Pradeep K. Aggarwal

Head, Isotope Hydrology Section

Isotope hydrology today is a fully recognized discipline of hydrologic sciences in its own right. Yet, the greatest measure of its success is perhaps the degree of integration of isotope techniques into other fields of hydrology. Based on a keyword scan of the Georef database between 1960 and 1965, less than 100 reports on groundwater hydrology in major scientific journals included the use of isotopes. However, more than 7000 reports published between 1995 and 2000 in the fields of groundwater hydrology and climate change studies used isotope techniques to various extents. Isotope hydrology, alongside physical and chemical hydrology, has evidently become an essential part of the hydrological tool kit. The present state of isotope hydrology is related in part to the efforts of the International Atomic Energy Agency (IAEA), an organization that has played a pivotal role in promoting and expanding the field of isotope hydrology over the last four decades. IAEA activities continue to facilitate development and integration of isotope techniques in hydrology through its major areas of activity which include: (i) research and development, (ii) global databases and reference materials, (iii) education and training; and (iv) technology transfer. Some recent initiatives in these areas are described below.

Research and development at IAEA today is aimed at isotope applications for global issues of water resources management. Nearly half of all freshwater used for drinking and irrigation worldwide is groundwater, linking the sustainability of groundwater resources to sustainable human development. Current estimates of the world’s groundwater resources are generally weak and reliable information on the proportion of renewable or non-renewable groundwater is sketchy. The IAEA has initiated a joint project with United Nations Education and Cultural Organization (UNESCO) and International Association of Hydrologists (IAH) to improve the understanding of the global distribution and amounts of non-renewable or fossil groundwater by utilizing the labelling properties of isotope data collected from groundwater aquifers worldwide. Most of these isotope data were collected as part of IAEA technical cooperation projects and the IAEA is in a unique position to make this contribution.

Estimates of renewable groundwater resources and understanding of related hydrological processes are critically dependent upon determining the presence and age of modern groundwater. As the strong signal of bomb tritium from thermonuclear atmospheric testing is now essentially washed out of the atmosphere, this ideal tool for characterizing the presence of modern groundwater is becoming more difficult or impossible to use. Radioactive decay of the tritium parent produces a daughter product, helium-3, which is being exploited more often and is a similar or better tool than tritium alone for age-dating of modern groundwater. Helium-3 measurements require expensive and high-maintenance mass spectrometry. This has kept helium-3 measurement facilities and applications in a limited number of laboratories even though hydrologic applications of helium-3 have been recognized for over 20 years. To facilitate further developments in this field, the IAEA has established a helium-3 facility in Vienna that is expected to undergo operational testing in 2003 and be ready for routine applications in 2004. Corresponding projects to conduct a survey of the age of baseflow in large rivers and shallow aquifers will be initiated during 2004-2005.

Climate change impacts on water resources are potentially significant and need to be a part of comprehensive water sustainability strategies. Isotopes are indispensable tools for understanding past climate change and impacts on the hydrological cycle. The IAEA/WMO Global Network of Isotopes in Precipitation (GNIP) has been a critical resource for isotope applications in climate studies. Isotopic composition of global precipitation has provided a new dimension for Atmospheric Global Circulation Models and is used to better represent and verify the simulation of hydrological processes in these models. Accessibility of the GNIP database has been greatly simplified and these data can be freely accessed through the internet. The network is also being strengthened through an increase in the geographical spread of the stations and through a greater integration with the climate research community.

While it has been recognized for some time that isotope monitoring of river discharge will improve our understanding of climate change and continental-scale hydrological processes, these data have not been available on a continental or global scale, except for the recently produced data from the United States. A new coordinated research project has been initiated to collect and analyse isotope data from large rivers world-wide for understanding hydrological processes in river basins and for improving continental-scale water balance estimations. About 15 research groups are participating in this project with monitoring stations in many of the large rivers in North America, South America, Africa, Asia, and Europe. In addition, the isotopic composition of atmospheric moisture, leaf water, and the terrestrial carbon dioxide flux will be investigated in a new coordinated research project to be initiated in 2004.

