U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3080

November 2013

Prepared in cooperation with the San Antonio Water System

Origin and Characteristics of Discharge at San Marcos 

Springs, South-Central Texas

Printed on recycled paper

Discharge from San Marcos Springs, the second largest spring complex in Texas, is used as a factor for enacting drought management 

strategies for the Edwards aquifer, although the hydrologic connection of the springs with this regional aquifer is not well understood.

Key Findings 

•	 Discharge at San Marcos Springs is dominated by regional recharge sources and groundwater flow paths;

the contribution to recharge from local streams is relatively minor.

•	 Different orifices of San Marcos Springs respond differently to changes in hydrologic conditions; Deep Spring is

less responsive to changes in hydrologic conditions than are Diversion Spring and Weissmuller Spring.

•	 San Marcos Springs discharge is influenced by mixing with a component of saline groundwater.

Spring Lake, created by the impounded 

discharge from San Marcos Springs, 

south-central Texas (right). 

A U.S. Geological Survey employee 

servicing water-quality monitoring 

equipment at one of the orifices of San 

Marcos Springs in Spring Lake, southcentral Texas (above).

The Edwards aquifer in south-central Texas is one of the most productive aquifers in the Nation and is the primary source of 

water for the rapidly growing San Antonio area. Springs issuing from the Edwards aquifer provide habitat for several threatened 

and endangered species, serve as locations for recreational activities, and supply downstream users. Comal Springs and San Marcos 

Springs are major discharge points for the Edwards aquifer (fig. 1), and their discharges are used as thresholds in groundwater 

management strategies. Regional flow paths originating in the western part of the aquifer are generally understood to supply discharge 

at Comal Springs. In contrast, the hydrologic connection of San Marcos Springs with the regional Edwards aquifer flow system is less 

understood. During November 2008–December 2010, the U.S. Geological Survey, in cooperation with the San Antonio Water System, 

collected and analyzed hydrologic and geochemical data from springs, groundwater wells, and streams to gain a better understanding 

of the origin and characteristics of discharge at San Marcos Springs.
1,000 mg/L

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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 

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Cibolo

Creek

Dry

Comal

Creek

Alligator

Creek

Blanco River

Onion Creek

San

Marcos River

Plum

Creek

York

Creek

CANYON

LAKE

Purgatory

Creek

Sink

Creek

ComalRiver

Guadalupe River

Guadalupe

River

Hueco Springs

Comal Springs

San Marcos Springs

0 5 7.5 10 MILES 2.50 5 7.5 10 KILOMETERS 2.5

Gulf of Mexico

EDWARDS PLATEAU TEXAS Study area

EXPLANATION

Edwards aquifer Contributing zone Recharge zone Confined zoneBalcones EscarpmentMunicipal boundaryGroundwater divideFreshwater/saline-water interface  (1,000 milligrams per liter [mg/L] dissolved solids concentration)

Streamflow-gaging station or surface-water site Groundwater well, Edwards aquiferGroundwater well, Edwards and Trinity aquifers Groundwater well, Trinity aquiferSpring Rain sample collection siteNational Weather Service cooperative station

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l l

da

98°00' 98°20' 98°40'

30°20'30°00'29°40'

d

d

d

d

d

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a

a

a

a

a

a

Figure 1. Study area and locations of data- and sample-collection sites, south-central Texas. Samples were collected periodicallyand in response to storms.

Figure 1. Study area and locations of data- and sample-collection sites, south-central Texas. Samples were collected periodically and in response to storms.
A wide range of climatic and hydrologic conditions during the study allowed for investigation of controls on 

discharge at San Marcos Springs over much of its historical range of discharge.

During the study, climatic and hydrologic conditions 

transitioned from exceptional drought to wetter than normal. 

Between November 1, 2008, and September 8, 2009, is referred 

to as the “dry period,” and between September 9, 2009, and 

December 31, 2010, is referred to as the “wet period.” The 

wide range of hydrologic conditions that occurred during this 

25-month study—and corresponding changes in surface-water, 

groundwater and spring discharge, and in physicochemical 

properties and geochemistry—provides insight into the origin of 

the water discharging from San Marcos Springs. Three orifices 

at San Marcos Springs (Deep, Diversion, and Weissmuller 

Springs) were selected to be representative of larger springs at 

the spring complex (fig. 2). Potential sources of discharge at San 

Marcos Springs include (1) regional groundwater flow that has 

bypassed Comal Springs, (2) local recharge from nearby streams, 

(3) groundwater from the saline zone of the Edwards aquifer 

(downgradient of the freshwater/saline-water interface) (fig. 1), 

and (4) interaquifer flow from the underlying Trinity aquifer.

