U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3035

May 2011 Printed on recycled paper

The area contributing recharge to Barton Springs is undergoing rapid growth, accompanied by increased generation of 

wastewater. This study found that nitrate, a major component of wastewater and a nutrient that can degrade water quality, 

has increased in Barton Springs and the creeks that provide its recharge.

Key Findings 

Nitrate concentrations in Barton Springs and the five streams that provide most of its recharge were much 

higher during 2008–10 than before 2008.

Biogenic nitrogen (nitrogen from human or animal waste, or both) is a probable source of nitrate measured 

in the recharging streams during 2008–10.

Septic systems and land-applied treated wastewater effluent are likely sources contributing nitrate to the 

recharging streams.

The area providing most of the recharge to Barton Springs has undergone rapid development since 2000 (upper left), with an associated 

increase in the generation of wastewater. Nitrate and other nutrients in treated wastewater effluent can cause algal blooms as shown 

here in Bear Creek, one of the five streams providing most of the recharge to Barton Springs, the principal discharge point for the Barton 

Springs segment of the karstic Edwards aquifer. (Photograph courtesy of Andrew Clamann, City of Austin.)

In cooperation with the City of Austin, City of Dripping Springs, Barton Springs/Edwards Aquifer Conservation District, Lower Colorado 

River Authority, Hays County, and Travis County

Nitrate Concentrations and Potential Sources in the Barton 

Springs Segment of the Edwards Aquifer and Its Contributing 

Zone, Central Texas
The Barton Springs Salamander 

(Eurycea sosorum) is one of many 

aquatic species that requires 

dissolved oxygen to survive. 

(Photograph courtesy of Lisa 

O’Donnell, City of Austin.)

Nutrients and Nitrate

Nutrients are those elements that are important to plant growth and 

survival and include nitrogen, phosphorus, and potassium. In water, 

these elements usually take the form of the ions nitrate, phosphate, 

and potassium. Of these, nitrate is the most soluble and phosphate 

is the least. Excessive concentrations of nutrients in waterways can 

cause algal blooms. As the algae die and decompose, dissolved 

oxygen in the water is consumed, which may cause other organisms, 

including fish, to die. Nutrients in water have many sources. Natural 

sources include soils, plant decomposition, animal waste, and rain. 

Human-related sources include fertilizers, sewage, wastewater, and 

vehicle exhaust.

Nitrate concentrations in Barton Springs and the streams that provide most of its 

recharge increased beginning around 2005.

Nitrate was measured in samples collected from five streams recharging the Barton Springs segment of the Edwards aquifer (Barton, Williamson, 

Slaughter, Bear, and Onion Creeks) near the downstream end of the contributing zone (fig. 1) during November 2008–March 2010. Nitrate concentrations were higher than those in samples collected at the same sites and analyzed by the U.S. Geological Survey (USGS) from the early 1990s until 

November 2008. Nitrate concentrations in samples collected at station 08158700 Onion Creek near Driftwood, Tex., during 2008–10 were about 6 to 

10 times higher than nitrate concentrations for similar streamflows at this station during 1990–2008; nitrate concentrations measured in samples collected at station 08155500 Barton Springs at Austin, Tex., during 2008–10 also were higher, on average, than nitrate concentrations measured during 

1990–2008 at this station for similar flow rates (fig. 2). The elevated nitrate concentrations likely resulted in part from the transition from dry to wet 

conditions in fall 2009, but similar transitions also occurred during 1990–2008, indicating that increased nitrogen loading likely also was a contributing factor. 

Figure 2. Nitrate concentrations measured in samples collected during 2008–10 (A) at 08158700 Onion Creek at Driftwood, Texas, were 

about 6 to 10 times higher than nitrate concentrations measured for similar streamflows at the same site during 1990–2008; and (B) at 

08155500 Barton Springs at Austin, Texas, also were higher, on average, than nitrate concentrations measured for similar streamflows at 

the same site during 1990–2008. (Photograph of Barton Springs courtesy of David Johns, City of Austin.)

Figure 1. Location of the Barton Springs segment of the Edwards 

aquifer (recharge and confined zones) and its contributing zone and 

sampling locations. Samples were collected monthly and in response 

to storms over 17 months from November 2008 to March 2010. 

