Effects of Drought and Wastewater on Nitrate Concentrations in the Barton Springs Zone

Barbara MahlerU.S. Geological Survey

bjmahler@usgs.gov

Hydrologic conditions: transition from drought to wet

Nitrate concentrations in streams increased when the drought broke in Sept. 2009

0.00

0.50

1.00

1.50

2.00

2.50 Bear Creek

Nit

rate

con

cent

rati

on (m

g/L)

Jan-88

Jan-91

Jan-94

Jan-97

Jan-00

Jan-03

Jan-06

Jan-09

Jan-12

0.00

0.50

1.00

1.50

2.00

2.50

Onion Creek

Nitr

ate

conc

entr

ation

(mg/

L)

time

Drought

….and were high relative to measured streamflow

1.0 10.0 100.0 1,000.00.00

0.50

1.00

1.50

2.00

2.50

Bear Creek

1993-20082008-2010

Streamflow (ft3/s)

Nitr

ate

conc

entr

ation

(mg/

L)

0.0 0.1 1.010.0

100.0

1,000.0

10,000.00.00

0.50

1.00

1.50

2.00

2.50

Onion Creek

1993-20082008-2010

Streamflow (ft3/s)

Nitr

ate

conc

entr

ation

(mg/

L)

Flow rate

10,000

Nitrate concentrations in groundwater had contrasting responses to the break in the drought (Sept. 2009)

Sep-08 Dec-08 Mar-09 Jul-09 Oct-09 Jan-10 May-100

0.5

1

1.5

2

2.5

3

Nitr

ate

conc

entr

ation

, mg/

L

Buda well

Marbridge well

Barton Springs

Barton Springs: Nitrate concentrations were higher relative to historical levels

Jan-88

Jan-91

Jan-94

Jan-97

Jan-00

Jan-03

Jan-06

Jan-09

Jan-12

0.0

0.5

1.0

1.5

2.0

Nitr

ate

conc

entr

ation

(mg/

L)

0 20 40 60 80 100 120 1400.0

0.5

1.0

1.5

2.0

Barton Springs discharge (ft3/s)

Nitr

ate

conc

entr

ation

(mg/

L)1988 to 2010

1993-20082008-10

Barton

Springs

What’s changed?

Septic systems permitted by year

1985 1990 1995 2000 2005 2010 20150

100

200

300

400

500

600

BartonWilliamsonSlaughterBearOnion

No.

of s

eptic

sys

tem

s pe

rmitt

ed

Irrigation volume permitted by year

1985 1990 1995 2000 2005 2010 20150

200,000

400,000

600,000

800,000

1,000,000

BartonSeries3SlaughterBearOnion

Volu

me

(gal

lons

/day

)

Williamson

Cumulative N loading

• Organic N is being converted to nitrate in the aquifer

• Total N is being stored (conservatively?) in the aquifer

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

20,000

40,000

60,000

80,000

100,000

Organic NNitrate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

20,000

40,000

60,000

80,000

100,000

Stream Recharge

Barton Springs

Nitr

ogen

, in

kilo

gram

s

Nov 08

Jan 09Mar 0

9May 09

Jul 09Sep 09

Nov 09Jan 10

Mar 10

Nov 08

Jan 09Mar 0

9May 09

Jul 09Sep 09

Nov 09Jan 10

Mar 10

Implications of nitrification

• Organic nitrogen NH4+ (ammonification)

• NH4+ + 1.5 O2 2H + + 2H2O + NO2

-

• NO2- + 0.5 O2 NO3

-

• For every mg of ammonia oxidized to nitrate, 4.18 mg of oxygen are consumed

• Nitrification lowers the pH

Infiltration and discharge to surface water

Partial ammonification and nitrification; recharge to groundwater

Continued ammonification and nitrification of organic nitrogen

Related publications• Nitrate concentrations and potential sources in the Barton Springs segment of the

Edwards Aquifer and its contributing zone, Central Texas. 2011. Mahler, B.J., Musgrove, M., and Herrington, C. USGS Fact Sheet 2011-3035 http://pubs.usgs.gov/fs/2011/3035/

• 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. 2011. Mahler, B.J., Musgrove, M., Herrington, C., and Sample, T.L. USGS Scientific Investigations Report 2011-5018. http://pubs.usgs.gov/sir/2011/5018/

• Recent (2008–10) water quality in the Barton Springs segment of the Edwards Aquifer and its contributing zone, Central Texas, with emphasis on factors affecting nutrients and bacteria. 2011. Mahler, B.J., Musgrove, M., Wong, C.J., and Sample, T.L. USGS Scientific Investigations Report 2011-5139. http://pubs.usgs.gov/sir/2011/5139/

• bjmahler@usgs.gov; 512-927-3566