U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the San Antonio Water System

Origin and Characteristics of Discharge at San Marcos 

Springs Based on Hydrologic and Geochemical Data 

(2008–10), Bexar, Comal, and Hays Counties, Texas

Scientific Investigations Report 2012–5126
Front cover: U.S. Geological Survey personnel and a diver from Texas State University near Weissmuller Spring, Spring Lake, San Marcos Springs, 

Hays County, Texas.

Back cover:

Left, Spring Lake, San Marcos Springs, near Deep Spring, Hays County, Texas.

Right, The Blanco River, downstream from Halifax Ranch, Hays County, Texas.
Origin and Characteristics of Discharge at 

San Marcos Springs Based on Hydrologic 

and Geochemical Data (2008–10), Bexar, 

Comal, and Hays Counties, Texas

By MaryLynn Musgrove and Cassi L. Crow

Prepared in cooperation with the San Antonio Water System

Scientific Investigations Report 2012–5126

U.S. Department of the Interior

U.S. Geological Survey
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 (2008–10), Bexar, Comal, and Hays Counties, Texas: U.S. Geological Survey 

Scientific Investigations Report 2012–5126, 94 p.
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Hydrogeologic Setting .........................................................................................................................4

Summary of Previous Studies.............................................................................................................6

Methods of Investigation..............................................................................................................................9

Study Design..........................................................................................................................................9

Sample Collection, Analytical Methods, and Quality Control......................................................16

Rainfall Estimation ..............................................................................................................................17

Streamflow Measurements and Recharge Estimation ................................................................17

Numerical and Statistical Methods.................................................................................................18

Climatic and Hydrologic Conditions .........................................................................................................19

Storm Characteristics ........................................................................................................................19

Rainfall Characteristics ....................................................................................................................19

Stream Recharge................................................................................................................................29

Geochemistry of San Marcos Springs and Nearby Hydrologic Features..........................................37

Hydrologic and Physicochemical Data...........................................................................................37

Surface Water............................................................................................................................37

Groundwater...............................................................................................................................37

Springwater................................................................................................................................47

Geochemical Variability Associated with Routine Sampling .....................................................53

Surface Water............................................................................................................................53

Groundwater...............................................................................................................................53

Edwards Aquifer ...............................................................................................................53

Trinity Aquifer ....................................................................................................................56

Springwater................................................................................................................................56

Geochemical Variability in Response to Storms............................................................................57

Surface Water............................................................................................................................57

Springwater ...............................................................................................................................58

Interaction Between Surface Water and Groundwater........................................................................58

Specific Conductance and Spring Discharge ...............................................................................58

Tracers of Geochemical Evolution Processes ...............................................................................60

Endmember Mixing Using PHREEQC...............................................................................................62

Modeling Associated with Routine Sampling.......................................................................65

Modeling Based on Storm Sampling......................................................................................76

Endmember Mixing Using Conservative Tracers ..........................................................................77

Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs .........................80

Factors Affecting Local Recharge Sources ...................................................................................80

Relation of Spring Geochemistry to Hydrologic Conditions ........................................................81

Sources of Water to San Marcos Springs......................................................................................86

Summary........................................................................................................................................................87

References ...................................................................................................................................................89
iv

Figures

1. Study area and locations of water-quality sampling and 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, 2008–10...................................26

8. Time series (November 2008–December 2010) of gain (positive values) 

and loss (negative values) for the Guadalupe River, south-central Texas........................27

9. Relation between deuterium and oxygen isotopes for rainfall samples 

collected at U.S. Geological Survey station 293146982941, Bexar County, 

south-central Texas (2008–10)..................................................................................................28

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 ......................................................................................................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

12. 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
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
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 (2008–10) ...........................................................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 (m2)

acre 0.004047 square kilometer (km2)

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

million gallons (Mgal) 3,785 cubic meter (m3)

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/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)
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 (m3) 264.2 gallon (gal)

cubic centimeter (cm3) 0.06102 cubic inch (in3)

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 (m3/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=(1.8×°C)+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 

(μS/cm at 25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
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):

δX = (Rsample /Rstandard -1) x 1,000, 

where 

δ 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 δ 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

HCO3 bicarbonate

B boron

Br bromide

Ca calcium

Cl chloride

δ13C delta carbon-13

δD delta deuterium 

δ18O delta oxygen-18

F fluoride

FNU formazin nephelometric units

Mg magnesium

Mg/Ca magnesium to calcium, molar ratio

μg/L micrograms per liter

μS/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

NO3 nitrate

NO2 nitrite

NO2+NO3 nitrite plus nitrate

N nitrogen

K potassium

Na sodium

Sr strontium 87Sr/86Sr strontium isotope ratio

Sr/Ca strontium to calcium, molar ratio

SO4 sulfate
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 

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 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 

Springs; variability at San Marcos Springs was intermediate. 

Comal Springs and Hueco Springs are representative of 

two endmember Edwards aquifer spring types, with Hueco 

Springs predominantly affected by local flowpaths and locally 

sourced recharge and Comal Springs predominantly affected 

by regional flowpaths and regionally sourced recharge. The 

geochemistry of discharge at San Marcos Springs from three 

of its orifices—Deep, Diversion, and Weissmuller Springs—

differed among the orifices; these differences in geochemistry 

are indicative of differences in the sources and flow paths of 

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 

by regional and less-variable flow paths. Diversion Spring 

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 

Deep Spring; the nature of the geochemical changes indicates 

that increased discharge included an increased component of 

saline groundwater. Weissmuller Spring was sampled only 

during the wet period; its geochemistry was similar to that of 

Diversion Spring, indicating that Weissmuller and Diversion 

Springs are likely supplied by common flow paths. 

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 

groundwater flow, is needed to account for the composition 

of water from Deep, Diversion, and Weissmuller Springs. 

Potential sources of saline groundwater are the downdip 

Edwards aquifer saline zone and the Trinity aquifer; while 

both sources are hydrologically and geochemically plausible, 

model results indicate that mixing with groundwater from the 
2 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Edwards aquifer saline zone is likely a better geochemical 

fit. Little local recharge occurred during the dry period. 

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 

0 to less than 30 percent; the proportion was smaller for 

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, 

the geochemical response at San Marcos Springs was minor; 

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 

sources and flow paths, even during wet hydrologic

 conditions when aquifer recharge is likely occurring from 

local streams. Local surface-water recharge sources do not 

strongly influence the geochemistry of San Marcos Springs 

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 

(Sharp and Banner, 1997; U.S. Environmental Protection 

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 

users. Comal Springs and San Marcos Springs, the first and 

second largest spring complexes in Texas, are major 

discharge points for the Edwards aquifer (Brune, 1975) 

(fig. 1). Spring discharges from the Comal and San Marcos 

Springs complexes, along with water-table altitudes in the 

Bexar County index well (hereinafter, J–17; fig. 1), are used as 

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 

supplied by regional flow paths originating in the western part 

of the aquifer (Puente, 1976; Guyton and Associates, 1979; 

LBG-Guyton Associates, 2004; Johnson and Schindel, 2008). 

Although San Marcos Springs has been hypothesized to be 

supplied by both regional and local recharge sources (Guyton 

and Associates, 1979; Ogden and others, 1986), the hydrologic 

interconnection of San Marcos Springs with the regional 

Edwards aquifer flow system is uncertain. 

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. 

(fig. 1). The primary objective of the study was to identify and 

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 

flow where present (White, 2002). Transport of water 

through the primary aquifer matrix is typically diffuse and 

much slower (Atkinson, 1977; Martin and Screaton, 2001; 

Desmarais and Rojstaczer, 2002). As a result, karst aquifers 

tend to be heterogeneous with large variability and rapid 

changes in aquifer recharge, flow, and discharge rates. Karst 

springs are integrators of input from various sources and 

processes that occur along flow paths to the spring and, as 

such, are ideal sites for studying aquifer processes and surfacewater/groundwater interaction (Quinlan, 1989). Karst springs 

that are dominantly supplied by focused recharge (quick flow) 

moving through a well-developed conduit system can be 

highly variable in discharge quantity and quality. Karst springs 

dominantly supplied by matrix (diffuse) flow (slow flow) tend 

to exhibit less temporal variability in spring discharge and 

geochemistry; recharge moves more slowly along diffuse flow 

paths than it does in conduits, and has more opportunity to 

interact with the rock matrix and mix with matrix groundwater 

(White, 1988; Ford and Williams, 2007). In karst aquifers, 

changes in climatic and hydrologic conditions directly affect 
1,000 mg/L

l l l l l l l l l l l l l l l l l l l

l

l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l

Kyle

Austin

San Antonio

New Braunfels

San

Marcos

Seguin

Boerne

Wimberley

Bulverde

Selma

Sattler

Base modified from U.S. Geological Survey 1:24,000 digital dataGeographic Coordinate SystemNorth American Datum of 1983

HAYS COUNTY

COMAL COUNTY

KENDALL COUNTY

BLANCO COUNTY

BEXAR

 COUNTY

TRAVIS COUNTY

GUADALUPE

COUNTY 

! !

#

#

#

#

#

# #

#

#

#

#

#

#

Q13

Q12

Q11

Q10

Q9

Q8

Q7

Q6

Q5

Q4

Q3

Q2

Q1

S3

S2 S1

S4

S5, S6, S7

W21

W20

W19

W17 W18

W14 W16 W13 W15

W10 W11

W6

W12

P1

W9

W8

W5

W4 W3

W2

W1

W22

"

"

" " "

""" " " "

" " " " "

"

"

"

"

"

"

W7

Cibolo

Creek

Dry

ComalCreek

Alligator

Creek

Blanco River

Onion Creek

San

Marcos River

Plum

Creek

York Creek

CANYON LAKE

Purgatory

Creek

Sink

Creek

ComalRiver

Comal Springs

San Marcos Springs

Guadalupe River

Guadalupe

River

Hueco Springs

0 2.5 5 7.5 10 MILES

0 2.5 5 7.5 10 KILOMETERS

Gulf of Mexico

EDWARDS PLATEAU

Bexar County

index well (J–17)

TE

X

AS Study

area

EXPLANATION

Edwards aquifer (Ashworth and Hopkins, 1995) Contributing zone Recharge zone Confined zoneBalcones Escarpment (modified from Abbott and Woodruff, 1986)Municipal boundaryGroundwater divide (modified from Maclay, 1995, plate 1; Mahler and others, 2006, fig. 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 1Groundwater well, Edwards aquifer and map identifier—Table 1Groundwater well, Edwards and Trinity aquifers and map identifier—Table 1Groundwater well, Trinity aquifer and map identifier—Table 1Spring and map identifier—Table 1Rain sample collection site—Table 1National Weather Service cooperative station and identifier

Q3 #

W5 "

S5

W10 "W21

P1

l l

417983

98°40' 98°20' 98°00'

30°20'30°00'29°40'

418544

411429

419815

412585 

417983 

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.
4 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

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 

Bexar, Comal, and Hays Counties, Tex.; (2) evaluate changes 

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 

during the dry and wet periods; and (3) synthesize the 

findings to identify and characterize sources of recharge and 

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 

groundwater flow within the Edwards aquifer. Most recharge 

to the Edwards aquifer occurs in the recharge zone, where 

the porous and permeable Edwards Group outcrops. Streams 

flowing south and east toward the Gulf of Mexico drain the 

Edwards Plateau and lose most or all of their flows into the 

faulted and fractured limestone as they cross the recharge 

zone, supplying from 60 to 80 percent of aquifer recharge 

(Klemt and others, 1979; Maclay and Land, 1988; Thorkildsen 

and McElhaney, 1992; Ockerman, 2005). The majority of 

recharge occurs west of Bexar County with additional recharge 

occurring in Bexar, Comal, and Hays Counties (fig. 1). 

Regional groundwater flow is generally eastward to discharge 

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 

and Land, 1988) (fig. 1). San Marcos Springs is downgradient 

from Comal Springs (fig. 1), and regional groundwater flow 

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 

can either facilitate or impede flow of groundwater between 

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 

Edwards aquifer, resulting in subsurface, interaquifer flow 

(Lindgren and others, 2004). The downdip limit of potable 

water in the Edwards aquifer, the freshwater/saline-water 

interface, is defined by a dissolved solids concentration of 

1,000 milligrams per liter (mg/L) (Schultz, 1994; Groschen 

and Buszka, 1997), which also defines the transition from 

a zone of more active flow with fresh groundwater to the 

downdip zone of less active flow with saline groundwater. 

Comal, San Marcos, and Hueco Springs are karst 

springs associated with large-displacement faults (fig. 2). 

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) (1927–2010) has ranged from 0 to 534 

cubic feet per second (ft3/s) with an average value of 292 ft3/s 

(U.S. Geological Survey, 2011); discharge during the study 

period (November 2008–December 2010) ranged from 158 to 

391 ft3/s with an average value of 293 ft3/s (median of 

306 ft3/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 

complex emerges from three large spring orifices, including 

Comal Spring 1, in the upthrown block of the fault; the 

remaining discharge emerges from numerous small orifices 

beneath the lake in the downthrown block of the fault (Ogden 

and others, 1986; LBG-Guyton Associates, 2004), which sits 

at an average altitude of about 623 ft (Guyton and Associates, 

1979). The water flows over the dam and forms the headwaters 

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 

(figs. 1 and 2). The springs are located just south of the 

Hueco Springs Fault, which has 380–400 ft of offset in the 

vicinity of the springs. Long-term discharge from the Hueco 

Springs complex (USGS station 08168000) (2002–2010) has 

ranged from 1.3 to 148 ft3/s with an average value of 53 ft3/s; 

discharge during the study period (November 2008–December 

2010) ranged from 1.3 to 121 ft3/s with an average discharge 

of 44 ft3/s (median of 40 ft3/s) (U.S. Geological Survey, 2011). 

Hueco Springs comprises two spring orifices that emerge from 

the alluvium along the west side of the Guadalupe River and 

then flow into the Guadalupe River. With an altitude of about 

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, 

1979). Discharge from Hueco Springs flows south along 

an approximately 0.3-mi-long channel that empties into the 

Guadalupe River. 
FAULT

FAULT

MOUNTAIN CITY 

Sink Creek

Blanco River

 Comal

Springs

Canyon

Lake San Marcos Springs

 Hueco

Springs

Infiltration

and recharge 

SOUTHWEST

Onion Creek

York Creek

Base modified from U.S. Geological Survey digital data, 1:24,000Universal Transverse Mercator projection, zone 14Datum, North American Datum of 1983

Not to scale

Interaquifer flow

from Trinity

into Edwards 

Regional

 Edwards

 aquifer

 flow

San Marco s River San Marco s River GuadalupeR

iv

e

r

Purgatory Creek

SAN MARCOS SPRINGS

FAULT

COMAL SPRINGS FAULT

HUECO SPRINGS

B

FAULT EAR CREEK FAULT

BAT CAVE FAULT

MUSTANG BRANCH ACADEMY FAULT

Upper confining units (Ashworth and Hopkins, 1995)Edwards aquifer (Ashworth and Hopkins, 1995)Trinity aquifer (Ashworth and Hopkins, 1995)Fault—Dashed where approximately located. Arrow indicates relative direction of movementStream—Dashed where ephemeralSpring

EXPLANATION

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.
6 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

San Marcos Springs is a complex of springs that 

issue from the San Marcos Springs Fault (figs. 3 and 4), a 

normal fault with as much as 470 ft of offset (LBG-Guyton 

Associates, 2004) that juxtaposes the Edwards Group against 

younger upper confining units. Long-term discharge from 

the San Marcos Springs complex (USGS station 08170000) 

(1955–2010) has ranged from 46 to 451 ft3/s with an average 

value of 174 ft3/s (U.S. Geological Survey, 2011); discharge 

during the study period (November 2008–December 2010) 

ranged from 83 to 270 ft3/s with an average discharge of 

160 ft3/s (median of 176 ft3/s) (U.S. Geological Survey, 2011). 

LBG-Guyton Associates (2004) estimated that approximately 

25 percent of the total discharge from San Marcos Springs 

emerges from well-defined orifices and the remaining 

75 percent from sand boils. Water discharging from San 

Marcos Springs is impounded by a concrete dam to form 

Spring Lake (fig. 4) and then spills over two outflow 

structures. The two channels converge downstream from the 

outflow structures as the headwaters of the San Marcos River. 

Because San Marcos Springs is 49 ft lower in altitude than 

is Comal Springs, downgradient aquifer flow of water past 

Comal Springs to San Marcos Springs likely occurs (Guyton 

and Associates, 1979; McKinney and Sharp, 1995). 

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 

discharge. Based on a comparison of spring flow curves, 

it has been previously noted that the correlation between 

water-table altitudes for well J–17 and San Marcos Springs 

discharge is not as well correlated as that between J–17 and 

Comal Springs; additionally, the relation between discharge at 

Comal Springs and at San Marcos Springs indicates that they 

might have differences in discharge sources (LBG-Guyton 

Associates, 2004; Johnson and Schindel, 2008).

Studies of Comal Springs have identified mostly regional 

sources for spring discharge. Puente (1976) used relations 

between water levels, spring discharge, and stream discharge 

to develop statistical correlations and concluded that Comal 

Springs was supplied primarily by flow from the regional 

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 

springs, low coefficients of variation for different geochemical 

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 

springflow augmentation, used information from previous 

studies to conclude that Comal Springs is supplied by regional 

flow with long residence times from the western part of the 

aquifer, with little to no evidence for local recharge sources. 

Otero (2007) used geologic, hydrologic, and geochemical 

data to identify and characterize three separate flow paths 

conveying water to Comal Springs. 

Studies of Hueco Springs have identified a mostly local 

recharge source for spring discharge (Guyton and Associates, 

1979; Ogden and others, 1986), which might also include 

some unknown percentage of water derived from the Trinity 

aquifer (LBG-Guyton Associates, 1995; Lindgren and others, 

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 

do the other major springs (Guyton and Associates, 1979; 

Maclay, 1995; Lindgren and others, 2004). Hueco Springs has 

a smaller mean discharge than do other major springs in the 

Edwards aquifer and generally does not flow during extended 

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 

flow from the Comal Springs area and local recharge in 

northern Comal and Hays Counties. Using historical tritium 

concentrations and spring discharge correlations, Guyton and 

Associates (1979) concluded that roughly 55–60 percent of 

discharge from San Marcos Springs originates from regional 

groundwater flowing past Comal Springs to San Marcos 

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 

Lake are supplied by regional flow from the Comal Springs 

Fault Block and that those in the northern part of the lake are 

supplied by local recharge from the Blanco River and Sink 

Creek; they proposed that the northern and southern parts 

of the lake are separated by faults or a “pressure boundary” 

that might shift in response to changing flow conditions. 

Wanakule (1988) accounted for a weaker correlation between 

the water-table altitude in well J–17 and discharge from San 

Marcos Springs, relative to discharge from Comal Springs, on 

the basis of the longer flow paths and the influence of local 

recharge from the Blanco River Basin. McKinney and Sharp 

(1995) concluded that, although supplied mainly by regional 

flow, a component of recharge to San Marcos Springs might 
Introduction 7

#

!

! !

" "

"

R

W8 W7

S7

S6

S5 COMAL SPRINGS FAULT

SAN MARCOS SPRINGS FAULTSan

Spring

Lake

Marcos River

Sink

Creek

U

D

U

D

D

U

U

D

U

D

U

D

A'

A

0 0.2 0.4 0.8 MILES

0 0.2 0.4 0.6 0.8 KILOMETERS

0.6

97°55'

97°56'

29°54'

29°53'

Kp

Kp

ucu

ucu

Qal

Qal

Qal

Qal

A A'

EXPLANATION

System

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 0.2 0.4 0.8 MILES

0 0.2 0.4 0.6 0.8 KILOMETERS

0.6

San Marcos Springs 

SAN MARCOS SPRINGS FAULT

Spring Lake

COMAL SPRINGS FAULT

A A'

Kp

Kp

Kp

ucu

Kp ucu

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

NORTH AMERICAN VERTICAL DATUM OF 1988 Modified from Guyton and Associates (1979)

VERTICAL EXAGGERATION

EXPLANATION

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

 movement

Qal

ucu

Kp

Kk

Kgr

A

B

Figure 3. San Marcos Springs, Hays County, south-central Texas. A, Surface geology. B, Geologic section.
8 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

SPRING LAKE

DAM

U

D

U

D

SAN MARCOS SPRINGS FAULT

Spring

 Lake

Diversion

 Spring (S7)

Weissmuller

 Spring (S6) 

Deep

 Spring (S5)

0 0.05 0.1 MILE

0 0.05 0.1 KILOMETER Fault—U, upthrown; D, downthrown

 (Guyton and Associates, 1979)

Spring sample collection location—

 See table 1

EXPLANATION

U

D

97°56' 97°55'50"

29°53'40"

29°53'30"

29°53'20"

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 10–30 mi west of the springs. 

