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. 84 Sr, 86 Sr, 87 Sr, and 88 Sr 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/NdOxygen Re/OsHydrogen K/ArCarbon U-Th-PbSulfur Rb/SrNitrogen

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 85 Rb and 87 Rb 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: 84 Sr, 86 Sr, 87 Sr, 88 Sr 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. SrCO 3 and SrSO 4

Rubidium to Strontium Decay Scheme where : ß - is a beta particle is an antineutrino Q is the decay energy = MeV

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 Creek Watershed Sampling sites are bright green circles

87 Sr/ 86 Sr 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