Stable Isotope Applications in Hydrologic Studies

Abstract

The dominant use of isotopes in shallow hydrologic systems in the last few decades has been to trace sources of waters and solutes. Generally, such data were evaluated with simple mixing models to determine how much water or solute was derived from either of two (sometimes three) constant-composition sources. The world does not seem this simple anymore. With the expansion of the field of isotope hydrology in the last decade, made possible by the development and increased availability of automated sample preparation and analysis systems for mass spectrometers, we have documented considerable heterogeneity in the isotopic compositions of rain, soil water, groundwater, and solute sources. In addition, hydrologists who utilize geochemical tracers recognize that the degree of variability observed is highly dependent upon the sampling frequency, and more effort is being placed on event-based studies with high-frequency sampling. We are still grappling with how to deal with this heterogeneity in our hydrologic and geochemical models. A major challenge is to use the variability as signal, not noise; the isotopes and chemistry are providing very detailed information about sources and reactions in shallow systems, and the challenge now is to develop appropriate models to use the data. In the past, much reliance was placed upon the stable isotopes of water (δ18O, δ2H) to reveal all of the information about the hydrologic processes taking place in a catchment. Today, we acknowledge that the best approach is to combine as many tracers as possible, including solutes and solute isotopes. This integration of chemical and isotopic data with complex hydrologic and geochemical models constitutes an important frontier of hydrologic research.