Isotopic Analysis of Austin
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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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Isotopic Analysis of Austin

Surface Water and Storm

Discharge

Lance Christian

CE 394K - Surface Water Hydrology

April 29, 1999


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  • Outline

  • Background on Isotopes

  • Rb - Sr Isotope System

  • Case Study of Selected Austin Storm

  • Events


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  • 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.


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  • 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)


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Isotope Systems of Interest:

Radiogenic IsotopesStable Isotopes

Sm/Nd Oxygen

Re/Os Hydrogen

K/Ar Carbon

U-Th-Pb Sulfur

Rb/Sr Nitrogen


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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.


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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


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Rubidium to Strontium Decay Scheme

where:

ß-is a beta particle

 is an antineutrino

Q is the decay energy = 0.275 MeV



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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?


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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.


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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


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87Sr/86Sr Ratios Plotted with Storm Discharge

Gage Height Data


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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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