Central

Austin

Colorado River

Onion Creek

Williamson Creek

Bear Creek

Little Bear

Creek

Slaughter Creek

N

0 5 MILES

TEXAS

Study area

U.S. Geological Survey

station and number

Spring

EXPLANATION

08155500

Barton Springs

08155500 08155240

08158920

08158840

08158810

08158700

BartonCreek

CONTRIBUTING ZONE

RECHARGE ZONE

CONFINED ZONE

Onion Creek Onion Creek

Barton Springs on Springs 1990

1992

1994

1996

1998

2000

2002

20042006

2008

2010

1990

1992

1994

1996

1998

2000

2002

20042006

20082010

0.01 0.1 1 10 100 1,000 10,000

0 20 40 60 80 100 120 140

DISCHARGE, IN CUBIC FEET PER SECOND

0

0.5

1.0

1.5

2.0

2.5

0

0.5

1.0

1.5

2.0

2.5

0

0.5

1.0

1.5

2.0

2.5

0

0.5

1.0

1.5

2.0

2.5

0

20

40

60

80

100

120

140

0

1,000

2,000

3,000

4,000

5,000

6,000

NITRATE CONCENTRATION, IN

MILLIGRAMS PER LITER

NITRATE CONCENTRATION, IN

MILLIGRAMS PER LITER

NITRATE CONCENTRATION, IN

MILLIGRAMS PER LITER

NITRATE CONCENTRATION, IN

MILLIGRAMS PER LITER

DISCHARGE, IN CUBIC FEET PER SECOND DISCHARGE, IN CUBIC FEET PER SECOND

1990–2008

mean concentration

2008–10

mean concentration

1990–2008

mean concentration

2008–10

mean concentration

1990–2008

2008–10

1990–2008

2008–10

A. Onion Creek

B. Barton Springs

TIME DISCHARGE
Isotopes of nitrogen and oxygen in nitrate were measured 

in water samples collected from Barton Springs at station 

08155500 and the five streams recharging the aquifer during 

November 2008–March 2010. During November 2008–September 2009 (the dry period), only Barton Creek flowed continuously. Nitrate in water samples from station 08155240 Barton 

Creek at Lost Creek Blvd. near Austin, Tex., during the dry 

period had an isotopic signature very similar to that for wastewater effluent, indicating a contribution of biogenic nitrate that 

has undergone some denitrification (fig. 3). The isotopic signature of nitrate in samples collected from Barton Springs during 

the dry period indicates a soil source of nitrate with a potential 

contribution from biogenic sources.

Isotopic signatures indicate human or 

animal waste, or both, as a likely source 

of some of the nitrate measured during 

2008–10.

Isotopic Ratios of Nitrate as a Tracer of Sources

Isotopes of a particular element have the same 

number of protons in the atomic nucleus but different 

numbers of neutrons, resulting in different atomic masses. 

Stable isotopes of nitrogen (nitrogen-14 [14N] and nitrogen-15 [15N]) and the principal stable isotopes of oxygen 

(oxygen-16 [16O] and oxygen-18 [18O]) are incorporated 

into compounds in different proportions depending on 

the nature of the reactions that produce the compounds. 

As a result, different sources or reservoirs of nitrate have 

characteristic isotopic ”signatures” (15N relative to 14N, 

expressed as δ15N, and 18O relative to 16O, expressed as 

δ18O). The ratios of the isotopes of nitrogen and oxygen in 

nitrate often are useful for determining sources of nitrate 

in groundwater and surface water. Isotopic ratios are 

expressed as the ratio of the heavier isotope to the lighter 

isotope relative to a standard (nitrogen in air for δ15N and 

oxygen in standard mean ocean water (SMOW) for δ18O), 

in parts per thousand (per mil). 

During mid-September 2009 to March 2010 (the wet period), flow was continuous in all five streams, and the isotopic signatures 

of the nitrate in recharging creeks and in Barton Springs discharge were relatively similar. The isotopic composition of nitrate in water 

samples from the five recharging creeks indicates a mix of nitrate from soils and biogenic sources at Onion Creek, with a progressively 

increasing component of biogenic nitrate in the samples collected from USGS stations on Barton, Bear, Slaughter, and Williamson 

Creeks. 

At Barton Springs, the values of δ15N during the wet period increased progressively, indicating an increasing contribution of 

biogenic nitrate. This is consistent with an increased amount of recharge from the creeks, which flowed continuously throughout the 

wet period. 

Figure 3. The isotopic composition of the nitrate in water samples from creeks recharging the Barton 

Springs segment of the Edwards aquifer and from Barton Springs during the dry period (November 2008–

September 2009) and the wet period (September 2009–March 2010). (Isotopic fields modified from Silva 

and others, 2002.)

