ENVIRONMENTAL ISOTOPE APPLICATIONS IN HYDROLOGY: AN OVERVIEW OF THE IAEA'S ACTIVITIES, EXPERIENCES, AND PROSPECTS

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1 Tracers in Hydrology (Proceedings of the Yokohama Symposium, July 1993) IAHS Publ. no. 215, 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 collected 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 prospective 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 hydrological 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 dissemination 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 often referred to as "Environmental Isotopes". Included in this group are also isotopes released 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

2 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 quantitative 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 methodologies. 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.

3 IAEA and Environmental Isotope Applications in Hydrology 5 TABLE I Stable isotopes in water resources investigations. Isotope Oxygen-18( 18 0) and Deuterium ( 2 H) Carbon-13 ( 13 C) Sulphur-34 ( 34 S) Nitrogen-15 ( 15 N) Potential Application Genesis of water Source of replenishment to groundwater and process tracing Component tracing - mixing proportion of different components of flows', hydraulic interconnections Paleohydrological indicators Geothermal activity Origin of carbon compounds Correction for 14 C age-dating Natural tracer for sulfates in water Identification of source of pollution Origin of nitrates Identification of sources of pollution TABLE 2 Environmental radioactive isotopes in water resources investigations (in the order of increasing half-life). Isotope Half-life Source Present Limitations S5 Kr J H Ji Si iy Ar 14 C 51 Kr /«y J6 C1 (years) (origin) Nuclear reactors Cosmic rays Thermonuclear Nuclear reactors Cosmic rays Thermonuclear Crustal (?) Cosmic rays Crustal Cosmic rays Thermonuclear Crustal Cosmic rays Decay chain Interactions Cosmic rays Nuclear tests Crustal (?) Sampling, counting Initial activity Sample size Counting time Sample size Counting time Complex geochemistry Isotope exchange processes Analytical Initial activity Initial activity (?) Sources and in-situ 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

4 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 ( 18 0), deuterium ( 2 H) and tritium ( 3 H) are determined 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, ; 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 3 H per atoms of ] H, which is equivalent to a specific activity of Bq l" 1 of water. It has a half-life of 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 hemisphere. 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 maximum values in Kaitoke were observed in At present, tritium contents in precipitation have almost returned to pre-thermonuclear levels, 5-20 TU in mid-high northern hemisphere and less then 10 TU in tropical areas and in the southern hemisphere. This natural level was significantly modified during 1952 to 1963 due to the high amount of tritium released into the atmosphere by thermonuclear explosions. The atmospheric bomb tests ceased in 1963, and since then, the tritium concentration in precipitation has been decreasing towards natural levels (Fig. 2). The observed rate of decrease of the tritium levels in precipitation also provides information on the apparent residence time of tritium in the atmosphere and the transport between the stratosphere and troposphere. This unintended tracer experiment was used not only in studies related to atmospheric circulation and inter-hemispheric transport, but also was the basis for many hydrological applications based on the presence of tritium. In recent times, when tritium levels in most places are close to pre-thermonuclear natural levels, thecnogenic tritium emitted from nuclear facilities and consumer products, has been detected in some stations. These emissions can affect the pattern on a local or regional scale. Stable Isotope variations in precipitation The stable isotope content of water 2 H/ 1 H and ls O/ 16 0 is expressed by convention as parts per thousand deviation relative to the standard VSMOW (Vienna Standard Mean Oceanic Water). Delta notation, commonly used to report isotope concentration, is defined as: V STANDARD 'wherer SAMP i andr STANDARD refer to the isotopic ratios 2 H/H and 18 0/ J

5 IAEA and Environmental Isotope Applications in Hydrology 1 r ^ f ^ ~^ > F/G. 1 Location of the meteorological stations belonging to the IAEA! WMO network "Isotopes in precipitation" ? 3 H Z o < as W U z o u 1000= 100: Ottawa, Canada N W 2 10: Kaitoke, N. Zealand S, E YEAR FIG. 2 Temporal variations of tritium concentrations at selected long-term stations in each hemisphere.

