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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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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-
4
4- Honey Creek
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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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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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NASA image of Saharan Dust over Canary Islands
NASA 3/6/98
Barb speleo Sr v time eps
Banner et al. (1996)