LBG-Guyton Associates (2004) concluded that San Marcos 

Springs is located at the end of the regional groundwater flow 

path in Edwards aquifer fault blocks on the upthrown side of 

the Comal Springs Fault that is bypassing discharge to Comal 

Springs; they also concluded that San Marcos Springs might 

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 

Springs is sourced primarily from regional flow paths from 

the western part of the aquifer (more than 90 percent of the 

discharge during low-flow conditions), whereas local recharge 

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 

flow paths to San Marcos Springs. Results from dye tracing 

efforts by Ogden and others (1986) indicate that, during 

below-average discharge conditions at San Marcos Springs 

(approximately 140 ft3/s), groundwater moved from Ezell’s 

Cave, located approximately 2 mi southwest of Spring Lake, 

to spring orifices in the southern part of Spring Lake within 

11 days; the dye was not detected in the spring orifices in the 

Figure 4. San Marcos Springs complex, Hays County, south-central Texas.
Methods of Investigation 9

northern part of Spring Lake. The Edwards Aquifer Authority 

conducted numerous tracer tests near San Marcos Springs in 

2002 through 2005 under a range of flow conditions, which 

confirmed recharge and groundwater flow from a number of 

sites to the south, southwest, and west of San Marcos Springs 

to both northern and southern spring orifices; their results 

indicate that a highly transmissive groundwater flow path 

with groundwater velocities of approximately 1,400 ft per 

day exists parallel to the Hueco Springs Fault (Johnson and 

Schindel, 2008). They delineated flow paths from locations to 

the southwest and west of San Marcos Springs during belowaverage to average discharge conditions (approximately 168–

196 ft3/s) in 2005 that had relatively long traveltimes and were 

detected at spring orifices in both the southern and northern 

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 ft3/s), and the dye was detected 

in spring orifices in both the northern and southern parts of 

Spring Lake 40 days after injection. Johnson and Schindel 

(2008) replicated the Rattlesnake Cave dye-tracing test during 

average discharge conditions (approximately 150 ft3/s), and 

the dye was detected in spring orifices in both the northern 

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 

a year-long traveltime at spring orifices in both the northern 

and the southern parts of Spring Lake; discharge at San 

Marcos Springs ranged from 100 to 218 ft3/s during the year 

between dye injection and detection with an average discharge 

of 141 ft3/s. The year-long traveltime was viewed skeptically 

at the time (Ogden and others, 1986) and indicates that flow 

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 

similar timeframe as the current study (2008–10; Johnson and 

others, 2012), the Edwards Aquifer Authority confirmed flow 

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, 

flow paths to San Marcos Springs are likely slow. Their results 

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 

regional flow paths (Johnson and others, 2012). 

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, 

21 wells, 3 orifices of San Marcos Springs, and orifices of 

Comal and Hueco Springs (fig. 1; table 1). Twenty-four rain 

samples (from one site in northern San Antonio) also were 

collected for stable isotope analysis (fig. 1, map identifier 

P1). Continuous water-quality monitors were installed at one 

stream site, in two Edwards aquifer wells, and in three orifices 

at San Marcos Springs. Water-table altitude was continuously 

measured in the two wells. Three streamflow-gaging stations 

were installed, and continuous gage height was recorded; 

stage-discharge ratings were developed for two of the stations 

to compute continuous discharge data. Three spring orifices 

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 

7 spring orifices at Comal, Hueco, and San Marcos Springs 

(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 3–7 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 

orifice each at Comal and Hueco Springs, and three spring 

orifices at San Marcos Springs (table 2; fig. 5). All samples 

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 

base-flow conditions. It should also be noted that spring 

discharge values for Comal, Hueco, and San Marcos Springs 

represent discharge for the spring complexes, whereas 

physicochemical and geochemical data were collected from 

individual spring orifices.
Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Te(November 2008–December 2010). [NAD 83, North American Datum of 1983; NAVD 88, North American Vertical Datum of 1988; dd, degrees; mm, minutes; ss, seconds; --, not available; N/A, not applicable; X, S, screened; QW, water-quality samples (routine and [or] storm); WL, continuous groundwater altitude; M, continuous water-quality monitor; G, continuous gage height; Q, concontinuous discharge; TSU, Texas State University; do, ditto; USGS, U.S. Geological Survey] Map iden- tifier (fig. 1) U.S. Geological Survey station number U.S. Geological Survey station name (surface- water 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 ope

interva

(feet) 

Surface-water sites Q1 08167990 Guadalupe River at River Road near Sattler, Tex. Guadalupe at River Road 29°45'55" 98°08'31" 640 N/A N/A N/A 

Q2 08168500 Guadalupe River above Comal River at New Braunfels, Tex. Guadalupe above Comal 29°42'54" 98°06'36" 587 N/A N/A N/A Q3 08169500 Guadalupe River at New Braunfels, Tex. Guadalupe at New Braunfels 29°41'53" 98°06'24" 572.9 N/A N/A N/A Q4 08169932 Sink Creek near San Marcos, Tex. Sink Creek 29°55'45.57" 97°59'39.33" 742 N/A N/A N/A Q5 08169958 Purgatory Creek at Mountain High Drive near San Marcos, Tex. Purgatory Creek 29°52'21.12" 98°00'14.1" 690 N/A N/A N/A Q6 08171000 Blanco River at Wimberley, Tex. Blanco at Wimberley 29°59'40" 98°05'20" 797.6 N/A N/A N/A Q7 08171290 Blanco River at Halifax Ranch near Kyle, Tex. Blanco at Halifax 30°00'20" 97°57'09" 675 N/A N/A N/A Q8 08171300 Blanco River near Kyle, Tex. Blanco near Kyle 29°58'46" 97°54'36" 620.5 N/A N/A N/A Q9 08184300 Cibolo Creek at Farm Road 1863 below Bulverde, Tex. Cibolo at 1863 29°43'57.6" 98°21'22.98" 941 N/A N/A N/A 
Q10 08167800 Guadalupe River 

at Sattler, Tex.

Guadalupe at 

Sattler

29°51'33" 98°10'48" 742 N/A N/A N/A 

Q11 08168797 Dry Comal Creek 

at Loop 337 near New Braunfels, Tex.

Dry Comal 29°41'16.8" 98°09'17.4" 610 N/A N/A N/A 

Q12 08183890 Cibolo Creek at 

Cibolo Nature Center near Boerne, Tex.

Cibolo Nature 

Center

29°46'52.07" 98°42'46.19" 1358 N/A N/A N/A 

Q13 08185000 Cibolo Creek at 

Selma, Tex.

Cibolo at 

Selma

29°35'39" 98°18'40" 728 N/A N/A N/A 

Groundwater wells

W1 294604098060701 DX–68–16–707 4D well -- -- 802 Edwards 400 195–400 W2 294739098075301 DX–68–15–605 Bonem well 29°47'39" 98°07'53" 860 Edwards 375 220–375 W3 295019097592701 LR–67–09–113 Fish Hatchery 

1 well

29°50'19.46" 97°59'26.65" 714 Edwards 280 216–280 

W4 295033098041201 DX–68–16–201 Mendez well 29°50'33" 98°04'12" 991 Trinity 640 580–640 W5 295052098070801 DX–68–16–101 Sac-N-Pac 

well

29°50'52" 98°07'08" 1,020 Trinity 408 256–408 

W6 295314097565701 LR–67–01–826 TSU-West 

Campus well

29°53'14.04" 97°56'57.47" 751 Edwards 210 103–210 

W7 295323097561101 LR–67–01–828 TSU-Artesian 

well

29°53'22.5" 97°56'11.2" 577 Edwards 600 0–199 

W8 295325097564301 LR–67–01–827 TSU-Jackson 

1 well

29°53'25" 97°56'42.9" 740 Edwards 191 135–19

W9 295345098001001 LR–68–08–902 SMBA 1 well 29°53'45" 98°00'10" 770 Edwards 335 200–335 

Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Te(November 2008–December 2010).—Continued[NAD 83, North American Datum of 1983; NAVD 88, North American Vertical Datum of 1988; dd, degrees; mm, minutes; ss, seconds; --, not available; N/A, not applicable; X, S, screened; QW, water-quality samples (routine and [or] storm); WL, continuous groundwater altitude; M, continuous water-quality monitor; G, continuous gage height; Q, concontinuous discharge; TSU, Texas State University; do, ditto; USGS, U.S. Geological Survey]

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 ope

interval 

(feet)

Surface-water sites—Continued
Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Te(November 2008–December 2010).—Continued [NAD 83, North American Datum of 1983; NAVD 88, North American Vertical Datum of 1988; dd, degrees; mm, minutes; ss, seconds; --, not available; N/A, not applicable; XS, screened; QW, water-quality samples (routine and [or] storm); WL, continuous groundwater altitude; M, continuous water-quality monitor; G, continuous gage height; Q, concontinuous discharge; TSU, Texas State University; do, ditto; USGS, U.S. Geological Survey] Map identifier (fig. 1) U.S. Geological Survey station number U.S. Geological Survey station name (surface- water 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 ope

interva

(feet) 

Groundwater wells—Continued W10 295352098071201 DX–68–08–701 Riedel well 29°53'52" 98°07'12" 1,150 Edwards 240 239–240 

W11 295406097551201 LR–67–01–818 Horton well 29°54'06" 97°55'12" 610 Edwards 80 10–80 W12 295443097554201 LR–67–01–809 Tipps well 29°54'43" 97°55'42" 601.3 Edwards 32.5 0–32.5 W13 295515097581801 LR–67–01–403 Solar well 29°55'15.1" 97°58'17.9" 688 Edwards -- -- W14 295524098114401 DX–68–07–505 Eagle Peak well -- -- 1,177 Trinity 732 200–732 W15 295530097563201 LR–67–01–503 Neff well 29°55'30" 97°56'32" 733 Edwards 280 180–240 do do do do do do do do do 240–280 W16 295538098042101 LR–68–08–502 Burns well 29°55'38" 98°04'21" 1,041 Trinity 700 400–700 W17 295709098000301 LR–68–08–602 Laguna well 29°57'08.5" 98°00'03" 906 Edwards 600 -- W18 295806097540901 LR–67–01–312 Aqua well -- -- 683 Edwards 520 300–520 W19 295915097525501 LR–67–01–309 City of Kyle 2 well -- -- 753 Edwards 658 328–658 W20 300041097563901 LR–58–57–808 Halifax well 30°00'41" 97°56'39" 740 Edwards 220 161–220 W21 300331097551601 LR–58–57–512 Ruby Ranch well -- -- 830 Edwards and Trinity 405 182–405 W22 300453097503301 LR–58–58–403 Buda well -- -- 710 Edwards 390 168–390 

Springs Hueco Springs 08168000 Hueco Springs near New Braunfels, Tex Hueco Springs 29°45'34" 98°08'24" 644.9 Edwards N/A N/A 

Comal Springs 08168710 Comal Springs at New Braunfels, Tex. Comal Springs 29°42'22" 98°07'21" 582.9 Edwards N/A N/A San Marcos 

Springs

08170000 San Marcos 

Springs at San Marcos, Tex.

San Marcos 

Springs

29°53'21" 97°56'02" 557.8 Edwards N/A N/A 
S1 294255098080501 DX–68–23–324 Comal 

Spring 3

29°42'55.38" 98°08'04.92" 625 Edwards N/A N/A 

S2 294300098080001 DX–68–23–301 Comal 

Spring 1

29°42'46" 98°08'15" 623.43 Edwards N/A N/A 

S3 294314098074101 DX–68–23–326 Comal 

Spring 5

29°43'14.22" 98°07'41.46" 626 Edwards N/A N/A 

S4 294533098082301 DX–68–15–901 Hueco 

Spring A

29°45'34" 98°08'24" 652.53 Edwards N/A N/A 

S5 295322097561000 LR–67–01–819 Deep 

Spring (San Marcos)

29°53'33" 97°55'54" 600 Edwards N/A N/A 

S6 295322097561002 LR–67–01–820 Weissmuller 

Spring (San Marcos)

29°53'36" 97°55'48" 600 Edwards N/A N/A 

S7 295336097555201 LR–67–01–825 Diversion 

Spring (San Marcos)

29°53'35.64" 97°55'51.9" 580 Edwards N/A N/A 

Rain sample collection site

P1 293146982941 San Antonio Subdistrict at San 

Antonio, Tex.

USGS San 

Antonio

29°31'47" 98°29'42" 970 N/A N/A N/A 

Table 1. Water-quality sampling and data-collection sites for hydrologic and geochemical characterization of San Marcos Springs, south-central Te(November 2008–December 2010).—Continued[NAD 83, North American Datum of 1983; NAVD 88, North American Vertical Datum of 1988; dd, degrees; mm, minutes; ss, seconds; --, not available; N/A, not applicable; X, S, screened; QW, water-quality samples (routine and [or] storm); WL, continuous groundwater altitude; M, continuous water-quality monitor; G, continuous gage height; Q, concontinuous discharge; TSU, Texas State University; do, ditto; USGS, U.S. Geological Survey]

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 ope

interval 

(feet)

Springs—Continued
14 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Table 2. Number of samples analyzed for water quality for characterization of San Marcos Springs, south-central Texas (November 

2008–December 2010).

[Bold indicates sites selected for routine sampling throughout the study; TSU, Texas State University]

Map identifier 

(fig. 1)

U.S. Geological Survey 

station number Short name1

Number of 

routine samples Number of 

Dry storm samples

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–68–16–707) 6 9 0

W2 294739098075301 Bonem well (DX–68–15–605) 1 0 0

W3 295019097592701 Fish Hatchery 1 well (LR–67–09–113) 3 1 0

W4 295033098041201 Mendez well (DX–68–16–201) 7 10 0

W5 295052098070801 Sac–N–Pac well (DX–68–16–101) 7 5 0

W6 295314097565701 TSU-West Campus well (LR–67–01–826) 3 9 0

W7 295323097561101 TSU-Artesian well (LR–67–01–828) 1 0 0

W8 295325097564301 TSU-Jackson 1 well (LR–67–01–827) 1 0 0

W9 295345098001001 SMBA 1 well (LR–68–08–902) 1 0 0

W10 295352098071201 Riedel well (DX–68–08–701) 1 0 0

W11 295406097551201 Horton well (LR–67–01–818) 1 0 0

W13 295515097581801 Solar well (LR–67–01–403) 6 8 0

W14 295524098114401 Eagle Peak well (DX–68–07–505) 1 0 0

W15 295530097563201 Neff well (LR–67–01–503) 7 10 0

W16 295538098042101 Burns well (LR–68–08–502) 3 0 0

W17 295709098000301 Laguna well (LR–68–08–602) 1 0 0

W18 295806097540901 Aqua well (LR–67–01–312) 7 5 0

W19 295915097525501 City of Kyle 2 well (LR–67–01–309) 1 0 0

W20 300041097563901 Halifax well (LR–58–57–808) 1 0 0

W21 300331097551601 Ruby Ranch well (LR–58–57–512) 6 3 0

W22 300453097503301 Buda well (LR–58–58–403 ) 2 0 0

Springs

S1 294255098080501 Comal Spring 3 (DX–68–23–324) 1 0 0

S2 294300098080001 Comal Spring 1 (DX–68–23–301) 7 2

10 8

S3 294314098074101 Comal Spring 5 (DX–68–23–326) 1 0 0

S4 294533098082301 Hueco Spring A (DX–68–15–901) 7 2

10 8

S5 295322097561000 Deep Spring (San Marcos) (LR–67–01–819) 7 9 17

S6 295322097561002 Weissmuller Spring (San Marcos) (LR–67–01–820) 0 5 6

S7 295336097555201 Diversion Spring (San Marcos) (LR–67–01–825) 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.
Introduction 15

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

Date

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

1

2

3

4

5

6

0

7

0

50

100

150

200

250

300

350

400

Stream discharge, in cubic feet per second

Spring discharge, in cubic feet per second

Daily average

rainfall, in inches

A

B

1 2 3

1 2 3

Dry period Wet period

Discharge (daily mean) at spring—

 Composite of individual orifices

 (table 1)

 San Marcos Springs

 ComalSprings

 Hueco 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—Sample collection at

 Deep and Diversion Springs and at Weissmuller

 Spring (USGS station 295322097561002)

 ComalSprings—Sample collection at Comal

 Spring 1 (USGS station 294300098080001)

 Hueco Springs—Sample collection 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).
16 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Stream sites selected for routine sampling were the 

Blanco River at Halifax Ranch near Kyle, Tex. (USGS station 

number 08171290; hereinafter, Blanco at Halifax) and the 

Guadalupe River at River Road near Sattler, Tex. (USGS 

station 08167990; hereinafter, Guadalupe at River Road). 

Wells selected for routine sampling (detailed in table 2) 

included wells completed in the Edwards aquifer and in the 

underlying Trinity aquifer. Spring orifices DX–68–23–301 

(Comal Spring 1) in the Comal Springs complex, and 

DX–68–15–901 (Hueco Spring A) were selected for routine 

sampling (fig. 1). Comal Spring 1 is the southernmost 

large spring that emerges from the Comal Springs Fault at 

the Comal Springs complex. Hueco Spring A is the lower 

elevation spring of the two orifices at Hueco Springs. Samples 

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 

Springs (figs. 1 and 4), Deep Spring (LR–67–01–819, 

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–67–01–825, submerged at a depth of about 

20 ft in the northeast part of Spring Lake) and Weissmuller 

Spring (LR–67–01–820, 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 2008–10 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 1–3) from nearby streams that 

potentially contribute recharge to San Marcos Springs, from 

Comal Spring 1, from Hueco Spring A, and from the three 

spring orifices at San Marcos Springs (Deep, Diversion, and 

Weissmuller Springs) (table 2; fig. 5); no groundwater samples 

were collected in response to storms. Storm samples from 

the streams were collected depending on flow conditions. 

At the springs, in response to storms 1 and 2, single samples 

were collected at Comal Spring 1 and Hueco Spring A; at San 

Marcos Springs, five (storm 1) or six (storm 2) samples were 

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 

December 2010 and at the three San Marcos Springs orifices 

(Deep Spring, from February 2009 through December 2010; 

Diversion Spring, from April 2009 through December 2010; 

and Weissmuller Spring, from June 2010 through December 

2010). Water temperature, specific conductance, pH, turbidity, 

and dissolved oxygen concentration were recorded at 

Blanco at Halifax; dissolved oxygen was not recorded at the 

springs. Water-table altitude, water temperature, and specific 

conductance were measured at hourly intervals in two wells 

near San Marcos Springs: (1) LR–67–01–809 (hereinafter, 

Tipps well; fig. 1, map identifier W12) from November 2008–

May 2010 and (2) LR–67–01–826 (hereinafter, 

TSU-West Campus well; fig. 1, map identifier W6) from 

January 2009–December 2010. Rain samples were collected 

at the USGS South Texas Program office in San Antonio 

(USGS station 293146982941; fig. 1, map identifier P1) 

periodically (March 2009–September 2010) and in response 

to storms 1–3 and were analyzed for stable isotope ratios of 

hydrogen (delta deuterium, δD) and oxygen (delta oxygen-18, 

δ18O). 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 

Water Information System (NWIS; U.S. Geological Survey, 

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

All associated data collected during this study—as well 

as descriptions of sample collection, sample processing, 

analytical methods, and quality-control assessment—are 

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 

values less than 0.3 FNU are shown on some figures in this 

report for completeness.
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

At USGS streamflow-gaging stations, streamflow 

(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 

of streamflow measurements and the stage of the stream at 

the time of measurement (Kennedy, 1984), which is used to 

compute a record of streamflow (Kennedy, 1983). Continuous 

streamflow data are available from NWIS (U.S. Geological 

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 

on streamflow measurements; the method, documented by 

Puente (1978), uses data collected from a network of USGS 

streamflow-gaging stations. For this study, streams near San 

Marcos Springs were evaluated as potential local recharge 

sources. Recharge estimates were computed daily for 

2008–10 for the Blanco River, Cibolo Creek, and Dry Comal 

Creek (fig. 1) by using discharge from USGS streamflowgaging 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 

streamflow-gaging stations upstream and downstream from the 

Edwards aquifer recharge zone in each basin. For the Blanco 

River, the upstream station was either 08171000 (Blanco 

River at Wimberley, Tex.; hereinafter, Blanco at Wimberley; 

fig. 1, map identifier Q6) or Blanco at Halifax (fig. 1); the 

downstream station was 08171300 (Blanco River near Kyle, 

Tex.; hereinafter, Blanco near Kyle; fig. 1, map identifier Q8). 

For Cibolo Creek (fig. 1), the upstream station was 08183890 

(Cibolo Creek at Cibolo Nature Center near Boerne, Tex.; 

hereinafter Cibolo Nature Center; fig. 1, map identifier Q12); 

the downstream station was 08185000 (Cibolo Creek at 

Selma, Tex.; hereinafter, Cibolo at Selma; fig. 1, map identifier 

Q13). For Dry Comal Creek (fig. 1), daily basin discharge was 

estimated from the discharge at station 08168797 (Dry Comal 

Creek at Loop 337 near New Braunfels, Tex., hereinafter, Dry 

Comal; fig. 1, map identifier Q11) and the upstream discharge 

estimated from the discharge occurring in the Guadalupe 

River Basin between station 08167800 (Guadalupe River at 

Sattler, Tex.; hereinafter, Guadalupe at Sattler; fig. 1, map 

identifier Q10) and station 08168500 (Guadalupe River above 

Comal River at New Braunfels, Tex.; hereinafter, Guadalupe 

above Comal; fig. 1, map identifier Q2) (R.N. Slattery, U.S. 