6 8 Y. Yurtsever & L. Araguas Araguas The global distribution pattern of stable isotope content in precipitation was reviewed (Dansgaard, 1964) during the first years of operation of the IAEA/WMO network. In this evaluation, the spatial distribution of heavy isotope content was related to environmental parameters, such as altitude, latitude, amount of precipitation, air temperature and degree of continentality. All of these factors reflect the degree of washout from the air mass, and to some extent, the water vapor history from the source to the site of precipitation. The observed latitudinal distribution is interpreted (Yurtsever & Gat, 1981) assuming that the tropical and subtropical oceans constitute the major source of water vapor and the poleward transport is connected with progressive rain-out due to decreasing temperature. Depleted isotopic values at high latitudes are a consequence of lower water vapor content in the atmosphere and lower temperatures of condensation. Superimposed to this general trend, local variations are produced by altitude and amount "effects". The continentality and the altitude "effects" represent a progressive removal of moisture from the original air mass moving from the ocean towards the continent. The preferential removal of heavy isotopes during the first stages of precipitation leads to a progressive depletion of precipitation moving inland or along the mountainous regions, when compared to coastal areas. The amount "effect" is clearly observed in areas where temperature does not show a clear seasonality during the year. On tropical islands, where the observed seasonal variation of temperature cannot account for isotopic variations, the observed variability is controlled by the amount of precipitation, indicating the degree of rain-out from the original air mass. The seasonal variability of ^/'H and 18 0/ O observed in temperate and high-latitude stations (depleted values in winter and enriched in summer) reflects the variations in temperature of condensation, and to some extent differences in air mass trajectories and sources of vapor. The observed temporal variations of values of precipitation and corresponding surface air temperatures for the station in Vienna, Austria, is shown in Fig. 3 as an example. The detailed elaboration of the spatial variations and distribution of the stable isotopes due to the above mentioned factors based on the data collected from IAEA/ WMO network has been alreadyreported (Yurtsever & Gat, 1981; Rozanski et ai, 1993). The data set accumulated during the last 30 years also provides the opportunity to study several relations between isotopic composition and relevant climatic parameters. Present values of the relation between some climatic parameters and the isotopic composition in precipitation are extrapolated to the isotopic signal stored in geological records. Most of the paleoclimatological studies are based on isotopic variations in ice cores, marine or lake sediments, and an increasing number of other indicators. Most of the information derived from these materials is based on the present relation between mean annual temperature and isotopic composition of precipitation in a given place. The coefficients obtained vary from 0.3 to 0.6 depending on the various temperature and isotope data sets used including both long-term and seasonal data. The characteristic relation between and Ô 2 H, as derived from long-term mean values of the IAEA/WMO network stations, is shown in Fig. 4, where the least-square regression line is also given. BASIC CONCEPTS AND APPLICATIONS OF ISOTOPE METHODS Environmental isotope techniques essentially rely on the hydrological evaluations to be made through observations of temporal and/or spatial variations of isotopic species - within and/or at the inflows/outflows of the system under study. Detailed description of the basic concepts and principles of the methodologies as applied to different hydrological problems

7 IAEA and Environmental Isotope Applications in Hydrology 9 FIG. 3 Smoothed temporal variation of$0 data of monthly precipitation and temperature at the station Vienna, Austria. 0 0 Deuterium Ox'ygen-18 [o/ool FIG. 4 The o l8 0/SD relation derived from long-term mean values of the stations included in the network "Isotopes in precipitation"

8 Y. Yurtsever & L. Araguas Araguas a) / Meteoric Water Line Global.. / Local^ ' / - '^r / r Sea water Surface evaporation Original Composition Geothermal exchange ^ / Paleowaters Oxygen-18 (per mil) b) Mixing with sea water Mixing with surface water /' Mixing with paleowaters Paleowaters Oxygen-18 (per mil) FIG. 5 Characteristic S 18 0/SD relations for different processes a) Related to processes b) Related to hydrological applications.

