Isotopes are the atoms of an element having same atomic number (Z) but different atomic weight (A). In other words, the atoms of an element having different number of neutrons (N) but same number of protons or electrons are called isotopes. For example, hydrogen has three isotopes having the same atomic number of 1 but different atomic masses or weights of 1, 2 and 3 respectively. Similarly, oxygen has eleven isotopes having atomic weight varrying from 12 to 22, and carbon has three isotopes 12C, 13C and 14C.

There are two more terms i.e., isobars and isotones that are used to differentiate and distinguish the atoms of different elements showing similarities in physical and chemical properties. Isobars have same atomic weight (Z + N).

Environmental isotopes, both stable and radioactive (unstable), occur in the Earth~s environment in varying concentrations with respect to location and time over which the investigator has no direct control. Environmental isotopes are neither required to be purchased nor to be injected as these are freely available and automatically injected in the hydrological cycle. Earlier only artificially produced radioactive isotopes were used but with the better instrumentation facilities, now-a-days envi ronmental isotopes are used more and more except in few cases where artificial radioisotopes can only be useful. The most commonly used environmental stable isotopes are deuterium (D), oxygen-18 (18O), carbon-13 (13C) and radioisotopes tritium (3H) and carbon-14 (14C), nitrogen-15 (15N), chlorine-36 etc. Silicon-32 (32 Si), caesium-137 (137Cs) and lead-210 (210Pb) etc. are also used as environmental radioisotopes for few specific studies in hydrology. Silicon-32 (32Si) is potentially attractive, because its half-life (100 year) is between that of 3H and 14C. Argon-39 (39Ar) has also been investigated and research is still in progress, but the disadvantage of using both 32Si and 39Ar is that large amount of water (a few tons) is required to provide required amount of sample for measurement. Environmental tritium is used to date the groundwater upto 50 years, while carbon-14 is used upto the age of 40,000 years.

Among the different properties of isotopic substances, one that is of particular importance for hydrologists is their slightly different physico-chemical behaviours that lead to isotopic fractionation effects. Isotopic fractionation is the basis for its utilization in stable isotope geochemistry, isotope geology, biogeochemistry, paleo- oceanography and others. For instance, the analysis of the ratio of stable oxygen isotopes in calcium carbonate, secreted by organisms like belemnites, mollusks and foraminifera and buried in deep-sea sediments, has permitted the reconstruction of paleo-temperatures for the last 150 million years or so (McCrea 1950; Epstein et al. 1953; Emiliani 1966.

According to classical chemistry, the chemical characteristics of isotopes, or rather of molecules that contain different isotopes of the same element (such as 13CO2 and 12CO2) are same. Largely this is true; however, if sufficiently accurate measurement are made using modern mass spectrometers, tiny differences in chemical as well as physical behaviour of so-called isotopic molecules or isotopic compounds can be observed. Differences in chemical and physical properties arising from variations in atomic mass of an element are called ~isotope effects~. This phenomenon can be observed as a result of change in isotopic composition by transition of a compound from one state to another (liquid water to water vapour), or by conversion of one compound into another compound (carbon dioxide into plant organic carbon), or due to difference in isotopic composition between two compounds in chemical equilibrium (dissolved bicarbonate and carbon dioxide).

It is well known that the electronic structure of an atom of an element essentially determines its chemical behaviour, whereas the nucleus is more or less responsible for its physical properties. Because all isotopes of a given element contain the same number and arrangement of electrons, a far-reaching similarity in chemical behaviour is the logical consequence. However, this similarity is not unlimited; certain differences exist in physicochemical properties due to mass differences. The replacement of any atom in a molecule by one of its isotopes produces a very small change in chemical behaviour. The addition of one neutron can, for instance, depress the rate of chemical reaction considerably. Furthermore, it may lead, for example, to a shift of the lines in the Raman and IR spectra. Such mass differences are most pronounced among the lightest elements. For example, some differences in physio-chemical properties of 1H2 16O, 2H2 16O, 1H2 18O are listed in Table 1. To summarize, the properties of molecules differing only in isotopic substitution are qualitatively the same, but quantitatively different.

Chua, J. H., and V. M. Ponce. 2010. Drainage basin of the Altiplano, South America. http://ponce.sdsu.edu/chua/altiplano_drainage_basin.html 15 October 2010.