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 2009–10, 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 2009–10 (hereinafter referred to as 

the “Halifax-to-Kyle method”) required that the drainage 

areas in Puente’s (1978) equations be modified to adjust the 

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 

above Comal station. Streamflow gain-loss was estimated 
18 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

from the measured streamflow value at the Guadalupe above 

Comal station less the measured values for streamflow at the 

Guadalupe at Sattler station and discharge at Hueco Springs. 

Gaining flow was interpreted to indicate that no aquifer 

recharge was occurring and correspondingly, losing flow was 

interpreted to represent aquifer recharge.

In addition to recharge from streamflow, recharge to 

the aquifer occurs through internal drainage sinkholes or as 

diffuse recharge through soils; while not well quantified, this 

type of recharge has been estimated to range from 20 to 40 

percent of total aquifer recharge for the San Antonio segment 

of the Edwards aquifer (Klemt and others, 1979; Maclay and 

Land, 1988; Thorkildsen and McElhaney, 1992). Additional 

recharge likely occurs by leakage from the underlying Trinity 

aquifer (Sharp and Banner, 1997; Mace and others, 2000) 

and has been estimated to range from 2 to 9 percent of total 

annual aquifer recharge (Lowry, 1955; Woodruff and Abbott, 

1986; LBG-Guyton Associates, 1995; Mace and others, 2000). 

This interaquifer flow has been estimated to occur to a lesser 

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 

by calculating the saturation index for specified minerals and 

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 

constituents (such as calcium [Ca], magnesium [Mg], 

strontium [Sr], and bicarbonate [HCO3]). The dissolution or 

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 (SO4)/

chloride (Cl) ratios and SO4 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 

associated mass transfers of specified sources and sinks of 

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 

geochemical reactions along a hypothetical flow path. An 

initial water composition or compositions (representing 

matrix groundwater) was mixed with other selected water 

compositions (representing surface-water recharge, salinezone groundwater, or Trinity aquifer water), and a final 

water composition was designated (representing San Marcos 

Springs discharge). Reactions along a hypothetical flow path 

with selected mineral and gas phases were allowed, and 

results were constrained by selected ion concentrations (for 

example, major ions and Sr). Concentration uncertainties 

were defined, and the maximum uncertainty from the 

different model scenarios was considered in evaluating 

results; models with relatively low uncertainties (<0.05, or 

5 percent) were considered a better geochemical fit and, 

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 

with carbonate aquifers and specifically with the Edwards 

aquifer (calcite, dolomite, gypsum, quartz, carbon dioxide, 

dissolved oxygen, and phases for Ca and sodium [Na] cation 

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 

considered for initial and final water compositions. 

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 

Kendall’s tau coefficient is a nonparametric, rank-based 

test used to measure the strength of the relation between x 

and y (linear and nonlinear) and is resistant to the effects 

of outliers (Helsel and Hirsch, 2002). The tau coefficient 

ranges from -1 to 1; a value of 0 indicates no correlation, and 

values approaching -1 or 1 indicate an increasing strength of 

correlation. The tau coefficient values generally will be lower 

than values of traditional correlation coefficients for linear 

associations of the same strength; for example, a strong linear 

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 
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 

considered statistically significant. 

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. 

Drought Monitor, 2011)—with sustained small amounts of 

rainfall, surface-water flow, and discharge at major springs 

(Comal, San Marcos and Hueco Springs)—to relatively wet 

conditions with regularly occurring rainfall, greater surfacewater flow, and above-average discharge at major springs (fig. 

5). 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 ft3/s on 

January 1, 2008, to 102 ft3/s on December 1, 2008, when the 

first sample was collected. Dry conditions persisted until early 

September 2009, resulting in discharge at San Marcos Springs 

falling to 86 ft3/s. During the dry period, 12.7 inches of rain 

fell in the study area; San Marcos Springs discharge ranged 

from 83 ft3/s to 106 ft3/s, with an average discharge of 95 

ft3/s; discharge for Blanco at Halifax ranged from 2.3 ft3/s to 

36 ft3/s, with an average discharge of 9.4 ft3/s (monitoring at 

Blanco at Halifax started on December 19, 2008). In contrast, 

during the wet period, 60.7 inches of rain fell in the study area; 

San Marcos Springs discharge ranged from 88 ft3/s to 270 ft3/s, 

with an average discharge of 202 ft3/s; discharge for Blanco 

at Halifax ranged from 14 ft3/s to an estimate of 3,620 ft3/s, 

with an average discharge of 204 ft3/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 

(discharge) response (table 3; fig. 6). Storm 1 marked the 

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 

and Comal Springs (fig. 6). Storms 2 and 3 followed similar 

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 

amount, and it resulted in the largest streamflows and largest 

recharge estimates for the Blanco River and Cibolo Creek. For 

Dry Comal Creek, storms 2 and 3 resulted in larger flows than 

did storm 1; unlike the other evaluated streams, discharge and 

estimated recharge were larger for storm 2 than for storm 3. 

Some flow occurred at Sink Creek during storm 2 but not at 

Purgatory Creek. In response to storm 3, some flow occurred 

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 

changes in discharge and estimated aquifer recharge (fig. 5; 

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 

Guadalupe River and Dry Comal Creek; estimated recharge 

from Dry Comal Creek was large in response to this rain 

event (fig. 7), whereas gain-loss estimates indicate that the 

Guadalupe River was gaining (fig. 8). The lack of response in 

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 

at the USGS South Texas Program office in San Antonio 

(USGS station 293146982941) covered a large range: δD 

values ranged from -139.0 to 8.9 parts per thousand (per 

mil), and δ18O values ranged from -19.13 to -0.32 per mil. 

These data are consistent with a meteoric origin based on 
Table 3. Characteristics of sampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, south-(November 2008–December 2010). [ft3/s, cubic feet per second; >, greater than; %, percent; max., maximum] Storm description Antecedent moisture conditions1 Hydrologic response2 Storm Date of storm onset Total (inches)1 Temporal distribution of rainfall Rainfall in prior 3 months (inches)1 Days since last storm1 (>1 inch of rain in a day) Spring discharge response (prestorm value to 10-day poststorm max.): ft3/s (% change) Stream discharge response (prestorm value to 7-day poststorm max.): ft3/s 1 9/9/2009 5.4 5.4 inches of rain fell between 9/9 and 9/13; largest proportion on any day was 39%, which fell on 9/11 2.5 134 San Marcos Springs: 86 to 97 (13%); Comal Springs: 173 to 210 (21%); Hueco Springs: 4.1 to 101 (2,363%) Blanco at Halifax: 2.8 to 211; Guadalupe at Sattler: 57 to 105; Sink Creek: no flow; Purgatory Creek: no flow; Cibolo near Boerne: 1.9 to 8.3; Dry Comal: 0 to 680 2 10/3/2009 2.5 2.5 inches of rain fell between 10/3 and 10/4; 91% of rain fell on 10/4; an additional 2.2 inches of rain fell between 10/9 and 10/14 10.2 20 San Marcos Springs: 96 to 147 (53%); Comal Springs: 210 to 274 (30%); Hueco Springs: 17 to 115 (576%) Blanco at Halifax: 21 to 220; Guadalupe at Sattler: 69 to 104; Sink Creek: 0 to 9.8 (estimated); Purgatory Creek: no flow; Cibolo near Boerne: 1.8 to 34; Dry Comal: 0.06 to 3,500 3 9/7/2010 8.1 8.1 inches of rain fell between 9/7 and 9/9; 77% of rain fell on 9/8; storm was a named tropical storm (Hermine) 10.1 4 San Marcos Springs: 207 to 223 (8%); Comal Springs: 313 to 348 (11%); Hueco Springs: 34 to 121 (256%) Blanco at Halifax: 46 to 3,620; Guadalupe at Sattler: 201 to 1,090 (9 day); Sink Creek: 0 to 25; Purgatory Creek, some flow3; Cibolo near Boerne: 3.8 to 835; Dry Comal: 0.7 to 2,660 

1Rainfall data from National Climatic Data Center for six National Weather Service meteorological stations (cooperative stations: Canyon Dam [411429], Dripping Springs 6 

New Braunfels [416276], San Marcos [417983], Spring Branch 2se [418544], and Wimberley 1 NW [419815]) (National Oceanic and Atmospheric Administration, 2011); dailfor these sites were averaged to obtain a composite rainfall record in the vicinity of San Marcos Springs. 2Short names are used here; see table 1 for complete U.S. Geological Survey station names and numbers. 3Gage height at Purgatory Creek rose from 0.06 to 1.7 feet; stage-discharge relation not determined for this site. 4Based on Halifax-to-Kyle method as described in the report in section "Streamflow Measurements and Recharge Estimation."
Climatic and Hydrologic Conditions 21

Aug. 28

Aug. 31

Sept. 15

Sept. 21

Sept. 3Sept. 6

Sept. 9

Sept. 12

Sept 18

2009

0

1

2

3

4

5

6

7

0

50

100

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300

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40

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4

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18

20

0

20

40

60

80

100

120

140

50

100

150

200

250

300

350

400

450

500

0

Daily 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 second

Rainfall

Blanco River*

, Cibolo Creek,

 and Dry Comal Creek

Hueco Springs

Comal Springs

San Marcos Springs

Cibolo Creek

Guadalupe River

Blanco River

A

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

 ComalSprings

 Hueco 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—Sample collection at

 Deep and Diversion Springs (USGS stations

 295322097561000 and 295336097555201,

 respectively)

 ComalSprings—Sample collection at Comal

 Spring 1 (USGS station 294300098080001)

 Hueco Springs—Sample collection 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 Creek

EXPLANATION

* 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). 
22 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Nov. 1

Sept. 20

Oct. 26

Sept. 26

Oct. 2Oct. 8

Oct. 14

Oct. 20

2009

0

0.5

1.0

1.5

2.0

2.5

0

100

200

300

400

500

600

700

20

40

60

80

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0

20

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80

100

120

140

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0

Daily 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 second

Rainfall

Blanco River*

,

Cibolo Creek,

and Dry Comal Creek

Hueco Springs

Comal Springs

San Marcos Springs

Cibolo Creek

Guadalupe River

Blanco River

B

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

 ComalSprings

 Hueco 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—Sample collection at

 Deep and Diversion Springs (USGS stations

 295322097561000 and 295336097555201,

 respectively)

 ComalSprings—Sample collection at Comal

 Spring 1 (USGS station 294300098080001)

 Hueco Springs—Sample collection 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 Creek

EXPLANATION

* 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
Climatic and Hydrologic Conditions 23

Sept. 3

Sept. 21

Sept. 6

Sept. 9

Sept. 12Sept. 15

Sept. 18

2010

0

20

40

60

80

100

120

140

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0

0

1

2

3

4

5

6

7

Daily 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 second 280

290

300

310

320

330

340

350

360

370

380

170

180

190

200

210

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0

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2,000

2,500

3,000

3,500

4,000

0

200

400

600

800

1,000

1,200

Rainfall

Blanco River*

,

Cibolo Creek,

and

Dry Comal

Creek

Hueco Springs

Comal Springs

San Marcos Springs

Cibolo Creek

Guadalupe River

Blanco River

C

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

 ComalSprings

 Hueco 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—Sample collection at

 Deep, Diversion, and Weissmuller Springs

 (USGS stations 295322097561000,

 295336097555201, and 295322097561002,

 respectively)

 ComalSprings—Sample collection at Comal

 Spring 1 (USGS station 294300098080001)

 Hueco Springs—Sample collection 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 Creek

EXPLANATION

* 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
Table 4. Characteristics of unsampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, sou(November 2008–December 2010). [ft3/s, cubic feet per second; >, greater than; %, percent; max., maximum] Antecedent moisture conditions1 Hydrologic response2 Date of storm onset Total (inches)1 Temporal distribution of rainfall Rainfall in prior 3 months (inches)1 Days since last storm1 (>1 inch of rain in a day) Spring discharge response (prestorm value to 10-day poststorm max.): ft3/s (% change) Stream discharge response (prestorm value to 7-day poststorm max.): ft3/s 

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: 155 to 178 (15%); Comal Springs 268 to 290 (8%); Hueco Springs: 76 to 102 (34%) Blanco at Halifax: 69 to 680; Guadalupe at Sattler: 77 to 136; Sink Creek: no flow; Purgatory Creek: no flow; Cibolo near Boerne: 8.5 to 193; Dry Comal Creek: 1.1 to 181 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: 178 to 184 (3%); Comal Springs 290 to 312 (8%) (13 days); Hueco Springs: 60 to 75 (23%) Blanco at Halifax: 123 to 631; Guadalupe at Sattler: 109 to 114; Sink Creek: no flow; Purgatory Creek: no flow; Cibolo near Boerne: 11 to 81; Dry Comal Creek: 1.1 to 232 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: 181 to 207 (14%); Comal Springs 309 to 340 (10%) (13 days); Hueco Springs: 22 to 98 (345%) Blanco at Halifax: 116 to 1,210; Guadalupe at Sattler: 102 to 113; Sink Creek: no flow; Purgatory Creek: no flow; Cibolo near Boerne: 20 to 105; Dry Comal Creek: 1.1 to 707 1/28/2010 3.1 Not a single rain event; 1.24 inches from 1/28 through 1/30; followed by additional 1.79 inches from 2/3 through 2/5 8.2 13 San Marcos Springs: 209 to 264 (26%); Comal Springs 338 to 364 (8%); Hueco Springs: 86 to 113 (31%) Blanco at Halifax: 280 to 1,780; Guadalupe at Sattler: 108 to 119; Sink Creek: no flow; Purgatory Creek: no flow; Cibolo near Boerne: 43 to 407; Dry Comal Creek: 3.4 to 636 
5/15/2010 4.5 Not a continuous 

rain event; 3.13 inches from 5/15 through 5/16; followed by additional 1.33 inches on 5/18

4.8 105 San Marcos Springs: 

233 to 235 (1%); Comal Springs: 341 to 359 (5%); Hueco Springs: 72 to 108 (50%)

Blanco at Halifax: 139 to 347; Guadalupe at 

Sattler: 389 to 2,580; Sink Creek: no flow; Purgatory Creek: 0 to 10; Cibolo near Boerne: 27 to 582; Dry Comal Creek: 25 to 575

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: 

222 to 228 (3%); Comal Springs: 359 to 399 (1%); Hueco Springs: 77 to 86 (12%)

Blanco at Halifax: 136 to 265; Guadalupe at 

Sattler: no flow increase (increase from 523 to 7,930 at downstream gage Guadalupe above Comal); Sink Creek: no flow; Purgatory Creek: no flow; Cibolo near Boerne: no flow increase; Dry Comal Creek: 1.8 to 3,050

1Rainfall data from National Climatic Data Center for six National Weather Service meteorological stations (cooperative stations: Canyon Dam [411429], Dripping Springs 6 

New Braunfels [416276], San Marcos [417983], Spring Branch 2se [418544], and Wimberley 1 NW [419815]) (National Oceanic and Atmospheric Administration, 2011); dailwere averaged to obtain a composite rainfall record in the vicinity of San Marcos Springs.

2Short names are used here; see table 1 for complete U.S. Geological Survey station names and numbers.3Based on Halifax-to-Kyle method as described in the report in section "Streamflow Measurements and Recharge Estimation."

Table 4. Characteristics of unsampled storms, antecedent moisture conditions, and hydrologic response in the vicinity of San Marcos Springs, sou(November 2008–December 2010).—Continued[ft3/s, cubic feet per second; >, greater than; %, percent; max., maximum]

Antecedent moisture 

conditions1 Hydrologic response2

Date of 

storm onset

Total 

(inches)1

Temporal 

distribution of 

rainfall

Rainfall 

in prior 3 

months 

(inches)1

Days since 

last storm1

(>1 inch 

of rain in 

a day)

Spring discharge 

response (prestorm value 

to 10-day poststorm max.): 

ft3/s (% change)

Stream discharge response (prestorm 

value to 7-day poststorm max.): ft3/s 
26 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

Date

0

100

200

300

400

500

600

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0

500

1,000

1,500

2,000

Estimated 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. Blanco River*

B. Cibolo Creek

C. Dry Comal Creek

1 Onset of major storm

 and identifier

EXPLANATION

Dry period Wet 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, 2008–10.
Climatic and Hydrologic Conditions 27

Date

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

Date

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

0

200

400

600

800

1,000

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

-200

Discharge, 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

B

Onset 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 Survey

EXPLANATION

1

Dry period Wet period

Dry period Wet 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.
28 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

-25 -20 -15 -10 -5 0 5

Delta oxygen-18, in per mil

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

Delta 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, 2010

EXPLANATION

Figure 9. Relation between deuterium and oxygen isotopes for rainfall samples collected at U.S. Geological Survey station 

293146982941, Bexar County, south-central Texas (2008–10). 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) 

(fig. 9). Rainfall samples collected during the dry period 

had significantly higher median δD and δ18O 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), 

results showed a range of values (fig. 9). Rainfall amount is an 

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 δD and δ18O 

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 1–3 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 δD and δ18O, 

respectively) were similar to median values for San Marcos 
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 δD and δ18O, respectively) and storm 3 (-131.0 and 

-17.8 per mil for δD and δ18O, 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 

this study (fig. 9), as well as isotopically lighter than rain 

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 

(Lawrence and Gedzelman, 1996; Lawrence, 1998). Recharge 

to the Edwards aquifer in response to storm 3 would likely 

reflect these low isotopic values and provide a tracer of 

recent recharge, as previously demonstrated in the Edwards 

aquifer by Musgrove and others (2010); this hypothesis is 

discussed further in the section “Endmember Mixing Using 

Conservative Tracers.”

Stream Recharge

Installation of a streamflow-gaging station (Blanco at 

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 

Kendall’s tau of 0.94 (fig. 10). 

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 

period (fig. 7). Based on average recharge estimates, Dry 

Comal Creek contributed the largest amount of recharge 

to the Edwards aquifer in the study area (average daily 

mean recharge of 102 ft3/s), followed by Cibolo Creek 

(average daily mean recharge of 46.9 ft3/s) and the Blanco 

River (average daily mean recharge of 28.4 ft3/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 ft3/s for the Blanco River [Halifax-to-Kyle], Cibolo 

Creek, and Dry Comal Creek, respectively) and significantly 

larger during the wet period (average daily mean recharge of 

44.2, 75.3, and 167.2 ft3/s for the Blanco River [Halifax-toKyle], Cibolo Creek and the Dry Comal River, respectively). 

Recharge estimates varied between the streams for different 

events (fig. 7); for example, estimated recharge for Cibolo and 

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 

(fig. 8) with a daily mean gain that averaged 27.2 ft3/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 ft3/s. The largest losses occurred 

during several days in mid-April 2010, with a maximum daily 

mean loss of 178.2 ft3/s. This period coincided with some 

of the largest streamflow on the Guadalupe River during the 

study (fig. 8), although similar streamflow losses did not occur 

during other large streamflow events during the wet period 

(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 ft3/s (ranging from a maximum daily 

mean loss of 26.0 ft3/s to a maximum daily mean gain of 

13.6 ft3/s). For most of summer 2009 (dry period), the 

Guadalupe River had small daily mean losses (8.4 ft3/s on 

average for May through August) (fig. 8). While these results 

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 

based on the low flow volumes.

Two ephemeral streams in the study area (Sink Creek 

and Purgatory Creek) were monitored for streamflow during 

the study (2009–10) (fig. 1). Discharge for Sink Creek was 

estimated from data collected at USGS station 08169932 

Sink Creek near San Marcos, Tex. (hereinafter, Sink Creek; 

fig. 1, map identifier Q4) from May 21, 2009, through the 

end of 2010. Sink Creek flowed at the gaged location on 

only 3 days (Crow, 2012). In response to storm 2, discharges 

of 9.8 (estimated) and 8.9 ft3/s on October 3 and 4, 2009, 

respectively, were measured for Sink Creek. In response to 

storm 3, Sink Creek discharge was 25 ft3/s on September 8, 

2010. Gage height for Purgatory Creek was collected at 

USGS station 08169958 (Purgatory Creek at Mountain 

High Drive near San Marcos, Tex.; hereinafter, Purgatory 

Creek; fig. 1, map identifier Q5) from August 12, 2009, 

through the end of 2010. Although data did not allow for a 

stage-discharge relation to be established for this site, gageheight records indicate that minor flow occurred in Purgatory 

Creek on only 2 days: May 16, 2010, and September 9, 2010 

(in response to storm 3). 
30 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

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

[Values in bold are statistically different between dry and wet hydrologic conditions based on Mann–Whitney U test. N, number of samples; IQR, interquartile 

range; ft3/s; μS/cm, microsiemens per centimeter; <, less than; FNU, formazin nephelometric units; oC, degrees Celsius; mg/L, milligrams per liter; μg/L, 

micrograms per liter; NO3+NO2, nitrate plus nitrite, as nitrogen; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/

strontium-86 isotopic ratio; CaCO3, calcium carbonate; Mg/Ca, magnesium to calcium molar ratio, Sr/Ca × 103, strontium to calcium molar ratio × 1,000; SO4/Cl, 

sulfate to chloride molar ratio; Mg/Na, magnesium to sodium molar ratio; -- none or not applicable] 

Discharge, 

physicochemical property, 

or constituent 

concentration

Unit

Hydrologic 

condition

Surface water1, 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 

period3

7 -- 7 -- 3 -- 6 -- 7 -- 7 -- 6 --

Wet 

period4

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 ft3/s Dry 

period

1

62

(312)

15 11 

(264)

9 -- -- -- -- -- -- -- -- -- --

Wet 

period

1

124

(479)

287 146

(479)

135 -- -- -- -- -- -- -- -- -- --

Specific 

conductance

μS/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 oC 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 

CaCO3

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 μg/L 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).