9 IAEA and Environmental Isotope Applications in Hydrology 11 are given in (IAEA, 1981; IAEA, 1983). Essential concepts and some of the main applications will be briefly reviewed in the following sections with few illustrative examples. Stable Isotopes The most commonly employed stable isotopes are 18 0 and 2 H, which are often used for assessment of the "genesis" (origin) of water, particularly in groundwater systems; processes involved in the replenishment (process-tracing); for estimating mixing proportions of different sources or component flows (component-tracing); and studying hydraulic relations between groundwater and surface waters or between different aquifer units within a given groundwater system. One of the most important factors governing the use of these isotopes is the isotopic fractionation occurring during phase changes, i.e. condensation or evaporation, which is mainly a temperature dependent phenomena. The isotopic changes thus induced, is a conservative property of the water during its circulation in the hydrological systems, and it is a finger-print of the history of the processes involved in its formation and circulation. The 18 0 and 2 H variations in natural waters show a linear relation as a consequence of the fact that their behavior during the fractionation processes are similar. The equation derived in this regard was first given (Craig, 1961) (using annual average values of 18 0 and 2 H)as, h 2 H = O+10 (2) which is generally referred to as the "Meteoric Water Line", and it is very close to the theoretically expected relation. The similar equation derived from the longer-term data now available from the IAEA/WMO network also confirms this equation (Fig. 4). The possible other H linear relations expected for various other processes of practical interest in hydrology are shown in Fig. 5. It should also be noted that, while the equation given above is valid as a global average relation, it may however have different characteristic values in different climatic regions, particularly with respect to the intercept (often referred to as deuterium excess) of the line. The deuterium excess, is also a function of the relative humidity during evaporation at the source of moisture, and could vary i.e. between the range of 4 to 23, for relative humidity variation range of 60 to 90 percent and temperature of condensation range of 0 to 20 C. The H relations given for different processes in Fig. 5 provide the basis for the above cited type of applications. These isotopes are also effectively used for paleo-hydrological studies, and delineating the origin of groundwaters replenished mainly during the earlier pluvial periods, which is relevant particularly to the occurrence of groundwater in arid regions. Such paleowaters are often characterized by the relatively low deuterium excess values in addition to their identification through age-dating. A typical example of the use of stable isotopes of 18 0 and 2 H in studying groundwater genesis is shown in Fig. 6 (Gat, 1983), where groundwaters replenished through recent precipitation and paleowaters in different aquifer systems are identified. The major regional aquifer systems in arid regions of the Middle East and northern Africa, such as Dammam Formation and Umm Er Rhaduma aquifers in the Saudi Arabian peninsula; Nubian Sandstone and Continental Intercalaire aquifer systems in north Africa; have been found to contain paleowaters mainly replenished during earlier pluvial periods, through isotope field applications conducted by the IAEA within its Technical Cooperation program in these regions (IAEA, 1980). The variations induced in stable isotopic composition of precipitation due to altitude

10 12 Y. Yurtsever & L. Araguas Araguas So (%o) FIG. 6 Illustrative example of the isotopic composition of precipitation and groundwater in the East-Mediterranean region (Gat, 1983). effect provide label for the recharge to the groundwaters at different elevations, thus enabling assessment to be made of the replenishment areas. The actual field data collected in this regard in various parts of the world indicate that the gradient of the altitude effect is in the range of to %o per 100 m elevation increase in the case of ls O isotope. Similarly, the isotopic composition of river waters draining higher altitude precipitation often have significantly different isotopic content than the adjacent aquifer, providing the basis for study of hydraulic relations between river-aquifer system, or assessing the recharge to the adjacent aquifer through such line-sources. The enrichment of the 18 0 and 2 H isotopic contents of surface water bodies in lakes or reservoirs due to direct surface evaporation provides a natural label for them, so that hydraulic inter-relations between such water bodies with groundwater can be investigated. The theoretical description of the isotope effects during the evaporation process is well established (Gonfiantini, 1986). An extensive number of case histories in all such applications are available in the literature (IAEA, 1967, 1970, 1974, 1979, 1983, 1987, 1992). The basic concepts and the theoretical framework for use of stable isotopes of 18 0 and 2 H in water balance of lakes and reservoirs, and the application of them to studies related to the dynamics of such surface-water bodies are well established (Gonfiantini, 1986). It is noteworthy to mention the potential applications of the isotopes of oxygen and hydrogen in studying rainfall-runoff processes. The component flows involved in the runoff process such as baseflow and overland flow within a given basin can effectively be quantified through simple mass balance considerations of the stable isotopic composition of the river water prior to and during the individual rainfall events. The results of

