98o 98o 98 15' o

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29o

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0 10 20 MILES

I-10

US 281

Loop

1604

US 90

I-35

Loop

Loop U

S 281

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410

I-10

US 90

I-35 State

123

State 123

US

87

US

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State

97

 SAN

 ANTONIO

 NEW

 BRAUNFELS

 SAN

 MARCOS

4

3

2 1

TEXAS

Study

 area

LOCATION MAP

The Edwards aquifer is the sole source of water for about 

1.3 million people in and near San Antonio, Texas, as well as for 

ranchers and farmers throughout south-central Texas. Because 

of the demand for this resource, various studies have been conducted to better understand the Edwards aquifer and how the 

aquifer reacts to environmental changes and human influences. 

In July 1996, the U.S. Geological Survey (USGS), in cooperation 

with the Edwards Aquifer Research and Data Center (EARDC) at 

Southwest Texas State University (SWTSU) and the Texas Water 

Development Board (TWDB), began a study to investigate possible changes in water quality of the Edwards aquifer near the 

freshwater/saline-water interface that might result if drought 

occurs. The continuing study is part of the USGS National WaterQuality Assessment (NAWQA) Program in south-central Texas. 

The freshwater/saline-water interface, known locally as the 

“bad-water line,” marks the beginning of the transition in the 

aquifer from freshwater (updip) to saline water (downdip). The 

interface is defined as the 1,000-milligram-per-liter (mg/L) line of 

equal dissolved solids concentration as delineated by Maclay 

and others (1980, fig. 7). Water in the freshwater zone of the 

Edwards aquifer tends to have dissolved solids concentrations 

of 250 to 300 mg/L (Pavlicek and others, 1987, p. 3). In contrast, 

water in the saline-water zone of the Edwards aquifer commonly 

has dissolved solids concentrations greater than 10,000 mg/L, 

and sample concentrations as large as 232,000 mg/L have been 

measured (Groschen and Buszka, 1997, p. 21).

The freshwater/saline-water interface is the convergence of two 

flow systems within the Edwards aquifer. Large transmissivity 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS–103–98

In cooperation with the Edwards Aquifer Research and Data Center and the

Texas Water Development Board

Monitoring of Selected Water-Quality Constituents Near the Freshwater/SalineWater Interface of the Edwards Aquifer, July 1996–December 1997

Figure 1. Location of monitoring sites near the freshwater/saline-water interface of the Edwards aquifer, south-central Texas, July 1996–

December 1997.

Recharge zone of the Edwards aquifer—Modified 

from Ashworth and Hopkins (1995)

Artesian zone of the Edwards aquifer—Modified 

from Ashworth and Hopkins (1995)

Freshwater/saline-water interface

Monitoring site

1 Southwest Texas State University

 research well

2 San Marcos Springs

3 Comal Springs

4 Atascosa municipal well

EXPLANATION
every 15 minutes and automatically downloaded daily by computer. Monitoring equipment at the SWTSU research well is 

shown in figure 2.

Similar telephone telemetry instrumentation was installed at 

the public water-supply well at Atascosa in June 1997. Data are 

recorded every 30 minutes while the pump is operating and automatically downloaded daily by computer. The pump operates 

when the public water-supply storage tank needs refilling. 

All four sites are visited every 2 to 3 weeks for probe recalibration and an on-site systems check. In addition to these visits, the 

data logger performs a systems check every 24 hours that allows 

for remote identification of equipment problems.

In January 1998, the EARDC assumed responsibility for maintenance and data collection at all four sites for a period of at least 

1 year. The EARDC provides public access to the data through the 

Internet on their homepage at <http://www.eardc.swt.edu>.

Data Collection July 1996–December 1997

Data collected during July 1996–December 1997 from the four 

monitoring sites are listed in table 1. Median pH at all four sites 

appears to be stable, ranging from 6.8 to 7.6 standard units. Mean 

[200,000 to 2,000,000 feet squared per day (ft2

/d) (Maclay and 

Small, 1986, p. 61)] characterizes the freshwater zone. By comparison, the saline-water zone has small transmissivity [about 

10,000 ft2

/d or less (Groschen and Buszka, 1997, p. 11)] and 

longer residence times. Although the freshwater and saline-water 

zones are in contact with each other, mixing of water between the 

two is believed to be limited because of the difference in transmissivity and other geologic constraints (Sharp and Banner, 1997).

Increased withdrawals from the Edwards aquifer because of 

population growth have raised the issue of saline water encroaching into the freshwater zone and contaminating public watersupply wells and a series of natural springs, including San Marcos 

and Comal Springs. These springs are home to several endangered 

species, which because of their protected status, further complicates issues related to public water-supply withdrawals and the 

subsequent effects of possible saline-water encroachment.

In addition to increased withdrawals, seasonal droughts that 

affect the freshwater/saline-water interface are possible. During 

droughts, withdrawals from public water-supply wells increase, 

and recharge to the aquifer decreases, resulting in a decrease in 

storage in the freshwater zone of the aquifer. This decrease in storage, if large enough, might make saline-water intrusion hydraulically possible (Pavlicek and others, 1987).

