 294300098080001) H u e c o Springs—S a m p l e co l l e c t i on at Hueco Spring A (USGS station 294533098082301) USGS, U.S. Geological Survey EXPLANATION Onset of major storm Estimated recharge, by stream Blanco River Cibolo Creek Dry Comal CreekEXPLANATION* Based on Wimberley-to-Kyle method as described in the report in section “Streamflow Measurements and Recharge Estimation.” Figure 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. A , Storm 1 (September 2009). B , Storm 2 (October 2009). C , Storm 3 (September 2010).

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22 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Nov. 1 Sept. 20 Oct. 26 Sept. 26 Oct. 2 Oct. 8 Oct. 14 Oct. 2020090 0.5 1.0 1.5 2.0 2.5 0 100 20 0 300 40 0 500 600 70 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 40 160 90 100 110 120 1 30 1 40 1 50 1 60 1 70 1 80 200 210 220 230 240 250 2 60 2 70 2 80 2 90 300 0 20 40 60 80 1 00 1 20 1 40 1 60 1 80 2 00 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 5 00 1,000 1,500 2,000 2,500 3,00 0 3,500 4,00 0 0Daily average rainfall, in inches Stream discharge, in cubic feet per second Estimated daily recharge, in cubic feet per second Spring discharge, in cubic feet per secondRainfall Blanco River*, Cibolo Creek, and Dry Comal Creek Hueco Springs Comal Springs San Marcos Springs Cibolo Creek Guadalupe River Blanco RiverB October 4, 2009 10,022 cubic feet per second Onset of major storm Discharge at surface-water site, by stream Blanco River—Halifax Ranch near Kyle, Texas (USGS station 08171290) Guadalupe River—Sattler, Tex. (USGS station 08167800) Cibolo Creek—Cibolo Nature Center near Boerne, Tex. (USGS station 08183890) Discharge (daily mean) at spring—Composite of individual orifices (table 1) San Marcos Springs Comal S p r i n gs H u e c o Springs Sample, by stream or spring Blanco River—Sample collection at Halifax Ranch near Kyle, Tex. (USGS station 08171290) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) San Marcos Springs—S a m p l e co l l e c t i o n at Deep and Diversion Springs (USGS stations 295322097561000 and 295336097555201, respectively) Comal S p r i n gs—S a m p l e co l l e c t i o n at Comal Spring 1 (USGS station 294300098080001) H u e c o Springs—S a m p l e co l l e c t i on at Hueco Spring A (USGS station 294533098082301) USGS, U.S. Geological Survey EXPLANATION Onset of major storm Estimated recharge, by stream Blanco River Cibolo Creek Dry Comal CreekEXPLANATION* Based on Wimberley-to-Kyle method as described in the report in section “Streamflow Measurements and Recharge Estimation.” Figure 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. A , Storm 1 (September 2009). B , Storm 2 (October 2009). C , Storm 3 (September 2010). —Continued

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Climatic and Hydrologic Conditions 23 Sept. 3 Sept. 21 Sept. 6 Sept. 9 Sept. 12 Sept. 15 Sept. 1820100 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 5 00 1,000 1,500 2,000 2,500 3,00 0 3,500 4,00 0 0 01 2 3 4 5 6 7Daily average rainfall, in inches Stream discharge, in cubic feet per second Estimated daily recharge, in cubic feet per second Spring discharge, in cubic feet per second2 8 0 2 9 0 3 0 0 3 1 0 3 2 0 3 3 0 3 4 0 3 5 0 3 6 0 3 7 0 3 8 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 2 3 0 2 4 0 2 5 0 2 6 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 0 5 0 0 1 , 0 0 0 1 , 5 0 0 2 , 0 0 0 2 , 5 0 0 3 , 0 0 0 3 , 5 0 0 4 , 0 0 0 0 2 0 0 4 0 0 6 0 0 8 0 0 1 , 0 0 0 1 , 20 0 Rainfall Blanco River*, Cibolo Creek, and Dry Comal Creek Hueco Springs Comal Springs San Marcos Springs Cibolo Creek Guadalupe River Blanco RiverC Onset of major storm Discharge at surface-water site, by stream Blanco River—Halifax Ranch near Kyle, Texas (USGS station 08171290) Guadalupe River—Sattler, Tex. (USGS station 08167800) Cibolo Creek—Cibolo Nature Center near Boerne, Tex. (USGS station 08183890) Discharge (daily mean) at spring—Composite of individual orifices (table 1) San Marcos Springs Comal S p r i n gs H u e c o Springs Sample, by stream or spring Blanco River—Sample collection at Halifax Ranch near Kyle, Tex. (USGS station 08171290), and sample collection near Kyle, Tex. (USGS station 08171300) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) San Marcos Springs—S a m p l e co l l e c t i o n at Deep, Diversion, and Weissmuller Springs (USGS stations 295322097561000, 295336097555201, and 295322097561002, respectively) Comal S p r i n gs—S a m p l e co l l e c t i o n at Comal Spring 1 (USGS station 294300098080001) H u e c o Springs—S a m p l e co l l e c t i on at Hueco Spring A (USGS station 294533098082301) USGS, U.S. Geological Survey EXPLANATION Onset of major storm Estimated recharge, by stream Blanco River Cibolo Creek Dry Comal CreekEXPLANATION* Based on Wimberley-to-Kyle method as described in the report in section “Streamflow Measurements and Recharge Estimation.” Figure 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. A , Storm 1 (September 2009). B , Storm 2 (October 2009). C , Storm 3 (September 2010). —Continued

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24 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 4. Characteristics of unsampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010). [ft 3 Antecedent moisture conditions 1 Hydrologic response 2 Date of storm onset Total (inches) 1 Temporal distribution of rainfall Rainfall in prior 3 months (inches) 1 Days since last storm 1 (>1 inch of rain in a day) Spring discharge response (prestorm value to 10-day poststorm max.): ft 3 /s (% change) Stream discharge response (prestorm value to 7-day poststorm max.): ft 3 /s Estimated stream recharge (ft 3 /s): range (5-day total unless otherwise noted) 10/22/2009 4 Not a continuous rain event: 1.69 inches on 10/22, followed by additional 2.31 inches on 10/26 through 10/27 14.7 13 San Marcos Springs: Comal Springs Hueco Springs: 76 1.1 to 181 Blanco River 3 : 26 to 382 Comal: 105 to 1,879 (5,096) (6-day totals) 11/20/2009 1.6 0.47 inches of rain on 11/20, followed by additional 1.08 inches on 11/21 19.2 24 San Marcos Springs: Comal Springs 290 Hueco Springs: 60 1.1 to 232 Blanco River 3 : 56 to 343 140 to 764 (1,863) 1/14/2010 2.5 0.21 inches of rain on 1/14, 1.39 inches on 1/15, followed by additional 0.93 inches on 1/16 9.8 53 San Marcos Springs: Comal Springs 309 Springs: 22 1.1 to 707 Blanco River 3 : 15 to 201 Comal: 46 to 589 (1,355) 1/28/2010 3.1 Not a single rain from 1/28 through additional 1.79 inches from 2/3 through 2/5 8.2 13 San Marcos Springs: Comal Springs Hueco Springs: 86 3.4 to 636 Blanco River 3 : 40 to 301 Dry Comal: 54 to 551 (2,141) (10-day totals)

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Climatic and Hydrologic Conditions 25 5/15/2010 4.5 Not a continuous inches from 5/15 followed by additional 1.33 inches on 5/18 4.8 105 San Marcos Springs: Comal Springs: Hueco Springs: 72 25 to 575 Blanco River 3 : 23 to 82 Dry Comal: 48 to 556 (1,704) (6 day totals) 6/9/2010 3.0 2.91 inches of rain (localized rainfall of 6.59 inches at New Braunfels site) 9.6 21 San Marcos Springs: Comal Springs: Hueco Springs: 77 523 to 7,930 at downstream gage Guada Creek: 1.8 to 3,050 Blanco River 3 : 19 to 55 35 to 9,105 (10,788) 1 were averaged to obtain a composite rainfall record in the vicinity of San Marcos Springs. 2 3 Table 4. Characteristics of unsampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued [ft 3 Antecedent moisture conditions 1 Hydrologic response 2 Date of storm onset Total (inches) 1 Temporal distribution of rainfall Rainfall in prior 3 months (inches) 1 Days since last storm 1 (>1 inch of rain in a day) Spring discharge response (prestorm value to 10-day poststorm max.): ft 3 /s (% change) Stream discharge response (prestorm value to 7-day poststorm max.): ft 3 /s Estimated stream recharge (ft 3 /s): range (5-day total unless otherwise noted)

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26 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Nov. 2008 Oct. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010Date0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 0 5 0 0 1 ,0 0 0 1 ,5 0 0 2 ,0 0 0 2 ,5 0 0 3 ,0 0 0 3 ,5 0 0 4 ,0 0 0 0 5 0 0 1 ,0 0 0 1 ,5 0 0 2 ,0 0 0Estimated daily recharge, in cubic feet per second 1 2 3 1 2 3 1 2 3 October 4, 2009 10,022 cubic feet per second June 9, 2010 9,105 cubic feet per second September 9, 2010 3,200 cubic feet per second A. B l a n c o River*B. C i b o l o CreekC. D r y C o m a l Creek 1Onset of major storm and identifierEXPLANATIONWet period Dry period* Based on Wimberley-to-Kyle method as described in the report in section “Streamflow Measurements and Recharge Estimation.” Figure 7. Estimated recharge to the Edwards aquifer from A , the Blanco River, B , Cibolo Creek, and C , Dry Comal Creek, southcentral Texas, 2008.

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Climatic and Hydrologic Conditions 27 DateFeb. 2009 May 2009 Aug. 2009 May 2010 Aug. 2010 Nov. 2008 Nov. 2009 Feb. 2010 Oct. 2010 Dec. 2010DateFeb. 2009 May 2009 Aug. 2009 May 2010 Aug. 2010 Nov. 2008 Nov. 2009 Feb. 2010 Oct. 2010 Dec. 20100 2 0 0 4 0 0 6 0 0 8 0 0 1 ,0 0 0 0 5 0 0 1 , 0 0 0 1 , 5 0 0 2 , 0 0 0 2 , 5 0 0 3 , 0 0 0 3 , 5 0 0 4,00 0 -2 0 0Discharge, in cubic feet per second Guadalupe River estimated gain/loss (daily mean), in cubic feet per second 1 2 3 1 2 3 GAIN LOSS June 9, 2010 3,834 cubic feet per second June 9, 2010 7,930 cubic feet per second A BOnset of major storm and identifier Guadalupe River estimated gain/loss Stream discharge at Guadalupe River at Sattler, Texas (USGS station 08167800) Stream discharge at Guadalupe River above Comal River at New Braunfels, Tex. (USGS station 08168500) USGS, U.S. Geological SurveyEXPLANATION 1 Wet period Dry period Wet period Dry period Figure 8. Time series (November 2008–December 2010) of gain (positive values) and loss (negative values) for the Guadalupe River, south-central Texas. A , Estimated stream gain (positive values) and loss (negative values). B , Stream discharge at two U.S. Geological Survey stations.

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28 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) -25 -20 -15 -10 -5 0 5Delta oxygen-18, in per mil-160 -140 -120 -100 -80 -60 -40 -20 0 20 40Delta deuterium, in per mil Global meteoric water line (Craig, 1961) Local meteoric water line (Pape and others, 2010) Rainfall sample Dry period—November 1, 2008, to September 8, 2009 Wet period—September 9, 2009, to December 31, 2010 Storm 1—September, 2009 Storm 2—October, 2009 Storm 3—September, 2010EXPLANATION Figure 9. Relation between deuterium and oxygen isotopes for rainfall samples collected at U.S. Geological Survey station 293146982941, Bexar County, south-central Texas (2008). Local (Pape and others, 2010) and global (Craig, 1961) meteoric water lines are shown for comparison. comparison with the global meteoric water line (Craig, 1961) and the local meteoric water line (Pape and others, 2010) 18 O values (5.1 and -0.94 per mil, respectively) than did samples collected during the wet period (-23.1 and -4.46 per mil, respectively). As discussed by Pape and others (2010), variations in meteoric precipitation in the midlatitudes can be controlled by a variety of factors, including temperature, rainfall amount, and the moisture source of individual rain events. When more than one sample was collected during the storms (storms 1 and 3), important control on the isotopic composition of rain during summertime periods in central Texas (Pape and others, 2010). Rainfall stable isotope values from samples collected during this study correlate with rain amounts measured nearby at the San Antonio International Airport (NWS Cooperative Station 417945) (Kendall’s tau = -0.34 and -0.46 for 18 O values, respectively), indicating that lighter stable isotope values for rainfall were generally associated with larger events. Stable isotope results for rainfall samples collected during storms 1 were different for the different storms. Stable isotope results for the single rain sample collected during storm 2 (-23.1 and -4.2 per mil for 18 O, respectively) were similar to median values for San Marcos

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Climatic and Hydrologic Conditions 29 Springs (table 5). In contrast, median stable isotope values for rain samples collected during storm 1 (-37.2 and -6.4 per mil for 18 O, respectively) and storm 3 (-131.0 and -17.8 per mil for 18 O, respectively) were isotopically distinct from most groundwater and spring discharge samples collected during the study (table 5). Rain samples collected during storm 3 in particular were isotopically lighter (lower isotopic values) than other rain samples collected during samples collected in Austin, Tex., over an 8-year period by Pape and others (2010). Rainfall from tropical cyclone storms such as storm 3 has been shown to have isotopically light stable isotope values relative to normal summer rainfall to the Edwards aquifer in response to storm 3 would likely recent recharge, as previously demonstrated in the Edwards discussed further in the section “Endmember Mixing Using Conservative Tracers.” Stream Recharge Halifax) for this study allowed for a detailed assessment of potential recharge to the Edwards aquifer from the Blanco River as it crosses the recharge zone, as well as for comparison of recharge in the Wimberley-to-Halifax and Halifax-to-Kyle reaches of the Blanco River. A comparison of estimated recharge between Wimberley-to-Kyle and Halifax-to-Kyle indicates that recharge estimates are generally similar for the two different methods, with a Recharge estimates for major streams in the study area that might provide recharge to San Marcos Springs (Blanco River, Cibolo Creek, and Dry Comal Creek) indicate that the amount of recharge varied markedly through the study Comal Creek contributed the largest amount of recharge to the Edwards aquifer in the study area (average daily mean recharge of 102 ft 3 /s), followed by Cibolo Creek (average daily mean recharge of 46.9 ft 3 /s) and the Blanco River (average daily mean recharge of 28.4 ft 3 /s). Recharge estimates were small and similar for the different streams during the dry period (average daily mean recharge of 2.7, 3.0, and 2.3 ft 3 larger during the wet period (average daily mean recharge of 44.2, 75.3, and 167.2 ft 3 /s for the Blanco River [Halifax-toRecharge estimates varied between the streams for different Dry Comal Creeks were much larger than for the Blanco River in response to storm 2. A gain-loss summary was estimated for the Guadalupe River between the Guadalupe at Sattler and Guadalupe above Comal stations. These results indicated that the Guadalupe River was largely a gaining stream for most of the study 3 /s. During the wet period, the Guadalupe River was a gaining stream for all but 7 days (1.5 percent of the wet period), with a daily mean gain that averaged 47.0 ft 3 /s. The largest losses occurred during several days in mid-April 2010, with a maximum daily mean loss of 178.2 ft 3 /s. This period coincided with some (for example, mid-May 2010 and mid-September 2010). In contrast to the wet period, during the dry period the Guadalupe River was, on average, a losing stream, with an average value for daily mean loss of 3.0 ft 3 /s (ranging from a maximum daily mean loss of 26.0 ft 3 /s to a maximum daily mean gain of 13.6 ft 3 /s). For most of summer 2009 (dry period), the Guadalupe River had small daily mean losses (8.4 ft 3 /s on indicate that during dry hydrologic conditions the Guadalupe River might contribute small amounts of recharge to the aquifer, they are consistent with the hypothesis that the Guadalupe River does not contribute substantial recharge to the Edwards aquifer (Puente, 1978) or to San Marcos Springs Two ephemeral streams in the study area (Sink Creek estimated from data collected at USGS station 08169932 only 3 days (Crow, 2012). In response to storm 2, discharges of 9.8 (estimated) and 8.9 ft 3 /s on October 3 and 4, 2009, respectively, were measured for Sink Creek. In response to storm 3, Sink Creek discharge was 25 ft 3 /s on September 8, 2010. Gage height for Purgatory Creek was collected at USGS station 08169958 (Purgatory Creek at Mountain through the end of 2010. Although data did not allow for a stage-discharge relation to be established for this site, gageCreek on only 2 days: May 16, 2010, and September 9, 2010 (in response to storm 3).

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30 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 5. Summary statistics for discharge, physicochemical properties, and selected geochemical constituents for surface-water, groundwater, and spring sites sampled during dry and wet hydrologic conditions in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued 3 3 +NO 2 18 13 87 Sr/ 86 Sr, strontium-87/ 3, 3 4 /Cl, Discharge, physicochemi cal property, or constituent concentration Unit Hydro logic condition Surface water 1, 5 Groundwater wells (Edwards aquifer) 2, 5 Guadalupe at River Road Blanco at Halifax TSU-West Campus Solar Neff Aqua 4D N (unless otherwise noted) Dry period 3 7 -7 -3 -6 -7 -7 -6 -Wet period 4 11 -13 -9 -8 -10 -6 -9 -median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR Discharge ft 3 /s Dry period 1 62 (312) 15 11 (264) 9 ----------Wet period 1 124 (479) 287 146 (479) 135 ----------conductance S/cm Dry period 393 17 381 (209) 41 810 (227) 16 536 14 574 4 630 3 574 5 Wet period 436 30 455 (476) 63 656 (448) 83 548 17 576 6 622 5 575 6 Turbidity FNU Dry period 6.7 1.2 6.6 (30) 4.7 0.3 6.2 <0.3 (5) 0.1 <0.3 0.0 <0.3 0.1 <0.3 0.1 Wet period 5.8 8.1 45.5 (380) 106 <0.3 1.3 <0.3 0.5 <0.3 0.0 <0.3 0.8 <0.3 0.0 Dissolved oxygen mg/L Dry period 8.6 1.2 7.7 (155) 0.4 5.8 0.2 7.4 1.6 5.4 0.3 4.0 2.1 7.4 0.4 Wet period 9.2 2.4 8.9 (479) 2.1 5.9 0.1 6.7 2.5 5.4 0.2 4.9 0.4 6.9 1.3 Temperature C Dry period 21.5 8.7 27.6 (209) 8.7 22.3 (226) 0 21.9 0.4 22.4 0.7 24.0 0.1 22.0 0.2 Wet period 19.2 11.1 20.7 (478) 12.1 22.3 (451) 0 21.9 0.2 22.2 0.3 24.0 0.3 22.0 0.4 Calcium mg/L Dry period 41.0 3.5 43.5 15.5 89.0 4.5 105.5 12.0 84.5 4.3 55.5 2.3 89.9 3.1 Wet period 56.7 7.0 61.2 15.4 92.9 5.2 104.0 30.6 85.8 5.8 55.6 5.0 87.7 7.4 Magnesium mg/L Dry period 18.3 1.2 19.8 1.9 16.7 0.3 3.8 0.8 19.8 1.0 31.8 0.7 16.1 0.6 Wet period 17.2 1.4 15.5 2.7 16.8 0.6 6.4 13.4 20.3 0.7 32.5 1.4 16.6 0.9 Alkalinity mg/L as CaCO 3 Dry period 160 26 156 48 277 26 281 12 277 33 224 28 265 19 Wet period 189 15 194 26 265 19 275 17 280 18 236 12 273 13 Strontium Dry period 434 33 584 87 476 20 66 13 270 28 42,400 (6) 2,400 466 42 Wet period 372 73 307 115 485 13 121 362 252 18 41,900 600 500 43 Sodium mg/L Dry period 10.4 0.7 8.8 2.0 10.7 0.2 3.2 0.2 5.5 0.3 6.5 0.4 7.0 0.1 Wet period 9.3 1.3 7.2 1.8 10.3 0.2 3.4 0.5 5.4 0.3 6.6 0.6 6.8 0.4 Table 5. Summary statistics for discharge, physicochemical properties, and selected geochemical constituents for surface-water, groundwater, and spring sites sampled during dry and wet hydrologic conditions in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010). 3 3 +NO 2 18 13 87 Sr/ 86 3, 3 , strontium to calcium molar 4