11 IAEA and Environmental Isotope Applications in Hydrology 13 hydrograph separation based on such observations in different sizes of surface catchment basins indicate that the contribution of the groundwater to the total hydrograph of the basin can be substantially higher than that envisaged through classical concepts so far applied (Hino & Hasebe, 1986; Hooper & Shoemaker, 1986). These studies are also important contributions to delineation of the fluxes and their pathways in the basin, which is most relevant to understanding of the processes involved in stream acidification and pollution due to diffused sources. In addition to most commonly used isotopes of 18 0 and 2 H, the stable carbon isotope ratio 13 C to 12 C in total dissolved carbonate species is commonly employed for evaluation of the radioactive 14 C isotope data in groundwaters, and also indicator of the origin of the carbon compounds. Similarly, the isotope ratio 15 N/ 14 N in nitrate and 34 S/ 32 S in sulfate and other sulphur compounds, can be employed to study the origin of dissolved materials in water as a means to identify the sources of water pollution. Tritium ( 3 H) and Carbon-14 ( 14 C) The environmental radioisotopes of 3 H and 14 C have transient concentrations in the hydrological system due to both their radioactive decay properties (which is a function of time) and also variable input - concentrations. This facilitates the study of water movement dynamics in the "time" domain. In general, the basic information to be obtained from these isotopes refers to "travel time" of water within a given system and/or to its distribution. In the case of tritium, it can be readily used for qualitative (or semi-quantitative) assessment of the presence of recent recharge to groundwater systems, since the history of the tritium concentrations in the precipitation is fairly well defined (Section-3.1, Fig. 2). Groundwaters containing measurable tritium concentrations provide clear evidence of recharge occurring into the system during the last three to four decades. The case of absence of tritium, however, could be indicative of either recharge being not significant during the above cited period, or the travel times involved in the system being longer than the time required for the decay of the isotope during its transport. One of the most important contributions of environmental tritium has been the study of moisture movement in the unsaturated zone as a means of estimating the amount of direct rainfall recharge to unconfined aquifers. The basic principle underlying this application is to detect the 1963 tritium peak in the vertical moisture profile in the unsaturated zone so that the moisture stored above the location of the peak would be direct measure of the replenishment rate. One of the earlier applications of the method was in a sand dune area at Dahna, Saudi Arabia (Dinçer, 1974). The thermonuclear tritium peak was observed at a depth of about 4 m from the surface, providing an estimated annual recharge rate of about 20 mm. Similarly, in a project being implemented by the IAEA in Senegal, the tritium peak was detected at a depth of 12 m within the unsaturated zone of about 34 m total thickness. The measured tritium concentration profile was used to simulate moisture transport, using the tritium input for the region, and an annual recharge rate of about 28 mm was estimated. Measured tritium and moisture content at the experimental site of this study and the model simulation results are shown in Fig. 7, as an example. In cases where thickness of the unsaturated zone is relatively small, the method may not be applicable any more, since the thermonuclear tritium peak may have already reached the saturated zone. In this case, use of artificially injected tritium at the experimental site can be employed. In rather slow circulation systems where the time-span involved is too long for use of tritium (water does not contain any tritium), estimation of the travel time of groundwater is

12 14 Y. Yurtsever & L. Araguas Araguas trilium TU humidity %vol. tritium TU expérimental _ modehmg FIG. 7 Results of tritium and moisture profile measurements along depth. Unsaturated zone at experimental site in Senegal: a) Observed variations at site, and b) Model simulation. commonly based on the use of 14 C isotope. The radioactive isotope 14 C, is also naturally produced in the atmosphere by cosmic radiation. It is readily oxidized to carbon dioxide and enters into the carbon cycle. Its natural production is rather constant and its input to hydrological systems can be assumed 14r to be steady-state for practical purposes. The concentration of C is expressed as "percent

