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Home , Hydrology, Isotope hydrology

Technical co-operation techniques - practical tools to solve problems by B.R. Payne*

At the time of the creation of the International . Today the mostcommonly used environmental Atomic Energy Agency, there were relatively few appli- are the stable isotopes and -18, cations of nuclear techniques to hydrological problems. and the radioactive isotopes tritium and carbon-14. The Over the last twenty-five years, the range and character first three isotopes are part of the water molecule and of techniques have broadened considerably. therefore are almost ideal tracers of water. In the early days, techniques were The potential usefulness of the naturally present used to measure the flow-rates of . Only relatively tritium in , hydrology, and recently has it become clear that, when it comes to a was pointed out in 1957 by the late Willard Libby, choice between radioisotope techniques and non-nuclear Nobel Laureate for his work on carbon-14 dating. But methods, the radioisotope techniques are particularly before this usefulness could be realized, it was clear that suited for fast-flowing turbulent rivers. This year the the temporal and geographical variations of tritium in Agency is using tritium as a tool in just such a project rainfall and other would have to be studied. in Tanzania under the IAEA's Technical Co-operation As a result, the Agency, in co-operation with the World programme. Unlike the gamma emitters which were used Meteorological Organization, set up a global precipita- in the early work, tritium cannot be measured immedi- tion sampling network. Initially, the samples were • ately in the field. The analyses will be made in the measured by some of the relatively few national tritium Agency's Isotope Hydrology Laboratory. laboratories which existed at that time. However, this arrangement could not cope with the analytical load Although non-nuclear methods were available to and furthermore there was an obvious need for an inter- measure flow-rates of rivers in the early 1950s, this was national comparison of measurements to ensure not so for movement. Over the years, nuclear standardization. The Agency thus set up a low-level techniques have been developed to the stage that quanti- tritium laboratory, with considerable financial support tative estimates of transport are possible. The Agency, from the Atomic Energy Commission in the Commissariat a l'energie atomique, and Singapore the early days. The samples from the precipitation have been co-operating recently in the study of the survey were not only analysed for tritium, but also for movements of associated with land reclama- deuterium and oxygen-18. Although it had been known tion projects, and the Agency receives requests for for a long time that the concentrations of these isotopes Technical Co-operation on this subject from other in natural waters varied, precise study only really became countries. possible with the advent of the mass-spectrometer, just a few years before the precipitation study got under way. Radioisotope tracers were already being used in the late 1950s for studying seepage through , measuring At an early stage the Agency recognized that the the flow-rate and direction from the dilu- introduction and development of these new techniques tion of a radioisotope injected into a section of a . required close collaboration between isotope specialists These techniques have also been developed and refined and hydrologists. Not only was the staff enlarged to over the years and are employed in a wide variety of include hydrologists, but an agreement was reached with seepage problems from dams to subways. FAO in 1961 to introduce isotope techniques in large- These examples use intentionally injected tracers, scale groundwater projects funded by the United Nations which were being developed for the hydrologist when Development Programme. The agreement, which included the Agency was drawing up its first scientific programme. reimbursement of Agency costs, was one of the first However, there is a group of completely different examples of inter-organization collaboration in the methods, known as environmental isotope techniques, United Nations system. Subsequently this type of which were not an established tool for the hydrologist arrangement has been broadened to include a number of at the time of the creation of the Agency. Environmental other organizations of the UN system, such as UNESCO, isotope techniques depend upon the variations of both WHO, UNICEF, and the United Nations*. the stable isotope and radioisotope content of natural * United Nations Educational, Scientific and Cultural Organization (UNESCO). * Mr Payne is Head, Isotope Hydrology Section, in the World Health Organization (WHO). Agency's Division of Research and Laboratories. United Nations Children's Fund (UNICEF).

IAEA BULLETIN, VOL.24, No.3 Technical co-operation-

The analytical system in the Agency's Isotope Hydrology Laboratory for measure- ment of the environmental carbon-14 content of groundwater. Such measure- ments are being made for studying groundwater movement in arid regions in several Member States.