The purpose of the USGS study is to (1) monitor selected 

water-quality constituents at sites near the interface for early 

detection of saline-water encroachment into the freshwater zone 

of the Edwards aquifer, and (2) provide information on the seasonal variation of general water-quality conditions at the sites. 

This fact sheet describes water-quality monitoring at four sites 

near the freshwater/saline-water interface and presents data 

collected at the monitoring sites during July 1996–December 

1997.

The City of New Braunfels and Atascosa Rural Water Supply 

provided access to Comal Springs and the Atascosa public watersupply well, respectively. 

Water-Quality Monitoring

Water-quality monitors were installed at one research water 

well (at SWTSU), two natural springs (San Marcos and Comal), 

and one public water-supply well (at Atascosa) (fig. 1).

Water-quality instrumentation was installed at a SWTSU 

research well and at San Marcos and Comal Springs in July 1996. 

The monitors have self-contained data loggers that collect continuous measurements of pH, temperature, and specific conductance.

Monthly data evaluation was judged inadequate for early 

detection of saline-water encroachment. Therefore, in May 1997, 

the self-contained data loggers at the SWTSU research well 

and San Marcos and Comal Springs were converted to standalone telephone telemetry systems to accomplish near real-time 

data collection. This system uses a data logger, modem, cellular 

telephone, and three sensors (pH, temperature, and specific 

conductance). Power is supplied by a 12-volt battery, which is 

recharged by a solar panel. Data at these three sites are recorded 

Figure 2. Monitoring equipment at Southwest Texas State 

University research well, San Marcos, Texas.
temperature also appears to be stable at all four sites, ranging from 

20.4 to 23.5 degrees Celsius (°C) at the SWTSU research well and 

San Marcos and Comal Springs and ranging from 27.0 to 27.6 °C 

at the public water-supply well. Mean specific conductance at the 

four sites ranged from 511 to 633 microsiemens per centimeter at 

25 °C (μS/cm), which indicates a range of dissolved solids concentration of about 300 to 400 mg/L (Hem, 1989, p. 67). 

The monthly mean temperature and monthly total precipitation 

data for July 1996–December 1997 and the mean monthly data for 

1961–90 for San Antonio are listed in table 2 (National Oceanic 

and Atmospheric Administration, National Climatic Data Center, 

1998). Because precipitation during the monitoring period was 

above normal for the region, substantial changes in water quality 

as a result of drought conditions were neither expected nor 

observed at any of the monitoring sites. Therefore, the waterquality data collected during July 1996–December 1997 at the 

four sites are considered a baseline that can be used for comparison with new data that might be collected during a future drought. 

References

Ashworth, J.B., and Hopkins, Janie, 1995, Aquifers of Texas: 

Texas Water Development Board Report 345, 69 p.

Groschen, G.E., and Buszka, P.M., 1997, Hydrogeologic framework and geochemistry of the Edwards aquifer saline-water 

zone, south-central, Texas: U.S. Geological Survey WaterResources Investigations Report 97–4133, 47 p.

Hem, J.D., 1989, Study and interpretation of the chemical characteristics of natural water: U.S. Geological Survey WaterSupply Paper 2254, 263 p.

Maclay, R.W., Rettman, P.L., and Small, T.A., 1980, Hydrochemical data for the Edwards aquifer in the San Antonio area, 

Texas: Texas Department of Water Resources LP–131, 38 p.

Table 1. Monthly water-quality data collected at monitoring sites near the freshwater/saline-water interface of the Edwards 

aquifer, July 1996–December 1997

[Number in parentheses refers to figure 1. SWTSU, Southwest Texas State University; median pH in standard units; T, mean temperature in degrees Celsius; SC, mean specific conductance in microsiemens per centimeter at 25 degrees Celsius; --, no data because of 

malfunctioning equipment] 