[Values in bold are statistically different between dry and wet hydrologic conditions based on Mann–Whitney U test. N, number of samples; IQR, interquartile 

range; ft3/s; -- none or not applicable;μS/cm, microsiemens per centimeter; <, less than; FNU, formazin nephelometric units; oC, degrees Celsius; mg/L, 

milligrams per liter; μg/L, micrograms per liter; NO3+NO2, nitrate plus nitrite, as nitrogen; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/strontium-86 isotopic ratio; CaCO3, calcium carbonate; Mg/Ca, magnesium to calcium molar ratio, Sr/Ca × 103

, strontium to calcium molar 

ratio × 1,000; SO4/Cl, sulfate to chloride molar ratio; Mg/Na, magnesium to sodium molar ratio]
Climatic and Hydrologic Conditions 31

Discharge, 

physicochemical property, 

or constituent 

concentration

Groundwater wells (Trinity aquifer)2, 5 Springs1, 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

Specific 

conductance

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
32 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

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

[Values in bold are statistically different between dry and wet hydrologic conditions based on Mann–Whitney U test. N, number of samples; IQR, interquartile 

range; ft3/s; μS/cm, microsiemens per centimeter; <, less than; FNU, formazin nephelometric units; oC, degrees Celsius; mg/L, milligrams per liter; μg/L, 

micrograms per liter; NO3+NO2, nitrate plus nitrite, as nitrogen; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/

strontium-86 isotopic ratio; CaCO3, calcium carbonate; Mg/Ca, magnesium to calcium molar ratio, Sr/Ca × 103, strontium to calcium molar ratio × 1,000; SO4/Cl, 

sulfate to chloride molar ratio; Mg/Na, magnesium to sodium molar ratio; -- none or not applicable] 

Discharge, 

physicochemical property, 

or constituent 

concentration

Unit

Hydrologic 

condition

Surface water1, 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 

period3

7 -- 7 -- 3 -- 6 -- 7 -- 7 -- 6 --

Wet 

period4

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 μg/L 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

NO3+NO2 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

87Sr/86Sr -- 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

δD 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

[Values in bold are statistically different between dry and wet hydrologic conditions based on Mann–Whitney U test. N, number of samples; IQR, interquartile 

range; ft3/s; -- none or not applicable;μS/cm, microsiemens per centimeter; <, less than; FNU, formazin nephelometric units; oC, degrees Celsius; mg/L, 

milligrams per liter; μg/L, micrograms per liter; NO3+NO2, nitrate plus nitrite, as nitrogen; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/strontium-86 isotopic ratio; CaCO3, calcium carbonate; Mg/Ca, magnesium to calcium molar ratio, Sr/Ca × 103

, strontium to calcium molar 

ratio × 1,000; SO4/Cl, sulfate to chloride molar ratio; Mg/Na, magnesium to sodium molar ratio]
Climatic and Hydrologic Conditions 33

Discharge, 

physicochemical property, 

or constituent 

concentration

Groundwater wells (Trinity aquifer)2, 5 Springs1, 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

NO3+NO2 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

87Sr/86Sr 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

δD -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
34 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

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

[Values in bold are statistically different between dry and wet hydrologic conditions based on Mann–Whitney U test. N, number of samples; IQR, interquartile 

range; ft3/s; μS/cm, microsiemens per centimeter; <, less than; FNU, formazin nephelometric units; oC, degrees Celsius; mg/L, milligrams per liter; μg/L, 

micrograms per liter; NO3+NO2, nitrate plus nitrite, as nitrogen; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/

strontium-86 isotopic ratio; CaCO3, calcium carbonate; Mg/Ca, magnesium to calcium molar ratio, Sr/Ca × 103, strontium to calcium molar ratio × 1,000; SO4/Cl, 

sulfate to chloride molar ratio; Mg/Na, magnesium to sodium molar ratio; -- none or not applicable] 

Discharge, 

physicochemical property, 

or constituent 

concentration

Unit

Hydrologic 

condition

Surface water1, 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 

period3

7 -- 7 -- 3 -- 6 -- 7 -- 7 -- 6 --

Wet 

period4

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

δ18O 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

δ13C 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

Sr/Ca × 103 -- 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

SO4/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 

Springs measured continuously at 15-minute intervals; summary statistics based on daily mean values. Stream discharge for Guadalupe at River Road (gage 

height/discharge not measured) reflects values measured at upstream Guadalupe at Sattler site.

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

[Values in bold are statistically different between dry and wet hydrologic conditions based on Mann–Whitney U test. N, number of samples; IQR, interquartile 

range; ft3/s; -- none or not applicable;μS/cm, microsiemens per centimeter; <, less than; FNU, formazin nephelometric units; oC, degrees Celsius; mg/L, 

milligrams per liter; μg/L, micrograms per liter; NO3+NO2, nitrate plus nitrite, as nitrogen; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/strontium-86 isotopic ratio; CaCO3, calcium carbonate; Mg/Ca, magnesium to calcium molar ratio, Sr/Ca × 103

, strontium to calcium molar 

ratio × 1,000; SO4/Cl, sulfate to chloride molar ratio; Mg/Na, magnesium to sodium molar ratio]
Climatic and Hydrologic Conditions 35

Discharge, 

physicochemical property, 

or constituent 

concentration

Groundwater wells (Trinity aquifer)2, 5 Springs1, 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

δ18O -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

δ13C -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

Sr/Ca × 103 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

SO4/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

Short names are used here; see table 1 for complete U.S. Geological Survey station names and numbers.

6

Discharge data for San Marcos, Comal, and Hueco Springs (complexes); physicochemical and geochemical data for individual spring orifices Deep Spring, 

Diversion Spring, Weissmuller Spring, Comal Spring 1, and Hueco Spring A.
36 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

0 100 200 300 400 500 600 700

Date

Estimated daily recharge (based on Wimberley-to-Kyle method), 

in cubic feet per second

Oct. 2010

Dec. 2010

0

100

0

200

300

400

500

600

700

100

200

300

400

500

600

700

Estimated daily recharge,

in cubic feet per second

Estimated daily recharge (based on Halifax-to-Kyle method), 

in cubic feet per second

1 2 3

1:1 correlation

Dry period Wet period

Onset of major storm and identifier

Blanco 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 Survey

EXPLANATION

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. 
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 

were recorded beginning in February 2009; daily mean 

values for discharge, selected physicochemical properties, 

and rainfall through December 2010 are shown in figure 

11. Discharge at Blanco at Halifax ranged from a low of 

2.3 ft3/s on July 28 and 29, 2009, to an estimated high of 

3,620 ft3/s on September 8, 2010, in response to storm 3. 

Water temperature fluctuated seasonally between 4.5 and 31.9 

degrees Celsius (°C) with warmer temperatures occurring 

in summer and colder temperatures occurring in winter; 

some sudden decreases in temperature were associated with 

rain events. Specific conductance ranged from 258 to 515 

μS/cm; large and rapid decreases in specific conductance 

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 

returned to less than 100 FNU by late September 2010; the 

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 

concentrations occurring in the winter. Discharge, specific 

conductance, turbidity, and concentrations of dissolved oxygen 

were significantly higher during the wet period than during 

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 

from the Guadalupe at River Road site are shown in figures 

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), 

as well as those measured throughout the study (fig. 12). The 

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 

specific conductance values for the Guadalupe at River Road 

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 

specific conductance was lower at stream sites (table 5). 

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) 

(fig. 1; table 1) collected during storms 1–3 are shown on 

figures 12 and 13. In response to storms 1–3, physicochemical 

properties at all sites where measured were characterized 

by relatively low specific conductance and relatively high 

turbidity values compared to routine samples.

Groundwater

Water-table altitude and physicochemical properties 

(water temperature and specific conductance) were monitored 

continuously at hourly intervals at two wells in the vicinity of 

San Marcos Springs: Tipps and TSU-West Campus (fig. 14). 

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 

2010 at both wells; the total variation was 6.42 and 4.88 ft at 

the Tipps and TSU-West Campus wells, respectively. At both 

wells, the water-table altitude gradually decreased after peak 

values in February 2010 (fig. 14). 

Specific conductance at the Tipps well covered a large 

range (591–984 μS/cm) and increased through much of the 

dry period, with the highest values occurring at the end of 

the dry period. Two rapid increases in specific conductance 

occurred during the dry period in July and September 2009; 

these increases had no discernible relation with water level 

or hydrologic conditions. Specific conductance dropped 

rapidly with the onset of the wet period. At the TSU-West 

Campus well, specific conductance ranged from 617 to 834 

μS/cm; the highest values occurred during the later part of the 

dry period, with specific conductance generally decreasing 

through the wet period. Specific conductance was significantly 

higher at both wells during the dry period than during the 
38 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

Date

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

5

10

15

20

25

30

300

350

400

450

500

500

1,000

1,500

2,000

2,500

3,000

6

8

10

12

14

0

1

2

3

4

5

6

Specific 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

Dry period Wet period

Discharge

Water temperature

Specific conductance

Turbidity

Dissolved oxygen

Rainfall

1 Major storm and

 identifier

EXPLANATION

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).
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

May 2010

Aug. 2010

Dec. 2010

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Dec. 2010

102030405060708090

05

1015202530

50

7090

110130150170190210230250

0

100200300400500600700

0

500

1,0001,5002,0002,5003,0003,500

02468

1012

02468

101214161820

05

101520253035404550

0

0.020.040.060.08

0.10

0.120.140.160.18

02468

101214

05

101520253035

100150200250300350400450500550

0

50

100150200250

Formazin 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

Dry period Wet period Dry period Wet period Dry period Wet period

XX X

1

X

O S

 

S

               N   U

Alkalinity

Strontium

ChlorideSulfate

Bromide

Dissolved oxygenWater temperatureSpecific conductance

Calcium

Sodium

Turbidity Magnesium

Figure 12. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for surface-water sicharacterization of San Marcos Springs, south-central Texas.
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

May 2010

Aug. 2010

Dec. 2010

102030405060708090

-16-14-12-10

-8-6-4-2

0

-120-100

-80-60-40-20

20

0

0.70760.7078

0.7080

0.70820.70840.70860.7088

02468

10

Concentration, in milligrams per liter Concentration, in

micrograms per liter

Strontium-87 (87Sr) to

strontium-86 (86Sr), isotopic ratio

Delta deuterium (δD),

in per mil

Delta carbon-13 (δ13C),

in per mil

Magnesium (Mg) to

calcium (Ca), molar ratio

Strontium (Sr) to calcium (Ca),

molar ratio x103

0.200.400.600.801.001.201.40

0 0

0.20.40.6

0.8

1.0

1.21.41.61.8

2.0

2.2

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Dry period Wet period Dry period Wet period

X

X X XX 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

EXPLANATION

Onset 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—Sample

collection at Sink Creek

near San Marcos, Tex.

(USGS station 08169932)

 Purgatory Creek—Sample

collection at Mountain High

 Drive near San Marcos, Tex.

(USGS station 08169958)

Nondetection,  shown at the method  reporting levelUSGS, U.S. Geological Survey

Nitrate plus nitrite

Boron δD

δ13 Fluoride C

Mg/Ca

87Sr/86Sr Sr/Ca

Figure 12. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for surface-water scharacterization of San Marcos Springs, south-central Texas.—Continued
8/28/099/1/099/5/099/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/29/09 9/20/09 9/3/10 9/7/109/11/109/15/109/19/109/23/10 10/14/0910/22/0910/30/09 9/28/0910/6/09 Date

100150200250300350400450500550

0500

1,0001,5002,0002,5003,000

23456789

10

Concentration, in

milligrams per liter

152025

30

Degrees Celsius

0

20406080

100

120

Formazin nephelometric 

units (springs)

Microsiemens per centimeter 

(streams)

Formazin nephelometric 

units (streams)

450470490510530550570590610630650

Microsiemens per centimeter at

25 degrees Celsius (springs)

XXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XX XX XX XXXXX XXX XXXXX XXXXX XX XX XX XX XX XX XX XX XX XX XX

Storm 1 Storm 2 Storm 3

Dissolved oxygen

Turbidity

Specific conductance

Dissolved oxygen

Turbidity

Specific conductance

Dissolved oxygen

Turbidity

Specific conductance

Water temperature Water temperature Water temperature

X

Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spreceding and in response to storm 1 (September 2009), storm 2 (October 2009), and storm 3 (September 2010), south-central Texas.
8/28/099/1/099/5/099/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/29/09 9/20/09 9/3/10 9/7/109/11/109/15/109/19/109/23/10 10/14/0910/22/0910/30/09 9/28/0910/6/09 Date 050 100 150 200 250 3000 100 200 300 400 500 600 70002468101214161820020406080 100 120 050 100 150 200 250 300 3500 100 200 300 400 500 600 70002468101214161820020406080 100 120 050 100 150 200 250 300 3500 100 200 300 400 500 600 700051015202530020406080 100 120

Concentration, in milligrams per liter

Concentration, in 

micrograms per liter

Storm 1 Storm 2 Storm 3

Alkalinity

Strontium

Magnesium

Calcium

Alkalinity

Strontium

Magnesium

Calcium

Alkalinity

Strontium

Magnesium

Calcium

OnSp    WSu

               US

Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spreceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued
8/28/099/1/099/5/099/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/29/09 9/20/09 9/3/10 9/7/109/11/109/15/109/19/109/23/10 10/14/0910/22/0910/30/09 9/28/0910/6/09 Date

02468

1012

05

10152025

05

101520253014

02468

101214

05

10152025

05

101520253035

02468

101214

05

10152025

05

1015202530354045

50

0

0.02

0.040.06

0.08

0.10

0.12

0.140.16

0.18

0

0.020.040.06

0.08

0.10

0.120.140.160.18

0

0.020.040.06

0.08

0.10

0.120.140.160.18

Concentration, in milligrams per liter

X X XX

Storm 1 Storm 2 Storm 3

Sodium Sodium Sodium

Chloride Chloride Chloride

Sulfate Sulfate Sulfate

Bromide Bromide Bromide

E

OnsSpri S De Di W

CoHu

Surf

 S

Bl

  

Bl

   Gu    Ci  

 

 Si Pu Non thUSG

X

Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spreceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued
8/28/099/1/099/5/099/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/29/09 9/20/09 9/3/10 9/7/109/11/109/15/109/19/109/23/10 10/14/0910/22/0910/30/09 9/28/0910/6/09 Date 01020304050607080900 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 0.70870 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0102030405060700 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 0.70870 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0102030405060700 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 0.70870 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Concentration, 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 3

Boron

Nitrate plus nitrite

(as nitrogen)

87Sr/86Sr

Fluoride Fluoride Fluoride

Nitrate plus nitrite

(as nitrogen)

Nitrate plus nitrite

(as nitrogen)

87Sr/86Sr 87Sr/86Sr

Boron Boron

X

Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spreceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued
8/28/099/1/099/5/099/9/09 9/13/09 9/17/09 9/21/09 9/25/09 9/29/09 9/20/09 9/3/10 9/7/109/11/109/15/109/19/109/23/10 10/14/0910/22/0910/30/09 9/28/0910/6/09 Date

-16

-14-12-10

-8-6-4-2

-50-40-30-20-10

0

10

01234

56

Delta deuterium (δD),

in per mil

Delta carbon-13 (δ13C)

,

in per mil

Magnesium (Mg) to

calcium (Ca), molar ratio

Strontium (Sr) to calcium (Ca),

molar ratio x10

3

0.100.200.300.400.500.600.70

0

-16

-14-12-10

-8-6-4-2

-35-40-30-25-20-15

-10

0123456

0.100.200.300.400.500.600.70

0

-16

-14-12-10

-8-6-4-2

-120-100

-80-60-40-20

0

0123456

0.100.200.300.400.500.600.70

0

Blanco River 9/2/09=1.23Blanco River9/2/09=8.35

Storm 1 Storm 2 Storm 3

δD

δ13C

Mg/Ca

Sr/Ca

δD

δ13C

Mg/Ca

Sr/Ca

δD

δ13C

Mg/Ca

Sr/Ca

OS    S

               U

Figure 13. Time series (November 2008–December 2010) of physicochemical properties and geochemical constituents for surface-water sites and spreceding and in response to storm 1 (September 2009), storm 2 (October 2009) and storm 3 (September 2010), south-central Texas.—Continued
46 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

Date

15

20

25

30

400

500

600

700

800

900

1,000

1,100

18

19

20

21

22

23

24

165

170

175

Water-table altitude, in feet below land surface 

180

(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

1 Onset of major storm and identifier

Groundwater well (table 1)—Continuous data

 Tipps well

 TSU-West Campus well

EXPLANATION

Dry period Wet period

Figure 14. Times series (November 2008–December 2010) of hydrologic and physicochemical data for two groundwater wells 

(U.S. Geological Survey stations LR–67–01–809 [Tipps well] and LR–67–01–826 [TSU-West Campus well]) near San Marcos Springs, 

south-central Texas.
Geochemistry of San Marcos Springs and Nearby Hydrologic Features 47

wet period. Specific conductance values in the TSU-West 

Campus well declined rapidly when the well was pumped for 

routine water-quality sampling during the dry period. After 

sampling, specific conductance values over about a 4-week 

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 

specific conductance during the dry period at the Tipps well 

likely reflects mixing with a more saline groundwater source. 

Specific-conductance values at both wells during the dry 

period (median value of 692 μS/cm at the Tipps well and 810 

μS/cm at the TSU-West Campus well) were notably higher 

than for other Edwards aquifer wells (table 5) as well as higher 

than the median value for confined groundwater from the 

regional aquifer (525 μS/cm; Musgrove and others, 2010), 

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 

specific conductance. 

The water-table altitude in both of the wells responded 

similarly to storms 1 and 2. After storms 1 and 2, water 

levels rose in both wells; the change was more substantial in 

response to storm 2. The water-quality monitor in the Tipps 

well malfunctioned during storm 2, and specific conductance 

was not recorded; monitoring of the Tipps well was 

discontinued before storm 3 occurred. Specific conductance 

in the Tipps well decreased substantially in response to storm 

1. Changes in specific conductance at the TSU-West Campus 

well in response to storms 1–3 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 

Trinity aquifer (figs. 15 and 16, respectively; tables 1 and 

2). Well LR–58–57–512 (Ruby Ranch well) is completed 

in both the Edwards and Trinity aquifers; it is considered in 

this study as a Trinity aquifer wells since its geochemistry 

is dominantly influenced by the Trinity aquifer. Dry period 

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 

were not significantly different between the dry period and 

the wet period. Results for routine samples from the Edwards 

aquifer wells consistently indicated that a decrease in specific 

conductance occurred in late July 2009 (fig. 15). These lower 

specific conductance values might be erroneous, as little 

rainfall and aquifer recharge occurred in July 2009 (figs. 5 and 

7), and recorded values were qualified as estimated; major ion 

concentrations do not show notable corresponding decreases 

for these samples (fig. 15). Several high values for turbidity 

(>2 FNU) were recorded at wells in both the dry and wet 

periods (fig. 15); these were generally not preceded or followed 

by high values and are likely anomalous (some are qualified as 

estimated). Most of the Edwards wells had higher-than-normal 

turbidity values (about 3 FNU) in December 2009. 

Water temperature and specific conductance were notably 

higher at well LR–67–01–312 (hereinafter, the Aqua well) 

relative to other Edwards aquifer wells (fig. 15). The Aqua well 

had an anomalously low dissolved oxygen concentration on 

August 28, 2009, of 0.4 mg/L that was qualified as estimated. 

Some of the Edwards aquifer wells had specific conductance 

values similar to Comal Spring 1 (for example, wells 

DX–68–16–707 and LR–67–01–503, hereinafter the 4D and 

Neff wells, respectively) (fig. 15; table 5). Higher specific 

conductance values at the Aqua well were similar to values 

at San Marcos Springs, particularly at Deep Spring (fig. 15; 

table 5). Differences in physicochemical properties between 

the different wells likely reflect differences in flow paths 

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 

orifices. Spring discharge was significantly higher at San 

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 

physicochemical properties (fig. 17). At Comal Spring 1 and 

Hueco Spring A, physicochemical properties were measured 

with routine and storm samples (fig. 18). At San Marcos 

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 

fluctuated seasonally with the highest values occurring during 

the summer months and the lowest values occurring during 

the winter months. Diversion Spring did not show seasonal 

variations in temperature. 

