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Isotopic Analysis of Austin Surface Water and Storm Discharge

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Isotopic Analysis of Austin Surface Water and Storm Discharge. Lance Christian CE 394K - Surface Water Hydrology April 29, 1999. Outline Background on Isotopes Rb - Sr Isotope System Case Study of Selected Austin Storm Events. What is an isotope?

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Presentation Transcript
slide1
Isotopic Analysis of Austin

Surface Water and Storm

Discharge

Lance Christian

CE 394K - Surface Water Hydrology

April 29, 1999

slide2
Outline
  • Background on Isotopes
  • Rb - Sr Isotope System
  • Case Study of Selected Austin Storm
  • Events
slide3
What is an isotope?
  • They are atoms which have the same number of protons but
  • different numbers of neutrons
  • e.g. 84Sr, 86Sr, 87Sr, and 88Sr
  • Therefore, the mass number (protons + neutrons) varies due to
  • increasing amounts of neutrons while the atomic number (number
  • of protons) remains constant.
slide4
Why do we use isotopes?
  • “Isotopes are everywhere.”
  • Isotopes may behave comparatively/differently within the same
  • isotope family - fractionation is a very important tool.
  • They can provide both age determinations as well as
  • environmental data such as temperature, flow patterns, degree of
  • alteration.
  • In hydrologic studies (e.g. storm discharge), one can expect
  • concentration to vary with flow, however, isotope ratios will not
  • vary unless affected by other variables (e.g. flow paths or a
  • change in contributing sources)
slide5
Isotope Systems of Interest:

Radiogenic IsotopesStable Isotopes

Sm/Nd Oxygen

Re/Os Hydrogen

K/Ar Carbon

U-Th-Pb Sulfur

Rb/Sr Nitrogen

slide6
Rubidium / Strontium System
  • Rubidium (element 37) is an alkali metal belonging to Group
  • IA giving it a +1 ionic charge. It has two naturally occurring
  • isotopes 85Rb and 87Rb whose isotopic abundances are 72.2%
  • and 27.8% respectively.
  • Strontium (element 38) is a member of the alkaline earths of
  • Group IIA giving it a +2 ionic charge. It has four naturally
  • occurring isotopes: 84Sr, 86Sr, 87Sr, 88Sr whose isotopic
  • abundances are 0.6%, 9.9%, 7.0%, and 82.5% respectively.
slide7
Rubidium / Strontium System
  • Rb has an ionic radius of 1.48Å compared to that of K (1.33Å).
  • Given that both elements are members of the alkali metals group,
  • meaning they both have a +1 ionic charge, the Rb often
  • substitutes for K in the crystal lattice of K bearing minerals.
  • Sr often substitutes for Ca (ionic radius of 1.13Å and 0.99Å
  • respectively) in lattice sites and can form its own minerals.
  • e.g. SrCO3 and SrSO4
slide8
Rubidium to Strontium Decay Scheme

where:

ß-is a beta particle

 is an antineutrino

Q is the decay energy = 0.275 MeV

slide10
Analysis of Austin Surface Water
  • Compare high discharge events to baseflow.
  • Is there a trend? Differences between isotopic ratios
  • of natural baseflow vs.. storm runoff induced by impermeable
  • surface coverage.
  • Can a signal be found relating to the discharge hydrograph?
slide11
Geology of the Area
  • West Austin bedrock is comprised of a number of limestone
  • units
  • Edwards Fm.
  • Bee Creek Member
  • Bull Creek Member
  • Glen Rose Fm.
  • Central Austin is crosscut by a series of normal faults resulting
  • from subsidence in the Gulf. The bedrock geology of central
  • Austin is therefore complex but is generally comprised
  • of limestone and shale/clays.
slide12
Location of Sampling Sites

Tan: West Bull Creek

Watershed

Yellow: Bull Creek

Watershed

Pink: Shoal Creek

Watershed

Green: Waller Creek

Watershed

Sampling sites are bright green circles

slide14
Conclusions
  • Isotope ratios do appear to be affected by an increase in discharge
  • e.g. storm events.
  • The effects of the Sr ratio variances are believed to be attributable
  • to input from storm runoff which are otherwise “outside sources”
  • The contributing runoff sources are believed to be predominantly
  • impervious surface coverage coupled with potential anthropogenic
  • effects.
  • The next stage is to look for spatial patterns as well as using
  • conventional cation data for an independent verification.
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