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Climatic and Hydrologic Conditions 31 Discharge, physicochemi cal property, or constituent concentration Groundwater wells (Trinity aquifer) 2, 5 Springs 1, 2, 5, 6 Mendez Sac-N-Pac Ruby Ranch San Marcos Springs Diversion Spring orifice San Marcos Springs Deep Spring orifice San Marcos Springs Weissmuller Spring orifice Comal Springs Hueco Springs N (unless otherwise noted) 7 -7 -6 -7 -7 -0 -7 -7 -10 -5 -3 -26 -26 -11 -17 -17 -median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR Discharge ------96 (312) 9 96 (312) 9 96 (312) 9 265 (312) 103 6.3 (312) 2.5 ------207 (479) 48 207 (479) 48 207 (479) 48 335 (479) 39 76 (479) 29 conduc tance 458 8 505 43 552 29 600 (188) 8 623 (140) 7 --555 4 573 18 460 3 506 9 564 6 593 (405) 12 611 (396) 6 595 (183) 6 559 (18) 4 595 (18) 52 Turbidity <0.3 0.2 2.1 9.2 0.4 1.2 <0.3 (170) 0.00 <0.3 (191) 0.08 --<0.3 0.0 <0.3 0.8 <0.3 (9) 0.0 0.3 2.2 <0.3 3.0 <0.3 (406) 0.01 <0.3 (314) 0.05 0.6 (166) 0.2 <0.3 (18) 0.00 6.1 (18) 7.7 Dissolved oxygen 5.4 (6) 2.8 5.0 0.7 8.2 0.5 5.7 0.2 5.6 1.2 --5.6 0.2 5.7 1.6 5.4 (9) 0.1 5.5 2.1 8.2 0.2 4.9 (15) 0.3 5.7 (15) 1.4 4.4 (9) 0.0 5.8 (18) 0.2 6.1 (18) 1.0 Temperature 24.4 1.5 23.4 0.5 21.5 0.2 21.9 (190) 0.2 22.3 (149) 0.1 --23.3 0.1 21.2 2.9 24.3 0.2 23.1 0.2 2.2 0.3 21.6 (406) 0.0 22.1 (398) 0.2 21.5 (183) 0.3 23.2 (18) 0.1 22.0 (18) 0.8 Calcium 52.5 4.7 55.5 12.4 63.9 4.6 89.9 6.2 92.1 3.9 --82.8 2.0 76.8 5.8 54.5 3.2 70.3 21.2 64.3 7.2 87.8 4.1 93.5 3.2 85.6 2.1 82.2 5.0 99.5 6.8 Magnesium 23.2 1.2 25.6 1.6 31.1 0.9 17.0 1.1 16.4 0.6 --16.3 1.1 18.9 1.1 23.7 0.7 18.8 6.0 31.8 3.0 18.0 0.8 16.6 0.5 18.3 0.4 15.7 0.6 10.6 3.1 Alkalinity 227 22 243 25 272 18 268 26 270 14 --243 24 247 40 227 19 256 22 243 35 263 28 269 24 247 20 234 15 263 19 Strontium 1,280 90 811 960 6,245 1,850 480 28 507 38 --579 34 523 27 1,210 60 548 489 5,370 1,510 514 33 517 17 549 16 572 9 239 157 Sodium 4.3 0.4 4.3 0.7 6.1 0.6 9.8 0.7 12.1 2.2 --10.2 0.9 11.4 0.8 4.4 0.2 4.3 0.5 6.3 0.7 9.9 0.8 12.1 0.6 11.1 0.9 10.4 0.6 8.7 1.6

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32 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 5. Summary statistics for discharge, physicochemical properties, and selected geochemical constituents for surface-water, groundwater, and spring sites sampled during dry and wet hydrologic conditions in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued 3 3 +NO 2 18 13 87 Sr/ 86 Sr, strontium-87/ 3, 3 4 /Cl, Discharge, physicochemi cal property, or constituent concentration Unit Hydro logic condition Surface water 1, 5 Groundwater wells (Edwards aquifer) 2, 5 Guadalupe at River Road Blanco at Halifax TSU-West Campus Solar Neff Aqua 4D N (unless otherwise noted) Dry period 3 7 -7 -3 -6 -7 -7 -6 -Wet period 4 11 -13 -9 -8 -10 -6 -9 -median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR Chloride mg/L Dry period 16.1 0.4 14.0 2.7 18.5 0.7 3.1 0.1 9.8 0.2 10.1 0.1 12.6 0.3 Wet period 15.9 0.9 13.9 2.9 18.5 0.9 3.9 2.3 10.3 0.6 10.4 0.7 12.7 0.6 Sulfate mg/L Dry period 24.1 3.5 41.3 7.7 27.1 1.0 4.3 0.2 17.6 0.4 91.1 0.7 17.3 0.4 Wet period 22.1 1.8 28.6 10.3 24.8 0.8 5.8 7.1 16.8 1.2 86.4 4.9 16.4 0.8 Bromide mg/L Dry period 0.12 0.00 0.07 0.07 0.11 0.02 0.05 0.01 0.06 0.01 0.05 0.01 0.06 0.04 Wet period 0.10 0.01 0.07 0.02 0.11 0.01 0.05 0.01 0.07 0.01 0.06 0.00 0.07 0.01 Boron Dry period 74.0 5.0 62.0 29.0 54.0 8.0 20.5 2.0 34.0 2.0 62.0 (6) 6.0 43.0 4.0 Wet period 61.0 7.0 48.0 8.0 52.0 2.0 20.5 6.5 34.5 3.0 62.0 1.0 43.0 3.0 Fluoride mg/L Dry period 0.24 0.03 0.23 0.02 0.22 0.06 0.08 0.03 0.18 0.04 2.77 0.15 0.18 0.03 Wet period 0.21 0.03 0.18 0.05 0.22 0.02 0.16 0.19 0.19 (9) 0.02 2.78 0.22 0.19 0.02 Potassium mg/L Dry period 2.1 0.1 1.5 0.4 1.3 0.1 0.6 0.0 0.8 0.0 1.4 0.1 1.1 0.1 Wet period 1.7 0.3 1.3 0.4 1.3 0.0 0.7 0.1 0.8 0.1 1.4 0.1 1.0 0.1 Silica mg/L Dry period 11.8 2.2 8.2 14.9 12.1 0.4 11.4 0.4 10.8 1.1 11.4 0.7 11.6 0.7 Wet period 9.9 1.4 9.9 1.8 12.0 0.5 11.4 0.8 10.6 0.6 11.3 0.5 11.6 0.4 NO 3 +NO 2 mg/L Dry period 0.045 (6) 0.0 0.000 0.020 1.50 0.01 2.43 0.03 0.89 0.02 0.00 0.00 1.54 0.02 Wet period 0.32 0.46 0.45 0.48 1.64 0.07 1.99 0.94 0.86 0.03 0.00 0.00 1.50 0.09 87 Sr/ 86 Sr -Dry period 0.70783 0.00004 0.70783 0.00004 0.70792 0.00003 0.70860 0.00004 0.70787 0.00002 0.70770 0.00003 0.70779 0.00002 Wet period 0.70787 0.00003 0.70791 0.00010 0.70792 0.00002 0.70808 0.00066 0.70789 0.00003 0.70774 0.00004 0.70777 0.00003 per mil () Dry period -9.3 1.6 -12.0 18.1 -23.6 1.0 -26.9 0.6 -22.6 1.0 -21.1 0.6 -22.9 1.0 Wet period -16.1 3.5 -23.9 3.4 -23.1 0.7 -25.5 1.4 -23.4 1.2 -21.2 0.8 -22.7 0.3 Table 5. Summary statistics for discharge, physicochemical properties, and selected geochemical constituents for surface-water, groundwater, and spring sites sampled during dry and wet hydrologic conditions in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued 3 3 +NO 2 18 13 87 Sr/ 86 3, 3 , strontium to calcium molar 4

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Climatic and Hydrologic Conditions 33 Discharge, physicochemi cal property, or constituent concentration Groundwater wells (Trinity aquifer) 2, 5 Springs 1, 2, 5, 6 Mendez Sac-N-Pac Ruby Ranch San Marcos Springs Diversion Spring orifice San Marcos Springs Deep Spring orifice San Marcos Springs Weissmuller Spring orifice Comal Springs Hueco Springs N (unless otherwise noted) 7 -7 -6 -7 -7 -0 -7 -7 -10 -5 -3 -26 -26 -11 -17 -17 -median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR Chloride 7.4 0.1 6.5 1.6 11.3 0.2 16.3 0.4 20.6 3.0 --16.8 0.3 18.1 2.1 7.7 0.4 8.4 1.2 12.2 1.5 18.1 1.7 21.1 0.9 19.9 0.9 17.3 0.3 16.3 3.0 Sulfate 11.2 0.3 11.8 4.0 24.0 12.4 24.7 0.2 27.8 1.9 --25.6 0.5 31.5 3.3 11.6 1.0 11.6 2.9 21.7 4.4 25.9 1.3 26.8 23.0 26.8 0.5 24.8 0.8 21.4 5.4 Bromide 0.04 0.01 0.04 0.02 0.07 0.01 0.11 0.01 0.13 0.04 --0.09 0.02 0.08 0.04 0.04 0.00 0.05 0.01 0.07 0.01 0.12 0.01 0.12 0.01 0.13 0.00 0.10 0.00 0.09 0.02 Boron 30.0 3.0 41.0 16.0 39.0 3.0 45.0 4.0 54.0 5.0 --52.0 1.0 68.0 18.0 31.0 3.0 30.0 8.0 41.0 6.0 51.5 3.0 56.5 3.0 55.0 3.0 55.0 3.0 52.0 12.0 Fluoride 0.27 0.04 0.40 0.18 0.32 0.02 0.22 0.03 0.22 0.03 --0.23 0.05 0.30 0.04 0.28 0.05 0.28 0.14 0.26 0.00 0.22 0.02 0.21 0.01 0.23 0.01 0.23 0.02 0.18 0.05 Potassium 0.8 0.1 1.4 0.6 1.3 0.2 1.3 0.2 1.4 0.2 --1.4 0.1 1.6 0.1 0.8 0.0 1.1 0.3 1.2 0.2 1.3 0.1 1.4 0.1 1.4 0.1 1.4 0.1 1.4 0.2 Silica 12.1 0.4 12.1 0.4 11.2 0.8 11.5 0.5 11.9 0.8 --12.4 0.8 12.3 4.5 12.2 0.7 11.4 1.0 10.9 0.9 11.0 0.1 11.9 0.3 10.8 0.2 12.1 0.6 10.7 0.8 NO 3 +NO 2 0.34 0.01 0.24 0.2 0.61 0.14 1.21 0.08 1.50 0.27 --1.85 0.05 0.84 0.16 0.34 0.01 0.5 0.65 0.65 0.02 1.05 0.12 1.53 0.12 0.95 0.02 1.98 0.07 1.49 0.45 87 Sr/ 86 Sr 0.70764 0.00004 0.70780 0.00003 0.70749 0.00003 0.70788 0.00003 0.70794 0.00006 --0.70793 0.00004 0.70784 0.00002 0.70765 0.00004 0.70786 0.00015 0.70750 0.00007 0.70789 0.00004 0.70794 0.00008 0.70786 0.00003 0.70790 0.00003 0.70800 0.00010 -23.0 0.6 -25.2 1.1 -22.5 0.5 -23.0 1.0 -23.1 1.1 ---23.3 1.1 -19.4 4.8 -23.4 0.5 -23.9 2.0 -22.9 0.9 -22.3 0.9 -23.2 0.7 -22.3 0.8 -23.5 0.5 -26.0 11.6

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34 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 5. Summary statistics for discharge, physicochemical properties, and selected geochemical constituents for surface-water, groundwater, and spring sites sampled during dry and wet hydrologic conditions in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued 3 3 +NO 2 18 13 87 Sr/ 86 Sr, strontium-87/ 3, 3 4 /Cl, Discharge, physicochemi cal property, or constituent concentration Unit Hydro logic condition Surface water 1, 5 Groundwater wells (Edwards aquifer) 2, 5 Guadalupe at River Road Blanco at Halifax TSU-West Campus Solar Neff Aqua 4D N (unless otherwise noted) Dry period 3 7 -7 -3 -6 -7 -7 -6 -Wet period 4 11 -13 -9 -8 -10 -6 -9 -median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR 18 O per mil () Dry period -1.12 0.24 -2.01 4.65 -4.23 0.09 -4.94 0.06 -4.14 0.05 -3.73 0.02 -4.15 0.06 Wet period -2.61 0.42 -4.25 0.62 -4.24 0.06 -4.75 0.17 -4.25 0.09 -3.76 0.05 -4.16 0.08 13 C per mil () Dry period -5.51 0.51 -3.96 1.24 -8.96 0.62 -11.57 0.71 -8.96 1.11 -3.38 0.76 -9.14 0.10 Wet period -8.57 1.42 -8.31 2.11 -9.48 0.12 -12.13 3.95 -9.44 (7) 0.33 -3.98 0.11 -9.19 0.35 Mg/Ca -Dry period 0.747 0.051 0.687 0.548 0.309 0.020 0.058 0.009 0.381 0.026 0.940 0.078 0.297 0.011 Wet period 0.502 0.075 0.418 0.127 0.297 0.003 0.101 0.613 0.393 0.033 0.955 0.037 0.308 0.023 3 -Dry period 4.80 0.46 5.85 0.55 2.38 0.15 0.27 0.03 1.47 0.12 351 (6) 30.5 2.36 0.22 Wet period 3.02 0.38 2.28 1.61 2.33 0.25 0.53 2.29 1.36 0.15 350 18.0 2.68 0.44 SO 4 /Cl -Dry period 0.56 0.07 1.02 0.18 0.52 0.02 0.52 0.03 0.67 0.01 3.31 0.06 0.51 0.01 Wet period 0.54 0.09 0.70 0.06 0.49 0.03 0.58 0.23 0.60 0.06 3.03 0.16 0.47 0.04 Mg/Na -Dry period 1.66 0.11 2.12 0.15 1.48 0.06 1.04 0.20 3.44 0.27 4.65 0.23 2.20 0.11 Wet period 1.83 0.28 2.03 0.13 1.56 0.06 1.78 3.27 3.55 0.28 4.65 0.12 2.32 0.17 Calcite saturation index -Dry period 0.73 0.15 0.73 0.23 -0.13 0.12 -0.05 0.43 0.04 0.39 0.08 0.40 0.21 0.34 Wet period 0.65 0.15 0.72 0.40 0.09 0.08 0.06 0.12 0.10 0.08 0.21 0.05 0.15 0.19 Dolomite saturation index -Dry period 1.42 0.40 1.54 0.61 -0.65 0.28 -1.27 0.72 -0.24 0.75 0.22 0.79 -0.01 0.68 Wet period 0.96 0.33 0.94 0.45 -0.20 0.16 -0.51 1.07 -0.16 0.18 0.53 0.11 -0.07 0.42 Gypsum saturation index -Dry period -2.38 0.07 -2.22 0.16 -2.09 0.01 -2.78 0.02 -2.28 0.02 -1.79 0.31 -2.26 0.02 Wet period -2.30 0.08 -2.18 0.19 -2.12 0.03 -2.66 2.19 -2.30 0.02 -1.81 0.05 -2.30 0.02 1 Stream discharge for Blanco at Halifax, spring discharge for San Marcos Springs, and selected physicochemical data for Blanco at Halifax and San Marcos 2 Physicochemical properties for groundwater wells, Hueco Springs, and Comal Springs measured at time of routine sample collection. 3 Dry period: November 1, 2008, through September 8, 2009. Table 5. Summary statistics for discharge, physicochemical properties, and selected geochemical constituents for surface-water, groundwater, and spring sites sampled during dry and wet hydrologic conditions in the vicinity of San Marcos Springs, south-central Texas (November 2008–December 2010).—Continued 3 3 +NO 2 18 13 87 Sr/ 86 3, 3 , strontium to calcium molar 4

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Climatic and Hydrologic Conditions 35 Discharge, physicochemi cal property, or constituent concentration Groundwater wells (Trinity aquifer) 2, 5 Springs 1, 2, 5, 6 Mendez Sac-N-Pac Ruby Ranch San Marcos Springs Diversion Spring orifice San Marcos Springs Deep Spring orifice San Marcos Springs Weissmuller Spring orifice Comal Springs Hueco Springs N (unless otherwise noted) 7 -7 -6 -7 -7 -0 -7 -7 -10 -5 -3 -26 -26 -11 -17 -17 -median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR median (N) IQR 18 O -4.27 0.07 -4.48 0.11 -4.17 0.01 -4.17 0.06 -4.22 0.14 ---4.15 0.04 -3.27 1.05 -4.23 0.04 -4.39 0.14 -4.13 0.08 -4.08 0.16 -4.23 0.04 -4.06 0.06 -4.18 0.06 -4.60 1.33 13 C -7.04 1.11 -8.80 2.43 -5.56 1.02 -9.32 0.50 -9.28 0.20 ---8.49 1.08 -7.40 1.29 -7.42 0.07 -9.91 2.14 -6.40 0.16 -9.38 0.29 -9.37 0.18 -9.41 0.07 -8.70 0.13 -11.20 1.52 Mg/Ca 0.710 0.064 0.743 0.201 0.794 0.062 0.312 0.016 0.294 0.024 --0.323 0.017 0.403 0.033 0.717 0.004 0.474 0.221 0.815 0.020 0.338 0.018 0.290 0.014 0.353 0.009 0.314 0.014 0.176 0.069 3 10.61 0.96 7.12 8.94 42.78 15.99 2.41 0.21 2.49 0.16 --3.12 0.24 3.14 0.21 10.34 0.90 3.87 3.47 37.39 7.89 2.70 0.25 2.52 0.17 2.92 0.10 3.21 0.13 1.03 0.98 SO 4 /Cl 0.56 0.02 0.64 0.32 0.77 0.41 0.56 0.01 0.50 0.03 --0.56 0.01 0.66 0.10 0.55 0.07 0.59 0.13 0.66 0.21 0.53 0.06 0.48 0.03 0.50 0.01 0.54 0.02 0.49 0.07 Mg/Na 5.14 0.58 5.35 0.70 4.83 0.58 1.65 0.14 1.28 0.34 --1.50 0.10 1.54 0.14 5.10 0.22 4.39 1.46 4.77 0.09 1.71 0.18 1.30 0.08 1.53 0.12 1.46 0.09 1.30 0.33 Calcite saturation index 0.10 0.46 -0.11 0.62 0.19 0.13 0.00 0.27 -0.11 0.30 ---0.01 0.50 0.02 0.57 0.02 0.11 0.09 0.12 0.24 0.19 0.07 0.19 0.14 0.21 0.17 0.29 0.12 0.17 0.15 0.25 Dolomite saturation index 0.16 0.83 -0.20 1.13 0.38 0.25 -0.39 0.54 -0.64 0.59 ---0.41 1.01 -0.25 1.06 0.00 0.22 -0.03 0.15 0.47 0.38 -0.24 0.37 -0.16 0.42 -0.03 0.59 -0.10 0.33 -0.39 0.47 Gypsum saturation index -2.64 0.03 -2.57 0.07 -2.28 0.20 -2.12 0.01 -2.06 0.04 ---2.12 0.02 -2.05 0.05 -2.62 0.06 -2.48 0.06 -2.32 0.12 -2.11 0.02 -2.07 0.03 2.10 0.01 -2.14 0.02 -2.14 0.10 4 Wet period: September 9, 2009, through December 31, 2010. 5 6 Diversion Spring, Weissmuller Spring, Comal Spring 1, and Hueco Spring A.

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36 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Feb. 2009 May 2009 Aug. 2009 May 2010 Aug. 2010 Nov. 2008 Nov. 2009 Feb. 20100 100 200 300 400 500 600 700Date Estimated daily recharge (based on Wimberley-to-Kyle method), in cubic feet per secondOct. 2010 Dec. 20100 100 0 200 300 400 500 600 700 100 200 300 400 500 600 700Estimated daily recharge, in cubic feet per secondEstimated daily recharge (based on Halifax-to-Kyle method), in cubic feet per second 1 2 3 1:1 correlationWet period Dry period Onset of major storm and identifierBlanco River estimated daily mean aquifer recharge based on USGS stations 08171000 (Blanco River at Wimberley, Texas) and 08171300 (Blanco River near Kyle, Tex.) Blanco River estimated daily mean aquifer recharge based on USGS stations 08171290 (Blanco River at Halifax Ranch near Kyle, Tex.) and 08171300 (Blanco River near Kyle, Tex.)USGS, U.S. Geological SurveyEXPLANATION 1 A B Figure 10. Estimated daily recharge to the Edwards aquifer from the Blanco River, south-central Texas, computed from two pairs of U.S. Geological Survey streamflow-gaging stations. A , Time series (November 2008–December 2010). B , Relation between estimated recharge from the two station pairs.

