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?

• 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.

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)

Isotope Systems of Interest:

Radiogenic Isotopes Stable Isotopes

Sm/Nd Oxygen

Re/Os Hydrogen

K/Ar Carbon

U-Th-Pb Sulfur

Rb/Sr Nitrogen

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.

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

Rubidium to Strontium Decay Scheme

where:

ß- is a beta particle

is an antineutrino

Q is the decay energy = 0.275 MeV

Qνβ Sr Rb -8738

8737

Strontium Seawater Curve - Burke et al. (1982)

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?

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.

Location of Sampling Sites

Tan: West Bull Creek Watershed

Yellow: Bull Creek Watershed

Pink: Shoal Creek Watershed

Green: Waller CreekWatershed

Sampling sites are bright green circles

87Sr/86Sr Ratios Plotted with Storm Discharge Gage Height Data

• 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.

Conclusions