Introduction
Isotopes (from the Greek iso-, equal, and topos, place; in reference to isotopes of an element having the same position in the periodical table of elements) are forms of a given chemical element that have different atomic masses. The nuclei of isotopes of an element contain identical numbers of protons, and so the isotopes have the same atomic number. Each isotope has a different number of neutrons and thus has a different atomic mass.
Most elements have both stable and radioactive isotopes. Radioactive isotopes of an element are commonly used as tracers in medical, biological, and industrial studies to gain information about physical and mechanical processes. In geology and archaeology, radioactive isotopes are used to determine the age of a sample. Hydrologists find isotopes useful in their research in a variety of ways. They are described in detail on this site.
For additional general information on isotopes and hydrology, see the January/February 2003 issue of Southwest Hydrology focusing on "Tracking Groundwater with Isotopes." The issue includes articles by Brenda Ekwurzel, "Dating Groundwater With Isotopes," and by James Hogan, ""Isotope Hydrology: Web and Print Resources".
Types of Isotopes
Stable
Introduction
Stable isotopes are those isotopes that do not undergo radioactive decay; thus, their nuclei are stable and their masses remain the same. However, they may themselves be the product of the decay of radioactive isotopes. (See "radiogenic" isotopes discussion on the Radioactive Isotopes section). The isotopic composition of stable isotoes is, however, subject to natural variation due to mass dependent fractionation. That is to say, mass differences between isotopes result in isotopic fractionation during chemical processes. In hydrological (as well as biological) studies, the stable isotopes of interest are generally H, C, N, O, S, B, and Li.
Isotopic fractionation
During isotopic fractionation, heavy and light isotopes partition differently between two compounds or phases. Isotope fractionation occurs because the bond energy of each isotope is slightly different, with heavier isotopes having stronger bonds and slower reaction rates. The difference in bonding energy and reaction rates are proportional to the mass difference between isotopes. Thus, light elements are more likely to exhibit isotopic fractionation than heavy isotopes. For example, the relatively light 12C and 13C isotopes have an 8% mass difference and undergo stable isotope fractionation. In contrast, the heavy isotopes 87Sr and 86Sr have a 1.1% mass difference and do not exhibit detectable mass fractionation. Isotopes especially susceptible to fractionation are of the elements that are among the most abundant on earth: H, C, N, O, and S.
Equilibrium fractionation
Equilibrium fractionation describes isotopic exchange reactions that occur between two different phases of a compound at a rate that maintains equilibrium, as with the transformation of water vapor to liquid precipitation.Although the process is in equilibrium, the rate of these exchanges is different so that the result is an enrichment of one of the isotopes. Such an exchange can be expressed as:
where A and B are phases, and superscripts 1 and 2 are isotopes.
The equilibrium constant may be expressed by
This can also be expressed as a ratio of the isotopes in each phase:
and 
where aA-B is the fractionation factor, the ratio of the numbers of any two isotopes in one chemical compound A divided by the corresponding ratio for compound B.
Other factors come into play to influence equilibrium fractionation and isotope effects, chiefly vibrational energy, which is related to the zero-point energy difference and is dependent on temperature. Different isotopes have different zero point energies for the vibrational mode of a bond. Temperature is a measure of energy in a system, translated to the energy of the bond. The zero point of energy changes with temperature increases. The difference in zero point energy between two isotopes decreases. Typically, the heavier isotope has a lower zero point energy, thus it takes more energy to break the bond of a heavy isotope compared to the light isotope. One may expect greater isotopic fractionation at low temperatures, and no isotopic fractionation at very high temperatures.
Kinetic fractionation
Kinetic fractionation is fractionation that is unidirectional, where equilibrium is not attained. This type of fractionation applies to evaporation of surface waters and to most biogeochemical reactions, where the lighter isotope is faster reacting and becomes concentrated in the products. More information on kinetic fractionation is provided under the discussion of oxygen and hydrogen isotopes.
