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? • 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 IsotopesStable 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
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 Creek Watershed Sampling sites are bright green circles
87Sr/86Sr Ratios Plotted with Storm Discharge Gage Height Data
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.