13 IAEA and Environmental Isotope Applications in Hydrology 15 of the 14 C of Modern Carbon" (pmc). It has a half-life of 5730 years. Unlike tritium, 14 C is not a conservative tracer providing direct indication of the travel time, due to complex chemical reactions involved during the transport process. The concentration of this isotope in water is often measured as the 14 C activity in the Dissolved Inorganic Carbon (DIC), which is altered due to interactions of water with the aquifer matrix. Various chemical and isotopic models have been suggested to account for these chemical reactions as to arrive at true travel time estimates of the water, rather than the apparent value (Fontes, 1983). One of the simplest approaches is based on the use of ^C content of the DIC, so that the chemical dilution of C activity due to water-matrix interaction is accounted for, and the initial 14 C activity (C 0 )is estimated (Salem et ai, 1980). The relation providing the estimate of this initial activity is: 100(5-5 C ), 2e >. C = 1 H + (3) 0 5 G -S C + E V loooj where: 5 is 13 C concentration of DIC 5 C is 13 C concentration of the aquifer carbonate matrix G is 13 C concentration of soil CO2 e is the fractionation factor for 13 C during the dissolution of soil C0 2 in the groundwater aeration zone. Further reduction of this initial activity to the levels at the measured section is then assumed to be only due to radioactive decay, since the natural input to the system is essentially steady-state. The travel time (age) of water can, then, be calculated with the use of following equation: t = 8267 In (4) C The main assumption inherent in the above formulation is that the dispersion and mixing during the transport is negligible (i.e. piston flow model as will be discussed in later sections), and the lower concentration observed at the measuring section is entirely due to radioactive decay. This would, consequently, limit the applicability of 14 C to confined aquifer systems, where the above cited assumption could possibly be justified. Furthermore, physical retardation processes which may possibly occur due to diffusion into aquitard porous matrix in such slow moving systems, may also add uncertainty to the estimates of groundwater travel times based on 14 C isotope (Sudicky & Frind, 1981). When the age difference between two measurement points along the flow line is considered, the travel time estimates (between these two points) are less susceptible to the above cited uncertainties. In this case, the age difference can be further used to make an estimate of the permeability, if data on hydraulic gradient and aquifer porosity are available, which would be a space-integrated average value for the aquifer. Considering the minimum detectable level of 14 C activity with the present analytical capabilities ( pmc), the time span covered by this isotope extends to a maximum of about years. In spite of the above mentioned complexities involved in the present methodology of using 14 C, it is still a valuable tool as a means to obtain groundwater travel time estimates over such a long timespan. Most of the other environmental radioactive isotopes cited earlier are of potential use in determination of the groundwater travel times over different time scales, but their routine applications are presently hindered due to either analytical efforts required or to lack of understanding of their natural production rates in different hydrogeological conditions. Detailed appraisal made in this regard through field research carried out under the

14 16 Y. Yurtsever & L. Araguas Araguas coordination and support of the IAEA has enabled an assessment to be made of their applicability (Ivanovich et al., 1992). Remarks on concepts and methodologies for quantitative interpretation of isotope data Commonly employed mathematical modelling procedures for quantitative interpretation of environmental isotope data in hydrology stem from the theory of linear systems approach, where the tracer input-output relation in "time domain" are linked through the convolution integral, C o (t) = JC,.(/-T) h{%) e~ Xx dx (5) where: Cj is tracer input concentrations C 0 is tracer output concentrations t is chronological time x is transit time h (x) is the weighting function or system response function X is the radioactive decay correction factor The system response function in the above equation, represents the compositional response of the system to the tracer input, which is the so-called transit time distribution function of the water in the system. The mean transit time obtained from an ideal tracer will also be equivalent to the hydrological turnover time (volume/inflow or volume/outflow ratio under steady-state flow conditions). Models commonly referred to in the literature, (i.e. piston-flow, dispersive, exponential) are all based on this general linear system theory, where different models have their respective transit time distribution functions. System response functions for the three most commonly adopted models are (Zuber, 1986): - Piston flow model: Exponential model: Dispersive model: hix) A(T) = h(x) - t. = 8(f-T ) 1 1 T = ±. e - X m 1 J4ntD ) L-M 1-2 -n I ~\ VX zj 4Dt_ A/ VXX V m where the additional parameters, v is the flow velocity in a uniform flow field, D is the longitudinal dispersion coefficient, and x is the distance. The above formulations are often used for quantitative interpretation of tritium data to arrive at the estimate of mean-transit time of water from the observed concentrations within or at the outlet of the system. For piston flow model, the earlier given convolution equation will reduce to: (6) (7) (8) C (t) = C.{t-i ) -e'^" (9) which is the basic equation used for age-dating. It is clear, however, that the information obtained from isotope data as regards the travel times of the water (and its distribution)