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 37 Geochemistry of San Marcos Springs and Nearby Hydrologic Features Hydrologic and Physicochemical Data Continuous measurements for discharge and physicochemical properties were recorded at selected stream and spring sites. Continuous measurement of water-table altitude and some physicochemical properties were recorded at selected groundwater sites. Summary statistics for discharge and (or) physicochemical properties from stream, well, and spring sites are shown in table 5. Surface Water Continuous discharge was recorded at Blanco at Halifax beginning in December 2008, and physicochemical properties values for discharge, selected physicochemical properties, 11. Discharge at Blanco at Halifax ranged from a low of 2.3 ft 3 /s on July 28 and 29, 2009, to an estimated high of 3,620 ft 3 /s on September 8, 2010, in response to storm 3. degrees Celsius ( C) with warmer temperatures occurring some sudden decreases in temperature were associated with occurred with increased stream discharge in response to rain events. Turbidity values of generally less than 100 FNU were punctuated with sudden large increases, as large as 2,890 FNU, in response to rain events. Variations in turbidity were small during the dry period. During the wet period, daily mean turbidity values increased from less than 100 FNU in mid-July 2010 to 700 FNU in early September 2010 then cause of this sustained increase in turbidity occurring over several months is unknown and does not correlate with stream discharge. Dissolved oxygen concentrations varied seasonally between 6.7 and 12.9 mg/L and inversely with temperature, with lower concentrations occurring in the summer and higher conductance, turbidity, and concentrations of dissolved oxygen the dry period, and water temperature was lower (table 5). Discharge and all measured physicochemical properties were more variable in the wet period relative to the dry period and had larger interquartile ranges (table 5). Physicochemical properties measured for grab samples 12 and 13. Physicochemical properties measured at the Guadalupe at River Road site were mostly similar to those measured continuously at the Blanco at Halifax gage, both with respect to changes from the dry and wet periods (table 5), median water temperature at the Guadalupe at River Road site during the dry period was cooler than at the Blanco at Halifax gage (table 5). In particular, during summer 2010, water temperatures at Guadalupe at River Road were cooler than those at Blanco at Halifax, which might have resulted from increased releases of colder water into the Guadalupe River from Canyon Lake. Similar to the Blanco at Halifax, median were higher during the wet period than during the dry period (table 5). Turbidity values at Guadalupe at River Road were similar between the dry and wet periods, unlike at Blanco at Halifax, although turbidity at Guadalupe at River Road was not measured in response to storms. Turbidity and dissolved oxygen concentrations at the stream sites were generally higher than at the groundwater well and spring sites, whereas Continuous discharge was measured by the USGS for other stream sites in the study area, including additional sites on the Guadalupe River, Cibolo Creek, and Dry Comal Creek (U.S. Geological Survey, 2011). Physicochemical properties measured for grab samples from other stream sites (Blanco near Kyle, Cibolo Creek, Sink Creek, and Purgatory Creek) properties at all sites where measured were characterized turbidity values compared to routine samples. Groundwater Water-table altitude and physicochemical properties continuously at hourly intervals at two wells in the vicinity of Water-table altitude at both wells was relatively stable during the dry period and increased similarly at both wells during the wet period. The lowest water-table altitude occurred at the end of the dry period, and the highest values occurred in February the Tipps and TSU-West Campus wells, respectively. At both wells, the water-table altitude gradually decreased after peak range (591 S/cm) and increased through much of the dry period, with the highest values occurring at the end of these increases had no discernible relation with water level rapidly with the onset of the wet period. At the TSU-West higher at both wells during the dry period than during the

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38 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Nov. 2008 Oct. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010Date5 0 0 1 , 0 0 0 1 , 5 0 0 2 , 0 0 0 2 , 5 0 0 3 , 0 0 0 3 , 5 0 0 4 , 0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 0 0 1 , 0 0 0 1 , 5 0 0 2 , 0 0 0 2 , 5 0 0 3 , 0 0 0 6 8 1 0 1 2 1 4 0 1 2 3 4 5 6Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Turbidity, in formazin nephelometric units Water temperature, in degrees Celsius Stream discharge, in cubic feet per second Daily average rainfall, in inches Dissolved oxygen, in milligrams per liter 1 2 3 Wet period Dry periodDischarge Water temperature Specific conductance Turbidity Dissolved oxygen Rainfall 1Major storm and identifierEXPLANATION Figure 11. Times series (November 2008–December 2010) of stream discharge, water temperature, specific conductance, turbidity, and dissolved oxygen (daily means) for U.S. Geological Survey station 08171290 (Blanco River at Halifax Ranch near Kyle, Texas), and daily average rainfall in the vicinity of San Marcos Springs, south-central Texas (mean for National Weather Service Cooperative Stations 411429, 412585, 416276, 417983, 418544, and 419815, National Oceanic and Atmospheric Administration, 2011).

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 39 May 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 201010 20 30 40 50 60 70 80 90 0 5 10 15 20 25 30 50 70 90 110 130 150 170 190 210 230 250 0 100 200 300 400 500 600 700 0 500 1,000 1,500 2,000 2,500 3,000 3,500 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 18 20 0 5 10 15 20 25 30 35 40 45 50 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0 2 4 6 8 10 12 14 0 5 10 15 20 25 30 35 100 150 200 250 300 350 400 450 500 550 0 50 100 150 200 250Formazin nephelometric units (Blanco) Concentration, in milligrams per liter Concentration, in micrograms per liter Concentration, in milligrams per liter Concentration, in milligrams per liter Concentration, in milligrams per liter Degrees Celsius Microsiemens per centimeter Formazin nephelometric units 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 Wet period Wet period Dry period Dry period Wet period Dry period X X X X 1 X EXPLANATIONOnset of major storm and identifierSurface water— Continuous data Blanco River at Halifax Ranch near Kyle, Texas (USGS station 08171290)Surface water— Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Tex. (USGS station 08171290) Blanco River—Sample collection near Kyle, Tex. (USGS station 08171300) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) Sink Creek—S a m p l e co l l e c t i o n at Sink Creek near San Marcos, Tex. (USGS station 08169932) Purgatory Creek—S a m p l e co l l e c t i o n at Mountain High Drive near San Marcos, Tex. (USGS station 08169958) Nondetection, shown at the method reporting level USGS, U.S. Geological Survey Alkalinity Strontium Chloride Sulfate Bromide Dissolved oxygen Water temperature Specific conductance Calcium Sodium Turbidity Magnesium Figure 12. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for surface-water sites sampled for the characterization of San Marcos Springs, south-central Texas.

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40 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) May 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 Aug. 201010 20 30 40 50 60 70 80 90 -16 -14 -12 -10 -8 -6 -4 -2 0 -120 -100 -80 -60 -40 -20 20 0 0.7076 0.7078 0.7080 0.7082 0.7084 0.7086 0.7088 0 2 4 6 8 10Concentration, in milligrams per liter Concentration, in micrograms per liter Strontium-87 (87Sr) to strontium-86 (86Sr), isotopic ratio Delta deuterium (D), in per mil Delta carbon-13 (13C), in per mil Magnesium (Mg) to calcium (Ca), molar ratio Strontium (Sr) to calcium (Ca), molar ratio x1030 . 2 0 0 . 4 0 0 . 6 0 0 . 8 0 1 . 0 0 1 . 2 0 1 . 4 000 0 . 2 0 . 4 0 . 6 0 . 8 1.0 1 . 2 1 . 4 1 . 6 1 . 8 2.0 2.20 0 . 0 5 0 . 10 0 . 1 5 0 . 20 0 . 2 5 0 . 30 0 . 3 5 Wet period Wet period Dry period Dry period X X X X X X 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 X EXPLANATIONOnset of major storm and identifierSurface water— Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Texas (USGS station 08171290) Blanco River—Sample collection near Kyle, Tex. (USGS station 08171300) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) Sink Creek—S a m p l e co l l e c t i o n at Sink Creek near San Marcos, Tex. (USGS station 08169932) Purgatory Creek—S a m p l e co l l e c t i o n at Mountain High Drive near San Marcos, Tex. (USGS station 08169958) Nondetection, shown at the method reporting level USGS, U.S. Geological Survey Nitrate plus nitrite Boron D 13C Fluoride Mg/Ca Sr/Ca87Sr/86Sr Figure 12. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for surface-water sites sampled for the characterization of San Marcos Springs, south-central Texas.—Continued

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 41 9/23/10 9/3/10 9/7/10 9/11/10 9/15/10 9/19/10 9/29/09 8/28/09 9/1/09 9/5/09 9/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/20/09 10/30/09 10/22/09 10/14/09 10/6/09 9/28/09Date100 150 200 250 300 350 400 450 500 550 0 500 1,000 1,500 2,000 2,500 3,000 2 3 4 5 6 7 8 9 1 0Concentration, in milligrams per liter15 20 2530Degrees Celsius0 20 40 60 80 100 120Formazin nephelometric units (springs) Microsiemens per centimeter (streams) Formazin nephelometric units (streams)4 5 0 4 7 0 4 9 0 5 1 0 5 3 0 5 5 0 5 7 0 5 9 0 6 1 0 6 3 0 6 5 0Microsiemens per centimeter at 25 degrees Celsius (springs) X X X X X X X X X X X X X X X X X X X X X XX X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X XX X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X XStorm 1 Storm 2 Storm 3Dissolved oxygen Turbidity Specific conductance Dissolved oxygen Turbidity Specific conductance Dissolved oxygen Turbidity Specific conductance Water temperature Water temperature Water temperature X EXPLANATION Onset of major storm Spring, by orifice (table1)— Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g ASurface water— Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Texas (USGS station 08171290) Blanco River—Sample collection near Kyle, Tex. (USGS station 08171300) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) Sink Creek—S a m p l e co l l e c t i o n at Sink Creek near San Marcos, Tex. (USGS station 08169932) Purgatory Creek—S a m p l e co l l e c t i o n at Mountain High Drive near San Marcos, Tex. (USGS station 08169958) Nondetection, shown at the method reporting level USGS, U.S. Geological Survey Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spring sites sampled preceding and in response to storm 1 (September 2009), storm 2 (October 2009), and storm 3 (September 2010), south-central Texas.

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42 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) 9/23/10 9/3/10 9/7/10 9/11/10 9/15/10 9/19/10 9/29/09 8/28/09 9/1/09 9/5/09 9/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/20/09 10/30/09 10/22/09 10/14/09 10/6/09 9/28/09Date0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 0 5 1 0 1 5 2 0 2 5 3 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0Concentration, in milligrams per liter Concentration, in micrograms per liter Storm 1 Storm 2 Storm 3Alkalinity Strontium Magnesium Calcium Alkalinity Strontium Magnesium Calcium Alkalinity Strontium Magnesium Calcium EXPLANATIONOnset of major storm Spring, by orifice (table1)— Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g ASurface water— Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Texas (USGS station 08171290) Blanco River—Sample collection near Kyle, Tex. (USGS station 08171300) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) Sink Creek—S a m p l e co l l e c t i o n at Sink Creek near San Marcos, Tex. (USGS station 08169932) Purgatory Creek—S a m p l e co l l e c t i o n at Mountain High Drive near San Marcos, Tex. (USGS station 08169958) USGS, U.S. Geological Survey Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spring sites sampled preceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 43 9/23/10 9/3/10 9/7/10 9/11/10 9/15/10 9/19/10 9/29/09 8/28/09 9/1/09 9/5/09 9/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/20/09 10/30/09 10/22/09 10/14/09 10/6/09 9/28/09Date0 2 4 6 8 1 0 1 2 0 5 1 0 1 5 2 0 2 5 0 5 1 0 1 5 2 0 2 5 3 0 1 4 0 2 4 6 8 1 0 1 2 1 4 0 5 1 0 1 5 2 0 2 5 0 5 1 0 1 5 2 0 2 5 3 0 3 5 0 2 4 6 8 1 0 1 2 1 4 0 5 1 0 1 5 2 0 2 5 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8Concentration, in milligrams per liter X X X XStorm 1 Storm 2 Storm 3Sodium Sodium Sodium Chloride Chloride Chloride Sulfate Sulfate Sulfate B r o m i d e B r o m i d e B r o m i d e EXPLANATIONOnset of major storm Spring, by orifice (table1)— Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g ASurface water— Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Texas (USGS station 08171290) Blanco River—Sample collection near Kyle, Tex. (USGS station 08171300) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) Sink Creek—S a m p l e co l l e c t i o n at Sink Creek near San Marcos, Tex. (USGS station 08169932) Purgatory Creek—S a m p l e co l l e c t i o n at Mountain High Drive near San Marcos, Tex. (USGS station 08169958) Nondetection, shown at the method reporting level USGS, U.S. Geological Survey X Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spring sites sampled preceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued

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44 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) 9/23/10 9/3/10 9/7/10 9/11/10 9/15/10 9/19/10 9/29/09 8/28/09 9/1/09 9/5/09 9/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/20/09 10/30/09 10/22/09 10/14/09 10/6/09 9/28/09Date0 10 20 30 40 50 60 70 80 90 0 0.5 1.0 1.5 2.0 2.5 0.7078 0.7079 0.7080 0.7081 0.7082 0.7083 0.7084 0.7085 0.7086 0.7087 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0 10 20 30 40 50 60 70 0 0.5 1.0 1.5 2.0 2.5 0.7078 0.7079 0.7080 0.7081 0.7082 0.7083 0.7084 0.7085 0.7086 0.7087 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0 10 20 30 40 50 60 70 0 0.5 1.0 1.5 2.0 2.5 0.7078 0.7079 0.7080 0.7081 0.7082 0.7083 0.7084 0.7085 0.7086 0.7087 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35Concentration, in milligrams per liter Concentration, in micrograms per liter Strontium-87 (87Sr) to strontium-86 (86Sr), isotopic ratio X X Storm 1 Storm 2 Storm 3B o r o n N i t r a t e p l u s ni t r i t e (as nitrogen)8 7S r /8 6S r F l u o r i d e F l u o r i d e F l u o r i d e N i t r a t e p l u s ni t r i t e (as nitrogen) N i t r a t e p l u s ni t r i t e (as nitrogen)8 7S r /8 6S r8 7S r /8 6S r B o r o n B o r o n EXPLANATIONOnset of major storm Spring, by orifice (table1)— Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g ASurface water— Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Texas (USGS station 08171290) Blanco River—Sample collection near Kyle, Tex. (USGS station 08171300) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) Sink Creek—S a m p l e co l l e c t i o n at Sink Creek near San Marcos, Tex. (USGS station 08169932) Purgatory Creek—S a m p l e co l l e c t i o n at Mountain High Drive near San Marcos, Tex. (USGS station 08169958) Nondetection, shown at the method reporting level USGS, U.S. Geological Survey X Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spring sites sampled preceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 45 9/23/10 9/3/10 9/7/10 9/11/10 9/15/10 9/19/10 9/29/09 8/28/09 9/1/09 9/5/09 9/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/20/09 10/30/09 10/22/09 10/14/09 10/6/09 9/28/09Date-16 -14 -12 -10 -8 -6 -4 -2 -50 -40 -30 -20 -10 0 10 0 1 2 3 4 5 6Delta deuterium (D), in per mil Delta carbon-13 (13C), in per mil Magnesium (Mg) to calcium (Ca), molar ratio Strontium (Sr) to calcium (Ca), molar ratio x1030.10 0.20 0.30 0.40 0.50 0.60 0.70 0 -16 -14 -12 -10 -8 -6 -4 -2 -35 -40 -30 -25 -20 -15 -10 0 1 2 3 4 5 6 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0 -16 -14 -12 -10 -8 -6 -4 -2 -120 -100 -80 -60 -40 -20 0 0 1 2 3 4 5 6 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0 B l a n c o River 9/2/09=1.23 B l a n c o River 9/2/09=8.35 Storm 1 Storm 2 Storm 3D 13C Mg/Ca Sr/Ca D 13C Mg/Ca Sr/Ca D 13C Mg/Ca Sr/Ca EXPLANATIONOnset of major storm Spring, by orifice (table1)— Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g ASurface water— Sample, by stream Blanco River—Sample collection at Halifax Ranch near Kyle, Texas (USGS station 08171290) Blanco River—Sample collection near Kyle, Tex. (USGS station 08171300) Guadalupe River—Sample collection at River Road near Sattler, Tex. (USGS station 08167990) Cibolo Creek—Sample collection at Farm Road 1863 below Bulverde, Tex. (USGS station 08184300) Sink Creek—S a m p l e co l l e c t i o n at Sink Creek near San Marcos, Tex. (USGS station 08169932) Purgatory Creek—S a m p l e co l l e c t i o n at Mountain High Drive near San Marcos, Tex. (USGS station 08169958) USGS, U.S. Geological Survey Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spring sites sampled preceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued

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46 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Nov. 2008 Oct. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010Date15 20 25 30 400 500 600 700 800 900 1,000 1,100 18 19 20 21 22 23 24 165 170 175 180Water-table altitude, in feet below land surface (Tipps well) Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Water temperature, in degrees Celsius Water-table altitude, in feet below land surface (TSU-West Campus well) 1 2 3 1 2 3 1 2 3 1Onset of major storm and identifier Groundwater well (table 1)—Continuous data Tipps well TSU-West Campus wellEXPLANATION Wet period Dry period Figure 14. Times series (November 2008–December 2010) of hydrologic and physicochemical data for two groundwater wells (U.S. Geological Survey stations LR [Tipps well] and LR [TSU-West Campus well]) near San Marcos Springs, south-central Texas.

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 47 Campus well declined rapidly when the well was pumped for routine water-quality sampling during the dry period. After period returned to prepumping values and, in one instance, continued to increase beyond the prepumping values until the well was pumped for the next sample. The increase in period (median value of 692 S/cm at the Tipps well and 810 S/cm at the TSU-West Campus well) were notably higher than for other Edwards aquifer wells (table 5) as well as higher indicating that both wells might be affected by mixing with a saline groundwater source. Water temperature at the Tipps well varied seasonally between 18.7 C and 21.5 C with warmer temperatures in summer months and cooler temperatures in winter months. Water temperature at the TSU-West Campus well remained constant at 22.3 C during the entire study period, which is unexpected given the variability in water level and The water-table altitude in both of the wells responded similarly to storms 1 and 2. After storms 1 and 2, water response to storm 2. The water-quality monitor in the Tipps in the Tipps well decreased substantially in response to storm well in response to storms 1 were minor, indicating that the well was not affected by pulses of freshwater recharge moving rapidly to the well. Physicochemical properties were measured for routine samples collected from four additional wells completed in the Edwards aquifer and three wells completed in the 2). Well LR (Ruby Ranch well) is completed this study as a Trinity aquifer wells since its geochemistry and wet period statistics are summarized in table 5. With few exceptions, physicochemical properties for routine samples at both Edwards aquifer and Trinity aquifer wells the wet period. Results for routine samples from the Edwards concentrations do not show notable corresponding decreases estimated). Most of the Edwards wells had higher-than-normal turbidity values (about 3 FNU) in December 2009. higher at well LR (hereinafter, the Aqua well) had an anomalously low dissolved oxygen concentration on values similar to Comal Spring 1 (for example, wells conductance values at the Aqua well were similar to values table 5). Differences in physicochemical properties between supplying the wells. Springwater As noted previously, spring discharge values represent discharge for the spring complexes, whereas physicochemical and geochemical data were collected from individual spring Marcos, Comal, and Hueco Springs during the wet period relative to the dry period (table 5). Deep, Diversion, and Weissmuller Springs, in the San Marcos Springs complex, were monitored continuously for Hueco Spring A, physicochemical properties were measured Springs, water temperature was generally stable with variations of 0.9 C at Deep Spring, 0.6 C at Diversion Spring and less than 0.1 C at Weissmuller Spring (which had a shorter monitoring interval). Water temperature at Deep Spring the summer months and the lowest values occurring during the winter months. Diversion Spring did not show seasonal variations in temperature. conductance values (median of 613 S/cm) than did Diversion or Weissmuller Springs (median for each of 595 S/cm) the wet period than in the dry period at both Deep and conductance at San Marcos Springs is discussed in detail Spring Discharge.”) Turbidity values were low (<1 FNU) at Deep, Diversion, and Weissmuller Springs throughout the study period and did not vary consistently from the dry period to the wet period (table 5). At both Deep and Diversion Springs, turbidity values higher turbidity values were measured at Weissmuller Spring, with a median of 0.6 FNU. After a rain event in mid-July 2010, turbidity values at Weissmuller Spring increased to 0.8 FNU

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48 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) May 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010Date400 450 500 550 600 650 700 750 800 850 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5Degrees CelsiusMicrosiemens per centimeterFormazin nephelometric units0 1 2 3 4 5 6 7 8 9 1 0 0 2 4 6 8 1 0 1 2 1 4Concentration, in milligrams per liter0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 , 0 0 0Concentration, in milligrams per liter Concentration, in micrograms per liter1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 03 0 , 0 0 0 3 2 , 0 0 0 3 4 , 0 0 0 3 6 , 0 0 0 3 8 , 0 0 0 4 0 , 0 0 0 4 2 , 0 0 0 4 4 , 0 0 0Concentration, in micrograms per liter (Aqua)0 0 0 2 4 6 8 1 0 1 2 5 1 0 1 5 2 0 2 5 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8Concentration, in milligrams per liter 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Wet period Dry period Wet period Dry period Wet period Dry periodAlkalinity Strontium Chloride Sulfate Bromide Dissolved oxygen Specific conductance Calcium Sodium Turbidity Magnesium Water temperature 1XOnset of major storm and identifierEdwards aquifer well (table 1)—Continuous data TSU-West CampusEdwards aquifer well (table 1)—Sample TSU-West Campus Solar Neff Aqua 4D Nondetection, shown at the method or laboratory reporting levelEXPLANATION Figure 15. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer groundwater wells sampled for the characterization of San Marcos Springs, south-central Texas.

