A Primer on Stable Isotopes and Some Common Uses in Hydrology

Created by Monica Z. Bruckner, Montana State University, Bozeman

What is a Stable Isotope?

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Scientists extrude an ice core from its barrel with the utmost care. Ice cores can be analyzed for stable isotope ratios of oxygen to determine temperatures in the Earth's past. Photo by Kendrick Taylor, Desert Research Institute, University and Community College System of Nevada (image courtesy of NOAA Image Gallery).

Isotopes are atoms of the same element that have different numbers of neutrons; that is, they have the same number of protons (positive charge) and electrons (negative charge), but differ in molecular weight due to different numbers of neutrons (neutral charge). Isotopes may be radioactive, or unstable in the natural environment and prone to decay to another state known as a daughter product, or stable. Common stable isotopes include ²H/H, ¹⁸O/¹⁶O, and ¹³C/¹²C. These isotopes occur naturally in the environment, but their natural abundance differs with different environmental conditions. For instance, the ratio of ¹⁸O to ¹⁶O in ice cores and fossil remains of microorganisms is commonly used to identify colder vs. warmer periods in the Earth's past; higher abundances of ¹⁶O indicate warmer periods (increased evaporation), whereas higher abundances of ¹⁸O indicate cooler periods (decreased evaporation). Hence, the partitioning of isotopes between substances during reactions or processes can be used to characterize processes in the biological, geological, and hydrological realm, both past and present. This partitioning, known as isotope fractionation is governed by the principle that lighter isotopes, or those with a lower molecular weight, will be favored in evaporation processes and biological uptake, leaving the source material "heavier," or with the heavier isotope more abundant.

How Are Stable Isotopes Used in Hydrology?

Stable isotopes are useful tools for characterizing several different water dynamics within a watershed. These applications include:

• constraining residence time, or the time it takes for a molecule of water to move from point a to point b,
• characterizing how water moves within the watershed, including indication of potential inputs (e.g. precipitation vs. groundwater) of water to a system, what happens to the water within the system, and outflows (e.g. water lost to groundwater vs. streamflow) of the water from the system, and
• determining mixing and flow paths of water within a system.

Stable isotopes are powerful tools in hydrology, in that they:

• are naturally occurring within a catchment, and are ideal conservative tracers as ²H and ¹⁸O are essentially the water molecule itself,
• do not readily chemically react with rocks and minerals at temperatures encountered at or near Earth's surface,
• undergo fractionation during evaporation/condensation and through biological processes, with light isotopes preferentially evaporated or taken up by biota and heavy isotopes condensing with precipitation or being left by biota.
  

What is Stable Isotope Fractionation and What Does it Tell Us?

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A schematic diagram of the isotope fractionation process via evaporation, condensation, and evapotranspiration (combination of evaporation and transpiration). Notice that waters are lighter when they evaporate and are relatively heavier when condensed in the form of precipitation.

Stable isotope fractionation occurs naturally through evaporation/condensation. Lighter isotopes preferentially evaporate while heavier isotopes condense to form precipitation preferentially to light isotopes. This said, the journey of a water molecule can be traced from its source to a given catchment based on fractionation ratios. The greater the distance between the ocean and inland storm event are, the more likely that rain will be lighter due to rain out of heavier isotopes from processes such as orographic uplift or previous storm events.

Results Analysis - How Can We Tell Where the Water is From?

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This graph illustrates a hydrograph (discharge, or the volume of water flowing through the system per unit time) of old and new water flowing through a New Zealand stream (data from McGlynn and McDonnell (2003)). 
The separation is based on the relative concentrations of ¹⁸O between the old and new water. The bars at the top of the graph show precipitation, where the longer line indicates greater rainfall; new water, or water recently fallen as precipitation, contribution to total stream discharge is shown in light blue whereas old water, or water that has been stored in the watershed for a period of time, is indicated in blue and total stream discharge is shown in green.

The stable isotope ratios in water samples can be analyzed to determine "new" water, or the water that falls directly from a storm event, from "old" water, or water that had fallen in a storm event in the past and may be stored in plants, soils, or groundwater. These ratios are represented by the notation per mil (represented with the symbol ‰) are compared with a standard, and are considered "heavy" or "enriched" (more of the heavier isotope) or "light" or "depleted" with respect to the standard. Each standard corresponds to a particular isotope or isotope pair; these standards (and their respective abbreviations) are:

• Standard Mean Ocean Water (SMOW) - used for hydrogen and oxygen,
• Pee Dee Belemnite (PDB) - used for carbon and oxygen,
• Atmospheric Air - used for nitrogen,
• The Canyon Diablo meteorite (CD) - used for sulfur analysis.

Ratios of heavy to light isotopes from sample can be compared with these standards to determine if the sample is enriched or depleted relative to these standards. The degree of enrichment or depletion can indicate the source and age of the water.

Related Links

• Isotope Tracers in Catchment Hydrology- A USGS website that provides links to select book chapters that discuss isotopes as tracers in hydrology. The book is edited by C. Kendall and J.J. McDonnell (1998).
• A Review of Isotope Applications in Catchment Hydrology- A review paper (PDF format) by Vitvar, T., Aggarwal, P.K., and McDonnell, J.J. (2004).
• Stable Isotope Overview- A general overview of the stable isotope method for tracer studies. This site includes a list of references for further information.