15 IAEA and Environmental Isotope Applications in Hydrology 17 depends on the model adopted. The above given equation, for example, used for 14 C age dating under constant tracer input concentration., is convenient for practical purposes, since "dating" requires only one measurement to obtain a singular solution in this formulation. In depth elaboration of the modelling approaches and their applicability for different hydrological systems are reported in the literature (IAEA, 1986; Zuber, 1986; Yurtsever, 1990). The above discussed formulation requires that the flow through the system is steadystate and it is a lumped-parameter modelling approach, in which the spatial variations observed for isotopic content within or at the inflows/outflows can not be incorporated. In this respect, compartmental models (mixing-cell models) have been proposed to be used as a distributed parameter modelling approach in the evaluation of isotope data, and several case studies are already reported (Przewlocki & Yurtsever, 1974; Simpson, 1975; Yurtsever & Payne, 1978; Campana & Mahin, 1985; Yurtsever & Payne, 1985; Van Ommen, 1985; Yurtsever & Payne, 1986; Adar et al., 1992; Yurtsever & Buapeng, 1992). At present, isotope data collected within the framework of hydrological and hydrogeological applications are used, to a large extent, for improved understanding of processes involved in the occurrence and transport of water, and for qualitative evaluations as regards system identification. Impact of the isotope methods and quantitative information to be derived from them could be improved if proper modelling approaches are further developed. Further required developments and IAEA's future activities Efforts devoted to development of methodologies in isotope hydrology during the last few decades, the substantial amount of isotope data collected, and experience gained from field applications have already resulted in proven techniques, which are now a recognized scientific discipline as an integral part of the basic hydrological and hydrogeological investigations within the framework of water resources assessment, development and management activities. One of the most commonly employed "dating" methods based on 14 C isotope, could be improved through measurement of its activity in the dissolved organic matter (DOC) rather than DIC, since the former does not interact with the aquifer matrix. A few recent attempts made in this direction (Murphy et al, 1989, Wassenaar et al., 1991) indicate the method to be most promising, and it should be further pursued through basic research directed towards understanding of the full geochemical behavior of the DOC. In addition to the two commonly used two natural radioactive isotopes ( 3 H and 14 C), further improvements in the use of other environmental radioisotopes (Table 2), will greatly improve the spectrum of "daring" applications. In mis regard, CI is probably the most promising one, and further research, particularly for full understanding of its in-situ production, is needed. Because of its very long half-life, the time-span of groundwater agedating will be very much expanded. The analytical precision achieved to date is quite sufficient for most of the commonly employed environmental isotopes. The factors such as prolonged counting time required, and sample sizes involved, and the need for accelerator mass spectrometry are limiting factors for routine use of some of them. The new technique suggested for measurement of tritium through mass spectrometric analyses of helium-3, produced by the tritium decay, would be a desirable achievement. This will improve the present level of analytical accuracy for H by an order of magnitude, allowing more effective use of tritium isotope,

16 18 Y. Yurtsever & L. Araguas Araguas since the natural tritium concentrations in precipitation reaches its relatively low prethermonuclear levels. Improved quantitative approaches through fully coupled flow and geochemicalreaction models will certainly provide a sound basis for evaluation of isotope data in their hydrogeological applications. They will facilitate detailed understanding of the various processes involved in their transport, proper description of these processes in the conceptual models and refined quantitative estimates of relevant hydrological parameters based on isotope data. Future development work in this particular field should be directed towards distributed parameter modelling procedures under steady-state and transient flow conditions. These developments will also substantially contribute to the studies related to water pollution and water quality management of groundwater resources. TABLE 3 IAEA ongoing and future Coordinated Research Programs (CRP) in Isotope Hydrology. Ongoing: CRP on Nuclear techniques in the study of pollutant transport in the environment ( ). CRP on the application of isotopes and geochemical techniques in geothermal exploration in Africa, Asia and the Middle East ( ). CRP on mathematical models for quantitative evaluation of isotope data in hydrology ( ). CRP on isotope variations of carbon dioxide and other trace gases in the atmosphere ( ). CRP on continental isotopic indicators of paleoclimates ( ). CRP on application of tracer techniques in the study of the Black Sea ( ). Future planned: CRP on the use of isotopes in studying water and pollutant dynamics in lakes ( ). CRP on isotope techniques in groundwater pollution studies ( ). CRP on isotope techniques in water resources investigations in arid and semi-arid regions ( ). CRP on the use of isotopes for validating flow and mass transport models in groundwater ( ). CRP on the application of isotopes to study soil erosion and sedimentation rate in lakes and reservoirs ( ). One of the most desirable developments refers to the use of environmental isotopes for validation of groundwater flow models, particularly for large scale aquifer systems. The distribution of natural isotopes (both stable and radioactive) within a given groundwater system comprises a natural analogue, reflecting the replenishment and transport processes, and they could be employed for independent verification of the groundwater flow models. A recent attempt in this regard is reported for the use of carbon-isotopes in Dogger aquifer system of Paris basin (Marsily, 1991). The IAEA/WMO database on isotope survey of precipitation has proved useful in recent years also in connection with climatological investigations (in particular, in palaeoclimatology - ice cores and lake sediments), and with the verification and improvement of atmospheric circulation models (GCM), (Jouzel et ai, 1991). Environmental isotopes could also contribute significantly to provide improved understanding of dynamics of atmospheric circulations and employed in environmental studies related to atmosphere, and its interaction with the hydrosphere. These will be most relevant also at local, regional and global scale research being carried out in relation to the announced climatic changes.

