More than half of the surface area of the continents is covered with hard rocks of low permeability. These rocks may acquire moderate to good permeability on account of fracturing and hence are broadly grouped under the term fractured rocks, in the context of hydrogeology. The importance of systematic hydrogeological studies in fractured rocks was realised by the international community about 50years back when an international symposium was held at Dubrovink in the erstwhile Yugoslavia (UNESCO-IAHS 1967).

Traditionally, hydrogeology is defined as the area of geologic sciences, as well as engineering, that in its broadest sense deals with water below the land surface. Hydro is the Greek word for water, and geology is the science of Earth: geo being a prefix derived from the Greek word gaia (meaning the earth, but also Gaia – a primordial Earth goddess in Greek myth), whereas logy is derived from the Greek word logos which means word, thought, principle, or speech, but in modern times is most commonly understood to mean science. Thus, geology literally means the science of Earth.

Hydrogeology (from the Greek hydra: water, ge: earth, and logos: discourse) can be defi ned as the science of groundwater. That said, water forms a single unit: precipitation, surface water, ice, and subterranean water are all parts of a continuous cycle. Hydrogeology therefore cannot be separated from surface hydrology, climatology, geology, and geography. Furthermore, as with all modern sciences, it calls on the countless domains included in physics, chemistry, and biology. As water becomes more and more valuable, hydrogeologists are also confronted with social and political problems. More, perhaps, than any other science, hydrogeology today requires an interdisciplinary approach. Defi ning it precisely is therefore a diffi cult exercise. Where does the subterranean world of the hydrogeologist begin and end? Is water contained in magma a part of it? Do hot springs at midocean ridges fall under the domain of hydrogeology? Does one classify as subterranean a river crossing a cave/tunnel such as the Mas d’Azil (Ariège)? Is soil, where important physico-chemical reactions take place and where water can be extracted by plants, a subterranean domain? The reader will therefore understand that the simple defi nition “science of subterranean water” is intentionally vague, so to be all the more precise. <...>

Over the years, NGWA Press, the official text publishing imprint of the National Ground Water Association, has produced a number of texts with glossaries. For the first time, we have gathered all of these various glossaries into a single source, and for many of the terms we have added illustrations and photographs. The individuals named as contributing to this compilation have all contributed over the years to NGWA Press titles that have featured glossaries.