General trend of isotopic change from denitrification

0 5 10 15 20 25 30

DELTA NITROGEN-15 OF NITRATE, IN PER MIL

-5

0

20

40

50

30

10

-10

60

DELTA OXYGEN-18 OF NITRATE, IN PER MIL

Nitrate in

precipitation

Synthetic

nitrate fertilizers

Biogenic nitrate (human or

animal waste, or both) Soil nitrate

Nitrate from

ammonium

fertilizers

BARTON CREEK

WILLIAMSON CREEK

SLAUGHTER CREEK

BEAR CREEK

ONION CREEK

BARTON SPRINGS

Dry Wet Minor Major

Storm

sampling

Routine

sampling

no flow

no flow

EXPLANATION
Publishing support provided by

Lafayette Publishing Service Center

For additional information, contact 

Director

USGS Texas Water Science Center

http://tx.usgs.gov/

gs-w-txpublic-info@usgs.gov 

—Barbara J. Mahler,1 MaryLynn Musgrove,1 and Chris Herrington2

1

 U.S. Geological Survey.

2

 City of Austin.

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

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

Figure 4. Permits for on-site sewage facilities (OSSFs) (septic systems) and land application of treated wastewater (Texas Land 

Application Permits [TLAPs]) on the Barton Springs contributing and recharge zones have increased greatly since 1990.

Increases in nitrate concentrations coincide with rapid increases in number of 

septic systems (OSSFs) and volume of land 

application of treated wastewater (TLAPs), 

sources of biogenic nitrogen, in the Barton 

Springs contributing zone. 

Rapid development over the contributing zone since 2000 has 

been accompanied by an increase in the generation of wastewater. 

Currently (November 2010), septic systems (also referred to as on-site 

sewage facilities, or OSSFs) and land application of treated wastewater effluent (Texas Land Application Permits, or TLAPs) are the two 

methods used for disposal of treated wastewater effluent in the Barton 

Springs contributing zone, and are likely sources of biogenic nitrate. 

The number of OSSFs and TLAPs increased greatly from 2000 to 2010 

(fig. 4). Other sources of biogenic nitrate (livestock and domesticated 

dogs and cats) are unlikely to be the source of increased nitrate in any 

of the watersheds in the contributing zone.

Septic systems. There were 6,217 OSSFs permitted during 2001–

10, about 4.5 times the total number permitted prior to 2001. The most 

permits were issued in 2001. Although the most permits were issued 

for properties in the Barton and Onion Creek watersheds, the greatest 

density of OFFSs (systems per acre) is in the Bear Creek watershed.

Land application of treated wastewater. Currently (November 

2010), the permitted volume of irrigated flow is 3,300,000 gallons per 

day. About 95 percent of that volume was permitted during 2005–10. 

The volume of new irrigation permitted annually peaked in 2005. Both 

the greatest irrigation volume and rate (volume per day per acre) are 

in the Barton Creek watershed. There are no irrigation facilities in the 

Williamson Creek watershed, and no new facilities have been permitted 

in the Slaughter Creek watershed since 1997.

What Does the Future Hold?

Population over the Barton Springs contributing and recharge 

zones is projected to double between 2010 and 2035, with most of this 

growth projected to occur in the Barton and Onion Creek watersheds 

(Capital Area Metropolitan Planning Organization, 2010). Projected 

population increases are prompting plans for direct discharge of treated 

wastewater into contributing zone streams. Currently (November 2010) 

there are no direct discharges of treated wastewater into the recharging creeks, but one wastewater discharge permit has been issued for 

the Bear Creek watershed. Small streams in Central Texas receiving 

wastewater effluent have been demonstrated to have elevated nutrient 

concentrations and to be eutrophic (Mabe, 2007).

References

Capital Area Metropolitan Planning Organization, 2005, 2005 to 2035 

Demographics: accessed on December 12, 2010, at http://www.

campotexas.org/Word/2035%20Final%20TSZ.zip.

Mabe, J.A., 2007, Nutrient and biological conditions of selected small 

streams in the Edwards Plateau, Central Texas, 2005–06, and implications for development of nutrient criteria: U.S. Geological Survey 

Scientific Investigations Report 2007–5195, 46 p.

Silva, S.R., Ging, P.B., Lee, R.W., Ebbert, J.C., Tesoriero, A.J., and 

Inkpen, E.L., 2006, Forensic applications of nitrogen and oxygen 

isotopes in tracing nitrate sources in urban environments: Environmental Forensics, v. 3, no. 2, p. 125–130.

This fact sheet is based on the following report:

Mahler, B.J., Musgrove, MaryLynn, Herrington, Chris, and Sample, 

T.L., 2011, Recent (2008–10) concentrations and isotopic compositions of nitrate and concentrations of wastewater compounds in the 

Barton Springs zone, south-central Texas, and their potential relation 

to urban development in the contributing zone: U.S. Geological 

Survey Scientific Investigations Report 2011–5018, 39 p.

TLAP OSSF

Barton Springs contributing and recharge zones

1990 1995 2000 2005 2010

Contributing

zone

Recharge zone

Contributing

zone

Recharge zone

Onsite sewage facility

Facility for land application of treated wastewater

N

N