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 49 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Nov. 2008 Nov. 2008Date10 20 30 40 50 60 70 80 90 0 0.5 1.0 1.5 2.0 2.5 3.0 -14 -12 -10 -8 -6 -4 -2 0 -30 -28 -26 -24 -22 -20 -18 -16 -17 0 0.2 0.4 0.6 0.8 1.0 0.7076 0.7078 0.7080 0.7082 0.7084 0.7086 0.7088 0 2 4 6 8 10 0.10 0 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 200 250 300 350 400 2.0 2.2 2.4 2.6 2.8 3.0Concentration, in milligrams per liter (Aqua) Concentration, in milligrams per liter Concentration, in micrograms per liter Strontium-87 (87Sr) to strontium-86 (86Sr), isotopic ratio Delta deuterium (D), in per mil Delta carbon-13 (13C), in per mil Magnesium (Mg) to calcium (Ca), molar ratio Strontium (Sr) to calcium (Ca), molar ratio x103Strontium to calcium, molar ratio x103 (Aqua) 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 X X X X X X X X X X X X Wet period Dry period Wet period Dry periodNitrate plus nitrite Boron D 13C Fluoride Mg/Ca Sr/Ca87Sr/86Sr 1XOnset of major storm and identifierEdwards aquifer well (table1)— Sample TSU-West Campus Solar Neff Aqua 4D Nondetection, shown at the method or laboratory reporting levelEXPLANATION Figure 15. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer groundwater wells sampled for the characterization of San Marcos Springs, south-central Texas.—Continued

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50 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) May 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010DateDegrees CelsiusMicrosiemens per centimeter at 25 degrees CelsiusFormazin nephelometric units Concentration, in milligrams per liter Concentration, in milligrams per liter Concentration, in micrograms per liter Concentration, in milligrams per liter0 2 4 6 8 10 12 0 5 10 15 20 25 0 5 10 15 20 25 30 35 40 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 35 40 50 100 150 200 250 300 350 400 0 2,000 4,000 6,000 8,000 10,000 12,000 0 2 4 6 8 10 12 14 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0 0 5 10 15 20 25 400 450 500 550 600 650 700 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 X X X X X X X X X X X X X X X X X X X X X X X X X Dry period Dry period Dry period Wet period Wet period Wet period Alkalinity Strontium Chloride Sulfate Bromide Dissolved oxygen Specific conductance Calcium Sodium Turbidity Magnesium Water temperature 1XOnset of major storm and identifierTrinity aquifer well (table 1)—Sample Mendez Sac-N-Pac Ruby Ranch Nondetection, shown at the method or laboratory reporting levelEXPLANATION Figure 16. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Trinity aquifer groundwater wells sampled for the characterization of San Marcos Springs, south-central Texas.

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 51 May 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 Aug. 2010 Dec. 2010Date10 20 30 40 50 60 70 80 90Concentration, in milligrams per liter Concentration, in micrograms per liter Strontium-87 (87Sr) to strontium-86 (86Sr), isotopic ratio0 0.5 1.0 1.5 2.0 2.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.7074 0.7075 0.7076 0.7077 0.7078 0.7079 0.7080 0.7081 0.7082 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 10 20 30 40 50 60 70 80 90 -14 -12 -10 -8 -6 -4 -2 0 -30 -28 -26 -24 -22 -20 -18 -16 -17Delta deuterium (D), in per mil Delta carbon-13 (13C), in per mil Magnesium (Mg) to calcium (Ca), molar ratio Strontium (Sr) to calcium (Ca), molar ratio x103 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 Dry period Dry period Wet period Wet period Nitrate plus nitrite Boron D 13C Fluoride Mg/Ca Sr/Ca87Sr/86Sr 1Onset of major storm and identifierTrinity aquifer well (table 1)—Sample Mendez Sac-N-Pac Ruby RanchEXPLANATION Figure 16. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Trinity aquifer groundwater wells sampled for the characterization of San Marcos Springs, south-central Texas.—Continued

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52 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Nov. 2008 Oct. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010Date0 1 2 3 4 5 6 590 600 610 620 630 21.2 21.4 21.6 21.8 22.0 22.2 22.4 22.6 22.8 100 150 200 250 300 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Turbidity, in formazin nephelometric units Water temperature, in degrees Celsius Discharge, in cubic feet per second Daily average rainfall, in inches 1 2 3 Wet period Dry periodDischarge Specific conductance Turbidity Rainfall Water temperature 1Onset of major storm and identifier San Marcos Springs, by orifice (table 1) Deep Spring Diversion Spring Weissmuller SpringEXPLANATION Figure 17. Times series (November 2008–December 2010) of discharge at San Marcos Springs (U.S. Geological Survey station 08170000 San Marcos Springs at San Marcos, Texas), selected physicochemical properties at San Marcos Springs orifices (Deep, Diversion, and Weissmuller Springs), and daily average rainfall in the vicinity of San Marcos Springs, south-central Texas (mean for National Weather Service Cooperative Stations 411429, 412585, 416276, 417983, 418544, and 419815, National Oceanic and Atmospheric Administration, 2011).

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 53 and remained higher throughout the rest of the study period. On 2 days, turbidity values at Deep Spring were anomalously high: on May 12, 2010, the daily mean turbidity value was 0.5 The 15-minute data for these 2 days show an increase and subsequent decrease in turbidity over the entire day. The source of these higher turbidity values is unknown. Dissolved oxygen values were inversely correlated with spring discharge at Diversion Spring (Kendall’s tau of -0.44) Deep Spring, dissolved oxygen values were similar during the dry and wet periods. Physicochemical properties showed little variability from the dry period to the wet period at Comal Spring 1 high value is unknown, but it is inconsistent with low (<0.3 FNU) turbidity values measured at this site throughout the study. Weissmuller Springs. Median dissolved oxygen values at Comal Spring 1 were slightly higher during the wet period than during the dry period (table 5). At Hueco Spring A, physicochemical conductance and turbidity values responded rapidly to storm Geochemical Variability Associated with Routine Sampling Samples were collected periodically during December 2008–December 2010 (routine samples) from two stream and Comal Spring 1). Geochemical changes during this period normal conditions. Surface Water Selected geochemical data from routine water-quality samples collected at Blanco at Halifax and Guadalupe at River in table 5. Samples from both streams showed some similar variations from the dry period to the wet period. Wet period 13 18 higher Ca and nitrate plus nitrate (NO 3 +NO 2 ) concentrations and Sr isotope ratios ( 87 Sr/ 86 Sr) (table 5). Samples from the Blanco at Halifax also had lower SO 4 concentrations during the wet period relative to the dry period. During the dry period, samples from the Blanco at Halifax had notably higher concentrations of Sr and SO 4 relative to samples from the Guadalupe at River Road. These differences might result from Trinity aquifer sourced the basins of the Blanco and Guadalupe Rivers might also contribute to differences between the geochemistry of the two streams. As discharge at Blanco at Halifax and Guadalupe at River Road increased during the transition from the dry period to the wet period, substantial changes in geochemistry occurred in the streams. Nineteen of 28 constituents (physicochemical and geochemical) from samples collected at Blanco at Halifax and at Guadalupe at River Road were statistically different when comparing the dry period to the wet period (table 5). In general, as stream discharge increased from the dry period to the wet period, dissolved and Ca concentrations increased in both streams 4 , B, and F resulting from the dilution effect of recent rainfall and runoff. Nitrate plus nitrite concentrations increased from the dry period to the wet period but remained lower than concentrations observed in Edwards aquifer wells and springs (table 5). Concentrations of NO 3 +NO 2 increased markedly at the onset of the wet period then generally decreased throughout the remainder of the wet period from the rewetting of soils following a drought (Lucey and been previously demonstrated to have occurred for the same sampling period in the Barton Springs segment of the Edwards aquifer (Mahler and others, 2011). Higher 87 Sr/ 86 Sr values for stream samples during the wet period relative to the dry with more radiogenic (higher 87 Sr/ 86 Sr values) surface soils (Musgrove and Banner, 2004). Groundwater Edwards Aquifer Selected geochemical data from routine samples wells, there are some differences in water chemistry that that supply the wells and, consequently, different water sources and different extents of water-rock interaction. Three of the wells (TSU-West Campus, Neff, and 4D) had generally similar compositions for numerous constituents TSU-West Campus well had notably higher Na, Cl, and bromide (Br) concentrations than did the other Edwards aquifer wells. The geochemistry of the Aqua well was distinct in comparison to the other Edwards aquifer wells, with higher concentrations of Mg, Sr, SO 4 3 +NO 2 13

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54 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) May 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 Aug. 2010 Dec. 2010 Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010Date0 2 0 4 0 6 0 8 0 1 0 0 1 2 0Formazin nephelometric units (Hueco A Spring)0 2 4 6 8 1 0 1 2 1 4Concentration, in milligrams per liter2 0.0 2 0 . 5 2 1.0 2 1 . 5 2 2.0 2 2 . 5 2 3.0 2 3 . 5 2 4.0 2 4 . 5 2 5.0Degrees Celsius4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0Microsiemens per centimeter at 25 degrees Celsius0 0 . 5 1.0 1 . 5 2.0 2 . 5 3.0 3 . 5 4.0 4 . 5 5.0Formazin nephelometric units0 5 1 0 1 5 2 0 2 5 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0Concentration, in milligrams per liter0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0Concentration, in micrograms per liter Concentration, in milligrams per liter0 5 1 0 1 5 2 0 2 5Concentration, in milligrams per liter0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8 0 2 4 6 8 1 0 1 2 1 4 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 X X X X X X X X X X X X X X X X X X X X X X X X X X Wet period Dry period Wet period Dry period Wet period Dry periodAlkalinity Strontium Chloride Sulfate Bromide Dissolved oxygen Specific conductance Calcium Sodium Turbidity Magnesium Water temperature 1XOnset of major storm and identifier Spring, by orifice (table 1)—Continuous data Deep Spring Diversion Spring Weissmuller Spring Spring (table1)—Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g A Nondetection, shown at the method or laboratory reporting levelEXPLANATION Figure 18. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer springs sampled for the characterization of San Marcos Springs, south-central Texas.

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 55 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Dec. 2010 Nov. 2008 Nov. 2008Date0 0.5 1.0 1.5 2.0 2.5 3.0 10 20 30 40 50 60 70 80 90 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.7076 0.7077 0.7078 0.7079 0.7080 0.7081 0.7082 0.7083 0.7084Concentration, in milligrams per liter Concentration, in micrograms per liter Strontium-87 (87Sr) to strontium-86 (86Sr), isotopic ratio-70 -60 -50 -40 -30 -20 -10 -14 -12 -10 -8 -6 -4 -2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 1 2 3 4 5 6 7 8Delta deuterium (D), in per mil Delta carbon-13 (13C), in per mil Magnesium (Mg) to calcium (Ca), molar ratio Strontium (Sr) to calcium (Ca), molar ratio x103 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 Wet period Dry period Wet period Dry periodNitrate plus nitrite Boron D 13C Fluoride Mg/Ca Sr/Ca87Sr/86Sr 1Onset of major storm and identifier Spring, by orifice (table 1)—Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g AEXPLANATION Figure 18. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer springs sampled for the characterization of San Marcos Springs, south-central Texas.—Continued

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56 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) The low NO 3 +NO 2 concentrations and lower dissolved oxygen concentrations relative to the other Edwards aquifer wells indicate that groundwater from the Aqua well likely includes a component of reduced water. Strontium concentrations for the Aqua well (median of 41,900 g/L) were particularly high relative to the other Edwards aquifer wells, as well as to aquifer with high Sr concentrations in a narrow “transition” zone near the downdip limit of the aquifer and proposed that water-rock interaction with Sr-rich minerals such as celestite or strontianite along fault contacts between Edwards aquifer and Trinity aquifer carbonates might account for these transitional compositions. The Sr composition of the Aqua well cannot be solely accounted for by mixing between Edwards aquifer fresh and saline-zone groundwater. The composition of well LR (Buda well) has also been proposed to be consistent with Oetting’s (1995) description of transitional water (Mahler and others, 2011). During the dry period, numerous geochemical constituents for well LR (hereinafter, Solar well) were distinct from the other Edwards aquifer wells (Mg, Sr, Na, Cl, SO 4 , B, F, and NO 3 +NO 2 Sr/Ca ratios, 87 Sr/ 86 Sr, 13 C isotopic compositions) Edwards aquifer wells showed little variability from the that groundwater from these wells was largely not affected by changes in hydrologic conditions and, thus, likely not the Solar well, however, numerous geochemical constituents 15), indicating that groundwater from this well was affected by mixing with a more saline groundwater source and (or) the latter part of the wet period, starting around May 2010, the geochemical composition of the Solar well returned to a composition similar to that observed during the dry period 4 /Cl ratio at the Solar well also changed whereas SO 4 /Cl ratios in the other Edwards wells decreased (table 5). Previous studies have indicated that Trinity aquifer groundwater is characterized by higher SO 4 /Cl ratios than is Edwards aquifer saline-zone groundwater (Sharp and others, 2010). The increase in SO 4 /Cl ratios in the Solar well at the onset of the wet period, accompanied by increases in ion Trinity aquifer groundwater. Trinity Aquifer Selected geochemical data for routine samples collected in table 5. The Trinity aquifer wells exhibited differences in their geochemistry that, similar to the Edwards aquifer wells, water sources or different amounts of water-rock interaction. Groundwater from the Trinity aquifer is spatially variable and generally more mineralized than that from the Edwards aquifer (Fahlquist and Ardis, 2004). Nonetheless, Cl collected from the Trinity aquifer wells were generally similar to those for samples collected from the Edwards aquifer wells (table 5), indicating that the Trinity aquifer wells were likely sourced from relatively fresh parts of the Trinity aquifer. Strontium concentrations were generally higher for the Trinity aquifer wells than for the Edwards aquifer wells (with the exception of the Aqua well) (table 5). Most geochemical to the wet period at the Trinity aquifer wells (table 5), indicating that groundwater from these wells was largely not affected by changes in hydrologic conditions or dilute surface-water recharge. Springwater Selected geochemical data for routine water-quality samples collected from San Marcos Springs (Deep, Diversion, and Weissmuller Springs), Comal Spring 1, and Hueco Spring table 5. The geochemistry of Comal Spring 1 varied little from that Comal Springs is not generally responsive to local recharge sources. These results are consistent with previous studies that have proposed that Comal Springs is largely 2008). Samples collected from Comal Spring 1 exhibit some differences in chemical composition compared to samples conductance values, higher water temperature, and higher Sr and NO 3 +NO 2 the geochemistry of samples collected from Comal Spring 1, the geochemistry of samples collected at Hueco Spring A (19 of 28 physicochemical and geochemical) were wet period (table 5). Concentrations of some constituents decreased immediately following the onset of the wet period

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 57 (for example, Mg, Sr, Na, SO 4 rapid dilution by recharging surface water. These results indicate that Hueco Springs is very responsive to changes local recharge sources. Higher turbidity values and lower concentrations of most major ions from the dry period to the wet period are consistent with dilution by recently recharged surface water. sampled during both the dry and wet periods (Deep and Diversion Springs) showed different responses to changes in hydrologic conditions. At Deep Spring, similar to Comal local recharge sources. In contrast, results for Diversion Spring indicate that it was more responsive to changes in hydrologic conditions (16 of 28 selected physicochemical dry period to the wet period). The number of constituents that similar to that for Hueco Spring A, although the geochemical variability at Diversion Spring was more muted than at Hueco Spring A, with smaller changes in most geochemical constituents (table 5). These results indicate that Diversion Spring is subject to more variability in recharge sources changes in geochemical constituents that occurred at Diversion Spring from the dry period to the wet period, however, are not SO 4 , potassium (K), B, and Br during the wet period indicate groundwater source. Samples were collected at Weissmuller composition from the dry period to the wet period could not be compared. The chemical composition of Weissmuller Spring during the wet period was similar to that of Diversion Spring Geochemical Variability in Response to Storms Samples were collected at stream and spring sites in the wet period. A series of samples was collected for all three storms from Deep and Diversion Springs. A single sample was collected from Comal Spring 1 and Hueco Spring A for storms 1 and 2. For storm 3, a series of six samples were collected from Deep, Diversion, Weissmuller, Comal Spring 1, and Hueco Spring A. Samples were collected from streams (well) samples were collected in response to storms. The three major storms evaluated for this investigation varied in size, antecedent moisture conditions, and resulting stream sampled storms, other storms during the study affected stream and spring discharge and estimated aquifer recharge (table 4). Surface Water Rapid increases in stream discharge occurred in response cm in response to storms 1 and 3 and by a lesser amount (20 S/cm) in response to storm 2, possibly because the smaller storm with wetter antecedent conditions. Increases in turbidity values were greater than 1,000 FNU for both storms 1 and 3 (no turbidity data were recorded for storm 2). Samples were collected from the Blanco at Halifax, Blanco near Kyle, Guadalupe at River Road, Cibolo Creek, Sink Creek, and Purgatory Creek sites in response to storms the Guadalupe River for storms 1 and 2, from Sink Creek for storms 2 and 3, and from Purgatory Creek for storm 3. Geochemical variability in samples from the stream sites was which marked the transition from the dry period to the wet period, geochemical changes likely were larger because of the effect of prolonged dry antecedent conditions associated with the preceding drought. For storm 3, geochemical changes likely were larger as a result of storm characteristics: storm 3 (tropical storm Hermine) was the largest storm to occur during the study period (table 3). Storm 3 resulted in the largest stream discharge increase for Blanco at Halifax and was the only storm event during the study period for which both representative of similar large storms, resulted in substantial of rainfall and runoff to the stream. Both storms 1 and 3 likely and relatively low concentrations of numerous geochemical constituents, including Mg, Sr, Na, Cl, SO 4 , Br, B, and F in the stream samples collected in response to storms relative to in response to storms, there are no nonstorm water-quality constituents measured in storm samples collected from these streams were generally low compared to those measured in

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58 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Springwater At Comal Spring 1, few changes in geochemistry were most notable difference when comparing the sample collected prior to the storm to the sample collected in response to the storm was a 0.28-mg/L increase in NO 3 +NO 2 concentration. In response to storm 3, for which a series of samples were collected from Comal Spring 1, the geochemistry of spring discharge did not change substantially. These results are consistent with the hypothesis that Comal Springs is affected by local recharge. In contrast to Comal Spring 1, substantial changes in geochemistry in response to storms comparing results for the sample collected prior to and in S/cm, turbidity increased 109 FNU, and dissolved oxygen 4 , Br, B, and F concentrations decreased substantially, the NO 3 +NO 2 concentration increased, and isotopic compositions also varied. These changes are indicative of dilution with recent surface-water recharge. Similar, although generally larger in magnitude, geochemical changes were observed at Hueco Spring A in response to storm 3, with a notable response in spring geochemistry occurring within 2 days. In contrast with storm 1, NO 3 +NO 2 concentrations at Hueco Spring A in response to storm 3 initially decreased then subsequently increased. Within several days subsequent to storm 3, most geochemical constituents at Hueco Springs had begun to return to prestorm values. The response to storms at San Marcos Springs was more similar to Comal Springs than to Hueco Springs. Spring discharge increased at Comal, Hueco, and San Marcos as in response to unsampled storms that occurred during the study period (table 4). While spring discharge increased at Comal Springs and San Marcos Springs in response to storms 1, the rate of increase was generally more muted and gradual than at Hueco Springs, consistent with a response to regional hydrologic conditions and predominantly regional recharge sources. Continuous recording of physicochemical properties at San Marcos Springs provided the opportunity to evaluate the response to numerous storm events in addition to Diversion, or Weissmuller Springs in response to storms were generally small and inconsistent (for example, in response to and Diversion Springs). Similarly, few geochemical changes F concentrations in response to storms 1 and 2. In response to storm 3, dissolved oxygen concentrations at both Deep and Diversion Springs varied, with an initial increase and then decrease in concentration. Time-series water-quality data for Weissmuller Spring in response to storm 3 showed little Interaction Between Surface Water and Groundwater Karst aquifers are characterized by extensive surfacewater and groundwater interaction. In the Edwards aquifer, recharge predominantly occurs by way of losing streams that provide direct interaction between surface water and groundwater (Sharp and Banner, 1997). Recharge sources to karst aquifers can vary as a result of seasonal changes, Ford and Williams, 2007). Previous studies in the San Antonio and Barton Springs segments of the Edwards aquifer have documented extensive surface-water/groundwater interaction and corresponding variability in geochemistry in response to changes in recharge and hydrologic conditions (Mahler and Because recent recharge is typically geochemically distinct from groundwater, chemical mixing models using constituents ion and (or) contaminant concentrations, and carbonate mineral saturation indices are often useful for distinguishing groundwater and spring discharge (for example, Scanlon and Mahler and Garner, 2009). Variations in geochemistry at San Marcos Springs were used to distinguish potential sources endmembers: local sources such as recently recharged surface saline zone or the Trinity aquifer, was also considered. Routine sample conditions during the dry and wet periods, as well as the response to storms, were evaluated. Specific Conductance and Spring Discharge contributions of different water masses with different compositions moving through the aquifer (such as recent recharge and water in storage in the aquifer) (White, 1999). conductance and associated major-ion concentrations generally decrease with increasing discharge, largely as a result of dilution by rainfall runoff (Hem, 1989). Likewise,

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Geochemistry of San Marcos Springs and Nearby Hydrologic Features 59 1/12/10 1/16/10 1/20/10 1/24/10 1/28/10 2/1/10 2/5/10 2/9/10 2/14/10 6/7/10 6/11/10 6/15/10 6/19/10 6/23/10 6/27/10 7/1/10 7/5/10 7/11/10 7/9/10 11/18/09 11/22/09 11/26/09 11/30/09 12/4/09 12/8/09 12/12/09 12/16/09 12/20/09 5/13/10 5/17/10 5/21/10 5/25/10 5/29/10 6/2/10 6/6/10 6/10/10 6/14/10 10/20/09 10/24/09 10/28/09 11/1/09 11/5/09 11/9/09 11/13/09 11/17/09 11/20/09 1/26/10 1/30/10 2/3/10 2/7/10 2/11/10 2/15/10 2/19/10 2/23/10 2/28/10Date5 9 0 6 0 0 6 1 0 6 2 0 6 3 0 6 4 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8 5 9 0 6 0 0 6 1 0 6 2 0 6 3 0 6 4 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 85 9 0 6 0 0 6 1 06 2 06 3 0 6 4 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Turbidity, in formazin nephelometric units Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Turbidity, in formazin nephelometric units Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Turbidity, in formazin nephelometric units00 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8 5 9 0 6 0 0 6 1 0 6 2 0 6 3 0 6 4 0 5 9 0 6 0 0 6 1 0 6 2 0 6 3 0 6 4 0 5 9 0 6 0 0 6 1 0 6 2 0 6 3 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8Specific conductance, in microsiemens per centimeter at 25 degrees Celsius0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 10 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8Turbidity, in formazin nephelometric units Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Turbidity, in formazin nephelometric units Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Turbidity, in formazin nephelometric units6 4 0Turbidity, in formazin nephelometric units (Weissmuller)0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X XAll values below method reporting level (0.3 formazin nephelometric units) All values below method reporting level (0.3 formazin nephelometric units) All values below method reporting level (0.3 formazin nephelometric units) All values below method reporting level (0.3 formazin nephelometric units) All values below method reporting level (0.3 formazin nephelometric units) XTime period of rainfall associated with storm Spring, by orifice (table 1) Deep Spring Diversion Spring Weissmuller Spring Nondetection, shown at the method reporting levelEXPLANATION A D B E C F Figure 19. Times series (November 2008–December 2010) of specific conductance and turbidity values at San Marcos Springs orifices (Deep, Diversion, and Weissmuller Springs), south-central Texas, preceding and in response to unsampled storms. A , Storm onset October 22, 2009. B , Storm onset November 20, 2009. C , Storm onset January 14, 2010. D , Storm onset January 28, 2010. E , Storm onset May 15, 2010. F , Storm onset June 9, 2010.

