Application of isotope techniques in hydrology. A comprehensive report from a panel

Tracers for water are not a new innovation in hydrology, but have been used on a rather limited scale in the past for various purposes. For instance various dyes or salts have been used in tracing underground water in karstic limestone areas and for computing discharge rates of small turbulent rivers like mountain streams. There are, however, several limitations to their use. Dyes employed in tracing underground water are often sorbed by the ground and hence results are qualitative. For discharge measurements in streams the amount of salt needed limits the method to small streams. With the advance of nuclear science, suitable radioactive, isotopes became available which, because of the sensitivity of their detection, could be added in relatively small amounts. Also, when tritium was discovered, it was realized that it is an ideal tracer for water as it can be incorporated into the water molecule. During recent years fractionation effects have been discovered in several stable isotopes present in water which seem promising as tools for obtaining useful hydrological information, especially on the origin of water. Finally, with the discovery of cosmic-ray-produced radioactive isotopes, especially tritium, a method has been found for determining the age of groundwater which is of great value for estimating the age and hence the recharge-rates of groundwater bodies. There is thus, at present, a great variety of isotope techniques available, some of them in the development stage, others more or less ready for application. These various techniques will be discussed below. The presentation of the information follows the pattern envisaged by the Panel. Thus, for each chapter the problem is first presented and then followed by a section called 'discussion' which summarizes the information given by the Panel members and also expresses their opinions. Finally, a section called 'conclusions and recommendations' sums up the present situation and the need for further development work. The last chapter is a collection of examples of more intricate problems where isotope techniques could profitably be used. It is hoped that the readers concerned with hydrological problems will find this a guide for judging the possibility of isotope applications to their own hydrological problems