The term ``isotope hydrology'' is, perhaps, unfortunate. It refers to the application of techniques that measure isotopic abundances (i.e., relative numbers of atoms of an element having different numbers of neutrons in the nucleus) to problems in hydrology. However, ``isotopic'' techniques can be applied to a very wide range of hydrological problems. For example, isotopic abundances may be used as conservative tracers for transport by flowing groundwater, for estimating amounts of materials going from one phase of a system to another, for determining extents of chemical reactions in the subsurface, for identifying recharge areas of aquifers, or for estimating subsurface residence times. In general, neither the analytical techniques used nor the physical processes involved have much in common from application to application. Instead, the common thread throughout the field is a basic application of simple nuclear physics and a generally understood (within the specialty) jargon and notation.
The roots of isotope hydrology can be traced back to the pioneering work of Urey [1947] and Arnold and Libby [1949]. At that time the principles of stable isotope fractionation and cosmogenic nuclide production were at the cutting edge of nuclear chemistry and nuclear physics. Logically, the first practitioners were physicists and chemists who, as time passed, gradually transmuted themselves into geophysicists and geochemists. The hydrology community is progressively appropriating for itself the principles of nuclear physics and chemistry, and, as a result, isotopic techniques are moving from a status akin to alchemy (a field for which the fundamentals are difficult to understand, the terminology peculiar, and the results dubious) to the rank of a method that many might consider applying themselves for approaching groundwater problems. Most hydrologists classify their problems by setting and process, and this review will follow such a scheme. Application to problems and processes in the vadose zone will be considered first, followed by shallow water table flow systems. Next, studies on an aquifer scale will be considered, finally followed by methods appropriate for large, regional-scale systems.
The progression of ``isotopic'' techniques from the bailiwick of a few imported experts, to a minor subfield of hydrology, to a set of techniques used along with a wide range of others in routine hydrological investigations parallels a major theme of this volume: the movement of concepts from ``basic research'' to implementation on practical problems. Isotopic techniques in hydrology are, in general, only part way along this continuum. They have moved from the domain of researchers who applied them for the sake of understanding the phenomena themselves to the general possession of researchers working on applied hydrology problems. They have not yet, however, become widely recognized and applied by the ``commercial'' community. Given the example of other hydrological techniques (e.g., numerical modeling), this acceptance can eventually be expected. Probably, this acceptance can most effectively be fostered by making sure that undergraduate and masters-level hydrology students understand the rudiments of nuclear physics, and by making a case that the basics of isotope hydrology be included in textbook chapters on chemical fundamentals instead of being relegated to some small, isolated section where the concepts are presented bereft of hydrological context. Finally, the addition of a few case study examples of isotopic applications to practical problems might launch the march down the road that leads to implementation!