Deep Spring had significantly higher specific 

conductance values (median of 613 μS/cm) than did Diversion 

or Weissmuller Springs (median for each of 595 μS/cm) 

(fig. 17). Specific conductance values were slightly lower in 

the wet period than in the dry period at both Deep and 

Diversion Springs (table 5). (The variability of specific 

conductance at San Marcos Springs is discussed in detail 

later in the report in the section “Specific Conductance and 

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 

were mostly below detection (<0.3 FNU) (fig. 17); slightly 

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 
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

May 2010

Aug. 2010

Dec. 2010

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Dec. 2010

Date

400450500550600650700750800850

20.0

20.5

21.0

21.5

22.0

22.5

23.0

23.5

24.0

24.5

Degrees Celsius Microsiemens per centimeter Formazin nephelometric units 0

123456789

10

02468

101214

Concentration, in

milligrams per liter

0

100200300400500600700800900

1,000

Concentration, in milligrams per liter

Concentration, in

micrograms per liter

102030405060708090

100110120130

05

10152025303540

50

100150200250300350400

30,00032,00034,00036,00038,00040,00042,00044,000

Concentration, in micrograms

per liter (Aqua)

0002468

1012

5

10152025

102030405060708090

100

0

0.020.040.060.08

0.10

0.120.140.160.18

Concentration, in milligrams per liter

1 2 31 2 3

1 2 3

1 2 3

1 2 31 2 3

1 2 3

1 2 3

1 2 31 2 3

1 2 3

1 2 3

XXXXXX XX XXX XXXX XXXX X XX XXXXXXX XX XX X XXX XXX

Dry period Wet period Dry period Wet period Dry period Wet period

Alkalinity

Strontium

ChlorideSulfate

Bromide

Dissolved oxygen

Specific conductance Calcium

Sodium

Turbidity Magnesium

Water temperature

1

X

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.
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

May 2010

Aug. 2010

Dec. 2010

Date

102030405060708090

0

0.51.01.52.02.53.0

-14-12-10

-8-6-4-2

0

-30-28-26-24-22-20-18-16-17

0

0.20.40.60.8

1.0

0.70760.7078

0.7080

0.70820.70840.70860.7088

02468

10

0.10

0

0.200.300.400.500.600.700.800.901.00

200250300350400

2.0

2.22.42.62.8

3.0

Concentration, 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 (δD),

in per mil

Delta carbon-13 (δ13C),

in per mil

Magnesium (Mg) to

calcium (Ca), molar ratio

Strontium (Sr) to calcium (Ca),

molar ratio x103

Strontium 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 XX X X X X X X X X X

Dry period Wet period Dry period Wet period

Nitrate plus nitrite

Boron δD

δ13C

Fluoride

Mg/Ca

Sr/Ca 87Sr/86Sr

1

X

Onset of major storm and identifierEdwards aquifer well (table1)

—

 Sample TSU-West Campus Solar Neff Aqua 4DNondetection, shown at the method  or laboratory reporting level

EXPLANATION

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
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

May 2010

Aug. 2010

Dec. 2010

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Dec. 2010

Date

Degrees Celsius Microsiemens per centimeter

at 25 degrees Celsius Formazin nephelometric units

Concentration, in

milligrams per liter

Concentration, in milligrams per liter

Concentration, in

micrograms per liter

Concentration, in milligrams per liter

02468

1012

05

10152025

05

10152025303540

0

0.020.040.060.08

0.10

0.120.140.160.18

10

2030405060708090

100

05

10152025303540

50

100150200250300350400

0

2,0004,0006,0008,000

10,00012,000

02468

101214

20.521.021.522.022.523.023.524.024.525.0

05

10152025

400450500550600650700 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 XX X X X X XX X X X X X X

Dry period Wet period Dry period Wet period Dry period Wet period

Alkalinity

Strontium

ChlorideSulfate

Bromide

Dissolved oxygen

Specific conductance Calcium Sodium

Turbidity Magnesium

Water temperature

1

X

Figure 16. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Trinity aquifer grsampled for the characterization of San Marcos Springs, south-central Texas.
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

May 2010

Aug. 2010

Dec. 2010

Date

102030405060708090

Concentration, in milligrams per liter Concentration, in

micrograms per liter

Strontium-87 (87Sr) to

strontium-86 (86Sr), isotopic ratio

0

0.51.01.52.02.5

0

0.10.20.30.40.50.60.7

0.70740.70750.70760.70770.70780.7079

0.7080

0.70810.7082

0

0.20.40.60.81.01.21.4

0

102030405060708090

-14-12-10

-8-6-4-2

0

-30-28-26-24-22-20-18-16-17

Delta deuterium (δD),

in per mil

Delta carbon-13 (δ13C),

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 Wet period Dry period Wet period

Nitrate plus nitrite

Boron δD

δ13 Fluoride C

Mg/Ca

Sr/Ca 87Sr/86Sr

1 Onset of major storm

 and identifierTrinity aquifer well (table 1)

—Sample

 Mendez Sac-N-Pac Ruby Ranch

EXPLANATION

Figure 16. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Trinity aquifer grsampled for the characterization of San Marcos Springs, south-central Texas.—Continued
52 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

Date

0

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

0.9

Specific 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

Dry period Wet period

Discharge

Specific conductance

Turbidity

Rainfall

Water temperature

1 Onset of major storm

 and identifier

San Marcos Springs,

 by orifice (table 1)

 Deep Spring

 Diversion Spring

 Weissmuller Spring

EXPLANATION

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).
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 

FNU, and on December 26, 2010, it was 0.7 FNU (fig. 17). 

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) 

and were significantly lower during the wet period (table 5). At 

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 

(table 5; fig. 18). An anomalously high turbidity value of 3.8 

FNU was measured on December 3, 2009; the reason for this 

high value is unknown, but it is inconsistent with low (<0.3 

FNU) turbidity values measured at this site throughout the study. 

Specific conductance values at Comal Spring 1 (median of 558 

μS/cm) were significantly lower than at Deep, Diversion, and 

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 

properties varied considerably over the study period; specific 

conductance and turbidity values responded rapidly to storm 

events, and temperature varied seasonally (fig. 18).

Geochemical Variability Associated with 

Routine Sampling 

Samples were collected periodically during December 

2008–December 2010 (routine samples) from two stream 

sites, eight groundwater wells (five completed in the Edwards 

aquifer and three completed in the Trinity aquifer), and five 

springs (three orifices of San Marcos Springs, Hueco Spring A, 

and Comal Spring 1). Geochemical changes during this period 

reflect a transition from exceptional drought to wetter-thannormal conditions. 

Surface Water

Selected geochemical data from routine water-quality 

samples collected at Blanco at Halifax and Guadalupe at River 

Road are shown in figure 12; summary statistics are shown 

in table 5. Samples from both streams showed some similar 

variations from the dry period to the wet period. Wet period 

samples had significantly lower Mg, Na, Sr, boron (B), and 

fluoride (F) concentrations, Mg/Ca and Sr/Ca ratios, and stable 

isotopes (carbon [δ13C], δD, δ18O); wet period samples also had 

higher Ca and nitrate plus nitrate (NO3+NO2) concentrations and 

Sr isotope ratios (87Sr/86Sr) (table 5). Samples from the Blanco at 

Halifax also had lower SO4 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 SO4 relative to samples from the Guadalupe at River Road. 

These differences might result from Trinity aquifer sourced 

springs contributing to base flow in the Blanco River during 

the dry period; differences in stratigraphy and lithology of 

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 

oxygen concentrations, specific conductance, turbidity, 

and Ca concentrations increased in both streams 

(fig. 12). Concentrations of Mg, Sr, Na, SO4, B, and F 

decreased as stream discharge increased (fig. 12), likely 

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 NO3+NO2 increased 

markedly at the onset of the wet period then generally 

decreased throughout the remainder of the wet period 

(fig. 12); this pattern is consistent with a flush of nitrification 

from the rewetting of soils following a drought (Lucey and 

Goolsby, 1993; Reynolds and Edwards, 1995), which has 

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 87Sr/86Sr values 

for stream samples during the wet period relative to the dry 

period likely reflect increased interaction of rainfall and runoff 

with more radiogenic (higher 87Sr/86Sr values) surface soils 

(Musgrove and Banner, 2004).

Groundwater

Edwards Aquifer

Selected geochemical data from routine samples 

collected from five wells completed in the Edwards aquifer 

(fig. 1) are shown in figure 15; summary statistics are shown 

in table 5. When comparing the geochemistry of the five 

wells, there are some differences in water chemistry that 

likely reflect different flow paths within the Edwards aquifer 

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 

during both the dry and wet periods (fig. 15), although the 

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, SO4, F, and B; higher ratios of 

Mg/Ca and Sr/Ca; lower concentrations of Ca and NO3+NO2; 

and higher water temperature and δ13C values (fig. 15). 
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

May 2010

Aug. 2010

Dec. 2010

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Dec. 2010

Date

0

20406080

100120

Formazin nephelometric 

units (Hueco A Spring)

02468

101214

Concentration, in

milligrams per liter 20.0

20.5

21.0

21.5

22.0

22.5

23.0

23.5

24.0

24.5

25.0

Degrees Celsius 400

450500550600650700

Microsiemens per centimeter

at 25 degrees Celsius

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Formazin nephelometric units

05

10152025

50

100150200250300350

Concentration, in milligrams per liter

0

100200300400500600700

2030405060708090

100110120

Concentration, in

micrograms per liter

Concentration, in

milligrams per liter

05

1015

20

25

Concentration, in milligrams per liter 0

5

10152025303540

0

0.020.040.060.08

0.10

0.120.140.160.18

02468

101214 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 XX X X X X XX X XXXXX X X X X X X

Dry period Wet period Dry period Wet period Dry period Wet period

Alkalinity

Strontium

ChlorideSulfate

Bromide

Dissolved oxygen

Specific conductance Calcium

Sodium

Turbidity Magnesium

Water temperature

1

X

Figure 18. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer characterization of San Marcos Springs, south-central Texas.
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

May 2010

Aug. 2010

Dec. 2010

Date

0

0.51.01.52.02.53.0

102030405060708090

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.70760.70770.70780.7079

0.7080

0.70810.70820.70830.7084

Concentration, 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

0.10.20.30.40.50.60.70.8

012345678

Delta deuterium (δD),

in per mil

Delta carbon-13 (δ13C),

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 Wet period Dry period Wet period

Nitrate plus nitrite

Boron δD

δ13 Fluoride C

Mg/Ca

Sr/Ca 87Sr/86Sr

1 Onset of major storm

 and identifierSpring, by orifice (table 1)—Sample Deep Spring Diversion Spring Weissmuller Spring

Comal Spring 1Hueco Spring A

EXPLANATION

Figure 18. Time series (November 2008–December 2010) of selected physicochemical properties and geochemical constituents for Edwards aquifer characterization of San Marcos Springs, south-central Texas.—Continued
56 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

The low NO3+NO2 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 μg/L) were particularly 

high relative to the other Edwards aquifer wells, as well as to 

the median value for samples from regional confined wells 

(median of 540 μg/L; Musgrove and others, 2010). Oetting 

(1995) identified examples of groundwater from the Edwards 

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–58–58–403 (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–67–01–403 (hereinafter, Solar well) 

were distinct from the other Edwards aquifer wells (Mg, Sr, 

Na, Cl, SO4, B, F, and NO3+NO2 concentrations; Mg/Ca and 

Sr/Ca ratios, 87Sr/86Sr, δD, and δ13C isotopic compositions) 

(table 5; fig. 15). With the exception of the Solar well, the 

Edwards aquifer wells showed little variability from the 

dry period to the wet period (table 5; fig. 15), indicating 

that groundwater from these wells was largely not affected 

by changes in hydrologic conditions and, thus, likely not 

influenced by local recharge sources or conduit flow paths. At 

the Solar well, however, numerous geochemical constituents 

changed markedly at the beginning of the wet period (fig. 

15), indicating that groundwater from this well was affected 

by mixing with a more saline groundwater source and (or) 

the influence of different geochemical processes. Toward 

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 

(fig. 15). The SO4/Cl ratio at the Solar well also changed 

significantly (increased) from the dry period to wet period, 

whereas SO4/Cl ratios in the other Edwards wells decreased 

(table 5). Previous studies have indicated that Trinity aquifer 

groundwater is characterized by higher SO4/Cl ratios than 

is Edwards aquifer saline-zone groundwater (Sharp and 

Clement, 1988; Garner and Mahler, 2007; Musgrove and 

others, 2010). The increase in SO4/Cl ratios in the Solar well 

at the onset of the wet period, accompanied by increases in ion 

concentrations (fig. 15), might be indicative of mixing with 

Trinity aquifer groundwater. 

Trinity Aquifer

Selected geochemical data for routine samples collected 

from the three Trinity aquifer wells (fig. 1, including Ruby 

Ranch) are shown in figure 16; summary statistics are shown 

in table 5. The Trinity aquifer wells exhibited differences in 

their geochemistry that, similar to the Edwards aquifer wells, 

likely reflect different flow paths and, consequently, different 

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 

concentrations and specific conductance values for samples 

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 

constituents did not change significantly from the dry period 

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 

A are shown in figure 18. Summary statistics are shown in 

table 5. The geochemistry of Comal Spring 1 varied little from 

the dry period to the wet period (table 5; fig. 18), indicating 

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 

supplied by regional flow paths originating in the western 

parts of the aquifer (Puente, 1976; Guyton and Associates, 

1979; LBG-Guyton Associates, 2004; Johnson and Schindel, 

2008). Samples collected from Comal Spring 1 exhibit some 

differences in chemical composition compared to samples 

collected from San Marcos Springs, notably lower specific 

conductance values, higher water temperature, and higher Sr 

and NO3+NO2 concentrations (table 5; fig. 18). In contrast to 

the geochemistry of samples collected from Comal Spring 1, 

the geochemistry of samples collected at Hueco Spring A 

was quite variable; the majority of selected constituents 

(19 of 28 physicochemical and geochemical) were 

significantly different during the dry period than during the 

wet period (table 5). Concentrations of some constituents 

decreased immediately following the onset of the wet period 
Geochemistry of San Marcos Springs and Nearby Hydrologic Features 57

(for example, Mg, Sr, Na, SO4, B, and F), likely reflecting 

rapid dilution by recharging surface water. These results 

indicate that Hueco Springs is very responsive to changes 

in hydrologic conditions and is strongly influenced by 

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. 

The two orifices of San Marcos Springs that were 

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 

Spring 1, few geochemical constituents changed significantly 

from the dry period to the wet period (table 5; fig. 18), 

indicating that Deep Spring is likely not influenced by 

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 

and geochemical constituents changed significantly from the 

dry period to the wet period). The number of constituents that 

varied significantly from the dry period to the wet period was 

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 

or flow paths than are Comal Springs or Deep Spring. The 

changes in geochemical constituents that occurred at Diversion 

Spring from the dry period to the wet period, however, are not 

consistent with dilution from recently recharged surface water; 

significantly higher median concentrations of Mg, Sr, Cl, 

SO4, potassium (K), B, and Br during the wet period indicate 

that Diversion Spring was likely influenced by a more saline 

groundwater source. Samples were collected at Weissmuller 

Spring only during the wet period; thus, changes in chemical 

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 

(table 5; fig. 18).

Geochemical Variability in Response to Storms

Samples were collected at stream and spring sites in 

response to three major storms (storms 1–3; table 3) during 

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 

in response to storms 1–3 depending on flow. No groundwater 

(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 

and spring response (table 3; fig. 6). In addition to the three 

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 

to storms 1–3 (fig. 6). At Blanco at Halifax, most increases 

in discharge were accompanied by rapid decreases in specific 

conductance and rapid increases in turbidity (figs. 11 and 13). 

Specific conductance values decreased by more than 100 μS/

cm in response to storms 1 and 3 and by a lesser amount 

(20 μS/cm) in response to storm 2, possibly because the 

stream had not recovered from storm 1; also, storm 2 was a 

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 

1–3 depending on flow (fig. 13); samples were collected from 

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 

largest in response to storms 1 and 3 (fig. 13). For storm 1, 

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 

Sink and Purgatory Creeks flowed. Storm 3, which is likely 

representative of similar large storms, resulted in substantial 

dilution of prestorm streamflow with the increased proportion 

of rainfall and runoff to the stream. Both storms 1 and 3 likely 

resulted in dilution of prestorm streamflow as indicated by the 

rapid decrease in specific conductance at Blanco at Halifax 

and relatively low concentrations of numerous geochemical 

constituents, including Mg, Sr, Na, Cl, SO4, Br, B, and F in 

the stream samples collected in response to storms relative to 

nonstorm (routine) samples (fig. 13). Because Cibolo Creek, 

Sink Creek, and Purgatory Creek only flowed in the study area 

in response to storms, there are no nonstorm water-quality 

data for comparison; however, concentrations of geochemical 

constituents measured in storm samples collected from these 

streams were generally low compared to those measured in 

routine samples from other stream sites (fig. 12).
58 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Springwater 

At Comal Spring 1, few changes in geochemistry were 

observed in response to storms 1–3 (fig. 13). For storm 1, the 

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 NO3+NO2 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 

predominantly supplied by regional flow paths and not 

affected by local recharge. In contrast to Comal Spring 1, 

substantial changes in geochemistry in response to storms 

were observed at Hueco Spring A (fig. 13). For storm 1, 

comparing results for the sample collected prior to and in 

response to the storm, specific conductance decreased 102 

μS/cm, turbidity increased 109 FNU, and dissolved oxygen 

concentrations increased by 3 mg/L; Mg, Sr, Na, Cl, SO4, Br, 

B, and F concentrations decreased substantially, the NO3+NO2

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, NO3+NO2 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 

Springs in response to storms 1–3 (fig. 6; table 3), as well 

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 

1–3, 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 

storms 1–3; few changes were observed in physicochemical 

properties in response to storms 1–3 (fig. 13) or other storms 

(fig. 19) that occurred during the study period. Changes 

in specific conductance or turbidity that occurred at Deep, 

Diversion, or Weissmuller Springs in response to storms were 

generally small and inconsistent (for example, in response to 

some storms, specific conductance increased slightly at Deep 

and Diversion Springs). Similarly, few geochemical changes 

were observed at San Marcos Springs orifices in response to 

storms 1–3 (fig. 13); some variability was observed in Br and 

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 

variability (fig. 13). 

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, 

as well as individual storm events, and are reflected in 

corresponding changes in spring geochemistry (White, 1988; 

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 

others, 2006; Garner and Mahler, 2007; Massei and others, 

2007; Musgrove and others, 2010; Mahler and others, 2011). 

Because recent recharge is typically geochemically distinct 

from groundwater, chemical mixing models using constituents 

such as specific conductance, isotopic compositions, major 

ion and (or) contaminant concentrations, and carbonate 

mineral saturation indices are often useful for distinguishing 

the proportion of recent recharge (quick flow) in karst 

groundwater and spring discharge (for example, Scanlon and 

Thrailkill, 1987; Lakey and Krothe, 1996; Pinault and others, 

2001; Vesper and White, 2004; Mahler and Massei, 2007; 

Mahler and Garner, 2009). Variations in geochemistry at San 

Marcos Springs were used to distinguish potential sources 

of spring discharge, specifically mixing between potential 

endmembers: local sources such as recently recharged surface 

water (quick flow) from the Blanco River and regional aquifer 

sources such as matrix-flow groundwater (slow flow). Mixing 

with a more saline groundwater source, specifically from the 

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 

Variations in specific conductance in karst groundwater 

and springwater are often interpreted to reflect varying 

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). 

Rainfall has very low specific conductance (Herczeg 

and Edmunds, 2000). In surface-water systems, specific 

conductance and associated major-ion concentrations 

generally decrease with increasing discharge, largely as a 

result of dilution by rainfall runoff (Hem, 1989). Likewise, 
Geochemistry of San Marcos Springs and Nearby Hydrologic Features 59

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Date

590

600

610

620

630

640

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

590

600

610

620

630

640

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

590

600

610

620

630

640

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

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 units

Specific conductance, 

in microsiemens per centimeter 

at 25 degrees Celsius

Turbidity, in formazin 

nephelometric units

0

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

590

600

610

620

630

640

590

600

610

620

630

640

590

600

610

620

630

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Specific conductance, 

in microsiemens per centimeter 

at 25 degrees Celsius

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

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 units 640

Turbidity, 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 XX X

X

X X X

X X X X X X X X

X XX

XXXXXXX XXXXXX

XXXXXX XXXXXXXXX

XXX

XXXXXXXXXXXXXXXXXXXXXX

XXXXXXXXXXXX XXXXXXXX XXXXXXXXXXXXXXX XXX XXX XXX

XXXXXXXXX

X X

XX

X X X X X X XX X

X X X

X X X X X X X X X X XX X

X

X X XX X X X X XX X X X X XX X XX X X X X XX X X X X X

X X

X X XX X

X XX XX

X XXXXX XX XX XXXXXXXXXXXXXXXXXXXX

X XX XXX XXXXXXXX XX

XX XXX

XX XXX XX XXX

XX XXX XX XXX

XX XXX XX XXX

XXX

X X X

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)

All values below

method reporting

level (0.3 formazin

nephelometric units)

X

Time period of rainfall 

 associated with

 storm

Spring, by orifice

 (table 1)

 Deep Spring

 Diversion Spring

 Weissmuller Spring

Nondetection, 

 shown at

 the method 

 reporting level

EXPLANATION

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.
60 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

specific conductance of groundwater can act as a tracer 

of recently infiltrated runoff and associated surface water 

that recharges a karst aquifer (for example, Scanlon and 

Thrailkill, 1987; Desmarais and Rojstaczer, 2002; Massei 

and others, 2007). In response to rain and recharge events, 

karst springs and groundwater wells that are affected by 

focused recharge generally show a rapid decrease in specific 

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 

in specific conductance is typically observed less than a day 

after the onset of rainfall and recharge (City of Austin, 1997; 

Mahler and others, 2006, 2011), and similar responses have 

been documented at groundwater wells affected by focused 

recharge and conduit flow (Garner and Mahler, 2007). 