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60 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) that recharges a karst aquifer (for example, Scanlon and and others, 2007). In response to rain and recharge events, karst springs and groundwater wells that are affected by conductance, followed by a gradual increase to a value more representative of interaction with the aquifer matrix (Garner and Mahler, 2007). In the Edwards aquifer, this process is well documented: at Barton Springs, for example, a decrease Mahler and others, 2006, 2011), and similar responses have been documented at groundwater wells affected by focused Results of this study indicate that, similar to Barton Springs, Hueco Springs is affected by a large component of focused large changes in other geochemical constituents occurring in response to recharge events (for example, storms 1 and 3) at Comal Springs were small during the study period (within a range of 27 S/cm, or about 5 percent) and showed little the interpretation that Comal Springs discharge is supplied appreciably by local recharge sources. and Diversion) varied during the study period within a range and Comal Springs (because Weissmuller was monitored for a shorter period only during the wet period, this discussion is focused on Deep and Diversion Springs at San Marcos did not show rapid changes indicative of the effect of dilution San Marcos Springs occurred during longer periods such as months, which is not indicative of recharge from individual conductance at Diversion Spring decreased initially at the start of the wet period and then remained relatively constant for conductance values at Deep and Diversion Springs had a similar rise and subsequent drop during summer 2009 (in the karst spring for which discharge is dominated by a continuum conductance generally decreases with increased discharge, discharge at San Marcos Springs indicates that additional conductance generally decreased with increasing discharge for low discharge conditions (<100 ft 3 3 /s, values of more than 150 ft 3 Diversion, Weissmuller, and Deep Springs, respectively) and is consistent with an increase in more saline water contributing to San Marcos Springs. These relations between discharge Springs likely is affected by a variable mixture of water sources contributing to spring discharge that vary depending on hydrologic conditions, and that the contribution of more saline groundwater increased at higher discharge values of more than about 150 ft 3 /s. Tracers of Geochemical Evolution Processes Geochemical evolution processes that occur as a result of interactive processes between water and rock in carbonate aquifers provide insight into groundwater residence time and solution reactions, and mixing processes. Previous work in the Edwards aquifer provides a framework of understanding for geochemical evolution processes that affect geochemical and isotopic tracers such as Mg/Ca and Sr/Ca ratios and Sr Musgrove and others, 2010). As discussed in Musgrove and others (2010), Mg/Ca and Sr/Ca ratios in carbonate water-rock interaction and progressive groundwater evolution processes (for example, calcite recrystallization, incongruent dolomite dissolution, and prior precipitation of calcite along Banner, 2004). Higher Mg/Ca and Sr/Ca ratios are consistent with longer residence times and greater extents of mineralsolution reaction. Strontium isotope ratios ( 87 Sr/ 86 Sr) in the Edwards aquifer have been applied in conjunction with Mg/ Ca and Sr/Ca ratios as tracers of water-rock interaction, soil composition on groundwater geochemistry (Oetting and Wong and others, 2011). As demonstrated in these studies, Sr

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Interaction Between Surface Water and Groundwater 61 Date0 50 100 150 200 250 300Spring discharge, in cubic feet per secondNov. 2008 Oct. 2010 Dec. 2010 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010580 590 600 610 620 630 640 650Specific conductance, in microsiemens per centimeter at 25 degrees Celsius580 590 600 610 620 630 640Specific conductance, in microsiemens per centimeter at 25 degrees Celsius50 100 150 200 250 300Spring discharge, in cubic feet per second 1 2 3 B A San Marcos Springs, by orifice (table 1) Deep Spring Diversion Spring Weissmuller SpringEXPLANATION 1 Onset of major storm (sampled) and identifier Onset of major storm (unsampled) Discharge (daily mean) for San Marcos Springs (table 1) Specific conductance (daily mean), by orifice (table 1) Deep Spring Diversion Spring Weissmuller SpringEXPLANATION Figure 20. Specific conductance at San Marcos Springs, south-central Texas. A , Time series (November 2008–December 2010) of specific conductance for Deep, Diversion, and Weissmuller Springs orifices, and discharge at San Marcos Springs. B , Relation of specific conductance (Deep, Diversion, and Weissmuller Springs orifices) with discharge at San Marcos Springs.

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62 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) isotope ratios in the Edwards aquifer generally decrease with increasing water-rock interaction, approaching values similar to those of the Cretaceous-age limestone aquifer rocks, which have values ranging from 0.7074 to 0.7077 (Koepnick 87 Sr/ 86 Sr values for groundwater relative to the aquifer rocks are indicative of a source of more radiogenic Sr (enriched in 87 Sr) to the groundwater, which has been previously proposed to result from chemical interaction with overlying soils (Musgrove and Banner, 2004). Lower 87 Sr/ 86 Sr values are indicative of proportionally more interaction with limestone aquifer rocks as a result of longer groundwater residence time. At a local scale, variations in limestone composition, soil composition, these geochemical constituents. Variations in Mg/Ca and Sr/Ca ratios and Sr isotope values for the springs were evaluated to distinguish potential for Hueco Springs were consistent with marked changes in recharge sources from the dry period to the wet period. Values for these constituents at Hueco Spring A were covered a larger range than at Comal Spring 1 or San Marcos Sr isotope values during the wet period are consistent with a large component of recently recharged, less geochemically contrast with Hueco Spring A, Mg/Ca and Sr/Ca ratios and 87 Sr/ 86 Sr values at Comal Spring 1 and at Deep Spring were and wet period (table 5) but covered a small range of values 87 Sr/ 86 Sr values at Diversion and wet periods (table 5). Ratios of Mg/Ca and Sr/Ca at Diversion Spring during the dry period were similar to values ratios at Diversion Spring shifted to higher values relative to ). This shift in Mg/Ca and Sr/Ca values during the wet period is consistent with a change in the proportion of different water sources contributing to Diversion Spring: the shift is consistent with an increased contribution of more geochemically evolved, longer residence time groundwater, rather than recent recharge, such as is observed at Hueco Springs. Results for Weissmuller Spring for Mg/Ca and Sr/Ca ratios and 87 Sr/ 86 Sr values (collected during only the wet period) are similar to water supplying both Diversion and Weissmuller Springs. These results indicate that discharge sources to Diversion Spring changed from the dry period to the wet period, whereas discharge sources to Deep Spring were more constant. Endmember Mixing Using PHREEQC Results of inverse modeling with the geochemical model PHREEQC (Parkhurst and Appelo, 1999) (table 6) provide insight into interactions between surface water and groundwater, aquifer processes under different hydrologic conditions, mixing, and potential sources of discharge from San Marcos Springs. Model results approximate mixing proportions of designated source (endmember) water recharge, saline-zone groundwater, and Trinity aquifer groundwater) and mass-transfer processes along hypothetical discharge at San Marcos Springs. Inverse modeling was considered for a range of hydrologic conditions (dry, wet, endmember combinations. Endmember water compositions used in inverse modeling to account for the composition of or Weissmuller Springs) were as follows: were used to represent Edwards aquifer regional discharge (Comal Spring 1) and the composition of groundwater from well 4D, which is located in the Comal Springs Fault Block to the north (downgradient) both compositions were used (that is, both were initial solutions available to the model to include, and the model solutions might include one or both based on included either composition or a mixture of both. Both Comal Springs and well 4D are located upgradient proposed that Comal Springs is largely supplied by western parts of the aquifer and that San Marcos Springs is supplied at least in part by the same regional Johnson and Schindel, 2008). The geochemistry of Comal Springs and well 4D are similar and varied little little in response to storms that supply local recharge. On the basis of these results, the composition of Comal Springs and well 4D were considered representative of Local (stream) recharge: The composition of surface water from the Blanco River was used to represent local recharge. Saline-zone groundwater: The saline zone has multiple distinct hydrochemical facies throughout the

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Interaction Between Surface Water and Groundwater 63 0.70780 0.70785 0.70790 0.70795 0.70800 0.70805 0.70810 0.70815 0.70820Strontium-87/strontium-86 isotopic ratio Magnesium to calcium, molar ratio0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Magnesium to calcium, molar ratio0 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00Strontium to calcium, molar ratio X 103 2-sigma uncertainty B A Spring, by orifice (table 1) and period Deep Spring—Dry period Deep Spring—Wet period Diversion Spring—Dry period Diversion Spring—Wet period Weissmuller Spring—Wet period Comal Spring 1—Dry period Comal Spring 1—Wet period Hueco Spring A—Dry period Hueco Spring A—Wet periodEXPLANATION Figure 21. Relations among selected geochemical constituents for samples collected from Comal Spring 1, Hueco Spring A, and San Marcos Springs orifices (Deep, Diversion, and Weissmuller Springs), south-central Texas (November 2008–December 2010). A , Magnesium to calcium (molar ratio) and strontium to calcium (molar ratio x 10 3 ). B , Strontium-87/strontium-86 values and magnesium to calcium (molar ratio); 2-sigma uncertainty is the external error based on analyses of strontium-isotope standard.

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64 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010Oct. 2010 Dec. 2010DateStream discharge, in cubic feet per second Spring discharge, in cubic feet per second5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 0 5 0 0 1 , 0 0 0 1 , 5 0 0 2 , 0 0 0 2 , 5 0 0 3 , 0 0 0 3 , 5 0 0 4 , 0 0 0 1 2 3 1 Onset of major storm and identifier Discharge (daily mean) San Marcos Springs at San Marcos, Texas (USGS station 08170000) Blanco River at Halifax Ranch near Kyle, Tex. (USGS station 08171290) Routine sample for USGS geochemical modeling program PHREEQC Storm response sample for PHREEQC USGS, U.S. Geological Survey EXPLANATIONWet period Dry period Figure 22. Time series (November 2008–December 2010) of stream discharge for the Blanco River (U.S. Geological Survey station 08171290 Blanco River at Halifax Ranch near Kyle, Texas), spring discharge at San Marcos Springs (U.S. Geological Survey station 08170000 San Marcos Springs at San Marcos, Tex.), and timing of sample collection used for PHREEQC (Parkhurst and Appelo, 1999) geochemical modeling. Oetting and others, 1996). Several transects of wells across the freshwater/saline-water interface are located within the study area in Hays County (Fish Hatchery, San Marcos, and Kyle transects). The composition of a sample collected from a well in the San Marcos transect in May 2007 (well LR– 01A) (Lambert and others, 2009) was used to represent saline-zone groundwater in the vicinity of San Marcos Springs. Trinity aquifer groundwater: The composition of groundwater sampled from two Trinity aquifer wells was used to represent the Trinity aquifer: well Comal County and well LR (hereinafter, Eagle Peak well was sampled for the regional synoptic sampling on December 16, 2008. The Burns well was sampled three times in the dry period between

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Interaction Between Surface Water and Groundwater 65 December 2008 and March 2009 (the composition of the sample collected on December 29, 2008, was used for inverse modeling). The composition of the Trinity aquifer is spatially variable, and groundwater from the Trinity aquifer is generally more mineralized than that from the Edwards aquifer (Fahlquist and Ardis, 2004). The composition of groundwater from the Eagle Peak well was considered representative of more mineralized Trinity aquifer water in general (labeled “regional/mineralized” in table 6), whereas the composition of groundwater from the Burns well was considered representative of Trinity aquifer water in the vicinity of San Marcos Springs. Both the Eagle Peak well and the Burns well compositions had high 4 and Sr concentrations relative to Edwards aquifer groundwater (Eagle Peak: 797 S/cm, 77.7 g/L SO 4 , 4 , 2,010 g/L Sr, respectively) (Crow, 2012). Similar models including the Trinity aquifer, both Trinity aquifer endmember compositions were used (that is, both were initial solutions available to the model to include, and the model solutions might include one and model solutions included either composition or a mixture of both. Modeling Associated with Routine Sampling Four sampling periods during the dry period were selected for inverse modeling (table 6). During the dry and groundwater from well 4D) and either the saline-zone groundwater or Trinity aquifer endmembers (groundwater from the Eagle Peak and Burns wells) were mixed to approximate the composition of spring discharge from Deep and Diversion Springs. Separate models included either the saline-zone groundwater endmember or the Trinity aquifer endmembers. Additionally, although little to no surface recharge was occurring, for comparison, all models were also run with a local recharge component (the Blanco River) included in the initial water compositions. Inverse modeling results for the dry period generally indicate that the composition of discharge at both Deep and Diversion Springs that has mixed with a small (up to 0.2 percent) component of saline-zone groundwater. Some models were consistent with mixing of regional groundwater with a small component of Trinity aquifer groundwater (up to 0.6 percent). Some of the models based on mixing of regional and Trinity aquifer groundwater, however, resulted in no plausible model solutions for uncertainties up to 10 percent. Additionally, model uncertainties were typically larger for mixtures that included the Trinity aquifer endmembers relative to those that included the saline-zone groundwater endmember. Thus, model results for the dry period indicate that mixing saline-zone groundwater, rather than with a small component of Trinity aquifer groundwater, more likely accounts for the composition of San Marcos Springs. The possibility of mixing with a small component of Trinity aquifer groundwater, however, cannot be eliminated. Model results consistently indicated that contribution from a more saline water source is necessary to account for the composition of both Deep and Diversion Springs during the dry period. Better constraints on the composition and variability of both saline-zone groundwater and Trinity aquifer groundwater in the vicinity of San Marcos Springs might help distinguish between these two potential sources of saline water to San Marcos Springs. Although little to no local surface recharge was likely occurring during the dry period based on hydrologic conditions, all dry-period models were also run with a local recharge component (the Blanco River) for comparison purposes. For these model runs, the modeled proportion of local recharge ranged from 0 to 25.9 percent, although for most models the local recharge component was low (less than 5 percent). It is hydrologically implausible during the dry period that local recharge could contribute as date with the largest modeled component of local recharge 10 percent of San Marcos Springs discharge. Additionally, geochemical variability during the dry period, for example with respect to stable isotope values, is not consistent with local recharge contributing a discernable component to discharge at San Marcos Springs. A mixture including 25 percent of recharge from the Blanco River (with a D value D value of spring discharge, on the order of 4 per mil, which is not seen in the isotopic composition of either Deep or Diversion Springs during the considered hydrologically implausible. One of the dates during the dry period (September 1, 2009) was modeled with and without Br as a model-balancing constraint for Deep Spring because a larger uncertainty was needed for the model to balance with Br included. The Br concentration at Deep Spring on this date was anomalously high (0.17 mg/L) relative to other measured Br concentrations the Br concentration on this date was not accompanied by also be associated with saline-zone groundwater. Although anthropogenic Br sources might also be important source in urban areas (Hem, 1989) such as San Marcos, the source of Br at Deep Spring on this date is unknown.

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66 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008).—Continued [See table 1 for complete U.S. Geological Survey station names and numbers. ft 3 Sample collection Hydrologic condition (dry period or wet period) Date for listed dis charge condi tions San Marcos Springs discharge (ft 3 /s) (mean daily) Comal Springs discharge (ft 3 /s) (mean daily) Blanco River (Halifax Ranch station) discharge (ft 3 /s) (mean daily) PHREEQC model details 1 Initial water compositions 3 Final water composition 8 Routine sampling Dec. 2008 dry 12/1/08 102 285 10 13 Regional (Comal Springs, 12/2/08, and/ Deep Spring (12/1/08) Regional (Comal Springs, 12/2/08, and/ local (Blanco River 12/9/08) Regional (Comal Springs, 12/2/08, and/ Regional (Comal Springs, 12/2/08, and/ Diversion Spring (12/1/08) Regional (Comal Springs, 12/2/08, and/ local (Blanco River 12/9/08) Regional (Comal Springs, 12/2/08, and/ Regional (Comal Springs, 12/2/08, and/ May/June 2009 dry 5/29/09 93 220 8.1 Regional (Comal Springs, 5/27/09, and/ Deep Spring (5/27/09) Regional (Comal Springs, 5/27/09, and/ local (Blanco River 5/29/09) Regional (Comal Springs, 5/27/09, and/ Regional (Comal Springs, 5/27/09, and/ local (Blanco River 5/29/09) Regional (Comal Springs, 5/27/09, and/ Diversion Spring (5/27/09) Regional (Comal Springs, 5/27/09, and/ cal (Blanco River 5/29/09) Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008). [See table 1 for complete U.S. Geological Survey station names and numbers. ft 3

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Interaction Between Surface Water and Groundwater 67 Sample collection Model results, proportion of initial water compositions (percent) 1 Local re charge (Blanco River) 3 Regionalflow ground water (Comal Springs) 4 Region alflow ground water (well 4D) 4 Salinezone ground water 5 Trinity aquifer ground water (regional/ mineral ized) 6 Trinity aquifer ground water (local) 7 Num ber of models found, minimal models found 9 Maximum fractional uncertain ty in model (element concentrations) Comments Routine sampling Dec. 2008 40.4– 53.2 46.6– 59.4 0.1.2 12, 2 0.04 0–mi nor 40.4– 53.2 46.6– 59.4 0.1.2 12, 3 0.04 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison. 99.5 0.5 0 0.5 61, 1 0.10 include Trinity aquifer. 0.6 99.5 0.5 0–minor 0.5 34, 1 0.10 Little to no surface recharge occurring for these hydrologic conditions, but local recharge includ ed for comparison. Relatively high uncertainty needed. Few models include Trinity aquifer. 39.6– 58.3 41.7– 60.3 0.1 30, 9 0.06 0.0 46.9– 58.9 41.1– 51.5 0.1 14, 7 0.06 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison. 58.3– 59.7 40.3– 41.7 0 0.13 16, 2 0.06 0.5 50.2– 59.7 40.3– 49.8 0 0 9, 1 0.06 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison. No models include Trinity aquifer. May/June 2009 55.4– 67.0 33.0– 44.5 0.1 55, 1 0.05 10.4– 20.3 42.4– 48.6 34.4– 42.2 0.1 23,1 0.04 Little to no surface recharge occurring for these hydrologic conditions, but local recharge of local recharge unlikely based on hydrologic conditions. 96.9 3.1 0 0 33, 1 0.06 No models include Trinity aquifer. 18.4– 25.9 71.0– 77.4 0.0 0 0 13, 1 0.05 Little to no surface recharge occurring for these hydrologic conditions, but local recharge of local recharge unlikely based on hydrologic conditions. No models include Trinity aquifer. 69.4– 69.9 30.0– 30.6 0.1 57, 1 0.06 4.5– 4.6 6.5 30.4 0.1 7, 1 0.06 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison.

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68 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008).—Continued [See table 1 for complete U.S. Geological Survey station names and numbers. ft 3 Sample collection Hydrologic condition (dry period or wet period) Date for listed dis charge condi tions San Marcos Springs discharge (ft 3 /s) (mean daily) Comal Springs discharge (ft 3 /s) (mean daily) Blanco River (Halifax Ranch station) discharge (ft 3 /s) (mean daily) PHREEQC model details 1 Initial water compositions 3 Final water composition 8 Routine sampling—Continued Regional (Comal Springs, 5/27/09, and/ Regional (Comal Springs, 5/27/09, and/ local (Blanco River 5/29/09) July 2009 dry 7/21/09 86 170 3 Regional (Comal Springs, 7/16/09, and/ Deep Spring (7/21/09) Regional (Comal Springs, 7/16/09, and/ cal (Blanco River 7/17/09) Regional (Comal Springs, 7/16/09, and/ Regional (Comal Springs, 7/16/09, and/ local (Blanco River 7/17/09) Regional (Comal Springs, 7/16/09, and/ Diversion Spring (7/21/09) Regional (Comal Springs, 7/16/09, and/ cal (Blanco River 7/17/09) Regional (Comal Springs, 7/16/09, and/ Regional (Comal Springs, 7/16/09, and/ local (Blanco River 7/17/09) Aug./Sept. 2009 dry 9/1/09 85 163 2.4 Regional (Comal Springs, 8/31/09, and/ Deep Spring (9/1/09) Regional (Comal Springs, 8/31/09, and/ Regional (Comal Springs, 8/31/09, and/ cal (Blanco River 9/2/09)

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Interaction Between Surface Water and Groundwater 69 Sample collection Model results, proportion of initial water compositions (percent) 1 Local re charge (Blanco River) 3 Regionalflow ground water (Comal Springs) 4 Region alflow ground water (well 4D) 4 Salinezone ground water 5 Trinity aquifer ground water (regional/ mineral ized) 6 Trinity aquifer ground water (local) 7 Num ber of models found, minimal models found 9 Maximum fractional uncertain ty in model (element concentrations) Comments Routine sampling—Continued 90.0 0 0 0 17, 1 0.08 Relatively high uncertainty needed. No models include Trinity aquifer. 0.3– 1.2 91.3– 92.0 7.4.8 0 0–mi nor 15, 1 0.07 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included in model for comparison. Relatively high uncertainty needed. July 2009 41.9– 51.9 47.9– 57.9 0.2 9, 1 0.04 6.0– 9.2 11.8– 32.6 61.2– 78.8 0.2 8,1 0.03 Little to no surface recharge occurring for these hydrologic conditions, but local proportion of local recharge unlikely based on hydrologic conditions. -----0.10 No models. -----0.10 No models. 11.2– 14.6 85.3– 88.7 0.1 7, 1 0.05 2.6– 6.8 0.5 86.8– 96.4 0.1 18, 3 0.04 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison. 78.3– 87.2 12.8– 21.7 -0.1.6 21, 1 0.09 Relatively high uncertainty needed. 13.7– 15.6 36.1– 42.2 44.1– 48.4 -0.2 10, 1 0.07 Little to no surface recharge occurring for these hydrologic conditions, but local recharge proportion of local recharge estimated is unlikely based on hydrologic conditions. Relatively high uncertainty needed. Aug./Sept. 2009 99.8 0–minor 0.2 9, 1 0.08 Relatively high uncertainty needed. 70.2– 70.5 29.4– 29.7 0.1 8, 1 0.04 No Br in model balances (composition of Deep Spring on 9/1/09 has anomalously high Br). 0.3 66.0– 70.5 29.3– 30.9 0.1 14, 1 0.04 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison. No Br in model balances (composition of Deep Spring on 9/1/09 has anomalously high Br).