An important part of the continental water balance involves submarine groundwater discharge (SGD). Though poor, current estimates of SGD range from about 5 to 12% of continental runoff. A coordinated research project was recently initiated in cooperation with UNESCO to develop isotope techniques for monitoring SGD. Together with the University of Palermo, Italy, the Sicilian coast was chosen for a pilot study that is expected to be complete by 2003. Further studies may be conducted in coastal regions of India and southern Africa.

Another important area of work at IAEA is the development and maintenance of reference materials and quality assurance. About 40 different isotope reference materials, including VSMOW and its new successor, are distributed to laboratories world-wide. Inter-laboratory comparisons to evaluate the quality of stable isotope and tritium analysis of water samples are conducted on a regular basis. Recently, 86 laboratories from 44 countries participated in a tritium inter-comparison exercise. Participating laboratories included those using electrolytic enrichment and helium in-growth techniques. Results showed that the performance of less than half (40) of the laboratories was sufficient for isotope hydrology applications in terms of accuracy and precision of measurements. Nearly one third of the participating laboratories may have systematic errors in their analytical procedures. The range of results from helium in-growth laboratories was as much as the overall range of all measurements. However, an improvement of about 10% could be detected in the sensitivity and performance compared to the last tritium inter-laboratory comparison of 1995.

In addition to scientific efforts, integration of isotopes into mainstream hydrology requires a closer interaction between isotope hydrologists and the hydrological community at large. To this end, a new IAEA-UNESCO Joint International Isotopes in Hydrology Programme (JIIHP) has been launched to foster cooperation between hydrologists and isotope professionals at the national level who have been traditionally associated with the IAEA or UNESCO’s International Hydrological Programme.

Over the years, the IAEA has built a cadre of trained isotope hydrologists in developing countries. In the past, efforts focused on training after formal education, rather than formal education at the university level. As a result, there is a continuing demand for manpower development even in countries where the Agency has built capacity in the past. Lack of sufficient academic training for hydrologists in the use and application of isotope techniques has been identified as one of the major constraints that limits integration of isotope hydrology in the water sector of many developing countries. To overcome these constraints, the University of Roorkee in India received assistance to offer a semester course on isotope hydrology within their post-graduate programme on water resources management. This course is expected to serve as a model for initiating similar courses in other parts of Asia and Africa. A series of six volumes on various aspects of isotope hydrology was recently published by the IAEA in cooperation with UNESCO. A CD-ROM version of these volumes that integrates utilities for graphical display of the various equations in the text and geochemical models NETPATH and SOLMINEQ.88 is expected to be released in 2003.

In the technical cooperation area, IAEA provides assistance to developing countries in application of isotope techniques for water resources management. The emphasis of this work is to build local capacity in terms of human resources and analytical facilities while helping to solve current problems. About 65 projects in more than 44 countries are operational for every two year cycle with a funding of about $8 M. A large number of these projects are related to groundwater development and management. In addition, projects related to surface water management, such as characterization of leakage from dams and reservoirs, and of geothermal reservoirs are also implemented.

The IAEA has hosted quadrennial symposia on latest developments in isotope hydrology since 1963. The 40^th anniversary symposium will be held in May 2003. We hope that more and more isotope and non-isotope hydrologists will participate in this symposium and make it a successful event.

As water resources sustainability and water development become more important issues on the world agenda, the IAEA, as a United Nations Organization will be expected to play a coordinating role in water resources research and development, with a mandate for promoting wider use and wider integration of isotope hydrology techniques.

Additional information on isotope hydrology activities at the IAEA can be found at http://www.iaea.org/programmes/ripc/ih/

Acknowledgements

I would like to thank my colleagues in the Section and the non-IAEA colleagues around the world who collaborate with us, contributing to a successful program of the Agency.

(preprint of a manuscript submitted to "Hydrological Processes")