Results of this study indicate that, similar to Barton Springs, 

Hueco Springs is affected by a large component of focused 

recharge, with a rapid decrease in specific conductance and 

large changes in other geochemical constituents occurring 

in response to recharge events (for example, storms 1 and 3) 

(figs. 13 and 18). In contrast, changes in specific conductance 

at Comal Springs were small during the study period (within 

a range of 27 μS/cm, or about 5 percent) and showed little 

variability from the dry period to the wet period (fig. 18) 

or in response to storms (fig. 13). The lack of variability in 

specific conductance at Comal Springs is consistent with 

the interpretation that Comal Springs discharge is supplied 

largely by regional groundwater flow paths and is not affected 

appreciably by local recharge sources. 

Specific conductance values at San Marcos Springs (Deep 

and Diversion) varied during the study period within a range 

of about 40 μS/cm (fig. 18), intermediate to that for Hueco 

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 

Springs). Specific conductance at Deep and Diversion Springs 

did not consistently change in response to storms 1–3 (fig. 

13) or in response to other rain events (fig. 19) and generally 

did not show rapid changes indicative of the effect of dilution 

by recent recharge. Variability in specific conductance at 

San Marcos Springs occurred during longer periods such as 

months, which is not indicative of recharge from individual 

rainfall and recharge events; rather, specific conductance 

variability seems to reflect a complex relation with spring 

discharge (fig. 20) that departs from the typical conceptual 

understanding of karst aquifers. For example, specific 

conductance at Diversion Spring decreased initially at the start 

of the wet period and then remained relatively constant for 

several months until January 2010 (fig. 20). In January 2010 

specific conductance increased at both Deep and Diversion 

Springs, following a similar pattern at the two orifices, then 

leveled off, and then decreased through spring 2010. Specificconductance values at Deep and Diversion Springs had a 

similar rise and subsequent drop during summer 2009 (in the 

dry period, when little to no recharge was occurring) (fig. 20). 

The relation between specific conductance and discharge for a 

karst spring for which discharge is dominated by a continuum 

between two endmembers of quick flow and slow flow would 

likely be characterized by an inverse relation, where specific 

conductance generally decreases with increased discharge, 

reflecting a larger component of quick flow at higher discharge 

values. The relation between specific conductance and 

discharge at San Marcos Springs indicates that additional 

water sources contribute to spring discharge. Specific 

conductance generally decreased with increasing discharge 

for low discharge conditions (<100 ft3/s); at around 150 ft3/s, 

however, a change in slope occurred, and specific conductance 

increased with discharge for higher discharge values (fig. 20). 

This increase in specific conductance with higher discharge 

values of more than 150 ft3/s was significant for all three 

monitored orifices (Kendall’s tau of 0.51, 0.51, and 0.41 for 

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 

and specific conductance (fig. 20) indicate that San Marcos 

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 ft3/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 

flow paths, diffuse and conduit recharge sources, mineralsolution 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 

isotopes (for example, Oetting, 1995; Sharp and Banner, 

1997; Musgrove and Banner, 2004; Wong and others, 2011; 

Musgrove and others, 2010). As discussed in Musgrove 

and others (2010), Mg/Ca and Sr/Ca ratios in carbonate 

groundwater typically increase along flow paths as a result of 

water-rock interaction and progressive groundwater evolution 

processes (for example, calcite recrystallization, incongruent 

dolomite dissolution, and prior precipitation of calcite along 

flow paths) (Plummer, 1977; Trudgill and others, 1980; 

Lohmann, 1988; Fairchild and others, 2000; Musgrove and 

Banner, 2004). Higher Mg/Ca and Sr/Ca ratios are consistent 

with longer residence times and greater extents of mineralsolution reaction. Strontium isotope ratios (87Sr/86Sr) in the 

Edwards aquifer have been applied in conjunction with Mg/

Ca and Sr/Ca ratios as tracers of water-rock interaction, 

groundwater residence time, recharge, and the influence of 

soil composition on groundwater geochemistry (Oetting and 

others, 1996; Musgrove and Banner, 2004; Garner, 2005; 

Wong and others, 2011). As demonstrated in these studies, Sr 
Interaction Between Surface Water and Groundwater 61

Date

0 50 100 150 200 250 300

Spring discharge, in cubic feet per second

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

580

590

600

610

620

630

640

650

Specific conductance, in microsiemens

per centimeter at 25 degrees Celsius

580

590

600

610

620

630

640

Specific conductance, in microsiemens

per centimeter at 25 degrees Celsius

50

100

150

200

250

300

Spring discharge, in cubic feet per second

1 2 3

B

A

San Marcos Springs, by

 orifice (table 1)

 Deep Spring

 Diversion Spring

 Weissmuller Spring

EXPLANATION

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 Spring

EXPLANATION

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.
62 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

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 

and others, 1985; Oetting, 1995). Higher 87Sr/86Sr values 

for groundwater relative to the aquifer rocks are indicative 

of a source of more radiogenic Sr (enriched in 87Sr) to the 

groundwater, which has been previously proposed to result 

from chemical interaction with overlying soils (Musgrove 

and Banner, 2004). Lower 87Sr/86Sr 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, 

flow paths, and residence time can affect individual values of 

these geochemical constituents. 

Variations in Mg/Ca and Sr/Ca ratios and Sr isotope 

values for the springs were evaluated to distinguish potential 

differences in residence time, flow paths, and components 

of diffuse versus conduit flow affecting the springs. Results 

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 

significantly different between the dry and wet periods and 

covered a larger range than at Comal Spring 1 or San Marcos 

Springs (table 5); lower Mg/Ca and Sr/Ca ratios and higher 

Sr isotope values during the wet period are consistent with a 

large component of recently recharged, less geochemically 

evolved water contributing to Hueco Springs (fig. 21). In 

contrast with Hueco Spring A, Mg/Ca and Sr/Ca ratios and 

87Sr/86Sr values at Comal Spring 1 and at Deep Spring were 

not significantly different between the dry and wet periods 

(table 5; fig. 21). At Diversion Spring, Mg/Ca and Sr/Ca 

ratios were significantly different between the dry period 

and wet period (table 5) but covered a small range of values 

relative to Hueco Spring A; 87Sr/86Sr values at Diversion 

Spring were not significantly different between the dry 

and wet periods (table 5). Ratios of Mg/Ca and Sr/Ca at 

Diversion Spring during the dry period were similar to values 

at Deep Spring; during the wet period, Mg/Ca and Sr/Ca 

ratios at Diversion Spring shifted to higher values relative to 

the dry period (table 5; fig. 21). 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 87Sr/86Sr 

values (collected during only the wet period) are similar to 

those for Diversion Spring during the wet period (table 5; 

fig. 21) and imply a similar origin and similar flow paths for 

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 

compositions (regional groundwater flow, local stream 

recharge, saline-zone groundwater, and Trinity aquifer 

groundwater) and mass-transfer processes along hypothetical 

flow paths that could feasibly produce the composition of 

discharge at San Marcos Springs. Inverse modeling was 

considered for a range of hydrologic conditions (dry, wet, 

and storm conditions) (fig. 22) and a variety of different 

endmember combinations. Endmember water compositions 

used in inverse modeling to account for the composition of 

San Marcos Springs discharge (specifically Deep, Diversion, 

or Weissmuller Springs) were as follows:

• Regional groundwater flow: Two endmembers 

were used to represent Edwards aquifer regional 

groundwater flow: the composition of Comal Springs 

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) 

of Comal Springs (figs. 1 and 2; table 1). Because 

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 

specific mass-balance constraints), model solutions 

included either composition or a mixture of both. Both 

Comal Springs and well 4D are located upgradient 

from San Marcos Springs; previous studies have 

proposed that Comal Springs is largely supplied by 

regional groundwater flow paths originating in the 

western parts of the aquifer and that San Marcos 

Springs is supplied at least in part by the same regional 

groundwater flow paths (Puente, 1976; Guyton and 

Associates, 1979; LBG-Guyton Associates, 2004; 

Johnson and Schindel, 2008). The geochemistry of 

Comal Springs and well 4D are similar and varied little 

from the dry period to the wet period (table 5; figs. 15 

and 18); the geochemistry of Comal Springs varied 

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 

regional groundwater flow.

• 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 

regional extent of the Edwards aquifer (Clement, 1989; 
Interaction Between Surface Water and Groundwater 63

0.70780 0.70785 0.70790 0.70795 0.70800 0.70805 0.70810 0.70815 0.70820

Strontium-87/strontium-86 isotopic ratio

Magnesium to calcium, molar ratio

0 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

0.50

Magnesium to calcium, molar ratio

0

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Strontium 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 period

EXPLANATION

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 103). 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.
64 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

Date

Spring discharge, in cubic feet per second

Stream discharge, in cubic feet per second

50

100

150

200

250

300

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

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 

EXPLANATION

Dry period Wet 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–67–

01–813A) (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 

DX–68–07–505 (hereinafter, Eagle Peak well) in 

Comal County and well LR–68–08–502 (hereinafter, 

Burns well) in Hays County (fig. 1; table 1). The 

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 
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 

specific conductance values and notably high SO4

and Sr concentrations relative to Edwards aquifer 

groundwater (Eagle Peak: 797 μS/cm, 77.7 μg/L SO4, 

9,970 μg/L Sr; Burns: 1,030 μS/cm, 214 μg/L SO4, 

2,010 μg/L Sr, respectively) (Crow, 2012). Similar 

to the regional groundwater flow endmembers, for 

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 

or both based on specific mass-balance constraints), 

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 

period, little to no surface recharge was occurring; regional 

groundwater flow endmembers (Comal Springs discharge 

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 

is consistent with a dominant component of regional flow 

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 

of regional groundwater flow with a small component of 

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 

much as 25.9 percent of spring discharge; for example, for the 

date with the largest modeled component of local recharge 

(May 29, 2009), flow in the Blanco River was less than 

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 δD value 

on May 28, 2009, of -7.2 per mil [Crow, 2012]) would yield 

a significant increase in the δD 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 

dry period (fig. 18). As a result, dry-period model results 

with a relatively large component of local recharge (>5%) are 

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 

at this site and other San Marcos Springs orifices throughout 

the study (fig. 18). Higher Br concentrations are associated 

with saline-zone groundwater; however, the increase in 

the Br concentration on this date was not accompanied by 

increases in other dissolved constituents (fig. 18) that would 

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.
66 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008–10).—Continued

[See table 1 for complete U.S. Geological Survey station names and numbers. ft3/s, cubic feet per second; X, not used or considered in model; minor, less than 

0.001 percent; Br, bromide; ~, approximately; --, not applicable, as no models found]

Sample 

collection

Hydrologic 

condition 

(dry period 

or wet 

period)

Date for 

listed 

discharge 

conditions

San Marcos 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Comal 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Blanco 

River (Halifax 

Ranch station) 

discharge

(ft3/s) 

(mean 

daily) 

PHREEQC model details1–8

Initial water 

compositions3–7

Final water 

composition8

Routine sampling

Dec. 2008 dry 12/1/08 102 285 1013 Regional (Comal Springs, 12/2/08, and/

or well 4D, 12/16/08); saline zone

Deep Spring 

(12/1/08)

Regional (Comal Springs, 12/2/08, and/

or well 4D, 12/16/08); saline zone; 

local (Blanco River 12/9/08)

Regional (Comal Springs, 12/2/08, and/

or well 4D, 12/16/08); Trinity aquifer

Regional flow (Comal Springs, 12/2/08, 

and/or well 4D, 12/16/08); Trinity 

aquifer; local (Blanco River 12/9/08)

Regional (Comal Springs, 12/2/08, and/

or well 4D, 12/16/08); saline zone

Diversion 

Spring 

(12/1/08)

Regional (Comal Springs, 12/2/08, and/

or well 4D, 12/16/08); saline zone; 

local (Blanco River 12/9/08)

Regional (Comal Springs, 12/2/08, and/

or well 4D, 12/16/08); Trinity aquifer

Regional (Comal Springs, 12/2/08, and/

or well 4D, 12/16/08); Trinity 

aquifer; local (Blanco River 12/9/08)

May/June 

2009

dry 5/29/09 93 220 8.1 Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09); saline zone

Deep Spring 

(5/27/09)

Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09); saline zone; 

local (Blanco River 5/29/09)

Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09);Trinity aquifer

Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09); Trinity aquifer; 

local (Blanco River 5/29/09)

Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09); saline zone

Diversion 

Spring 

(5/27/09)

Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09); saline zone; local (Blanco River 5/29/09)

Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008–10).

[See table 1 for complete U.S. Geological Survey station names and numbers. ft3/s, cubic feet per second; X, not used or considered in model; minor, less than 

0.001 percent; Br, bromide; ~, approximately; --, not applicable, as no models found]
Interaction Between Surface Water and Groundwater 67

Sample 

collection

Model results, proportion of initial water compositions (percent)1–8

Local 

recharge 

(Blanco 

River)3

Regionalflow 

groundwater 

(Comal 

Springs)4

Regionalflow 

groundwater 

(well 

4D)4

Salinezone 

groundwater5

Trinity 

aquifer 

groundwater 

(regional/

mineralized)6

Trinity 

aquifer 

groundwater 

(local)7

Number of 

models 

found, 

minimal 

models 

found9

Maximum 

fractional 

uncertainty in model 

(element 

concentrations)

Comments

Routine sampling

Dec. 2008 X 40.4–

53.2

46.6–

59.4

0.1–0.2 X X 12, 2 0.04

0–minor40.4–

53.2

46.6–

59.4

0.1–0.2 X X 12, 3 0.04 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge 

included for comparison.

X 99.5–100 0–0.5 X 0 0–0.5 61, 1 0.10 Relatively high uncertainty needed; few models 

include Trinity aquifer.

0–0.6 99.5–100 0–0.5 X 0–minor 0–0.5 34, 1 0.10 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge included for comparison. Relatively high uncertainty 

needed. Few models include Trinity aquifer.

X 39.6–

58.3

41.7–

60.3

0–0.1 X X 30, 9 0.06

0–3.0 46.9–

58.9

41.1–

51.5

0.1 X X 14, 7 0.06 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge 

included for comparison.

X 58.3–

59.7

40.3–

41.7

X 0 0–0.13 16, 2 0.06

0–0.5 50.2–

59.7

40.3–

49.8

X 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

X 55.4–

67.0

33.0–

44.5

0.1 X X 55, 1 0.05

10.4–

20.3

42.4–

48.6

34.4–

42.2

0.1 X X 23,1 0.04 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge 

included for comparison; estimated proportion 

of local recharge unlikely based on hydrologic 

conditions.

X 96.9 3.1 X 0 0 33, 1 0.06 No models include Trinity aquifer.

18.4–

25.9

71.0–

77.4

0–5.0 X 0 0 13, 1 0.05 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge 

included for comparison; estimated proportion 

of local recharge unlikely based on hydrologic 

conditions. No models include Trinity aquifer.

X 69.4–

69.9

30.0–

30.6

0.1 X X 57, 1 0.06

4.5–

4.6

6.5 30.4 0.1 X X 7, 1 0.06 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge 

included for comparison.
68 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008–10).—Continued

[See table 1 for complete U.S. Geological Survey station names and numbers. ft3/s, cubic feet per second; X, not used or considered in model; minor, less than 

0.001 percent; Br, bromide; ~, approximately; --, not applicable, as no models found]

Sample 

collection

Hydrologic 

condition 

(dry period 

or wet 

period)

Date for 

listed 

discharge 

conditions

San Marcos 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Comal 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Blanco 

River (Halifax 

Ranch station) 

discharge

(ft3/s) 

(mean 

daily) 

PHREEQC model details1–8

Initial water 

compositions3–7

Final water 

composition8

Routine sampling—Continued

Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09); Trinity aquifer

Regional (Comal Springs, 5/27/09, and/

or well 4D, 6/01/09); Trinity aquifer; 

local (Blanco River 5/29/09)

July 2009 dry 7/21/09 86 170 3 Regional (Comal Springs, 7/16/09, and/ 

or well 4D, 7/20/09); saline zone

Deep Spring 

(7/21/09)

Regional (Comal Springs, 7/16/09, and/

or well 4D, 7/20/09); saline zone; local (Blanco River 7/17/09)

Regional (Comal Springs, 7/16/09, and/ 

or well 4D, 7/20/09); Trinity aquifer

Regional (Comal Springs, 7/16/09, and/ 

or well 4D, 7/20/09); Trinity aquifer; 

local (Blanco River 7/17/09)

Regional (Comal Springs, 7/16/09, and/ 

or well 4D, 7/20/09); saline zone

Diversion 

Spring 

(7/21/09)

Regional (Comal Springs, 7/16/09, and/ 

or well 4D, 7/20/09); saline zone; local (Blanco River 7/17/09)

Regional (Comal Springs, 7/16/09, and/ 

or well 4D, 7/20/09); Trinity aquifer

Regional (Comal Springs, 7/16/09, and/ 

or well 4D, 7/20/09); Trinity aquifer; 

local (Blanco River 7/17/09)

Aug./Sept. 

2009

dry 9/1/09 85 163 2.4 Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); saline zone

Deep Spring 

(9/1/09)

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); saline zone

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); saline zone; local (Blanco River 9/2/09)
Interaction Between Surface Water and Groundwater 69

Sample 

collection

Model results, proportion of initial water compositions (percent)1–8

Local 

recharge 

(Blanco 

River)3

Regionalflow 

groundwater 

(Comal 

Springs)4

Regionalflow 

groundwater 

(well 

4D)4

Salinezone 

groundwater5

Trinity 

aquifer 

groundwater 

(regional/

mineralized)6

Trinity 

aquifer 

groundwater 

(local)7

Number of 

models 

found, 

minimal 

models 

found9

Maximum 

fractional 

uncertainty in model 

(element 

concentrations)

Comments

Routine sampling—Continued

X 90.0–100 0–10 X 0 0 17, 1 0.08 Relatively high uncertainty needed. No models 

include Trinity aquifer.

0.3–

1.2

91.3–

92.0

7.4–7.8 X 0 0–minor15, 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 X 41.9–

51.9

47.9–

57.9

0.2 X X 9, 1 0.04

6.0–

9.2

11.8–

32.6

61.2–

78.8

0.2 X X 8,1 0.03 Little to no surface recharge occurring for 

these hydrologic conditions, but local 

recharge included for comparison; estimated 

proportion of local recharge unlikely based on 

hydrologic conditions. 

X -- -- X -- -- -- 0.10 No models.

X -- -- X -- -- -- 0.10 No models.

X 11.2–

14.6

85.3–

88.7

0.1 X X 7, 1 0.05

2.6–

6.8

0–10.5 86.8–

96.4

0.1 X X 18, 3 0.04 Little to no surface recharge occurring for 

these hydrologic conditions, but local 

recharge included for comparison.

X 78.3–

87.2

12.8–

21.7

X -- 0.1–0.6 21, 1 0.09 Relatively high uncertainty needed.

13.7–

15.6

36.1–

42.2

44.1–

48.4

X -- 0–0.2 10, 1 0.07 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge 

included for comparison; relatively high 

proportion of local recharge estimated is 

unlikely based on hydrologic conditions. 

Relatively high uncertainty needed.

Aug./Sept. 

2009

X 99.8 0–minor 0.2 X X 9, 1 0.08 Relatively high uncertainty needed.

X 70.2–

70.5

29.4–

29.7

0.1 X X 8, 1 0.04 No Br in model balances (composition of Deep 

Spring on 9/1/09 has anomalously high Br).

