Isotope Hydrology Techniques - Practical Tools to Solve Water Problems by B.R
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Technical co-operation Isotope hydrology techniques - practical tools to solve water problems by B.R. Payne* At the time of the creation of the International waters. Today the mostcommonly used environmental Atomic Energy Agency, there were relatively few appli- isotopes are the stable isotopes deuterium and oxygen-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, isotope hydrology techniques were The potential usefulness of the naturally present used to measure the flow-rates of rivers. Only relatively tritium in meteorology, hydrology, and oceanography 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 precipitation 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 sediment 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 United States 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 sediments 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 dams, measuring At an early stage the Agency recognized that the the groundwater flow-rate and direction from the dilu- introduction and development of these new techniques tion of a radioisotope injected into a section of a borehole. 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- Isotope analysis 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. Infiltration 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 river. 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 Sudan. 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 confluence to the groundwater to be studied.