In the course of time the Agency has become a focal considerable error and it may not even be possible to point for the development of isotope hydrology. Many tell whether the groundwater is actually being recharged. of the world's environmental isotope hydrological labora- In the 1950s and early 1960s large amounts of tritium tories - seventeen of them set up with the help of the were injected into the atmosphere by nuclear bomb tests. IAEA — are modelled on the Agency's own laboratory. As a result, precipitation has been labelled so that if a Indeed many of the staff received their training in Vienna. sample of groundwater contains a significant amount of Microprocessors are now part of everyday life and the tritium that is unequivocal proof that the water has been Agency's laboratory is currently automating analytical recharged during the last two or three decades. procedures by using microprocessors, and expects that An example of this is provided by a collaborative laboratories in the developing countries will be able to study between FAO and the Agency in Qatar. The benefit from these developments in the future. mean annual precipitation is about 80 mm in the northern part of Qatar and about 50 mm in the south. Tracking rainfall The spatial and temporal occurrences of rainfall are In arid regions precipitation is subject to irregularities variable. It was believed that short, intense downpours in terms of amount, intensity and location. For example, recharge the groundwater system. However, such storms there may be no precipitation for many years in some occur quite randomly. Therefore it was important to places. To assess the potential groundwater resources, demonstrate the occurrence of recharge by an independent the rate at which the groundwater is replenished has to method. be estimated. Because of the small amounts of precipita- revealed that more than 60 per cent tion in arid regions, estimates of the rate of replenishment, of the groundwater samples contained significant amounts using standard hydrological methods are subject to of tritium and thus that these waters had been recharged

10 IAEA BULLETIN, VOL.24, No.3 -Technical co-operation since 1952. There was more tritium in the northern part very useful routine hydrological tool. An IAEA co- of the peninsula where the amount of precipitation is ordinated research programme on a comparison of higher. It was estimated that about 8 per cent of the different dating methods will be completed early next annual precipitation since 1952 has been effective in year. replenishing the groundwater reserves. of precipitation may not be the only source of recharge to a groundwater system. Recharge Carbon-14 dating of water may also come from a . Losses to groundwater may The relatively short half-life of tritium, 12.43 years, be indicated by differences in flow-rates over a certain means that it is only of use in relatively fast-flowing reach of the river and by the gradient of groundwater groundwater systems or, as outlined above, in studies of levels in the vicinity of the river. However, neither recharge. In 1962 carbon-14 dating was first applied in approach provides a proof of actual transfer of water an attempt to measure the age of groundwater in the from the river to the groundwater system. Stable- western desert of Egypt. The carbon-14 technique is isotope data provide a unique tool to answer this more difficult to apply because carbon is not part of the question. Most of the water flowing in a big river will water molecule. It occurs in groundwater in the come from higher elevations and so will contain less of dissolved inorganic carbon species, and chemical changes the heavy isotopes deuterium and oxygen-18 than local generally tend to dilute the carbon-14 and thus give rainfall. Thus each potential source of recharge is erroneously high ages. However, corrections can be characterized by a different isotopic composition, so made using chemical and carbon-13 (a stable isotope of that the contribution of river-water to the groundwater carbon) data, to provide good estimates of the time a can be clearly demonstrated. The Agency has worked water was recharged. Even though a groundwater on this type of problem in Ecuador, Mexico, Republic system may not be recharged under the present climatic of Korea, and . conditions, it is important in the assessment of a ground- water resource to have information on past recharge The investigation in Sudan was near Khartoum where conditions*. Research is being carried out on the dating the Blue Nile and White Nile rivers merge. The two possibilities of other radioisotopes, such as silicon-32, rivers are distinctly different in isotopic composition argon-39, and chlorine-36; but carbon-14 remains a which enabled the extent of infiltration from both rivers below their to the groundwater to be studied. * See Investigating of the desert by The isotope data did not provide any evidence of R. Gonfiantini, IAEA Bulletin Vol.23 No.l, pp 3-6 (1981). significant recharge by infiltration of recent precipita-