Previous studies have hypothesized that discharge at 

San Marcos Springs might include notable contributions of 

recharge from nearby streams, including the Guadalupe River, 

Cibolo Creek, Dry Comal Creek, Sink Creek, Purgatory Creek, 

York Creek, Alligator Creek, and, in particular, the Blanco 

River (Guyton and Associates, 1979; Ogden and others, 1986; 

Johnson and Schindel, 2008). Results from this study indicate 

that recharge from these local streams is not a major source of 

San Marcos Springs discharge.

Rather, 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. Geochemical modeling results using the 

program PHREEQC (Parkhurst and Appelo, 1999) indicate 

that the proportion of local stream recharge contributing to San 

Marcos Springs discharge increased from the dry period to the 

wet period, but even under wet conditions the proportion was 

less than 30 percent, and for most hydrologic conditions it was 

less than 10 percent (fig. 3). 

SPRING LAKE

DAM

U

D

U

D

SAN MARCOS SPRINGS FAULT

Spring

 Lake

Spring

Diversion

 Spring

Weissmuller

 Spring

Deep

 Spring

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

EXPLANATION

U

D

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

Figure 2. Location of Spring Lake, San Marcos Springs complex, and spring orifices sampled during this study.

East outflow

structure

West outflow

structure

Figure 2. Spring Lake, San Marcos Springs complex, and location of sampled spring orifices.
The Blanco River in Hays County, Texas. The Guadalupe River in Comal County, Texas.

Geochemical modeling results indicate that the proportion of local stream recharge contributing to 

San Marcos Springs discharge was less than 10 percent for most hydrologic conditions.

0 50 100 150 200 250

San Marcos Springs daily mean discharge, 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

 Diversion Spring

 Weissmuller Spring

Figure 3. Relation between San Marcos Springs discharge and the modeled proportion of spring discharge composed of

stream recharge from the Blanco River based on inverse geochemical modeling.

Figure 3. Relation between San Marcos Springs discharge and the modeled proportion of spring discharge composed of stream recharge 

from the Blanco River based on inverse geochemical modeling.

Geochemical modeling results also indicate that the effect 

of storm-related recharge from local focused recharge sources 

was small. For example, mixing models developed by using 

conservative tracers for the largest storm that occurred during 

the study, a named tropical storm (Hermine, September 7, 2010), 

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. 4).

Modeling results for this and other storms indicate 

that San Marcos Springs was not notably affected by 

recharge events from local streams moving rapidly through 

transmissive flow paths. This hypothesis is also supported 

by a lack of marked changes in geochemistry from the dry 

period to the wet period in time-series data from wells located 

to the north of San Marcos Springs along possible flow paths 

from the Blanco River to the springs.
0 5 10 15 20 25

Chloride, in milligrams per liter

0 5 10 15 20 25

Chloride, in milligrams per liter

-90

-80

-70

-60

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-40

-30

-20

-10

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-90

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Delta deuterium, in per mil Delta deuterium, in per mil

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

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

Sample collected in response to storm 3

 San Marcos Springs—Deep Spring

 San Marcos Springs—Diversion Spring

 San Marcos Springs—Weismuller Spring

Routine sample collected after storm 3,

 by date

 September 29, 2010

 October 29, 2010

 December 1, 2010

EXPLANATION

Figure 4. Relation between chloride concentration and deuterium isotopes for two-component mixing models showing

proportional mixing between surface-water (stream recharge) and springwater endmembers for San Marcos Springs

samples collected immediately following and several months after Tropical Storm Hermine in September 2010. 

Deep Spring Diversion Spring

Weissmuller Spring

Figure 4. Relation between chloride concentration and deuterium isotopes for two-component mixing models showing proportional mixing 

between surface-water (stream recharge) and springwater endmembers for San Marcos Springs samples collected immediately following 

and several months after Tropical Storm Hermine in September 2010.

Different spring orifices at San Marcos Springs, Comal Springs, and Hueco Springs respond 

differently to temporal changes in hydrologic conditions.

Differences in the geochemistry and variability of Comal 

Springs, Hueco Springs, and San Marcos Springs were compared 

to better understand flow paths and sources of spring discharge. 

