environmental isotope applications in hydrology: an ... · iaea and environmental isotope...

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 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 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 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

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.

IAEA and Environmental Isotope Applications in Hydrology 5

TABLE I Stable isotopes in water resources investigations.

Isotope Potential Application

• Genesis of water • Source of replenishment to groundwater and process tracing • Component tracing - mixing proportion of different compo

nents of flows', hydraulic interconnections • Paleohydrological indicators • Geothermal activity

Carbon-13 (13C)

Sulphur-34 (34S)

Nitrogen-15 (15N)

• Origin of carbon compounds

• Correction for 14C 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

S5Kr JH

J iSi

iyAr

14C

5 1Kr

/ « y

J 6 C 1

Half-life

(years)

10.8

12.43

100

269

5730

210 000

250 000

306 000

Source

(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 (?)

Present Limitations

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 (?)

Oxygen-18(180)

and

Deuterium (2H)

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


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 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, 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 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 1964.

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 2H/1H and l sO/ 1 60 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 J

'whereRSAMPi£andRSTANDARD refer to the isotopic ratios 2 H / H and 1 8 0/ 1 6 0.


_ _ r ^ f ^ ~^—>

F/G. 1 Location of the meteorological stations belonging to the IAEA! WMO network "Isotopes in precipitation".

10000?

3 H Z

o < as W U z o u

1000=

100:

2 10:

.13

Ottawa, Canada 45.32 N - 75.67 W

Kaitoke, N. Zealand 41.10 S, 175.17 E

60 65 70 75 YEAR

80 85 90

FIG. 2 Temporal variations of tritium concentrations at selected long-term stations in each hemisphere.


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 1 8 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 5180 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 51 80 and Ô2H, 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


FIG. 3 Smoothed temporal variation of$0 data of monthly precipitation and temperature at the station Vienna, Austria.

0 0

0

Deu

teri

um

.

-50

-100

-150

-200

-250 -

-300 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10

Ox'ygen-18 [o/ool FIG. 4 The ol80/SD relation derived from long-term mean values of the stations included in the network "Isotopes in precipitation"

Y. Yurtsever & L. Araguas Araguas

a)

Meteoric Water Line

Original Composition

^

Global .. •• /

Local^ • ' / - '^r

/ Paleowaters

/ r

/

Sea water

Surface evaporation

Geothermal exchange

Oxygen-18 (per mil)

b)

Mixing with sea water

Mixing with surface water

/ ' Mixing with paleowaters

Paleowaters

Oxygen-18 (per mil)

FIG. 5 Characteristic S180/SD relations for different processes a) Related to processes b) Related to hydrological applications.


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 1 8 0 and 2H, 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 1 8 0 and 2H 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 1 8 0 and 2H)as,

h2H = 8-518O+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 180-2H 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 180-2H 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 1 80 and 2H 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


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 -0.15 to -0.35 %o per 100 m elevation increase in the case of l s 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 1 8 0 and 2H 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 1 8 0 and 2H 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


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 180 and 2H, the stable carbon isotope ratio 13C to 12C in total dissolved carbonate species is commonly employed for evaluation of the radioactive 14C isotope data in groundwaters, and also indicator of the origin of the carbon compounds. Similarly, the isotope ratio 15N/14N in nitrate and 34S/32S 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 (3H) and Carbon-14 (14C)

The environmental radioisotopes of 3H and 14C have transient concentrations in the hydro-logical 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


trilium TU humidity %vol.

tritium TU

20 30 50

25+

30

. 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 14C isotope. The radioactive isotope 14C, 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


of the 14C of Modern Carbon" (pmc). It has a half-life of 5730 years. Unlike tritium, 14C 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 14C 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 14C activity (C0)is estimated (Salem et ai, 1980). The relation providing the estimate of this initial activity is:

100(5-5C) , 2 e >. 1 H C = 1 + (3)

0 5 G - S C + E V lOOOJ

where: 5 is 13C concentration of DIC 5C is 13C concentration of the aquifer carbonate matrix §G is 13C concentration of soil CO2 e is the fractionation factor for 13C during the dissolution of soil C02 in the ground

water 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 14C 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 14C 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 14C activity with the present analytical capabilities (0.5-1.0 pmc), the time span covered by this isotope extends to a maximum of about 40 000 years. In spite of the above mentioned complexities involved in the present methodology of using 14C, 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


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,

Co(t) = J C , . ( / - T ) • h{%) • e~Xx • dx (5)

where: Cj is tracer input concentrations C0 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:

(6)

(7)

(8) )

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:

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)

Exponential model:

Dispersive model:

hix)

A(T) =

h(x) -

t.

= 8 ( f - T )

1

1 T = ±.e -X m

1

J4ntD

L-M1-2 -n

I ~\ VX

zj 4Dt_


depends on the model adopted. The above given equation, for example, used for 14C 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 hy-drological systems are reported in the literature (IAEA, 1986; Zuber, 1986; Yurtsever, 1990).

The above discussed formulation requires that the flow through the system is steady-state 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 hydrogeo-logical 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 14C 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 (3H and 14C), 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 age-dating 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,


since the natural tritium concentrations in precipitation reaches its relatively low pre-thermonuclear levels.

Improved quantitative approaches through fully coupled flow and geochemical-reaction 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 (1987-1992). CRP on the application of isotopes and geochemical techniques in geothermal exploration in Africa, Asia and the Middle East (1990-1993). CRP on mathematical models for quantitative evaluation of isotope data in hydrology (1990-1993). CRP on isotope variations of carbon dioxide and other trace gases in the atmosphere (1991-1994). CRP on continental isotopic indicators of paleoclimates (1992-1995). CRP on application of tracer techniques in the study of the Black Sea (1992-1995).

Future planned:

CRP on the use of isotopes in studying water and pollutant dynamics in lakes (1993-1996). CRP on isotope techniques in groundwater pollution studies (1993-1996). CRP on isotope techniques in water resources investigations in arid and semi-arid regions (1993-1996). CRP on the use of isotopes for validating flow and mass transport models in groundwater (1994-1997). CRP on the application of isotopes to study soil erosion and sedimentation rate in lakes and reservoirs (1994-1997).

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 palaeo-climatology - 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.


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.

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