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70 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008).—Continued [See table 1 for complete U.S. Geological Survey station names and numbers. ft 3 Sample collection Hydrologic condition (dry period or wet period) Date for listed dis charge condi tions San Marcos Springs discharge (ft 3 /s) (mean daily) Comal Springs discharge (ft 3 /s) (mean daily) Blanco River (Halifax Ranch station) discharge (ft 3 /s) (mean daily) PHREEQC model details 1 Initial water compositions 3 Final water composition 8 Routine sampling—Continued Regional (Comal Springs, 8/31/09, and/ Regional (Comal Springs, 8/31/09, and/ local (Blanco River 9/2/09) Regional (Comal Springs, 8/31/09, and/ Diversion Spring (9/1/09) Regional (Comal Springs, 8/31/09, and/ local (Blanco River 9/2/09) Regional (Comal Springs, 8/31/09, and/ Regional (Comal Springs, 8/31/09, and/ local (Blanco River 9/2/09) Nov. 2009 wet (after storm 2) 11/2/09 178 289 188 Regional (Comal Springs, 11/3/09, and/ Deep Spring (11/2/09) Regional (Comal Springs, 11/3/09, and/ Regional (Comal Springs, 11/3/09, and/ Diversion Spring (11/2/09) Regional (Comal Springs, 11/3/09, and/ Feb. 2010 wet (between storms 2 and 3) 2/4/10 223 354 1,780 Regional (Comal Springs, 2/2/10, and/or Deep Spring (2/4/10) Regional (Comal Springs, 2/2/10, and/or Regional (Comal Springs, 2/2/10, and/or Diversion Spring (2/4/10) Regional (Comal Springs, 2/2/10, and/or

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Interaction Between Surface Water and Groundwater 71 Sample collection Model results, proportion of initial water compositions (percent) 1 Local re charge (Blanco River) 3 Regionalflow ground water (Comal Springs) 4 Region alflow ground water (well 4D) 4 Salinezone ground water 5 Trinity aquifer ground water (regional/ mineral ized) 6 Trinity aquifer ground water (local) 7 Num ber of models found, minimal models found 9 Maximum fractional uncertain ty in model (element concentrations) Comments Routine sampling—Continued ----0 0.10 No models (with or without Br in constraints). -----0 0.10 No models (with or without Br in constraints). 60.4– 71.1 28.8– 39.5 0.1 25,1 0.08 Relatively high uncertainty needed. 0–mi nor 56.9– 57.7 42.2– 43.0 0.1 12, 1 0.07 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison. Relatively high uncertainty needed. 84.9– 86.8 13.2– 15.1 0 0 25, 1 0.08 include Trinity aquifer. 0–mi nor 84.9– 99.7 0.3.1 0–minor 0.3 17, 1 0.09 Little to no surface recharge occurring for these hydrologic conditions, but local recharge included for comparison. Relatively high uncertainty needed. Nov. 2009 01.9 78.7– 88.9 0.2 0.1 12, 6 0.04 In time period of large and rapid increase in San 0.5 93.5 0–minor 0.0 0.7 11, 3 0.09 In time period of large and rapid increase in San relatively high uncertainty needed. 8.2– 9.6 90.4– 91.8 0–minor 0.05– 0.06 8, 1 0.04 In time period of large and rapid increase in San 3.6– 23.5 75.3– 95.7 0–minor 0.7.2 0.8 7, 1 0.05 In time period of large and rapid increase in San Feb. 2010 15.9 40.7 43.2 0.2 4, 1 0.03 In time period of large and rapid increase in San 0.1 99.9 0–minor 0.0 0–mi nor 24, 1 0.08 In time period of large and rapid increase in San aquifer included in only one model. 6.6– 10.3 72.9– 73.9 15.7– 20.4 0.1 5, 1 0.03 In time period of large and rapid increase in San -----0 0.10 In time period of large and rapid increase in San models.

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72 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008).—Continued [See table 1 for complete U.S. Geological Survey station names and numbers. ft 3 Sample collection Hydrologic condition (dry period or wet period) Date for listed dis charge condi tions San Marcos Springs discharge (ft 3 /s) (mean daily) Comal Springs discharge (ft 3 /s) (mean daily) Blanco River (Halifax Ranch station) discharge (ft 3 /s) (mean daily) PHREEQC model details 1 Initial water compositions 3 Final water composition 8 Routine sampling—Continued June 2010 wet 6/7/10 220 355 139 Regional (Comal Springs, 6/1/10, and/or Deep Spring (6/7/10) Regional (Comal Springs, 6/1/10, and/or Regional (Comal Springs, 6/1/10, and/or Diversion Spring (6/7/10) Regional (Comal Springs, 6/1/10, and/or Regional (Comal Springs, 6/1/10, and/ Weissmuller Spring (6/7/10) Regional (Comal Springs, 6/1/10, and/or Nov./Dec. 2010 wet 12/1/10 171 332 55 Regional (Comal Springs, 11/30/10, and/ Deep Spring (12/1/10) Regional (Comal Springs, 11/30/10, and/ Regional (Comal Springs, 11/30/10, and/ Diversion Spring (12/1/10) Regional (Comal Springs, 11/30/10, and/ Regional (Comal Springs, 11/30/10, and/ Weissmuller Spring (12/1/10) Regional (Comal Springs, 11/30/10, and/

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Interaction Between Surface Water and Groundwater 73 Sample collection Model results, proportion of initial water compositions (percent) 1 Local re charge (Blanco River) 3 Regionalflow ground water (Comal Springs) 4 Region alflow ground water (well 4D) 4 Salinezone ground water 5 Trinity aquifer ground water (regional/ mineral ized) 6 Trinity aquifer ground water (local) 7 Num ber of models found, minimal models found 9 Maximum fractional uncertain ty in model (element concentrations) Comments Routine sampling—Continued June 2010 2.5– 17.5 43.9– 76.5 21.0– 38.9 0.1 12, 4 0.04 near date (following) of highest discharge during study. -----0 0.10 near date (following) of highest discharge 8.1– 17.7 40.1– 57.7 33.3– 42.7 0.1 14, 4 0.03 near date (following) of highest discharge during study. 0.1 97.8 0.2 0.8 0.3 14, 2 0.09 near date (following) of highest discharge 4.9– 12.6 17.3– 59.0 34.6– 69.8 0.2 5, 1 0.03 Time period associated with falling highest discharge during study. -----0 0.10 near date (following) of highest discharge Nov./Dec. 2010 0.7 16.7– 21.4 75.8– 83.1 0.2 12, 3 0.04 tropical storm Hermine. 0.5 95.9 0.4 0 0–mi nor 9, 1 0.10 needed. 21.0– 24.4 74.4– 79.0 0.1 0.1 22, 1 0.06 tropical storm Hermine. 0.1 90.9 0.1 0 0.9 14, 1 0.10 uncertainty needed. 25.3– 28.9 68.1– 74.7 0.0 0.1 12, 1 0.06 tropical storm Hermine. -----0 0.10

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74 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008).—Continued [See table 1 for complete U.S. Geological Survey station names and numbers. ft 3 Sample collection Hydrologic condition (dry period or wet period) Date for listed dis charge condi tions San Marcos Springs discharge (ft 3 /s) (mean daily) Comal Springs discharge (ft 3 /s) (mean daily) Blanco River (Halifax Ranch station) discharge (ft 3 /s) (mean daily) PHREEQC model details 1 Initial water compositions 3 Final water composition 8 Storm sampling Storm 1 (9/9/09) start of wet response to storm 1 9/20/09 97 (storm range 86) 210 (storm range 173) 16 (storm range 2.81) Regional (Comal Springs, 9/9/09, and/ Diversion Spring (9/20/09) Regional (Comal Springs, 9/9/09, and/ 9/16/09 96 (storm range 86) 211 (storm range 173) 27 (storm range 2.81) Regional (Comal Springs, 9/9/09, and/ Diversion Spring (9/16/09) (most dilute) Regional (Comal Springs, 9/9/09, and/ Storm 2 (10/3/09) response to storm 2 10/9/09 142 (storm range 96) 265 (storm range 210) 196 (storm range 21) Regional (Comal Springs, 10/7/09, and/ Diversion Spring (10/9/09) (most dilute) Regional (Comal Springs, 10/7/09, and/ aquifer Storm 3 (9/7/10) response to storm 3 9/22/10 218 (storm range 207) 357 (storm range 313) 203 (storm range 46,620) Regional (Comal Springs, 9/8/10, and/ Diversion Spring (9/22/10) Regional (Comal Springs, 9/8/10, and/ 9/29/10 218 (storm range 207) 352 (storm range 313) 159 (storm range 46,620) Regional (Comal Springs, 9/8/10, and/ Diversion Spring (9/29/10) (most dilute) Regional (Comal Springs, 9/8/10, and/ 1 2 2 3 Composition of representative local recharge water based on samples from Blanco River at Halifax Ranch near Kyle, Texas (station number 08171290). 4 5 Composition of regional/mineralized Trinity aquifer groundwater based on sample from well LR13A, as discussed in text. 6

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Interaction Between Surface Water and Groundwater 75 Sample collection Model results, proportion of initial water compositions (percent) 1 Local re charge (Blanco River) 3 Regionalflow ground water (Comal Springs) 4 Region alflow ground water (well 4D) 4 Salinezone ground water 5 Trinity aquifer ground water (regional/ mineral ized) 6 Trinity aquifer ground water (local) 7 Num ber of models found, minimal models found 9 Maximum fractional uncertain ty in model (element concentrations) Comments Storm sampling Storm 1 (9/9/09) 8.7– 10.2 60.7– 68.1 21.7– 30.6 0.03 8, 1 0.05 Storm marks transition from dry period to wet period. 10.6 67.4 2.1 0 0 4, 1 0.05 Storm marks transition from dry period to wet 12.9– 13.1 64.1– 65.3 21.6– 23.0 0.1 4, 1 0.04 Storm marks transition from dry period to wet period. 14.8 78.1 7.0 0 0–mi nor 4, 1 0.04 Storm marks transition from dry period to wet period. Storm 2 (10/3/09) 6.4– 8.9 91.0– 93.6 0 0 5, 1 0.06 No models include saline zone. 0.5 98.3 0.3 0 0.5 19, 1 0.09 Relatively high uncertainty needed. Storm 3 (9/7/10) 0.6 19.0– 83.1 16.8– 79.9 0.1.2 10, 2 0.05 0–mi nor 99.9 0.1 0–minor 0.1 18, 2 0.09 Relatively high uncertainty needed. 0.7 15.2– 54.3 45.1– 84.6 0.1 12, 4 0.05 0.1 94.9 0.1 0–minor 0–mi nor 31, 1 0.10 Relatively high uncertainty needed. 7 Composition of local Trinity aquifer groundwater based on sample from well LR02 (Burns well) in Hays County, as discussed in text. 8 Composition of San Marcos Springs based on samples from Deep (LR–01), Diversion (LR), or Weissmuller (LR) Springs 9 1999). 10 12/19/2008.

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76 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Four sampling periods during the wet period were selected for inverse modeling, which included a large range in hydrologic conditions (table 6). One of the modeled sampling periods (February 2010) coincided with an unsampled storm D ). A similar modeling approach was used as for sampling periods the local recharge endmember, and either the saline-zone groundwater or Trinity aquifer endmembers were mixed to approximate the composition of spring discharge from Deep, Diversion, and, when sampled, Weissmuller Springs. Similar to those for the dry period, inverse modeling results for the wet period indicate that the composition of discharge at San that has mixed with a small (up to 0.2 percent) component of saline-zone groundwater. Also similar to those for the dry period, some wet-period models were consistent with mixing model uncertainties were typically larger for mixtures that included the Trinity aquifer relative to those that included the saline zone. As a result, although mixing with the Trinity aquifer is possible, modeling results indicate that mixing with hydrologically plausible. model results for the wet period generally included a component of local recharge, which ranged from 0 to 28.9 percent. For each modeled routine sampling date (dry and wet periods), the modeled range of the proportion of local recharge (for all models that included saline-zone groundwater only, not Trinity aquifer groundwater) was compared with hydrologic conditions to evaluate potential changes in response to hydrologic conditions. The median value (for the midpoint of the modeled range of the proportion of local recharge for each modeled date) for the dry period was less than that for the wet period for both Deep (3.1 and 8.0 percent, respectively) and Diversion (1.9 and 10.9 percent, respectively) Springs, which indicates that the proportion of local stream recharge contributing to San Marcos Springs likely increased from the dry period to the wet period. The difference between the dry was not sampled during the dry period and therefore was not compared). The modeled proportion of local recharge tended to be larger with larger spring discharge values, although Weissmuller Springs were considered collectively. The highest values for the modeled proportion of surface water (larger than 20 percent) were associated with the northern spring same sampling date (December 1, 2010) (table 6), but not to the wettest hydrologic conditions. These higher values are further considered in the section “Endmember Mixing Using Conservative Tracers.” In summary, inverse modeling results for routine samples indicate that San Marcos Springs discharge includes a small component of local recharge from the Blanco River. The component of local recharge is typically less than 10 percent but might be as much as 20 percent under some local stream recharge were generally associated with wetter local recharge are considered hydrologically implausible on and associated aquifer recharge. Inverse modeling results for routine samples indicate that San Marcos Springs discharge includes a small component of more saline groundwater. As a proportion of spring discharge, the percentage of salinezone groundwater from model results was relatively constant (for models considering saline-zone groundwater and not Trinity aquifer groundwater) (table 6). As a result, as spring discharge increased, the associated rate of discharge attributed results, however, estimates of the saline-zone groundwater contribution were less than 0.5 ft 3 /s. Modeling Based on Storm Sampling For storms 1, the geochemical composition of was more responsive to changes in hydrologic conditions) in response to the storms was modeled (table 6). Similar to modeling for routine samples, surface-water samples collected during storm events were used to represent the local recharge component, and the most recent samples collected from Comal Springs and well 4D were used to represent sample collected from Diversion Spring in response to each storm was modeled on the basis of the hypothesis that it might plausibly represent discharge associated with the maximum contribution of dilute surface-water recharge. For storms 1 and 3, two additional samples collected from Diversion Spring were also modeled. Inverse modeling results for storms 1 indicate that the proportion of local recharge contributing to Diversion Spring ranged from 0 to 13.1 percent (table 6). Note that this discussion is based on models including saline-zone groundwater but not Trinity aquifer groundwater (the proportion of the local recharge contribution for models incorporating Trinity aquifer groundwater was similar, from 0 to 14.8 percent). The modeled local recharge contribution range is within that modeled for routine samples and is consistent with little geochemical variability at San Marcos Springs in response to individual storm and recharge events

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Interaction Between Surface Water and Groundwater 77 0 50 100 150 200 250San Marcos Springs daily mean discharge (USGS station 08170000), in cubic feet per second0 5 10 15 20 25 30Modeled proportion of spring discharge composed of stream recharge, in percent EXPLANATIONSan Marcos Springs—Midpoint and range of stream recharge proportion, by orifice (except where range is smaller than symbol size) Deep Spring (USGS station 295322097561000) Diversion Spring (USGS station 295336097555201) Weissmuller Spring (USGS station 295322097561002) USGS, U.S. Geological Survey Springs is not notably affected by storm recharge from local recharge sources moving rapidly through transmissive to Diversion Spring was largest for storm 1, which marked the transition from the dry period to the wet period and was preceded by the driest antecedent moisture conditions during the study (table 3) (models including Trinity aquifer groundwater also had the largest proportion of local recharge contribution for storm 1). The proportion of modeled local largest storm but had the smallest proportion. Endmember Mixing Using Conservative Tracers As noted earlier in the section “Rainfall Characteristics,” stable isotope values for rain samples collected during storm 1 and, in particular, storm 3 (tropical storm Hermine) were distinctive and isotopically light relative to other rain samples. As a result, stable isotope values might provide a useful geochemical tracer of recent recharge to the aquifer. Deuterium isotopes ( D) and Cl are conservative tracers that can be used to distinguish mixing processes when endmembers are chemically distinct. Both D values and Cl Figure 23 Relation between discharge at San Marcos Springs (U.S. Geological Survey station 08170000 San Marcos Springs at San Marcos, Texas) and the modeled proportion of discharge at Deep, Diversion, and Weissmuller Springs (San Marcos Springs orifices) that is composed of stream recharge from the Blanco River based on PHREEQC (Parkhurst and Appelo, 1999) inverse modeling results (detailed in table 6). For model results that included a range of stream recharge proportions, the midpoint is shown and used for statistical analysis (included in the range for dry period conditions are model results that excluded stream recharge; thus, the minimum value for local recharge for these conditions was 0).

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78 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) concentrations tended to markedly decrease in the streams was the largest storm that occurred during the study period. Isotopically light values for rainwater during storms 1 and composition of surface-water samples collected in response to storm 3, which were also isotopically light relative to other Edwards aquifer and to wells and springs affected by recent recent recharge. Several groundwater wells appear to have been affected example, the D value for groundwater from the Solar well collected about 3 weeks after storm 3 was -28.9 per mil, a 4.3 per mil drop from the previous sample collected before about 7 weeks after storm 3), the D value for groundwater from the Solar well had largely recovered and had risen to -25.8 per mil, about 1 per mil lower than the prestorm value. This decrease and subsequent rise in D values, like a breakthrough curve, is indicative of a pulse of isotopically light recharge water mixing with groundwater. Mixing calculations based on a storm 3 recharge endmember (with a D value of -79.5 per mil, the value measured for the Blanco at the Solar well indicate that a mixture including about 8 percent of recharge from the Blanco River would account for the change in D at the Solar well in response to local recharge from storm 3. Other geochemical constituents, however, indicate that the Solar well is affected by mixing as discussed earlier in the section Geochemical Variability Associated with Routine Sampling. Other groundwater wells (for example, Neff and TSU-West Campus) showed similar but more muted responses to storm 3, with decreases of about 2 per mil in D values (the analytical uncertainty of D measurements is per mil) might be indicative of mixing with a minor component of isotopically light recent recharge. Springs (Deep, Diversion, and Weissmuller Springs) in response to storms 1 showed little variability in D values following the storms was not substantially affected by mixing with local surface-water recharge. Effects of mixing with surface-water recharge would likely be most evident for storm 3, for which stable isotope values in surface-water recharge to storm 3, D values at San Marcos Springs showed little D values was measured in samples from Diversion Spring: the prestorm D value was D decreased by 1.3 per mil (to -22.9 per mil) 2 days after the onset of storm 3, returned to a value (-21.8 per mil) similar to the prestorm value 4 days after the onset of storm 3, and dropped to -24.0 per mil 3 months after the onset of storm 3 (a 2.4-per mil decrease from the prestorm value). A decrease of 1.3 per mil would be accounted for by mixing prestorm spring discharge with about 2 percent of the Blanco River endmember collected during storm 3 (with a be accounted for by mixing prestorm spring discharge with about 4 percent of the Blanco River endmember. Similar to those from Diversion Spring, samples collected from Weissmuller Spring showed little change in D values in the D value for Weissmuller Spring had decreased by 1.7 per mil (which would be accounted for by mixing prestorm spring discharge with about 2 percent of the Blanco River endmember). These proportions of local recharge based on isotopic mixing for storm 3 are similar to the range of values determined by using PHREEQC inverse modeling (0.7 percent) for storm 3. It should be noted, however, that the measured changes in D to analytical uncertainty ( per mil), and thus the changes in response to storm 3 should be interpreted with caution. A two-component mass-balance mixing model was developed on the basis of the conservative tracers D and Cl to estimate the proportion of local (stream) recharge to spring discharge in response to storm 3. Endmember water compositions were the surface-water recharge endmember (based on the storm sample collected from Blanco at Halifax in response to storm 3) and the springwater sample collected preceding the storm. The compositions of Deep, Diversion, Weissmuller, and Hueco Spring A were evaluated in response percent increments. Model results were also compared with spring samples collected for several months after storm 3 Blanco River was a minor (less than 10 percent) component of San Marcos Springs discharge for Deep, Diversion, and Weissmuller Springs both immediately following storm 3 and River recharge increased slowly during the subsequent 3 months following the storm, approaching a maximum value results indicate that a small amount (less than 10 percent) of local recharge might contribute to San Marcos Springs hydrologic conditions but that such recharge travels slowly contribution of local recharge following storm 3 based on the two-component mixing model is less than that estimated by two-component mixing model results place further constraints on estimates of local recharge sources from PHREEQC inverse models and indicate that the proportion of local recharge is likely lower than that estimated by PHREEQC for this time period.

