Isotope Hydrology Jay Banner Department of Geological Sciences Environmental Science Institute April 19, 2005 Outline 1. Background: Isotope systematics stable & radiogenic 2. Tracing atmospheric vs. hydrologic processes 3. Edwards aquifer examples 4. The temporal evolution of groundwater Isotopes Different isotopes of the same element have the same number of protons and electrons, but different numbers of neutrons.
Stable Isotope Systems Many elements of low atomic weight have two or more stable isotopes, e.g., Hydrogen - 1 H, 2 H (= D) δd Carbon - 12 C, 13 C δ 13 C Oxygen - 16 O, 18 O δ 18 O The different masses cause isotopes to behave differently in physical and chemical processes. Consider two water molecules: H 2 18 O and H 2 16 O 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.
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
Map of North America showing contours of the average hydrogen isotope values (δd or 2 H/ 1 H values) of meteoric surface waters. An identical pattern is found for oxygen isotopes (δ 18 O or 18 O/ 16 O) in meteoric waters. Hydrogen isotopes in meteoric water Taylor (1974) Oxygen in hydrologic cycle cartoon
Barbados Monthly Rainfall Jones et al. (2000) H-O isotopes in rain and groundwater on Barbados
Barb Rain oxygen vs time Groundwaters Groundwater Rain Jones and Banner (2003)
What is strontium? An alkaline earth element with 4 naturally occurring isotopes ( 84 Sr, 86 Sr, 87 Sr, 88 Sr). The 87 Sr/ 86 Sr ratio (δ 87 Sr) 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
0 87 Rb 87 Sr radioactive decay system 87 87 + β - 37 Rb 38 Sr Half life = 48.8 b.y. 87 Sr = 87 Sr o + 87 Rb (e λt 1) 87 Sr/ 86 Sr = ( 87 Sr/ 86 Sr) o + 87 Rb/ 86 Sr (e λt 1) y = b + m x Sr isotope evolution of the Earth 0.720 0.716 87 Sr 86 Sr 0.712 0.708 Proterozoic Crust Fig. 1 Archean Crust 0.704 0.700 Mantle Mantle Initial Earth 1 2 3 4 5 Age (Ga) Banner (2004)
Sr isotopes in the oceans through time 0.710 0.709 87Sr/ 86Sr 0.708 0.707 0.706 Q Tertiary Cretaceous Jurassic Triassic Permian Penn. Miss. Devonian Silurian Ordovician Cambrian PC 0 100 200 300 400 500 600 Age (Ma) Burke et al. (1982) Musgrove et al. 2001 4 1-2- 3-4- Honey Creek
www.edwardsaquifer.net/
Soils = High Sr isotope ratios Low Mg/Ca and Sr/Ca values Limestone = Low Sr isotope ratios High Mg/Ca and Sr/Ca values Methods Soils are leached using a NH 4 Ac 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 thermalionization mass spectrometry.
Edwards Aquifer: Regional groundwater compositions Water-calcite interaction Water-dolomite interaction phreatic vadose Soil waters Cretaceous limestones Edwards Aquifer: Local groundwater compositions Natural Bridge dripwaters Inner Space dripwaters NB soil waters IS soil waters
0.70868 Inner Space Cavern drip site ISST: Temporal Variations in 87 Sr/ 86 Sr and Effective Precipitation 8 0.70866 87 Sr/ 86 Sr 6 87 Sr/ 86 Sr 0.70864 0.70862 0.7086 0.70858 0.70856 effective precipitation 4 2 0 monthly effective precipitation (inches) -2-4 -6-8 0.70854-10 0.70852-12 4/19/2001 April 2001 May 2002 June 2003 Aug. 2004 11/5/2001 5/24/2002 12/10/2002 6/28/2003 1/14/2004 8/1/2004 2/17/2005 Edwards Limestone
Vadose Flow Routes in Karst After Smart & Friederich (1987) Vadose Groundwater Flow Model After Smart and Friederich (1987)
Rainfall Flow route model summary High recharge = conduit flow = high 87 Sr/ 86 Sr Low recharge = diffuse flow = low 87 Sr/ 86 Sr Honey Creek Study Area and Setting Preliminary data: Sr isotopes for spring waters in the control watersheds (Bravo and Delta Springs, and the treatment watershed) Collaborators: MaryLynn Musgrove Libby Stern Honey Creek Guadalupe River treatment Bravo Spring Cotton Spring Delta Spring Echo Spring Other springs Rainfall simulation site 20 contour control
HCSNA Sr Isotope Variations NH 4 Ac soil leachates HCSNA spring waters HCSNA Sr Isotope Variations NH 4 Ac soil leachates HCSNA spring waters control watershed treatment watershed
Temporal Variations in Spring Water 87 Sr/ 86 Sr and Effective Precipitation Effective preciptation: rainfall less evaporation Impact of Urbanization on Austin-area streams Lance Christian Geological Sciences MS thesis
Current Study: Street Indicators of Urbanization Bull Creek West Bull Creek Shoal Creek Barton Creek Waller Creek Williamson Slaughter Onion Creek Big and Little Bear Source data: City of Austin Amy Hobbs, Dept. of Architecture, Univ. of TX Strontium Isotope Versus Building Area Urbanization Indicator (for watersheds: Waller, Shoal Bull, West Bull, Williamson, and Slaughter) 0.70900 0.70880 R 2 = 0.923 0.70860 87 Sr/ 86 Sr 0.70840 0.70820 0.70800 0.70780 0.70760 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 (%)
Temporal Evolution of Groundwater: An Isotopic Perspective
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
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
Tectonic Setting of Barbados Torrini et al. (1985) Barbados, West Indies After Scoffin, 1987
1 mm 1 mm Southeast Barbados
Modern Barbados Reef
Horse Hill Saharan dust transport patterns The New York Times Oct 29, 1992
NASA image of Saharan Dust over Canary Islands NASA 3/6/98 Barb speleo Sr v time eps Banner et al. (1996)
Vadose Flow Routes in Karst After Smart & Friederich (1987) Barb Speleo Sr record
Holocene climate records, Mesoamerican tropics Lake levels Lake chemistry: mineralogy stable isotopes Pollen, vegetation Fires Archeology