Month

(1) SWTSU

research well (2) San Marcos Springs (3) Comal Springs (4) Atascosa

municipal well

pH T SC pH T SC pH T SC pH T SC

July 1996 6.8 22.4 624 7.1 22.4 592 -- -- --

Aug. 1996 7.1 22.4 625 7.1 22.4 609 -- -- --

Sept. 1996 7.0 22.4 621 7.0 22.4 602 7.1 23.4 525

Oct. 1996 7.0 22.4 624 7.1 22.4 610 7.1 23.4 524

Nov. 1996 7.0 22.4 631 7.1 22.4 611 7.1 23.4 523

Dec. 1996 7.2 22.4 633 7.0 22.4 621 7.0 23.4 526

Jan. 1997 -- -- -- -- -- -- 7.2 23.4 533

Feb. 1997 7.1 22.4 591 7.1 22.4 607 7.2 23.5 533

Mar. 1997 7.1 22.4 592 7.0 22.4 606 7.2 23.4 539

Apr. 1997 7.1 22.4 614 7.2 22.4 602 7.1 23.4 538

May 1997 7.2 22.3 618 7.2 21.5 608 7.5 23.5 534

June 1997 7.1 22.4 611 7.2 20.8 609 7.6 23.5 532 -- 27.0 575

July 1997 7.0 22.2 601 7.1 20.5 606 7.1 23.5 530 7.0 27.6 624

Aug. 1997 7.0 22.0 604 7.1 20.6 607 7.5 23.5 533 7.1 27.6 630

Sept. 1997 6.9 21.9 610 7.0 20.6 608 -- 23.5 531 7.0 27.6 620

Oct. 1997 7.0 21.8 615 7.0 20.4 610 7.2 -- -- 7.2 27.4 630

Nov. 1997 6.9 21.8 619 7.1 20.4 612 6.8 -- -- 7.2 27.3 630

Dec. 1997 7.0 21.9 621 7.0 20.4 614 7.2 23.5 511 6.9 27.3 627

Water-quality monitor not

installed until June 1997
Sharp, J.M., Jr., and Banner, J.L., 1997, The Edwards aquifer—

A resource in conflict: Geological Society of America Today, 

v. 7, no. 8, p. 1–9.

—J.R. Cederberg, P.B. Ging, and R.T. Ourso

Any use of trade, product, or firm names is for descriptive 

purposes only and does not imply endorsement by the U.S. 

Government.

Information on technical reports and hydrologic data related 

to this study can be obtained from:

San Antonio Subdistrict Chief

U.S. Geological Survey

435 Isom Road, Suite 234

San Antonio, TX 78216

Phone: (210) 321–5200

FAX: (210) 530–6008

E-mail: gbozuna@usgs.gov

World Wide Web: http://tx.usgs.gov

Maclay, R.W., and Small, T.A., 1986, Carbonate geology and 

hydrology of the Edwards aquifer in the San Antonio area, 

Texas: Texas Water Development Board Report 296, 90 p.

National Oceanic and Atmospheric Administration, National 

Climatic Data Center, 1998, Regional climatic data: 

Asheville, N.C., U.S. Department of Commerce.

URL: <http://www.ncdc.noaa.gov/ol/climate/

climatedata.html>

Pavlicek, Dianne, Small, T.A., and Rettman, P.L., 1987, 

Hydrogeologic data from a study of the freshwater 

zone/saline-water zone interface in the Edwards aquifer, 

San Antonio region, Texas: U.S. Geological Survey OpenFile Report 87–389, 108 p.

November 1998

The National Water-Quality Assessment Program

In 1991, the U.S. Geological Survey, U.S. Department of the 

Interior, began a National Water-Quality Assessment (NAWQA) 

Program. The long-term goals of the NAWQA Program are to describe 

the status of and trends in the quality of a large representative part of 

the Nation’s surface- and ground-water resources and to identify the 

major factors that affect the quality of these resources. In addressing 

these goals, the NAWQA Program will produce water-quality 

information that is useful to policymakers and managers at Federal, 

State, and local levels.

Studies of 59 hydrologic systems that include parts of most major 

river basins and aquifer systems are the building blocks of the national 

assessment. The study units range in size from less than 1,000 to more 

than 60,000 square miles and represent 60 to 70 percent of the Nation’s 

water use and population served by public water supplies. Twenty 

investigations began in fiscal year 1991, 16 in fiscal year 1994, 13 in 

fiscal year 1997, and 2 are scheduled to begin in fiscal year 1999.

Table 2. Monthly mean temperature and monthly total 

precipitation, July 1996–December 1997, and mean 

monthly temperature and mean monthly total precipitation, 

1961–90, for San Antonio, Texas

[Data from National Oceanic and Atmospheric Administration, 

National Climatic Data Center, 1998. Temperature in degrees 

Celsius and precipitation in inches]

Month

Monthly

mean

temperature

Monthly

total precipitation

Month

1961–90

mean

monthly

temperature

1961–90

mean

monthly

total precipitation

July 1996 30.1 1.34 July 29.4 2.20

Aug. 1996 28.1 2.89 Aug. 29.4 2.50

Sept. 1996 25.4 4.72 Sept. 26.3 3.40

Oct. 1996 21.4 .39 Oct. 21.2 3.20

Nov. 1996 15.9 3.09 Nov. 15.8 1.50

Dec. 1996 12.3 1.41 Dec. 11.2 1.40

Jan. 1997 9.4 .49 Jan. 9.6 1.70

Feb. 1997 11.6 2.52 Feb. 11.9 1.80

Mar. 1997 17.4 1.90 Mar. 16.5 1.50

Apr. 1997 17.7 5.25 Apr. 20.7 2.50

May 1997 22.8 3.94 May 24.2 4.20

June 1997 26.0 6.48 June 27.9 3.80

July 1997 28.9 .00

Aug. 1997 29.3 .59

Sept. 1997 27.2 1.85

Oct. 1997 20.9 4.12

Nov. 1997 14.1 1.74

Dec. 1997 10.1 4.37