0–3.3 66.0–

70.5

29.3–

30.9

0.1 X X 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).
70 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008–10).—Continued

[See table 1 for complete U.S. Geological Survey station names and numbers. ft3/s, cubic feet per second; X, not used or considered in model; minor, less than 

0.001 percent; Br, bromide; ~, approximately; --, not applicable, as no models found]

Sample 

collection

Hydrologic 

condition 

(dry period 

or wet 

period)

Date for 

listed 

discharge 

conditions

San Marcos 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Comal 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Blanco 

River (Halifax 

Ranch station) 

discharge

(ft3/s) 

(mean 

daily) 

PHREEQC model details1–8

Initial water 

compositions3–7

Final water 

composition8

Routine sampling—Continued

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); Trinity aquifer

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); Trinity aquifer; 

local (Blanco River 9/2/09)

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); saline zone

Diversion 

Spring 

(9/1/09)

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); saline zone;

local (Blanco River 9/2/09)

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); Trinity aquifer

Regional (Comal Springs, 8/31/09, and/

or well 4D, 8/31/09); Trinity aquifer; 

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/

or well 4D, 12/3/09); local (Blanco 

River 11/4/09); saline zone

Deep Spring 

(11/2/09)

Regional (Comal Springs, 11/3/09, and/

or well 4D, 12/3/09); local (Blanco 

River 11/4/09); Trinity aquifer

Regional (Comal Springs, 11/3/09, and/

or well 4D, 12/3/09); local (Blanco 

River 11/4/09); saline zone

Diversion 

Spring 

(11/2/09)

Regional (Comal Springs, 11/3/09, and/

or well 4D, 12/3/09); local (Blanco 

River 11/4/09); Trinity aquifer

Feb. 2010 wet (between 

storms 2 

and 3)

2/4/10 223 354 1,780 Regional (Comal Springs, 2/2/10, and/or 

well 4D, 2/2/10); local (Blanco River 

2/4/10); saline zone

Deep Spring 

(2/4/10)

Regional (Comal Springs, 2/2/10, and/or 

well 4D, 2/2/10); local (Blanco River 

2/4/10); Trinity aquifer

Regional (Comal Springs, 2/2/10, and/or 

well 4D, 2/2/10); local (Blanco River 

2/4/10); saline zone

Diversion 

Spring 

(2/4/10)

Regional (Comal Springs, 2/2/10, and/or 

well 4D, 2/2/10); local (Blanco River 

2/4/10); Trinity aquifer
Interaction Between Surface Water and Groundwater 71

Sample 

collection

Model results, proportion of initial water compositions (percent)1–8

Local 

recharge 

(Blanco 

River)3

Regionalflow 

groundwater 

(Comal 

Springs)4

Regionalflow 

groundwater 

(well 

4D)4

Salinezone 

groundwater5

Trinity 

aquifer 

groundwater 

(regional/

mineralized)6

Trinity 

aquifer 

groundwater 

(local)7

Number of 

models 

found, 

minimal 

models 

found9

Maximum 

fractional 

uncertainty in model 

(element 

concentrations)

Comments

Routine sampling—Continued

X -- -- X -- -- 0 0.10 No models (with or without Br in constraints).

 -- -- -- X -- -- 0 0.10 No models (with or without Br in constraints).

X 60.4–

71.1

28.8–

39.5

0.1 X X 25,1 0.08 Relatively high uncertainty needed.

0–minor56.9–

57.7

42.2–

43.0

0.1 X X 12, 1 0.07 Little to no surface recharge occurring for these 

hydrologic conditions, but local recharge 

included for comparison. Relatively high 

uncertainty needed.

X 84.9–

86.8

13.2–

15.1

X 0 0 25, 1 0.08 Relatively high uncertainty needed; no models 

include Trinity aquifer.

0–minor84.9–

99.7

0.3–15.1 X 0–minor 0–0.3 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 0–11.9 78.7–

88.9

0–13.2 0.1 X X 12, 6 0.04 In time period of large and rapid increase in San 

Marcos Springs discharge; rising hydrograph.

0–6.5 93.5–100 0–minor X 0.0 0–0.7 11, 3 0.09 In time period of large and rapid increase in San 

Marcos Springs discharge; rising hydrograph; 

relatively high uncertainty needed.

8.2–

9.6

90.4–

91.8

0–minor 0.05–

0.06

X X 8, 1 0.04 In time period of large and rapid increase in San 

Marcos Springs discharge; rising hydrograph.

3.6–

23.5

75.3–

95.7

0–minor X 0.7–1.2 0–0.8 7, 1 0.05 In time period of large and rapid increase in San 

Marcos Springs discharge; rising hydrograph.

Feb. 2010 15.9 40.7 43.2 0.2 X X 4, 1 0.03 In time period of large and rapid increase in San 

Marcos Springs discharge; approaching highest 

discharge during study; rising hydrograph.

0–0.1 99.9–100 0–minor X 0.0 0–minor24, 1 0.08 In time period of large and rapid increase in San 

Marcos Springs discharge; approaching highest 

discharge during study; rising hydrograph; 

relatively high uncertainty needed; Trinity 

aquifer included in only one model.

6.6–

10.3

72.9–

73.9

15.7–

20.4

0.1 X X 5, 1 0.03 In time period of large and rapid increase in San 

Marcos Springs discharge; approaching highest 

discharge during study; rising hydrograph.

 -- -- -- X -- -- 0 0.10 In time period of large and rapid increase in San 

Marcos Springs discharge; approaching highest 

discharge during study; rising hydrograph; no 

models.
72 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008–10).—Continued

[See table 1 for complete U.S. Geological Survey station names and numbers. ft3/s, cubic feet per second; X, not used or considered in model; minor, less than 

0.001 percent; Br, bromide; ~, approximately; --, not applicable, as no models found]

Sample 

collection

Hydrologic 

condition 

(dry period 

or wet 

period)

Date for 

listed 

discharge 

conditions

San Marcos 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Comal 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Blanco 

River (Halifax 

Ranch station) 

discharge

(ft3/s) 

(mean 

daily) 

PHREEQC model details1–8

Initial water 

compositions3–7

Final water 

composition8

Routine sampling—Continued

June 2010 wet 6/7/10 220 355 139 Regional (Comal Springs, 6/1/10, and/or 

well 4D, 6/3/10); local (Blanco River 

6/2/10); saline zone

Deep Spring 

(6/7/10)

Regional (Comal Springs, 6/1/10, and/or 

well 4D, 6/3/10); local (Blanco River 

6/2/10); Trinity aquifer

Regional (Comal Springs, 6/1/10, and/or 

well 4D, 6/3/10); local (Blanco River 

6/2/10); saline zone

Diversion 

Spring 

(6/7/10)

Regional (Comal Springs, 6/1/10, and/or 

well 4D, 6/3/10); local (Blanco River 

6/2/10); Trinity aquifer

Regional (Comal Springs, 6/1/10, and/

or well 4D, 6/3/10); Blanco River 

(6/2/10); saline zone

Weissmuller 

Spring 

(6/7/10)

Regional (Comal Springs, 6/1/10, and/or 

well 4D, 6/3/10); local (Blanco River 

6/2/10); Trinity aquifer

Nov./Dec. 

2010

wet 12/1/10 171 332 55 Regional (Comal Springs, 11/30/10, and/

or well 4D, 11/30/10); local (Blanco 

River 12/1/10); saline zone

Deep Spring 

(12/1/10)

Regional (Comal Springs, 11/30/10, and/

or well 4D, 11/30/10); local (Blanco 

River 12/1/10); Trinity aquifer

Regional (Comal Springs, 11/30/10, and/

or well 4D, 11/30/10); local (Blanco 

River 12/1/10); saline zone

Diversion 

Spring 

(12/1/10)

Regional (Comal Springs, 11/30/10, and/

or well 4D, 11/30/10); local (Blanco 

River 12/1/10); Trinity aquifer

Regional (Comal Springs, 11/30/10, and/

or well 4D, 11/30/10); local (Blanco 

River 12/1/10); saline zone

Weissmuller 

Spring 

(12/1/10)

Regional (Comal Springs, 11/30/10, and/

or well 4D, 11/30/10); local (Blanco 

River 12/1/10); Trinity aquifer
Interaction Between Surface Water and Groundwater 73

Sample 

collection

Model results, proportion of initial water compositions (percent)1–8

Local 

recharge 

(Blanco 

River)3

Regionalflow 

groundwater 

(Comal 

Springs)4

Regionalflow 

groundwater 

(well 

4D)4

Salinezone 

groundwater5

Trinity 

aquifer 

groundwater 

(regional/

mineralized)6

Trinity 

aquifer 

groundwater 

(local)7

Number of 

models 

found, 

minimal 

models 

found9

Maximum 

fractional 

uncertainty in model 

(element 

concentrations)

Comments

Routine sampling—Continued

June 2010 2.5–

17.5

43.9–

76.5

21.0–

38.9

0.1 X X 12, 4 0.04 Time period associated with falling hydrograph; 

near date (following) of highest discharge 

during study.

 -- -- -- X -- -- 0 0.10 Time period associated with falling hydrograph; 

near date (following) of highest discharge 

during study; no models.

8.1–

17.7

40.1–

57.7

33.3–

42.7

0.1 X X 14, 4 0.03 Time period associated with falling hydrograph; 

near date (following) of highest discharge 

during study.

0–0.1 97.8–100 0–0.2 X 0–0.8 0–1.3 14, 2 0.09 Time period associated with falling hydrograph; 

near date (following) of highest discharge 

during study; relatively high uncertainty needed.

4.9–

12.6

17.3–

59.0

34.6–

69.8

0.2 X X 5, 1 0.03 Time period associated with falling 

hydrograph; near date (following) of 

highest discharge during study.

 -- -- -- X -- -- 0 0.10 Time period associated with falling hydrograph; 

near date (following) of highest discharge 

during study; no models.

Nov./Dec. 

2010

0–5.7 16.7–

21.4

75.8–

83.1

0.2 X X 12, 3 0.04 Time period associated with falling hydrograph; 

~3 months post large recharge event from 

tropical storm Hermine.

0–3.5 95.9–100 0–2.4 X 0 0–minor9, 1 0.10 Time period associated with falling hydrograph; 

~3 months post large recharge event from tropical storm Hermine; relatively high uncertainty 

needed.

21.0–

24.4

74.4–

79.0

0–1.1 0.1 X X 22, 1 0.06 Time period associated with falling hydrograph; 

~3 months post large recharge event from 

tropical storm Hermine.

0–9.1 90.9–100 0–6.1 X 0 0–0.9 14, 1 0.10 Time period associated with falling hydrograph; 

~3 months post large recharge event from 

tropical storm Hermine; relatively high 

uncertainty needed.

25.3–

28.9

68.1–

74.7

0–3.0 0.1 X X 12, 1 0.06 Time period associated with falling hydrograph; 

~3 months post large recharge event from 

tropical storm Hermine.

 -- -- -- X -- -- 0 0.10 Time period associated with falling hydrograph; 

~3 months post large recharge event from 

tropical storm Hermine; no models.
74 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Table 6. Summary of PHREEQC (Parkhurst and Appelo, 1999) inverse geochemical modeling results for San Marcos Springs, southcentral Texas (2008–10).—Continued

[See table 1 for complete U.S. Geological Survey station names and numbers. ft3/s, cubic feet per second; X, not used or considered in model; minor, less than 

0.001 percent; Br, bromide; ~, approximately; --, not applicable, as no models found]

Sample 

collection

Hydrologic 

condition 

(dry period 

or wet 

period)

Date for 

listed 

discharge 

conditions

San Marcos 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Comal 

Springs 

discharge 

(ft3/s) 

(mean 

daily) 

Blanco 

River (Halifax 

Ranch station) 

discharge

(ft3/s) 

(mean 

daily) 

PHREEQC model details1–8

Initial water 

compositions3–7

Final water 

composition8

Storm sampling

Storm 1 

(9/9/09)

start of wet 

period; 

response 

to storm 1

9/20/09 97 (storm 

range 

86–97)

210 (storm 

range 

173–210)

16 (storm 

range 

2.8–211)

Regional (Comal Springs, 9/9/09, and/

or well 4D, 8/31/09); local, storm 

(Blanco River 9/9/09); saline zone

Diversion 

Spring 

(9/20/09)

Regional (Comal Springs, 9/9/09, and/

or well 4D, 8/31/09); local, storm 

(Blanco River 9/9/09); Trinity aquifer

9/16/09 96 (storm 

range 

86–97)

211 (storm 

range 

173–210)

27 (storm 

range 

2.8–211)

Regional (Comal Springs, 9/9/09, and/

or well 4D, 8/31/09); local, storm 

(Blanco River 9/9/09); saline zone

Diversion 

Spring 

(9/16/09) 

(most dilute)

Regional (Comal Springs, 9/9/09, and/

or well 4D, 8/31/09); local, storm 

(Blanco River 9/9/09); Trinity aquifer

Storm 2 

(10/3/09)

wet period; 

response 

to storm 2

10/9/09 142 (storm 

range 

96–147)

265 (storm 

range 

210–274)

196 (storm 

range 21–220)

Regional (Comal Springs, 10/7/09, and/

or well 4D, 10/5/09); local, storm 

(Blanco River 10/4/09); saline zone

Diversion 

Spring 

(10/9/09) 

(most dilute)

Regional (Comal Springs, 10/7/09, and/

or well 4D, 10/5/09); local, storm 

(Blanco River 10/4/09); Trinity 

aquifer

Storm 3 

(9/7/10)

wet period; 

response 

to storm 3

9/22/10 218 (storm 

range 

207–223)

357 (storm 

range 

313–348)

203 (storm 

range 

46–3,620)

Regional (Comal Springs, 9/8/10, and/

or well 4D, 10/1/10); local, storm 

(Blanco River 9/8/10); saline zone

Diversion 

Spring 

(9/22/10)

Regional (Comal Springs, 9/8/10, and/

or well 4D, 10/1/10); local, storm 

(Blanco River 9/8/10); Trinity aquifer

9/29/10 218 (storm 

range 

207–223)

352 (storm 

range 

313–348)

159 (storm 

range 

46–3,620)

Regional (Comal Springs, 9/8/10, and/

or well 4D, 10/1/10); local, storm 

(Blanco River 9/8/10); saline zone

Diversion 

Spring 

(9/29/10) 

(most dilute)

Regional (Comal Springs, 9/8/10, and/

or well 4D, 10/1/10); local, storm 

(Blanco River 9/8/10); Trinity aquifer

1

Mineral and gas phases included for all models: calcite, dolomite, gypsum, quartz, carbon dioxide (gas), oxygen (gas), CaX2 (calcium exchange site), NaX 

(sodium exchange site); phase transfer options for dolomite and gypsum included only dissolution (no precipitation).

2

Models balanced on ions included in phases, plus boron, Br, chloride, fluoride, potassium, sodium, and strontium, unless otherwise noted.

3

Composition of representative local recharge water based on samples from Blanco River at Halifax Ranch near Kyle, Texas (station number 08171290).

4

Composition of representative regional flow water based on samples from Comal Spring 1 (DX–68–23–301) and well 4D (DX–68–16–707).

5

Composition of regional/mineralized Trinity aquifer groundwater based on sample from well LR–67–01–813A, as discussed in text.

6

Composition of regional Trinity aquifer groundwater based on sample from well DX–68–07–505 (Eagle Peak well) in Comal County, as discussed in text.
Interaction Between Surface Water and Groundwater 75

Sample 

collection

Model results, proportion of initial water compositions (percent)1–8

Local 

recharge 

(Blanco 

River)3

Regionalflow 

groundwater 

(Comal 

Springs)4

Regionalflow 

groundwater 

(well 

4D)4

Salinezone 

groundwater5

Trinity 

aquifer 

groundwater 

(regional/

mineralized)6

Trinity 

aquifer 

groundwater 

(local)7

Number of 

models 

found, 

minimal 

models 

found9

Maximum 

fractional 

uncertainty in model 

(element 

concentrations)

Comments

Storm sampling

Storm 1 

(9/9/09)

8.7–

10.2

60.7–

68.1

21.7–

30.6

0–0.03 X X 8, 1 0.05 Storm marks transition from dry period to 

wet period.

10.6 67.4 2.1 X 0 0 4, 1 0.05 Storm marks transition from dry period to wet 

period; no models include Trinity aquifer.

12.9–

13.1

64.1–

65.3

21.6–

23.0

0.1 X X 4, 1 0.04 Storm marks transition from dry period to 

wet period.

14.8 78.1 7.0 X 0 0–minor4, 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 X X 5, 1 0.06 No models include saline zone.

0–1.5 98.3–100 0–0.3 X 0 0–0.5 19, 1 0.09 Relatively high uncertainty needed.

Storm 3 

(9/7/10)

0–1.6 19.0–

83.1

16.8–

79.9

0.1–0.2 X X 10, 2 0.05

0–minor99.9–100 0–0.1 X 0–minor 0–0.1 18, 2 0.09 Relatively high uncertainty needed.

0–2.7 15.2–

54.3

45.1–

84.6

0.1 X X 12, 4 0.05

0–1.1 94.9–100 0–5.1 X 0–minor 0–minor31, 1 0.10 Relatively high uncertainty needed.

7

Composition of local Trinity aquifer groundwater based on sample from well LR–68–08–502 (Burns well) in Hays County, as discussed in text.

8

Composition of San Marcos Springs based on samples from Deep (LR–67–01–819), Diversion (LR–67–01–825), or Weissmuller (LR–67–01–820) Springs 

orifices.

9

Minimal model is one with a minimum number of phases; that is, none of the phases can be removed and still obtain a feasible model (Parkhurst and Appelo, 

1999).

10For Blanco River at Wimberley, Tex. (station number 08171000); station at Halifax Ranch (08171290) installed 12/2008 with data available beginning 

12/19/2008.
76 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

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 

event (January/February 2010) (tables 4 and 6; fig. 19D). A 

similar modeling approach was used as for sampling periods 

during the dry period: regional groundwater flow endmembers, 

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 

Marcos Springs is dominated by regional groundwater flow 

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 

of regional groundwater flow with a small component of 

Trinity aquifer groundwater (up to 1.3 percent); however, 

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 

water from the saline zone is a better geochemical fit and more 

hydrologically plausible. 

Although dominated by regional groundwater flow, 

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 

and wet periods, however, was statistically significant for only 

Diversion Spring (not for Deep Spring; Weissmuller Spring 

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 

the relation was not statistically significant for either Deep 

or Diversion Springs (fig. 23). The relation was statistically 

significant, however, when all values for Deep, Diversion, and 

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 

orifices (Diversion and Weissmuller) and corresponded to the 

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 20–30 percent under some 

conditions; model results that indicated a larger component of 

local stream recharge were generally associated with wetter 

hydrologic conditions (fig. 23; table 6). Model results for the 

dry period with a relatively large (that is, >5%) component of 

local recharge are considered hydrologically implausible on 

the basis of hydrologic conditions and nominal streamflow 

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 

to saline-zone groundwater also increased; for all model 

results, however, estimates of the saline-zone groundwater 

contribution were less than 0.5 ft3/s.

Modeling Based on Storm Sampling

For storms 1–3, the geochemical composition of 

Diversion Spring (the San Marcos Springs orifice that 

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 

the regional groundwater flow component. The most dilute 

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 1–3 

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 

(figs. 13 and 19); these results indicate that San Marcos 
Interaction Between Surface Water and Groundwater 77

0 50 100 150 200 250

San Marcos Springs daily mean discharge (USGS station 08170000), in cubic feet per second

0

5

10

15

20

25

30

Modeled proportion of spring discharge 

composed of stream recharge, in percent

EXPLANATION

San 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 

flow paths. The proportion of local recharge contributing 

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 

recharge decreased from storm 1 to storm 3; storm 3 was the 

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 (δD) and Cl are conservative tracers 

that can be used to distinguish mixing processes when 

endmembers are chemically distinct. Both δD 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).
78 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

concentrations tended to markedly decrease in the streams 

in response to storms (figs. 12 and 13). Storm 1 marked the 

transition from the dry period to the wet period; storm 3 

was the largest storm that occurred during the study period. 

Isotopically light values for rainwater during storms 1 and 

3, and in particular for storm 3 (fig. 9), were reflected in the 

composition of surface-water samples collected in response 

to storm 3, which were also isotopically light relative to other 

surface-water samples (figs. 12 and 13). Local recharge to the 

Edwards aquifer and to wells and springs affected by recent 

recharge (via quick flow) in response to storm 3 should 

reflect these light isotopic values and provide a tracer of 

recent recharge. 

Several groundwater wells appear to have been affected 

by mixing with recent recharge from storm 3 (fig. 15). For 

example, the δD 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 

storm 3 (fig. 15). After 4 additional weeks (October 28, 2010, 

about 7 weeks after storm 3), the δD 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 δD 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 

δD value of -79.5 per mil, the value measured for the Blanco 

River during storm 3 [Crow, 2012]) and the prestorm value 

at the Solar well indicate that a mixture including about 8 

percent of recharge from the Blanco River would account 

for the change in δD 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 

with more saline groundwater during the wet period (fig. 15), 

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 δD values (fig. 15). These small decreases in 

δD values (the analytical uncertainty of δD measurements 

is ±1 per mil) might be indicative of mixing with a minor 

component of isotopically light recent recharge.

Samples collected from the three orifices of San Marcos 

Springs (Deep, Diversion, and Weissmuller Springs) in 

response to storms 1–3 showed little variability in δD values 

or Cl concentrations (fig. 13), indicating that spring discharge 

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 

showed the largest variation (figs. 12 and 13). In response 

to storm 3, δD values at San Marcos Springs showed little 

variability; the largest change in δD values was measured in 

samples from Diversion Spring: the prestorm δD value was 

-21.6 per mil; δD 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 δD value of -79.5 per mil); a 2.4-per mil decrease would 

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 δD values in the 

weeks after storm 3; 3 months after storm 3, the δD 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 (0–2.7 percent) for storm 3. It 

should be noted, however, that the measured changes in δD 

values for the San Marcos Springs orifices are within or close 

to analytical uncertainty (±1 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 δD 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 

to storm 3 (fig. 24). The model estimated mixing in 10 

percent increments. Model results were also compared with 

spring samples collected for several months after storm 3 

(fig. 24). Mixing-model results indicate that recharge from the 

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 

for up to 3 months after storm 3; the proportion of Blanco 

River recharge increased slowly during the subsequent 3 

months following the storm, approaching a maximum value 

of 10 percent at Diversion and Weissmuller (fig. 24). These 

results indicate that a small amount (less than 10 percent) 

of local recharge might contribute to San Marcos Springs 

(specifically, Diversion and Weissmuller Springs) under some 

hydrologic conditions but that such recharge travels slowly 

and is not moving through highly transmissive flow paths. The 

contribution of local recharge following storm 3 based on the 

two-component mixing model is less than that estimated by 

using PHREEQC (table 6; results for Nov./Dec. 2010). The 

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. 
Interaction Between Surface Water and Groundwater 79

0 5 10 15 20 25 0 5 10 15 20 25

Chloride, 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

0

Delta deuterium, in per mil Delta deuterium, in per mil

-120

-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, Texas

EXPLANATION

A. Deep Spring

(USGS station 295322097561000)

B. Diversion Spring

(USGS station 295336097555201)

D. Hueco Spring A

(USGS station 294533098082301)

C. Weissmuller 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, δD 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 

(fig. 18). Concentrations of Cl at Hueco Spring A dropped 

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 

(>50 percent) by recent surface-water recharge (fig. 24). 

