First International Conference on Saltwater Intrusion and Coastal Aquifers— Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001 Isotope techniques for monitoring groundwater salinization. Cheikh B. Gaye Isotope Hydrology Section, Department of Nuclear Sciences And Applications, International Atomic Energy Agency, Wagramersrasse 5, P.O. Box 100, A-1400 Vienna, Austria ABSTRACT Salinization of water resources is one the most widespread processes that degrades water-quality and endangers future water exploitation. In many areas, particularly in arid and semi-arid zones, ground-water salinization limits the supply of potable fresh water. This problem is intensified in coastal aquifers where human activities result in accelerating water-quality deterioration. Also in in-land basins ground-water salinization can be one of the most important factor that affects the water quality. Monitoring and identifying the origin of the salinity are crucial for both water management and remediation. Yet the variety of salinization sources, particularly in unconfined aquifers, makes this task difficult. The International Atomic Energy Agency has been actively involved in the development and application of isotope techniques to a wide spectrum of hydrological problems including the development and management of water resources facing salinity risk. The paper provides a brief overview of the role of isotopes in groundwater salinization investigations. A new co-ordinated research project aimed at optimising the application of isotope methods for groundwater salinity studies is also presented. INTRODUCTION Steady increase in the salinity of most of the major aquifers being used for water supply in the arid and semi-arid regions of Africa, Asia and West Asia provides evidence of water quality deterioration. This salinization is often due to inflow of saline dense water during heavy withdrawals of fresh water from coastal aquifers and / or mobilisation of saline formation waters by over-exploitation of inland aquifer systems. Water pollution (salinization) cases due to extensive irrigation and use of fertilisers and other pesticides are also growing. Since there are several sources for the groundwater quality deterioration, it is necessary to characterise the specific processes involved. Isotope techniques are particularly effective for identifying the source of salinity and renewability of groundwater ROLE OF ISOTOPE APPLICATIONS IN GROUNDWATER SALINIZATION INVESTIGATIONS The fundamental relationships between δ 18 O and δ 2 H (fig.1) and between δ 18 O and salinity (fig. 2) have been used in previous studies to identify different salinization pathways. These includes wash and flushing of airborne salts by precipitation or by dissolution of evaporitic minerals from the surface, the soil or aquifer formations; and encroachment of sea water or admixture of connate brine pockets. Although this basic approach is still valid, in the past decade, new isotope tools for investigating the dissolved component (such as δ 37 Cl, δ 11 B) have appeared,