Changes in hydrologic conditions that occurred at the beginning 

of the wet period resulted in large changes in discharge from the 

springs (fig. 5). At Hueco Springs, this change was accompanied 

by large changes in geochemistry in response to local rainfall 

and recharge events. At Comal Springs, changes in geochemistry 

were relatively minor as discharge increased, reflecting regional 

flow paths and discharge sources.

The geochemical response at San Marcos Springs as 

discharge increased was intermediate between Hueco and 

Comal Springs and was different for the different orifices (fig. 

5). Deep Spring, similar to Comal Springs, was not responsive 

to changes in hydrologic conditions, which indicates that 

discharge at Deep Spring was likely dominated by regional and 

less variable flow paths. The geochemical response at Diversion 

Spring was more variable than at Deep Spring. Previous studies 

have also described differences between springs in the southern 

and northern parts of Spring Lake (Ogden and others, 1986). 

Diversion Spring was more affected by changes in recharge 

sources, but the nature of the geochemical changes, such as the 

increase in chloride concentration (fig. 5), indicates that increased 

discharge at Diversion Spring included an increased component 

of saline groundwater rather than recharge from local streams. At 

higher discharge, the composition of Diversion Spring became 

more like that of Deep Spring. Weissmuller Spring, which was 

sampled during only the wet period, had a composition and 

variability similar to Diversion Spring (fig. 5), indicating that 

these two springs were likely supplied by common flow paths.
0 100 200 300 400

Spring daily mean discharge, in cubic feet per second

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15

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23

25

Chloride, in milligrams per liter

Spring, by orifice

 San Marcos Springs (Deep Spring)

 San Marcos Springs (Diversion Spring)

 San Marcos Springs (Weissmuller Spring)

 Comal Springs (Spring 1)

 Hueco Springs (Spring A)

EXPLANATION

Figure 5. Relation between spring discharge for Comal, Hueco, and San Marcos Springs

and chloride concetration for samples collected from orifices of those springs.

Figure 5. Relation between spring discharge for Comal, Hueco, 

and San Marcos Springs and chloride concentration for samples 

collected from orifices of those springs.

U.S. Geological Survey personnel and a diver from Texas State 

University above Weissmuller Spring, an orifice of San Marcos 

Springs in Spring Lake, south-central Texas.

Geochemical modeling results consistently indicate that, 

in addition to a dominant component of regional groundwater 

flow, a small contribution of saline water (more saline than 

the freshwater Edwards aquifer) is needed to account for the 

composition of San Marcos Springs. Both the Edwards aquifer 

saline zone and the underlying Trinity aquifer were considered 

as hydrologically plausible saline-water sources. Although 

the Trinity aquifer cannot be eliminated as a potential source, 

modeling results indicate that mixing with a small component 

(less than 1 percent) of saline-zone groundwater more likely 

accounts for the composition of San Marcos Springs discharge.

This fact sheet is based on the following report:

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., http://pubs.usgs.gov/

sir/2012/5126/.

References

Guyton, W.F., and Associates, 1979, Geohydrology of Comal, 

San Marcos, and Hueco Springs: Austin, Texas Department of 

Water Resources Report 234, 85 p.

Johnson, S.B., and Schindel, G.M., 2008, Evaluation of the 

option to designate a separate San Marcos pool for critical 

period management: San Antonio, Tex., Edwards Aquifer 

Authority, 109 p.

Ogden, A.E., Quick, R.A., Rothermel, S.R., and Lundsford, D.L., 

1986, Hydrological and hydrochemical investigation of the 

Edwards aquifer in the San Marcos area, Hays County, Texas: 

San Marcos, Tex., Edwards Aquifer Research and Data Center, 

364 p.

Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide to 

PHREEQC (v. 2)—A computer program for speciation, 

reaction-path, one-dimensional transport, and inverse 

geochemical calculations: U.S. Geological Survey WaterResources Investigations 99–4259, 312 p.

—By MaryLynn Musgrove and Cassi L. Crow

For additional information contact:

Director, Texas Water Science Center

U.S. Geological Survey

1505 Ferguson Lane

Austin, Texas 78754–4501

http://tx.usgs.gov/

Discharge at San Marcos Springs is influenced by mixing with a component of saline groundwater.

Publishing support provided by

Lafayette Publishing Service Center

ISSN 2327–6916 (print)

ISSN 2327–6932 (online)

http://dx.doi.org/10.3133/fs20133080