Preface It has been of some concern that in spite of over four decades of development and demonstration, isotope hydrology is yet to find its rightful place in field practice in India and in many other countries. Isotope hydrology is still not taught as part of hydrology syllabus in many universities. It is probably not surprising when you notice the level of application of geochemistry in groundwater investigations. In real practice, chemistry is generally restricted to determinations of water quality, more specifically to potability. Geochemistry, particularly trace element chemistry, is rarely used asa tool to studygroundwater evolution and residence. Isotope hydrology, being an extension of geochemistry, cannot possibly claim a better status. This is particularly unfortunate in view of India's ever-increasing need for utilizable water, particularly groundwater. Nearly 60% of India's groundwater occurs in hard rocks. The field hydrologist knows how frustrating it is to depend on methods developed for characterizing alluvial aquifers. India's hydrological diversity is immense from glaciers to humid tropics to deserts with attendant huge range of water resources problems. With our long-standing experience in the management of water resources, it should be evident India needs to use all available tools. Isotope hydrochemistry is one such tool. Isotope hydrology is definitely not a panacea for all hydrological ills. It has value when integrated with the other well-established tools like geology, hydrogeology and geophysics and, of course geochemistry and remote sensing. It is tempting to compare the level of absorption of scientific developments in medical practice with the level in hydrological practice. The speed with which the medical establishments in India adapted to computed tomography, magnetic resonance imaging, nuclear medicine and radiation sterilization of medical products is commendable. The same cannot be said about the water resources scene in India. Computer modeling of hydrology data and remote sensing are the exceptions. Here again, diversity and the quality of data available for modeling need to be strengthened and modernized. If one browses through refereed international journals on water resources, the scientific talent in the country would be evident. India has also accumulated significant experience in isotope hydrology, but not wide enough to have a regular say in the country's water resources scene. There are a few institutions with facilities for isotope work. Some of them are the Bhabha Atomic Research Centre, Mumbai; Physical Research Laboratory, Ahmedabad; National Geophysical Research Institute, Hyderabad; National Institute of Hydrology, Roorkee; Indian Agricultural Research Institute, Delhi and Centre for Water Resources Development and Management, Kozhikode. But, oganisations like the Central Ground Water Board who are directly responsible for water resources development in India are still to have in-house facilities for isotope hydrology. Then what are the constraints? There is a certain lack of stress on basic sciences in hydrology education. Geochemistry, as mentioned earlier, is not fully exploited as a tool in groundwater studies. Isotope hydrology, being closely associated with nuclear sciences, may not be easily usable by a field hydrologist who had no earlier training. Field hydrologists are rarely prepared to use techniques not taught in the university. Compared to conventional equipment and instruments used in hydrology investigations, isotope analytical equipment like mass spectrometers and scintillation counters are relatively more expensive. But, when compared with our national budget for water resources development, the benefit to cost ratio would make the expense negligible. Decision makers need to be properly briefed on these aspects.

“Hydrology is the science that treats the waters of the earth, their occurrence, circulation and distribution, their chemical and physical properties, and their reaction with their environment, including their relation with living things. The domain of hydrology embraces the full life history of water on the earth.” (Federal Council for Science and Technology, “Scientific Hydrology”, Washington, 1962) This definition puts in a nutshell what is most often inadequately done before undertaking exploitation of water resources. In short, it is the scientific inputs that are likely to be missed while planning water resources management projects. It is the “full life history of water” part of the above definition that is of specific relevance to the theme of this book, Isotope Hydrology. Conventional hydrology is strong on measurement and quantification of different parts of the hydrological cycle (Fig.1.1), be it precipitation, run-off, evaporation, transpiration, infiltration, groundwater dynamics etc. Through chemistry and isotopes, we try to understand the processes themselves in order to delineate the history of that part of the hydrological cycle that is being studied. For example, if we are measuring river flow, we would like to know the contribution of different components; base flow, rainfall run-off and snow melt, if any. Let’s see what we know and what do not know in the conventional hydrological practice. We know surface waters evaporate, form into clouds and precipitation occurs under favourable weather conditions. What we often do not know is the source of moisture, oceanic or inland.

In the last chapter, we have broadly introduced the concept of tracer hydrology and also the concepts of isotopes as tracers, their characteristics and measurement techniques. In this chapter let us see the basis of application of each group of isotope tracers, injected and environmental. What are the different groups of isotope tracers used in hydrology and how are they classified?

The range of isotope applications in surface water hydrology is not as wide as in groundwater hydrology as the surface waters are more easily amenable to conventional hydrological methods. Following are the areas in which isotopes have a significant role in either improving the accuracy of measurements or in providing additional information, which is otherwise not easily available.

After precipitation, run-off and surface water storage in lakes and man-made reservoirs, the next steps in the hydrological cycle are infiltration through the unsaturated zone and groundwater dynamics in the saturated zone. Well-established applications of isotopes in groundwater hydrology are

5.1 The Need In spite of the considerable experience and quite a few facilities available in the country, the growth of application of isotope techniques is not commensurate with India’s size, its climatic and hydrogeological diversity and the sheer range of its water resources problems. The situation may not be unique to India and other developing countries. Australia also appears to be facing a similar problem. A recent publication [1] says ‘the limited uptake of the method may be due to a number of factors. Until relatively recently, isotope hydrology has remained a largely academic pursuit, or measurements were the domain of national atomic energy agencies. A large fraction of information lies buried in the grey literature or in arcane technical journals. Interaction with local or regional water resources agencies has been the exception rather than the rule’ India has probably a stronger case for a more effective use of isotope techniques. A large portion of groundwater occurs in hard rocks in India. Normal hydrogeological and geophysical methods cannot fully describe such groundwater regimes to effectively plan their exploitation. It is essential to apply tracer hydrology methods, especially isotope geochemical techniques, for a comprehensive understanding of the systems. The examples of Manikaran geothermal springs, seepage studies in Salal tunnel and in some dams, given in the earlier chapters come under this category. Studies on percolation tanks in basaltic terrain and on recharge processes need isotope support. It will not be long before we would be planning to exploit very deep ground waters (>500 m), both in the hard rock terrains of peninsular India as well as in the Ganga plains to meet our ever growing need for water. We would need good understanding of the recharge processes to these deep ground waters. Isotopes will have a large role in such investigations. Arid zone hydrology is yet another area where the potential of isotope techniques has been effectively demonstrated for the characterization of the recharge processes, both palaeo and modern, and including the role of ancient rivers.

Index A Adsorbable Tracers 84 Age Dating 5, 25, 43, 44, 49, 50 Alpha Decay 8 Alpha Rays 7 Altitude Effect 36, 38, 58, 126 Amount Effect 38 Anionic Tracers 4 Anthropogenic 40, 42, 105, 167, 194 Applications 4, 12, 21, 23, 24, 26, 33, 42, 49, 51, 53, 98, 99, 187, 192, 193,194 Aquifers26,28,47,103,112,113, 118, 119, 128, 133, 134, 141, 151, 153,