Isotopic and chemical analyses carried out in co-operation with the Agency have helped in delineating both the source and deep- temperature regime for discharges from this recently completed geothermal in Los Azufres, Mexico. (Photo: Payne, IAEA)

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IAEA BULLETIN, VOL.24, No.3 11 Technical co-operation" tion. Carbon-14 data demonstrated that groundwater part of the study in Qatar cited earlier in this article flowing to the area from the east was recharged some dealt with this type of problem. The inferior quality tens of thousands of years ago and is therefore not a of water from a number of in the south-western renewable source of supply. The stable isotopic composi- part of the country was proved to be due to the tion of this ancient groundwater is markedly different leakage of brackish water from an underlying deeper from that of the Blue Nile and thus provides an excellent . However, for some wells it was shown that indicator of the mixing with river-water. -water also contributed to the salinity problem. This type of problem may also arise within the con- text of in industrialized areas. An example Alternative energy is an investigation of groundwater in the neighbourhood of Seoul in the Republic of Korea. The study is part of The global concern for conservation of energy and an RCA** Isotope Hydrology Programme financed by development of alternative energy sources has prompted Australia. The first phase of the project focused on the exploration for geothermal energy resources. Stable metropolitan area and the isotope data showed that isotopes can help provide information on the origin infiltrated river-water is the dominant source of recharge of the geothermal waters. However, the key question to groundwater. In contrast, the groundwater in the in the assessment of any system is the temperature of valley downstream of Seoul is recharged by infiltration the fluid. This will determine whether the fluid can be of precipitation. The work is currently being extended used to produce electricity or whether it is suitable only to cover a larger area and also to study the origin of for domestic and agricultural space heating. Isotope groundwater in deeper crystalline rocks. geothermometers may be used to estimate temperatures at depth. The principle depends upon the fact that the distribution of isotopes among the phases and compo- Waste disposal nents of a geothermal system is a function of temperature. The application of isotope techniques to the hydrology Thus from measurements of a given isotope in two of crystalline rocks has recently gathered momentum. components an estimate of the temperature may be On the one hand this has been due to the fact that, in derived. An example of an isotope geothermometer is many areas, water in fractured crystalline rocks is the the sulphate-water exchange reaction in which the only source of supply. However, in general these rocks oxygen-18 distribution is measured. This geothermo- are unable to transmit significant volumes of ground- meter has been used in a number of different geothermal water. In the extreme situation of essentially no fields in different parts of the world. The Agency has , these rocks are of interest as potential already been involved in the use of isotope techniques host rocks for storage of radioactive waste. In conjunc- in geothermal exploration in, for example, Costa Rica, tion with other hydrological and conventional methods, , Italy, Mexico, and Thailand. one may expect an increased use of isotope techniques not only in the assessment of potential sites for storage of radioactive waste, but also for the disposal of Technology transfer municipal wastes. Isotope techniques are now an established tool for Groundwater often occurs in distinct horizons which the hydrologist. Some of the techniques can provide are separated the one from the other by impermeable unique information which is unobtainable by other layers. One of the water-bearing horizons may contain methods. Although much work remains to be done on water of inferior quality, such as high salinity, and it is the refinement of existing methods and development important to know whether this poorer quality water and proving of new techniques, there remains a need is leaking to the other water-bearing horizon, or for continuing effort to transfer the knowledge of aquifer, which is being used as a supply of water. One already established techniques to the developing countries. In this respect the Agency's efforts are * Regional Co-operative Agreement for Research, benefiting from specific financial contributions from Development and Training Related to Nuclear Science and some Member States such as Australia and the Federal Technology. Republic of .

12 IAEA BULLETIN, VOL.24, No.3