Isotope techniques in water resources : an advanced tool

46-49Isotopes are the atoms of an element with same atomic number but different atomic weight. Isotopes may be radioactive and stable by nature. Now a days, environmental isotopes (stable and radioactive) are widely used for hydrological investigations. Isotope Hydrology deals with the application of isotopes as tracers in water resources development and management. Applications of isotopes in hydrology and water resources are relatively a new subject, but its importance has been felt more in recent years. This is due to tremendous increase of problems in water sector, particularly depleting groundwater quantity, deterioration in water quality and many other unpredictable natural events that affect the hydrological cycle. The conventional methods often fail to provide insight to these problems, while isotope techniques provide a clear picture and helps in finding a suitable solution. Applications of isotopes in hydrology are based on the general concept of “tracing”, in which either intentionally introduced isotopes or naturally occurring (environmental) isotopes are employed. Environmental isotopes (either radioactive or stable) have a distinct advantage over injected (artificial) tracers and these facilitate the study of various hydrological processes on a much larger temporal and spatial scale through their natural distribution in a hydrological system. Thus, environmental isotope methodologies are unique in regional studies of water resources to obtain time and space integrated characteristics whereas the artificial tracers generally are effective for site-specific, local applications. Generally, isotope tracers are not used as an independent tool but to supplement hydrogeological, geophysical and geochemical information for a better understanding of the processes taking place in a hydrological system. Therefore, in hydrological investigations, isotope techniques should be used routinely along with hydrochemical and hydrogeological techniques. As all isotopic, hydrogeological and hydrodynamic interpretations are space and time related, it is imperative that one should consider all the related aspects of water sampling and prevailing hydrogeological conditions in a study area. A large variety of environmental stable and radioactive isotopes are employed for hydrological studies (e.g., 2H, 3H, 3He, 6Li, 11B, 13C, 14C, 15N, 18O, 34S, 36Cl, 37Cl, 81Br, 81Kr, 87Sr, 129I, 137Cs, 210Pb etc.) However, the stable isotopes have the distinct advantage over injected (artificial) tracers 3H, 46Sc, 60Co, 82Br, 131I, 198Au, etc. is that they facilitate the study of various hydrological processes on a much larger temporal and spatial scale through their natural distribution in a system. Earlier, artificially produced radioactive isotopes which were being used with a very limited isotopes are widely used for a variety of applications with no fear of health hazards. Environmental isotopes are freely available in the atmosphere and automatically injected in to the hydrologic cycle. Therefore, the users have neither to purchase these isotopes nor to inject them in the hydrological system.