G-WADI aims to provide a basic understanding and facilitate the use of chemical and isotopic tracers in addressing hydrological questions relevant to arid and semi-arid regions around the world. Although not all methods and applications discussed herein will be available to all users, many of these techniques are routinely used and widely available.
Chemical tracers refer generally to inert (e.g. Cl-) and reactive (Ca2+) ions alone, as ratios (Br/Cl), or as major anions and cations. Chemical tracers may also include trace elements, nutrients, organics and dissolved gasses. Isotopic tracers are more specialized and are often used in conjunction with geochemical analysis.
What is an isotope? Let’s begin with water. An isotope describes a variant of a single element based on its number of neutrons. Each variant of an element has the same atomic number but a different atomic mass. For example, oxygen may form 16O, 18O, and much less frequently, 17O. 18O has an atomic mass of 18 and is therefore heavier than 17O. Hydrogen may form 1H,2H—which is frequently referred to as deuterium (D)—and 3H, a radioactive variant known as tritium. We can write “water” as H2O, HDO (1H2HO), HTO (1H3HO), or any combination of 1H, 2H, 3H and 16O, 17O, and 18O.
Isotopes are not uniformly distributed over and throughout the earth. Some isotopes are abundant while others are very rare. The natural abundance of oxygen illustrates how this works. 16O is the most abundant, comprising 99.76% of all oxygen. 18O, the heavier isotope, comprises only 0.2%, and 17O, even rarer, makes up 0.04%. Due to these variations in natural abundance and the preference certain chemical reactions have for one isotope over another, isotopes make good tracers.
By analyzing the isotopic composition of water, researchers can answer questions such as, how old is this water? Where does this water come from? What are the rates of recharge? Isotopic analysis can target water from the atmosphere, surface, or ground, water from oceans and ice cores, and even water extracted from leaves, stems, tree trunks, and soil.
Water samples may be analyzed for dissolved elements such as N, S, and Si, and these elements can be analyzed to answer questions related to environmental contamination, geologic origin, and hydrologic flowpaths. In other cases, solid material extracted from aqueous (or past aqueous) environments is crushed and heated to release isotopes that date and define the material. Because there are so many uses for isotopic tracers, isotope analysis is used in fields such as hydrology, geology, oceanography, climatology, botany, microbiology, molecular biology, oncology, and archeology, as well as quaternary and forensic science.
For the dryland hydrologist, the most commonly used isotopes may be the stable isotopes, those isotopes whose nuclei are stable and whose mass remains constant. Radioactive isotopes, which have unstable nuclei that decay, are also used by the dryland hydrologist to determine geologic origin, flowpath, and residence time. We begin to have a more complex understanding of water when we realize the potential of these chemical and isotopic tracers to answer questions about the water’s age, origin, and movement.
To continue reading about isotopic tracers on the G-WADI website, please click “Introduction” on the left-hand menu.
To explore free, downloadable material on specific aspects and applications of isotopes, follow this link to the IAEA website: http://www-pub.iaea.org/books/IAEABooks/Subject_Areas/0402/Hydrology