Radioactive
Introduction
Radioactive isotopes are nuclides (isotope-specific atoms) that have unstable nuclei that decay, emitting alpha, beta, and sometimes gamma rays. Such isotopes eventually reach stability in the form of nonradioactive isotopes of other chemical elements, their "radiogenic daughters." Decay of a radionuclide to a stable radiogenic daughter is a function of time measured in units of half-lives.
Types of radioactive decay:
- alpha (a) decay results from an excess of mass. In this type of decay, alpha particles (consisting of two protons and two neutrons) are emitted from the nucleus. Both the atomic number and neutron number of the daughter are reduced by two, so the mass number decreases by four. An example is the decay of 238U:
- ß+ - or "positron decay" results from an excess of protons. In this type of decay, a positively charged beta particle and a neutrino are emitted from the nucleus. The atomic number decreases by one and the neutron number is increased by one. An example is the decay of radioactive 18F to stable 18O:
where ß+ is the positron, v is the neutrino, and Q is the total energy given off.
- ß- - or "negatron decay" results from an excess of neutrons. In this type of decay, a negatively charged beta particle and a neutrino are emitted from the nucleus. The atomic number increases by one and the neutron number is reduced by one. An example is the decay of radioactive 14C to stable 14N:
where ß- is the beta particle, v is the antineutrino, and Q is the end point energy (0.156 MeV).
- electron capture also results from an excess of protons. In this type of decay, an electron is spontaneously incorporated into the nucleus and a neutrino is emitted from the nucleus. The atomic number decreases by one and the neutron number increases by one. Electron capture may be followed by the emission of a gamma ray. An example is the decay of 123I to 123Te:
Types of radioactive isotopes by origin
Long-lived radioactive nuclides. Some radioactive nuclides that have very long half lives were created during the formation of the solar system (~4.6 billion years ago) and are still present in the earth. These include 40K (t½ = 1.28 billion years), 87Rb (t½ = 48.8 billion years), 238U (t½ = 447 billion years), and 186Os (t½ = 2 x 106 billion years, or 2 million billion years).
Cosmogenic. Cosmogenic isotopes are a result of cosmic ray activity in the atmosphere. Cosmic rays are atomic particles that are ejected from stars at a rate of speed sufficient to shatter other atoms when they collide. This process of transformation is called spallation. Some of the resulting fragments produced are unstable atoms having a different atomic structure (and atomic number), and so are isotopes of another element. The resulting atoms are considered to have cosmogenic radioactivity. Cosmogenic isotopes are also produced at the surface of the earth by direct cosmic ray irradiation of atoms in solid geologic materials.
Examples of cosmogenic nuclides include 14C, 36Cl, 3H, 32Si, and 10Be. Cosmogenic nuclides, since they are produced in the atmosphere or on the surface of the earth and have relatively short half-lives (10 to 30,000 years), are often used for age dating of waters.
Anthropogenic. Anthropogenic isotopes result from human activities, such as the processing of nuclear fuels, reactor accidents, and nuclear weapons testing. Such testing in the 1950s and 1960s greatly increased the amounts of tritium (3H) and 14C in the atmosphere; tracking these isotopes in the deep ocean, for instance, allows oceanographers to study ocean flow, currents, and rates of sedimentation. Likewise, in hydrology it allows for the tracking of recent groundwater recharge and flow rates in the vadose zone. Examples of hydrologically useful anthropogenic isotopes include many of the cosmogenic isotopes mentioned above: 3H, 14C, 36Cl, and 85Kr.
Radiogenic. Radiogenic isotopes are typically stable daughter isotopes produced from radioactive decay. In the geosciences, radiogenic isotopes help to determine the nature and timing of geological events and processes. Isotopic systems useful in this research are primarily K-Ar, Rb-Sr, Re-Os, Sm-Nd, U-Th-Pb, and the noble gases (4H, 3H-3He, 40Ar).
Because of their stable evolution in groundwater, such naturally occurring isotopes are useful hydrologic tracers, allowing evaluation of large geographic areas to determine flowpaths and flow rates. Consequently, they are helpful in building models that predict fracturing, aquifer thickness, and other subterranean features.