Isotope Hydrology Outline 1. Background: Isotope systematics – stable & radiogenic 2. Tracing atmospheric vs. hydrologic processes 3. Edwards aquifer examples 4. The temporal evolution of groundwater Jay Banner Department of Geological Sciences Environmental Science Institute April 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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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
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.