17 IAEA and Environmental Isotope Applications in Hydrology 19 In the light of the foregoing considerations, future program components of the IAEA related to research and development activities in the water sector, include Coordinated Research Programs, planned to be carried out by joint efforts of various national institutions in its Member Countries. The subject matter of the ongoing and future envisaged activities are listed in Table 3. REFERENCES Adar, E. M., Rosenthal, E., Issar, A. S. & Batelaan, O. (1992) Quantitative assessment of the flow pattern in the southern Arava Valley (Israel) by environmental tracers and a mixing cell model. J. Hydrol. 136: Campana, M. E. & Mahin, D. A. (1985) Model-derived estimate of groundwater mean ages, recharge rates, effective porosities and storage in a limestone aquifer, J. Hydrol. 76: Craig, H. (1961) Isotopic variations in meteoric waters. Science 133: Dansgaard, W. (1964) Stable isotopes in precipitation. Tellus 16: Dinçer, T., Al Mugrim, A. & Zimmermann, U. (1974) Study of the infiltration and recharge through the sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium. J. Hydrol. 23: Fontes, J. Ch. (1983) Dating of groundwater. In: Guidebook on nuclear techniques in hydrology edition. Technical Report Series 91: IAEA, Vienna. Gat, J.R. (1983) Precipitation, groundwater and surface waters: control of climate parameters on their isotopic composition and their utilization as palaeoclimatological tools. In: Palaeoclimates and Palaeowaters: A collection of environmental Isotope Studies, IAEA, Vienna, Gonfiantini, R. (1986) Environmental isotopes in lake studies. In: Handbook of environmental isotope geochemistry, Vol. 2, P. Fritz and J. Fontes (eds.), Elsevier. Amsterdam. Hino, M. & Hasebe, M. (1986) Separation of a storm hydrograph into runoff components by both filter-separation AR method and environmental tracers. J. Hydrol. 85: Hooper, R. P. & Shoemaker, C. A. (1986) A comparison of chemical and isotopic hydrograph separation. Water Resour. Res. 22(10): International Atomic Energy Agency (1969,1970,1971,1973,1975, 1979,1983,1986, 1990) Environmental isotope data: World survey of isotope concentration in precipitation. Vol. No. 1 to 9. Technical Report Series 96,117,129, 147, 165,192, 226, 264, and 311. IAEA, Vienna. International Atomic Energy Agency (1967) Isotopes in Hydrology. (Proc. Vienna Symposium, 1966). IAEA, Vienna. International Atomic Energy Agency (1970) Isotope hydrology 1970 (Proc. Vienna Symposium, 1970) IAEA, Vienna. International Atomic Energy Agency (1974) Isotope techniques in groundwater hydrology (Proc. Vienna Symposium, 1974). IAEA, Vienna. International Atomic Energy Agency (1979) Isotope hydrology (Proc. Neuherberg Symposium, 1978). IAEA Vienna. International Atomic Energy Agency (1980) Arid-zone hydrology: Investigations with isotope techniques. IAEA, Vienna. International Atomic Energy Agency (1981) Stable isotope hydrology. Deuterium and oxygen-18 in the water cycle. Technical Report Series 210. IAEA, Vienna. International Atomic Energy Agency (1981) Statistical treatment of environmental isotope data in precipitation. Technical Report Series 206. IAEA, Vienna. International Atomic Energy Agency (1983) Guidebook on nuclear techniques in hydrology Edition. Technical Report Series 91. IAEA, Vienna. International Atomic Energy Agency (1984) Isotope hydrology (Proc. Vienna Symposium, 1983). IAEA, Vienna. International Atomic Energy Agency (1984) Mathematical models for interpretation of tracer data in groundwater hydrology. IAEA-TECDOC-381. IAEA, Vienna. International Atomic Energy Agency (1987) Isotope techniques in water resources development. (Proc. Vienna Symposium, 1987). IAEA, Vienna. International Atomic Energy Agency (1992) Isotope techniques in water resources development (Proc. Vienna Symposium, 1991). IAEA, Vienna.