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).
80 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Synthesis of the Origin and 

Characteristics of Discharge at 

San Marcos Springs

The wide range of hydrologic conditions that occurred 

during this 25-month study—and corresponding changes 

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 

Blanco River (Ogden and others, 1985a, 1985b, 1986; Johnson 

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 (1934–2009) 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 

source. Temporal correspondence and similar flashy behavior 

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 

(LBG-Guyton Associates, 2004; Johnson and Schindel, 2008). 

Daily mean recharge estimates for the Blanco River computed 

for this study and daily mean discharge at San Marcos Springs 

show both similarities and differences (fig. 25); values are 

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 

(fig. 25). San Marcos Springs discharge during the study 

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 

that it is difficult to distinguish local and regional recharge; 

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 

contribute substantial recharge to the Edwards aquifer (fig. 

8), which is consistent with previous interpretations of the 

small role of the Guadalupe River in recharging the Edwards 

aquifer (Puente, 1978; Edwards Aquifer Authority, 2010). In 

response to a large and geographically isolated flow event on 

the Guadalupe River and Dry Comal Creek on June 9, 2010, 

estimated recharge to the Edwards aquifer from Dry Comal 

Creek was large (fig. 7), whereas the Guadalupe River was 

gaining (that is, not contributing recharge) (fig. 8). At San 

Marcos Springs following this event there was no notable 

response in physicochemical properties (fig. 19), which 

indicates little contribution to San Marcos Springs discharge 

likely occurred from these streams.

The short time periods of flow and the relatively minor 

amounts of flow at Sink Creek and Purgatory Creek over 

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 

Springs. Although streamflow was not measured at York and 

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 

or recharge from surface water. Specific conductance values 

were consistently higher at San Marcos Springs (median 

values of 613, 595, and 595 μS/cm for Deep, Diversion, 

and Weissmuller Springs, respectively) than at Comal 

Springs (median value of 558 μS/cm) (fig. 18) and were 

also consistently higher than the median value for confined 

groundwater from the regional aquifer (525 μS/cm; Musgrove 

and others, 2010). Higher specific conductance values at San 

Marcos Springs are consistent with modeling results that 

indicate that San Marcos Springs is affected by the influx of a 

small component of more saline groundwater (table 6).
Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs 81

Date

Nov. 2008

Feb. 2009

May 2009

Aug. 2009

Nov. 2009

Feb. 2010

May 2010

Aug. 2010

Oct. 2010

Dec. 2010

1

10

100

1,000

Estimated stream recharge, in cubic feet per second

50

100

150

200

250

300

Spring discharge, in cubic feet per second

1 2 3

1 Onset 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 Survey

EXPLANATION

Dry period Wet 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 

wet period provide information on flow paths and sources of 

spring discharge. During the dry period, little recharge was 

occurring regionally or locally, and spring discharge from 

all of the springs predominantly reflects draining of matrix 

groundwater. There were, however, some notable geochemical 

differences between the springs during the dry period that 

likely reflect differences in flow paths and sources of spring 

discharge. The geochemistry of Hueco Springs during the 

dry period was different from both Comal Springs and San 

Marcos Springs for numerous constituents (fig. 18; table 

5), with higher Mg, SO4, B, and F concentrations and Mg/

Ca ratios; higher δD and δ13C values; and lower NO3+NO2

concentrations. Hueco Springs discharges from the upthrown 

side of the Hueco Springs Fault Block (Johnson and Schindel, 

2008) (fig. 2) and thus is likely supplied by different flow 

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 

the dry period, water temperature increased; Cl, Na, Br, and 

B concentrations and Mg/Ca ratios increased; and δD values 
82 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

increased. The increases in values of conservative constituents 

such as Cl and δD 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, 

which is consistent with evaporation (fig. 26). 

Comal Springs and San Marcos Springs had generally 

similar geochemistry during the dry period (fig. 18), consistent 

with similar regional flow paths supplying both springs. 

Results of geochemical modeling support this hypothesis, 

indicating that discharge at San Marcos Springs during the dry 

period was dominated by regional groundwater flow (table 6). 

Nonetheless, Comal Springs had higher water temperatures 

and higher Sr and NO3+NO2 concentrations than did either 

Deep or Diversion Springs. These results indicate that San 

Marcos Springs was influenced by additional discharge 

sources with lower Sr and NO3+NO2 concentrations and 

lower water temperatures. Well 4D, located between Comal 

Springs and San Marcos Springs in the Comal Springs Fault 

Block and likely along regional flow paths that supply San 

Marcos Springs, also had lower temperatures and lower Sr and 

NO3+NO2 concentrations in the dry period than did Comal 

Springs, with values closer to San Marcos Springs (figs. 15 

and 18; table 5). Differences in the geochemistry of Comal 

Springs and well 4D indicate that there is some variability in 

regional flow paths, which is consistent with previous studies 

such as Otero (2007). Geochemical modeling results indicate 

that mixing of these regional flow paths (represented by the 

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 

indicate that Deep Spring was more influenced by mixing with 

a component of saline groundwater: Deep Spring had slightly 

higher specific conductance values and higher Cl, Na, SO4, Sr, 

and B concentrations (table 5; fig. 18). Geochemical modeling 

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 

in geochemistry (table 5; fig. 18). At Comal and San Marcos 

Springs, changes in geochemistry were relatively minor with 

fewer significant differences or smaller ranges of variability 

between the dry and wet periods (table 5; fig. 18) and mostly 

nominal changes in response to storms (fig. 13).

The source of recharge and the length of flow paths 

supplying spring discharge are key factors that affect the 

chemical variability of spring discharge (Scanlon and 

Thrailkill, 1987; White, 1999; Vesper and White, 2004). 

The characterization of karst springs ranges from those with 

considerable variations in springwater chemistry, resulting 

from higher proportions of quick flow through conduits, to 

those with little to no variations in springwater chemistry 

despite fluctuations in discharge, resulting from lower 

proportions of quick flow and higher proportions of slow 

flow (Shuster and White, 1971; Atkinson, 1977). Relations 

between respective spring discharge and results for selected 

geochemical constituents for Comal, Hueco, and San Marcos 

Springs (table 7; fig. 27) indicate that Comal and Hueco 

Springs are representative of two endmember spring types for 

major Edwards aquifer springs: Hueco Springs discharge is 

dominated by locally sourced conduit flow (quick flow), and 

Comal Springs discharge is dominated by more regionally 

sourced flow paths (slow flow). These endmember spring 

types are consistent with a measure of spring “flashiness” 

defined by the ratio of maximum discharge to average 

discharge (White, 1988): for the overlapping period of 

record (2002–2010) 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 

Comal Springs (fig. 18). The geochemistry of discharge at 

Hueco Springs is highly variable in time (fig. 18) and shows 

distinct and rapid changes in response to local rainfall and 

recharge events (fig. 13). Geochemical variability of Hueco 

Springs discharge for numerous selected constituents (19 of 

24) was significantly correlated with changes in hydrologic 

conditions as represented by changes in spring discharge 

(table 7). In contrast, the geochemistry of discharge at Comal 

Springs varied little throughout the study period (fig. 18), 

including in response to storms (fig. 13). The geochemical 

variability of Comal Springs discharge was significantly 

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 

constituents (19 of 24) showed a significant correlation with 

spring discharge (table 7). The response at Deep Spring was 

intermediate between Diversion Spring and Comal Springs; 

Deep Spring was less responsive to changes in hydrologic 

conditions and more similar to Comal Springs, with 10 of 

24 selected geochemical constituents showing a significant 

correlation with spring discharge (table 7). These results 

indicate that Deep Spring was not strongly influenced by 

changes in hydrologic conditions; similar to Comal Springs, 

discharge at Deep Spring is likely dominated by regional 

flow paths. 

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 

Diversion Spring relative to Hueco Springs (fig. 27; table 5). 
Synthesis of the Origin and Characteristics of Discharge at San Marcos Springs 83

-10 -8 -6 -4 -2 0

Delta oxygen-18, in per mil

-15 -10 -5 0 5

Delta oxygen-18, in per mil

-120

-100

-80

-60

-40

-20

0

10

Delta deuterium, in per mil

-80

-70

-60

-50

-40

-30

-20

-10

0

Delta 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

 Blanco River at Halifax Ranch near

 Kyle, Tex.—Dry period

 Blanco River at Halifax Ranch near

 Kyle, Tex.—Wet period

 Blanco River near Kyle, Tex.

 Cibolo Creek at Farm Road 1863

 below Bulverde, Tex.

 Sink Creek near San Marcos, Tex.

 PurgatoryCreek 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

 Guadalupe River at River Road near

 Sattler, Texas—Dry period

 Guadalupe River at River Road near

 Sattler, Tex.—Wet period

EXPLANATION

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. 
84 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

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).

[ns, not statistically significant based on a p-value of 0.05; NO3+NO2, nitrate plus nitrite, as nitrogen; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/strontium-86 

isotopic ratio; Mg/Ca, magnesium to calcium molar ratio, Sr/Ca x103, strontium to calcium molar ratio x 1,000]

Constituent

Spring1

San Marcos 

Springs—Deep 

Spring orifice

San Marcos 

Springs—Diversion 

Spring orifice

San Marcos Springs—

Weissmuller 

Spring orifice2

Comal 

Spring 1 

Hueco 

Spring A

Kendall’s tau

Specific conductance3 ns -0.27 ns ns ns

Turbidity3 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

NO3+NO2 0.40 -0.65 ns ns 0.43

Phosphorus -0.50 -0.52 ns ns 0.67

87Sr86Sr -0.44 ns ns -0.32 0.55

δ13C -0.25 ns ns -0.52 -0.57

Mg/Ca ns 0.52 ns ns -0.59

Sr/Ca x103 ns 0.65 ns ns -0.59

1

See table 1 for complete U.S. Geological Survey station names and numbers.

2

Weissmuller sampled only during the wet period (September 9, 2009, through December 31, 2010) and does not reflect similar range of hydrologic conditions 

as other springs.

3

Relation based on continuous data collection at 15-minute intervals (mean daily values).
Spring discharge, in cubic feet per second

0 100 200 300 400 0 100 200 300 400 0 100 200 300 

0

20406080

100120

Formazin nephelometric 

units (Hueco Spring A)

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Formazin nephelometric units

579

1113151719212325

2468

101214

Concentration, in milligrams per liter

0

100200300400500600700

Concentration, in

micrograms per liter

0

0.51.01.52.02.53.03.54.0

Strontium to calcium,

molar ratio x103

0.51.01.52.02.53.0

Concentration, in

milligrams per liter

0.70770.70780.7079

0.7080

0.70810.7082

Strontium-87/strontium-86 

isotopic ratio

0.100.150.200.250.300.350.400.450.50

Magnesium to calcium, molar ratio

Concentration, in

milligrams per liter

Concentration, in

micrograms per liter Concentration, in milligrams per liter

0

0.020.040.060.08

0.10

0.120.140.160.18

5

101520253035

30

405060708090

0.10

0.15

0.20

0.25

0.30

0.35

X

XX

XX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXX X

Turbidity

Bromide

Chloride

Strontium

Sodium

Fluoride

Sulfate

Boron

Nitrate plus nitrite

87Sr/86Sr

Mg/Ca

Sr/Ca

Figure 27. Relation between spring discharge (daily mean) for Comal, Hueco, and San Marcos Springs and selected physicochemical and geochemsamples collected from Comal Spring 1, Hueco Spring A, and San Marcos Springs orifices (Deep, Diversion, and Weissmuller Springs), south-central 2008–December 2010).
86 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Hueco Springs is influenced by mixing with a large component 

of local and recent surface-water recharge, whereas Diversion 

Spring is not (fig. 24). Second, for many geochemical 

constituents, the correlation of constituent concentration 

with spring discharge at Diversion Spring was the inverse 

of the correlation between constituent concentrations and 

spring discharge observed at Hueco Springs (table 7; fig. 

27). For example, constituents such as Cl, Na, Sr, Br, and 

lithium (Li) had relatively strong (Kendall’s tau ≥ 0.50) 

positive correlations with discharge at Diversion Spring; other 

constituents such as SO4 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 

discharge at Hueco Springs (table 7; fig. 27). These results are 

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 

also indicates that Diversion Spring is influenced by changing 

discharge sources with changes in hydrologic conditions. In 

contrast with Hueco Springs, however, these results indicate 

that Diversion Spring was influenced by mixing with saline 

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 

correlated (Kendall’s tau ≥ 0.50) with San Marcos Springs 

discharge, consistent with an increasing component of more 

geochemically evolved water as hydrologic conditions became 

wetter; in contrast, the relation between these constituents at 

Hueco Springs with spring discharge was inverse (table 7). 

Sources of Water to San Marcos Springs

The orifices of San Marcos Springs sampled during 

this study exhibited geochemical characteristics that reflect 

differences in the water sources and flow paths to the orifices, 

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, 

Deep Spring had higher specific conductance and higher 

concentrations of numerous constituents associated with more 

saline groundwater sources (including Sr, Na, Cl, SO4, B, and 

Br) than did Diversion Spring (table 5); this indicates that 

during the dry period Deep Spring was influenced by mixing 

with a component of saline groundwater. Potential sources of 

saline groundwater to San Marcos Springs are the downdip 

Edwards aquifer saline zone and the Trinity aquifer (figs. 1 and 

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 

orifices at San Marcos Springs (table 6). Model results do not 

readily distinguish the source of saline groundwater to San 

Marcos Springs; although groundwater from both sources is 

geochemically plausible, mixing with saline-zone groundwater 

is a better geochemical fit (with generally lower mass-balance 

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 

Diversion Spring. This relation is somewhat counterintuitive; 

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 

the San Antonio (Harden, 1968; Mahler, 2008) and the Barton 

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 

(fig. 1; table 1), also is affected by mixing with a more saline 

groundwater source during the wet period; concentrations of 

geochemical constituents such as Cl, SO4, B, and F increased 

at the start of the wet period (fig. 15). These changes were 

accompanied by decreases in NO3+NO2 and dissolved oxygen 

concentrations (indicative of mixing with more reducing 

groundwater), a decrease in 87Sr/86Sr values and increases 

in Mg/Ca and Sr/Ca ratios (indicative of mixing with more 

geochemically evolved groundwater) (fig. 15). An increase 

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. 

The spring orifices at San Marcos Springs sampled 

during this study responded differently to temporal changes 

in hydrologic conditions, which reflects differences in their 

water sources and contributing flow paths. Deep Spring was 

less responsive to changes in hydrologic conditions than 

was Diversion Spring (fig. 27; table 7), which indicates that 

discharge at Deep Spring is likely dominated by regional 

and less variable flow paths; Diversion Spring is likely 

more affected by changes in recharge sources, which might 

include local surface-water recharge sources. Increases in 

the concentrations of Sr, Cl, SO4, B, and Br at Diversion 

Spring from the dry period to the wet period (fig. 18; table 5), 

however, imply that discharge during the wet period had an 

increased component of saline groundwater; the composition 

of discharge from Diversion Spring through the wet period 

became more like that of Deep Spring. Weissmuller Spring 

was sampled during only the wet period; the geochemistry 

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 

surface-water sources does not strongly influence the 

geochemistry of San Marcos Springs discharge. Rather, 

discharge at San Marcos Springs is dominated by regional 

recharge sources and flow paths, even during wet hydrologic 
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 

Marcos Springs discharge from 0 to 28.9 percent; the range 

was narrower for Deep Spring (0–17.5 percent) than for 

Diversion Spring (0–24.4 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 

wet period (fig. 23; table 6). The geochemical response at 

San Marcos Springs to storm events, when focused local 

recharge is most likely to occur, was minor (figs. 13 and 19). 

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 

afterwards (fig. 24). These results place further constraints 

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 

transmissive flow paths. This hypothesis is supported by 

time-series data for wells located to the north of San Marcos 

Springs that might be along flow paths between the Blanco 

River and San Marcos Springs (Neff and Aqua wells) and 

that do not show marked changes in geochemistry from the 

dry period to the wet period (table 5; fig. 15). Local recharge 

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 

characteristics, aquifer levels, and flow paths. For the large 

range of hydrologic conditions that occurred during this 

study, results indicate that discharge at San Marcos Springs is 

dominated under all conditions by regional flow.

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 

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 regional flow paths. In contrast, the hydrologic 

connection of San Marcos Springs with the regional Edwards 

aquifer flow system is less understood, and there is interest in 

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, 

Comal, and Hays Counties. Springs included three orifices 

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 

sites was selected for periodic (routine; every 3–7 weeks) 

sampling to characterize temporal changes in water quality in 

response to hydrologic conditions; these were two streams, 

eight wells, one spring orifice each at Comal and Hueco 

Springs, and the three spring orifices at San Marcos Springs 

(Deep, Diversion, and Weissmuller Springs). To characterize 

changes in water quality in response to storms, samples were 

collected (depending on flow) in response to three major 

storms (storms 1–3) 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 

amount and resulted in large streamflows and aquifer recharge. 

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 

the three San Marcos Springs orifices; water-table altitude 

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. 
88 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10)

Streams in the vicinity of San Marcos Springs were 

evaluated as potential recharge sources. Recharge estimates 

were computed daily for the Blanco River (2009–10), Cibolo 

Creek (2008–10), and Dry Comal Creek (2008–10) 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 

Springs on the basis of their short periods of flow and the 

relatively minor amounts of flow that occurred. 

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 

in specific conductance and increases in turbidity occurred in 

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. 

Water-table altitude and specific conductance values at 

two groundwater wells near San Marcos Springs changed 

following the onset of the wet period: water-table altitudes 

increased, reflecting increasing water levels, and specific 

conductance decreased, reflecting dilution. Both wells had 

higher specific conductance values than did other Edwards 

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 

transmissive (karst conduit) flow paths but were dominated 

by matrix (diffuse) flow. An exception was the Solar well 

(LR–67–01–403), 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 

influence of different geochemical processes. Toward the latter 

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 

wet period provide information on flow paths and recharge 

sources supplying the springs. During the dry period, little 

recharge was occurring regionally or locally, and spring 

discharge from all of the springs predominantly reflects 

draining of matrix groundwater. There were, however, some 

notable geochemical differences between the springs during 

the dry period that likely reflect differences in flow paths 

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 

through the dry period, likely reflecting evaporation of the 

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 

flow paths supplying both springs. There were some notable 

differences between the geochemistry of Comal Springs and 

San Marcos Springs, however, which indicate that San Marcos 

Springs also was influenced by mixing with other water 

sources (specifically, a source or sources with lower water 

temperature and lower concentrations of strontium and nitrate 

plus nitrite). Well 4D (DX–68–16–707), located between 

Comal Springs and San Marcos Springs in the Comal Springs 

Fault Block and likely along regional flow paths that supply 

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 

groundwater flow paths that contribute to San Marcos Springs. 

At San Marcos Springs, the geochemistry of the Deep and 

Diversion Springs orifices during the dry period was similar, 

although some differences indicate that Deep Spring was more 

influenced by a small component of saline groundwater.

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 

significant differences or smaller ranges of variability 

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 

conduit flow (quick flow) and Comal Springs dominantly 

affected by more regionally sourced flow paths (slow flow). 

These endmember spring types are consistent with timeseries results of geochemical variability for Hueco Springs 

and Comal Springs. At San Marcos Springs, Deep and 

Diversion Springs orifices responded differently to changes 
References 89

in hydrologic conditions. Deep Spring was not strongly 

influenced by changes in hydrologic conditions, which 

indicates that, similar to Comal Springs, discharge at Deep 

Spring is likely dominated by regional groundwater flow 

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 

surface-water recharge, Diversion Spring was influenced by a 

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 

flow, local stream recharge, saline-zone groundwater, and 

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 

orifices). Modeling results for the routine samples collected 

during the wet period yielded a range for the contribution of 

local stream recharge (specifically from the Blanco River) to 

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 

other samples; recharge to the Edwards aquifer in response to 

storm 3 would reflect these low isotopic values and provide a 

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 

recharge sources moving rapidly through transmissive flow 

paths to San Marcos Springs is small. This conclusion is 

further supported by time-series data for wells located to the 

north of San Marcos Springs that might be located along flow 

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 

sources does not strongly influence the geochemistry of 

San Marcos Springs discharge. Rather, results of this study 

indicate that discharge at San Marcos Springs is dominated 

by regional recharge and groundwater flow paths, even during 

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.

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Musgrove and Crow—Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data—SIR 2012–5126

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