4.1 Existing Information and Knowledge about Large Aquifer Systems

The preceding sections form only an introduction to the subject of large aquifer systems. They intend to clarify what is meant by ‘large aquifer systems’, and to make the reader more familiar with the world’s largerategst aquifer systems. The latter is done by a systematic inventory, review and comparison of several important macroscopic characteristics of a selected set of very large aquifers (mega aquifer systems).

What is presented in these sections summarizes only a very minor fraction of what is known about these systems. Numerous scientists and organizations have been and are still engaged in exploring, assessing and studying large aquifer systems or parts of such systems, at a variety of spatial scales and covering a wide range of themes or aspects. A significant part of the knowledge and information they acquire in this way is publicly shared in the form of reports and papers that are increasingly accessible via the internet. For some large aquifer systems, it is difficult to find such sources of information, perhaps because of the language used for reporting or restrictive policies on sharing information, or simply because the aquifer system has been hardly explored and studied so far. For other aquifers, on the other hand, it is possible to access large numbers of publications. In some cases, organizations and scientists have made special efforts to concentrate relevant information on the aquifer systems of their concern in the form of dedicated information systems (e.g., USGS, 2021 and BGS, 2021), monographs (e.g., UN-ESCWA and BGR, 2013; Ransley et al., 2015) or special issues of scientific journals (e.g., IAH, 2018, 2020).