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Interaction Between Surface Water and Groundwater 79 0 5 10 15 20 25 0 5 10 15 20 25Chloride, in milligrams per liter Chloride, in milligrams per liter-90 -80 -70 -60 -50 -40 -30 -20 -10 0 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 -90 -80 -70 -60 -50 -40 -30 -20 -10 0Delta deuterium, in per mil Delta deuterium, in per mil120 100 80 60 40 20 0 50:50 mix 50:50 mix 50:50 mix 50:50 mix 30:70 mix 10:90 mix 70:30 mix 90:10 mix 100 percent surface water 100 percent springwater Routine sample collected after storm 3, by date September 29, 2010 October 29, 2010 December 1, 2010 USGS, U.S. Geological Survey Sample collected in response to storm 3 San Marcos Springs—Deep Spring San Marcos Springs—Diversion Spring San Marcos Springs—Weismuller Spring Hueco Spring A Hueco Spring A, modeled sample composition (described in report section “Endmember Mixing Using Conservative Tracers”) Modeled mixing line—In 10 percent increments between springwater endmember (100 percent) and surface-water endmember (100 percent) Mixing-model springwater endmember— Prestorm sample Mixing-model surface-water endmember— Storm sample, Blanco River at Halifax Ranch near Kyle, TexasEXPLANATION A. De e p Spring (USGS station 295322097561000) B. D i v e r s i o n Spring (USGS station 295336097555201) D. H u e c o Spring A (USGS station 294533098082301) C. We i ss m u l l e r Spring (USGS station 295322097561002) The modeled response at San Marcos Springs using conservative tracers is notably in contrast with the response at Hueco Springs following storm 3, where, 2 days after the onset of that storm, D values dropped from -23.9 (prestorm) to a low of -62.7, a drop of 38.8 per mil, and then gradually returned to near-prestorm values over the subsequent 3 months from a prestorm value of 17.2 mg/L to a low of 5.88 mg/L. Mixing models for the composition of Hueco Spring A in response to storm 3, based on (1) the prestorm composition of Hueco Spring A (from the sample collected on September 6, 2010) and (2) the composition of Cibolo Creek in response to storm 3, indicate that the composition of Hueco Spring A following storm 3 was dominated Mixing model results for Hueco Spring A also indicate that other sources might contribute to Hueco Springs discharge as sample results do not fall directly on the mixing line between these two endmembers. Figure 24. Relation between chloride concentration and deuterium isotopes for two-component mixing models showing proportional mixing between surface-water (stream recharge) and springwater endmembers and for samples collected in response to and subsequent to storm 3 (September 2010). A , Deep Spring (San Marcos Springs orifice). B , Diversion Spring (San Marcos Springs orifice). C , Weissmuller Spring (San Marcos Springs orifice). D , Hueco Spring A (Hueco Springs).

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80 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs The wide range of hydrologic conditions that occurred in surface-water, groundwater and spring discharge, physicochemical properties, and geochemistry–provides insight into the origin of the water discharged from San Marcos Springs. Factors Affecting Local Recharge Sources Previous studies have hypothesized that discharge at San Marcos Springs might include contributions of recharge from nearby recharging streams, including the Guadalupe River, Cibolo Creek, Dry Comal Creek, Sink Creek, Purgatory Creek, York Creek, Alligator Creek, and, in particular, the and Schindel, 2008). Estimates of annual groundwater recharge to the regional Edwards aquifer indicate that the Blanco River Basin is not a major recharge contributor relative to other recharge basins. The long-term (1934) average annual recharge estimate for the Blanco River Basin, as a proportion of total estimated aquifer recharge, is 7.1 percent of annual groundwater recharge (Edwards Aquifer Authority, 2010). Nonetheless, with a historical median recharge estimate of 35,200 acre feet per year (Edwards Aquifer Authority, 2010), the Blanco River might be a substantial local recharge in response to local storms have been previously noted in comparisons of discharge of the Blanco River and San Marcos Springs and have been interpreted to indicate that the Blanco River might be a source of discharge at San Marcos Springs Daily mean recharge estimates for the Blanco River computed for this study and daily mean discharge at San Marcos Springs correlated with a Kendall’s tau of 0.61. Similar to many surface-water features in the region, the Blanco River has large responses in recharge estimates to rain events, while discharge responses at San Marcos Springs tend to be more attenuated period is similarly correlated with recharge estimates for other potential local recharge sources, namely Cibolo Creek and Dry Comal Creek (Kendall’s tau of 0.59 and 0.70, respectively), as well as with discharge at Comal Springs, an indicator of regional hydrologic conditions (Kendall’s tau of 0.79). These correlations, however, are not necessarily indicative of the effects of local recharge. It is important to note that both the regional and local aquifer systems respond to regional rainfall and recharge events. For example, discharge at Comal Springs is similarly correlated with local upgradient recharge sources such as the Blanco River (Kendall’s tau with estimated Blanco River recharge is 0.55), which is not a hydrologically likely source of recharge to Comal Springs. These results indicate recharge events often occur throughout the regional aquifer system, and individual basin contributions are not readily distinguishable solely on the basis of temporal variations in spring discharge. The Guadalupe River has been hypothesized as a possible source of discharge at San Marcos Springs (Johnson and Schindel, 2008). As discussed in the section Climatic and Hydrologic Conditions, the Guadalupe River likely does not 8), which is consistent with previous interpretations of the small role of the Guadalupe River in recharging the Edwards the Guadalupe River and Dry Comal Creek on June 9, 2010, estimated recharge to the Edwards aquifer from Dry Comal Marcos Springs following this event there was no notable indicates little contribution to San Marcos Springs discharge likely occurred from these streams. the range of hydrologic conditions that occurred during the study indicate that these ephemeral streams did not contribute substantial recharge to the Edwards aquifer or to San Marcos Alligator Creeks, similar to Sink and Purgatory Creeks they are relatively minor ephemeral streams and likely did not contribute substantial recharge to the Edwards aquifer or to San Marcos Springs during this study. Other potential sources of discharge from San Marcos Springs include groundwater from the Trinity aquifer and from the saline zone, both of which are generally more saline in composition than is Edwards aquifer groundwater were consistently higher at San Marcos Springs (median values of 613, 595, and 595 S/cm for Deep, Diversion, and Weissmuller Springs, respectively) than at Comal Marcos Springs are consistent with modeling results that small component of more saline groundwater (table 6).

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Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs 81 DateNov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010Oct. 2010 Dec. 20101 1 0 1 0 0 1 , 0 0 0Estimated stream recharge, in cubic feet per second5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0Spring discharge, in cubic feet per second 1 2 3 1Onset of major storm and identifier San Marcos Springs daily mean discharge (USGS station 08170000) Blanco River estimated daily mean recharge— Based on USGS stations 08171000 (Blanco River at Wimberley, Texas) and 08171300 (Blanco River near Kyle, Tex.) USGS, U.S. Geological SurveyEXPLANATION Wet period Dry period Figure 25. Time series (November 2008–December 2010) of San Marcos Springs daily mean discharge (U.S. Geological Survey station 08170000 San Marcos Springs at San Marcos, Texas) and estimated daily mean recharge to the Edwards aquifer from the Blanco River, south-central Texas. Relation of Spring Geochemistry to Hydrologic Conditions Differences in the geochemistry of Comal Springs, Hueco Springs, and San Marcos Springs from the dry period to the spring discharge. During the dry period, little recharge was occurring regionally or locally, and spring discharge from groundwater. There were, however, some notable geochemical differences between the springs during the dry period that discharge. The geochemistry of Hueco Springs during the dry period was different from both Comal Springs and San 5), with higher Mg, SO 4 , B, and F concentrations and Mg/ 13 3 +NO 2 concentrations. Hueco Springs discharges from the upthrown side of the Hueco Springs Fault Block (Johnson and Schindel, paths than are Comal or San Marcos Springs. Unlike that of Comal and San Marcos Springs, the geochemistry of Hueco Springs discharge varied notably through the dry period for some constituents. For example, at Hueco Springs through D values

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82 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) increased. The increases in values of conservative constituents such as Cl and D are consistent with evaporation of recharge sources supplying Hueco Springs. Stable isotope values for streams in the study area and Hueco Springs during the dry period diverge from the global and local meteoric water lines, Comal Springs and San Marcos Springs had generally Results of geochemical modeling support this hypothesis, indicating that discharge at San Marcos Springs during the dry Nonetheless, Comal Springs had higher water temperatures and higher Sr and NO 3 +NO 2 concentrations than did either Deep or Diversion Springs. These results indicate that San sources with lower Sr and NO 3 +NO 2 concentrations and lower water temperatures. Well 4D, located between Comal Springs and San Marcos Springs in the Comal Springs Fault Marcos Springs, also had lower temperatures and lower Sr and NO 3 +NO 2 concentrations in the dry period than did Comal Springs and well 4D indicate that there is some variability in such as Otero (2007). Geochemical modeling results indicate composition of Comal Spring 1 and well 4D) largely accounts for the composition of San Marcos Springs during the dry period. The geochemistry of Deep and Diversion Springs during the dry period was similar, although some differences a component of saline groundwater: Deep Spring had slightly 4 , Sr, results indicate that, during the dry period, discharge from Deep and Diversion Springs included a minor component (<1 percent) of saline groundwater (table 6). Changes in hydrologic conditions that occurred at the beginning of the wet period, as evidenced by changes in estimated aquifer recharge and water-level altitudes, resulted in large changes in discharge from all of the studied springs (table 5). At Hueco Springs, increases in discharge during the wet period were accompanied by large changes Springs, changes in geochemistry were relatively minor with supplying spring discharge are key factors that affect the chemical variability of spring discharge (Scanlon and The characterization of karst springs ranges from those with considerable variations in springwater chemistry, resulting those with little to no variations in springwater chemistry between respective spring discharge and results for selected geochemical constituents for Comal, Hueco, and San Marcos Springs are representative of two endmember spring types for major Edwards aquifer springs: Hueco Springs discharge is Comal Springs discharge is dominated by more regionally discharge (White, 1988): for the overlapping period of record (2002) this ratio is 1.5 for Comal Springs and 2.8 for Hueco Springs. The ratio for San Marcos Springs is intermediate, with a value of 2.2. Additionally, these endmember spring types are consistent with time-series results of geochemical variability for Hueco Springs and distinct and rapid changes in response to local rainfall and Springs discharge for numerous selected constituents (19 of conditions as represented by changes in spring discharge (table 7). In contrast, the geochemistry of discharge at Comal correlated with changes in spring discharge for few constituents (5 of 24) (table 7). At San Marcos Springs, Deep and Diversion Springs responded differently to changes in hydrologic conditions (table 7). The response at Diversion Spring was similar to that at Hueco Springs in that the majority of selected geochemical spring discharge (table 7). The response at Deep Spring was Deep Spring was less responsive to changes in hydrologic conditions and more similar to Comal Springs, with 10 of correlation with spring discharge (table 7). These results discharge at Deep Spring is likely dominated by regional Although geochemical variability at Diversion Spring was correlated with discharge for the same number of constituents as at Hueco Springs, there were several notable differences in this comparison of Diversion Spring and Hueco Springs. First, the range of variability for most geochemical constituents was much less at

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Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs 83 -10 -8 -6 -4 -2 0Delta oxygen-18, in per mil-15 -10 -5 0 5Delta oxygen-18, in per mil-120 -100 -80 -60 -40 -20 0 10Delta deuterium, in per mil-80 -70 -60 -50 -40 -30 -20 -10 0Delta deuterium, in per mil Spring sample, by orifice (table 1) and period Deep Spring—Dry period Deep Spring—Wet period Diversion Spring—Dry period Diversion Spring—Wet period Weismuller Spring—Wet period Comal Spring 1—Dry period Comal Spring 1—Wet period Hueco Spring A—Dry period Hueco Spring A—Wet period B l a n c o River at Halifax Ranch near Kyle, Tex.—Dry period B l a n c o River at Halifax Ranch near Kyle, Tex.—Wet period B l a n c o River near Kyle, Tex. C i b o l o Creek at Farm Road 1863 below Bulverde, Tex. S i n k C r e e k near San Marcos, Tex. P u r g a t o r y C reek at Mountain High Drive near San Marcos, Tex.Global meteoric water line (Craig, 1961) Local meteoric water line (Pape and others, 2010) Stream sample, by surface-water site (table 1) and period G u a d a l u p e River at River Road near Sattler, Texas—Dry period G u a d a l u p e River at River Road near Sattler, Tex.—Wet periodEXPLANATION B. Springs A. Surface-water sites Figure 26. Relation between deuterium and oxygen isotopes for surface-water and spring samples, south-central Texas (November 2008–December 2010). A , Surface-water sites. B , Comal Spring 1, Hueco Spring A, and San Marcos Springs orifices (Deep, Diversion, and Weissmuller Springs). Local (Pape and others, 2010) and global (Craig, 1961) meteoric water lines are shown for comparison.

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84 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Table 7. Statistical relations for selected geochemical constituents with spring discharge for San Marcos Springs (Deep, Diversion, and Weissmuller Springs), Comal Springs, and Hueco Springs, south-central Texas (November 2008–December 2010). 3 +NO 2 13 87 Sr/ 86 Sr, strontium-87/strontium-86 3 Constituent Spring 1 San Marcos Springs—Deep Spring orifice San Marcos Springs—Diversion Spring orifice San Marcos Springs— Weissmuller Spring orifice 2 Comal Spring 1 Hueco Spring A Kendall’s tau 3 ns -0.27 ns ns ns Turbidity 3 ns ns ns ns 0.64 Dissolved oxygen ns -0.44 ns 0.36 ns Calcium ns -0.45 ns ns ns Magnesium ns 0.41 ns ns -0.64 Alkalinity ns -0.36 ns ns ns Strontium ns 0.72 ns ns -0.61 Sodium ns 0.50 ns ns -0.66 Chloride -0.24 0.62 ns ns -0.52 Sulfate -0.42 0.35 ns -0.60 -0.64 Bromide -0.28 0.67 ns ns -0.33 Boron ns 0.44 ns ns -0.56 Fluoride ns 0.42 ns ns -0.59 Potassium -0.31 ns ns ns -0.31 Silica ns -0.41 -0.55 ns ns Barium -0.45 -0.71 ns -0.47 -0.47 Lithium ns 0.51 ns ns -0.62 Uranium -0.24 ns ns ns -0.45 NO 3 +NO 2 0.40 -0.65 ns ns 0.43 Phosphorus -0.50 -0.52 ns ns 0.67 87 Sr 86 Sr -0.44 ns ns -0.32 0.55 13 C -0.25 ns ns -0.52 -0.57 Mg/Ca ns 0.52 ns ns -0.59 Sr/Ca x10 3 ns 0.65 ns ns -0.59 1 See table 1 for complete U.S. Geological Survey station names and numbers. 2 as other springs. 3 Relation based on continuous data collection at 15-minute intervals (mean daily values).

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Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs 85 Spring discharge, in cubic feet per second0 100 200 300 400 0 100 200 300 400 0 100 200 300 4000 20 40 60 80 100 120Formazin nephelometric units (Hueco Spring A)0 0 . 5 1.0 1 . 5 2.0 2 . 5 3.0 3 . 5 4.0 4 . 5 5.0Formazin nephelometric units5 7 9 1 1 1 3 1 5 1 7 1 9 2 1 2 3 2 52 4 6 8 1 0 1 2 1 4Concentration, in milligrams per liter0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0Concentration, in micrograms per liter0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Strontium to calcium, molar ratio x1030.5 1.0 1.5 2.0 2.5 3.0Concentration, in milligrams per liter0.7077 0.7078 0.7079 0.7080 0.7081 0.7082Strontium-87/strontium-86 isotopic ratio0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Magnesium to calcium, molar ratio Concentration, in milligrams per liter Concentration, in micrograms per liter Concentration, in milligrams per liter0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 5 10 15 20 25 30 35 30 40 50 60 70 80 90 0.10 0.15 0.20 0.25 0.30 0.35 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X TurbidityBromide Chloride Strontium Sodium Fluoride Sulfate Boron Nitrate plus nitrite87Sr/86Sr Mg/Ca Sr/Ca XSpring, by orifice (table 1)—Sample Deep Spring Diversion Spring Weissmuller Spring C o m a l S p r i n g 1 H u e c o S p r i n g A Nondetection, shown at the method or laboratory reporting levelEXPLANATION Figure 27. Relation between spring discharge (daily mean) for Comal, Hueco, and San Marcos Springs and selected physicochemical and geochemical constituents for samples collected from Comal Spring 1, Hueco Spring A, and San Marcos Springs orifices (Deep, Diversion, and Weissmuller Springs), south-central Texas (November 2008–December 2010).

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86 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) of local and recent surface-water recharge, whereas Diversion constituents, the correlation of constituent concentration with spring discharge at Diversion Spring was the inverse of the correlation between constituent concentrations and 27). For example, constituents such as Cl, Na, Sr, Br, and constituents such as SO 4 and F were also positively correlated with discharge at Diversion Spring (table 7). Elevated concentrations of all of these constituents are associated with more saline groundwater and were negatively correlated with consistent with Hueco Springs being diluted by an increasing component of local surface-water recharge as spring discharge increased. The positive correlation between numerous geochemical constituents and discharge at Diversion Spring discharge sources with changes in hydrologic conditions. In contrast with Hueco Springs, however, these results indicate groundwater, rather than surface-water recharge, as hydrologic conditions became wetter. Ratios of Mg/Ca and Sr/Ca at Diversion Spring are also positively and relatively strongly discharge, consistent with an increasing component of more geochemically evolved water as hydrologic conditions became Hueco Springs with spring discharge was inverse (table 7). Sources of Water to San Marcos Springs which is consistent with previous studies that have described differences between springs in the southern and northern parts of the lake (Ogden and others, 1986). During the dry period, concentrations of numerous constituents associated with more saline groundwater sources (including Sr, Na, Cl, SO 4 , B, and with a component of saline groundwater. Potential sources of saline groundwater to San Marcos Springs are the downdip 2). Geochemical modeling results consistently indicate that a small amount of saline-zone groundwater (up to 0.2 percent) and (or) Trinity aquifer groundwater (up to 1.3 percent) is needed to account for the composition of all of the modeled readily distinguish the source of saline groundwater to San geochemically plausible, mixing with saline-zone groundwater uncertainties). Relations between selected geochemical constituents and hydrologic conditions indicate that the proportion of saline groundwater contributing to San Marcos Springs increased from dry to wet conditions, in particular for with wetter hydrologic conditions, higher water levels in the Edwards aquifer, and larger spring discharges, mixing with fresh recharge sources might be expected to increase and mixing with saline groundwater sources to decrease. Previous studies, however, have noted similar relations in wells in both Springs (Garner and Mahler, 2007) segments of the aquifer. Garner and Mahler (2007) proposed that proportionally higher hydraulic heads in the Trinity aquifer relative to the Edwards aquifer during wetter hydrologic conditions might occur. The Solar well, several miles northwest of San Marcos Springs geochemical constituents such as Cl, SO 4 , B, and F increased accompanied by decreases in NO 3 +NO 2 and dissolved oxygen concentrations (indicative of mixing with more reducing groundwater), a decrease in 87 Sr/ 86 Sr values and increases in Mg/Ca and Sr/Ca ratios (indicative of mixing with more in the proportion of saline groundwater contributing to San Marcos Springs during the wet period, however, is not evident from geochemical modeling results (table 6), possibly because the proportion of saline water is generally small compared to the mass-balance uncertainty. during this study responded differently to temporal changes less responsive to changes in hydrologic conditions than discharge at Deep Spring is likely dominated by regional more affected by changes in recharge sources, which might include local surface-water recharge sources. Increases in the concentrations of Sr, Cl, SO 4 , B, and Br at Diversion however, imply that discharge during the wet period had an of discharge from Diversion Spring through the wet period became more like that of Deep Spring. Weissmuller Spring of discharge from Weissmuller Spring was similar to that of Diversion Spring, which indicates a similar origin. Results of this study indicate that recharge from local geochemistry of San Marcos Springs discharge. Rather, discharge at San Marcos Springs is dominated by regional