18 20 Y. Yurtsever & L. Araguas Araguas International Atomic Energy Agency (1992) Statistical treatment of data on environmental isotopes in precipitation. Technical Report Series 331. IAEA, Vienna. Ivanovich, M., Frohlich, K., Hendry, M.J., Andrews, J.N., Davis, S.N., Drimmie, R.J., Fabryka-Martin, J., Florkowski, T., Fritz, P., Lehmann, B.E., Loosli, H.H. & Nolle, E. (1992) Evaluation of isotopic methods for the dating of very old groundwaters: A case study of the Milk River Aquifer. In: Isotope techniques in water resources development (Proc. Vienna Symposium). IAEA, Vienna Jouzel, I., Koster, R.D., Suozzo, R.I., Russell, G.L., White, J.W.C. & Browcker, W.S. (1991) Simulations of the HDO and H atmospheric cycles using the NASA GIS S General Circulation Model: Sensitivity experiments for present-day conditions. /. Geophys. Res. 96(D4): Marsily, G. de (1991) Validation of conceptual models of flow and transport in porous or fractured media. In: Geoval (Proc. NEA/KI Symposium on validation of geosphere flow and transport models. Stockholm) OECD, Paris Murphy, E.M., Davis, S.N., Long, A., Donahue, D. & Ml, A. T. J. (1989) Characterization and isotopic composition of organic carbon in the Milk River Aquifer. Water Resour. Res. 25(8): Przewlocki, K. & Yurtsever, Y. (1974) Some conceptual mathematical models and digital simulation approach in the use of tracers in hydrological systems. In: Isotope techniques in groundwater hydrology (Proc. Vienna Symposium). IAEA, Vienna. 425^-50. Rozanski, K., Araguas, L. & Gonfiantini, R. (1993) Isotopic patterns in modem global precipitation. Geophysical Monograph, American Geophysical Union, Washington DC (in press). Salem, O., Visser, J.H., Dray, M. & Gonfiantini, R. (1980) Groundwater flow patterns in the Western Libyan Arab Jamahiriya evaluated from isotopic data. Arid-Zone Hydrology: Investigations with isotope techniques, IAEA, Vienna. Simpson, S.E. (1975) Finite state mixing-cell models. In: Bilateral United States-Yuguslavian Seminar in Karst Hydrology and water resources, Dubrovnik. Sudicky, E. A. & Frind, E. O. (1981) Carbon 14 dating of groundwater in confined aquifers: Implications of aquitard diffusion. Water Resour. Res. 17(4): Van Ommen, H. C. (1985) The mixing-cell concept applied to transport of non-reactive and reactive components in soils and groundwater. J. Hydro!. 78: Wassenaar, L., Aravena, R., Hendry, J. & Fritz, P. (1991) Radiocarbon in dissolved organic carbon, a possible groundwater dating method: case study from western Canada. Water Resour. Res. 27(8): Yurtsever, Y. (1990) Concepts and model formulations for quantitative evaluation of isotope data in hydrology. Second International Groundwater Conference, Khota Baru, Malaysia. D1-D25. Yurtsever, Y. & Buapeg, S. (1992) Compartmental modelling approach for simulation of spatial isotopic variations in the Study of groundwater dynamics. A case study of a multi-aquifer system in the Bangkok Basin, Thailand. In: Isotope techniques in water resources development (Proc. Vienna Symposium), IAEA, Vienna, Yurtsever, Y. & Gat, J.R. (1981) Atmospheric waters. In: Stable isotope hydrology: Deuterium and oxygen-18 in the water cycle. Technical Report Series 210: IAEA, Vienna. Yurtsever, Y. & Payne, B. R. (1978) A digital simulation approach for a tracer case in hydrological systems. Multicompartmental mathematical model. In: Second International Conference on Finite Elements in Water Resources, London 4: Yurtsever, Y. & Payne, B. R. (1985) Time-variant linear compartmental model approach to study flow dynamics of karst groundwater systems by the aid of environmental tritium. IAHS Publ Yurtsever, Y. & Payne, B. R. (1986) Mathematical models based on compartmental simulation approach for quantitative interpretation of tracer data in hydrological systems. In: Fifth International Symposium on Underground Water Tracing. Athens Zuber, A. (1986) Mathematical models for interpretation of environmental radioisotopes in groundwater systems. In: Handbook of environmental isotope geochemistry. Vol. II. (Fritz, P. & Fontes, J. Ch. eds) Elsevier, Amsterdam.