Tracers in Hydrology (Proceedings of the Yokohama Symposium, July 1993) IAHS Publ. no. 215, 1993. ENVIRONMENTAL ISOTOPE APPLICATIONS IN HYDROLOGY: AN OVERVIEW OF THE IAEA'S ACTIVITIES, EXPERIENCES, AND PROSPECTS Y. YURTSEVER & L. ARAGUAS ARAGUAS Isotope Hydrology Section Department of Research & Isotopes, International Atomic Energy Agency (IAEA), Vienna, Austria ABSTRACT Development and applications of isotope methodologies in hydrology have been an integral part of the program component of the IAEA over the last three decades, within the framework of its overall activities related to peaceful nuclear applications. The use of environmental isotopes as a means of tracing water movement in the hydrology including surface and ground water is much of the Agency's work in this field. This paper provides an overview of the temporal and spatial variations of the above cited isotopes in precipitation based on the long-term data collect­ ed from the global network, and reviews the concepts and formulations of environmental isotope applications to specific problems in hydrology and hydrogeology. Results of a few case studies are provided to illustrate their use. Activities of the IAEA in this particular field, together with future pro­ spective developments in the use of environmental isotopes in hydrology and environmental studies are briefly discussed. INTRODUCTION The methodologies based on the use of naturally occurring isotopes for various hydrologi- cal problems encountered in water resources assessment, development and management activities is an already established field recognized as "Isotope Hydrology". Together with the techniques based on the employment of radioactive isotopes and sealed radioactive sources for in-situ experiments related to water movement, they comprise the overall scientific discipline of "Nuclear Techniques in Hydrology". During the last three decades, the International Atomic Energy Agency (IAEA) has been directly involved in efforts related to research and development of nuclear techniques in the water sector, their actual field applications, and in providing a forum for dissemina­ tion information internationally, within a framework of peaceful nuclear applications. This paper provides an overview of the basic concepts and methodologies of environmental isotope applications in hydrology with a some highlights from a few case studies, and briefly elaborates on the future required developments in this field, based on the knowledge and experiences of IAEA. GENERAL CONSIDERATIONS Isotopes which are naturally produced and incorporated into the hydrological cycle, are of­ ten referred to as "Environmental Isotopes". Included in this group are also isotopes re­ leased due to man-made activities, but distributed in the environment at regional or global scale due to natural processes. The potential contribution of isotope methods to studies in water resources can be grouped into the following categories: 3 4 Y. Yurtsever & L. Araguas Araguas - determination of physical parameters related to flow dynamics and system structure, - delineation of processes involved (process tracing) during flow and circulation of water, - study of origin (genesis) of water, mixing ratios of component flows (component tracing), - study of 'Time-scale" of events. Information obtained from isotope data can provide, either improved understanding of the processes associated with the source of water and dynamics of the system, or quantita­ tive estimates related to flow dynamics and transport parameters. The type of information to be obtained in groundwater and surface water systems can be summarized as follows: In groundwater systems: - System boundaries - Origin (genesis) of water - Hydraulic connections with surface waters or between different aquifer units - Source(s), processes and rate of replenishment - Source(s) and mechanisms of salinization - Mixing proportions of component flows originating from different sources - Transit times of groundwater flow and its distribution - Dynamics of geothermal systems - Parameters related to mass transport characteristics In surface water systems: - Dynamics of catchment basins (rainfall-runoff processes) - Distribution of travel limes of water in the catchments, or within a surface water body - Surface water and groundwater hydraulic interrelations - Catchment soil erosion and reservoir siltation rates - Mass-transport characteristics of surface waters It should be noted that, while the use of artificial radioactive tracers (or chemical tracers) are still an effective tool for in-situ studies of short-term processes, environmental isotope applications are unique for investigating hydrological processes over much larger scale (spatial) and longer time spans. Consequently, they