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Isotope Hydrology

Outline1. Background: Isotope systematics – stable & radiogenic2. Tracing atmospheric vs. hydrologic processes3. Edwards aquifer examples4. The temporal evolution of groundwater

Jay BannerDepartment of Geological Sciences

Environmental Science InstituteApril 19, 2005

Isotopes

• Different isotopes of the same element have the same number of protons and electrons, but different numbers of neutrons.

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Stable Isotope Systems

• Many elements of low atomic weight have two or more stable isotopes, e.g.,Hydrogen - 1H, 2H (= D) δDCarbon - 12C, 13C δ13COxygen - 16O, 18O δ18O

• The different masses cause isotopes to behave differently in physical and chemical processes.

• Consider two water molecules: H218O and H2

16O

Radiogenic Isotope Systems

Some elements undergo radioactive decay due to inherent instability of their nuclei, producing a daughter product of another element.

The daughter product produced is termed ‘radiogenic’. The relative amounts of radiogenic vs. stable isotopes of the same element in rocks, minerals, and waters provide information about age and sources of dissolved ions.

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Isotope Fractionation

• Isotope fractionation: the development of differences in isotopic composition as a result of physical and chemical processes.

• The degree of fractionation depends on the relative weights of the isotopes.– Commonly fractionated: H, C, N, O, S (stable)– Negligible fractionation: Sr, Nd, U, Pb (radiogenic)

H and O isotopes in the hydrologic cycle

• Distillation and fractionation during cycle• Effects of

– Evaporation– Rainfall amount– Topography– Temperature– Climate change

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Map of North America showing contours of the average hydrogen isotope values (δD or 2H/1H values) of meteoric surface waters. An identical pattern is found for oxygen isotopes (δ18O or 18O/16O) in meteoric waters.

Taylor (1974)Hydrogen isotopes in meteoric water

Oxygen in hydrologic cycle cartoon

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Barbados Monthly Rainfall

Jones et al. (2000)

H-O isotopes in rain and

groundwater on Barbados

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Barb Rain oxygen vs time

Groundwaters

Rain

Groundwater

Jones and Banner (2003)

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What is strontium?

• An alkaline earth element with 4 naturally occurring isotopes (84Sr, 86Sr, 87Sr, 88Sr).

• The 87Sr/86Sr ratio (δ87Sr) varies in nature with age and composition of aquifer rock and soil.

• Groundwaters reacting with rock and soil may reflect these variations.

Radiogenic isotope variations in a karst aquifer

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87Rb – 87Sr radioactive decay system

87 → 87 + β-

Half life = 48.8 b.y.

87Sr = 87Sro + 87Rb (eλt – 1)

87Sr/86Sr = (87Sr/86Sr)o + 87Rb/86Sr (eλt – 1)

37Rb 38Sr

y = b + m x

Sr isotope evolution of the Earth

0.720

0.700

0.704

0.708

0.712

0.716

543210

Archean Crust

Mantle

87Sr86 Sr

Age (Ga)

Mantle

Proterozoic Crust

Initial Earth

Fig. 1

Banner (2004)

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0.710

0.709

0.708

0.707

0.706Q Tertiary Cretaceous Jurassic Triassic Permian Penn. Miss. Devonian Silurian Ordovician Cambrian PC

0 100 200 300 400 500 600

Age (Ma)

87S

r/86

Sr

Sr isotopes in the oceans through time

Burke et al. (1982)

Musgrove et al. 2001

1-

2-3-

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4- Honey Creek

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www.edwardsaquifer.net/

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Soils = High Sr isotope ratiosLow Mg/Ca and Sr/Ca values

Limestone = Low Sr isotope ratiosHigh Mg/Ca and Sr/Ca values

Methods• Soils are leached using a

NH4Ac solution to approximate the ion exchange by infiltrating rainwater as it moves through the soil.

• Cave waters and soil leachates are analyzed for elemental composition using ICP-mass spectrometry.

• Sr is extracted from waters using ion-exchange chemistry.

• Waters are analyzed for Sr isotope ratios using thermal-ionization mass spectrometry.