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Summary 87 conditions when aquifer recharge is likely occurring from local streams. Geochemical modeling results for the wet period (routine samples) yielded a range for the contribution of local recharge sources (based on the Blanco River) to San was narrower for Deep Spring (0.5 percent) than for Diversion Spring (0.4 percent), which is consistent with a more muted response at Deep Spring to changes in hydrologic conditions (table 7). The median value for the midpoint of the range of the local recharge contribution for modeled dates during the wet period was 7.8 percent for Deep Spring and 10.9 percent for Diversion Spring. Model results indicate that the proportion of local stream recharge contributing to San Marcos Springs increased from the dry period to the San Marcos Springs to storm events, when focused local For example, mixing models for storm 3, a named tropical storm (Hermine) and the largest storm to occur during the study, indicate that recharge from the Blanco River composed less than 10 percent of discharge at San Marcos Springs immediately following the storm and for several months on the higher proportion of local recharge estimated by PHREEQC and indicate that the local recharge component is likely not more than 10 percent. Mixing models indicate that San Marcos Springs is not notably affected by storm recharge from local focused recharge sources moving rapidly through time-series data for wells located to the north of San Marcos River and San Marcos Springs (Neff and Aqua wells) and that do not show marked changes in geochemistry from the sources contributing to San Marcos Springs would likely vary in their contribution with changes in hydrologic conditions, antecedent conditions for rainfall and recharge events, storm range of hydrologic conditions that occurred during this study, results indicate that discharge at San Marcos Springs is Summary The Edwards aquifer in south-central Texas is a productive and important water resource. Several large springs issuing from the aquifer are major discharge points, provide habitat for threatened and endangered species, and are locations for recreational activities. Spring discharges from two of these springs, Comal and San Marcos Springs (the as thresholds in groundwater management strategies for the Edwards aquifer. Comal Springs is generally understood to be connection of San Marcos Springs with the regional Edwards improving the understanding of the hydrogeology and sources of water to San Marcos Springs. The U.S. Geological Survey (USGS) conducted a hydrologic and geochemical study of San Marcos Springs in cooperation with the San Antonio Water System during November 2008–December 2010. The primary objective of the study was to identify and characterize sources of discharge at San Marcos Springs by evaluating hydrologic and geochemical data from streams, groundwater, and springs in the vicinity of San Marcos Springs in Bexar, at San Marcos Springs (Deep, Diversion, and Weissmuller Springs) that were selected to be representative of larger springs within the spring complex. An initial sampling effort characterized surface water, groundwater, and springs in the study area. A subset of sampling to characterize temporal changes in water quality in (Deep, Diversion, and Weissmuller Springs). To characterize changes in water quality in response to storms, samples were storms (storms 1) from nearby streams that might contribute recharge to San Marcos Springs and from Comal, Hueco, and San Marcos Springs. The storms varied in size, antecedent moisture conditions, and resulting stream (discharge and recharge) and spring (discharge) response. Storm 1 marked the transition from the dry period to the wet period and occurred following the driest antecedent moisture conditions. Storm 3, a named tropical storm (Hermine), was the largest climatic and hydrologic event during the study with respect to rainfall Collection of routine and storm-associated samples from streams, wells, and springs over the 25 months of the study provided an opportunity to investigate the hydrogeology of San Marcos Springs under a large range of hydrologic conditions. In addition to routine and storm sample collection, discharge and selected physicochemical properties were measured continuously at a site on the Blanco River and at and selected physicochemical properties were measured continuously at two wells near San Marcos Springs. During this study, hydrologic conditions changed from exceptional drought to wetter-than-normal conditions. In this report, the period between November 1, 2008, and September 8, 2009, is referred to as the “dry period,” and the period between September 9, 2009, and December 31, 2010, is referred to as the “wet period.” Hydrologic and geochemical variability at San Marcos Springs was compared with that at Comal Springs and Hueco Springs, which is illustrative based on the small range of variability observed at Comal Springs and the large range of variability observed at Hueco Springs.

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88 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Streams in the vicinity of San Marcos Springs were evaluated as potential recharge sources. Recharge estimates were computed daily for the Blanco River (2009), Cibolo Creek (2008), and Dry Comal Creek (2008) by using discharge at stations in each basin. Gain/loss estimates also were computed for the Guadalupe River. For the Blanco River, recharge estimates were compared for two station pairs (at Wimberley and Kyle and at Halifax and Kyle), and results were similar. Recharge estimates for these local streams indicate that the amount of recharge to the aquifer varied markedly through the study period with the largest recharge occurring from Dry Comal Creek and the smallest from the Blanco River. The Guadalupe River was largely a gaining stream, which is consistent with previous hypotheses that it does not contribute substantial recharge to the Edwards aquifer or to San Marcos Springs. Sink Creek and Purgatory Creek were dry during most of the study and did not contribute substantial recharge to the Edwards aquifer or to San Marcos The geochemistry of surface water in sampled streams varied markedly through the study period from the dry period to the wet period and in response to changes in rainfall and corresponding stream discharge. Large and rapid decreases response to rain events. Geochemical constituents in surfacewater samples, including major ions, trace elements, and isotopic compositions, changed following the onset of the wet period in response to dilution from increased rainfall and runoff. two groundwater wells near San Marcos Springs changed following the onset of the wet period: water-table altitudes aquifer groundwater wells during the dry period, indicative of contributions from a saline groundwater source. Most groundwater wells in the Edwards aquifer and the Trinity aquifer showed few geochemical changes from the dry period to the wet period. These results indicate that sampled wells were not affected by focused local recharge moving along (LR), where numerous geochemical constituents change markedly at the beginning of the wet period, indicating that groundwater from this well was affected by mixing with a different and more saline groundwater source and (or) the part of the wet period to the end of the study, the geochemical composition of the Solar well returned to a composition similar to that observed during the dry period. Differences in the geochemistry of Comal Springs, Hueco Springs, and San Marcos Springs from the dry period to the sources supplying the springs. During the dry period, little recharge was occurring regionally or locally, and spring draining of matrix groundwater. There were, however, some notable geochemical differences between the springs during and sources of spring discharge. The geochemistry of Hueco Springs during the dry period differed from that of Comal Springs and San Marcos Springs and also varied notably recharge sources supplying Hueco Springs. The geochemistry of Comal Springs and San Marcos Springs was generally similar during the dry period, which is consistent with regional differences between the geochemistry of Comal Springs and San Marcos Springs, however, which indicate that San Marcos temperature and lower concentrations of strontium and nitrate Comal Springs and San Marcos Springs in the Comal Springs San Marcos Springs, also had lower temperature and lower concentrations of strontium and nitrate plus nitrite. Samples from Comal Springs and well 4D, which are upgradient from San Marcos Springs, are likely representative of regional At San Marcos Springs, the geochemistry of the Deep and although some differences indicate that Deep Spring was more Changes in hydrologic conditions at the beginning of the wet period were characterized by large changes in spring discharge at all of the springs (Comal, Hueco, and San Marcos Springs). At Hueco Springs, increases in discharge during the wet period were accompanied by large changes in geochemistry. Changes in geochemistry at Comal and San Marcos Springs were minor in comparison, with fewer between the dry and wet periods and mostly nominal changes in response to storms. Comal and Hueco Springs are representative of two endmember spring types, with Hueco Springs dominantly affected by more locally sourced These endmember spring types are consistent with timeseries results of geochemical variability for Hueco Springs and Comal Springs. At San Marcos Springs, Deep and

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References 89 in hydrologic conditions. Deep Spring was not strongly indicates that, similar to Comal Springs, discharge at Deep paths. Diversion Spring was more responsive to changes in hydrologic conditions than was Deep Spring, although the range of variability for most geochemical constituents was small, indicating that Diversion Spring was affected by small changes in discharge sources as hydrologic conditions changed. For many geochemical constituents the correlation with spring discharge at Diversion Spring was inverse to that for Hueco Springs, which indicates that, rather than dilute more saline groundwater component. Inverse modeling with the geochemical model PHREEQC was used to evaluate the potential for mixing of different source-water compositions (regional groundwater Trinity aquifer groundwater) and mass-transfer processes (mineral dissolution/precipitation and ion exchange) that could account for the composition of discharge from San Marcos Springs (Deep, Diversion, and Weissmuller Springs during the wet period yielded a range for the contribution of San Marcos Springs discharge from 0 to less than 30 percent. Additional two-component mixing models using conservative tracers further constrain these results and indicate that the proportion of local recharge is likely lower than the highest values estimated by PHREEQC. The modeled contribution of local stream recharge was narrower for Deep Spring than for Diversion Spring, which is consistent with a more muted response at Deep Spring to changes in hydrologic conditions. The median value for the midpoint of the range of the local recharge contribution for modeled dates (using PHREEQC) during the wet period was 7.8 percent for Deep Spring and 10.9 percent for Diversion Spring. The modeled proportion of local stream recharge accounting for San Marcos Springs discharge increased from the dry period to the wet period. The geochemical response at San Marcos Springs to storm events, when focused local recharge is most likely to occur, was small. Stable isotope values for rainfall and stream samples associated with storm 3 were distinct from tracer of recent recharge. Mixing models for storm 3 indicate that recharge from the Blanco River composed less than 10 percent of discharge at San Marcos Springs directly following the storm and for several months afterwards. These results indicate that the effect of storm recharge from local focused paths to San Marcos Springs is small. This conclusion is further supported by time-series data for wells located to the paths between the Blanco River and San Marcos Springs and that do not show marked changes in geochemistry from the dry period to the wet period. The geochemistry of water samples collected routinely and in response to storms from streams, groundwater wells, and springs was used to characterize sources of discharge from San Marcos Springs. Recharge from local surface-water San Marcos Springs discharge. Rather, results of this study indicate that discharge at San Marcos Springs is dominated wet hydrologic conditions when aquifer recharge is occurring from local streams. A small component of saline groundwater contributes to San Marcos Springs discharge under all hydrologic conditions. References Abbott, P.L., and Woodruff, C.M., Jr., 1986, eds., The social development in central Texas: Geological Society of America, 200 p. Ashworth, J.B., and Hopkins, Janie, 1995, Aquifers of Texas: Texas Water Development Board Report 345, 69 p. limestone terrain in the Mendip Hills, Somerset (Great Britain): Journal of Hydrology, v. 35, p. 9310. Barker, R.A., and Ardis, A.F., 1996, Hydrogeologic framework of the Edwards-Trinity aquifer system, west-central Texas: Brune, Gunnar, 1975, Major and historical springs of Texas: Texas Water Development Board Report 189, 94 p. City of Austin, 1997, The Barton Creek report: City of Austin, Water Quality Report Series, 460 p. Clement, T.J., 1989, Hydrochemical facies of the badwater zone of the Edwards aquifer, central Texas: Austin, Tex., University of Texas at Austin, M.A. thesis, 168 p. Coplen, T.B., Hopple, J.A., Bhlke, J.K., Peiser, H.S., Rieder, S.E., Krouse, H.R., Rosman, K.J.R., Ding, T., Vocke, R.D., Jr., Rvsz, K.M., Lamberty, A., Taylor, P.D.P., and De Bivre, P., 2002, Compilation of minimum and maximum isotope ratios of selected elements in naturally occurring terrestrial materials and reagents: U.S. Geological Survey Water-Resources Investigations Report 01, 98 p.

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90 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Craig, Harmon, 1961, Isotopic variations in meteoric waters: Science, v. 133, p. 1702. Crow, C.L., 2012, Geochemical and hydrologic data for San Marcos Springs recharge characterization near San Marcos, Texas, November 2008–December 2010: U.S. Geological Survey Data Series 672, 19 p., 16 appendixes. DeCook, K.J., 1956, Geology of San Marcos Springs quadrangle, Hays County, Texas: Austin, Tex., University of Texas at Austin, M.A. thesis, 90 p. DeCook, K.J., 1960, Geology and ground-water resources of Hays County, Texas: Texas Board of Water Engineers, Bulletin 6004, 167 p. Desmarais, Kathryn, and Rojstaczer, Stuart, 2002, Inferring source waters from measurements of carbonate spring responses to storms: Journal of Hydrology, v. 260, p. Edwards Aquifer Authority, 2010, Hydrologic Data Report for 2009, Edwards Aquifer Authority, San Antonio, Texas, 340 p. Edwards Aquifer Research and Data Center, 2010, Threatened and endangered species in the Edwards aquifer system: accessed January 29, 2010, at http://www.eardc.txstate.edu/ about/endangered.html . Fahlquist, Lynne, and Ardis, A.F., 2004, Quality of water in the Trinity and Edwards aquifers, south-central Texas, Report 2004, 17 p. Fairchild, I.J., Borsato, Andrea, Tooth, A.F., Frisia, Silvia, Hawkesworth, C.J., Huang, Yiming, McDermott, Frank, and Spiro, Baruch, 2000, Controls on trace element Implications for speleothem climatic records: Chemical Geology, v. 166, p. 255. Ford, Derek, and Williams, Paul, 2007, Karst hydrogeology and geomorphology: Chicester, England, Wiley, 562 p. Fritz, Peter, and Fontes, J.C., eds., 1980, Handbook of environment: Amsterdam, Elsevier, 545 p. Garner, B.D., 2005, Geochemical evolution of ground water in the Barton Springs segment of the Edwards aquifer: Austin, Tex., University of Texas at Austin, M.S. thesis, 317 p. by wells, and associated major ion and nitrate geochemistry, Barton Springs segment of the Edwards aquifer, Austin, Investigations Report 2007, 39 p. Groschen, G.E., and Buszka, P.M., 1997, Hydrogeologic framework and geochemistry of the Edwards aquifer salinewater zone, south-central Texas: U.S. Geological Survey Water-Resources Investigations Report 97, 47 p. Guyton, W.F. and Associates, 1979, Geohydrology of Comal, San Marcos, and Hueco Springs: Austin, Tex., Texas Department of Water Resources Report 234, 85 p. Harden, R.W., 1968, Review of water quality changes in the Edwards reservoir, especially near the bad water line: Austin, Tex., R.W. Harden and Associates, Inc., 23 p. Helsel, D.R., and Hirsch, R.M., 2002, Hydrologic analysis U.S. Geological Survey Techniques of Water-Resources Investigations, book 4, chap. A3, accessed September 2008, at http://pubs.usgs.gov/twri/twri4a3/html/pdf_new.html . Hem, J.D., 1989, Study and interpretation of the chemical characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 264 p. Herczeg, A.L., and Edmunds, W.M., 2000, Inorganic ions as tracers, in Cook, P.G., and Herczeg, A.L., eds., Environmental tracers in subsurface hydrology: Dordrecht, The Netherlands, Kluwer Academic Publishing, p. 31. Johnson, S.B., and Schindel, G.M., 2008, Evaluation of the option to designate a separate San Marcos pool for critical period management: San Antonio, Tex., Edwards Aquifer Authority, 109 p. San Marcos Springs, Texas: San Antonio, Tex., Edwards Aquifer Authority, 139 p. Kennedy, E.J., 1983, Computation of continuous records Water-Resources Investigations, book 3, chap. A13, 53 p., available at http://pubs.usgs.gov/twri/twri3-a13/.

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References 91 Kennedy, E.J., 1984, Discharge ratings at gaging stations: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A10, 59 p., available at http://pubs.usgs.gov/twri/twri3-a10/. Klemt, W.B., Knowles, T.R., Edler, G.R., and Sieh, T.W., 1979, Ground-water resources and model applications for the Edwards (Balcones fault zone) aquifer in the San Antonio region: Texas Water Development Board Report 239, 88 p. Koepnick, R.B., Burke, W.H., Denison, R.E., Hetherington, E.A., Nelson, H.F., Otto, J.B., and Waite, L.E., 1985, Construction of the seawater 87 Sr/ 86 Sr curve for the Geology (Isotope Geoscience Section), v. 58, p. 55. Lakey, Barbara, and Krothe, N.C., 1996, Stable isotopic variation of storm discharge from a perennial karst spring, Indiana: Water Resources Research, v. 32, Lambert, R.B., Hunt, A.G., Stanton, G.P., and Nyman, M.B., 2009, Water-level, borehole geophysical log, and waterquality data from wells transecting the freshwater/salinewater interface of the San Antonio segment of the Edwards aquifer, south-central Texas, 1999: U.S. Geological July 26, 2011, at http://pubs.usgs.gov/ds/403/ . Lawrence, J.R., 1998, Isotopic spikes from tropical cyclones paleoclimatology: Chemical Geology, v. 144, p. 153. Lawrence, J.R., and Gedzelman, S.D., 1996, Low stable isotope ratios of tropical cyclone rains: Geophysical LBG-Guyton Associates, 1995, Edwards aquifer ground-water divides assessment, San Antonio region, Texas: San Antonio, Tex., Edwards Underground Water District Report 95, 35 p. LBG-Guyton Associates, 2004, Evaluation of augmentation methodologies in support of in-situ refugia at Comal and San Marcos Springs, Texas: Report prepared for the Lindgren, R.J., Dutton, A.R., Hovorka, S.D., Worthington, S.R.H., and Painter, Scott, 2004, Conceptualization and simulation of the Edwards aquifer, San Antonio region, Report 2004, 143 p. Lohmann, K.C., 1988, Geochemical patterns of meteoric diagenetic systems and their application to studies of paleokarst, in James, N.P., and Choquette, P.W., eds., Paleokarst: New York, Springer-Verlag, p. 58. Lowry, R.L., 1955, Recharge to Edwards ground-water reservoir: Consultant report to San Antonio City Water Board, 66 p. Lucey, K.J., and Goolsby, D.A., 1993, Effects of climatic variations over 11 years on nitrate-nitrogen concentrations in the Raccoon River, Iowa: Journal of Environmental Quality, v. 22, no. 1, p. 38. Mace, R.E., Chowdhury, A.H., Anaya, Roberto, and Way, S.C., 2000, Groundwater availability of the Trinity aquifer, Hill Texas Water Development Board Report 353, 169 p. Maclay, R.W., 1995, Geology and hydrology of the Edwards aquifer in the San Antonio area, Texas: U.S. Geological Survey Water-Resources Investigations Report 95, 64 p. in the Edwards aquifer, San Antonio region, Texas, and Maclay, R.W., and Small, T.A., 1983, Hydrostratigraphic subdivisions and fault barriers of the Edwards aquifer, south-central Texas, U.S.A.: Journal of Hydrology, Mahler, B.J., 2008, Statistical analysis of major ion and trace element geochemistry of water, 1986, at seven wells transecting the freshwater/saline-water interface of the Edwards aquifer, San Antonio, Texas: U.S. Geological Mahler, B.J., and Garner, B.G., 2009, Using nitrate to

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92 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008) Mahler, B.J., Garner, B.D., Musgrove, M., Guilfoyle, Amber, and Rao, M.V., 2006, Recent (2003) water quality of Barton Springs, Austin, Texas, with emphasis on factors affecting variability: 2006, 83 p., 5 appendixes. Mahler, B.J., and Massei, Nicolas, 2007, Anthropogenic contaminants as tracers in an urbanizing karst aquifer: Mahler, B.J., Musgrove, M., Sample, T.L., and Wong, C.I., 2011, Recent (2008) water quality in the Barton Springs segment of the Edwards aquifer and its contributing zone, central Texas, with emphasis on factors affecting Investigations Report 2011, 66 p. Martin, J.B., and Screaton, E.J., 2001, Exchange of matrix and conduit water with examples from the Floridan aquifer, in Kuniansky, E.L., ed., U.S. Geological Survey Karst Interest U.S. Geological Survey Water-Resources Investigations Massei, Nicolas, Mahler, B.J., Bakalowicz, Michel, Fournier, Matthieu, and Dupont, J.P., 2007, Quantitative interpretation augmentation of Comal Springs and San Marcos Springs, Texas: Phase I Feasibility Study, Center for Research in Water Resources Technical Report 247, 416 p. Musgrove, M., and Banner, J.L., 2004, Controls on the spatial and temporal variability of vadose dripwater Geochimica et Cosmochimica Acta, v. 68, p. 1007. Musgrove, M., Fahlquist, L., Houston, N.A., Lindgren, R.J., and Ging, P.B., 2010, Geochemical evolution processes and water-quality observations based on results of the National Water-Quality Assessment Program in the San Antonio segment of the Edwards aquifer, 1996: http://pubs.usgs.gov/ sir/2010/5129/ .) National Aeronautics and Space Administration , 2012, Hurricanes/tropical cyclones, past year archives: accessed May 28, 2012, at http://www.nasa.gov/mission_pages/ hurricanes/archives/2010/past-years-2010.html. National Oceanic and Atmospheric Administration, 2011, National Climatic Data Center, climatological data for cooperative stations 411429, 412585, 416276, 417983, 418544, and 419815: accessed February 3, 2011, at http://www.ncdc.noaa.gov/oa/ncdc.html . estimation of recharge to the Edwards aquifer in the Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, 1951: U.S. Geological Survey Oetting, G.C., 1995, Evolution of fresh and saline interaction: Austin, Tex., University of Texas at Austin, M.A. thesis, 204 p. Oetting, G.C., Banner, J.L., and Sharp, J.M., Jr., 1996, Geochemical evolution of saline groundwaters in the tectonic, and hydrodynamic controls: Journal of Hydrology, v. 181, p. 251. Ogden, A.E., Quick, R.A., Rothermel, S.R., and Lundsford, D.L., 1986, Hydrological and hydrochemical investigation of the Edwards aquifer in the San Marcos area, Hays County, Texas: San Marcos, Tex., Edwards Aquifer Research and Data Center, 364 p. Ogden, A.E., Spinelli, A.J., and Horton, Jack, 1985a, Hydrologic and hydrochemical data for the Edwards aquifer in Hays and Comal Counties, October 1981 to September 1983: San Marcos, Tex., Southwest Texas State University, Edwards Aquifer Research and Data Center Report R1, 102 p. Ogden, A.E., Spinelli, A.J., and Horton, Jack, 1985b, Hydrologic and hydrochemical data for the Edwards aquifer in Hays and Comal Counties, October 1983 to June 1985: San Marcos, Tex., Southwest Texas State University, Edwards Aquifer Research and Data Center Report R2, 83 p.

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