enable derivation of integrated (both in space and time) basic characteristics relevant to occurrence and circulation of waters. Their value also lies in facilitating confirmation (or elimination) among alternative conceptual models of a given hydrological system, as well as in preliminary assessment of large scale systems in the absence of adequate basic data. Environmental isotopes of potential use in hydrological sciences are listed in Tables 1 and 2. A substantial amount of background data has already been collected in the applications of environmental isotopes in hydrological sciences so as to understand the cause/effect relations of their occurrence and distribution, and to develop sound evaluation methodolo­ gies. Characteristic features of the isotope-input have been mainly derived from systematic data collected from long-term monitoring being undertaken by the IAEA on the isotope content of precipitation involving a global scale network of stations. ISOTOPIC CONTENT OF PRECIPITATION The IAEA, in cooperation with the World Meteorological Organization (WMO) has been conducting a world-wide survey of hydrogen and oxygen isotope content in precipitation. The objective of the program is the systematic basic data collection at a global scale to determine spatial and temporal variations of environmental isotopes in precipitation and, therefore, to provide basic information for the use of isotope techniques in hydrological investigations. IAEA and Environmental Isotope Applications in Hydrology 5 TABLE I Stable isotopes in water resources investigations. Isotope Potential Application • Genesis of water Oxygen-18(180) • Source of replenishment to groundwater and process tracing • Component tracing - mixing proportion of different compo­ and nents of flows', hydraulic interconnections Deuterium (2H) • Paleohydrological indicators • Geothermal activity Carbon-13 (13C) • Origin of carbon compounds • Correction for 14C age-dating Sulphur-34 (34S) • Natural tracer for sulfates in water • Identification of source of pollution Nitrogen-15 (15N) • Origin of nitrates • Identification of sources of pollution TABLE 2 Environmental radioactive isotopes in water resources investigations (in the or­ der of increasing half-life). Isotope Half-life Source Present Limitations (years) (origin) S5Kr 10.8 Nuclear reactors Sampling, counting JH 12.43 Cosmic rays Thermonuclear Nuclear reactors JiSi 100 Cosmic rays Initial activity Thermonuclear Sample size Crustal (?) Counting time iyAr 269 Cosmic rays Sample size Crustal Counting time 14C 5730 Cosmic rays Complex geochemistry Thermonuclear Isotope exchange Crustal processes 51Kr 210 000 Cosmic rays Analytical /«y 250 000 Decay chain Initial activity Interactions J6C1 306 000 Cosmic rays Initial activity (?) Nuclear tests Sources and in-situ Crustal (?) production (?) The data set accumulated during 30 years of operation contains information for more than 410 meteorological stations, distributed in more than 80 countries. The number of 6 Y. Yurtsever & L. Araguas Araguas stations in operation during the last 30 years has been varying between 120 and 200. An increasing number of stations belonging to national networks are also providing their results to the IAEA. The location of the stations included in the database, which have a reasonable length of record, is shown in Fig. 1. The concentrations of oxygen-18 (180), deuterium (2H) and tritium (3H) are deter­ mined on monthly composite samples. Relevant meteorological information (amount of precipitation, surface air temperature and water vapor pressure) are included in the database and regularly published by the IAEA (IAEA, 1969-1990; IAEA, 1981; IAEA, 1992). Tritium in precipitation Tritium is produced in the upper atmosphere due to cosmic radiation, and is oxidized and transported to the troposphere where it enters the water cycle. Natural inventory in the atmosphere is estimated to be 3.6 kg, establishing an equilibrium between the amount of tritium generated and the removal by decay. The tritium content is expressed in tritium units (TU). One TU is defined as one atom of 3H per 1018 atoms of ]H, which is equivalent to a specific activity of 0.118 Bq l"1 of water. It has a half-life of 12.43 years. The long-term variations of tritium content for both hemispheres is presented in Fig. 2. The stations of Ottawa and Kaitoke represent the most complete record for each hemi­ sphere. The highest tritium values in the northern hemisphere were recorded in 1963, few months before "The Limited Test Ban Treaty" on atmospheric explosions came into force. Much lower contents were measured in the southern hemisphere mainly because most of the thermonuclear tests took place at high latitudes in the northern hemisphere. The maxi­ mum values in Kaitoke