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Cretaceous limestones

Water-calcite interaction

Water-dolomite interaction

Edwards Aquifer: Regionalgroundwater compositions

Soil waters

vadosephreatic

Edwards Aquifer: Local groundwater compositions

Natural Bridge dripwaters

IS soil waters

Inner Space dripwaters

NB soil waters

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Inner Space Cavern drip site ISST: Temporal Variations in 87Sr/86Sr and Effective Precipitation

0.70852

0.70854

0.70856

0.70858

0.7086

0.70862

0.70864

0.70866

0.70868

4/19/2001 11/5/2001 5/24/2002 12/10/2002 6/28/2003 1/14/2004 8/1/2004 2/17/2005

87Sr

/86Sr

-12

-10

-8

-6

-4

-2

0

2

4

6

8

mon

thly

effe

ctiv

e pr

ecip

itatio

n (in

ches

)

April 2001 May 2002 June 2003 Aug. 2004

effective precipitation

87Sr/86Sr

Edwards Limestone

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Vadose Flow Routes in Karst

After Smart & Friederich (1987)

Vadose Groundwater Flow Model

After Smart and Friederich (1987)

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Rainfall – Flow route model summary

High recharge = conduit flow = high 87Sr/86Sr

Low recharge = diffuse flow = low 87Sr/86Sr

Honey Creek

treatment

Guadalupe River

control

●

●● ● ●●

●

Preliminary data: Sr isotopes for spring waters in the control watersheds (Bravo and Delta Springs, and the treatment watershed)

Honey CreekStudy Area and Setting

●Bravo Spring●Cotton Spring●Delta Spring●Echo Spring●Other springs● Rainfall simulationsite20’ contour

Collaborators: MaryLynn MusgroveLibby Stern

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HCSNA Sr Isotope Variations

HCSNA spring waters

NH4Ac soil leachates

HCSNA Sr Isotope Variations

HCSNA spring waters

NH4Ac soil leachates

treatment watershed

control watershed

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Temporal Variations in Spring Water 87Sr/86Srand Effective Precipitation

Effective preciptation: rainfall less evaporation

Impact of Urbanization on Austin-area streams

Lance ChristianGeological Sciences

MS thesis

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Source data:City of AustinAmy Hobbs,

Dept. of Architecture, Univ. of TX

Current Study: Street Indicators of Urbanization

Waller Creek

Shoal Creek

Bull Creek

Onion CreekBig and Little Bear

Williamson

Slaughter

Barton Creek

West Bull Creek

Strontium Isotope Versus Building Area Urbanization Indicator(for watersheds: Waller, Shoal Bull, West Bull, Williamson, and Slaughter)

R2 = 0.923

0.70760

0.70780

0.70800

0.70820

0.70840

0.70860

0.70880

0.70900

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

building area per watershed (%)

87Sr/86Sr

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Temporal Evolution of Groundwater:An Isotopic Perspective

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Speleothems

Speleology, n. [Greek speleon, cave; ology, study of] the exploration and study of caves

Speleothems, n. [Greek speleon, cave; them, them things] them things that grow in caves

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Applicability of Speleothems to Studies of the Temporal Evolution of Groundwater

• Wide geographic coverage

• Dateable and continuous on a range of time scales

• Range of tracers: Stable and radiogenic isotopes Trace elements, inclusions

Pittman, 1999

The West Indies

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Tectonic Setting of Barbados

Torrini et al. (1985)

Barbados, West Indies

After Scoffin, 1987

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1 mm 1 mm

Southeast Barbados

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Modern Barbados Reef

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Horse Hill

Saharan dust transport patterns

The New York Times

Oct 29, 1992

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NASA image of Saharan Dust over Canary Islands

NASA 3/6/98

Barb speleo Sr v time eps

Banner et al. (1996)

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Vadose Flow Routes in Karst

After Smart & Friederich (1987)

Barb Speleo Sr record

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Holocene climate records, Mesoamerican tropics

• Lake levels• Lake chemistry:

mineralogystable isotopes

• Pollen, vegetation

• Fires • Archeology