1 Aquifers and Aquifer Systems

The physical setting of groundwater can be viewed at different spatial scales, ranging from microscopic to global. Each level has its own merits for studying specific types of groundwater features and processes, making use of concepts and parameters tailored to the corresponding spatial resolution. This section focuses on aquifers and aquifer systems. It outlines their position in the hierarchy of spatial scales that are of interest for groundwater science and assessment; reviews how they are defined and how they are classified according to their hydraulic conditions; depicts briefly their diversity in terms of lithological aquifer categories, geological and climatic setting, and aquifer productivity; and gives an impression of the enormous variation in the size of aquifers and aquifer systems. The section ends with the presentation of a map of seventy large and very large aquifer systems spread across the world.